Exotic materials and innovative concepts for photovoltaics

Resume : The bulk photovoltaic effect exploits the intrinsic polarization field in ferroelectrics to separate charges upon exciton dissociation into electrons and holes. For several decades this effect was considered mostly for its fundamental interest as the power conversion efficiencies (PCE) were extremely low, mostly due to the large bandgap of most ferroelectrics and to their insulating nature. There has been a resurgent interest in this effect during the last decade, when it was shown that multiferroics, i.e. materials that exhibit two ferroic properties (in this case ferroelectricity and magnetism) exhibit higher efficiencies, mostly attributed to their smaller bandgaps [1-8]. In addition to these latest results, which brought efficiencies close to double digits [4], we also discuss the application of ferroelectric and multiferroic nanostructures for hydrogen generation [9-13]. References [1] R. Nechache et al., Appl. Phys. Lett. 98, 202902 (2011); [2] J. Chakrabartty et al., J. Am. Ceram. Soc. 97, 1837–1840 (2014); [3] J. Chakrabartty et al., Optics Express 22, A80–A89 (2014); [4] R. Nechache et al., Nature Photonics 9, 61 (2015); [5] R. Nechache et al., Nanoscale 8, 3237 (2016); [6] J. Chakrabartty et al., Nanotechnology 27, 215402 (2016); [7] W. Huang et al., J. Mater. Chem. A 5, 10355–10364 (2017); [8] J. Chakrabartty et al., Nature Phot. 12, 271–276 (2018); [9] S. Li et al., Chem. Comm. 49, 5856 (2013); [10] S. Li et al., Small 11, 4018 (2015); [11] S. Li et al., Nano Energy 35, 92–100 (2017); [12] J. Chakrabartty et al., J. Power Sources 365, 162–168 (2017); [13] W. Huang et al., ACS Appl. Mater. Int. 11, 13185–13193 (2019).

Resume : The symmetry lies at the heart of the law of nature and forms the basis for modern physics. Breaking of inversion symmetry by external stimuli would enable in certain materials exotic phenomena which are otherwise are prohibited. Here, we would like to introduce a new physical concept to harvest solar energy into electricity by inversion asymmetry engineering, i.e., the flexo-photovoltaic effect. The flexo-photovoltaic effect is the generalized version of the well-known bulk photovoltaic effect mediated by the strain gradient. The strain gradient, namely inhomogeneous strains, induces electric polarization in materials of all symmetry and at the same time enables the manifestation of the bulk photovoltaic effect that is free from any symmetry limitation. Being distinctive from the conventional photovoltaic effect based on built-in field (such as p-n junctions), the non-equilibrium charge separation mechanism therein originates from the inversion symmetry breaking induced asymmetric distribution of the non-equilibrium photo-generated carriers in momentum space. The flexo-photovoltaic effect not only provides a new route to boost the energy conversion efficiency in the present solar cell technologies from a wide pool of established semiconductors, but also offers new insights into the understanding of the structure-property correlations in photo-active materials.

Resume : The creation of a photo-current in a uniform bulk material is referred to as bulk photovoltaic effect (BPVE). The origin of the BPVE is found in an internal asymmetry of the lattice that leads to a shift current upon photo-excitation. Interestingly, and in contrast to p-n junction- based photovoltaics, many materials exhibiting a BPVE have shown open circuit voltages far beyond the bandgap of the material. This means that the Shockley-Queisser limit for p-n junction solar cells does not apply to the BPVE. However, no energy conversion limit has been formulated. In this contribution, we theoretically analyse I-V characteristics for ideal BPVE materials and use these to derive monochromatic and broadband solar energy conversion limits. Shift currents as instantaneous spatial shifts of the electron wavefunctions have successfully been calculated for various materials. However, the shift current calculations do not reveal any voltage-dependence, which makes it currently impossible to calculate energy conversion efficiency limits, or limits on the current-voltage product, using this formalism. The BPVE is also described as arising from an asymmetric electron distribution in momentum space that is created by the absorption of photons. Ballistic transport then leads to a net current in a particular direction. The voltage dependence of the current arises from the acceleration of all electrons and holes, both photo-excited and intrinsic, in the electric field generated by the applied voltage. The result is a linear I-V curve and materials with low intrinsic carrier densities can show extremely large open circuit voltages. This linear I-V behaviour is typically found in experiment. The magnitude of the short circuit current is proportional to the scattering lifetime of the carriers. In contrast, the magnitude of the open circuit voltage is inversely proportional to the scattering lifetime as the increased carrier mobility resulting from this leads to a larger carrier acceleration with the applied electric field. This combination of dependencies leads to an energy conversion limit that is independent of scattering lifetime and, in the ideal case, corresponds to the device being able to extract half of the kinetic energy of the photo-excited carriers. This implies that the smaller the bandgap of the material, the higher the potential solar energy conversion efficiency. However, the presence of intrinsic carriers also leads to smaller voltages as they too are accelerated in the photo-generated electric field. Therefore, materials with small bandgaps, which have high intrinsic carrier densities, quickly become extremely inefficient at generating a bulk photovoltage. Our analysis shows that the BPVE has an ideal energy conversion efficiency limit that is smaller than the Shockley-Queisser limit despite the potential for extremely large open circuit voltages.

Resume : Although Si-based solar cells are widely used to convert light into electric current, their efficiency reached already 26%, close to the theoretical Shockley-Queisser limit which is about 30%. Alternative materials are expected to further improve the performances of solar cells. Among these materials, ferroelectric (FE) oxides are particularly interesting since they are highly stable and do not need a p-n junction to separate the charges as this is the case in conventional photovoltaic devices. For such applications, Bi2FeCrO6 (BFCO) double perovskite seems to be a good choice. In epitaxial films, the gap has already been modulated between 1.4 and 2.6 eV by controlling the growth conditions and by improving the Fe-Cr order.1 In order to obtain PV devices, highly Fe-Cr ordered BFCO thin films are needed along with the deep understanding of their electrical and optical properties. Epitaxial BFCO films are grown by pulsed laser deposition on SrTiO3(001) that presents a mismatch of 0.77%. The films are therefore strained on the substrate which has impact on the Fe-Cr order and the electrical properties.2 In this work, we show the influence of strains on the FE properties of BFCO with an impact on the polarization anisotropy and the FE domain polarization. Conductive-AFM measurements confirm the potential of BFCO for FE solar cells applications. [1] R. Nechache et al, Nature Photonics 9, 61 (2015). [2] M.V. Rastei et al, ACS Applied Energy Materials 2, 8550 (2019).

Resume : Cuprous oxide thin films are potential candidates for various optoelectronic devices. The Cu2O material has been up to now delivering the highest solar cell efficiencies among many oxides. In general, oxides can provide the advantages of abundance, stability, non-toxicity and low-cost manufacturing with a wide range of thin film deposition methods. Here we successfully prepared p-type Cu2O thin films by pulsed laser deposition at both low temperature and high temperature by carefully controlling the oxygen partial pressure. The growth conditions were carefully optimized to provide the best conductivity and mobility. The properties of these films were analyzed structurally, optically and electrically by different techniques. X-ray diffraction patterns of the films deposited on quartz substrates showed significant peaks for (200), (311), (220) and (111) indicating good crystallization. Approximately 100 nm thick cuprous oxide films deposited at high temperature were found to have resistivity of 26 ohm.cm, a high mobility of 25 cm2/(V.s) and a bandgap of around 2 eV. We will describe the fabrication of all-oxide solar cells based on these films and their characterization and optimization.

Resume : Cuprous oxide (Cu2O) is a non-toxic and abundant p-type semiconductor with a direct band gap around 2.1 eV and a large visible absorption coefficient. It has been studied and developed for several devices such as solar cells, thin film transistors or batteries. In this study, an innovative technique for depositing conformal and high-quality thin films, AP-SALD (Atmospheric Pressure Spatial Atomic Layer Deposition), is used to deposit Cu2O at low temperatures (up to 260 °C), under atmospheric pressure for photovoltaic applications. AP-SALD is an alternative approach to conventional ALD in which the precursors are separated in space rather than in time, allowing fast deposition rates as compared to conventional ALD (up to nm/s in some cases). The aim is to optimize the low-cost Cu2O deposition by AP-SALD on different substrates, even flexible, with a control over growth rate and transport properties (mobility and concentration of carriers). The effect of deposition parameters has been carefully studied, and mobility values of 91 cm²V-1s-1 have been obtained, close to values associated to epitaxial Cu2O thin films or Cu2O single crystals. Optimized Cu2O thin films, combined with n-type ZnO also deposited by AP-SALD, lead to all-oxide solar harvesters with efficiency rivaling values for similar devices made with high temperature and/or vacuum approaches. This shows that AP-SALD is a suitable approach to fabricate all-oxide solar harvesters, on both glass and flexible substrates.

Resume : Cu2O with intrinsic p-type conductivity is one of the most attractive p-type transparent conductive oxides (TCO). It holds an enormous potential in the applications of transparent displays, invisible integrated circuits, photovoltaics, photocatalysis and sensor arrays. Its advantages are related to its direct band gap of 2.1 eV, the abundance of the source materials, its non-toxicity and low production costs. In addition, cuprous oxide shows a relatively high mobility, around 100 cm2 V-1 s -1, motivating its potential use in the development of transparent electronics. Nevertheless, Cu2O thin films present a poor p-type conductivity, lower than 10−2 S × cm-1, which limits the electrical properties of Cu2O-based heterojunctions. In order to address this issue, the introduction of other cations such as magnesium (Mg) into Cu2O is considered as a promising way to increase the material conductivity. Previous works have indeed shown that Mg-doped Cu2O thin films exhibit enhanced opto-electrical properties. Techniques for the deposition of Cu2O thin films include pulsed laser deposition, magnetron sputtering, thermal evaporation, chemical vapor deposition, sol-gel, spray pyrolysis and electrodeposition. Among all these methods, sputtering has many advantages, yielding high-quality uniform film at relatively low substrate temperatures, using cost-effective processes allowing for an accurate control over the deposition parameters. In this work, we deposit undoped and Mg-doped Cu2O (Mg:Cu2O) films by radio-frequency magnetron sputtering and we study the effects of the argon pressure in the sputtering chamber (PAr) on the surface morphology as well as the optical and electrical characteristics, using a systematic approach based on material characterization by scanning electron microscopy, X-ray diffraction, optical absorption spectroscopy and sheet resistance measurements. We demonstrate the existence of a strong dependence between the film properties and the sputtering pressure. The experimental results reveal that an increasing of the sputtering pressure from 5 to 15 mTorr leads to a significant effect on the electrical properties. In particular, hole concentration in Mg-doped Cu2O films reaches values up to 4.3 1016 cm-3, an increase of at least 4 orders of magnitude compared to undoped films. In addition, opto-electrical properties of Mg:Cu2O were improved compared to intrinsic Cu2O thanks to a strong reduction of the resistivity from 100 to 6 Ω.cm. These results demonstrate the potential of magnesium incorporation into sputtered Cu2O for the production of an efficient p-type material in transparent photovoltaic heterojunctions.

Resume : Flexible transparent electrodes are indispensable components in optoelectronic devices such as solar cells, meeting the requirements for low-cost fabrication, portability or wearability. To this end, dielectric-metal-dielectric (DMD) stacks have emerged as compelling alternative to traditional transparent conductive oxides like ITO, due to the high ductility of the thin metal layer and reduced material costs. However, low temperature processing techniques are required to account for the thermal sensitivity of flexible substrates like PET, which demands temperatures below 150°C. In this study, sputter deposition of niobium-doped titanium oxide (NTO) from a conductive oxide target in DC magnetron mode allows for high deposition rates at room temperature, resulting in transparent NTO layers even in non-reactive, oxygen-free atmosphere. The NTO is then combined with ultrathin Au and Ag layers to create flexible and transparent DMD electrodes on PET, reaching transmittance values above 70% and sheet resistances below 10 Ohm/sq. Bending tests of the DMD electrodes show their flexibility and mechanical stability under tensile stress throughout 3000 bending cycles, proving their compatibility with roll-to-roll production requirements and flexible devices. These electrodes are then combined with low-temperature, solution-processed oxide scaffold layers, in order to deposit perovskite solar cells. The approach yielded indium-free, flexible solar cells with a power conversion efficiency of 9%.

Resume : The understanding of processing-structure-properties relationship in organic solar cells is severely limited by lack of experimental tools suitable for characterizing the bulk heterojunction (BHJ) morphology. More specifically, techniques that rely on order are impractical for the disordered domains abundant in the film; surface analysis techniques do not necessarily represent the underlying bulk structure, and the all-organic composition imposes inherent low z-contrast hindering imaging by electron microscopy. Recently, we suggested the use of vapor phase infiltration (VPI) to characterize the internal morphology of organic solar cells. VPI infuses inorganic materials into an organic matrix by exposure to gaseous precursors that diffuse into the film and in-situ convert to an inorganic product. The diffusion process proceeds selectively through domains with high free-volume leading to inorganic deposition selectively along the diffusion paths. The diffusion network is easily visualized using electron microscopy because of the high contrast between the organic matrix and the accumulated inorganic phase. This labelling approach is in concept similar to the staining approach used to image low contrast biological specimens in electron microscopy. Furthermore, the inorganic phase deposited in the organic film also provides material stability for HRTEM imaging and tomography. We used this newmethodology to characterize the morphologies polymer:fullerene and polymer:non-fullerene BHJs as a function of composition, thermal treatments and processing technique. We were able to visualize crucial characteristics of BHJ morphologies, such as domains size, shape and distribution, 3D-connectivity and interface area through fast and straightforward HRSEM and more advanced HRTEM imaging and tomography. The spatial mapping of the phase morphology and the phase behavior of each system were then correlated with device performances to provide insight on the processing-structure-property relationship towards directing improved devices.

Resume : To enable terawatt-scale photovoltaics, resource and cost efficiency are mandatory. Perovskite silicon tandem solar cells can achieve both goals, by exceeding the efficiency limit of 29.4% of single junction silicon solar cells, with only little added production costs. For highest efficiencies, important material-related choices concern the absorber material, the contacting and passivation layers, and the sequence of deposition. We apply a photo-stable mixed cation mixed halide perovskite FA0.75Cs0.25Pb(I0.8Br0.2)3 absorber with a bandgap of 1.7 eV, in the optimal range for monolithic silicon-based tandem devices. This absorber is used in both n-i-p and p-i-n devices. In the n-i-p-configuration, an n-type silicon heterojunction (SHJ) bottom solar cell is connected via an ITO recombination contact to the perovskite top solar cell. Evaporated compact TiO2 and UV-treated mesoporous TiO2 form the electron contact, passivated with spin-coated PCBM and PMMA. The hole contact is formed by evaporated Spiro-TTB and directly sputtered ITO, capped with an evaporated MgF2 antireflection coating (ARC). This configuration reaches 23.4% stabilized efficiency. In the p-i-n-configuration, a p-type SHJ solar cell with an ITO recombination contact is coated with spincoated PTAA and PFN, the perovskite absorber, evaporated C60, ALD-deposited SnOx, sputtered ITO and again MgF2 ARC. Despite a lower voltage, this configuration achieves 25.1% stabilized efficiency due to a higher current. Our optical simulations show that with adaptions of the layer thickness more than 29% efficiency should be possible with this structure in the near future.

Resume : Optical strategies create a conducive path for perovskite solar cell (PSC) technology development, not only to increase their efficiency, but also to enable less material usage - in view of enhanced flexibility and improve the cells´ stability [1-2]. We developed and studied two novel photonic concepts in the wave-optics regime and applied them to PSCs with distinct thicknesses of the perovskite absorber (250-500 nm). First, front-located photonic-structured electron transport layers (ETLs) demonstrated the best photocurrent improvement (up to 27% - value rather close to the fundamental Lambertian limit), together with a proclivity to protect the absorber layer from harmful UV radiation which is a vital issue concerning the PSCs stability. Secondly, PSCs conformally deposited on photonic-structured substrates show also a comparable photocurrent enhancement to the previous structure (up to 25%). Furthermore, this latter configuration also offers considerable practical advantages regarding its applicability in the PSCs’ process technology. Moreover, optimal wavelength-sized semi-spheroidal structured substrates also show a significant omni-directional optical response for angles up to 70º, therefore underlining the viability of this light trapping scheme for implementation in flexible photovoltaic (PV) devices that need optimal operation under bending. In addition, both photonic-embedded ultra-thin solar cells designed here ultimately support the reduction of material usage in PSC technology, that positively accommodates the reduction of the absorber thickness to more than 2-fold, thereby improving the intrinsic mechanical flexibility, as well as the mitigation of lead usage, without impacting the device’s performance. References: [1] S. Haque et al. Nano Energy. 59 (2019) 91–101 and [2] S. Haque et al. Applied Materials Today, Volume 20, September 2020, 100720

Resume : Addressing the mixed conducting properties of hybrid perovskites is a critical challenge for the development of solar cells based on this class of materials. The significant ionic conduction in these compounds is responsible for the long timescale electrical response of solar cells as well as an important factor limiting device lifetime. Mapping mixed conduction as a function of environmental and bias conditions is a key target for guaranteeing the stable function of hybrid perovskites and to establish them as a reliable photovoltaic technology. One of the most critical knowledge-gaps which lie between this vision and the current status of the field concerns defect chemical models able to describe and predict device behavior on the basis of ionic and electronic properties. This includes knowledge of the mechanisms underlying bias dependent polarization processes, trap-mediated bulk and interfacial recombination of electronic carriers. Including these aspects in models that encapsulate both the defect chemistry and the device physics of solar cells will benefit the interpretation of experimental data and the progress of the field towards mitigating reliability issues due to ionic conduction. In this contribution, we address such questions using a combined experimental and theoretical approach. We evaluate the contribution of ion conduction under different conditions using electrochemical conductivity measurements.[1] Control on the iodine partial pressure during the measurement, on the device contact materials and on the electrode geometry allows to discriminate between bulk and interfacial contributions to the polarization phenomena in the mixed conductor, under dark and under light. Next, we develop a model describing the ionic and electronic properties of perovskite-based devices. After deriving the analytical description of the recombination currents in perovskite semiconductors as a function of recombination mechanism and working conditions, we use drift diffusion equations of electronic and ionic charge carriers to obtain a model bearing analytical consistency with the expected semiconductor and mixed conductor physics of hybrid perovskites’ bulk. The contribution from ionic-to-electronic current amplification to the device response is generalized.[2] We finally develop equivalent circuit models of mixed conducting devices and discuss its generality at describing solar cell absorbers with different properties. [1] T. Y. Yang et al. Angew. Chemie - Int. Ed. 54, 7905 (2015). [2] D. Moia et al. Energy Environ. Sci. 12, 1296 (2019).

Resume : P-type dye-sensitized solar cells (DSCs) offer the possibility to directly convert sunlight into electrical energy at a low cost. However, these devices have rather poor efficiencies (PCE?2.5%) compared to the n-type DSCs (PCE?14%) for reasons that are not yet clarified (slow hole injection, fast back recombination pathways).[1] P-type DSCs are based on the hole injection from the HOMO of the photoexcited dye (e.g. C343) into the valence band of a semiconductor (e.g. NiO) and the driving force (the difference between the VB energy and the HOMO energy) has a key role on the overall efficiency of the DSCs.[1-2] As far as dye-sensitized NiO is concerned, relatively few computational works, based on density functional theory (DFT) calculations, have been reported.[1-5] A common outcome of these studies is that the interfacial energetics, dictating the driving force for hole injection from the dye to the NiO VB, turns out to be extremely sensitive to the dye anchoring mode and solvation effects, thus indicating that the structural and electronic properties of complex interface between dye, semiconductor and electrolyte play a major role in the properties of DSCs. In this framework, we carry out a theoretical characterization of the C343-sensitized NiO surface in water, by combining ab-initio molecular dynamics simulations with GW (G0W0) calculations along the MD trajectory, to reliably describe the structure and energetics of the interface when explicit solvation and finite temperature effects are accounted for. [1] Piccinin, S. et al. J. Phys. Chem. C 2017, 121, 22286?22294. [2] Pavone, M. et al. Phys. Chem. Chem. Phys. 2015, 17, 12238?12246. [3] Sun, L. L. et al. RSC Adv. 2015, 5, 39821?39827. [4] Naik, P. et al. Sol. Energy 2017, 157, 1064-1073. [5] Zhang, T. et al. Phys. Chem. Chem. Phys. 2015,17, 5459?5465

Resume : Perovskites have emerged in recent years as promising materials for PV solar cell applications.1-3 The two-step method has been proved to be an effective approach to convert binary compounds into high-quality perovskites for high-performance perovskite solar cells.4 In this work, we present a new method to produce high-quality thin films of methylammonium lead iodide (MAPbI3) perovskite via an all-solution two-step conversion of chemically deposited PbS thin films into PbI2 and consequently to MAPbI3 films. By controlling the conversion parameters (such as solvent, temperature, reactant concentrations, and conversion duration) we demonstrate control over the resultant perovskite film morphology. We have previously reported on the control over solution deposited PbS film morphology and its resulting physical properties by optimizing the deposition parameters (temperature, pH, growth duration, etc.).5–7 We used films with columnar morphology with a high density of vertical grain boundaries which can facilitate the penetration of iodine within the films. In the first conversion step, PbS films were converted into PbI2 by treating the films in polyiodide solutions dissolved in a mixture of water and isopropanol. Different ratios of water/isopropanol resulted in different PbI2 morphologies. In the second step, the PbI2 films were converted into MAPbI3 by immersion in an isopropanol solution of methylammonium iodide. Distinctive MAPbI3 grain sizes were obtained depending on the initial PbI2 morphology. The degree of conversion to perovskites was found to depend on the first step of conversion; the highest conversion to perovskite was obtained from PbI2 samples prepared with solutions containing 20% water. In order to confirm the quality of the perovskite films, photoluminescence (PL) measurements were carried out, showing a distinctive peak at ~772nm, with a rather small intensity. To increase PL intensity, different surface passivation treatments were studied. Concentrated pyridine was found to decompose the films immediately, while diluted pyridine (10% in isopropanol) was found to significantly enhance PL intensity. Although the intensity was increased and the sample did not decompose immediately, low concentrations of remaining pyridine still accelerated the decomposition of the film and one day later the samples showed yellow areas that correlated to PbI2. SEM showed that the pyridine reacts very strongly with the perovskite, altering the morphology of the film. However, 2-amylpyridine (2-py) was shown to preserve film morphology and composition while maintaining reasonably intense PL emission. It was previously reported that the alkyl chain near the nitrogen atom suppresses reaction with the lead ion, thus preventing corrosion of the perovskite while passivating the surface and enhancing stability8. SEM images showed well-defined facets of the perovskite grains, while the morphology remained unchanged. PL measurements exhibited an increase in intensity, with the peak position remaining at ~772 nm. Although the passivation using pyridine resulted in higher PL intensity compared to passivation with 2-py, the stability of the perovskite passivated with 2-py was better. 1. Shockley, W. & Queisser, H. J. Detailed balance limit of efficiency of p-n junction solar cells. J. Appl. Phys. 32, 510–519 (1961). 2. Best Research-Cell Efficiency. NREL accessed 23 September,2019 (2019). 3. Green, M. A., Ho-Baillie, A. & Snaith, H. J. The emergence of perovskite solar cells. Nat. Photonics 8, 506–514 (2014). 4. Chen, H. Two-Step Sequential Deposition of Organometal Halide Perovskite for Photovoltaic Application. Adv. Funct. Mater. 27, 1–19 (2017). 5. Osherov, A., Ezersky, V. & Golan, Y. The role of solution composition in chemical bath deposition of epitaxial thin films of PbS on GaAs (100). J. Cryst. Growth 308, 334–339 (2007). 6. Osherov, A., Shandalov, M., Ezersky, V. & Golan, Y. Epitaxy and orientation control in chemical solution deposited PbS and PbSe monocrystalline films. J. Cryst. Growth 304, 169–178 (2007). 7. Osherov, A. & Golan, Y. Chemical solution deposited PbS thin films on Si(100). Phys. Status Solidi Curr. Top. Solid State Phys. 5, 3431–3436 (2008). 8. Yue, Y., Salim, N., Wu, Y., Yang, X. & Islam, A. Enhanced Stability of Perovskite Solar Cells through Corrosion-Free Pyridine Derivatives in Hole-Transporting Materials. 10738–10743 (2016).

Resume : Significant growth in Pb-based hybrid perovskite (Pb-perovskite) solar cells in recent years suggests that Pb-perovskite could be the most promising substitute for the silicon widely used in the solar market. However, the presence of Pb and the instability the material are major obstacles to commercialization. Therefore, considerable research is currently underway to find alternative low-toxicity stable solar absorbers. Bismuth and antimony-based chalcoiodides, (Bi,Sb)SI, can be good candidates due to their low-toxicity and stability. In addition, the solar cells based on these are expected to exhibit high efficiency because they contain post-transition metals with ns2 electronic configuration making them defect-tolerant semiconductors. In this presentation, we will introduce the fabrication of (Bi,Sb)SI thin films for solar cells application [1,2]. The thin films were fabricated by two-step solution process of (Bi,Sb)2S3 fabrication (step I) and its conversion to (Bi,Sb)SI (step II). The (Bi,Sb)2S3 was prepared by a thiol-amine solution process and the (Bi,Sb)SI was finally formed by chemical reaction between (Bi,Sb)2S3 prepared in step I and (Bi,Sb)I3. Pure (Bi,Sb)SI phases with a high crystallinity were obtained at a low temperature of 200 °C. In addition, the structure, absorption, morphology, and electronic structure were controlled by tuning key experimental parameters, such as molar ratio and annealing temperature. Our work is expected to contribute to developing low-cost, environment-friendly, and stable solar cells. REFERENCES [1] Y. C. Choi, E. Hwang, and D.-H. Kim, APL Materials 2018, 6, 121108. [2] Y. C. Choi and E. Hwang, Nanomaterials 2019, 9, 1650.

Resume : Hybrid lead halide perovskite solar cells have been the vigorous subject of emerging photovoltaics for the last decade, with booming performance and low-cost solution process.1 One challenge in advancing this encouraging technology to commercialization is the toxicity of lead. Although efforts have been devoted to substitute lead, nontoxicity is the cost of the efficiency. Currently, tin based-perovskite is the most promising alternative among lead-free options, with narrow bandgap and higher carrier mobility.2 The new star is mainly suffering from the oxidation of Sn2 to Sn4 , which induces the self-doping and fast carrier recombination. Herein, the project focuses on the design, synthesis, and characterization of stable tin perovskite solar cells. Notably, it emphasizes on the low-dimensional tin perovskites that demonstrate the promising results of stability and device performance.3 By combining low-dimensional perovskite film with the novel organic hole-transporting materials, such hybrid heterojunctions will undergo a series of optical spectroscopic methods (steady state and time-resolved) for their stability and performance. (1) Jena, A.K. et al Chem.Rev, vol. 119, issue 5, pages 3036-3103 (2) Herz, L. M. et al ACS Energy Letters, vol 2, issue 7, pages 1539–1548 (2017) (3) Lanzetta et al ACS Energy Letters, vol 2, issue 7, pages 1662-1668

Resume : Perovskite solar cells (PSCs) have attracted considerable attention among various new generation photovoltaic technologies due to their easy and low-cost solution manufacturing process with high power conversion efficiency (PCE). Currently, several research works are going on to enhance the efficiency of perovskite solar cells to the Shockley–Queisser limit. However, the fabrication process is usually carried out inside a glovebox to avoid moisture, because methylammonium lead iodide (MAPbI3) perovskites are easily dissolved in atmospheric water vapor. In this study, we present a one-step fabrication under endurable humidity (relative humidity ~50%) to achieve high-quality perovskite grains by using F4TCNQ as an additive into the MAPbI3. The presence of F4TCNQ bridges the gaps between the MAPbI3 grain boundaries and improves the fill factor and short circuit current compared to the basic MAPbI3 devices. We fabricated both small area and large area devices with humid conditions. This method presents a new way of controlling the growth of perovskite films in humid environments.

Resume : Recent years, ZnO nanostructures have attracted much more interest to be used as photo-electrodes in dye-sensitized solar cells (DSSCs) because ZnO has very similar wide bandgap (3.37 eV), as well as higher electron mobility than TiO2. In addition, ZnO nanostructures are much easier to be fabricated in order to increase the absorption surface for dyes. Among various fabrication methods, chemical bath deposition (CBD) method is a simple and low cost technique to synthesis nanostructures. In this research, transparent and conductive Al-doped ZnO film (AZO) (300 nm-in-thickness) was deposited on flexible PEN substrate by radio frequency magnetron sputtering. The ZnO nanorods were fabricated on AZO films by CBD method. The substrates were immersed in a solution containing Zn(NO3)2 and hexamethylenetetramine (C6H12N4, HMTA) at 95 ͦ C. The concentration of HMTA was controlled at 7.5 mol/L while keeping concentration of Zn(NO3)2 at 15 mol/L. The deposition time were set at 5,10 and 15 hours respectively. It was found that ZnO nanorods with vertical growth direction grown on AZO/PEN substrates, which was attribute to the good crystallinity of AZO thin film. The growth of ZnO nanorods could keep in the same vertical direction with the increased length when the deposition time increasing. The crystallinity of ZnO nanorods was enhanced as well. The thermal annealing treatment for the ZnO nanorods led to the significantly improvement of crystallinity and the increase of the transmittance. The obtained ZnO nanorods have the high potential to be used as photo-electrodes in DSSCs.

Resume : PEDOT:PSS (PPSS) is a standard HTL in organic solar cells (OPV) but it possesses many drawbacks that do not allow the fabrication of an efficient and chemically stable OPV. A promising solution is represented by an exotic combination of organic and inorganic materials. Among the latter, transitions metal-based oxides represent an encouraging alternative improving stability and having other peculiar features that make them appealing applied as HTL. In this work, HTLs of CuCrO2 with various Cu contents, synthesized by Aerosol Assisted Chemical Vapor Deposition, were analyzed to find a good trade-off between transparency, conductivity, level energy, bandgap (Eg) and PV performances. The effect of CuCrO2 composition on PCE was studied. The partial recyclability of devices was tested and proved by reusing the substrate/ITO/HTL structure. CuCrO2 tunes its optoelectronic properties with stoichiometry. Resistivity and Eg are reduced for higher Cu content in the film with an optimal cationic content of at% Cu = 67%. The Series and Shunt Resistance link the OPV performances also to the HTL optical properties. We obtain an increasing PCE for greater Cu amount in the HTL. The maximum is found for Cu content = 70 at.% with a PCE of 3.75%, higher than PPSS standard. The reusability was tested by the elimination of top part Aluminum/ETL/Active layer, followed by the assembly of new cells reusing the substrate/ITO/ HTL. The PCE, not altered by recycling, is 15% above the PPSS reference, after Active layer optimization.

Resume : Hybrid lead halide perovskites have been revolutionary in the photovoltaic research field, reaching efficiencies comparable with the most established photovoltaic technologies, although they have not yet reached their competitors’ stability. The search for a stable configuration requires the engineering of the charge extraction layers; in this work, molecular doping is used as an efficient method for small molecules and polymers employed as hole transport materials in a planar heterojunction configuration on compact-TiO2. 1 The active layer deposition methodology relies on solvent engineering during the casting process, leading to an ultra-flat, uniform, and thick film ensuring an optimal interface connection with the charge-extracting layer, combined with post-deposition thermal and vacuum treatments, which merge the crystalline domains and cure the defects at the grain boundaries. This method allows obtaining perovskite active layer with superior optical properties. 2 We proved the viability of this approach, obtaining significantly increased performances and reduced hysteresis on compact titania-based devices. We investigated the photovoltaic performance correlated to the hole transport material structure. We have demonstrated that the molecular doping mechanism is more reliable than oxidative doping and have verified that molecular doping in polymeric hole transport materials leads to highly efficient perovskite solar cells, with long-term stability. (1) Trifiletti, V. D., T.; Manfredi, N.; Fenwick, O.; Colella, S.; Rizzo. Molecular Doping for Hole Transporting Materials in Hybrid Perovskite Solar Cells. Metals 2020, 10 (14). (2) Trifiletti, V.; Manfredi, N.; Listorti, A.; Altamura, D.; Giannini, C.; Colella, S.; Gigli, G.; Rizzo, A. Engineering TiO2/Perovskite Planar Heterojunction for Hysteresis-Less Solar Cells. Advanced Materials Interfaces 2016, 3 (22), 1600493.

Resume : Organometal halide perovskite solar cells (PSCs) have experienced the fastest efficiency growth in the history of photovoltaic research. Yet, their stability, which is sensitive to environmental factors (i.e., moisture, oxygen, ultraviolet light), represents one of the major issue preventing their further development. In this work, starting from colloidal synthesis to device fabrication, we present a hybrid layer based on carbon quantum dots (CQDs) and mesoporous TiO2 (me-TiO2) scaffold as a downshifting layer to alleviate the UV damage and enhance the stability of planar triple-cation Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 hybrid perovskite solar cells. Here, the me-TiO2 provides an effective scaffold for CQDs to decorate on. When applied on the transparent electrode of the solar cell, this hybrid layer can convert stability-harmful UV photons to visible photons which the perovskite layer can effectively harvest. We will compare in detail the device and stability characteristics of different perovskite devices applying me-TiO2 only, CQDs only, and the hybrid CQD/me-TiO2 to investigate the synergistic effect of the hybrid material. This work highlights the hybrid CQD/me-TiO2 layer as an effective means to enhance the stability of perovskite solar cells against UV degradation.

Resume : The success story of lead halide perovskite solar cells set off a tide of novel photovoltaic materials in the last decade. However, one prominent drawback for this emerging technique is the strong reliance on lead, a well-known toxic element. One of the most promising less-toxic substitutions is tin. While instability and unsatisfactory efficiency of tin-based solar cells are primary bottlenecks compared with its lead analogues. This project focuses on design, synthesis and characterization of high-efficiency tin perovskite solar cells. Particularly, we emphasize on exploring the potential of low- dimensional tin perovskite solar cells. some efforts like compositional engineering have been made to boost the efficiency of low-dimensional perovskite solar cells. But for tin-based perovskite solar cells, tackling fast crystallization is the basic and essential way to increase PCE and stability of every bunch of samples. Herein, we will investigate the DMSO/DMF solvent mixture first and then perhaps replace DMSO with other promising solvents to study the crystal growth process of perovskite film. After good film quality is ensured, that film will be incorporated in ITO/PEDOT:PSS/(PEA)0.2(FA)0.8SnI3/PC60BM/BCP/Ag device architecture for further photovoltaic measurements such as current-voltage (J-V) measurement, photoluminescence spectroscopy, transient absorption spectroscopy and time-resolved single-photon counting.

Resume : Over the past few years, the performance of perovskite solar cells was gradually improved up to >25%, which is close to the characteristics of crystalline silicon photovoltaics. However, the high toxicity and low stability of complex lead halides used as absorber materials hamper the commercialization of this technology. Compositional engineering of lead halide perovskites was actively pursued to improve their stability and/or performance. In particular, a partial or full replacement of Pb2+ in APbX3 is highly desirable in terms of developing more environmentally friendly and/or durable materials. Herein, we present a systematic study of lead substitution in CsPbI3 with >30 different cations introduced in atomic concentrations ranging from 1% to 20-30%. It was shown that a series of 25 cations enables the formation of the black orthorhombic γ-phase of CsPb1-xMxI~3 at relatively low temperatures of 100-200C, whereas non-modified CsPbI3 is obtained as a black polymorph above 340oC. Among these cations, we revealed 10 of the most promising systems surviving >500 h of continuous light soaking (0.85 suns at 50 oC). The XRD and EDS elemental mapping suggested the localization of Mn+ cations at the grain boundaries, thereby providing a deeper understanding of the stabilization mechanism of lead halide perovskites. The CsPb1-xMxI~3 formulations showing the best photostability were evaluated as absorber materials in perovskite solar cells and demonstrated encouraging performances: e.g. CsPb0.9Ba0.1I3 delivered efficiency of 11.4% in non-optimized devices. Thus, the obtained results suggest that the partial Pb2+ substitution in complex lead halides represents a highly promising strategy for designing efficient and stable perovskite solar cells.

Resume : The ever increasing demand for energy in third world countries has necessitated the need of coming up with measures of seeking alternative energy sources. Solar energy is one of the most important alternative sources of energy because of its abundance in these tropical regions. Up to this time, the use of the first and second generations solar cells made of silicon in making solar panels has notable shortcomings such as unaffordability and lack of longevity of the electric power generated. The band gap and the absorption coefficient of CH3NH3PbI3 are here calculated using density functional methods. The norm conserving pseudo potential generated using the Troullier-Martins scheme was used. Band gap was calculated as 1.58 eV which is close to the experimental value which is approximately 1.56 eV. The absorption coefficient was calculated and found to be 5.76x105cm-1 within the range of 0 – 6.0 eV. The values of the parameters were compared with the available experimental and theoretical values. There is a fairly good agreement between experimental and theoretical data and this computational work. These findings establish systematic design rules to achieve silicon like efficiencies in simple perovskite solar cell. Key Words: Perovskites, photovoltaic

Resume : Metal halide perovskites undergo several changes when illuminated. Perovskite layers show dynamic and static photoluminescence quenching effects under low illumination densities equivalent to 1 sun, with addition to passivation effects under different atmospheric gases. For this experiment, specific partial pressures of argon, nitrogen, oxygen and water-enriched argon where tested on perovskite layers. Using the Stern-Volmer relation, we found that all these gases show a primarily static quenching effect, meaning that they are adsorbed to the crystallites. Not only that, but there is also evidence of dynamic quenching and even passivation at specific pressure ranges, measurable already at partial pressures as low as 1 mbar. Furthermore, these phenomena are mostly reversible once the quenching molecules are removed. Our observations indicate that there is an interplay between the photon flux density used to excite the material and the resulting enhancement or quenching result. In particular, oxygen and nitrogen lead to a significant but reversible static quenching, while H2O atmosphere signals a reversible dynamic process. In the case of argon, it first shows a minor PL increase at low pressures, pointing to an immediate passivation effect on the surface, yet, at higher pressures, argon still shows a moderate quenching effect. These results highlight the importance of proper sample conditioning and measurement environment before, during and after material characterization.

Resume : Lead halide perovskites are one of the most exciting classes of optoelectronic materials due to their unique ability to form high quality crystals with tunable band gaps in the visible and near infrared using simple solution precipitation reactions. This facile crystallization is driven by their ionic nature; just as with other salts, it is challenging to form amorphous halide perovskites, particularly in thin-film form where they can most easily be studied. Here we show that rapid desolvation promoted by addition of acetate precursors is a general method for making amorphous lead halide perovskite films with a wide variety of compositions, including those using common organic cations (methylammonium, formamidinium) and anions (bromide, iodide). We show that by controlling the amount of acetate, it is possible to tune from fully crystalline to fully amorphous films, with an interesting intermediate state consisting of crystalline islands embedded in an amorphous matrix. The amorphous lead halide perovskite has a large and tunable optical band gap and improves the photoluminescence quantum yield and lifetime of incorporated crystalline perovskite, opening up the intriguing possibility of using amorphous perovskite as a passivating contact, as is currently done in record efficiency silicon solar cells.

Resume : Perovskite solar cells represent a highly promising 3rd generation photovoltaic technology. However, its practical implementation is hindered by a low device operational stability mostly related to facile degradation of the absorber materials under exposure to light and elevated temperatures. Improving the stability of complex lead halides is a big challenge, which might be addressed using various “molecular modifiers”. These modifiers usually undergo strong interactions with the perovskite absorber material resulting in enhancement of solar cell efficiency and/or operational stability. Herein, we present a derivative of 1,4,6,10-tetraazaadamantane NAdCl as a promising molecular modifier for lead halide perovskites. NAdCl spectacularly improved both thermal and photochemical stability of MAPbI3 films and, most importantly, prevented the formation of metallic lead Pb0 as a photolysis product. It was shown that NAdCl improves the electronic quality of perovskite films by healing the traps for charge carriers. Furthermore, it strongly interacts with the perovskite framework and most likely stabilizes the undercoordinated Pb2 ions, which are responsible for Pb0 formation under light exposure. The obtained results feature 1,4,6,10-tetraazaadamantane derivatives as highly promising molecular modifiers, which might help to improve the operational lifetime of perovskite solar cells and facilitate the practical implementation of this photovoltaic technology.

Resume : Perovskite solar cells (PSC) have known a surge of attention from both the scientific and industrial photovoltaic communities. PSCs have shown a high photovoltaic performance as they have reached a power conversion efficiency of 25.5 %. This burst of attention is mainly thanks to the numerous advantages they offer, such as their simple, easy and cheap manufacturing process. Nonetheless, they also face some limitation related to stability and degradation when in contact with humidity, and toxicity due their lead content. The goal of this work is to study Methylammonium Lead iodide (MAPI) based PSCs with the generic formula CH3NH3[(PbI2)1-x(CuI)x], where Lead Iodide (PbI2) is partially substituted by the inorganic compound Copper Iodide (CuI) to enhance the solar cell stability thanks to the hydrophobic properties of the latter. After the elaboration of the mixed perovskite material and complete solar cells, their respective structural and optoelectronic properties were investigated using different characterization techniques such as, X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), UV-Visible spectroscopy (UV), external quantum efficiency (EQE) and I/V characteristic under illumination. From the XRD results, it was observed that CuI was well incorporated into the perovskite lattice structure. The partial substitution showed a clear improvement of the MAPI formation and it was concluded that the highest CuI/PbI2 ratio, which is 20%, showed the best optical and PV properties.

Resume : Transparent conducting oxides (TCOs) are crucial component of solar cells. Indium-tin oxide (ITO) is the most employed TCO, but the scarcity and the high price of indium induce a search for lower cost TCOs with equivalent properties as a substitute ITO. Tin oxide films (SnO2) have many advantages, such as rich sources of material, low prices, and non-toxicity. SnO2 films present a high visible light transmittance, near-infrared light reflectivity and excellent electrical properties. They also have a higher chemical and mechanical stability compared with ITO. The aim of this work is to elaborate SnO2 films by RF-magnetron sputtering and use them as an electrode for Organic Solar Cells (OSCs). These SnO2 films were deposited on glass and quartz substrates in a mixed environment of Ar and O2. The structural, electrical, and optical properties of the films were investigated using different characterization techniques such as, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), UV-Visible spectroscopy (UV), Hall Effect measurement and Kelvin Probe (KP). The XRD measurement shows that the as-deposited SnO2 films are polycrystalline films with tetragonal structure and have preferred orientation along the (110) direction. The films are homogeneous and continuous with small grains. The electrical resistivity and optical transmittance of the sputtered SnO2 films are of about 10−3 Ω.cm and over 85%, respectively. The estimated optical band gap (Eg) is around 4.0 eV while the work function of the films is around 5.0 eV which is very close to the work function of the commercial ITO. Then, we studied the inverted OSCs using poly(3-hexylthiophene-2,5-diyl) : indene-C60 bisadduct (P3HT:ICBA) as active layer, where the SnO2 films are used as electrode. The device’s PV properties are investigated through the measurement of the I/V characteristic and external quantum efficiency (EQE).

Resume : Perovskite solar cells (PSCs) represent one of the most promising emerging technologies for the conversion of solar energy to electricity, which has attracted massive attention, in particular, due to the demonstration of high device efficiencies of >25%.1 Although PSCs have a good commercial potential i.e. due to the low cost of raw materials and ease of the device fabrication, low operational stability is severely impeding their practical application. Among different device architectures of PSCs, the p-i-n configuration with the fullerene-based top electron transport layer (ETL) is one of the most intensively studied. The fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) has been widely used as ETL material because of its high electron mobility and good solubility in non-polar solvents. Although PC61BM delivers high efficiencies in p-i-n PSCs, this material has such drawbacks as high cost and poor stability under environmental conditions as well as upon exposure to light or heat.2,3 Furthermore, it has been shown recently that PC61BM accelerates the degradation of perovskite solar cells due to parasitic interfacial reactions.4 Therefore, new ETL materials with optimal electronic and physicochemical properties need to be developed for efficient and stable p-i-n PSCs. In this contribution, we report on the synthesis of a series of fullerene derivatives and their systematic investigation as promising ETL materials for p-i-n perovskite solar cells. The devices fabricated using new fullerene derivatives demonstrated power conversion efficiencies approaching 17.2%, which is higher than for the reference cells assembled using PC61BM as ETL (16.7%). The improved photovoltaic performance of the devices incorporating new fullerenes derivatives originated from the decreased nonradiative recombination at the MAPbI3/ETLs interface, efficient electron extraction, and full coverage of the perovskite absorber layer. These obtained results feature new functionalized fullerene derivatives as promising ETLs for efficient and potentially stable (experiments underway) perovskite solar cells. References [1] Best Research-Cell Efficiency Chart | Photovoltaic Research | NREL, (n.d.). https://www.nrel.gov/pv/cell-efficiency.html (accessed January 15, 2021). [2] Shi Y.; Chen W.; Wu Z.; Wang Y.; Sun W.; Yang K.; Tang Y.; Woo H.Y.; Zhou M.; Djurišić A.B; He Z.; Guo X. Imide-functionalized acceptor-acceptor copolymers as efficient electron transport layers for high-performance perovskite solar cells, J. Mater. Chem. A. 8 (2020) 13754–13762. [3] Chen W.; Shi Y.; Wang Y.; Feng X.; Djurišić A.B; Woo H.Y; Guo X.; He Z. N-type conjugated polymer as efficient electron transport layer for planar inverted perovskite solar cells with power conversion efficiency of 20.86%, Nano Energy. 68 (2020) 104363. [4] Akbulatov A.F.; Frolova L.A.; Griffin M.P.; Gearba I.R.; Dolocan A.; Vanden Bout D.A.; Tsarev S.; Katz E.A.; Shestakov A.F.; Stevenson K.J.; Troshin P.A. Effect of Electron-Transport Material on Light-Induced Degradation of Inverted Planar Junction Perovskite Solar Cells, Adv. Energy Mater. 7 (2017) 1700476.

Resume : Hybrid lead halide perovskites have rapidly developed in recent years due to their outstanding performance in a range of applications, including photovoltaics (PV).1 Nevertheless, their toxicity and instability continue to be problematic and the research on bismuth-based counterparts is currently on high demand. Hybrid perovskite films are typically deposited by spin-coating, from which is hard to achieve large-scale applications. Although not commonly used in the fabrication of perovskite thin films, the aerosol-assisted variant of chemical vapour deposition (AACVD) - a widely industrially implemented method- has been widely applied in the thin film deposition of functional materials.2 Therefore, in order to overcome the toxic nature of Pb and deposit the perovskite films with ideal morphologies through a cost-effective and scalable deposition method, films of a novel hybrid lead-free perovskite, phenethylammonium bismuth iodide, were prepared via AACVD. The bulky organic ligand with hydrophobic nature could significantly improve the humidity resistance of the perovskite structure. Different kinds of substrates, including glass, titanium dioxide and fluorine-doped tin oxide, were employed in the deposition process, and their influence on morphologies and optoelectrical properties of films were investigated. Furthermore, films prepared via AACVD possessed different optical properties from reported spin-coated films.3 Band-bending was observed between the perovskite films and semiconducting substrates. This work entails a deep study of the structural and optoelectrical properties of a promising hybrid bismuth-based perovskite film with remarkable ambient stability, which is beneficial to facilitate further investigation for its applications in various up-to-date fields. 1 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am. Chem. Soc., 2009, 131, 6050–6051. 2 P. Marchand, I. A. Hassan, I. P. Parkin and C. J. Carmalt, Dalt. Trans., 2013, 42, 9406–9422. 3 M. Ghasemi, M. Lyu, M. Roknuzzaman, J. H. Yun, M. Hao, D. He, Y. Bai, P. Chen, P. V. Bernhardt, K. Ostrikov and L. Wang, J. Mater. Chem. A, 2019, 7, 20733–20741.

Resume : Omnidirectional absorption is important to non-tracking systems designed for the harvesting of solar energy. Presently, we examine both experimentally and numerically the broadband absorption under oblique illumination driven by deep sidewall subwavelength structures (DSSS) in silicon nanopillar arrays (DSSS arrays). Specifically, we target DSSS geometries that are a built-in side effect of the top-down cyclic Bosch dry etch process employed to realize high aspect ratio silicon nanopillar (NP) arrays. We numerically compare the DSSS array with an optically-optimized straight-sidewall nanopillar array (SSNP array) under oblique illumination. We show how the presence of DSSS generates a higher absorptivity particularly for the spectral range >600 nm, induces the formation of absorptivity peaks at the near-infrared spectral range, and overall provides an enhanced omnidirectional broadband absorption. We experimentally show that the specular reflectivity and the total reflectivity of silicon DSSS arrays that were fabricated using a top-down Bosch dry etch process is significantly lower compared with that of the corresponding SSNP arrays. Specifically, we show a decrement in the broadband specular reflection of up to 30% for certain angles of illumination, and about 13% decrement in the total reflectivity.

Resume : Photovoltaic devices that use metal halide perovskites as the light harvesting layer have strong potential to be both flexible and low cost. However, this potential can only be fully realised if the indium tin oxide (ITO) and fluorine doped tin-oxide (FTO) coated glass electrodes currently used to let light into these devices are replaced with a low cost, flexible alternative. Unfortunately, the precursors and decomposition products of metal halide perovskites are corrosive to most conventional electrode materials and so developing a viable alternative to conducting oxide electrodes for this class of emerging photovoltaics is particularly challenging. This talk will present a new high performance transparent electrode based on a nickel passivated copper grid that is suitable for use in metal halide perovskite PVs. The highly conductive metal grid is fabricated using the scalable process of micro-contact printing which enables metal line-widths of < 1.5 microns to be achieved - an order of magnitude lower than can be achieved using conventional printing methods. The promising results of device stability testing (under 1 sun simulated solar illumination and under load) in model tin perovskite photovoltaics with the structure: Anode|poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)|FA0.78GA0.20SnI31|C60|Cathode will be presented. This electrode offers a transparency of ≥ 80% together with a sheet resistance of ≤ 7.0 Ω/sq which makes it competitive with ITO and FTO coated glass and plastic. The findings presented are expected to be equally applicable to lead perovskite photovoltaics, and bode well for the prospect of using copper based grids as the substrate electrode in perovskite photovoltaics. References [1] E. Jokar, C. H. Chien, C. M. Tsai, A. Fathi and E. W. G. Diau, Adv. Mater., , DOI:10.1002/adma.201804835.

Resume : Co-evaporation can fabricate high quality perovskite absorbers for solar cells, but this fabrication route is still largely underexplored, as most research groups focus on cheap and fast, but industrially unattractive wet-chemical processes. At MLU, we develop dynamic co-evaporation processes for mixed halide perovskites and try to find new material combinations with better performance and higher stability. In this contribution, we investigate the film formation during the growth of hybrid lead halide perovskites by co-evaporation with the aid of in situ XRD diagnostics. The detailed analysis of phase evolution, nucleation/reaction kinetics and degradation modes allows to pinpoint structure-property relationships between the involved crystalline phases and the device photophysics. We are able to resolve secondary phases already during growth and our time-resolved measurements allows us to report on the kinetics of film formation and thermal degradation. Ultimately, we report on our progress in fabricating efficient devices with efficiencies exceeding 14%.

Resume : Tin Oxide (SnO2) films have been used as an electron transport layer (ETL) in formamidinium-cesium mixed-cation lead mixed-halide perovskites (FA0.83 Cs0.17 Pb I(3-x) Brx). This perovskite material has a bandgap of 1.74 eV. To better understand the sputtered SnO2/perovskite interface, Metal-Insulator-Semiconductor (MIS) devices were fabricated. The MIS stack consisted of gold (Au)/fluorine doped tin oxide (FTO)/SnO2/Perovskite/Au. In this work, MIS devices were produced with SnO2 deposited with: i) different thicknesses, in order to study its effects on the electronic properties of the fabricated devices; and ii) with and without annealing, to comprehend the effects of a heat treatment on the SnO2 optoelectronic and structural properties. Capacitance-voltage-frequency (C-V-f) and capacitance-conductance-frequency (C-G-f) measurements were carried out on the MIS structures, which allowed for: i) the estimation of the apparent number and polarity of fixed insulator charges (Qf) and ii) the estimation of the density of interface defects (Dit), respectively. Moreover, the C-G-f data was used to fit several circuit models and study the MIS AC electrical behaviour. The thickness and annealing were found to influence the extracted Qf values. The results suggest that the optimum SnO2 thickness to be used is 20 nm without annealing, in order to maximize Qf values. On the other hand, Dit values appear to be independent of thickness and/or annealing, as their values remained closed for all devices, within error calculation. The AC electrical circuit fitting also show differences between non-annealed and annealed SnO2 layers on MIS devices. This study was complemented with X-ray photoelectron spectroscopy (XPS) measurements, which allowed to correlate the differences observed on SnO2 electrical properties before and after annealing with the chemical state of the oxide.

Resume : The promising optoelectronic properties, broad tunability and facile synthesis of lead halide perovskites (LHPs) has led to an explosion in research over the last decade, resulting in photovoltaics with power conversion efficiencies fast-approaching their theoretical limit. Future advances will include utilising above-gap (‘hot’) carriers before they relax via carrier-carrier and carrier-phonon interactions. Here, we have applied various ultrafast, multi-pulse spectroscopic techniques to achieve optical control over hot carriers in bulk and nanocrystalline LHPs, isolating their cooling dynamics from the melee of excited state processes occurring on similar timescales. The effects of size, shape and composition of these (nano)materials have been explored in films and devices via optical and photocurrent detection, yielding new insights which could aid the design of next-generation photovoltaics based on hot carrier extraction and carrier multiplication methods.

Resume : Both bismuth (Bi) and tin (Sn) are considered promising candidate substitutes of lead (Pb) in perovskite materials due to their ionic radius, valence electronic shells, and their non-toxicity compared to lead. Out of the two, Bi additionally shows long term chemical/photochemical stability. Instead of the corner sharing octahedral structure of perovskites, Bi-based materials with edge-sharing octahedral receive the name of Rudorffites. These compounds are expected to be stable under humid environments and thermal stress. Two examples of these are based on the 3D trigonal structure formed by the connection of silver (Ag ) to iodobismuth ions, resulting in Ag2BiI5 and Ag3 BiI6. Here, we present ink dispersions (hexane/dodecane) of Ag2BiI5 and Ag3BiI6 suitable for inkjet printing. Thin films have been deposited with this technology on flexible substrates in ambient environment. A throughout characterization of both the inks and the printed layers has been carried out, aiming at their use as light absorbers in solar cells. By means of dynamic light scattering (DLS), the dimensions of the rudorffites nanocrystals present in the inks are estimated. Their stability is proved by turbiscan ageing test measurements. Additionally, the thermal treatments after printing are studied, including annealing in oven at 220 °C under N2 and annealing in vacuum at 120 °C. Homogeneous thin films with different controlled thicknesses between 400 and 900 nm are achieved and no pinholes have been detected by scanning electron microscopy (SEM). The optical characterization via UV-VIS measurements in terms of absorbance allowed the estimation of the band gap for both materials with 1.98 eV for Ag2BiI5 and 1.92 eV for Ag3BiI6, values suitable for photovoltaic applications and close to the theoretical and experimental ones found in the literature. From the optical absorption and the approximate thickness (calculated from interferences) we have also determined the absorption coefficient of the layers. In conclusion, this work paves the way for eco-friendly lead-free stable perovskite solar cells and their integration onto flexible substrates for future electronic-device requirements.

Resume : Solution processable methods have been used recently to produce large-area and low -cost optoelectronic devices and solar cells based on metal halide perovskites (MHPs). Those materials are especially well suited for such applications because of facile synthesis at low temperature, defect tolerance, high absorption, tunability of band-gap within visible and near infrared and high quantum yield for light emission. Hybrid metal halide perovskite solar cells have been reported with efficiencies higher than 25% and LEDs with external quantum efficiency in excess of 20%. However, these records have been obtained with deposition techniques difficult to scale-up. In contrast, ink jet printing is an industrial friendly technique that has not been extensively explored for MHPs devices due to the difficulty to print large area defect free layers without pinholes and the complexity to achieve high quality interfaces with transport layers. In this work we report the making of fully inkjet-printed functional devices for solar cells and light emission applications. For this, we have formulated suitable inks for printing starting from nanocrystalline suspensions of CsPbX3 (X=Br, I) in hexane and dodecane. Solution-based materials such as ZnO or SnO2 and NiO or PEDOT:PSS have also been formulated and printed as electron and hole transport layers, respectively. The morphology, composition and optical properties of the different printed layer stacks have been thoroughly studied and the quality of the layers and the interfaces has been optimized by fine tuning of solvents and post-processing annealing in vacuum. Full devices have been printed on ITO/glass substrates and topped with Ag or Au electrodes. Electrical characterization shows excellent diode rectification in several devices, as well as good light absorption and light emission, which is a novel finding, and depends largely on materials, transport layers and processing conditions. Successful realization of fully ink-jet printed devices based on halide perovskites is expected to boost low cost, large-area, and lithography free production of optoelectronic and photovoltaic devices.

Resume : Metal oxide heterojunctions find a multitude of applications in different fields, including transport layers for LEDs, solar cells, and photodetectors. Among them, ZnO, SnO2 and NiOx have wide direct-bandgaps, making them transparent to visible light, and ideal for optoelectronic applications. On the other hand, inkjet printing is an emerging deposition technique that allows great control of device parameters, while being a digital drop-on-demand method. In addition, it is a scalable approach where very few material is wasted. However, the variety of materials available for printing remains sparse. Many metal oxides have been only single printed for applications such as supercapacitors or gas sensors, but the heterostructures required in optoelectronics have remained comparatively unexplored. High control of layer and heterostructure properties is achieved thanks to a variety of printing parameters. In this work we present high quality inkjet-printed ZnO, SnO2 and NiOx thin films and corresponding heterojunctions. A variety of morphological measurements including FE-SEM, profilometry, and AFM allow to confirm the suitability of the uniform printed structures and low surface roughness and absence of pinholes; while XRD, XPS, Raman and PL indicate suitable material and chemical structure. Transparency measurements show excellent light transmission in the visible spectrum, making them ideal for use as charge transport layers for LEDs and solar cells. Electrical measurements of the single metal oxide layers and heterojunctions have also been performed. The heterojunction diode devices show excellent current rectification behavior, with a large current ratio between direct and indirect polarization. These results indicate their suitability for applications including UV light photodetection, and charge transport materials for LEDs.

Resume : Intermediate-band materials are predicted to increase the conversion efficiency of solar cells by increasing the photocurrent while maintaining a high output voltage. Based on theoretical calculations, In2S3 substituted with the transition metal vanadium has been proposed as a promising candidate for an intermediate-band material. However, the predicted favourable properties rely on the incorporation of the V atoms on octahedral rather than tetrahedral lattice sites in the defect spinel-type structure of In2S3. We have therefore used X-ray absorption spectroscopy to study the local structural environment of V substitutes in co-evaporated In2S3 thin films. Both, the V-S bond length and the coordination number clearly indicate a preferential incorporation on tetrahedral lattice sites. This is remarkable since the V atoms in vanadium sulphides are mostly octahedrally coordinated. However, the nearest neighbour distances of the tetrahedral sites in the In2S3 matrix are close to the V-S bond lengths in vanadium sulphides. In contrast, the nearest neighbour distances of the octahedral sites are significantly larger and therefore energetically unfavourable. Our results thus provide important insight for the growth of In2S3-based intermediate band materials in particular and contribute to a better fundamental understanding of the structure-property-relations in intermediate band materials in general.

Resume : The low operation stability of hybrid perovskite solar cells currently is the main obstacle for their practical implementation. Replacing fragile organic moieties with robust inorganic cations represents a promising approach to designing more stable absorber materials. Inorganic lead halide-based perovskites CsPbI3-xBrx (x=0-3) have demonstrated improved thermal and photochemical stability. Among them, CsPbI3 is considered the most promising inorganic perovskite for photovoltaic applications due to its narrow bandgap. Unfortunately, the cubic α-phase of CsPbI3 undergoes a fast transition to a non-active yellow δ-phase under realistic solar cell operation conditions thus limiting the practical application of this material. Efforts of many research groups working on improving CsPbI3 perovskite phase stability resulted in certain progress in this field achieved very recently. Here we addressed the problem of poor α-phase stability of CsPbI3 films by using properly designed additives. A group of additives was identified that suppress significantly the phase transition effects in CsPbI3. The lifetime of the stabilized CsPbI3 perovskite films exceeded 5000 h under 1 sun illumination at elevated temperatures. Furthermore, the solar cells based on CsPbI3 films modified with optimal additives showed remarkably improved operational stability under more than 4000 h of continuous light soaking in combination with the high photovoltaic performance. Possible mechanisms of the observed stabilization effects will be discussed. This work was supported by Russian Science Foundation (project 19-73-30020).

Resume : This article presents the implementation of an automatic spray equipment in open atmosphere for the creation of multilayer materials with 2D organic semiconducting materials, suitable to manufacturing Grätzel solar cells devices; uses of spray coating open atmosphere techniques causes different changes in physical and chemical thin film technology (TFT) properties mainly in morphology, mechanical, compositional, magnetic and optical ones, improving at the same time nanoparticle adhesion phenomena. Through the automatic spray equipment was developed to apply optimized coating technique when performing production TFT processes. Due to the development of this device were obtained significant results in the fabrication of solar cells using graphene-like material, TiO2, exotic molecules for sensitizing semiconducting materials (Bixin type molecules) on substrate slides glass with accuracy thickness in the order of nm. Above materials were used to performed photovoltaic Grätzel cell devices and was found a growth for 2D organic semiconductors between 75 nm and 438 nm, also maximum electrical potential between 120 mV and 520 mV, in a 7.0 cm2 on standard illumination conditions, these results depend on fabrication process and selected molecules.

Resume : Halide perovskite is a promising material that attracts attention in the field of optoelectronics for its excellent optical properties. However, toxicity and poor stability of halide perovskites are major challenges that hinder commercialization in wide applications. Here, we suggest a robust strategy to ensure long-term stability and environmental non-toxic of lead-free perovskite, Cs2SnI6, by applying ultraviolet (UV) curable polyethylene glycol dimethacrylate (PEGDMA). The oxygen lone pair electrons of PEGDMA capture the Cs+ cations of the CsI, which form a highly stable Cs2SnI6 phase. Moreover, the formation of the polymer network of PEGDMA through UV irradiation strongly contributes to maintaining the stable optical properties of Cs2SnI6 film over one month. The Cs2SnI6 precursor solution containing 5wt% PEGDMA forms a structurally and chemically stable photo-functional film. In particular, the photoconductor using a Cs2SnI6 perovskite with a fairly low bandgap (1.4eV) exhibits a broadband photodetector that operates not only in the visible region but also in the near-infrared region, and it shows excellent optical properties in near-infrared illumination (980 nm). These results demonstrate the significant potential of long-term phase engineering of non-toxic perovskites for high-performance electronic and optical applications.

Resume : High performance hybrid perovskite solar cell devices involve a stacked structure, where a perovskite thin film is deposited between a hole and an electron transporting layer.1 Several studies have emphasized the key role of the interfaces between the halide perovskite and the transporting layers at affecting charge carrier properties. In particular, surface recombination at these interfaces is often the main source of non-radiative recombination,2,3 which reduces the open circuit voltage and the overall photo-conversion efficiency of the device. Therefore, understanding and controlling interfacial effects in solar cells is critical to improve device performances. Our previous contributions have shown that CH3NH3PbI3 (MAPI) is a mixed ionic electronic conductor, where iodide vacancies are majority carriers and dominate the ionic response of the material, with a less significant contribution from transport of Pb2+ and MA+ defects.4 Since ionic defects are majority charge carriers in this material, space charges at interfaces can be expected to form by means of ionic interactions. In our previous study,5 we gave evidence for ionically generated space charges forming at the MAPI/TiO2 and MAPI/Al2O3 interfaces. In this study, we found that a space charge zone is formed in MAPI, because of positive ionic charges being adsorbed at the interface between MAPI and metal oxide particles. This results in the depletion of positive charge carriers (holes, iodide vacancies) and leads to the accumulation of negative charge carriers (electrons). In this study, we consider the interfacial effects between MAPI and copper(I) iodide (CuI). CuI is used as hole-transporting materials in metal halide perovskite solar cells. We used zeta potential for investigating the surface charge of CuI nanoparticles and we performed electrical measurement such as AC impedance and DC polarization to quantify the ionic and electronic conductivity in composite films of CuI:MAPI. The zeta potential results showed that CuI nanoparticles have lower value of surface charge compared to the oxides previously investigated, suggesting less pronounced interaction with charged ionic defects. Our conductivity data, performed for different concentrations of CuI nanoparticles in the MAPI precursor solution, indicate that the presence of the particles results in interfacial contribution to the sample’s conductivity compared with pure MAPI. We discuss these results in terms of defect-equilibration at the interface, considering the case of ionic and electronic contributions. Our study indicates that harnessing ionic interactions at interfaces and space charge effects induced by ionic charge charrier redistribution could be a promising approach for tuning electronic charge carrier behavior at interfaces involving hybrid perovskites, with potential application to solar cell devices. [1] D. Luo, R. Su, W. Zhang, Q. Gong, R. Zhu, Nature Review Materials 2020, 5, 44-60. [2] S. Zhang, P. E. Shaw, G. Zhang, H. et. al., ACS Appl. Mater. Interfaces, DOI: 10.1021/acsami.0c02960. [3] M. Stolterfoht, C. M. Wolff, J. A. Marquez, et al., Nat. Energy 2018, 3, 847-854. [4] T.-Y. Yang, G. Gregori, N. Pellet, M. Gratzel, J. Maier, Angew. Chem., Int. Ed. 2015, 54 ,27, 7905-7910. [5] G. Y. Kim, A. Senocrate, D. Moia, J. Maier, Adv. Funct. Mater. 2020, 2002426.

Resume : There are many new concepts to enhance the efficiency of solar cells were proposed in a recent decades. Each semiconductor material has its own absorption spectrum which depends on bandgap energy value and band structure. Unfortunately for engineers, spectral distribution of solar energy reaching the surface of the Earth is fixed and does not exactly match the spectral response of any semiconductor photovoltaic detector. To utilize larger part of solar energy different kinds of up-conversion, down-conversion and downshifting layers are used. They effectively trap photons with too low or too high energy and re-emit them in the range where the photovoltaic detector exhibits a significantly better response. Zinc oxide is the well-known semiconductor material which can be characterized by wide energy bandgap, high transparency in optical range and relatively good conductance defined by high concentration of intrinsic defects. Due to that facts zinc oxide is considered as suitable material for transparent conducting electrodes and functional layers for thin film and multijunction solar cells. Here we report on the study of synthesis and characterization of zinc oxide coatings doped with transitional metals fabricated by chemical hydrothermal method. Monocrystalline silicon substrates either with intact or oxidized surface were used for deposition. In both cases substrates were also covered with 30 nm undoped zinc oxide nucleation layer by atomic layer deposition method. Then the layer of doped zinc oxide were hydrothermally deposited from the aqueous zinc nitrate solution containing nickel or cobalt ions at temperature 80 – 90 °C. Continuous polycrystalline nanostructured layers with the thickness of 1 – 2 microns consisting of closely packed zinc oxide crystallites were obtained in the result. Concentration of the impurity atoms varied from 0.04 to 1.1 at.% for nickel and around 0.01 at.% for cobalt doped layers. Under ultraviolet excitation obtained coatings demonstrated photoluminescence in the optical range with the maximum at 550 – 625 nm corresponding to zinc oxide crystal lattice defects. However it was discovered that at high concentration of doping atoms additional photoluminescence band with intensity maximum located around 430 nm appears leading to broadening of the luminescence spectra. Doping of the zinc oxide with transitional metal atoms also decreases its resistivity to 0.7 Ohm•cm. Thus, obtained materials can be used as multifunctional coatings for thin film solar cells as both transparent conductive and downshifting layer with broad visible range luminescence corresponding to spectral response range of conventional silicon and chalcogenide photovoltaic detectors. Obtained doped zinc oxide coatings also can be used as luminescent layers in charged particles scintillation detectors and ultraviolet photodetectors. The work has been funded as a part of the Belarus Government Research Program “Photonics and electronics for innovations”.

Resume : The photophysical process of low photon energy up-conversion via triplet-triplet annihilation (TTA-UC) describes the capability of a multicomponent system to exhibit photoluminescence (PL) at wavelengths shorter than the wavelength used for its photoexcitation. Systems exhibiting TTA-UC luminescence are particularly attractive to a broad range of light management applications including sensitization of photodiode devices, activation of photocatalytic systems and photo-stimulation of optogenetic platforms. Particularly for the area of organic solar cell (OSC) and organic photodetector (OPD) devices, TTA-UC offers the possibility to generate photocurrent when low energy photons are interacting with the device, which would be otherwise lost by their transmission through the device photoactive layer. However, to integrate a TTA-UC layer both by optical and electrical means into an organic photodiode device architecture remains challenging. Here we present a methodology that enables the generation of TTA-UC induced photocurrent in OPD devices functionalized with an electrically and optically integrated TTA-UC layer. An interlayer of the organometallic sensitizer of (2, 3, 7, 8, 12, 13, 17, 18-octaethyl-porphyrinato) PtII (PtOEP) is used for extending the absorption profile of a planar OPD heterojunction to the red, thereby allowing for the capture of photons with energies lower than the absorption energy of the heterojunction. Planar OPD heterojunctions are used comprising the 9,10 diphenyl anthracene (DPA) electron donor interfaced with a C60 fullerene acceptor. In respect to the DPA/C60 reference system, a 7-fold enhancement is achieved in the photocurrent of the PtOEP/DPA/C60 device when photons of 532 nm are used. In addition, the electrical integration of the PtOEP interlayer in the device structure facilitates an optimum hole extraction thereby generating an open circuit voltage of 500 mV. Time-integrated PL spectroscopy on the fabricated devices confirms the occurrence of the TTA-UC process that manifests in detection of the characteristic DPA luminescence upon laser excitation at 532 nm. The applicability of our methodology to a wider set of materials is demonstrated by replacing the C60 acceptor with bis(8-hydroxy-2-methylquinoline)-(4-phenylphenoxy)aluminum (BAlq), a high-energy gap electron acceptor. Based on these findings we demonstrate TTA-UC sensitized PtOEP/DPA/C60 OPD heterojunctions with responsivity (R), noise-equivalent power (NEP) and specific detectivity (D*) values of R= 610 μA/W, NEP= 40 pW and D*= 5.8 × 109 Jones at 550 nm. We will discuss on the potential utilization of these devices in all-optically ternary logic circuits and TTA-UC sensitized OSC platforms. This work was co-funded by the European Regional Development Fund and the Republic of Cyprus through the Research and Innovation Foundation (Project: EXCELLENCE/1216/0010).

Resume : To enhance the stability of solar cells against drastic environmental conditions such as heat, humidity, etc., appropriate encapsulation material should be employed, which is normally ethylene vinyl acetate (EVA). Also, it is an important module component in ensuring the reliable performance of the solar module till the warranty period. In the present study, EVA composite was developed with zinc oxide (ZnO) at different weight ratios of ZnO to enhance the thermal conductivity, dielectric strength and electrical resistivity of EVA, without affecting its optical transmittance. ZnO doping was varied in the range of 0.01 - 4 wt. %. Solvent casting process was used to prepare EVA/ZnO composites using toluene as solvent. Then, the pure EVA and EVA/ZnO composites were characterized to evaluate the optical, electrical and thermal properties. A nominal reduction in the optical transmittance of EVA was noticed at lower (< 1 wt.%) concentration of ZnO. Thermal conductivity of EVA/ZnO composites was found to be enhanced with increasing concentrations of ZnO. Furthermore, EVA/ZnO composites were found to have low dielectric constant as compared to pure EVA. UV degradation of the composites was also analyzed using fluorescence spectroscopy. In conclusion, EVA/ZnO composites showed improved electrical, thermal with a nominal change in the optical properties in comparison with pure EVA.

Resume : The design criteria for sustainable thin-film photovoltaic devices includes the chemical (e.g. abundance, toxicity, stability, scalability) and physical (e.g. band gap, absorption, doping density, contact behaviour) properties of the underlying materials. Many non-conventional inorganic materials are currently being investigated including oxides (e.g. Cu2O) and sulphides (e.g. SnS); however, none are close to reaching their theoretical potential as defined by the Shockley?Queisser limit [1]. I will discuss the latest advances in materials modelling [2] for the discovery of new materials for solar energy conversion. The role of predictive simulations can vary from high-throughput screening of candidate compounds, rigorous assessment of physical responses, to the optimisation of device architectures. Particular attention will be paid to origins of non-radiative electron-hole recombination that limits the performance of many new technologies. Examples will be taken from our exploration of kesterite (e.g. Cu2ZnSnS4), herzenbergite (SnS), and antimonselite (Sb2Se3) systems [3,4]. [1] "Emerging inorganic solar cell efficiency tables" J Phys Energy (2019); https://doi.org/10.1088/2515-7655/ab2338 [2] ?Machine learning for molecular and materials science? Nature 559, 547 (2018); https://doi.org/10.1038/s41586-018-0337-2 [3] "Finding a junction partner for candidate solar cell absorbers enargite and bournonite from electronic band and lattice matching" J. Appl. Phys. 125, 055703 (2019); https://doi.org/10.1063/1.5079485 [4] "Upper limit to the photovoltaic efficiency of imperfect crystals from first principles" Energy Environ. Sci. 13, 1481 (2020); https://doi.org/10.1039/D0EE00291G

Resume : In recent years, the scientific community has emphasized materials with a 2D structure. These types of materials have been designed for various application areas, with photovoltaic energy production being one of them. The Sb2Se3 semiconductor falls into this family and preliminary studies have shown that it presents optical and electrical properties compatible with photovoltaic technology. In this work we describe a method that allows the growth of Sb2Se3 which is easily up scalable for industrial environment. The method consists of the Sb-Se precursor films deposition using RF magnetron sputtering and a subsequent annealing step in an H2Se atmosphere. The physical properties of the samples are studied using several techniques, such as scanning electron microscopy, energy dispersive spectroscopy, x-ray diffraction Rutherford Backscattering Spectrometry, and Raman, allowing to access the material structure, composition and morphology. The effect of the substrates, such as Si, Mo and soda lime glass in the Sb2Se3 growth and in the interface between the substrate and Sb2Se3 is also investigated. The optical characterization was also performed to extract the absorption coefficient and band gap energy.

Resume : Growing concern over resource availability and toxicity are leading to a paradigm shift towards truly sustainable materials for photovoltaics. Zn-group IV-nitrides are one potential class of materials fulfilling these criteria, which adopt a wurtzite-type derived structure. The ternary nitrides were postulated to allow a unique bandgap tuning mechanism through cation disorder [1] in addition to cation alloying. Fully ordered structure ZnGeN2 crystallises in a ß-NaFeO2 type structure in a subgroup of the wurtzite type. Interestingly, incorporating oxygen into ZnGeN2 also introduces an increased degree of disorder in the material. [2] We present a detailed study of the degree of cation disorder in oxygen containing Zn1 xGeN2Ox that is revealed through neutron powder diffraction. Studying samples with a variable degree of oxygen allows us to conclude on the role of oxygen and further comparing different samples of nominally similar composition allows to decorrelate the oxygen effect from intrinsic cation disorder. We will combine our results with optoelectronic and chemical properties of the materials and finally aim to answer the question, whether cation disorder exists independently of oxygen incorporation or is fundamentally linked to it. [1] A.D. Martinez, A.N. Fioretti, E.S. Toberer, A.C. Tamboli, J. Mater. Chem. A, 2017, 5, 11418. [2] J. Breternitz, Z.Y. Wang, A. Glibo, A. Franz, M. Tovar, S. Berendts, M. Lerch, S. Schorr, Phys. Status Solidi A, 2019, 1800885.

Resume : Significant efforts are made with intermediate band solar cells (IBSCs) for achieving high conversion efficiency [1]. Presently, research on IBSC using quantum dots (QDs) is continuing in three areas. The first is to establish methods to fabricate high-density QDs array of low defect density and with long radiative recombination lifetimes. The areal density of QDs has direct influence on the generation and recombination processes via IB states and for this, we have shown that strain-compensated growth improves the QDs quality and InAs/GaAs QDSC characteristics with QD stacks up to 100 layers in self-organized heteroepitaxy by MBE [1]. However, the average QD size prepared by such dry method is still large and in-plane density is low, typically limited to the range of 15-30 nm and 1e11-1e12 cm/2, respectively. To this end, optimized QD-IBSC structure prepared by a solution process in which PbS colloidal QDs of 4 nm in size were densely dispersed in a bulk perovskite matrix with a high energy bandgap of 2.4eV has been studied [2]. The second challenge is to increase the carrier lifetime in IB states by controlling the recombination rates such as by using a type-II QD heterostructure or a superlattice that allows fast spatial separation of carriers, as well as a photon-ratchet concept. Lastly, maximizing the photocurrent generation by 2-step photoabsorption (TSPA) by optimally doping the QDs to realize partially-filled IB states is important to improve the current matching between valence band (VB)-IB and IB-conduction band (CB) current production paths. The current developments of IBSCs will be reviewed. [1] Y. Okada et al, Appl. Phys. Rev. 2, 021302 (2015). [2] H. Hosokawa et al, Nature Commun. 10, 43 (2019).

Resume : To overcome the Shockley Queisser limit of single junction solar cells (SC), an intermediate band (IB) can be introduced between the valence band (VB) and the conduction band (CB) of a wide band gap material [1]. Thus, sub-bandgap photons can be absorbed by VB to IB and IB to CB transitions. Furthermore, thermalization losses can be reduced due to the higher band gap compared to single junction SC. A promising material to realize such an IBSC is In2S3 hyperdoped with Vanadium [2]. We use physical co-evaporation of the elements as a combinatorial approach to grow In2S3:V layers with up to 14 at% V. To initiate epitaxial growth we use p-Si wafers as substrates. As transparent top electrode n-ZnO:Al grown by pulsed laser deposition is used to realize heterostructure pin-SC. Reference SC without IB are prepared using undoped In2S3. Investigations of structural properties reveal epitaxial growth even for V-doped samples. We find high rectifications of up to almost 6 orders of magnitude by current-voltage characteristics in the dark. Measurements under illumination reveal no increase in the short circuit current density for V-doped samples, which indicates that the IB is full, in agreement with [3]. We investigate defect states that are induced by Vanadium using thermal admittance spectroscopy, to determine the energetic position of the IB. [1] Luque and Martí, Phys. Rev. Lett., 1997, 78. [2] Palacios et al., Phys. Rev. Lett., 2008, 101. [3] Ghorbani et al., J. Mater. Chem. A, 2019, 7.

Resume : Intermediate band solar absorber materials based on well-known chalcopyrite type semiconductors can be a low cost and efficient alternative in solar energy production [1]. A comprehensive analysis of the structural incorporation of manganese in the wide band gap CuGaS2 chalcopyrite type semiconductor is demonstrated by means of X-ray diffraction, neutron diffraction and photoluminescence spectroscopy. In this systematic study powder samples were prepared by solid state reaction of pure elements, following the pseudo-binary section of CuGa1-xMnxS2 and (CuGaS2)1-x(2MnS)x, respectively. The alloyed ternary phase consequently crystallizes in the chalcopyrite type structure with a maximum manganese content of 0.091 mol-%. It is shown that the actual incorporation of manganese follows rather the pseudo-binary section of a coupled substitution ((CuGaS2)1-x(2MnS)x) than of a unilateral substitution (CuGa1-xMnXS2) independent of their initial composition [2]. The increasing manganese content results in a continuous increase of the lattice parameters a and c, as respective marker for the incorporation into the crystal structure. From analysis of the average neutron scattering length of the cation sites a manganese incorporation is observed for both cation sites of the chalcopyrite host phase. The occurring extrinsic and intrinsic point defects,deduced from neutron diffraction data, were related to the photoluminescence spectra and correlated with the predictions given by density functional theory (DFT) [3]. [1] Marti et al., JAP 103, 073706 (2008) [2] Marquardt et al. Phys. Stat. Sol. A (2019) 1800882 [3] Zhao & Zunger, Phys. Rev. B 69 (2004)

Resume : Epitaxially-fused colloidal quantum dot (QD) superlattices (SL) promise to combine the unique photophysics of QDs with the efficient band transport of bulk-semiconductors. PbS and PbSe QD layers have demonstrated multiple exciton generation in response to photo excitation by high energy photons. However, devices made from epi-QDSL samples fail to demonstrate superior PV characteristics due to poor superlattice order. We would like to extend the multi-exciton generation to thick arrays of QDs, which is possible if the structural order of the film is improved to reduce electronic traps. We fabricate extended superlattice films with multi-layers of QDs and examine how the solution assembly fabrication method affects the QD structural and electronic local order. Specifically, we form ordered arrays of PbSe QDs at a liquid-liquid interface and then reduce the spacing between QDs using in-situ ligand exchange. The resulting films are characterized using high-resolution scanning transition electron tomography (STET). We are able to resolve the position of each QD in a sample encompassing 1846 QDs. In addition of the particle positions, we also resolve the presence (or lack of) of epitaxial connections between neighboring QDs and the thickness of the neck formed between neighboring QDs. We also examine a SL grain boundary and show the presence of necking across the grain boundary in spite of poor crystalline alignment between Qds. This morphological information is used to generate an electronic transport model. Our model shows that the energetic disorder caused by heterogeneous epitaxial bridging between QDs in a single domains is about one order of magnitude less important than the presence of disorder at grain boundaries between superlattice domains. This presentation will explore the link between structural and electronic order in these complex 3D samples.

Resume : The assembly of colloidal quantum dots (QDs) into dense superstructures holds great promise for novel optoelectronic devices. The rich and unique photophysics of QDs can benefit solar cells by harvesting hot carriers, thereby decreasing thermalization losses and significantly raising the maximum power conversion efficiency. However, the working principles of these devices rely on efficient light absorption and charge carrier transport, which are both limited for quantum dots and their aggregates. A recently developed emulsion-templated assembly technique, which yields QD supercrystals of extraordinary structural quality, expected to feature `artificial solids' with properties beneficial for PV devices. Here, we show that these supercrystals simultaneously enhance the absorption efficiency and inter-dot coupling of QDs. By comparing optical measurements with Mie theory, we demonstrate that supercrystals can exhibit resonant optical properties resulting in absorption efficiencies greater than 1 in the visible range. By performing ligand exchange, we increase the excitonic coupling, and show via ultrafast transient absorption (TA) spectroscopy that biexcitons transit from a bound to a free state as a consequence of the increased interdot coupling. These results reveal a new state of matter with properties architecturally designed to feature simultaneously photonically and electronically coupled QDs for future optoelectronic devices.

Resume : Global electricity generation is projected to be 5 times today’s level by 2070 [1]. Such growth defines a new era for photovoltaics (PV) because of the demand for innovative and sustainable distributed power sources. This creates an opportunity for inorganic thin film PV because of its stability and compatibility with a variety of substrates. Here, we present the results of two novel approaches to the fabrication of kesterite solar cells from scalable, Cu2ZnSnS4 (CZTS) nanoparticle inks. Firstly, CZTS nanoparticles are annealed in a Se-rich atmosphere for the first time inside a transmission electron microscope (TEM) [2]. This allows observation of the thin film PV absorber formation in real-time and provides the new insight that chalcogen atom exchange and grain growth occur in different temperature regimes. The technique is transferred to an industrially-similar furnace to achieve a 33% increase in the device VOC. The approach can be applied to any solar cell fabrication process that requires thermal annealing. Secondly, we demonstrate an approach to alkali doping Cu2ZnSn(S,Se)4 PV absorbers on flexible Mo-foil substrates [3] that is compatible with high volume manufacturing. Adding NaF directly to the CZTS nanoparticle ink promotes grain growth and improves the device efficiency to 4.5%. Importantly, the technique nearly eliminates a detrimental fine-grain CZTS layer associated with the nanoparticle ink synthesis. Finally, we present results from kesterite solar cells made with Cd-free buffer layers using the nanoparticle ink approach. These cells use In2S3 and offer 7.8% photovoltaic conversion efficiency. All results are discussed in the context of a new proposed sub-field, product integrated photovoltaics (PIPV). [1]. Shell International, B. V. Sky Scenario, www.shell.com/skyscenario [2]. Y. Qu, N. S. Beattie et al., ACS Appl. Energy Mater. (2019) https://doi.org/10.1021/acsaem.9b01732 [3]. X. Xu, N. S. Beattie et al., J. Mater. Sci.: Mater. Electron (2019) 30: 7883

Resume : The presence of scarce and expensive elements in the Cu(In,Ga)Se2 (CIGS) composition, such as In and Ga, present a strong limitation to the large-scale production of these solar-cell devices. Therefore, the continuous reduction of the absorber thickness is of outmost importance in order to further decrease the material consumption and production cost. However, with such reduction, problems in the solar cells light absorption efficiency become more evident. The implementation of light management architectures, such like surface texturization and the integration of plasmonic nanoparticles, can be used to mitigate such optical losses. In this work, we propose a novel way of introducing gold nanoparticles in a solar cell structure, where the nanostructures are encapsulated with a dielectric layer protecting them from the CIGS harsh growth conditions. The proposed architecture was implemented in an ultrathin CIGS solar cell device through the combination of a bottom up chemical approach to deposit the nanostructures and a top down photolithographic procedure to create electrical contacts. With the proposed architecture, an enhancement of the light effective optical path of approximately 4 times the absorber thickness is demonstrated in the infrared region, through optical simulations. The new architecture leads to a broadband external quantum efficiency enhancement corresponding to short circuit current increase of almost 4 mA/cm2, in comparison to a reference sample.

Resume : Silicon-based solar cells are of the greatest interest for terrestrial photovoltaic because Si is relatively low cost. However, Si solar cells with efficiency exceeding 26% approached closely to the theoretical limit of single junction solar cells leaving almost no possibility for further improvement. The most efficient way to overcome this theoretical limit related to thermalization losses is to use multijunction solar cells. According to theoretical estimations a combination of top subcell with a band gap of 1.6–1.7 eV and Si (1.12 eV) bottom subcell could give an efficiency of more than 35% for dual junction solar cells. Low-dimensional structures provide possibility to achieve desired band gap by varying the width of the wells in superlattice-type structures. Thus GaP/Si superlattice could be efficiently used as an absorption material for top subcell of Si based photovoltaic devices. Mass production of high-efficiency silicon solar cells requires structures that are made by easily scalable methods. One of such technologies is low temperature plasma-chemical atomic-layer deposition (PE-ALD), which makes it possible to obtain high-quality thin layers of binary compounds. The key feature of this method is the possibility of conformal deposition of thin layers with a smooth surface, which is extremely important in the formation of multilayer structures. Previously n-doped GaP was obtained by PE-ALD at the temperature of 380°C. This paper presents the results on the formation of p-i-n structures by low temperature PE-ALD using n- and p-doped GaP with GaP/Si superlattice as an absorbing layer. The results of structural analysis, optical and photovoltaic measurements of the obtained solar cells are presented. For the first time plasma deposited p-i-n structures with undoped region based on GaP/Si superlattices on a Si substrate showed a substantial internal quantum efficiency in the spectral range of 300-750 nm. A shift of the absorption edge to longer wavelengths is observed with an increase the thickness of silicon wells. Moreover almost complete absence of absorption at longer wavelengths was demonstrated, which indicates their perspective for use in solar cells as the top-junction. Thus, the use of multilayer GaP/Si structures with varied band gap can be used to create highly efficient tandem solar cells on silicon substrates.

Resume : Attaining high efficiency III-V solar cells on cost-effective silicon substrates is a long-sought objective. Besides, a high-throughput substrate removal process is key in the development of flexible and lightweight high efficiency solar cells. Embedding a porous silicon (pSi) layer in the silicon substrate can attain both objectives, if this layer achieves the required mechanical properties. Electrochemical etching and thermal treatment leading to void reorganization are used for this purpose.1 Subsequent epitaxial III-V growth can be performed, and final detachment through the porous interface can yield ultra-thin solar cells. This so-called layer transfer process (LTP) has been successfully transitioned from microelectronic to photovoltaic applications in the realm of standard silicon solar cells.2,3 A similar method is being developed for III-V solar cells on silicon, which can also benefit from the potential compliance attained through the porous layer to buffer the lattice constant difference between the III-V layers and the silicon substrate. However, optimization of synthetic parameters is still challenging. Etching of p-type Si wafers can form mesoporous or microporous pSi with dendritic or spongy structures, depending on the synthetic parameters.4 In addition, stacking of low/high porosity layers with different structures can also be achieved in p-type Si wafers.5 Annealing of freshly etched pSi enables surface silicon diffusion, which leads to pore coalescence and wafer surface reconstruction at high enough temperatures.4 Anneal of pSi smoothens the pore surface of low porosity layers (<30%) while widening the pore diameter of high porosity layers (>50%), and easily detachable silicon substrates can be prepared treating stacked low/high porosity layers. Oxidation of porous structures inhibits heat-driven pore redistribution,6 and therefore surface protection of porous structures to prevent oxidation is a key milestone for the development of LTP technology. Although H2 atmosphere enables reconstruction of slightly oxidized pSi7 and prevents the formation of surface defects,8,9 the extreme sensitivity to oxidation of highly porous pSi layers becomes a major challenge. Furthermore, the mechanisms of oxidation depend on layer microstructure and not just porosity,4 which makes development of protection mechanisms ever more necessary. This work features a comprehensive study of the independent effect of synthetic parameters involved in the electrochemical etching of p-type Si wafers. Surface protection of pSi is achieved via methylation using a tandem synthesis, which enables accurate structural and optical characterization of pSi layers regardless of porosity, some of them otherwise impossible to analyze or anneal. Thermal reconstruction of etched and chemically protected pSi films have different reconstruction mechanisms, and formation of low/high porosity stacking layers enable smooth surface reconstruction and minimal layer delamination. 1 G. Müller, M. Nerding, N. Ott, H. P. Strunk and R. Brendel, Phys. Status Solidi A, 2003, 197, 83–87. 2 R. Brendel, Jpn. J. Appl. Phys., 2001, 40, 4431–4439. 3 M. K. Sahoo and P. Kale, Microporous Mesoporous Mater., 2019, 289, 109619. 4 D. W. Zheng, Y. P. Huang, Z. J. He, A. Z. Li, T. A. Tang, R. Kwor, Q. Cui and X. J. Zhang, J. Appl. Phys., 1997, 81, 492–496. 5 N. Milenkovic, M. Drießen, C. Weiss and S. Janz, J. Cryst. Growth, 2015, 432, 139–145. 6 R. Herino, A. Perio, K. Barla and G. Bomchil, Mater. Lett., 1984, 2, 519–523. 7 V. Depauw, O. Richard, H. Bender, I. Gordon, G. Beaucarne, J. Poortmans, R. Mertens and J.-P. Celis, Thin Solid Films, 2008, 516, 6934–6938. 8 E. V. Astrova, N. E. Preobrazhenskiy, S. I. Pavlov and V. B. Voronkov, Phys. Status Solidi A, 2017, 214, 1700211. 9 E. V. Astrova, N. E. Preobrazhenskiy, S. I. Pavlov and V. B. Voronkov, Semiconductors, 2017, 51, 1153–1163.

Resume : Nanowires (NW) are a potential solution to grow III-V material on silicon (Si) without lattice-mismatch issues, and thus represent an attractive technology to fabricate high-efficiency III-V/Si tandem solar cells and overcome the efficiency limit of the mainstream Si technology. Fabricating NW solar cells presents several challenges: growing organized arrays of NWs with high optoelectronic quality, processing NW arrays into devices, and characterising those devices (consisting of up to 109cm-2 sub-µm sized junctions in parallel). GaAs-based NW solar cells have been fabricated on patterned Si substrates by MBE. Our innovative core-shell heterostructure design consists in a p-GaAs core grown using the VLS method, and a n-GaInP high-bandgap shell. For fabricating complete devices, the NWs are encapsulated in a polymer, and contacted by a transparent conductive oxide. Our best sample has a 3.7% conversion efficiency, with a Voc = 0.65V, which is above the current record for GaAs NW solar cells on silicon. To assess the intrinsic quality of the NWs, we have used a calibrated, hyperspectral imager to record the photoluminescence and determine without contact the quasi-Fermi level splitting. Under a 1 sun illumination, we find values up to 0.95eV. They represent the maximum achievable Voc and confirm the NWs high crystalline quality. We will discuss the considerable scope for improvement in carrier collection and device processing and the routes toward higher efficiency NW solar cells.

Resume : Selective contacts are attracting a lot of interest in crystalline silicon-based photovoltaics (c-Si) in order to still improve the solar cell performance and getting closer to the theoretical limit efficiency. One of the most promising concept has been demonstrated by silicon heterojunction cells (HET-Si), that use a stack consisting of a very thin layer of hydrogenated amorphous silicon (a-Si:H) ensuring a good chemical passivation of the surface of the c-Si, and a layer of doped a-Si:H, n-type or p-type, for contact selectivity to extract electrons or holes, respectively. However, there are some drawbacks as the low conductivity of a-Si:H, which requires use of an additional layer of Indium Tin Oxide (ITO). The use of indium for which long-term resources and prices are not guaranteed, may also be a drawback for large-scale industrial development of this technology. Another concept of passivated selective contacts, based on a stack of very thin tunnel oxide layer and highly doped polycrystalline silicon is also under development. Here we explore a new path based on phosphide-type materials, which have attractive characteristics for this application. Boron phosphide (BP) is an indirect material with band gap of 2 eV [1] being significantly larger than c-Si and higher than a-Si:H, which augurs well for a gain in the short-circuit current compared to the use of a-Si:H. Preliminary work has shown that BP could be p-type doped and that the BP/Si interface exhibits a valence band offset either slightly positive (type I straddling gap heterojunction) or slightly negative (type II staggered gap heterojunction), making it an excellent candidate as a selective hole contact, without requiring an additional ITO layer [2]. The electrophysical properties of BP/Si heterostructures will be presented. The BP layers were grown by plasma technique (PECVD and PE-ALD processes) using phosphine and trimethyl-boron (TMB). This is a reliable, industry-relevant technique that allows one to grow electronic-quality films at low temperature on large areas. The material and heterojunction properties will be analyzed by combining several types of characterization techniques together with modeling studies. They will be complemented by temperature dependent current-voltage measurements performed on the same phosphide layers deposited on n-type c-Si to study transport through the heterojunction. Additional conductance and capacitance techniques will be used: capacitance vs voltage to address the doping and band offset issues, admittance spectroscopy (capacitance and conductance as a function of frequency and temperature) and DLTS (Deep Level Transient Spectroscopy) to study defects in the layers and at the interface.

Resume : Surface plasmon resonance (SPR) can be generated in metal nanostructures such as gold, silver, copper and aluminium, which are promising materials in the fields of optoelectronics and plasmonics. The SPR in periodic arrays has aroused great interest because of its uniform optical properties which can be applied in integrated optoelectronics device, including solar cells (SC). The obtained arrays of filamentous silicon nanocrystals with diameters of 100 nm and a height of 2 microns. The optical spectra of structures with Si nanowires and Ag nanoparticles on surface of Si wafer were calculated. The simulation was performed for a silicon wafer with silver nanoparticles with a diameter of 100 nm with a different placement period (P = 125 nm and 250 nm). Electric field strength distribution at the interface of a metal nanoparticle / Si-nanowires over a wavelength corresponding to a surface plasmon resonance was calculated. The simulation was carried out using Finite-Difference Time-Domain method (FDTD). Technology of formation of the silicon nanowires by the Metal-Assisted Chemical Etching method depending on the deposition time of silver nanoparticles and etching time are investigated in detail. The light trapping of the structures with Si-NWs base on analysis of reflection spectra in dependence on Si-NWs formation parameters are considered. It was shown that optimal deposition time and etching time for such Si nanowire – based solar cell are 60 s and 30 min, respectively. The influence of Ag nanoparticle in the Si-NWs arrays on reflectance coefficient, photoelectric properties are also investigated. The prototypes of photoelectric converters based on structures with silicon nanowires are formed and their characteristics are studied.

Resume : Common forms of elemental silicon (mono-, multi-crystalline and amorphous) play a foundational role in the field of electronics and the underlying technologies are well mastered. Exotic, low-density forms of silicon obtained have recently regained interest due to alluring semiconducting properties as thin-films. Here we focus on an allotrope of silicon: silicon clathrate thin-films. Clathrates are structures defined by a 3D porous framework (Si or group IV elements) of polyhedrons, which can encloses guest-atoms (Na or alkali metals). Due to this specific organization, some silicon clathrates can exhibit an interesting direct band-gap of 1.8-2 eV, at low guest-atom occupancy, which is interesting for both optoelectronic and photovoltaic applications. Moreover, this band-gap is tunable by modifying the amount/nature of the guest-atoms or using Si/Ge alloys framework. In this work, we describe the preparation and characterization of silicon clathrate thin-films using different substrates (p-doped and intrinsic c-Si (100) and (111), a-Si on sapphire...). The structural, electrical, optical, surface properties are analyzed and the fabrication conditions optimized to give the best properties and mechanical integrity. Ion implantation experiments and other doping techniques are investigated to tune the clathrates properties. Furthermore, early devices are fabricated in order to estimate the potential of this material for photovoltaic and optoelectronic applications.

Resume : To fabricate large-area photodetection material with high sensitivity, low cost and easy processing for biomedical imaging and environmental applications, P-type CuO films with proper bandgap, high optical absorption and good electrical performance have been fabricated by DC reactive magnetron sputtering. Considering that Copper oxide has three different phases (CuO, Cu4O3 and Cu2O), it features the advantage of multiple bandgaps by tuning the oxygen concentration. By XRD, Raman and SEM measurements, the phases of sputtered layers can be determined. For the calculation of Bandgap and Urbach energy, UV-VIS-IR spectrophotometer has been used to measure the reflectance and transmittance of thin films. By increasing the oxygen to argon ratio from 33.3% to 66.7% during sputtering, CuO thin films exhibit a tunable bandgap from 1.39 to around 1 eV, high absorption coefficient of over 105 cm-1 in UV and near visible spectral range. Hall measurements have also been carried out to measure the carrier type, mobility, concentration, and sheet resistance. The best performing p-type CuO film (O2/Ar=10/20 SCCM) exhibits a low Urbach energy of 4.1 meV, a good carrier concentration of 1.75×1015 cm-3, an excellent hole mobility of 11.1 cm2V-1s-1, and a sheet resistance of 1.90×107 Ω·sq-1.These CuO films show a great potential to be used in the application of large-area photodetection in the UV and near visible spectral range.

Resume : Efficiency of the solar cells can be increased by harvesting both solar and mechanical energy. Recently, several concepts utilizing solar and mechanical energy by hybrid semiconductor devices were presented. We showed InP/Si heterostructure for triboelectric/photovoltaic generation where triboelectric current was produced by sliding a conductive tip across InP surface while photocurrent was generated at the InP/Si interface. Unfortunately, tribo- and photocurrents had an opposite polarity due to relatively high surface state density at the Me/InP interface. Recently Hao et al. [2] and Liu et al. [3] presented synergetic co-harvesting of the solar and mechanical energies at the interface between sliding metal tip and a perovskite or p-Si, respectively. Motion of the conductive electrode non-linearly increased a solar cell harvesting at the moving electrode/semiconductor interface. Here, we studied a triboelectric/photovoltaic generation by the moving of the conductive tip of the atomic force microscope across the bare surface of the p-a-Si/i-a-Si/n-Si/i-a-Si/n-mc-Si/ITO heterojunction solar cell. Motion led to the increasing of the open-circuit voltage (Voc) from 630 mV to 640 mV with simultaneous increasing of the short-circuit current (Isc) density by 15%. This work is supported by Russian Science Foundation (RSF) project 18-72-00104. [1] Sharov V.A., et al.//ACS Appl. Energy Mater., 2, 4395 (2019) [2] Hao Z., et al.//Matter, 1, 639 (2019) [3] Liu J., et al.//Matter, 1, 650 (2019)

Resume : Transparent Conductive Oxides (TCO) in solar cells are used as transparent electrodes. They must necessarily have a high optical transmission in order to allow efficient transport of photons to the active layer and also good electrical conductivity which is required to obtain the least losses of transport of photo-generated charges. Aluminum, Magnesium and Tin doped ZnO thin films are some of these. In this contribution, the TCO materials formed of Al, Mg and Sn doped ZnO thin films (AZO, MZO, TZO) were synthesized by spin coating method. The structural, optical and electrical properties of these films are investigated as a function of dopant concentration. X-ray diffraction patterns show that all AZO, MZO and TZO films exhibit the hexagonal würtzite structure with a preferential orientation along [002] direction with no additional peak corresponding to secondary phases. The surface morphologies show that AZO and MZO films are homogeneous and dense while TZO films showed different surface morphologies going from homogeneous to porous structure depending on Sn concentration. An enhancement of optical transmittance of about 95% with a blue shift of the gap energy was found for both Al and Mg doped ZnO layers against 70% of average level transmittance and a red shift of optical window for Sn doped ZnO films. Photoluminescence (PL) measurements at room temperature shows excitonic peaks corresponding to the AZO, MZO and TZO films with a low defects level for 1% Al doped ZnO compared to MZO and TZO films that showing a large intensive band defects centred around 514 nm characterizing the substitution defects of the zinc atoms by oxygen atoms. Electrical measurements show that all the films present an Ohmic comportment with a best electrical conductivity of 2.45 S.cm-1 obtained for 1% AZO in accordance with the photoluminescence analysis. The reduced electrical conductivity shown in our films is attributed to the low mobility of charge carriers due to their diffusion by the grain boundaries.

Resume : The concept of hot carrier photovoltaics (HCPV) is to extract carriers excited to energies above the bandgap (hot carriers), before they have time to relax to the band edge via electron-phonon scattering. Semiconductor nanowires offer a promising platform for such devices for several reasons: the ability to grow highly lattice mismatched heterostructures [1]; observations of increased hot carrier temperatures [2]; and a geometry well suited for spatial control of light absorption. A HCPV device based on single InAs nanowires containing a small axial segment of InP at the center of the nanowire has been realized, where the InP segment serves as a carrier-selective filter that transmits primarily hot electrons. [3] To further probe the nature and performance of this device, we have established a scanning probe microscopy set up that allows for atomic force -, scanning photocurrent -, and scanning gate- microscopy interchangeably on the same device. At the spring meeting we will present the principles of this setup, early results and ongoing measurements on single InAs/InP nanowires, and discuss what properties we expect to probe in future work using this setup. [1] Caroff, P. et al., Nanotechnology, 2009, 20(49), 49560 [2] Tedeschi, D. et al., Nano Letters, 2016, 16(5), 3085-3093. [3] Limpert, S., Burke, A., Chen, I. et al., Nanotechnology 28, 434001 (2017).

Resume : Germanium (Ge) is commonly used as a substrate for epitaxial III-V growth because of the almost perfect match of lattice parameters between Ge and GaAs. Here, thin (40 - 200 nm) epitaxial films of Ge are deposited on two different types of (100) silicon wafers, to be used as the virtual substrate for epitaxial growth of III-V layers instead of bulk crystalline germanium wafers [1]. Radiofrequency plasma-enhanced chemical vapour deposition (PECVD) at a substrate temperature of 175oC has been used to deposit these heteroepitaxial germanium films [2] from the dissociation of germane (GeH4) diluted at 1% in hydrogen. The as-grown films on silicon are characterised using spectroscopic ellipsometry, Raman spectroscopy, X-Ray Diffraction and Transmission Electron Microscopy. Optimisation of the thickness, growth rate and quality of the epitaxial films is undertaken by varying different deposition parameters such as RF power, GeH4 flow rate, pressure etc. The germanium films are observed to have grown directly on the silicon wafers without any intermediate silicon-germanium alloy formation. The suitiability of these substrates for III-V epitaxy has been tested by annealing them up to 800 °C and measuring their hydrogen desorption. The behaviour of the germanium heteroepitaxial films as virtual substrates is analysed by depositing GaAs and GaInAs layers on them by either chemical beam epitaxy (CBE) or metallo organic chemical vapour deposition (MOCVD). These studies revealed the importance of the quality of the c-Si wafers on which the processes were carried out, as well as of the various in situ cleaning procedures adopted prior to III-V growth. The properties of the III-V films deposited on the virtual substrates were comparable in terms of roughness, microstructure and crystallinity to these of the materials co-deposited on c-Ge substrates, pointing at the effectiveness of the Ge epitaxial layers to act as virtual substrates for epitaxial III-V growth.

Resume : Transparent conducting materials (TCM) play a pivotal role in many modern devices. However, the most used TCM (ITO, Indium Tin Oxide) suffers from two major drawbacks: brittleness and indium scarcity. Among emerging TCMs used as transparent electrodes, silver nanowire (AgNW) networks appear as a promising alternative to ITO, with typical sheet resistance of 10 Ω/sq and optical transparency of 90%, combined with good mechanical flexibility. The fabrication of these electrodes often involves low-temperature processing steps and scalable methods, thus making them ideal for low-cost transparent electrodes in flexible electronic devices. The reduced sheet resistance of AgNW networks, however is usually obtained by thermal annealing of the material up to 300 ◦C in air. Although this treatment drastically reduces junction resistances, this high temperature is not ideal for temperature-sensitive process steps in manufacturing. Moreover, reduced thermal budgets are desired in some applications. In this work, we present an alternative approach by sintering the as-deposited AgNW networks with plasma exposure. The high energy ions from an Ar plasma interacts with the AgNW and locally sinter the AgNW together, forming a conducting path across large distances over the network. The aim of such an approach is to reach similar electrical and optical performance characteristics as in thermally-treated materials but with a significantly lower thermal budget. Plasma exposure also offers a better surface homogeneity compared to thermal annealing, which is affected by the mismatch of thermal conduction between the glass substrate and the metallic nanowires, and reduces the impact on the integrity of flexible polymer substrates. In this work, we report on a study of the effect of the plasma power and the exposure time on the morphological, electrical and optical properties of AgNW networks and we demonstrate that a fine tuning of these parameters is needed to obtain transparent electrodes with competitive characteristics

Resume : The intermediate band solar cell stands as one of the most outstanding options to make launch the 3rd Generation of photovoltaics with improved efficiencies. High concentration doping with transition metals has been proposed previously as a useful method to obtain IB materials. Using the experience in this methodology, applied until now mainly in chalcogenides, the in some manner forgotten space of multinary nitrides is explored looking for the desired properties to favour the generation of an intermediate band. This way, the family of ternary nitrides with spinel structure and group 14 elements as cations has been selected as a promising set of materials to study in detail in search of that possibility. Specifically, Sn3Ge3N8 presents a very convenient electronic configuration with a direct gap of 2.1 eV centered in and a good charge carriers mobility. After a systematic screening of the effect of the different transition metals doping in the host structure, some of the final compounds, by means of a precise study using GW approximation, present huge improvement in their absorption features. The analysis of their absorption spectra, obtained with a BSE calculation which consider excitonic effects, shows high absorption rates below the gap energy, leading inequivocally to the IB configuration.

Resume : Defect-free epitaxial growth of materials with high lattice mismatch can be facilitated by limiting the interface area to nanoscale regions [1,2]. One material that stands to gain from such conditions is zinc phosphide (Zn3P2), which is an earth-abundant semiconductor with potential as a photovoltaic absorber due to its appropriate bandgap (1.50 eV), long minority carrier diffusion lengths (< 10 μm), and high absorption in the visible range [3,4]. Despite its advantageous properties, it has so far been limited by challenges in high-quality epitaxial growth as a consequence of its large lattice constant and high coefficient of thermal expansion relative to commercially available substrates. Zinc phosphide’s intrinsic p-type doping due to it readily forming self-interstitials and lack of controllable n-type doping has further limited it. Herein, we utilize selective area epitaxy (SAE) in nanoscale holes and molecular beam epitaxy to demonstrate a transferable method to grow zinc phosphide [5]. Using a nanopatterned oxide mask on indium phosphide substrates we demonstrate epitaxial growth without the formation of misfit dislocations. Initially, zinc phosphide grows into nanopyramids encompassed by (101) facets, the most stable configuration as predicted by our DFT calculations. However, if allowed to grow for a sufficiently long time (which is a function of hole size and pitch), the nanopyramids will laterally overgrow the mask and eventually coalesce to form a textured thin film. This pyramidal structuring has been shown to be beneficial for light-management.6 Transmission electron microscopy was used to further investigate the crystalline and compositional properties of the SAE grown zinc phosphide, which showed the presence of rotational core-shell structures and the influence of hole size on the stoichiometry. Finally, through conductive atomic force microscopy (CAFM) and photoluminescence spectroscopy (PL) we elucidated the functional properties of the material. A diode-like behavior was observed through CAFM on samples grown on n-type substrates. From the PL we could observe a clear bandgap emission at 1.53 eV, indicative of a high crystalline quality and ideal for photovoltaic applications.s To summarize, SAE has shown great potential for high-quality epitaxial growth despite high lattice mismatches. The approach was demonstrated on zinc phosphide, producing a material with functional properties suitable for photovoltaic applications. Upcoming studies aim to apply transfer this approach to the growth on earth-abundant substrates (e.g. silicon) and making prototype photovoltaic devices based on SAE grown zinc phosphide. References 1. F. Glas Phys. Rev. B, 74, 121302 (2006). 2. C. –Y. Chi et al. Nano Lett., 13, 2506–2515 (2013). 3. G. M. Kimball et al. Appl. Phys. Lett., 95, 112103 (2009). 4. M. Y. Swinkels et al. Phys. Rev. Applied, 14, 024045 (2020). 5. S. Escobar Steinvall et al. Nanoscale Advances, (2020), DOI: 10.1039/D0NA00841A 6. D. Liang et al. Adv. Energy Mater., 2, 1254-1260 (2012).

Resume : Despite ZnO is widely used in various applications such as photocatalysts, photoelectrodes, antibacterial agents, gas sensors, supercapacitors and other electronic devices further search for new methods of production enabling performance of the material due to geometrical configuration, oxygen vacancy concentration etc. appears important field. E.g. semiconducting properties ZnO nanowires (NW) show critical change in electrical and optical properties that are observed for diameters below 30 nm. Recently we developed non catalytic ZnO NW synthesis methods, allowing to obtain nanowires with diameters 5-30 nm2, based on which a new type of multifunctional coatings can be obtained. In this way, several ZnO NW layers with novel function can be used to make a multifunctional coatings for the objects having potential commercial applications. Anti-reflecting coatings are essential components in solar cells, since in case of crystalline Si solar cell it reflects up to 30% of the incident photons. Preliminary results with a single layer employing original ZnO tetrapod coating technology already showed 6.2% efficiency increase without any additional surface patterning. At the same time, the coating is supehydrophobic, with a contact angle of as high as 170° Moreover, ZnO tetrapod layer can be simply deposited on any surfaces, demonstrating the further application prospects. NW sensors have much higher stability compared to other nanostructure counterparts, therefore high efficiency of the sensor employing novel ZnO tetrapods coatings is expected. ZnO tetrapods sensors were also tested for biosensing, UV sensing and gas sensing, showing fast response times and high efficiency. In order to further improve the ZnO tetrapod based multifunctional coatings for solar harvesting, specific functionalization with other materials can be performed. REFERENCES 1. Rackauskas S, Jiang H, Wagner JB, et al., Nano letters 14, 10 (2014). 2. Rackauskas S, Klimova O, Jiang H, et al. J Phys Chem C 119, 28 (2015). 3. Sulciute A, et al. J Phys Chem C, (2021) 10.1021/acs.jpcc.0c08459. ACKNOWLEDGMENTS This research is funded by the European Regional Development Fund according to the supported activity ‘Attracting scientists from abroad to carry out research‘ project No. 01.2.2-LMT-K-718-02-0011.

Resume : Doped strontium titanate (SrTiO3, STO) is a classic material for photocatalytic water splitting and hydrogen production. Its performance, however, is limited by relatively high water molecule dissociation barrier. Lowering this barrier could greatly improve the overall performance of the energy conversion process. Recently synthesized STO faceted nanocrystals demonstrated an extremely high performance in comparison to the standard flat STO surface. This was achieved due to the complex topology of the nanocrystal surface, giving the well-known material a whole set of completely new properties. Experimental studies show that faceted STO nanocrystals provide plenty of photocatalytically active sites. One of the key features of these nanoparticles are stepped facets. Catalytically more active stepped facets significantly facilitate water molecule adsorption and dissociation, increasing the overall reaction rate. Also, the presence of different facets makes possible targeted doping, which creates a potential gradient for an efficient charge carrier separation, reducing unwanted recombination. The process of water splitting on the stepped STO nanoparticles was modelled deploying the DFT method implemented in the plane wave and localized orbital basis sets*. Various structural models of stepped surfaces were tested. A complete process of water splitting on the stepped surface, including adsorption, dissociation, and migration, was investigated. Energetic, atomic, and electronic properties were obtained for the essential reaction steps. *VASP6.1, CRYSTAL17

Resume : The key component of solar cells is the absorber layer which captures the photons and converts them into electron-hole pairs. In practice, not all generated electron-hole pairs are extracted and collected from this layer. Indeed, the presence of deep defect states within the band gap of the absorber facilitates the annihilation of these pairs, hence reducing the minority charge-carrier lifetime and thus the conversion efficiency. The characteristics of point defects and their concentrations differ for all semiconductors, which leads to different performances even for comparable band gap solar cells. The carrier lifetime is a fundamental parameter that determines the efficiency of the solar cell. Here, we propose an ab initio high-throughput screening approach for accelerating the discovery of potential high-performance photovoltaic absorbers. In addition to the common essential material properties for potential PV materials (thermal stability, abundance, environmentally friendly, band gap, absorption coefficient spectrum, effective mass), we add two key parameters to our criteria; the lifetime of charge carrier and doping density. The charge-carrier lifetime is estimated using the Shockley-Read-Hall recombination model and first-principles point defect computations. We show that our methodology is able to eliminate the poorly-efficient materials. As a prior step, we test our approach on several materials that have already been extensively studied, both experimentally and from first principles. We show that our methodology displays a high ability for predicting the good efficiency of absorbers such as Si, GaAs, and CIGS. Moreover, we have succeeded at identifying defect tolerant materials and promising PV candidates by conducting our approach on Cu-based semiconductors.

Resume : Quasi-1D materials are a new class of thin film absorbers; owing to their highly tunable properties, well beyond standard polycrystalline compounds, there is a potential for technological breakthrough in a world where innovative and disruptive emerging PV applications are poised to grow and reshuffle the idea of decentralized energy production. In the frame of the ERC project SENSATE, our group works on the improvement of quasi-1D absorbers to bridge the performance gap with state of the art thin film solar cells, and to deliver on the promise of highly tunable photovoltaic solutions. This study focuses on Sb2Se3, our current reference material, and aims at identifying the main factors limiting the current performance of solar cells. Using literature data and numerical modeling, we assess the prevalence of the dominant defects reported in reference Sb2Se3 solar cells. While the bulk defects, commonly ascribed as D1 and D2, are found to significantly alter the performance of the device, the interface defect D3 (ascribed to either lattice mismatch or Cd-Se interdiffusion) is identified as the most important limitation, leading to a detrimental pinning of the quasi-fermi level and thus reducing the voltage of the cell by at least 250mV. Using this analysis as starting point, a mitigating strategy based on the short chemical etching of the Sb2Se3 surface is devised, with a comparison between various etching processes: Br2, HCl, KMnO4, Nitric and phosphoric acid N-P, Ammonium sulfide, KCN, CS2 and a combination between Br2 and KCN. A subsequent study of the etched surfaces by X-Ray Photoelectron Spectroscopy, X-Ray diffraction and Raman spectroscopy, along with the electrical characterization of complete devices, reveals the complex interplay existing between the surface state and morphology, and the solar cells’ performance. Interfacial passivation layers such as TiO2 are also investigated as possible passivation layers at the absorber/buffer pn interface. Additionally, the bulk defect properties of state of the art Sb2Se3 films are investigated using Photothermal Deflection Spectroscopy (PDS), an original approach where the thermal deflection of a laser beam allows for the energy-dependent defect probing of a layer. This method has never, to the best of our knowledge, been applied to study the sub-gap absorption of Sb2Se3 thin films, and is valuable as it theoretically offers a direct observation of defect-related transition energies. Alkali doping strategies are considered and show a reduced sub-bandgap absorption, hinting at an improvement of the film’s bulk properties. Complementing the PDS analysis, results from Raman and XRD investigations will also be presented. The combination of numerical modeling, material and device characterization permits to devise strategies aiming at improving Sb2Se3 solar cells, along with providing valuable insights in the design of other quasi-1D thin film absorbers. The characterization of solar cells implementing such improvement strategies (interface etching, passivation, alkali doping…) will be discussed in regard to the aforementioned limitations. The latest set of results will be presented.

Resume : Earth-abundant and low-cost materials are essential for future large-scale deployment of thin-film photovoltaics (PV). However, many such materials face numerous challenges as the optimal performance requires fine-tuning of optoelectronic properties, which is usually achieved through defect engineering. Defect identification is a first step in this direction. This work gives an example of systematic defect identification in zinc phosphide (Zn3P2) using both theoretical and experimental perspectives that can be readily transferred to other materials. Zn3P2 is an earth-abundant, direct band gap (1.5 eV) and highly absorbent semiconductor (> 10^5), making it promising for PV applications. However, synthesis on commercially available substrates, which favours the formation of defects due to mismatching lattice parameters and thermal expansion coefficients, and controllable doping are challenging drawbacks that restrain device performance. To solve the biggest challenge of defect engineering, we have employed Raman spectroscopy and density functional theory (DFT) calculations in order to identify and understand the effect of defects on materials properties. First part of the study is focused on a comprehensive analysis of vibrational properties of tetragonally-structured Zn3P2 through DFT calculations and Raman measurements on single crystalline nanowires [1,2,3]. Different polarisation configurations have allowed selective enhancement Raman modes. A total of 34 peaks were identified and assigned to 8 A1g + 9 B1g + 3 B2g + 14 Eg theoretically predicted eigenmodes. DFT calculations have resolved three separate regions in the phonon dispersion diagram: (i) low-frequency region (<215 cm−1) - dominated by Zn-related vibrations, (ii) intermediate region (215–225 cm−1) - representing a true phonon gap with no observed vibrations, and (iii) high-frequency region (>225 cm−1) - attributed to primarily P-related vibrations. Further, each Raman mode was assigned to a distinct vibrational pattern, involving vibrations of either Zn or P atoms primarily. These results were used as a reference for the second part of the study, where a series of epitaxial single crystalline thin films with various Zn/P compositional ratios was produced. Comparison of the Raman spectra from the non-stoichiometric samples has allowed identifying modes which intensity is sensitive to presence of certain defects. For example, P interstitial incorporation into Zn empty sites in the Zn3P2 structure leads to overall depletion in intensity of Raman peak at 211 cm-1, which are related to in-plane vibrations of Zn atoms. Similarly, changes in other modes are identified and correlated to the presence of ZnP, PZn, VZn and VP defects. This has allowed formulation of a general methodology for defect identification in Zn3P2 and other semiconductors, which will be presented and discussed. [1] E. Z Stutz et al. Nanotechnology 32 085704 (2021) [2] S. Escobar Steinvall et al., Nanoscale Horizons, 5, 2, 274–282, (2020) [3] S. Escobar Steinvall et al., Nanoscale Adv., 2020, doi: 10.1039/D0NA00841A

Resume : The current search of low-cost and more environmentally friendly photovoltaic technologies and is a driving force for the development of thin-film Cu2O as absorber material. Indeed, by combining its earth abundance and potentially excellent electrical and optical properties, cuprous oxide is one of the highest potential metal oxides for the absorber layer of solar cells. Usually, photovoltaics based on bulk cuprous oxide were produced by forming the Cu2O absorber material from thermal oxidation of copper. However, the thermal oxidation method requires high energy consumption, which limits the development of Cu2O based solar cells at low cost and industrial scale. The ultrasonic spray pyrolysis (USP) technique meets these crucial requirements of low cost and compatibility with large area industrial manufacturing. Nevertheless, the deposition of Cu2O thin films by USP still suffers from the novelty of this elaboration process as their lower properties are detrimental to the performances of photovoltaic devices, mainly due to the high concentration of impurities. Very recently, the feasibility of using new precursor solution protocol was suggested by Plankensteiner et al. (ChemNanoMat 6 4 (2020). pp. 663–671) and consists in the substitution of the glucose oxidizer agent by D-sorbitol or glycerol to improve Cu2O thin film properties as this reactant might decrease carbon related by-products during the pyrolysis reaction and thus impurities in the Cu2O thin films. Beyond the feasibility and in order to investigate the Cu2O thin films obtained by USP using D-sorbitol as a new oxidizer agent and to optimize the material for all-oxide solar cell applications, the present work focus on the detailed study of thin film microstructural and optical properties with respect to elaboration conditions, mainly the concentration, pH and thickness, thanks to a specific design of experiment. The deposited thin films properties, using this new precursor solution, were compared to the standard solution processed Cu2O. Cuprous oxide thin films are thus characterized from transmission measurements to investigate the optical properties and evaluate the defects thanks to Urbach energy and absorption edge comparisons. High resolution confocal Raman mapping was used for phase analysis while the microstructure of thin films is analyzed by scanning electron microscopy. Finally, thanks to the correlation between the optical properties, Raman mapping and scanning electron microscopy, the evolution of the microstructure of the thin films with respect to the deposition conditions is presented and allow to better understand Cu2O thin film properties with the aim of enhancing all-oxide photovoltaic performances.

Resume : All-oxide photovoltaics is a new and very promising technology for low cost and efficient solar cells using earth-abundant and environmentally compatible materials. To match the requirements of this technology, beyond the optimization of the absorber layer, it is crucial to elaborate window and transport layers with excellent optical and structural properties in a wide range of thickness. Among the oxide materials, zinc oxide is a key material for the realization of these layers in the targeted all-oxide solar cells. Beside the low toxicity and abundance of ZnO, the reduction of the environmental footprint of photovoltaic technology could benefit from the low energy and low precursor consumption of the ultrasonic spray pyrolysis (USP) process. In addition to the environmental aspects, the USP process combines many advantages compared to conventional methods such as low-cost and large-scale production capability. The potential for elaboration of thin films of suitable structural and optical properties of this process combined with the previously mentioned advantages makes it of high interest for the future of photovoltaics. To date, the optical and structural properties of ZnO elaborated by USP have not been fully investigated with respect to the precursor solution concentration, which could have a strong impact on these properties and thus on the performances of the solar cell. ZnO thin films were deposited using the USP process after a precise optimization of the main growth conditions, namely the substrate temperature, spray pressure, nozzle speed and precursor solution flux, according to a specific design of experiment. ZnO films were confirmed to be conductive by showing resistivity values in the order of 1 Ω.cm. For this study, we used X-ray diffraction (XRD) and Raman spectroscopy to analyze the crystal structure, orientation and texture coefficient. Our results show that the preferential orientation can be controlled to be either (002) or (100) by adjusting the precursor solution concentration. On the one hand, the precise control of the orientation allows to better control the optical properties, that are crucial in our application, and, on the other hand, limit the defects at the heterointerface. The morphological properties were studied using atomic force microscopy and were correlated to the structural properties of the ZnO thin films. For instance, high consistency was evidenced between the AFM measured grain size and the XRD measured crystallite size. Finally, the optical properties were studied from UV/visible transmittance spectroscopy. The deposited ZnO films exhibit high transmittance above 90 % for the whole concentration range of the precursor solution with a maximum as high as 95 %. The resulting optical and structural properties could benefit to the performances of all-oxide solar cells deposited by USP.

Resume : Introduction: The transmittance of the photoanode is the key parameter that affects the conversion efficiency of dye-sensitized solar cell device (DSSCs) [1]. Compared to the TiO2 photoanodes, ZnO nanorods have attracted more attention due to their good conductivity, excellent optical properties, and large surface-to-volume ratio [1]. However, it is difficult to control the growth direction of ZnO nanorods on amorphous ITO substrate resulting in low transmittance. Therefore, the objective of this work is to improve the transmittance of ZnO nanorods synthesized on ITO substrate by introducing aluminum doped ZnO (AZO) thin films and, for further enhancement, ZnO thin films between ITO and AZO thin films. Experimental: 300 nm ITO thin films were deposited on glass substrate by direct current sputtering. Radio frequency (13.56 MHz) magnetron sputtering was utilized to deposit the two types of seed layers on ITO: 600 nm AZO thin films directly (AZO/ITO) and 60 nm ZnO thin films following 600 nm AZO thin films (AZO/ZnO/ITO). Subsequently, ZnO nanorods were grown by chemical bath deposition method with dissolving zinc nitrate hexahydrate (25 mM) and hexamethylenetetramine (12.5 mM) in 200 mL deionized water at 95℃ for 5 hours. X-ray diffraction (XRD, Rigaku ATX-G), FE-SEM (Hitachi SU8000), and UV-Visible/NIR spectrophotometer (Hitachi U-4100) were applied to investigate the crystallinity, the morphology, and the transmittance, respectively. Results and Discussion: A dominated (002) peak was observed from the XRD results of the ZnO nanorods grown on the two substrates, which indicated the ZnO nanorods had a preferred growth orientation along (0001) direction. The FE-SEM images of ZnO nanorods grown on these two substrates showed the average diameter was around 120 nm and the height was in the range of 900 nm to 1100 nm. Based on the transmittance spectra, the transmittance (42%) of ZnO nanorods grown on AZO/ZnO/ITO substrate was improved compared to that (30%) of ZnO nanorods fabricated on AZO/ITO substrate. The AZO thin films (seed layer) and the additional ZnO thin films (buffer layer) with good crystallinity deposited on the amorphous ITO substrate contributed to the vertical growth direction of ZnO nanorods. In summary, the transmittance of ZnO nanorods on ITO substrate was significantly improved with introducing AZO thin films with ZnO buffer layer. Reference: [1] S. Zhang, J. Jin, D. Li, Z. Fu, S. Gao, S. Cheng, X. Yu, Y. Xiong, RSC ADV., 9, 22092-22100 (2019).

Resume : Transparent Conductors Materials (TCM) are of great technological importance for many devices and applications in the renewable energy production field. Tin-doped indium oxide (ITO) is the most used transparent conductive oxide (TCO) due to its high transparency in the VIS region (~ 80%) and low resistivity (10^(-2)-10^(-3) Ωcm). However, its large-scale usage faces the issue of scarcity, price and toxicity of In, so promoting new materials with superior properties in terms of sustainability or, at least, in terms of performances. Doping indium oxide with transition metal (Mo, Zr, Hf, Ta) as dopant has been proposed as potential strategy to improve conductivity and transparency in ultra-thin films. In this work we show a further approach to improve ITO properties by using Mo co-doping (ITMO). Thin films (~75nm) of ITMO with Mo concentrations of about 1-2% were deposited by RF magnetron co-sputtering from ITO and Mo targets. Thermal annealings, up to 400°C, were also performed to improve optical and electrical properties of the as deposited films. Samples have been characterized by UV-VIS-NIR spectrophotometry, 4-points electrical probe, XPS and RBS. Although a high doping efficiency is measured already in the as deposited ITMO films, a drastic improvement in both electrical and optical properties has been observed for thermal annealing at about 300 °C, probably due to the annealing of structural defects limiting the carrier mobility. The XPS analysis, in particular, helps to understand the role of the Mo oxides in the ITMO matrix.

Resume : Atomic Layer Deposition (ALD) method found new applications in the field of photovoltaics. First of all, ALD deposited layers are used as passivating layers for silicon-based solar cells, as anti-reflective layers, and, as we show, as transparent conductive electrodes (TCO layers). Other possible applications include use of ZnO as a buffer layer and/or as a n-type emitter in modified Si-based solar cells. The latter application we demonstrated recently [1-3]. In our work, we show solar cells based on a n-type ZnO structures (layers and nanorods). ZnO/p-type Si photovoltaic cells were constructed on silicon wafers with thicknesses of 170 μm and 50 μm, using developed by us 3D electrode based on ZnO nanorods. For 170 μm thick solar cells, we received light conversion efficiency up to 14%. For thinner silicon wafers (50 μm) the Jsc value decreases from 38 mA/cm2 to 30 mA/cm2, whereas the Voc remains similar, of about 500 mV. The solar cells efficiency was ~10%. Further modification includes passivation of the rear contact (like in standard PERC solar cells). Thin aluminum oxide (Al2O3) layer is deposited between Al-contact and back side of silicon. Using laser cutter, small dot patterns are cut in the Al2O3 before deposition of Al contact. The results for these modified cells will be presented. This work was partially supported by the National Centre for Research and Development TECHMATSTRATEG1/347431/14/NCBR/2018. 1. R. Pietruszka, et al., Solar Energy 155 (2017) 1282-1288. 2. R. Pietruszka, et al., Solar Energy Materials & Solar Cells 147, (2016) 164-170. 3. R. Pietruszka, et al., Solar Energy Materials & Solar Cells 143, (2015) 99–104.

Resume : Copper oxides are a p-type semiconductor materials with a band gap energy in the range from 1.2 – 2.1 eV and great potential for use in thin-film solar cells. Common methods of copper oxides manufacturing are magnetron sputtering, chemical vapor deposition or thermal oxidation. However, their main problem is providing a non-single phase layer which is usually a mixture of tenorite (CuO) and cuprite (Cu2O) especially when thermal oxidation method is used. It should be mentioned that with oxidation temperature increasing the crystallinity and stoichiometry of the layer are improved. Nevertheless, the resistivity of the layer also goes up considerably. Therefore, to enhance conductivity of copper oxides the additional dopant atoms are introduced into their structure. In the literature sodium in the form of sodium chloride is most often reported. In our work we proposed a simple method of high temperature sodium doped copper oxide manufacturing by using bar coating technique. The Na precursor was metalorganic sodium compound with low decomposition temperature. The sodium doped copper oxide layer was obtained by one step thermal oxidation of copper sheet in IR belt furnace at 800°C in dry air atmosphere. This approach allowed to reduce the series resistance of the Cu/Cu2O/CuO system investigated with 4 cm2 surface from 22kΩ to 103Ω. From the other perspective, to accomplish a solar cell a high quality and energetically matched n-type emitter layer is necessary. For that matter the zinc oxide is a good candidate since it has low resistivity and energy bands matched to CuO. What is more it can be obtained by various methods. One of the convenient deposition method is ALD (atomic layer deposition), which allows for precise layer thickness control even at rough surfaces such as thermally obtained CuO. From electrical point of view the emitter layer could be as thin as required n-side junction width. However, it was confirmed that optoelectronic properties of ALD zinc oxide, are highly thickness dependent. The thickness effect was investigated in the range of 25 - 800 nm in three sets of different deposition temperature (150, 175 and 200°C). This investigation will help to optimize the emitter thickness and deposition conditions.

Resume : Zinc phosphide (Zn_3 P_2) has great potential for widespread thin film photovoltaic applications due to the abundance of its constituting elements and suitable opto-electronic properties. It offers high minority carrier mobility, diffusion length and absorption coefficient in combination with a direct bandgap of 1.5 eV, which is close to optimal value of the Shockley-Queisser limit [1]. Furthermore, it is thermodynamically stable with a simple phase diagram compared to other earth-abundant semiconductors such as kesterites. Separate source molecular beam epitaxy (MBE) is an ideal platform for optimizing the growth of Zn_3 P_2thin films as it provides precise control over the fluxes and substrate temperature. Furthermore, it facilitates low-temperature growth of this material, addressing the long standing issues stemming from the abnormally high thermal expansion coefficient of Zn_3 P_2, which result in crack formation upon cooling down from high growth temperatures [2]–[4]. Here, we present the epitaxial growth of zinc phosphide thin films on InP(100) substrates using MBE. We provide an overview of the thin film morphology and crystalline quality as functions of the zinc and phosphorous fluxes as well as V/II ratio, growth temperature and substrate preparation. The samples are investigated using multiple characterization methods, such as scanning electron microscopy (SEM), X-ray diffraction (XRD), (scanning) transmission electron microscopy (STEM/TEM), electron energy loss spectroscopy (EELS) and Raman spectroscopy. The insight gathers from these techniques help us establish the conditions for selectively changing the thin film type from amorphous to polycrystalline and monocrystalline. It is observed that elevated growth temperatures and V/II ratios result in amorphous films. The effects of interface oxide on the crystalline quality of the thin films is studied by STEM and EELS. Complete removal of interface oxide through in-situ degassing of the substrates at high temperature is found to be a key step for growing monocrystalline thin films. The functionality of the different thin film types is further evaluated by photoluminescence (PL) spectroscopy, conductive atomic force microscopy (c-AFM) and terahertz (THz) spectroscopy. This study provides information about the growth of zinc phosphide thin films that could be valuable for implementation of this material for photovoltaic applications. References [1] J. P. Bosco, G. M. Kimball, N. S. Lewis, and H. A. Atwater, “Pseudomorphic growth and strain relaxation of α-Zn3P 2 on GaAs(001) by molecular beam epitaxy,” J. Cryst. Growth, vol. 363, pp. 205–210, Jan. 2013, doi: 10.1016/j.jcrysgro.2012.10.054. [2] R. Paul et al., “Van der Waals Epitaxy of Earth-Abundant Zn3P2on Graphene for Photovoltaics,” Cryst. Growth Des., vol. 20, no. 6, pp. 3816–3825, Jun. 2020, doi: 10.1021/acs.cgd.0c00125. [3] S. Escobar Steinvall et al., “Multiple morphologies and functionality of nanowires made from earth-abundant zinc phosphide,” Nanoscale Horizons, vol. 5, no. 2, pp. 274–282, Feb. 2020, doi: 10.1039/c9nh00398c. [4] S. Escobar Steinvall et al., “Towards defect-free thin films of the earth-abundant absorber zinc phosphide by nanopatterning,” Nanoscale Adv., 2020, doi: 10.1039/D0NA00841A.

Resume : Metal-oxide semiconductors have gained a lot of attention owing to their good chemical stability, non-toxicity, abundance and cost-effective manufacturing processes. They are reported for multiple applications in a wide variety of domains such as photovoltaics, photodetection, water splitting and optoelectronics, among others. Furthermore, the development of all-oxide based devices is emerging as an important approach to minimize fabrication cost and environmental impact. Recently, cuprous oxide (Cu2O)-based heterojunctions have attracted an increased attention as this material is promising as a building block for such structures. For instance, the use of Cu2O as a p-type semiconductor was reported in heterojunctions with wide band gap compounds such as ZnO as n-type material. Despite of recent efforts to fabricate Cu2O/ZnO heterojunctions with decent electrical properties, there is no detailed study about the optimal values for layer thickness, bandgap, and doping concentrations of the constitutive thin films to produce devices with enhanced performances. In this work, we report results of the numerical simulation of Cu2O/ZnO and Cu2O:Mg/ZnO heterojunctions, aiming at unveiling the impact of variations in material properties on the electrical response of the devices in steady-state regime. In this numerical approach, a drift-diffusion model was derived from the basic semiconductor equations, in which a spatial discretization and a linearization of the system are performed. The algorithm chosen to solve this model is based on the Newton-Raphson iterative method that involves a series of corrections to an initial guess. Our results indicate that Mg cations play an important role in the improving of the heterojunction performance. In particular, the onset voltage was increased from 0.8 V to 1.15 V for Cu2O/ZnO and Cu2O:Mg/ZnO heterojunctions, respectively. Moreover, our study highlights the effectiveness of numerical simulations to accelerate the experimental workflow in view of optimizing the electrical characteristics of the heterojunctions through a detailed understanding of the direct correlation between material parameters and device output.

Resume : Surface arrangements of subwavelength formations are extensively discussed in the context of photocurrent enhancement for photovoltaic (PV) applications. Recently, the potential contribution of such arrangements toward photovoltage augmentation is suggested. This study numerically demonstrates the potential for photovoltage management based on arrays composed of inverted silicon cones, referred to as light funnel (LF) arrays. The transition from an optimized nanopillar (NP) array into an LF array is examined. It is shown that a decrease in NP bottom diameter (Db) is accompanied by an increase in open-circuit voltage (Voc). The highest photovoltage enhancement is recorded for the smallest considered Db ¼ 50 nm with a Voc increase of 75 mV and reflects a 22% Voc enhancement compared with the NP Voc. It is shown that this Voc increase is due to 250% increase in the excitation level, and that the spatially resolved excitation level of the array-nested LFs is more than two orders of magnitude higher than the highest spatially resolved excitation level in the array-nested NPs. Finally, it is shown that the suggested photovoltage management entails almost a factor of 2 increase in the nominal power conversion efficiency upon the transition from an NP PV cell into LF PV cell.

Resume : The properties of vertically aligned silicon structures have attracted increased attention lately. One of the properties is their low reflection coefficient and so these structures show strong second garmonic generation. These properties can be used in flexible silicon solar cells as well as in flexible transparent visualizers of laser radiation. To obtain vertically aligned structures on silicon, it is possible to use the processes of dry plasma etching in an SF6 / O2 gas at cryogenic temperatures, without applying a mask. In the process of cryogenic plasma etching, the surface of the silicon substrate is bombarded with SFx radicals (0 ≤ x ≤ 5), formed as a result of the dissociation of SF6 molecules, with the formation of volatile compounds SiFх and SiFхOy. Under standard conditions, (at temperatures > - 100° C), the process exhibits isotropic etching behavior. With a decrease in temperature ( ≤ 100° C), condensation of SiFхOy molecules occurs on the cooled surface of the silicon substrate, which creates a passivation layer on the silicon surface. Since in plasma etching, active SFx radicals have a strict direction of motion under the action of the pulling plasma field, the vertical etching rate is much higher than the lateral one. At a sufficiently low temperature and a high oxygen content in the gas mixture, the mode of passivation of silicon and vertically aligned structures (“black silicon”) could be formed. The addition of inert gases makes it possible to increase the intensity of the ion bombardment of the bottom etching and to increase the temperature of black silicon formation. The influence of Ar and He gas additives to “black silicon” formation is shown in this work. The addition of inert gases allows you to adjust the density of vertically aligned structures as well as control their size (diameter).

Resume : SnOx materials are very appealing TCOs owing to their higher earth-abundance, wide bandgap, chemical stability, optical transparency and low cost. SnO2 is known as an intrinsically n-type semiconductor and SnO as a p-type one. SnO2 is easier to synthesize and more stable compared to SnO. A high-quality p-type SnOx will enable all-oxide p-n junction, which can be used in different applications particularly in optoelectronics. In this work, we study Plasma Post Treatment (PPT) effect on nitrogen doping for sputtered SnOx to achieve a p-type semiconductor using PECVD set-up at six different conditions. Process gases used in PPT are NH3, N2O and N2. These nitrogen precursor gases can be decomposed by RF-plasma and release N ions, which will be incorporated within SnOx. The as-deposited SnOx thin films were intentionally grown in poor-oxygen condition to form oxygen vacancies and favor nitrogen incorporation. Results showed that nitrogen has been successfully incorporated in SnOx thin films in different chemical states namely both NOx and atomic N incorporation within O vacancy sublattice, and Sn nitrides N-Sn(IV) /N-Sn(II). Two different NH3 treatments have etched/reduced the SnOx. Two different N2O treatments have improved the n-type conductivity from 0.25 to 0.075 and 0.046 Ohm-cm while decreasing only slightly the wide bandgap of SnOx and the surface roughness remains unchanged. One of N2 post plasma treatments has made the SnOx highly resistive. The other N2 treatment has converted the n-type conductive to p-type (4.7 Ohm-cm) while the optical bandgap remains almost unchanged. The surface roughness after the N2 treatments has increased from 0.5 to around 2 nm. The optical transmittance (500~800 nm) has slightly decreased for N2O and N2 treatments from 77% to between 72 and 76%. These results have demonstrated that nitrogen doping using PPT has a strong potential for improving the TCOs and moving towards efficient p-type TCOs.

Resume : Silicon nanowire/organic hybrid solar cells are promising for next generation alternative energy sources due to their flexibility and low cost of manufacture. As well as the possibility to enhance the performance compared to traditional silicon based solar cell. The usage of Si nanowires (SiNWs) leads to an increase in the optical absorption of solar radiation in active layers. PEDOT: PSS films, in turn, possess high transmittance, conductivity and flexibility. These layers can be used as transparent electrode instead of ITO, which is not suitable for flexible devices. SiNWs solar cells embedded in polymer matrix have enhanced mechanical stability compared to conventional planar tandem solar cells deposited on flexible substrate. In addition, PEDOT: PSS is a more attractive emitter material for SiNWs compared to a-Si: H layer because it is difficult to deposit (p)a-Si:H/(i)a-Si:H layer stack with precise thickness control on vertically aligned silicon structures. In this paper, four types of structures that differ in the type of substrate surface (planar/NWs) and the method of PEDOT:PSS layer deposition (printing/ spin coating) are considered. An ohmic contact to PEDOT:PSS is formed using Ag paste. The open-circuit voltage (Voc) up to 0.6 V is reached for PEDOT:PSS/(n)Si structures. Spectral response measurements demonstrate an extremely high quantum efficiency in UV region spectrum below 400 nm meaning low absorption losses in PEDOT:PSS layer. High values of Voc and quantum efficiency at short wavelength region indicate sufficient passivation properties of PEDOT:PSS/(n)Si interface. Thus PEDOT:PSS is promising candidate to use as an emitter layer for SiNWs. However, there steel several problems need to be addressed including optimization of the photoelectrical properties and improving the adhesion between PEDOT: PSS/Si.

Resume : The increasing need of green energy sources pull the research to search for innovative photovoltaic devices, based on new materials. An important answer can be provided by All Oxides Solar Cells (AOSCs), due to the remarkable properties of the metal oxides, such as appropriate band gaps, availability and stability[1]. These materials still require further optimization to solve issues like low charge mobility or energy level position. We reported here on the successful tuning of the electronic properties of Co3O4 and MoO3, a light absorber layer[1][2] and a charge transporting material, respectively.[1] By exploiting a hydrothermal synthesis approach, they were chemically doped with V(V), thus obtaining the increase of the two oxides work function positions by the control of the amount of synthesis precursors. Preliminary results in the deposition of thin film of the two oxides using dip coating will be presented. [1] Rühle S, Anderson AY, Barad H-N, Kupfer B, Bouhadana Y, Rosh-Hodesh E, et al. All-Oxide Photovoltaics. J Phys Chem Lett 2012;3:3755–64. https://doi.org/10.1021/jz3017039. [2] Kupfer B, Majhi K, Keller DA, Bouhadana Y, Rühle S, Barad HN, et al. Thin Film Co 3 O 4 /TiO 2 Heterojunction Solar Cells. Adv Energy Mater 2015;5:1401007. https://doi.org/10.1002/aenm.201401007.

Resume : Development of new photovoltaic (PV) technologies based on novel green materials with higher efficiency and lower cost options could create a new industry with independent supply chains, foster IoT market diversification, and attenuate market volatility. Bi2S3 has emerged as one of promising inorganic materials for PV and other optoelectronic applications owing to its optimum direct band gap of 1.3 eV, high absorption coefficient (105 cm-1, and absorption onset in the infrared), exhibits both n-type and p-type conductivity, nontoxic and earth abundant elements. Related to its non-toxic nature, so far it has been very popular to deposit the material by chemical solution routes. Research investigations on development of the Bi2S3 as well as the solar cell structures by physical deposition methods are almost absent to date. Thus, the focus of the current study was to develop for the first time, Bi2S3 thin film and solar cells using robust, rapid and high yield close spaced sublimation (CSS) technique. Three sets of samples were fabricated: glass/Bi2S3, glass/FTO/CdS/Bi2S3/Au, and glass/FTO/TiO2/Bi2S3/Au. To find the optimal processing condition, the CSS substrate deposition temperature was varied from 250 to 450 oC. The absorber films deposited at 250 and 300 °C exhibit highly dispersed and porous ribbon-like structures, whereas deposition at 350-450 °C leads to denser absorber with larger grain size. The diffractograms showed the primary peaks from the (221), (141), and (431) planes corresponding to the orthorhombic Bi2S3 crystal structure (PDF card no: 00-017-0320, space group Pbnm (62)) with no evident preferential orientation. The band gap, resistivity, conductivity type and carrier concentration of Bi2S3 films on glass substrates were revealed by UV-Vis, hot probe and Hall effect measurements. The layers exhibited Eg~1.25 eV, n-type conductivity and carrier concertation of ~1017-1018 cm-3. Superstrate configuration glass/FTO/CdS/Bi2S3/Au and glass/FTO/TiO2/Bi2S3/Au isotype heterojunction solar cells were fabricated with efficiencies of 0.1% and 2%, respectively. I-V-T, EQE, XPS, low-temperature PL, C-V, C-F-T characterization methods were used to bring insight into band alignment, recombination mechanism, defect chemistry, and correlation between defect density and carrier concentration in Bi2S3 films and solar cells. The 2% efficient TiO2/Bi2S3 solar cell is the first-time reported performance by CSS deposition technique and it shows the high potential of such device architecture.

Resume : Nowadays solar cells become a part of everyday life. Giant solar farms produce electricity for the whole cities along with solar panels installed on roofs of private houses made people’s life less dependent on traditional fossil fuel like oil and coal. Silicon based solar cells are the most common due to well developed silicon technology. The world record efficiency of 26.7% is reached for laboratory samples [1] and 22.7% for commercially available solar panels [2]. Further market growth will be connected with non-traditional solar cells application like flexible panels or non-planar installations. There are already many concepts concerned to flexible solar cells development. However, presented at the moment flexible panels either have low bending angle or low efficiency or fast degradation. The problem may be solved with an approach where photoactive vertically aligned silicon nano-sized heterostructures are bonded within polymer flexible film. Polymer thin film specifies high flexibility. Silicon heterostructures due to its small size can’t be broken even with high bending angle. To fabricate vertically aligned silicon heterostructures a method of cryogenic ICP plasma etching of latex sphere patterned silicon wafer is used. Such approach makes it possible to form silicon nanostructures up to 10 microns with high aspect ratio and a diameter from 100 nm [3]. Recent studies have shown that ICP plasma etching at cryogenic temperature do not dramatically decrease carrier lifetime in high quality silicon wafers in contrast to RIE etching [4] that makes it possible to fabricate high efficiency solar cells. Si based heterostructures are formed by PECVD of a-Si:H. In this work new results such as heterojunction formation, its characterization and related problems will be demonstrated. Also aspects related to detachment of polymer membrane with silicon nanostructures will be discussed. This work was supported by the Russian Scientific Foundation under grant number 18 79 10059. [1] Yoshikawa K, Kawasaki H, Yoshida W, et al. Nat Energy. 2017;2(5):17032 [2] https://us.sunpower.com/products/solar-panels [3] I. Morozov et. al. // Phys. Status Solidi A, 217: 1900535 [4] D. Kudryashov et. al. // Phys. Status Solidi A 2019, 1900534

Resume : In this work, we propose to consider the system of iron and chromium oxides as a hybrid photovoltaic and thermoelectric harvester of solar energy. We also present the effect of the composition of Cr and Fe oxide-based multilayers on the photo-induced charge separation rate in the one. Cr and Fe oxide-based multilayer structures with enhanced photovoltaic properties were investigated. Nanometric iron and chromium oxide layers, their multilayer combinations and solid solutions were grown on silicon and glass substrates by the reactive pulsed laser deposition techniques at various technological parameters such as oxygen pressure in the reactor, substrate temperature, number of laser pulses. For surface and in-depth composition identi?cation of oxide-based structures XPS, XRD and TOF SIMS technique were successfully used. Systematic studies of the optical properties of these structures using spectral ellipsometry in the NIR?VIS?UV range and light reflectance and transmittance measurements in the VIS range allowed to describe investigated structures in the framework of the optical model of the multilayer structure with composite layers. It was shown that oxides are present on the surface in different phases, and the technological parameters and composition of these films significantly affect their optical properties in NIR?VIS?UV range. Methods of low-temperature photoluminescence and Raman spectroscopy techniques were used for the investigation of the recombination and vibrational properties of the samples studied. Investigation of the charge carriers? transport kinetics was carried out by surface photovoltage and impedance spectroscopy technique. The results reveal that optimal bilayer structure grown on a cold substrate has demonstrated photovoltage enhancement that corresponds to the high separation rate of photo-induced charges. Integral responsivity in the spectral region 360-1100 nm is about 500 V/W that is more than 100 times of that of the nonoptimized structures. A synergetic effect of thermoelectric and photovoltaic phenomena in multilayer combinations of Cr and Fe oxides is observed.

Resume : Combination of III-V compounds with Si provides a possibility to create new optoelectronic devices like light emitting diodes, optical IC and multijunction solar cells. GaP has the smallest lattice mismatch to Si among all III-V binary alloys making this material commonly used as a nucleation layer. However, GaP is an indirect semiconductor and there is still about (0.4%) lattice mismatch. Addition of few percent of nitrogen in GaP could compensate this difference in lattice constant and the most important that GaPN alloy becomes direct band material. Thus, GaP based diluted nitride is a perspective material for III-V/Si solar cells. Low temperature technique is one of the key issues for integration of III-V with Si electronics. Recently, an epitaxial growth of thin GaP film on Si substrate as well as GaN layers on sapphire was achieved by PE-ALD at temperature below 400 C. Here the plasma deposition technology is explored for fabrication of photoactive III-V layer. Two different approach to plasma deposition process were considered: conventional PECVD and digital alloys growth by PE-ALD. The first one allows to reach high growth rate while the second one could improve control of nitrogen incorporation. Conventional PECVD process was explored for GaP binary alloy. Deposition in continuous mode at 390 C using a gas mixture of phosphine and TMG (8% in H2) without additional dilution leads to the growth of amorphous GaP layers. Additional dilution of the gas mixture in hydrogen or argon at increased power first leads to a decrease in the growth rate, and then a transition to the growth of microcrystalline GaP occurs with a further decrease in the growth rate. In this case, a decrease in the power leads to the growth of amorphous layers with a low growth rate, and an increase in power leads to the growth of microcrystalline GaP at a higher rate. An analysis of the structural properties of GaP layers by relation of the TO and LO modes of the Raman spectra indicate their improvement in the structural quality with increasing plasma power and dilution. Comparing the effect of dilution in hydrogen and argon, it was found that dilution in argon allows one to achieve a much smoother layer surface, while for dilution in hydrogen, a better TO/LO ratio is observed. Study of the electrical properties of GaP layers grown on Si substrates demonstrates that use of a continuous process with argon plasma makes it possible to reduce the concentration of defects and reduce their activation energy, as well as increase the crystallinity and uniformity of the layer. Thus, the growth mode with dilution in an Ar/H2 mixture is of the greatest interest for further studies. For GaPN ternary alloy grown under continuous process the problem of nitrogen incorporation control appears. Due to strong deferens in electro negativity for V group atoms (N and P) the clusterization, local fluctuation of the composition and defect formation are observed. Digital alloys approach was proposed in this work to improve control of nitrogen incorporation. Using a combination of a sequence of layers in the form of short-period superlattices (digital alloys) GaP/GaN allows one to precise control of the band gap with compensation of elastic stresses arising from the difference in the constant lattice constant. A low-temperature technology of PE-ALD was successfully used as a synthesis method for growth of GaPN based p-i-n structure on Si. This work was supported in part by RFBR under grant #20-08-00870

Resume : One of the most efficient ways to improve the efficiency of solar cells is to increase the photons’ capture, thus enhancing the short-circuit current. In the last years, different strategies to improve light capture in silicon solar cells have been proposed. One of the techniques that have attracted more interest is metal-assisted chemical etching (MACE). The capability of this technique to produce very low reflective surfaces has been demonstrated [1]. In this abstract, are presented recent results of an optimized MACE based method which can produce large-area surfaces’ texturization with very low reflectance values over the spectral range of interest for silicon-based solar cells, 350 nm – 1050 nm. The lowest effective reflectivity (Reff), normalized to the AM 1.5 spectrum, obtained was 3% [2]. The method consists of a simple maskless etching based on the use of a solution of hydrogen peroxide (H2O2), hydrofluoric acid (HF), and silver nitrate (AgNO3). H2O2 and HF act as etching agents, while AgNO3 donates silver ions that will locally catalyze the etching reaction contributing to steep and anisotropic structures creation [3]. The reaction dynamics was previously characterized in terms of temperature sensibility, etchants molar ratio dependency, and etching time [2, 3]. In this work, the method was used to texturize both n- and p-type monocrystalline silicon (mono-c Si) wafers in order to compare reaction dynamics and resultant surfaces’ properties. Previous studies demonstrated differences between the etching of the two types of silicon with other MACE based methods [4]. The used mono-c Si p-type wafers had resistivities of 1-3 Ω.cm and n-type of 3-7 1-3 Ω.cm, and both surface orientations were (100). The obtained reflectance spectra were similar for the two types of wafers, and the lowest effective reflectivity measurements were below 5%. Although, as expected, significant differences in etching dynamics were observed. For the same etching conditions, texturing n-type substrates showed to be faster than p-type. In this way, very low reflective surfaces (Reff ~ 3 %) can be obtained in less than one minute for n-type monocrystalline wafers. Surface passivation by atomic layer deposition of alumina is been applied to the textured surfaces with promising first results. [1] K. Tsujino, M. Matsumura, Y. Nishimoto, ‘Texturization of multicrystalline silicon wafers for solar cells by chemical treatment using metallic catalyst’, Solar Energy Materials & Solar Cells 90 (1), 100-110 (2006). [2] Pera, David, et al. ‘Advanced light-trapping structures for back-contact solar cells produced by metal-assisted chemical etching.’ 37th European Photovoltaic Solar Energy Conference. 2020. [3] I. Costa, D. Pera, J. A. Silva, ‘Improving light capture on crystalline silicon wafers’, Materials Letters 272 (2020) 127825. [4] Z. Huang, N. Geyer, P. Werner, J. De Boor, and U. Gösele, ‘Metal-assisted chemical etching of silicon: A review’, Advanced Materials. 2011.

Resume : MoOx films deposited by reactive sputtering were studied as a hole-selective contact for Si wafer based solar cells. The films were deposited on n-Si and glass substrates at room temperature. It was understood that the oxygen partial pressure played a significant role in the defect density at the interface of MoO3/n-Si as evaluated using capacitance-voltage measurements, and also the selectivity of the charge carriers. The energy band gap of MoO3 layers was evaluated to be in the range, 3- 3.7 eV when O2 flow changed from 5 to 25 SCCM and the layers were highly transparent in the visible region of the solar spectrum. The layers deposited glass showed a high value of sheet resistance (~ 1010 Ω/sq), while it is ~104 Ω/sq for the layers deposited on silicon, indicating the presence of an inversion layer. Photoconductance measurements showed an implied open-circuit voltage (iVOC) of 640 mV, which could be improved further by improving the passivation quality of the deposited layers. The results demonstrates the potential of reactively sputtered MoOx layers as hole-selective contact and possibility for further improvement through optimization of the deposition parameters.

Resume : Reduced-dimensional (quasi-2D) perovskites have attracted widespread of attention recently, owing to its superior semiconducting properties. As far as we know, multiple -value species are created during the crystallization. Accordingly, efficient cascade energy transfer takes place among different -value species, and the carriers would eventually concentrate to the lowest bandgap specie. Great success has accomplished in perovskite light-emitting diodes (PeLEDs) and photovoltaics by taking advantage of this phenomenon. Here, I will summarize the recent progress in our group towards efficient optoelectronics including photovoltaics, LEDs and detectors using quasi-2D perovskites.

Resume : The power conversion efficiency (PCE) of perovskite solar cells (PSC) has increased impressively within the last years and exceeds 25% today. A remaining challenge is the limited stability of PSC. Previous work indicated that inorganic charge transport layers (CTL) can help to improve the stability. However, the choice of materials is limited due to the employed layer-by-layer deposition, since each layer deposition must preserve the already processed layer stack, e.g. from dissolving. In order to overcome this limitation, we present a novel lamination approach: the hot pressing of two independently processed half-stacks of the PSC [1]. The separation of the CTL deposition enables new material combinations, in particular full oxide contact materials with enhanced stability. To allow this architecture, a certain importance must be set on the adhesion of the half stacks which is shown to be improved by additional buffer layers. We demonstrate stable power outputs as high as 14.6% PCE with high stability during continuous maximal power point tracking over 90 h. Even at temperatures up to 80 °C or realistic temperature profiles, the champion device exhibits good thermal stability during prolonged illumination. Furthermore, flexible and semi-transparent PSC are successfully laminated, demonstrating the high versatility of the process in particular for future applications in the field of tandem photovoltaics. [1] R. Schmager, J. Roger et al., Adv. Funct. Mat., 2020, 30, 1907481

Resume : Hybrid perovskites remain one of the leading materials in photovoltaics, however, their limited operational stability poses challenges to practical applications. This stimulates the development of hybrid perovskite materials, such as low-dimensional perovskites, that feature superior stabilities under device operation conditions.[1-4] To this end, supramolecular chemistry provides a powerful tool for controlling the properties of hybrid organic-inorganic materials by tailoring noncovalent interactions. We have recently demonstrated its function in templating hybrid perovskite structures through halogen bonding[5] and π-interactions[4,6] of organic moieties as well as host-guest complexation.[7-8] This has been uniquely assessed by solid-state NMR spectroscopy[1,3,7-8] and NMR crystallography.[5-6] As a result, we have obtained perovskite solar cells that exhibit superior performances and enhanced operational stabilities. This has been investigated by using a combination of theoretical and experimental techniques to unravel material design principles, highlighting the utility of supramolecular engineering in advancing hybrid photovoltaics. Reference: [1] J. V. Milić et al. Adv. Energy Mater. 2019, 1900284; [2] Y. Li et al. Nano Lett. 2019, 19, 150; [3] L. Hong, et al. Angew. Chem. Int. Ed. 2020, 59, 4691; [4] Y. Liu et al. Sci. Adv. 2019, 5, eaaw2543; [5] M. A. Ruiz-Preciado et al. J. Am. Chem. Soc. 2020, 142, 1645; [6] M. A. Hope et al. J. Am. Chem. Soc. 2021, in press, doi: 10.1021/jacs.0c11563; [7] T-S. Su et al. J. Am. Chem. Soc. 2020, in press, doi: 10.1021/jacs.0c08592; [8] H. Zhang et al. 2021, under revision.

Resume : Understanding photogenerated carrier distribution and separation is key to improving performance gains even if perovskite solar efficiency exceeds 25%. Analysis of surface photovoltage images displays the photo-responses form the localized carrier separation that provide insight into the understanding the photovoltaics. Furthermore, the differences between the work function with and without illumination indicates the surface band bending variation. In that point, Kelvin probe force microscopy reveals not only the potential distribution in the surface which is known to be beneficial or harmful to carrier transport and recombination but also the surface photovoltage spatially. We grow halide dependent CH3NH3PbX3 (X= I, Br, Cl) single crystals using the low temperature technique which are considered an ideal form without interfacial effects to investigate the surface photovoltage imaging and the spectral surface band bending of perovskite material itself. We examine the surface photovoltage maps and the band bending with and without the external light sources and demonstrate the band structures of each crystal depending on the wavelength of the induced lights. Also, conductive atomic force microscopy is used to show the dark current and photocurrent to explain the carrier movement. Analysis of the current maps reveals the important role of surface band bending in the photovoltaic performance. Those results provide the local optoelectrical properties in the perovskite material which have the direct correlation with the efficient devices.

Resume : Perovskite solar cells (PCS) rank among the highest performing photovoltaic technologies. In contrast to wafer-based photovoltaics, the perovskite absorber is commonly infiltrated into a mesoscopic charge extraction layer like titania (m-TiO2) in n-i-p configurations. Drift-diffusion simulations are essential for the understanding of charge transport and recombination in PSC. So far, however, these models have treated the nanoporous m-TiO2/perovskite layers as effective media in one-dimensional simulations. We present for the first time a 2D electrical model of mesoscopic perovskite solar cells. Herein, the charge extraction pathways of electrons and holes are decoupled: After photogeneration in the perovskite layer, electrons are extracted into the m-TiO2 electron selective layer after migrating less than 10 nm inside the perovskite. In turn, holes need to travel along the entire perovskite layer thickness of several hundred nanometers to reach the back electrode. Our calculations show that – in contrast to the common perception in literature – due to the mesoscopic nature the conduction band minima of perovskite and the m-TiO2 nanoparticles are aligned. The same applies for the ultrathin compact c-TiO2 hole blocking layer at the front side. This notion is important for the ongoing discussion of junction formation in PSC as it implies that under open-circuit conditions the largest electric field can be built up at the TCO interface. The model is experimentally validated by the fabrication of printed solar cells where the perovskite is entirely embedded in nanoporous layers. This enables to vary the thickness of the nano-crystalline photoabsorber over a wide range from few 100 nm to 8 µm. With the 2D electrical model, we are able to explain why printed mesoscopic PSC with carbon-graphite back electrode reach remarkable high photovoltages of 1.0 V despite the poor charge selectivity of the perovskite/carbon interface. This is due to the diffusion limited surface recombination at the back electrode. Measurements of steady-state and transient characteristics of the photovoltage and photoluminescence reveal insights into the bulk and interface recombination processes. We show that the most detrimental non-radiative recombination channels are due to interfacial recombination, of which recombination at the selective perovskite/TiO2 interface can become dominant at both a high m-TiO2 thickness or a large electrode spacing. Apart from the specific application for optimization of perovskite solar cells, this 2D model can describe the physical working mechanisms of mesoscopic semiconductor devices in an accurate manner.

Resume : Tunability of optoelectronic properties of lead halide perovskites achieved through halide mixing can potentially enable their multiple applications e.g. in tandem solar cells and light-emitting diodes. However, mixed halide perovskites are unstable under illumination due to their segregation into Br-rich and I-rich phases, which negatively affects the performance characteristics and operational stability of devices. Research efforts over the past years provided a substantial understanding of the factors influencing light-induced halide phase segregation. While several mechanisms were proposed, none of them could account for all available experimental data; and hence the origin of the effect is still under active debate. Herein, we thoroughly investigated the photodegradation of CsPbI2Br and Cs1.2PbI2Br1.2 using a set of complementary techniques. In-situ atomic force microscopy provided a spectacular visualization of the real-time halide phase segregation dynamics demonstrating that iodoplumbate is selectively expelled from the mixed halide perovskite grains and nucleates as a separate I-rich phase at the grain boundaries. We propose a mechanism based on the reversible Pb2+/Pb0 and I-/I3- redox (photo)chemistry, which explains our experimental findings and other previously reported results. Furthermore, it sheds new insights on the underlying mechanisms of multiple phenomena related to light- or electric field-induced degradation of various lead halide perovskites. More details can be found in our recently accepted paper: L. A. Frolova et al., Reversible Pb2+/Pb0 and I-/I3- redox chemistry drives the light-induced phase segregation in all-inorganic mixed halide perovskites. Adv. Energy Mater. 2021, accepted

Resume : Perovskite-based tandem solar cells have been promising candidates from the viewpoint of industrial applications due to their capability to boost power conversion efficiencies (PCEs) at reasonable costs. Furthermore, monolithic two-terminal perovskite/silicon tandem devices have experienced a rapid increase in their certified PCEs. Based on the application of silicon subcells, perovskite/silicon tandem devices are classified into two types: heterojunction or homojunction. Most reported two-terminal perovskite/silicon tandem cells use a silicon heterojunction (SHJ) as the bottom cell due to its high voltage, which is a consequence of low recombination losses. SHJs benefit from using a transparent conductive oxide electrode, which also acts as a recombination layer in monolithic perovskite/silicon tandem devices. However, SHJs also suffer from disadvantages: they experience parasitic optical losses, are unable to tolerate high temperatures, and limit the fabrication temperature of the top cell at low temperatures. In contrast to SHJ solar cells, silicon homojunction solar cells are the most extensively fabricated solar cells and account for over 90% of industrially fabricated solar cells due to their low manufacturing cost and higher temperature tolerance. In homojunction solar cells, tunnel-oxide-passivated contacts (TOPCons) are an upcoming technology, which features a dielectrically passivating front homojunction and passivating rear contact; the passivated contact can reduce recombination losses and increase the open-circuit voltage (Voc) by effectively separating electrons and holes. Additionally, the processing steps concerning TOPCons are relatively facile and have lower manufacturing costs than other conventional cells and SHJs.Employing the TOPCon device as a bottom cell can effectively develop high-efficiency perovskite/Si tandem cells as alternatives to SHJ-based tandem cells. However, to date, there have been few reports regarding their efficiency or their fabrication using TOPCon-based tandem devices. Here, We demonstrates perovskite/Si tandem devices fabricated using the TOPCon structure. The devices exhibit a high voltage over 1780 mV in a large area of 25 cm2. Efficiencies up to 22.6% were achieved without additional deposition of recombination layer, of which the sturucture has n-i-p 1.63eV perovskite based on TOPCon cell. These results show the possibility of a large-area tandem solar cell using a next-generation silicon solar cell structure with a high commercialization potential.

Resume : Ion diffusion has been demonstrated to seriously affect the optoelectronic properties of Halide Perovskites (HaPs). Till now the fastest diffusion has been attributed to the movement of the halide anions, largely neglecting the contribution of mobile protons, on the basis of computed estimates of their density. Here we prove the process of proton diffusion inside HaPs following deuterium-hydrogen exchange and migration in MAPbI3, MAPbBr3 and FAPbBr3 single crystals through D/H NMR quantification, Raman spectroscopy and Elastic Recoil Detection Analysis, challenging the original assumption of halide-dominated diffusion. Our results are confirmed by impedance spectroscopy, where MAPbBr3-based solar cells respond at very different voltage frequencies compared to CsPbBr3-based ones. We find that water plays a key role in allowing the migration of protons as we do not detect deuteration in its absence. The water contribution is modeled to explain and forecast its effect as a function of its concentration in the perovskite structure. These findings are of great importance as they evidence how unexpected, water-dependent proton diffusion can be at the basis of the ~7 orders of magnitude spread of diffusion (attributed to I- and Br-) coefficient values, reported in the literature. The reported enhancement of the optoelectronic properties of HaP when exposed to small amounts of water, may be related to our finding.

Resume : Si clathrates are cage-like, crystalline Si inclusion compounds with potentially interesting optical and electronic properties. Typically, Na in clathrates is viewed as an interstitial guest. However, if the Na content is low enough, Si clathrate can become a semiconductor with Na as a dopant. Here we report a study of Type II Si clathrate prepared with low Na concentration, which provides new insights into the properties of this cage-like Si allotrope and into Na as an n-type dopant. Theoretical studies have previously predicted the clathrate to have a direct or nearly direct bandgap near 2.0eV. Absorption measurements support this view indicating a room temperature bandgap near 1.7ev and an above gap absorption coefficient nearly two orders of magnitude larger than diamond silicon. Room temperature photoluminescence (PL) is also observed. Temperature dependence of the PL intensity and of the Na hyperfine line observed in Electron Paramagnetic Resonance (EPR) spectra are consistent with Na as a shallow donor. EPR spectra also exhibit “super-hyperfine” lines surrounding the Na hyperfine lines due to interactions of the donor electron with a Si29 nucleus on the surrounding cage. Analysis of the strength of these interactions indicates only about 20% of electron density of the Na donor extends beyond the cage suggesting the donor does not have a simple hydrogenic wavefunction. EPR spectra exhibit additional features associated with Na clusters providing evidence that the Na may not be randomly distributed. A line arising from unterminated Si bonds in a disordered Si phase is identified in the EPR spectrum with further evidence of a disordered phase found in Raman scattering. The presence of this amorphous-like component helps explain thermally activated hopping-related conductivity that has long been observed in this system, but which is poorly understood. These results help to resolve outstanding questions about electron transport in Si clathrates and reinforce the view that Si clathrates are an alternative to diamond silicon with exciting optoelectronic properties. This work was supported by the NSF through grant 1810463.

Resume : There is current interest in finding non-toxic alternatives to lead-halide perovskites for optoelectronic applications. Silver-bismuth double perovskites (e.g., Cs2AgBiBr6) have recently gained attention because they maintain the perovskite crystal structure, whilst substituting the toxic lead cation for the more benign silver and bismuth cations. Recently, we showed that Cs2AgBiBr6 exhibits a long charge-carrier lifetime of 1.4 microseconds, which can sustain a large steady-state excess carrier density under 1 sun illumination [1]. However, the indirect band gap is 2.25 eV and is too wide for single-junction or multi-junction photovoltaic applications. Recent literature suggests that alloying the Bi3+ cation with Sb3+ can lower the band gap. This talk examines the potential and challenges of this approach when realised in solution-processed thin films. The ratio of Sb:Bi is varied over the entire composition range in solution, and the effect on the bulk and surface composition of the thin films measured, along with the effect on the phase-purity and charge-carrier lifetime, measured by transient absorption spectroscopy. Band gap bowing is observed (i.e., the smallest band gap is obtained using a mixed Sb-Bi alloy) and the origin of this effect examined [2]. A route to growing pinhole-free thin films is developed, and the effect of Sb content on the performance of the double perovskites in photovoltaic devices examined. [1] Robert L. Z. Hoye, et al., Adv. Mater. Interfaces, 2018, 5, 1800464 [2] Zewei Li, Seán R. Kavanagh, et al., J. Mater. Chem. A, 2020, 8, 21780

Resume : Traditional lead halide perovskites exhibit excellent photovoltaics properties and over the past few years have revolutionised the field of emerging photovoltaics technologies, reaching power conversion efficiencies of above 25%. Yet, the well-known instability of these materials when exposed to ambient conditions and/or illumination, and the concerns raised regarding the presence of the non-environmental friendly Pb-atoms in the structures are still challenges to be overcome. The search for stable non-toxic materials with high photo-conversion efficiency remains an active topic of research for the next-generation photovoltaics. In 2017 a new family of ternary Ag-Bi-I materials named rudorffites, with the formula AaBbXx (x = a + 3b), were shown to be highly stable and promising for solar cell applications, exhibiting low optical band gaps, impressive high short-circuit currents of 10.7mA/cm2, and achieved a photo-conversion efficiency up to 4.3%[1]. Contrary to standard ABX3 perovskites, rudorffites exhibit edge-shared AgI6 and BiI6 octahedra, thus breaking the paradigm of corner-sharing structures for good opto-electronic properties, and highlight a new class of lead-free perovskitoid materials to be explored. These new materials have also shown an ease of processing and have been successfully synthesised by spin-coating techniques [2], thermal co-evaporation [3], and more recently through a solution atomisation to produce aerosols [4]. However, to-date a systematic investigation of their structural and electronic properties is missing. Here, we thoroughly analyse the symmetry of the rudorffites structures and employ calculations from first-principles to unveil their electronic and optical properties. In practice, it’s possible to derive an infinity number of rudorffites structures. Among these, Ag3BiI6, Ag2BiI5, AgBiI4 and AgBi2I7 were previously experimentally investigated by Turkevych et al. and exhibit extremely similar structural properties and optical absorption spectra [1]. We use their reported symmetry groups (R3m, C2/m and Fd3m), and show that it is necessary to finely analyse the structure/symmetry relationship to accurately describe the lattices. Starting from AgBiI4 we establish a reference point to evaluate its opto-electronic properties, including quasi-particle effects by means of state-of-the-art GW calculations. We expand our approach to investigate other rudorffites, by varying the Ag/Bi ratio in the composition, as well as the number of B-site vacancies. In summary, by systematically exploring the rudorffite phase-space we use our calculations to propose key parameters to tune the properties of these compounds by controlling the stoichiometry at the time of synthesis. [1] ChemSusChem 10, 3754 (2017) [2] Angew. Chem 128, 9738 (2016) [3] J. Mater. Chem. A 7, 2095 (2019) [4] J. Phys. Chem. C 124, 23930 (2020) This work was partially funded by DROP-IT H2020 project (grant agreement 862656)

Resume : Zinc phosphide (Zn3P2) is an earth-abundant semiconductor with properties suitable for photovoltaic applications [1,2]. However, its applicability has been restrained by its large lattice parameter and thermal expansion coefficient in relation to substrates currently on the market, which in turn complicates the epitaxial growth of high-quality zinc phosphide crystals. One approach which has been shown to limit the influence of the lattice mismatch is the reduction of the interface area and radial stress relaxation by utilising the nanowire morphology [3,4]. In addition, this morphology is desirable for photovoltaic applications due to its enhanced light absorption cross section and potential to surpass the planar Shockley-Queisser limit [5,6]. In this study we demonstrate the epitaxial growth of zinc phosphide nanowires for the first time. The vapour-liquid-solid method is used to grow epitaxial zinc phosphide nanowires on indium phosphide (100) substrates through molecular beam epitaxy. The liquid catalyst is formed by a 5-minute zinc pre-deposition on a clean surface, where it reacts with surface defects to form indium nanoparticles that are liquid at the growth temperature (250 °C). By controlling the surface pre-treatment and growth conditions (temperature and relative phosphorus and zinc precursor fluxes), we are able control the nucleation density and achieve four different nanowire morphologies, namely vertical, zigzag, straight-tilted, and horizontal, with the first one being ideal for photovoltaic applications. Furthermore, we showed that varying the relative precursor flux also allows for the tuning of the composition of the nanowires, which in turn controls the formation of intrinsic dopants through the formation of self-interstitials. The optical properties of the nanowires, investigated by cathodoluminescence spectroscopy at 10 K, showed a dependence on morphology and stoichiometry. All luminescence observed at these temperatures were found to be defect related, and no bandgap recombination was observed. In short, we have demonstrated the tuneable epitaxial growth of zinc phosphide nanowires and elucidated the impact of growth conditions on the morphology, composition, and functional properties, opening up a route to achieve its potential as a photovoltaic absorber. References 1. G. M. Kimball et al. Appl. Phys. Lett., 95, 112103 (2009). 2. M. Y. Swinkels et al. Phys. Rev. Applied, 14, 024045 (2020). 3. F. Glas Phys. Rev. B, 74, 121302 (2006). 4. S. Escobar Steinvall et al. Nanoscale Horiz., 5, 274-282 (2020). 5. S. Mann et al. ACS Nano, 11, 1120 (2017). 6. R. Fredriksen et al. ACS Photonics, 4, 2235-2241 (2017).

Resume : The path towards inexpensive solar cells for large-scale photovoltaic deployment is based on the utilisation of earth-abundant, non-toxic, and chemically stable materials. Among these, wide-bandgap semiconductors (1.6 -1.8 eV) are especially interesting since they can be used as suitable partners for a bottom Si-based cell in tandem configuration. Inorganic-halide perovskites have already shown great potential in photovoltaics. However, concerns about their instability and toxicity might affect the commerciality of this technology. One promising candidate to overcome these issues is BaZrS3, with a tunable direct bandgap, high absorption coefficient ( > 105 cm-1), and excellent air stability. In order to bring this material to the forefront of PV, it is necessary to deepen the knowledge of its fundamental properties, as well as find pathways for low-cost and affordable synthesis. This work is focused on investigating the structural and vibrational properties of BaZrS3–BaZrO3 in order to develop predictive synthesis-structure-function relationships, which will enable the production of high performing devices. A series of thin film layers was synthesised by annealing of BaZrO3 amorphous films in H2S atmosphere at various temperatures (700 – 1000 °C). This has resulted in samples with varying compositions (S/(S+O) from 0.2 to 0.8) and bandgaps (2.0 – 3.4 eV). Lateral homogeneity of the thin films was confirmed by Scanning Electron Microscopy (SEM), electron-dispersive X-ray spectroscopy (EDX), and Raman mapping measurements. Transmission Electron Microscopy (TEM) of the cross-sections revealed the presence of phases at the back of the films, which were identified as BaZrO3. However, the near-surface area of each sample has shown high S incorporation. This fact was exploited for obtaining reference Raman spectra of the BaZrS3-BaZrO3 film series with multiwavelength excitation sources (488, 532, and 785 nm). Detailed analysis of the spectra has revealed three types of modes, which are identified as Ba-Zr-S, Ba-Zr-O, and Ba-Zr-S-O type vibrations. The appearance of Ba-Zr-S and Ba-Zr-O vibrations suggests the possible presence of microdomains with primarily BaZrO3 and BaZrS3-like structures. Further investigation by a combination of TEM and EDX, as well as the microdomain influence on the optoelectronic properties is presented and discussed.

Resume : The development of advanced photovoltaic concepts that might overcome the single junction efficiency limit, as well as the development of new materials, rely on advanced characterization methods. Among all the methods the optical ones are very well adapted to quantitatively probe optoelectronic properties at any stage. We here present the use of multidimensional imaging techniques that record spectrally and time resolved luminescence images. As a typical example, we show how to harvest the available solar energy using hot carrier devices and evidence a positive contribution of the hot carrier effect on the photovoltaic performances. We can extract temperature and quasi-Fermi level splitting (internal voltage) of photo-generated carriers at various lattice temperatures and excitation powers. The device might be seen as a combination of photovoltaic and thermoelectric device and the outputs of the optical method impact both communities. The reduce thermalization mechanisms seems to positively impact PV performances of our multi-quantum well devices. To go further, we show how to obtain individual temperatures of electrons and holes from spectral analysis. The individual thermalization coefficients of each carrier as well as the individual Seebeck coefficients are also determined. This quantitative thermodynamic study also help to develop the conception of other PV devices. Finally, the optical method favors the design of nanostructures to increase the light absorption in very thin layers. It therefore boosts PV performances by enhancing physical mechanisms such as the hot carrier effect.

Resume : The worldwide energy demand fosters the development of new strategies to improve energy conversion efficiencies in photovoltaic devices. The alternative we propose in this work is to achieve thermionic emission of electrons in a solid-state, cold-cathode device by the absorption of infrared (IR) photons in the unused region of the solar spectrum using plasmonic nanostructures patterned on the cathode. The plasmons launched by the absorption of the IR photons generate, upon fast relaxation, a photocurrent in the semiconductor. As a proof of concept, arrays of inverted Au-coated pyramids were etched over areas of 1 square centimeter of a silicon substrate using wet chemical methods, having lateral sizes of 0.2-4 microns and periods of 0.4-7 microns. The optical properties were characterized using Fourier-transform infrared spectroscopy (FTIR). Multiple absorption bands associated with different plasmon resonances were observed in the desired spectral range, which were tuned in frequency and amplitude by modifying the structural parameters of the array. Finite-difference time-domain (FDTD) methods were used for the design optimization of the fabricated arrays regarding their optical properties. In particular, we aimed at maximizing the electromagnetic field enhancements obtained at the plasmon resonance frequencies to boost electron emission from the cold cathode. Selected structures will be implemented in a proper thermionic device and evaluated measuring the produced photocurrents.

Resume : Many living organisms have structural colours due to periodic arrangements of micro- or nano-structures, such as butterfly wings, beetles, bird feathers. In some cases, these structures are photonic crystals and present a bandgap for some wavelengths. As an example, the butterfly Asterope Leprieuri features green and blue colours in some areas of its wings. With naked eye under white illumination, these green and blue parts are visibly iridescent and the colours depend on the observation angle. In this work, we explore the morphological structure and the optical properties of the wing of this butterfly by scanning electron microscopy (SEM), angular spectroscopy, and Fourier optics which allows directly imaging the Fourier plane and the k-space distribution. These experimental results were compared with simulations which have been conducted with a FDTD method. In parallel, thermal properties of this photonic structure were studied, for visible and infrared wavelengths, in normal incidence and with angular dependency. Correlation between optical and thermal properties will be considered. We will present experimental and simulated results, both for photonic and thermal properties. The understanding of such a biological structure at the origin of iridescent colours with angular dependence will allow to transpose it to solar cells which suffer currently from overheating and a lack of aestheticism.

Resume : Luminescent solar concentrators (LSCs) are a novel photovoltaic device introduced as a new solution for concentrating the disperse and discontinuous solar radiation. LSCs consist of a semi-transparent host matrix embedding luminescent particles that absorb the incident radiation on a large sun-facing area and re-emit it at a longer wavelengths. Some advantages can be highlighted, such as the low sensitivity to the radiation angle, allowing for higher efficiency in diffuse light conditions and their transparency, which allows the integration into windows or facades without interfering in the space lighting. Thus, they represent a great advance in the installation of the zero energy buildings. In this work, we present a computational model to analyze the performance of a novel LSC configuration consisting of stacked layers of micro-fiber arrays. For this purpose, we developed and validated a Monte-Carlo ray tracing model including the physical processes occurring in the LSC when absorbing, re-emitting and guiding solar radiation, which will serve for studying the efficiency of different configurations and optimizing this novel approach. The fiber LSC efficiency improvements are compared against a conventional planar LSC while varying some parameters of interest such as the fiber packing geometry, diameter, length, dopants distribution (homogenously doped or coated) or coating thickness. This work serves as a previous theoretical study for the future development of a new LSC based on ultra-flexible silica micro-fibers coated with Si-QDs doped PLMA layers. The exceptionally low absorption coefficient of the silica fibers (α ~10^-4 cm^-1) is expected to reduce significantly the host absorption impact on the LSC performance in contrast with the traditional polymer LSCs when their dimensions are scaled over 1 m^2, leading to the highest light concentration factors ever reported. Additionally, the silica micro-fibers have outstanding mechanical properties, allowing their practical implementation in structural elements as multi-functional fabrics composed of multiple layers of fiber arrays. We present the results and analyses of simulations of layers of microfibers arrays, with diameters from 10 μm up to 600 μm, thickness from 5 up to 800 layers of fibers, and lengths up to several meters. These results provide a practical insight into the factors affecting the performance of different configurations, providing an useful tool for their optimization. In this sense, the results obtained with this model provide guidance and stimulus for the development of a new type of multifunctional LSCs consisting of semitransparent fabrics with structural applications.

Resume : Anthracene derivatives are classic and unique luminophores which combine high photoluminescence quantum yields (PLQY), large exchange energies (ΔEST) and narrow Stokes shifts. This makes them ubiquitous in many applications, particularly organic light emitting diodes and triplet-triplet annihilation upconversion (TTAUC). Anthracenes should, therefore, be highly valuable building blocks for luminescent conjugated polymers, particularly if TTA is desired. Unfortunately this has been thwarted by the extended π system of anthracene which imparts a strong tendency for it to aggregate. This firstly suppresses materials solubility, restricting molecular weight and processability. Secondly, aggregation prompts photodimerisation,1 aggregation caused quenching (ACQ)2 and the formation of excimers in the solid state.3 These constitute PL quenching pathways and their exact extent is hard to predict and must be controlled. We have surmounted aggregation through molecular encapsulation to develop true conjugated anthracene polymers with high molecular weights and minimal ACQ which do not form excimers. These materials provide a new lease of life for an established luminophore and are ideal annihilators towards single material solid state TTAUC films. Results for this unprecedented application will be presented. 1 Zhu, L. et al., J. Am. Chem. Soc. 2011, 133, 12569–12575. 2 Egbe, D. A. M. et al., Macromolecules 2010, 43, 1261–1269. 3 Freeman, D. M. E. et al., Polym. Chem. 2016, 7, 722–730.

Resume : The efficiency potential of any solar cell absorber material is largely determined by three material parameters – the absorption coefficient α, charge carrier mobility μ and charge carrier lifetime τ. While the relevance of each parameter is unquestioned, their relative importance is not self-evident. One could ask for example, whether a tenfold increase in mobility or absorption coefficient is more beneficial in terms of solar cell performance. With parameter values varying over several orders of magnitude, such questions are of great practical relevance for emerging inorganic and organic solar cells. Here we present how α, μ and τ quantitatively affect solar cell efficiency. By performing numerical drift-diffusion simulations over a wide representative parameter space, we find different efficiency-(αμτ) relations for different mobility regimes. We find no fundamental difference between simulations based on monomolecular and bimolecular recombination, representative of inorganic and organic solar cells, respectively. Special attention is given to the impact of surface recombination, which may present an ultimate limit to solar cell efficiency. With a focus on organic solar cells, we assess the gain in efficiency obtained from an increase in absorption strength. As many organic absorber blends suffer from modest mobilities and recombination coefficients, we conclude that designing molecules with enhanced absorption strength are a path to overcome today’s efficiency limits.

Resume : We study photo-carrier escape from quantum wells and their contribution to current, as they reach the continuum of the conduction band. By considering a superlattice layer (SL), embedded in the bulk of a Graphene/Oxide/n-GaAs solar cell, we calculate excess current due to electrons escaping from the quantum wells of the SL layer. We therefore solve for collective thermionic current out of quantum wells (from low-gap GaAs layers) through the populations of excess electrons in the eigenenergies. We treat the device as a metal-insulator-semiconductor (MIS) solar cell, where monolayer graphene replaces the metal. In addition, by inserting an AlAs/GaAs superlattice layer in the region near the depletion edge of the diode, we expect excess photo-current due to electrons surmounting the Schottky barrier thermionically. The collective effect of current out of quantum wells is treated and represented as a thermionic emission from the embedded SL region(TE/SL) current density. In addition to the superlattice effect, photo-current develops from the bulk cell structure due photo-carriers from several regions of the device. Such photo-excited carriers come from (a) the bulk of the semiconductor as minority holes in the n-GaAs and (b) photo-excited carriers (holes) from the depletion region. Note that such a design offers reduction of recombination losses in both super-lattice and depletion regions due to carrier separation from the quantum wells and the electrostatic field at the depletion region. We therefore treat carrier contribution to total current as the sum of three current components due to super-lattice-generated photo-carriers and minority photo-holes from the bulk semiconductor region and photoholes from the depletion region respectively. We find that the Schottky solar cell J-V characteristic J = Jo (exp (qV/kT)-1) – JL includes a new prefactor and the sum of the three photocurrent terms as mentioned above. The cell structure is essentially a graphene/n-GaAs Schottky cell with a superlattice layer in the bulk of the semiconductor. The Jo prefactor in the fundamental solar cell expression is different due to graphene replacing the metal. A new Richardson constant appears due to graphene’s different properties (graphene treated as a 2D medium with different density of states (DOS)). The cell design is a Schottky barrier solar cell with the metal replaced by a monolayer graphene film as the window of the device. We demonstrate the advantage of current density increase in graphene/Oxide/n-GaAs solar cells by means of a short AlAs/GaAs superlattice (SL) embedded in the GaAs layer. Such a layer attracts photo-excited carriers of both kinds leading to optical gap increase from 1.42eV to 2.129eV due to ground state energy difference, leading to strong absorption at 582nm of incident solar wavelength. We study carrier escape in the thermionic emission model and find that excess carriers thermally escaping from quantum wells of the SL region contribute additional current depending on temperature and solar concentration. Specifically, a twenty-period SL region under one sun absorbs within 582nm <  < 873nm range, contributing 1.391mA/cm2 and 9.83mA/cm2 and near 20mA/cm2 at one, 50 and 200 suns respectively.