Perovskite solar cells

Resume : Perovskite solar cells are based on organolead halide compounds which are used as highly efficient solar cell absorber materials. However, their lead component is a high human health concern, toxic, and causes strong damages when disperse in the environment. In the present paper, silver iodobismuthate compounds are investigated as lead-free semiconductors in replacement of organolead halide in perovskite solar cells. We describe a new strategy for the preparation of AgBixI3x 1 layers with x ranging between 1 and 2.25. The preparation has been optimized to get well-crystallized well-covering layers. The prepared compounds are semiconductors, absorbing light below 700 nm. Ag2Bi3I11crystallized in the pure cubic phase, yields to the layer with the larger grains and is the composition giving the best solar cell power conversion efficiency (PCE). We show that the oxide used in the blocking layer is also an important parameter to get good efficiency. SnO2 with a conduction band minimum energy lower than the TiO2 one gives a higher photocurrent due to a better electron injection from the absorber to the electron transport layer. The role of the layer roughness on the charge separation is also investigated. Based on the results presented here, we conclude that the silver iodobismuthate compounds have promising properties for use in optoelectronic devices and specifically in perovskite solar cells.

Resume : Organolead halide perovskites are currently the most promising absorber materials for photovoltaic applications. Despite its good opto-electrical properties, MAPbI3 possesses two main drawbacks, that hampers further commercialization: (a) moisture sensitivity and (b) high toxicity due to lead content. Bi3+ is good replacement for Pb2+, because it is environment friendly and both ions possess isoelectronic structure. (1,2) Goal of presented work is to design new lead-free absorbers for photovoltaic cells, that are air and moisture stable. One of the most promising absorber is Cs2[AgBi]X6 (X = halide), but despite many efforts only chloride and bromide materials were obtained by solution based processes. Theoretical studies suggest that Cs2[AgBi]I6 is not thermodynamically unstabile and experiment suggest that Cs3Bi2I9 synthesis is favored. Systematic studies of several new perovskite and non-perovskite materials with composed of Cs, Ag and Bi halides were studied to set one step closer to a suitable bismuth based absorbers. Obtained materials were studied by diffractometric and spectroscopic methods. Clear evidence that there is a possibility of bismuth iodide perovskite will be presented. (3) 1. D. Prochowicz, M. Franckevičius, A. M. Cieślak, S. M. Zakeeruddin, M. Grätzel, J. Lewiński. Journal of Materials Chemistry A, 2015, 3, 20772–20777. 2. D. Prochowicz, P. Yadav M. Saliba, M. Saski, S. M. Zakeeruddin, J. Lewiński and M. Grätzel, Sustainable Energy and Fuels, 2017, 1, 689–693. 3. manusctipt in preparation

Resume : In the last few years halide perovskites emerged as a key material for third-generation photovoltaics (PV). Methylammonium lead iodide (MAPI) represented the prototypical system and devices with a total efficiency of ~20% have been fabricated based on it. New systems emerged with even better performance and superior stability [1]. Indeed, halide perovskite have potential applications in many optoelectronics applications, e.g. light emitting devices (LED), lasing, light detection, etc [2], which fostered an intense research activity on a broader set of halide systems. One of the major contributions to the increase of efficiency from the initial ~3% to > 20% is come from the improvement of the crystallization and deposition process. The sequential deposition strategy [3], consisting in first depositing a precursor phase, e.g. PbI2, and then infiltrating the complementary salt, e.g. MAI, has been one of the breakthroughs in the field. Despite the importance of crystallization and deposition step, not enough is known on the mechanism of the process, which limits further improvement of the preparation and/or fabrication of more complex systems [4]. In this talk I will present the results of our theoretical study on the sequential deposition mechanism. We considered MAPI and MAPBr (methylamonium lead bromide) and starting from the corresponding precursors have performed ab initio rare event simulations to identify the atomistic path to form the associated perovskites. Our results explain recent real-time X‐ray diffraction experiments [5]. I believe that the comparison between the nucleation mechanism of different types of perovskites will allow designing improved crystallization and deposition methods and doping approaches. [1] Yi et al, Energy Environ. Sci., 2016, 9, 656 (2016) [2] Zhang et al, Nat. Energy 1, 16048 (2016); https://www.nature.com/collections/fnnxcznnbb [3] Burschka et al, Nature 499, 316 (2013) [4] Abdi-Jalebi et al, Adv. Energy Mater 6, 1502472 (2016) [5] Miyadera et al, Nano Lett. 15, 5630 (2015)

Resume : Organic-inorganic perovskite solar cells, although efficient and easy for production, suffer many problems with structural stability and resistance to moisture. The introduction of organic layers - to form the heterostructures, so-called quasi 2D halide perovskites - improves the photovoltaic parameters and stability. The molecules used as organic layers have hydrophobic character – therefore, the stability of the device working at ambient condition achieves at least 35 days. The number of the perovskite layers is also essential for all the photovoltaic parameters: efficiency (PCE), open-circuit voltage, current, and I-V hysteresis. Conformation of the molecules is crucial for the performance of the device, and it depends on the perovskite thickness as well. Termination of the perovskite slab such that it is rich with the halide atoms causes the inter-gap states, which could be involved in the multiphoton transitions. We present combined experimental and theoretical studies on these systems, and show that this direction has a great potential for a design of the more stable and efficient solar cells. [1] B.E. Cohen, M. Wierzbowska, L. Etgar, High Efficiency and High Open Circuit Voltage in Quasi 2D Perovskite Based Solar Cells. Adv. Funct. Mater. 1604733 (2017), DOI:10.1002/adfm.201604733 [2] B-E Cohen, M. Wierzbowska, L. Etgar, Exploring barrier molecules in quasi 2D lead bromide perovskite solar cells. Submitted [3] D. Amgar, M. Wierzbowska, L. Etgar, Rubidium lead halide nanocrystals. Submitted

Resume : Even though lead-halide perovskite solar cells have shown a remarkable progress in the power conversion efficiency (PCE) from 3.8% in 2009 to more than 22% in 2017, a further improvement in PCE is still expected when taking into account the Shockley–Queisser (SQ) limit (≈30%) for photovoltaic efficiency with the absorber bandgap of ≈1.6 eV. In order to achieve such upper limit of PCE, it is highly required to discuss the marginal efficiency of these solar cells quantitatively. Until now, intensive research efforts have been devoted to discussing the upper limit of the short-circuit current density (JSC) and the open-circuit voltage (VOC) in perovskite solar cells. However, little is known about the upper limit of a fill factor (FF) in perovskite solar cells because of J–V hysteresis. Here, we have fabricated highly efficient CH3NH3PbI3 perovskite solar cells with different thicknesses of a hole-transporting layer by a fast deposition–crystallization method.[1] We have analyzed FF in these solar cells by using an empirical equation with diode and photovoltaic parameters depending on the scan directions separately. As a result, we found that this equation can well explain the thickness dependence of FF and hence can potentially be applied to the estimation of the upper limit of FF in these solar cells. Furthermore, we discuss the potential improvements in FF and PCE on the basis of the empirical equation with the experimental data. [1] H. D. Kim, H. Ohkita, H. Benten, and S. Ito, Adv. Mater., 28, 917 (2016). [2] H. D. Kim and H. Ohkita, Sol. RRL, DOI: 10.1002/solr.201700027.

Resume : Perovskite materials have already proven their ability to reach photo-electric power conversion efficiencies higher than 22% in appropriate devices. If their instability against time could be solved, they could quickly compete with silicon since they benefit from low-cost manufacturing processes. Our work is dedicated to this stability study using the benefit of X-ray Photoelectron Spectroscopy (XPS) as a main tool, and coupled with XRD and SEM-EDX analysis. Using XPS, it is possible to track the surface (10 nm) changes the perovskite undergoes, in terms of both composition and chemical environments, and is therefore efficient towards understanding the first steps of the degradation process. The first experiment was dedicated to demonstrate the feasibility of perovskite ageing study by XPS. For this purpose, samples of spin-coated thin-film perovskite without capping on the top (glass/ITO/PEDOT:PSS/MAPI), were aged in different conditions (ambient light with air and vacuum respectively), and analysed using XPS at different times. The obtained survey spectra show that all the expected elements can be identified with this spectrometry method. Starting from an initial known composition, the results obtained reveal interesting changes the material underwent during the ageing. For example, it was observed that both nitrogen and iodine gradually escape the surface. Also, metallic lead was observed in the final stages of the degradation process, and this finding is still under interpretation. Then, other experiments were designed to highlight the influence of oxygen and light as previously mentioned, but also to disentangle the influence of the bottom layers (hole transporting material, conductive electrode) on the degradation. Ageing under 100% oxygen, with and without light was for example undergone for different kind of samples, all with MAPI as a top layer. The monitoring of the ageing by XPS was also completed with X-ray diffraction characterization. This permitted to provide a rate of the bulk sample modification that can be compared to the one of the surface. This should therefore give access to interesting features regarding the degradation mechanisms of MAPI, which are still not completely clear up to now.

Resume : Perovskite solar cells (PSC) are one the most fast growing photovoltaic technologies. The power conversion efficiency (PCE) of these emerging cells increased from 3.8 % to 22.1 % in 8 years. While PSCs displayed high PCE, their stability remains a major challenge. PSCs are known to be highly sensitive to many external environmental factors such as humidity and photo- and thermal exposure. Stability issues such as photo- and thermal are mainly related to the components of the cell. However, the humidity can cause decomposition of the perovskite structure leading to a large PCE decrease. Even though application of protective layers on top of the device can improve the humidity related issues, the commercialization of PSCs requires more advanced encapsulation methods. The most common encapsulation methods for sealing PSCs are epoxy resins. However, epoxies are not hermetic and therefore not long-time stable. According to MIL-STD-883 standard of microcircuits a sealing should display < 5×10-8 atm cm3 s-1 of helium leak rate for being considered as hermetic8. Glass frit sealing is compatible to this hermeticity standard but the conventional bonding processes requires temperatures higher than 380°C. Alternatively, a selective local bonding via laser beam can lower the process temperature to extend the hermetic encapsulation for a wide range of applications. The present work, discloses a laser-assisted method for sealing glass substrates at process temperature of ca. 120°C.

Resume : Dynamic J-V hysteretic effects are consistently described by the dynamic electrical model (DEM) [1]. DEM explains the dependence of the hysteresis amplitude and short circuit current on the bias scan rate. It also reproduces the current overshoot in the reverse characteristics experimentally observed and its dependence on bias pre-poling. The basic assumption is that the rather slow process (ion migration) governing the time evolution of the polarization charge is described in the single relaxation time approximation. We investigate here the dynamic J-V characteristics of perovskite solar cells [2] and report the occurrence of normal hysteresis (NH) and inverted hysteresis (IH) in the J-V characteristics, the behavior strictly depending on the pre-poling bias (Vpol) [3]. Using a three step measurement protocol, which includes the stabilization of the open circuit bias (Voc), bias pre-poling at Vpol for a certain time interval, followed by a reverse-forward scan starting from Voc as actual measurement, we introduce a unified description of the dynamic hysteresis, which can be tuned from NH (Vpol>Voc) to IH (Vpol< 0). We also investigate additional measurement conditions and the J-V characteristics are matched closely by DEM calculations, showing the interplay between NH and IH. References: [1] G.A. Nemnes et al., Sol. Energy Mater. Sol. Cells 159, 197 (2017); [2] C. Besleaga et al., J. Phys. Chem. Lett. 7, 5168 (2016); [3] G.A. Nemnes et al., J. Phys. Chem. C 121, 11207 (2017)

Resume : The existence of slow dynamic processes in perovskite solar cells is well-known. This is most commonly observed as hysteresis in the current-voltage curve during device efficiency measurements. Slow processes have also been observed in a range of frequency and time domain measurements. Whilst there is considerable evidence linking the origins of these observed processes to the migration of ions within the perovskite, the exact nature of their interaction with the electronic structure of the device is still unclear. In this work a range of complimentary frequency and time domain characterization techniques have been utilized to help understand the wide range of effects this electronic-ionic coupling has upon recombination in perovskite solar cells. By combining the use of transient photovoltage (TPV) and intensity modulated voltage spectroscopies (IMVS), over a wide range of temperatures from 77 323 K, relaxations with time constants on microsecond, millisecond and second timescales have been observed. Measurements are complimented further with the use of impedance spectroscopy, large amplitude open-circuit photovoltage decay and current-voltage measurements. Excellent agreement is shown between the time and frequency domain results, with the combination of techniques being used to aid understanding. A range of architectures have been studied including planar and mesoporous based TiO2 and organic contact cells. Similar processes are observed in all types highlighting that the underlying physical process is intrinsic to the perovskite. The recombination rate is found to be frequency/time dependent, relating to the particular ionic environment and its impact on electronic band structure. The high frequency response is representative of recombination in a fixed ionic system. On longer timescales the relaxation of the electronic and ionic species to re-establish electrochemical equilibrium results in a reduction in recombination rate.

Resume : The effect of the imidazolium (IM) incorporation on different properties of methylamonium lead iodide perovskite thin films has been investigated. Methylammonium and imidazolium iodide powders (CH3NH3I = MAI and C3H5N2I = IMI) were previously synthesized and then mixed with PbI2 in different proportions. Afterwards, solution-processed MA1-xIMxPbI3 perovskite thin films were obtained. X-ray diffractograms show that the crystalline structure of MA1-xIMxPbI3 keeps the same structure of MAPbI3 up to IM concentrations of 10%. Higher ratios of IM result in two different crystallographic structures. Optical spectroscopy reveals a blue shift in the onset of the absorption of MA1-xIMxPbI3 films as the IM ratio increases. Band gap energy of thin films was estimated from absorbance spectral measurements and it was found to range between 1.58 eV and 2.86 eV for MA1-xIMxPbI3 (0?x?1). For energies above the band gap the absorption tends to decrease as IM ratio increase. According to FESEM imaging, the presence of IM in the mixed MA1-xIMxPbI3 films seems to improve the morphology of the films and they appear to be more compact and continuous than MAPbI3 films. Further, PL emission also exhibits a blue shift proportional to the IM ratio and confirms the values obtained for the band gap energies.

Resume : Within the last decade a continuous attention has been addressed to perovskite light absorbers as principal components in new generation hybrid organic-inorganic solid state devices. Nevertheless, many other functional materials have sustained the growth in efficiency of perovskite solar cells (PSCs), being coupled to the absorbing layer either as hole transporting or electron transporting layers (HTL and ETL, respectively). Among these, carbon nanostructures (CNSs) have attracted a considerable attention, [1] due to their outstanding electronic, thermal and mechanical properties. Furthermore, they appear very promising in boosting stabilities of PSCs, which is a key requisite for commercial applications. In this contest, we have worked recently at introducing CNSs within both HTLs and ETLs. For the first case, we reported on semiconducting polymer blends with single walled carbon nanotubes and reduced graphene oxide that, when used as HTLs within standard PSC architectures, allow to prolong lifetimes and efficiencies significantly with respect to the pure polymer. [2] For the second case, we focused on a fullerene derivative, bearing a siloxane moiety, which can be used for the direct covalent functionalization of a transparent conducting electrode. The material form a one-molecule thick monolayer, which acts as a robust and effective ETL for PSC, avoiding the use of the compact titania based ETL. This strategy simplify extremely the device architecture, allowing at the same time to obtain comparable results in terms of efficiency, but eliminating completely electrical hysteresis phenomena, which take place when titania is present. In addition, PSC stability to UV light is improved (which is a drawback also related to the presence of titania). We will report here about these works, taking care of outlining future perspectives. References: [1] For a brief summary see: S. Collavini et al. Adv. En. Mater. 2016, 1601000. [2] T. Gatti et al. Adv. Funct. Mater. 2016, 26, 7443. [3] P. Topolovsek et al. submitted paper

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Resume : A series of novel spiro-cyclopentadithiophene (CPD) based hole-transport materials (HTMs) with application in hybrid organic/inorganic perovskite solar cells (PSC) were designed and synthesized. The HTMs were designed with an emphasis to avoid dopants and additives in the resulting PSC devices. All materials consist of a CPD-core, which is spiro-linked to a acridine or thioxanthene moiety, to increase the glass transition temperature of the material. The CPD-core was substituted with different arylamine substituents to ensure a good alignment of the band structure with the perovskite. The resulting materials were characterized with optical and electrochemical methods in order to understand the structure-properties relationships. PSC devices were fabricated and characterized by photophysical methods to explore the scope and applicability of the new materials.

Resume : Over the past five years, a rapid progress in organometal halide perovskite solar cells (PSCs) has greatly influenced emerging solar energy science and technology. In PSCs, both the overlying hole transporter material (HTM) and a controlled interfacing layers are critical for achieving high power conversion efficiencies (PCEs) and for protecting the air-sensitive perovskite active layer. As the new candidate, acridone-based molecules were designed and synthesized from ready commercial precursor 9(10H)-acridone by simply synthetic procedures. By incorporating in 2,7-positions of acridone-core with various strong electron rich groups such as 4,4?-dimethoxy-diphenylamine, 4,4?-dimethoxy-triphenylamine or phenothiazine, these molecules were evaluated on photophysical, electrochemical and thermal properties. Because of donor-acceptor structure, these molecules provided intermolecular charge transfer and/or intramolecular charge transfer to each other. Modification electron-donating group lead to tunable HOMO level and keep maintained LUMO level, their energy level could be compatible adapted with conductance band of the TiO2 layer and the valence band of perovskite layer. The presence of methoxy group (-OCH3) or phenothiazine-sulfur rich, they would be supported to charge carriers, increased hole mobility and conductivity. A combination of experimental and computational methods, they supported to optimize the device fabrication in progress. We strong believe that these novel perovskite-based materials will be a promising candidate for photovoltaic solar cells and further in organic electronic devices.

Resume : The molecular structure of the hole transporting material (HTM) play an important role in hole extraction in a perovskite solar cells. It has a significant influence on the molecular planarity, energy level, and charge transport properties. Understanding the relationship between the chemical structure of the HTM's and perovskite solar cells (PSCs) performance is crucial for the continued development of the efficient organic charge transporting materials. Using molecular engineering approach series of the hole transporting materials with strategically placed aliphatic substituents were constructed in order to investigate the relationship between the chemical structure of the HTMs and the photovoltaic performance. Charge-transporting properties of the investigated HTMS were determined by the xerographic time-of-flight (XTOF) technique. Additionally, it was demonstrated that structure of the HTMs has a significant influence on the molecular planarity, energy level and charge transport properties. Cyclic voltammetry (CV) was used to determine the ground-state oxidation potentials of the HTMs and photoelectron emission spectroscopy was used to determine an ionization potentials. PSCs employing the investigated HTMs demonstrate power conversion efficiency values in the range of 9% to 16.8% highlighting the importance of the optimal molecular structure. An inappropriately placed side group could compromise the device performance. Due to the ease of synthesis and moieties employed in its construction, it offers a wide range of possible structural modifications. This class of molecules has a great potential for structural optimization in order to realize simple and efficient small molecule based HTMs for perovskite solar cells application.

Resume : Perovskite solar cells are considered to be a promising photovoltaic technology because of their high power conversion efficiency (PCE) and low material costs. Their impressive photovoltaic performance can be attributed to their high optical absorption properties and balanced charge-transport properties with long diffusion lengths. A number of processes have been developed to improve the coverage, uniformity, and morphology of the light absorber, such as one-step solution process, two-step solution process, vapor process, etc. In particular, the solution-processed technique using the two-step method, in which the PbI2-coated substrate is immersed in a CH3NH3I solution by spin-coating, has been proposed to improve the pore filling, coverage, and morphology of the perovskite films. However, there have been two main problems associated with the two-step process: namely, residual PbI2 and non-uniform morphology. In this study, we suggest the perovskite absorber growth mechanism of the two-step process could be explained by an Ostwald ripening growth model for planar-structure perovskite solar cells. We attempt to find out the source of two main problems such as unreacted PbI2 and non-uniformed morphology by proposed ripening growth mechanism and experimental results. This growth mechanism opens the way toward understanding a key aspect of the photovoltaic operation of high-efficiency, two-step perovskite solar cells.

Resume : Photovoltaic (PV) as renewable energy is a green energy that uses solar light energy to produce electricity. Many countries have invested heavily to develop cost-effective and highly efficient solar cells. In the past years, global PV industry has been shown a game of chicken due to expansion of Chinese PV module production and global low-growth trap. Various companies have been restricted with merger and division. However, the global PV market recently entering its path of stable growth. And, new PV technology like perovskite has been investigated to overcome the existing limitations. In a rapidly changing global environment, understanding the global status is important for accelerating PV technology innovation and effectively implementing national renewable policies. In this study, multidimensional analysis by integrating 3P (paper, patent and project) data for Perovskite solar cells as emerging PV technology were conducted with respect to research portfolio and network. The main objectives of this study were to understand the global status, and to suggest a direction for improving the efficiency of R&D investment in perovskite solar cells in Korea. From the this study, we found that light absorption materials for AMX3 perovskite has been strongly developed. The main target for R&D is to improve the efficiency and reliability of perovskite solar cells.

Resume : Exploration and implementation of the renewable energy sources attract steadily increasing attention of the research community and industry worldwide. Perovskite solar cells represent a highly promising emerging photovoltaic technology. Recently, power conversion efficiencies of the best laboratory lead halide based perovskite solar cells have exceeded 22%. While the efficiencies are that impressive, reaching decent stability of the perovskite solar cells under the realistic operation conditions represents a big challenge. A deep mechanistic understanding of the major degradation processes is required for developing approaches for their prevention, which is essentially important for commercialization of the perovskite photovoltaics. In the present work, we report a systematic analysis of the intrinsic photochemical and thermal stability of MAPbI3 films grown on different substrates under a variety of processing conditions. The degradation behavior of the perovskite films was monitored under anoxic environment inside the glove box (O2, H2O < 0.1 ppm). We have applied a set of complementary techniques involving UV-Vis-NIR absorption spectroscopy, scanning electron microscopy (SEM), X-ray diffraction analysis (XRD) and EDX microanalysis to reveal the evolution of the chemical composition and structure of the material under the aforementioned stress conditions. The obtained comprehensive set of the experimental data allowed us to propose substrate-dependent pathways of the thermal and photochemical degradation of the perovskite films. This work was performed in the frame of the Skoltech-MIT NGP program.

Resume : Hybrid organo-inorganic perovskite solar cells have attracted a great interest of the research community due to a simple, low-cost production technology and record-breaking power conversion efficiencies (PCE) exceeding 22% for the best laboratory devices. Despite this, perovskite solar cells haven't been implemented on industrial scale due to the poor stability of perovskite materials with respect to the elevated temperatures, moisture, and light exposure, resulting in poor device operation lifetimes. Replacing fragile organic moieties with robust inorganic cations can be considered as a promising approach to designing more stable materials. While some inorganic lead-halide perovskites CsPbX3 (X=I, Br) have shown enhanced resistance towards moisture in combination with improved thermal and photochemical stability, their photovoltaic performances remained inferior as compared to the hybrid perovskites (e.g. MA/FAPbI3). Here we report all inorganic CsPbI3 planar junction perovskite solar cells fabricated by thermal co-evaporation of CsI and PbI2 precursors. The best devices delivered PCE of 9.3%-10.5% thus coming close to the reference MAPbI3-based devices (PCE~12%). These results emphasize that all-inorganic perovskite materials can successfully compete in terms of photovoltaic performance with the most widely used hybrid materials such as MAPbI3. This work was supported by RFBR (project ?16-03-00793) and Skoltech-MIT NGP collaboration program.

Resume : Herein, we report four star-shaped HTMs that promotes excellent contact between perovskite and the underneath transparent electrode. The new HTMs, which we denote TTA1, TTA2, TTA3 and TTA4, consist of a C4h tetrathienoacene-core (TTA) with four arylamine groups. The new star shaped materials have been obtained through a facile synthetic route by crosslinking tretraarylamine-based donor groups with a tetrathienoacene (TTA) core. The new compounds were obtained as stable solids in a good overall yield. The molecular structure of the new HTMs was confirmed by NMR techniques and MALDI-TOF mass spectrometry. The effects of different linkage moieties between the core and arylamine groups on the optical and electrochemical properties were investigated. The compounds have good solubility in all of the common organic solvents. The TTA-based HTMs exhibited absorption peak in the region of 390-460 nm, in which the perovskite display lower light harvesting properties. Emission spectra show that all molecules have relatively large Stokes shifts of around 70-130 nm suggesting significant changes in geometrical configuration of the molecules upon excitation. The HOMO energy levels were calculated from the cyclic voltammetry (CV) data. The HOMO of TTA1, TTA2, TTA3 and TTA4 are 5.14, 5.11, 5.12 and 5.05 eV, respectively. The LUMO energy level is estimated according to its HOMO energy and band gap. The large barrier between the valence band edge of the perovskite and LUMO of the HTMs could block the electrons efficiently and thus reduce the recombination process. The new star-shaped molecules fulfil some general requirements to make it working efficiently in PSCs and will prompt us to use them as HTMs in MAPbI3-based photovoltaic devices

Resume : A large interest in semiconductors is observed due to their applications in electronics and renewable energy harvesting. One of the interesting semiconductors are perovskites. Curently the most commonly used perovskite in photovoltaics is CH3NH3PbI3 - however solar cells based on that material shows insuficient durability for comercial uses. It is expected that new MSnS3-type perovskites could be stable semiconductors with band-gaps of around 1,5 eV. The syntheses of MSnS3-type compounds (where M = Mn, Ni, Cr, Co, Fe) have been carried out by chemical transport method. The obtained materials have been characterized with X-ray powder diffraction, Raman Spectroscopy, Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy.

Resume : Perovskite solar cells (PSC) demonstrated impressive power conversion efficiencies going beyond 22%. However, very poor operational stability of these devices hampers significantly their practical application. In particular, perovskites represent solid-state redox active ionic conductors, which might undergo electrochemical degradation if electric bias is applied to the films. Obviously, the active layer of the operating solar cells has to sustain the electric fields induced by the built-in and light-induced potentials. Otherwise, the electrochemical degradation of the active layer material under the solar cell operation conditions would ruin the device performance. In this work, we report a systematic comparative study of the electrochemical stability of a series of hybrid and all-inorganic lead halide based perovskite materials - MAPbI3, MAPbBr3, FAPbI3, FAPbBr3, CsPbBr3, CsPbI2Br. Thin films of these materials were exposed to the potentiostatic polarization under anoxic conditions using the lateral and vertical two-electrode device architectures (electric fields of ~0.1 to 1 mV/nm). We have shown that polarization leads to the appearance of the field-induced gradients in the chemical composition of the films. Additionally, significant changes of the film morphology and the surface potential have been revealed using Kelvin probe force microscopy (KPFM). The analysis of the obtained data allowed us to draw some important correlations between the chemical composition of the perovskite materials and their electrochemical stability, which might guide further design of stable light absorbers for future generation photovoltaics.

Resume : In perovskite solar cells (PSCs), the hybrid perovskite light harvesting layer as well as materials for selective contacts serving for electron and hole extraction are the most important components. The use of HTM to extract holes from perovskite and transport them to the electrode, is indispensable in almost all the architectures utilized for PSCs. Spiro-OMeTAD has been commonly used as HTM, showing PCE over 22 %. However, the onerous synthetic pathway and costs of Spiro-OMeTAD is an obstacle for large-scale production. For this reason, alternative cost-effective HTMs for highly efficient PSCs are highly researched. In search for alternative to Spiro-OMeTAD, a huge number of molecular HTMs have been employed in PSC. A dozen of them have been found to give device with performance comparable, or even superior, to that of Spiro-OMeTAD (PCE ~20%.). Among them, carbazole derivatives are attractive classes of HTMs. We report herein the molecular conception, synthesis, characterization, and incorporation into PSC device of new bis(diphenylaminyl)carbazoles derivatives bearing polymerisable moiety. The effect of diphenylaminyl substitution position on the carbazole core, the nature of polymerisable chain will be discussed. The polymerized HTM were synthetized. The comparison of PSC used molecular HTM and polymerized HTM and differences between 3,6- and 2,7- substituted Carbazole isomers will be comparatively studied and reported. An encouraging efficiency of 16% with highly stable devices was obtained, which are comparable with that of Spiro-OMeTAD control devices.

Resume : Inverted perovskite solar cells (IPSCs) provide various advantages over the commonly chosen mesoporous cell geometry. Flexible substrates can be used and high-temperature processes during preparation are not needed. As PEDOT:PSS is mostly used as transparent hole conductor in IPSCs the charge transfer processes between the perovskite layer and PEDOT:PSS as charge-extraction layer is one of the key factors for well-functioning IPSCs. In this study, pump-probe absorption spectroscopy with sub-ps resolution was used to determine the injection-time of photo-generated holes from the perovskite to the p-type organic semiconductor PEDOT:PSS. By comparing the transient signals of photoexcited perovskite films with and without PEDOT:PSS, a characteristic signal for injected carriers in PEDOT:PSS was identified. This signal appears within the resolution of the measurement setup (< 200 fs) which proves the ultrafast character of the charge carrier injection. The observation is confirmed by complementary changes in the time constants of the commonly known perovskite signals indicating a decreased hole concentration in the perovskite. Ultrafast hole-injection from excited perovskite layers to PEDOT:PSS was thereby directly proven and explains the high performance of IPSCs using this contact.

Resume : Organic?inorganic hybrid perovskites have attracted a lot of interest due to opportunities for low-cost and high-performance solar cell. While full attention was focused on perovskite material itself and the efficiency increasing not many works were dedicated on very thin electron transport layer (ETL). Our recent investigation, shows that even ETL) can has significant influence on electrical parameters of a perovskite solar cell (PSC). The most often, titanium oxide is applied as ETL in view of its good transparency, non-toxicity, easy availability and long-term stability. It can be deposited by ALD, evaporation, magnetron sputtering or sol-gel method. In this work sol-gel method was used and it was observed that after solution formation and gelation, aging and coarsening take place. We have been found that age of precursor has an impact on electrical parameters of perovskite solar cells. The results shows 8% higher short circuit current value for perovskite solar cells obtained from aged titanium sol compared to fresh solution. Microstructural and morphological AFM analysis combined with electrochemical impedance spectroscopy and optical properties measurements enabled to the determine sources of these differences. Major differences was found in thickness and crystallinity of investigated hole blocking layers. The results suggest a significant impact of the sol history and the type of substrate on the proper design and preparation of ETL for perovskite solar cells.

Resume : The highly efficient photovoltaic conversion (up-to 21.6 %[1]) of alkylammonium lead halide perovskite solar cells has attracted the attention of the scientific community in the recent years. However, the toxicity of lead and the low stability of solar cells based on it are serious drawbacks for its large-scale application. To address this potential limitation, non-toxic metal-halide photovoltaics are now being explored. In this context, we have focused on bismuth based halides, which has been reported recently as emerging lead-free photovoltaic absorbers. [2-4] In this work we have carried out the synthesis, structural characterization and the study of the optical properties of a novel bismuth-halide family of organic-inorganic hybrid compounds with general formula ABiX5, where X is Cl-, Br- or I-, and A is a divalent organic cation (diimidazolium) with molecular formula [C9H14N4]2+. We have been able to determinate the crystal structure of these compounds, and we have observed that it is based on halobismuthate (Bi2X10)4- (X= Cl, Br, I) dimers and diimidazolium as counter cations. We have also determined the optical band gaps obtaining values in the range of 3.2 to 1.9 eV. It is worth to note that the optical band gap of this family of compounds makes them very promising for optoelectronic applications, such as photovoltaic absorbers, water splitting, LEDs, etc. Additionally, these compounds display a broad photoluminescence, with emission maxima in the range of 430 to 615 nm at room temperature. (1) Grätzel, M. et al. Science, 2016, 354, 206-209 (2) Park, B. W. et al. Adv. Mater, 2015, 27(43), 6806-6813 (3) Johansson, M. B. et al. J. Phys. Chem. Lett, 2016, 7(17), 3467-3471 (4) Öz, S. et al. Sol. Energ. Mat. Sol. C, 2016, 158, 195-201

Resume : Fully vacuum-deposited planar perovskite solar cells with efficiency approaching 20% can be prepared by using a precise combination of intrinsic and molecularly doped electron and hole transport layers (ETL, HTL). These transport layers can be kept very thin (< 40 nm) thanks to the smooth and homogeneous surface of vacuum-deposited methylammonium lead iodide films, which are formed by a densely packed mesh of nanocrystals. Grain sizes (50-200 nm) are, however, well below the reported carrier diffusion length (around 1.5 microns) and could hinder charge extraction. Besides the fine control over the thickness and composition of each constituent layer, vacuum deposition also allows to invert at will the configuration of the device. In order to correlate the electronic properties with the morphology of vacuum evaporated perovskites, methylammonium lead iodide layers deposited on either n-type ETL or p-type HTL have been investigated by Kelvin probe force microscopy. The surface potential maps of these samples showed that variations at the grain boundaries were less pronounced compared to previous reports for solution-processed perovskites. Increasing the time under air exposure evidenced aging effects involving a redistribution of the surface potential towards the grain boundaries. Performing the measurements under illumination allowed to obtain maps of the surface photovoltage which confirmed the photovoltaic effect in the perovskite films. The sign and the magnitude of the photovoltage were indicative of the charge transfer at each back contact.

Resume : Hybrid organic-inorganic perovskites (HOIPs) emerged as low-cost but high-performance materials for photovoltaic cells. The certified power conversion efficiencies of perovskite solar cells (PSCs) have been largely increased in the last years making this the fastest-advancing solar technology to date. Physical vapor deposition (PVD) is an efficient method of thin film deposition with a variety of advantages over solution processing: the absence of solvents allows depositing easily two or more materials; multilayers can be deposited on the top of each other; layer thickness can be controlled with nanometer precision; absence of solvents and vacuum processing reduces impurities.[1] In this work, thin film deposition is described for lead halide perovskites, using a novel physical vapor methodology. PVD was optimized based on the volatility of each perovskite precursor, from the mass flow of effusing vapor from a Knudsen cell maintained at phase equilibrium conditions.[2] Optical properties of thin films were investigated by UV-Vis-NIR and fluorescence spectroscopy. Scanning electron microscopy and X-ray diffraction evidenced the crystallinity of the prepared films on different surfaces. This methodology originates quite stable and crystalline perovskite films displaying encouraging solar to power conversion efficiencies. [1] José C. S. Costa, Rui M. Rocha, Inês C. M. Vaz, Manuel C. Torres, Adélio Mendes, Luís M. N. B. F. Santos; Journal of Chemical & Engineering Data, 60, 3776-3791, 2015. [2] José C. S. Costa, João Azevedo, Adélio Mendes, Luís M. N. B. F. Santos; Journal of Physical Chemistry C, 121, 2080-2087, 2017.

Resume : Perovskite solar cells (PSC) have emerged recently as a strong contender for the next generation of photovoltaic technologies having received the attention of the photovoltaic community, both scientists and industry. In the last four years, organic/inorganic lead halide PSCs have leapt from ca. 10 % at the beginning of 2013 to a certified value of 22.1 % in 2016. The best performing PSCs use gold as back-contact, prepared by thermal-evaporation under high- vacuum condition. Gold is a very good electrical conductor and does not oxidize in air but obviously introduces increased costs for large-scale production; moreover, vacuum deposition is also high-energy consuming. Carbon materials such as fullerenes, carbon black or carbon nanotubes can be used as back-contact with comparable performances. Moreover, carbon is a material that is rather inexpensive, environmentally friendly and compatible with perovskite solar cell inner-components. Here we present our first results concerning the preparation and optimization of carbon nanotubes and graphene-based films to be used as back contact in PSCs. The morphology of the different materials was evaluated by SEM and the electrical resistance/conductance was assessed through I-V curves. The work function by Kelvin probe was assessed to determine the compatibility between the hole transport and the carbon-based layers. By using the most promising materials, encouragingly high photovoltaic performances were obtained.

Resume : Efficiency, stability and cost are the three main pillars to accelerate an effective industrial exploitation of a given PV technology. Until now, perovskite solar cells (PSC) have demonstrated high power conversion efficiencies with cost-effective materials. However, their poor stability under oxygen but especially under moisture has challenging the research community to develop good sealing strategies for avoiding degradation mechanisms under outdoor operation. Typical encapsulation processes use either or cumulatively temperature, UV curing and pressure that may affect the power conversion efficiency stabilization [1] and materials’ stability. This work presents a triple cation perovskite solar cell hermetically sealed with a low- melting point glass frit by a laser assisted process at low temperature. The laser sealing process was performed in an in-house made equipment called “LaserBox”, comprehending an ytterbium laser source (1070 nm), a laser scan head and a controlled heating plate. The process temperature for sealing the above mentioned substrates is < 60 oC. Lab-size PSC devices using the mesoporous-conducting semiconductor scaffold and planar architectures were laser-sealed; the performance and stability of the devices were assessed before and after sealing for comparison. This encapsulation method, easily scalable, allows obtaining stable PSC devices operating under outdoor conditions at high values of oxygen and moisture. [1] Matteocci, F., et al., Encapsulation for long-term stability enhancement of perovskite solar cells. Nano Energy, 2016. 30: p. 162-172.

Resume : The organic-inorganic hybrid perovskite solar cells have shown a remarkable progress in their efficiencies in a very short period of time. However, the toxicity of lead in the most extensively studied MAPbI3 type perovskites has raised the concern for exploring alternative materials. Cu perovskites have recently been explored for this purpose due to the interesting properties of Cu2+. This oxidation state of Cu is stable in air and it can form compounds having large absorption coefficient in the visible region. Furthermore, due to the small ionic radius, the Cu atoms adapt to K2NiF4 structure (2-D layered perovskite). Here, we present the synthesis of different Cu based 2D-perovskites having the general formula A2BX4. We have prepared the perovskites with different stoichiometric ratios of halides by following a very simple and extremely efficient mechanosynthetic method. The properties of these materials such as band gap and the crystal structures were tuned by ion substitution at the cationic as well as anionic positions. Shifts in these properties were measured by diffratometric and spectroscopic measurement techniques.

Resume : Titanium dioxide (TiO2) is known to be an important n-type semiconducting material. It shows compelling characteristics among which bistable electrical resistance states, high photocatalytic activity, surface chemical stability and high electron drift mobility are desired for applications in photovoltaic devices [1-4]. TiO2-based materials are a broad group with distinct structural and geometrical features, among which mesoporous TiO2 architectures represent an important class of materials which have recently attracted an enhanced interest. Herein, we would like to present a promising approach of synthesis of mesoporous TiO2 scaffolds as a result of a merger of well-known hard-templating [5-6] method with use of naturally occurring, highly porous cinchona alkaloid compounds. Titanium alkoxide along with appropriate alkaloid were used to produce TiO2 precursor in one-step synthesis. Resulting complex was precipitated from the solution, dried and calcinated. Obtained precursor and oxide materials were analyzed with number of methods such as Powder X-ray Diffraction (PXRD), Accelerated Surface Area and Porosity Analysis (ASAP), Scanning Electron Microscopy (SEM) and other spectroscopic techniques. Acquired TiO2 was found to be of high active surface. Additionally, the different polymorphic forms of titanium oxide could be steered in temperature dependent calcination process. References: 1. Hossain M. F., Biswas S., Takahashia T., Kubota Y., Fujishima A. J. Vac. Sci. Technol. A 2008, 26, 1007-1011. 2. Mukherjee B., Karthik C., Ravishankar N., J. Phys. Chem. C 2009, 113, 18204-18211. 3. Fu T., Liu B. G., Zhou Y. M., Wu X. M., J. Sol-Gel Sci. Technol. 2011, 58, 307-311. 4. Bischoff B. L., and Anderson M. A., Chem. Mater. 1995, 7, 1772-1778. 5. Qiu J., Yu W., Gao X., Li X., J. Sol-Gel Sci. Technol. 2007, 44, 235-239. 6. Liu X., Gu Y., Huang J., Chem. Eur. J. 2010, 16, 7730-7740.

Resume : .

Resume : Lead halide perovskites are known for being able to function not only as light harvester, but also as charge transporter. Despite this fact, perovskite solar cells (PSC) generally employ an n-i-p arrangement, with the perovskite layer (so-called intermediate layer) being sandwiched between an n-type electron transport layer (ETL), such as TiO2, and a p-type hole transport layer (HTL), generally spiro-OMeTAD. Thus, selective charge transport is ensured, which results in higher cell performance. However, the adjacent layers have to display a suitable work function and a high conductivity. Besides, interface engineering is crucial for enhanced efficiencies, as lattice mismatch can provoke recombination and therefore lower efficiencies. While the organic HTL film is commonly doped with additives to enhance its conductivity, doping of the inorganic TiO2 electron transport layer is not yet commonly established due to its complexity. Furthermore, control over doping content and homogeneous dopant distribution through the TiO2 layer depends in many cases on fabrication conditions and might alter from sample to sample. Atomic layer deposition (ALD) is a very powerful technique for the fabrication of thin films with precisely fixed composition and thickness. In this work, ALD was used in a first step to optimize the thickness of TiO2 ETL, which led to new perspectives on the possible influence of lattice mismatch on PSC efficiencies. In a second time, lithium doping was performed with the aim to increase bulk conductivity and alter its band edge position. Several concentrations were screened and optimized towards enhanced cell efficiency and stability.

Resume : Perovskite solar cells (PSC) have shown a stunning performance increase in a very short time period, with latest published efficiencies surpassing 21 %. Such efficiencies make them already comparable to established photovoltaic technologies. For achieving high credibility and market implementation however, solar cells have to fulfill crucial requirements for outdoor application. In laboratory conditions, temperature, humidity level as well as light intensity and incident angle are controlled, but this is not the case in real-life applications. In this work, three different cell types were compared: crystalline Si cells that stand for a mature solar cell technology, dye-sensitized solar cells (DSC) were chosen as an intermediate technology at the verge to market implementation and finally PSC as recent technology. As humidity and temperature limitations of PSC already have been addressed, light induced variations were the scope of this work, as so far no comparable study has been performed to the best of the authors’ knowledge. Incident angle was varied for each cell type in small steps (from 0° to 90°) and results are discussed. Furthermore, light intensity was also varied from 1-sun to 0.25-sun and relative efficiencies were analyzed and compared, which delivered unexpected results. This study is thought to contribute to a better understanding of advantages and drawbacks of the different technologies under outdoor conditions and points to further optimization possibilities.

Resume : Mesoscopic photovoltaics have emerged as credible contenders to conventional p-n junction photovoltaics [1-3]. Mimicking light harvesting and charge carrier generation in natural photosynthesis, dye sensitized solar cells (DSCs) were the first to use three-dimensional nanocrystalline junctions for solar electricity production, reaching currently a power conversion efficiency (PCE) of over 14% in standard air mass 1.5 sunlight. Remarkably the PCE increase to 29% in ambient light exceeding the performance of GaAs photovoltaics. By now, large-scale DSC production and commercial sales have been launched on the multi-megawatt scale for application in building integrated PV and light-weight flexible power sources. Recently, the DSC has engendered the meteoric rise of perovskite solar cells (PSCs) [4-6]. Today’s state of the art devices employ metal halide perovskite of the general composition ABX3 as light harvesters, where A stands for methylammonium, formamidinium or caesium, B denotes lead or tin and X iodide or bromide. Carrier diffusion lengths in the 100 nm - micron range have been measured for solution-processed perovskites and certified power conversion efficiencies (PCEs) attain over 22 %, exceeding the PCE of polycrystalline silicon solar cells. These photovoltaics show intense electro-luminesence. and Voc values over 1.2 V for a 1.55 eV band gap material. This renders perovskite-based photosystem very attractive for applications in tandem cells and for the generation of fuels from sunlight mimicking natural photosynthesis [7,8]. 1. B.O’Regan and M. Grätzel. “A Low Cost, High Efficiency Solar Cell based on the Sensitization of Colloidal Titanium Dioxide,” Nature 353 (1991) pp 7377-7381. 2. M. Grätzel, “Photoelectrochemical Cells,” Nature 414 (2001). pp 332-344. 3. A.Yella, H.-W. Lee, H. N. Tsao, C. Yi, A.Kumar Chandiran, Md.K. Nazeeruddin, EW-G .Diau,,C.-Y Yeh, S. M. Zakeeruddin and M. Grätzel,“Porphyrin-based Solar Cell with Co(II/III) Redox Electrolyte Exceed 12% Efficiency,“ Science 629 (2011) pp 334-341. 4. M. Grätzel, Light and Shade of Perovskite Solar Cells, Nature Mat. 13 (2014) pp 838-842. 5. J. Burschka, N. Pellet, S.-J. Moon, R.Humphry-Baker, P. Gao1, M K. Nazeeruddin and M. Grätzel, „Sequential deposition as a route to high-performance perovskite-sensitized solar cells“ Nature 499, (2013),pp 316-3199. 6. X. Li, D. Bi, C. Yi, J.-D. Décoppet, J. Luo, S.M. Zakeeruddin, A. Hagfeldt, M. Grätzel, A vacuum flash–assisted solution process for high-efficiency large-area perovskite solar cells, Science 353 (2016), pp 58-62. 7. J. Luo, J.-H. Im, M.T. Mayer, M. Schreier, Md.K. Nazeeruddin, N.-G. Park, S.D.Tilley, H.J. Fan, M. Grätzel, Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth abundant catalysts Science, 345, (2014), pp 1593-1596. 8. M.Schreier L. Curvat, F. Giordano, L. Steier, A. Abate, S.M. Zakeeruddin, J. Luo, M. Mayer and M:Grätzel, "Efficient photosynthesis of carbon monoxide from CO2 using perovskite photovoltaics" Nature Commun. 6, (2015) , pp 7326-7332.

Resume : Hybrid perovskite solar cells have demonstrated impressive power conversion efficiencies exceeding 22%, while their practical application is hampered by poor operation stability. We have reported recently that hybrid MAPbX3 lead halides undergo facile thermal and photochemical degradation even under anoxic conditions without exposure to oxygen and moisture, while their all-inorganic counterparts CsPbX3 proved to be significantly more stable [1]. Here we will discuss our the most recent results on the intrinsic stability of a broad range of materials represented by various lead-based perovskites and lead-free complex halides of tin, germanium, bismuth and antimony. The revealed pathways of thermal, photochemical and electrochemical degradation processes will be presented and a conclusion on the potential of different groups of materials for practical application in PV technology will be drawn. We will also analyze interface degradation effects occurring between the electrodes, charge transport layer materials and the photoactive layer induced by electric field, elevated temperatures, solar light or a combination of these stress factors [2-3]. Finally, it will be shown that reaching any commercially interesting operation lifetimes for perovskite solar cells requires a considerable shift from the currently used device design paradigms as well as a comprehensive multiparametric optimization of all used materials and functional components. [1] A. F. Akbulatov et al., J. Phys. Chem. Lett. 2017, 8, 1211 [2] S.Yu. Luchkin et al., ACS App. Mater. Interfaces 2017, 9, DOI: 10.1021/acsami.7b01960 [3] A. F. Akbulatov et al., Adv. Energ. Mater. 2017, DOI: 10.1002/aenm.201700476

Resume : Organic-inorganic hybrid halide perovskites have recently emerged as a new class of light absorbers with a rapid progress and impressive efficiencies (>22%) for solar conversion applications. Despite the rapid progress demonstrated by these light absorbers, there is still a lack of understanding of some fundamental material/physical/chemical properties of these materials. There is also a significant processing gap between the lab-scale spin coating and scalable deposition methods toward future roll-to-roll manufacturing. The challenge results from the sensitivity of perovskite crystallization and film formation in different processing conditions. In this presentation, I will present our recent studies toward a better understanding and control of perovskite nucleation, grain growth, and microstructure evolution using solution processing. The precursor chemistry and growth conditions are found to affect significantly the structural and electro-optical properties of perovskite thin films. We also find that the precursor ink chemistry (solvent, coordination chemistry) of perovskite is critical for scalable deposition; but this has largely been underexplored. We present a rational design of perovskite precursor film formation to achieve highly specular films using scalable deposition methods. Using these high quality perovskite films, we have achieved device efficiencies approaching the values obtained on small scale by spin coating. These findings make a significant advance towards commercialization of the perovskite photovoltaic technology. These results and others will be discussed.

Resume : The increasing demand for clean energy has prompted researchers to intensively investigate environmentally friendly photovoltaic devices. The recent discovery of metal-organic halide perovskite crystals gave thin film photovoltaics a renaissance.1 The diversity of the hybrid perovskites in composition and preparation makes them excellent materials with a unique combination of properties and potential for low cost and easy processing along with relatively high power conversion efficiency.2 Crystallinity, density of defects and impurities are determining factors for (opto)electronic properties, and are also highly dependent on the materials formation processes for most inorganic semiconductors. Understanding this behavior and the structure/property relationship is crucial to a fundamental understanding of perovskite materials and to an eventual extension of their properties to other process-tolerant systems. In that context, the synthetic approach induced by mechanical forces has appeared as a new emerging methodology in materials science.3 The mechanochemical reactions in solid state offer a significant advance by avoid the use of solvent, dramatically shortening synthesis times and simultaneously increasing the purity and amount of product. Herein, we present a facile mechanochemical route for the preparation of various hybrid perovskite particles with the size of several hundred nanometers for high-efficiency thin-film photovoltaics.4,5 We also demonstrate that such approach applied for preparation of perovskite materials has advantage over a solution-based synthetic routes in terms of hysteresis and device performance. References [1] M. Grätzel, Nat. Mater. 2014, 13, 838; (b) N.-G. Park, Mater. Today 2015, 18, 65. [2] W. S. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Science (2015), 348, 1234. [3] S. L. James et. al., Chem. Soc. Rev. (2012), 41, 413. [4] D. Prochowicz, M. Franckevicius, A. M. Cie?lak, S. M. Zakeeruddin, M. Grätzel, J. Lewinski, J. Mater. Chem. A 2015, 3, 20772 [5] D. Prochowicz, P. Yadav, M. Saliba, M. Saski, S. M. Zakeeruddin, J. Lewinski, M. Grätzel, Sustainable Energy Fuels 2017, DOI: 10.1039/C7SE00094D.

Resume : Perovskite solar cells as a new generation of nanostructured cells, have attracted a great deal of attention due to their remarkable photovoltaic performance and easy device manufacturing within the last few years. Organometal halide perovskite as a light absorber plays a significant role in order to achieve high power conversion efficiencies. However inhomogeneous perovskite layer with poor film uniformity can lead to uncontrolled reproducibility of the devices. To overcome this problem, several solvent engineering methods and variable deposition parameters have been employed. Herein we have applied a two-step solution approach using a solvent engineering method. Parameters such as different solvents including dimethyl sulfoxide, acetone, isopropyl alcohol, temperature and time of annealing altered. Prepared cells and perovskite films were fully characterized by field emission scanning electron microscopy, x-ray diffraction, UV-visible spectroscopy and I-V (under AM 1.5 sunlight simulation). The results revealed improved morphology of perovskite layer, optimized conversion rate of PbI2 to the perovskite crystals and effective infiltration of PbI2 through the layer of mesoporous TiO2 substrate. Ultimately, eliminating the annealing process of the PbI2 layer led to higher quality perovskite films.

Resume : Some limitations are keeping perovskite solar cells (PSC) away from commercialization: high annealing temperatures and fabrication with laboratory scale environment. To overcome these limitation several strategies have been proposed: to use PEDOT:PSS [1] or to replace TiO2 with ZnO [2,3] or WO3 [4]. In this work, we propose the use of a printed WO3 electron transporting layer (ETL) in a direct device architecture: we demonstrated good performances (9.3%) using an inkjet-printed WO3 layer [5]. In our device, all the inner layers (ETL, perovskite, HTL) are printable and annealed below 110°C. To demonstrate the effective transposition of this device architecture to printing technologies we performed the inkjet printing of the WO3, CH3NH3PbClxI3-x and Spiro-OMeTAD. This allowed us to produce solar cells with a promising PCE of 7.2% (figure 1), constituting the first demonstration of PSC based on three inkjet-printed layers. Lately, a champion cell with a 11.6 % PCE was produced with optimized conditions, which indicates that these technologies could produce printed PSC with PCE around 9 %. When printing is involved the wetting conditions are prime parameters to investigate. With contact angle measurements and Owens, Wendt, Rabel and Kaelble method we determined the polar and dispersive components of the surfaces energy of mesoporous TiO2 and our WO3 layer. Were also calculated the polar and dispersive part of the surface tension of CH3NH3PbI3 and CH3NH3PbClxI3-x with or without diiodooctane (DIO) which is an often used additive [6], the solvent being DMF. Our result indicates that the use of chlorine tends to reduce the dispersive component of the surface tension, DIO would increase its polar component (Figure 2). These modifications would allow the surface tension of the ink to move away from the non-wettability limit and thus promote a better impregnation of the ink into the mesoporous layer. [1] J. You, Z. Hong, Y. M. Yang et al., ACS Nano (2014), 8, pp. 1674?1680. [2] D. Liu, T. L. Kelly. Nat. Photonics (2014), 8, pp. 133-138 [3] A. Dymshits, L. Iagher, L. Etgar, Materials (2016), 9, p. 60, [4] Y. Hou, C. O. R. Quiroz, S. Scheiner et al., Adv. Energy Mater. (2015), 5. [5] A. Gheno, T.T.T. Pham, C. Di Bin et al. Sol. Energ. Mat. Sol. C. 116 (2017), pp. 347 354 [6] P.-W. Liang, C.-Y. Liao, C.-C. Chueh et al. Adv. Mater. 26 (2014), pp 3748-3754

Resume : Back-contact concepts are well established in the field of silicon solar cells, where their implementation has resulted in significant efficiency gains, compared to conventional contacting architectures. Charge collection in these devices is typically facilitated by a set of two interdigitated finger electrode arrays, co-located on the backside of the silicon wafer. Here we describe the fabrication and study of back-contact perovskite solar cells (bc-PSCs) with novel contact geometries. The main advantage of back-contact concepts is that optical transmission losses can be avoided, arising from the top charge collection electrode, which for PSCs typically is a thin conducting oxide (TCO) layer. The back-contact design furthermore provides the opportunity to study the influence of post-processing treatments on device efficiency in situ. To demonstrate this we study the evolution of bc-PSCs performance during their exposure to pyridine vapors. Furthermore we report a novel photovoltaic device concept based on a gold-perovskite-gold Schottky-junction bc-PSCs in which the work-function of the gold electrodes is controlled by the presence of self-assembled molecular monolayers (SAM). We provide evidence of the successful workfunction tuning by means of Kelvin probe microscopy while also presenting the photovoltaic performance date of these devices. We show that the presence of these SAMs can produce photovoltages of up to 600 mV and photocurrents in excess of 12 mA/cm2 under simulated sunlight, despite a large center-to-center electrode spacing of 6.5 µm.

Resume : Perovskite-based materials combine unique optoelectronic properties with the ease of fabrication by a variety of methods. The potential for cheap fabrication with readily available equipment has lead most research groups to focus on layer deposition utilizing solution-based methods. However, the high number of process parameters in the letter is a severe bottle neck towards an industrial large-scale production of perovskite-based optoelectronic devices. Years of experience for other thin-film materials like CdTe, CIGS or organic materials have shown that thermal evaporation of the base materials is able to easily overcome these challenges. Some of the main benefits are the decoupling of the materials from the environment, the elimination of solvents, and a strong reduction of process parameters thus enabling a reproducible and homogeneous deposition even for larger areas. In this work, we present detailed studies on the influence of process parameters in the vacuum-based preparation of CH3NH3PbI3 thin films for high efficient solar cells. Thorough control of the lead iodide rate and the process pressure yields power conversion efficiencies above 16 % which could be demonstrated reproducibly for various substrate materials and device architectures. In addition, the influence of substrate temperature on the crystallization dynamics and the device performance is explored. These results can easily be transferred to perovskite variations with different organic and halide compounds.

Resume : Organometal halide perovskite solar cells have evolved in an exponential manner in high conversion efficiencyand promise a high pay back energy in the near future which make the possibility of using them in a large scale. In this investigation, a device with structure of FTO/Block layer/ mesoporous TiO2/ CH3NH3PbI3/ Spiro OMeTAD/ Au has been fabricated by two-step spin-spin solution process. The etched FTO glass were spin coated with 80 micro liter of titanium di isopropoxide solution in dried ethanol and HCl. The films were then dried at 120 ºC followed by annealing at 450ºC for 30 min. After cooling to room temperature, substrates were treated with a 40mM aqueous solution of TiCl4 for 30 min at 70 ºC to enhance the surface properties of TiO2 layer. To prepare a mesoporous TiO2 film (TiO2 ML), 400 micro liter of diluted TiO2 paste (1:10 wt% in ethanol), was spin coated and annealed at 45ºC for 30 min. The prepared samples were coated for 5 sec, at rotation speed of 5000 rpm and temperature of 70 ºC, with 1.2 M of PbI2 solution in dimethylformamide. Then samples were dried at 70 ºC for 30 min. After that they were dipped in CH3NH3I solutions (in 2-propanol solvent) with the concentrations of 8 mg/ mL for 10 min, rinsed with pure 2-propanol solution, and dried at 100 ºC for 10 min. The fabricated films were characterized by field emission scanning electron microscopy, X-Ray diffraction analysis and UV-Vis spectroscopy. Photovoltaic performance of fabricated cells exhibited high efficiency of 13% and superb stability.

Resume : Organometallic halide perovskite quantum dots (OHP-QDs) have attracted a great deal of attention because of their high photoluminescence quantum yields (PLQYs, up to 90%) and narrow-band colors. However, a precise control of the OHP-QDs’ size is difficult and they are unstable under ambient conditions, which limits their usage in applications such as lasers and displays. Herein, we introduce a novel method to prepare size-controllable and stable OHP (methylammonium lead trihalide, MAPbX3, X = Cl, Br, and I)-QDs/polymer films using polydimethylsiloxane (PDMS) as a template. The MAPbX3 QDs/PDMS films developed in this work are self-standing, flexible, transparent, and retaining the photophysical properties of OHP-QDs for at least several months. Moreover, the uniform size of OHP-QDs results in narrow photoluminescence (PL) emission (full-width-at-half-maximum, FWHM ≈ 20 nm) compared to that of QDs synthesized by conventional method (≈ 50 nm). The PL emission peak can be shifted by varying the size of QDs and also by changing the halide component (Cl, Br, and I) covering a wide range of absorption wavelengths. Furthermore, the higher PL intensity of larger QD was confirmed by time-correlated single photon counting (TCSPC) experiments.

Resume : Given the simple processability of perovskite?halides, typically from solutions at relatively low temperatures, a non?negligible level of unintentional structural and chemical defects at temperatures relevant for device operation are currently limiting the devices efficiencies and causing instability - i.e. hysteretic behaviours and formation of metastable phases upon photoexcitation which hampers the band-gap tunability - and low reproducibility, especially in view of large area deposition. Defects thus influence further progress towards reaching the highest possible power conversion efficiencies. So far, high quality colloidal nanocrystal of perovskites has been demonstrated. However, they all need bulky and insulating organic ligands to remain in suspension, thus hampering the fabrication of conductive thin films. We demonstrate, for the first time, the synthesis of ligand free metal-halide pervskite nano-crystal inks by Laser Ablation Synthesis in Solution (LASiS). This methodology, simple and easy to use for large scale materials production, allows to produce nano-crystals solutions to print conductive thin films electrically and photo-stable. In fact, we show that such films do not present any hysteretic behavior under polarization, typical in presence of ion migration and permit monotonic tunability of the band gap across the visible spectrum, in absorption and emission, without the formation of sub-band gap emissive phases upon photo-excitation.

Resume : Tandem silicon/perovskite solar cells are of particular interest in solar energy, so as it makes possible to obtain a solar cell efficiency of more than 25% [1]. Another potential application of the silicon/perovskite couple is photo- and electroluminescent devices, since perovskites in the form of nanocrystals have effective luminescence. Colloidal inorganic CsPbX3 (X = Cl, Br, I) quantum dots (QDs) have photoluminescence efficiency of up to 90%, luminescence maxima position tuning in the entire visible spectrum region, and a line width of less than 30 nm [2]. The structure of porous silicon/CsPbX3 QDs/transparent electrode with a variation in the architecture of porous silicon and the elementary composition of QDs is considered as the main motive. Porous silicon (por-Si) obtained by electrochemical etching provides both electrical contact to perovskite nanocrystals and a distributed heterojunction, where por-Si acts as a semiconductor with a smaller bandgap. The main difficulty in creating the effective electronic structures was the existence of a potential barrier between an oxide on the surface of porous silicon and ligands stabilizing the surface of nanocrystals. Certain procedures were undertaken to solve the problem of injection of charge carriers. 1. J.W.Lee et al. J. Phys. Chem. Lett., 2017, 8 (9), 1999-2011 2. L. Protesescu et al. Nano Lett., 2015, 15 (6), 3692-3696

Resume : Nucleation, growth, and ligand-controlled structural transformations of cesium lead halide nanocrystals Thumu Udayabhaskararao1, Miri Kazes*1, Lothar Houben2, Ayelet Teitelboim1, Hong Lin3, and Dan Oron1* 1Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel. 2Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel. 3State Key Laboratory of New Ceramics and Fine Processing, Department of Material Science and Engineering, Tsinghua University, Beijing 100084, P.R. China. e-mail: thumu.udayabhasararao@weizmann.ac.il Despite the recent surge of interest in lead halide perovskite nanocrystals, there are still significant gaps in the understanding of nucleation and growth processes involved in their formation. In addition, active control over the shape, composition and crystalline habit of nanocrystals is a long sought-after goal. Using CsPbX3 nanocrystals (NCs) as a model system, we systematically study the formation mechanism of cubic CsPbX3 nanocrystals, their growth via oriented attachment into larger nanostructures, and the associated phase transformations.1 Moreover, we show that for cesium lead halide perovskite nanoparticles, a reversible structural and compositional change can be induced at room temperature solely by modification of the ligand composition in solution.2 Reversible transformation of cubic CsPbX3 NCs to rhombohedral Cs4PbX6 NCs is achieved by controlling the ratio of oleylamine to oleic acid capping molecules. This scheme does not only enable fabrication of high purity monodisperse Cs4PbX6 nanoparticles with controlled sizes. Rather, dependent on the Cs4PbX6 nanoparticles final size afforded by reaction time, the back reaction yields CsPbX3 nanoplatelets with controlled thickness. These results provide insight on the very significant role of the exact ligand composition on surface stabilization (and, concomitantly, destabilization) in lead halide perovskites, and present a new post-synthetic pathway to control their morphology and composition. References: 1. Udayabhaskararao, T; Kazes, M; Houben, L; Lin, H; Oron, D (2017). Nucleation, Growth, and Structural Transformations of Perovskite Nanocrystals. Chemistry of Materials. 29:1302-1308. 2. Udayabhaskararao, T; Kazes, M; Houben, L; Ayelet T; Oron, D (2017). Dynamic ligand control of crystalline phase and habit in cesium lead halide nanoparticles arXiv:1702.05382.