Perovskite solar cells: surface, interface and materials aspects

Resume : Along with the intense effort to further improve the efficiency and other technological aspects, there is a considerable effort in understanding intrinsic properties of hybrid perovskite halides. Curiously enough, there does not appear to be any universally accepted understanding of even the most basic properties of these materials. For example, an intensely debated issue concerns the ability of permanent dipoles on organic moieties to give rise to polar fields, either in the normal state (as in any ferroelectric material) or in the photo-excited state, contributing to its spectacular photovoltaic properties. Even estimates of the excitonic binding energy in these materials have proven to be controversial with various estimates differing by more than an order of magnitude. I shall discuss our own efforts with various pure and solid solution samples of APbX_3 to understand physical properties of several of these hybrid materials. We use different techniques that are sensitive to the polar nature of any given material, probing time-scales from the static down to a few hundred femto-seconds, both without and in presence of photo-excitation to address several outstanding issues. This work is a result of collaborations with B Bhattacharyya, M Bokdam, C De, C Franchini, S Ghara, TN Guru Row, A Hossain, BP Kore, G Kresse, A Kumar, J Lahnsteiner, P Mahale, A Mohanty, S Mukherjee, S Pal, A Pandey, MS Pavan, S Picozzi, T Sander, Sharada G, A Stroppa, A Sundaresan, and D Swain. Relevant references: 1. M Bokdam et al., Sci. Rep. 2016, 6, 28618 2. J Lahnsteiner et al., Phys. Rev. B 2016, 94, 214114 3. Sharada G et al., J. Phys. Chem. Lett. 2016, 7, 2412 4. Sharada G et al., J. Phys. Chem. Lett. 2017, 8, 4113 5. Sharada G et al., J. Phys. Chem. C (ASAP, 2018)

Resume : Hybrid perovskite photovoltaics has received rapidly growing interest from the emerging solar-cell community due to its record increase in power-conversion efficiency. To further advance this technology, we need to understand the hybrid perovskite (HP) materials on the atomic scale, on which the light-to-energy conversion and transport processes occur. Currently, this atomic scale is riddled with controversies. For the detachment of methylammonium (MA) cations from the PbI3 cage in the most common HP material MAPbI3, very low as well as high (∼100 meV) activation energies (Ea) have been reported. Quasi-elastic neutron scattering measurements (QENS) for the orthorhombic phase found Ea=48 meV, which was attributed to the axial rotation of the whole MA cation [1]. To shed light on this controversy, we performed density-functional theory (DFT) calculations (PBE0 functional + van der Waals corrections + inclusion of nuclear quantum effects) for several rotational MA processes [2]. For the torsional rotation, we obtain Ea=42 meV in good agreement with the QENS result. We therefore ascribe this barrier to the rotation of CH3 against the NH3 unit, which remains bound to the PbI3 cage. For the full axial rotation, which breaks three hydrogen bonds, we obtain a much higher barrier (∼120 meV) and therefore conclude that this process is not likely to occur. [1] Chen et al., Phys. Chem. Chem. Phys. 17 31278 (2015). [2] Li et al., J. Phys. Chem. Lett. submitted.

Resume : The dynamic effects observed in the J-V measurements represent one important hallmark in the behavior of the perovskite solar cells. Proper measurement protocols (MPs) should be employed for the experimental data reproducibility, in particular for a reliable evaluation of the power conversion efficiency (PCE), as well as for a meaningful characterization of the type and magnitude of the hysteresis. We discuss several MPs by comparing the experimental J-V characteristics with simulated ones using the dynamic electrical model (DEM). Under certain re-poling conditions and bias scan rates a hysteresis-less behavior with relatively high PCEs may be observed, although the J-V characteristics are far away from the stationary case. Furthermore, forward-reverse and reverse-forward bias scans show qualitatively different behaviors regarding the type of the hysteresis, normal and inverted, depending on the bias pre-poling. We emphasize here that correlated forward-reverse or reverse-forward bias scans are essential for a correct assessment of the dynamic hysteresis. In this context, we define a hysteresis index which consistently assigns the hysteresis type and magnitude. Our DEM simulations, supported by experimental data, provide further guidance for an efficient and accurate determination of the stationary J-V characteristics, showing that the type and magnitude of the dynamic hysteresis may be affected by unintentional pre-conditioning in typical experiments. [arXiv:1803.00285]

Resume : Since the first report on the solid-state perovskite solar cell (PSC) with power conversion efficiency (PCE) of 9.7% and 500 h-stability in 2012 by our group, researches on PSCs increase exponentially. As a result, PCE approaching 23% was reported, which is higher than conventional inorganic thin film solar cells, and publications reached more than 9,000 as of May 2018. It is believed that PSC is promising next-generation photovoltaics due to superb performance and very low cost. In this talk, the history of perovskite photovoltaics will be presented along with their scientific progress and perspective. For reproducible and high quality perovskite film, Lewis acid-base adduct method was developed. Grain-boundary healing method was developed via non-stoichiometric approach. Grain boundary healing process yields PCE as high as 20.4%. Method for reduction in hysteresis and improving moisture stability was developed by interfacial engineering using 2-dimensioanl perovskite. A universal method to eliminate hysteresis even in normal mesoscopic structure was discovered. Large-area coating technology was developed to commercialize high efficiency PSCs via perovskite single crystals (or powder) based viscous liquid approach. A unique bifacial technique was developed to produce high PCE (>20%) MAPbI3, FAPbI3 and other compositions at mild condition. Halide perovskite is now extend to X-ray imaging, LED and resistive memory areas, which will be also discussed in this talk.

Resume : Perovskite solar cells have seen a tremendous increase in efficiencies to above 22% within few years. By now, the device parameter with the largest potential for further improvements is the open circuit voltage (Voc), which is mainly governed recombination losses at the interfaces. Here, a water-free poly(3,4-ethylenedioxythiophene) (PEDOT) layer doped with sulfonated copolymers is utilized as hole selective contact (HSC) on top of perovskite absorbers. Transient photoluminescence (PL) and steady-state PL quantum yield measurements reveal longer charge carrier lifetimes and larger quasi-fermi level splitting for perovskite films with PEDOT atop. PEDOT thin films therefore seem to suppress charge carrier recombination at the perovskite surface. By blending undoped Spiro-OMeTAD into the PEDOT dispersion, the surface energetics of the resulting films are shifted to larger ionisation energies as measured by photoelectron spectroscopy and the charge extraction is enhanced as implied by transient PL and impoved fill factors in complete devices. As a result, perovskite solar cells with mixtures of PEDOT and Spiro-OMeTAD as HSC achieve high Voc values up to 1.19 V and stabilized efficiencies over 16%. This exceeds both the Voc and PCE of reference devices with doped Spiro-OMeTAD or pure PEDOT as HSC in this study.

Resume : Planar heterojunction organic-perovskite hybrid solar cell is one of the most promising photovoltaic architecture to achieve high efficiency with low temperature process. Several processes to synthesis high quality perovskite film at room temperature were disclosed. Combining with high mobility hole transporting and electron transporting layers, the planar heterojunction perovskite-PC71BM solar cell achieves the power conversion efficiency above 20% and a reasonable stability. The champion cell shows no current hysteresis both in voltage scan directions and scan rates. The growth mechanism of the high quality perovskite film can be manipulated using several strategies under low temperature. Be able to control the nucleation/ growth of the perovskite crystals makes the fabrication of high-efficiency perovskite solar cell rather reproducible.

Resume : Metal halide perovskite solar cells are now effectively competing with their inorganic counterparts in terms of power conversion efficiencies. However, state of the art perovskite solar cells still suffer from limited fill factor (FF) and open circuit voltage (Voc), which has been related to non-geminate losses mostly happening at the surface of the perovskite absorber. Therefore, it is of imperative importance to address these types of losses and try to minimize them. One way to suppress this type of charge carrier recombination is to appropriately modify the perovskite surface in order to slow down surface recombination and, accordingly, improve its interface with the charge transporting layers as it was shown that this is the predominant loss channel. Here, we present the improvement of solar cell devices in a p-i-n solar cell architecture utilizing PTAA and C60 as hole and electron transporting layers by two different surface modifications. We combine this with an in-depth understanding of the physical processes involved in both actual devices and the bare material itself by the means of a series of detailed combined analyses. First, we show the enhancement of the Voc by adding Strontium (Sr) to a quadruple cation perovskite Rb5(Cs5(MA0.17FA0.83)95)95Pb(I0.83 Br0.17)3 in which the Sr mostly accumulates at the perovskite surface. The resulting material displays significantly enhanced photoluminescence (PL) lifetime and absolute PL yield, indicating strong reduction of non-radiative surface recombination. Consequently, in the actual devices, it has been found that the Voc drastically increases from 1.11 V to 1.18 V, along with an improvement of the electroluminescence efficiency of more than one order of magnitude, with resulting power conversion efficiency (PCE) over 20%. Upon detailed investigation of morphology (electron microscopy and secondary ion mass spectroscopy) and energetics (photoelectron spectroscopy), we found that Sr segregates at the surface where it induces a more n-type surface with a consequent strong band-banding. We propose that such a surface modification creates a back-surface field able to repel holes from the surface, thus reducing the probability of a trapped electron recombining with a hole in the valence band. Additionally, this screening effect brings a substantial improvement of the perovskite-C60 interface, compensating for the lack of selectivity of the latter. Second, we developed a treatment of the triple cation Cs5(MA0.17FA0.83)95)95Pb(I0.83 Br0.17)3 perovskite surface with a Polyimidazole derivative polyionic liquid. Here, this surface modification enables a beneficial improvement of the FF as well as an enhancement of the Voc. The resulting solar cell devices show outstanding FF values of up to 82.5% and extraordinarily PCE of over 21%. In conclusion, we propose that through exclusively addressing the surfaces it is possible to efficiently suppress the non-radiative recombination of charges and consequently reduce the energy losses. Additionally, our results can be representative of a more general methodology for device modification, and therefore potentially applicable to other compositions and cell architecture.

Resume : The power conversion efficiency of organic-inorganic perovskite solar cells has soared from 3.8% in 2009 to the current record exceeding 22%. Long-term stability of perovskite solar cells not only depends on the photoactive layer, but also on the proper selection of interfacial layers. Efficient and stable interfaces offer minimized energy-level offsets and good isolation of the perovskite layer preventing its decomposition. Here we report new fullerene-derivatives as more promising alternatives to the commonly used PCBM material, which improve the efficiency and stability of the devices in inverted configuration. Our findings show that new fullerenes: (1) provide good coverage atop of the MAPbI3 layer as confirmed by conductive AFM imaging; (2) yield efficient charge quenching confirmed by PL measurements; and (3) enhance the power conversion efficiency of PEDOT-based p-i-n devices reaching 15%. Most importantly, two of the new fullerene derivatives provided excellent isolation of the perovskite active layer resulting in largely improved thermal and ambient (60% RH in air) stability of the solar cells.

Resume : Over the last few years metal halide perovskite based solar cells have undergone a meteoric rise in cell efficiency to > 22%. Increasing importance of improving solar cell operation is reliant upon understanding and controlling thin-film crystallisation and controlling the nature of the heterojunctions between the perovskite with the p and n-type charge extraction layers. In addition, understanding and enhancing long term stability of the materials and devices is a key driver. In this talk, I will highlight the key factors which are important for reaching the maximum efficiencies and long-term operational stability, and also areas where we know require further improvement. I will specifically highlight recent advances in understanding the enhancement of long term stability through appropriate choice and adaptation of charge selective interlayers, interfacial doping and trap passivation[1?4]. I will also highlight the importance of tailoring the perovskite composition; especially introduce the creation of stable 2D-3D heterostructures within a perovskite film, for device stability improvement[5]. References: [1] Z. Wang, D. P. McMeekin, N. Sakai, S. van Reenen, K. Wojciechowski, J. B. Patel, M. B. Johnston, H. J. Snaith, Adv. Mater. 2017, 29, 1604186. [2] Y. Hou, X. Du, S. Scheiner, D. P. McMeekin, Z. Wang, N. Li, M. S. Killian, H. Chen, M. Richter, I. Levchuk, N. Schrenker, E. Spiecker, T. Stubhan, N. A. Luechinger, A. Hirsch, P. Schmuki, H. P. Steinrück, R. H. Fink, M. Halik, H. J. Snaith, C. J. Brabec, Science (80-. ). 2017, 358, 1192. [3] M. Kot, C. Das, Z. Wang, K. Henkel, Z. Rouissi, K. Wojciechowski, H. J. Snaith, D. Schmeisser, ChemSusChem 2016, 9, 3401. [4] M. Daskeviciene, S. Paek, Z. Wang, T. Malinauskas, G. Jokubauskaite, K. Rakstys, K. T. Cho, A. Magomedov, V. Jankauskas, S. Ahmad, H. J. Snaith, V. Getautis, M. K. Nazeeruddin, Nano Energy 2017, 32, 551. [5] Z. Wang, Q. Lin, F. P. Chmiel, N. Sakai, L. M. Herz, H. J. Snaith, Nat. Energy 2017, 2, 17135.

Resume : With power conversion efficiencies (PCEs) exceeding 22%, perovskite solar cells (PSCs) are considered to be a promising alternative to established photovoltaic technologies. The main challenge of the technology is the limited stability. In this study, we reveal the importance of the right choice of materials when considering outdoor applications of PSCs. We present a novel degradation mechanism in PSCs that is not apparent at constant temperatures but has a heavy impact on the performance when the temperature is varied. This degradation can lead to reductions in photocurrent and thus efficiency as high as 80% within less than three hours in the TiO2/perovskite/Spiro-MeOTAD architecture, that is one of the most commonly used architectures. Identifying the underlying physical process of this degradation mechanism and proposing possibilities to overcome it is in the focus of this contribution. By exchanging the electron and hole transport layer as well as the perovskite absorber, we systematically investigate the role of the interfaces on this novel degradation process. We show that one of several routes to avoid the temperature variation induced degradation is to use C60 instead of TiO2 as electron transport layer. Combining all of these results, we demonstrate a PSC architecture that is stable for several hours under realistic outdoor temperature variations with a PCE of up to 19.5%.

Resume : Predicting the stability of the perovskite structure remains a longstanding challenge for the discovery of new functional materials for photovoltaics, fuel cells, and many other applications. Using a novel data analytics approach based on SISSO (sure independence screening and sparsifying operator), an accurate, physically interpretable, and one-dimensional tolerance factor, τ, is developed that correctly classifies 92% of compounds as perovskite or nonperovskite for an experimental dataset containing 576 ABX3 materials (X = O, F, Cl, Br, I). In comparison, the widely used Goldschmidt tolerance factor, t, achieves a maximum accuracy of only 74% for the same set of materials. The probability of forming stable perovskites is mapped continuously as a function of the sizes of the A, B, and X ions revealing physical insights into how these relative sizes yield stable and unstable perovskite structures. Additionally, the new tolerance factor is shown to compare well with DFT-calculated decomposition enthalpies of single and double perovskite oxides and chalcogenides. τ is applied to identify more than a thousand inorganic (Cs2BB’Cl6) and hybrid organic-inorganic (MA2BB’Br6) double perovskites that are predicted to be stable.

Resume : All optical, time-resolved laser spectroscopy analysis of perovskite materials and photovoltaic devices have been widely used to assay photogenerated carrier dynamics and trap states in them. Analysis using transient absorption spectroscopy has however been complicated by the high dielectric constant of these materials compared to dyes and organic semiconductors, limiting our capabilities of extracting useful information especially for sub-bandgap signals. The application of photoluminescence spectroscopy techniques like time-correlated single photon counting has also been complicated by using vastly different photogenerated carrier densities, often giving misleading information of carrier trapping, recombination and extraction times. In this contribution, I will present our ongoing efforts to develop successful methods for analysis of perovskite materials and devices based on sub-bandgap transient absorption and photoluminescence spectroscopy techniques.

Resume : We are moving towards a sustainable society powered by renewal energy where solar photovoltaics is one of the most important players. In the past few years, emerging photovoltaic (PV) technologies have observed an exponential increase in power conversion efficiencies (PCE) with halide perovskite solar cells above 22 %, tandem photovoltaics reaching 26 % or dye sensitized solar cells for indoor lighting at the impressive 28.9 % PCE mark. Oxides in solar cells can be found as the main solar absorber responsible for photon-to-electron conversion, as interfacial layers for the transport of electron or holes, as part of the conductive metal electrodes (including transparent electrodes) and also as part of photon management. Among the many advantages is the ease of fabrication, low cost and enhanced stability that provide to the solar cell. Moreover, new-generation of oxides (e.g. doped or undoped, binary, ternary, ferroelectric, etc.) are slowly breaking ground providing competitive power conversion efficiencies, enhanced transport properties or improved UV-light stability, among others. We report our most recent studies on the application of classic oxides (binary, doped, nanostructured) and complex oxide compounds (ternary, ferroelectric, etc.) as transport layers in Halide Perovskite Solar Cells. We will show the effect of metal oxides and their surface functionalization on solar cell efficiency and long-term stability. [1] A. Hagfeldt, M. Lira-Cantu, Recent concepts and future opportunities for oxides in solar cells, Applied Surface Science, (2018) Submitted. [2] A. Perez-Tomas, A. Mingorance, Y. Reyna, M. Lira-Cantu, Metal Oxides in Photovoltaics: All-Oxide, Ferroic, and Perovskite Solar Cells, in: M. Lira-Cantu (Ed.) The Future of Semiconductor Oxides in Next Generation Solar Cells, Elsevier, 2017, pp. 566. [3] M. Lira-Cantú, Perovskite solar cells: Stability lies at interfaces, Nature Energy, 2 (2017) nenergy2017115. [4] M. Lira-Cantu, The future of semiconductor oxides in next generation solar cells, 1st ed., Elsevier, 2017. [5] Y. Reyna, M. Salado, S. Kazim, A. Pérez-Tomas, S. Ahmad, M. Lira-Cantu, Performance and Stability of Mixed FAPbI3(0.85)MAPbBr3(0.15) Halide Perovskite Solar Cells Under Outdoor Conditions and the Effect of Low Light Irradiation., Nano Energy, 30 (2016) 570–579.

Resume : Metal halide perovskites constitute a very attractive class of materials for optoelectronic applications, such as solar cells, light emitting diodes, lasers and photodetectors. Most notably, solid-state photovoltaic devices based on these materials have reached power conversion efficiencies (PCEs) of 22% after just five years of academic research. Perovskite solar cells have a great commercial potential, but there still remain few challenges, which need to be resolved to prove the viability of the technology. Some of the well-known issues include material stability. Furthermore, cost-effective, reliable fabrication process capable of delivering highly efficient, large-area perovskite modules is yet to be demonstrated. This talk will outline the industrial challenges of perovskite solar cells including legislative hurdles as well as the potential impact on the solar industry. Moreover, we will present fully scalable ink-jet printing process of the perovskite PV stack and fabrication of perovskite printable mini-modules of areas up to A4 size, complemented with a robust encapsulation methodology.

Resume : Combining inorganic?organic perovskites and crystalline silicon solar cells into monolithic tandem solar cells has recently attracted increased attention due to the high efficiency potential of this cell architecture. Promising results with certified efficiencies as high as 23.6% were realized and published in early 2017 [1]. To further increase the tandem device performance to a level well above the best silicon single junctions at 26.7%, optical and electrical optimizations as well as a detailed device understanding of this advanced tandem architecture need to be developed. Here we present our recent results on monolithic perovskite/silicon-heterojunction tandem solar cells that reach a certified conversion efficiency of 25.0%. This remarkable achievement was enabled by several experimental improvements as compared to our previous reports [2]. Guided by our optical simulations [3,4], we selected the front contact layer stack with less parasitic absorption utilizing an (?inverted?) perovskite top cell in the p-i-n contact design and combined it with an adapted rear-emitter silicon heterojunction cell into a tandem device. In addition, optical and electrical optimization of the n-type front contact stack together with improving the optoelectronic quality of the perovskite bulk and the interfaces to the adjacent charge selective layers were necessary to achieve the high efficiency. With the help of a detailed loss analysis, experimental guidelines for further improvements will be shown. The analysis shows that realistic efficiencies above 30% are within reach for monolithic tandem cells when the perovskite band-gap is enlarged and optimized light trapping/light management techniques are successfully implemented. [1] Bush, K. A.; Palmstrom, A. F.; Yu, Z. J.et al. Nature Energy 2017, 2, 17009. [2] Albrecht, S.; Saliba, M.; Correa Baena, J. P. et al. Energy Environ. Sci. 2016, 9, 81. [3] Jäger, K.; Korte, L.; Rech, B. et al. Optics Express 2017, 25, A473. [4] Mazzarella, L.; Werth, M.; Jäger, K. et al. Optics Express 2018, 26, A487.

Resume : For all thin film based tandem solar cells, perovskite solar cell (PSC) represents an ideal candidate for wide band gap top cell in combination with low band gap Cu(In,Ga)Se2 (CIGS) bottom cell. The possibility to develop highly efficient perovskite and CIGS solar cells on flexible substrates lays the foundations to lightweight flexible tandem devices by high-throughput roll-to-roll manufacturing. Here, we report a multi-stage perovskite deposition approach by combining vacuum- and solution-based techniques, where the morphology of vacuum deposited PbI2 is appropriately tailored to enhance organic cation intercalation. The preferential growth of the inorganic layer is the result of minimization of combined surface and interface energy. Depending on the specific substrate surface nature, compact or nanostructured PbI2 morphologies can be obtained. With this process, we have achieved steady-state efficiencies of 15.8% and 14.0% for opaque and NIR-transparent PSCs, respectively, on a flexible front sheet which is used for flexible CIGS module encapsulation. Further, flexible perovskite/CIGS tandem with 19.6% efficiency were measured in four-terminal configuration. Eventually, through optical loss analysis, we define potential pathways for more efficient flexible NIR-transparent PSCs. Our work represents a step forward towards new concepts for flexible thin film tandem devices, and provides insights into growth of layers and interfaces for high efficiency flexible solar cells.

Resume : Resistive switching memory devices based on 2-dimensional halide perovskites The halide perovskite is the crystal structure formed with the general formula ABX3, where A cation is larger than B cation, and X represents the halide (Cl, Br, or I) anion. And the ABX3 halide perovskite shows a 3-dimensional (3D) structure corresponding to a corner-sharing octahedral network. The halide perovskite shows unique property and fast ion migration, which plays a major role in resistive switching behavior and causes current–voltage (I–V) hysteresis. The halide perovskite materials have received great attention as potential alternatives to conventional materials and begun to be applied to resistive switching memory devices. However, in the resistive switching process which has repeatable formation and rupture of filaments in an insulating layer, conducting filaments in the 3D halide perovskite structure are difficult to control the strength, shape, and uniformity, because charged defects randomly migrate in 3D structure. To overcome these issues, 2D layered halide perovskites can be applied to the memory devices instead of 3D halide perovskites. The 2D layered perovskite structures are formed by inserting large molecule at A sites in the 3D frames, which means that the 3D structures are broken, then become layered structures. Herein, we successfully apply CsPbI3 with phenylethyl ammonium (PEA) for 2D layered-perovskite in resistive switching memory devices, and the device structure is Ag/polymethylmethacrylate (PMMA)/(PEA)2Csn−1PbnI3n+1/Pt/Ti/SiO2/Si stack. The uniform perovskite film is synthesized on the substrate using low temperature all-solution process and the 2D layered-perovskites exhibit bipolar resistive switching behavior. Previously, the characteristics of CsPbI3 (3D perovskite) switching memory devices exhibit the ON/OFF ratio of 109 and a root mean square (RMS) roughness of 9.883 nm. But interestingly, the switching memory devices based on (PEA)2Csn−1PbnI3n+1 (2D perovskite) shows ON/OFF ratio of 109 with ultralow operating voltage (<0.20 V) and a RMS roughness of 2.634 nm. This research will contribute to the improvement of switching behavior of memory devices and the better understanding on the resistive switching mechanisms based on the 2D layered halide perovskites.

Resume : Methylammonium lead tri-iodide (MAPbI3) is a polymorphic material with a remarkable potential for solar energy conversion. It is characterized by two temperature-induced phase transitions at 165 K and 327 K, accompanied by an orthorhombic-to-tetragonal and a tetragonal-to-cubic lattice modification, respectively. Understanding the origins of these transitions as well as their structural implications could be important for the technological optimization of this material. Here, we use ab initio molecular dynamics to study the phase changes of MAPbI3. We argue that the orthorhombic-to-tetragonal transition is characterized by an antiferroelectric to ferroelectric ordering, through the partial rearrangement of the organic cations that locally relaxes the stress arising from the thermal movement of atoms [1,2]. Besides, we propose a macroscopic model for the tetragonal phase that consists of rotated noncentrosymmetric domains where the MA ions are quasi-two-dimensionally confined around the a-b crystallographic plane. We finally show that the gradual disordering of the MA ions at higher temperatures triggers the tetragonal-to-cubic transition, resulting in a loss of the ferroelectric characteristics. [1] I. Deretzis, B.N. Di Mauro, A. Alberti, G. Pellegrino, E. Smecca, A. La Magna, Sci. Rep. 6, 24443 (2016) [2] I. Deretzis, A. La Magna, Nanoscale 9, 5896-5903 (2017)

Resume : Ferroelectric materials take advantage of spontaneous electric polarization separating photo-excited carriers and above-bandgap output voltage. Compared with organic-inorganic hybrid perovskites, ferroelectric oxide perovskites are much stable in a wide range of mechanical, chemical and thermal conditions and be able to fabricate using low-cost methods. The bottlenecks for ferroelectric photovoltaic applications are their poor visible optical absorption and low electrical conductivity owing to wide bandgap. In this work, using B-site chemistry, bandgap of BiFeO3-based oxide perovskites has been judiciously engineered in the range of 1.20-2.06 eV, that make ferroelectric semiconductors feasible for wide light absorption and high energy conversion photovoltaic solar cell applications. In particular, La and Mn co-substituted BiFeO3 solid solution exhibits a narrow direct bandgap of 1.22 eV, that allows photon absorptions of ~80% sunlight spectrum. The electronic subshell configuration, reduce mass of unit cell, and tolerant factor/octahedral factor related to ionic size are attempted as descriptors to classify and data-mine the relationship between composition and bandgap property of BiFeO3-based oxide perovskites. This essay provides an efficient approach to make ferroelectric semiconductor photovoltaic solar cells for human sustainable and clean energy resource with power conversion efficiency possible beyond the Shockley-Queisser limit for conventional p-n junction solar cell.

Resume : Tailoring the organic cations composition has often been considered as the most effective approach to dictate the morphology and crystal structure of inorganic-organic halide based perovskite film. In this work, we demonstrate the fabrication of (FAPbI3)1−x(MAPbBr3)x perovskite solar cells (PSCs) with various composition ratios of x, (x = 0, 0.10, 0.125, 0.15 and 0.20) employing spin-coating method. The as-fabricated PSCs exhibit device architecture as follows: glass/indium tin oxide (ITO)/tin oxide(SnO2)/composite perovskite/Spiro-oMeTAD/Ag. Specifically, the composite perovskite film acts as an active light-harvesting layer, whereas SnO2 and Spiro-oMeTAD function as electron and hole transporting layer, respectively. The device performance of the PSC are evaluated with respect to the structural and optical properties of the perovskite film at different x composition. It was found that the addition of MAPbBr3 into FAPbI3 stabilized the perovskite phase, leading to more uniform and dense morphology. The incorporation of MAPbBr3 into FAPbI3 stabilized the perovskite phase and helped to enhance the power conversion efficiency (PCE) of PSCs. The optimal PSC device (at x = 0.15) exhibits the lowest series resistance and highest fill factor, resulting in a PCE of 17.06% with negligible hysteresis. In summary, this study offers insights on how the incorporation of MAPbBr3 altered the properties of the perovskite films and the photovoltaic performance of PSCs.

Resume : Hole extraction layer (HEL) of the inverted planar perovskite solar cells (PSCs) plays a dominant role in determining the device performance. High short circuit current density, open circuit voltage and efficient charge collection with suppressed charge recombination can be achieved through controlled growth of the perovskite layer. In this work, the effect of sodium citrate modified poly (3,4-ethylenedioxythiophene): poly (4-styrenesulfonate) (PEDOT:PSS) HEL on improved perovskite growth and hence the performance of the PSCs was investigated. Power conversion efficiency (PCE) of >11.30% was achieved for PSCs made with a sodium citrate modified PEDOT:PSS HTL, which is >20% higher than that of a structurally identical control device (9.16 %). Incident-photon to current efficiency, light intensity-dependent J−V characteristics, scanning electron microscope (SEM) and X-ray diffraction (XRD) measurements were carried out to analyzed the performance enhancement of the PSCs. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements reveal that sodium citrate solution modification partially removes surface PSS content of PEDOT:PSS HTL. A significant increase in the ratio of PEDOT to PSS of about 0.197 was observed for the sodium citrate modified PEDOT:PSS, as compared to that of the pristine PEDOT:PSS layer (0.108). The increase in the ratio of PEDOT to PSS agrees with the observation of increase in the conductivity of sodium citrate modified PEDOT:PSS layer. We propose that the sodium citrate modified PEDOT:PSS HTL benefits the efficient operation of PSCs in two ways: (1) it assists the crystal growth to increase in the perovskite domain size, and (2) it favors the charge collection in the PSCs.

Resume : In recent years, perovskite solar cells (PSCs) have attained state-of-the-art efficiency comparable to that of silicon solar cells. To further enhance the power conversion efficiency (PCE) and air stability of PSCs, it is imperative to optimize the device architecture, surface and interfacial morphology and composition of the active perovskite layer. Herein, we demonstrate the use of methylammonium bromide (MABr) as a passivation layer on top of MAPbI₃ film surface to effectively suppress charge recombination and simultaneously improve charge collection. The MAPbI₃ PSCs were prepared via one-step anti-solvent technique, in which tin oxide (SnO₂) and spiro-OMeTAD were used as electron (ETL) and hole transporting layer (HTL), respectively. Our findings show that the presence of MABr passivation layer has remarkably reduced the excessive PbI₂ phase, grain boundaries and surface defects of MAPbI₃ film. This facilitated the charge transfer in the device, resulting in > 10% increment in the PCE of the device (13.5%). In overall, this work manifests the defect-passivation and charge facilitator roles of interfacial passivation layer on the rear surface of perovskite film.

Resume : Planar p-i-n type (“inverted”) perovskite solar cells based on the mixed perovskite “triple cation” absorber (Cs0.05((CH3NH3)0.17(HC(NH2)2)0.83)0.95Pb(I0.83Br0.17)3 with an initial efficiency of more than 17% were cycled in a temperature range between room temperature and 180°C. Modulated photovoltage spectra were measured during and after each heating cycle of 40 min duration and correlated with current-voltage characteristics, which were measured at room temperature. Surprisingly, a decrease of the energy parameter of the exponential defect states near the band-gap was observed with increasing temperature up to 80°C. The photovoltage signals related to deep defect states increased strongly between 80 and 100°C and decreased between 100 and 120°C, in correlation with a local minimum of the parameters of the solar cell at 100°C. However, strong degradation of the efficiency of the solar cells set on at temperatures above 150°C. This can be related to formation of defects in the perovskite and/or instability of the electron-selective contact layer.

Resume : Wurtzite structure Cu2ZnSnS4 (CZTS) quantum dots (QDs) are successfully synthesized by the hot-injection method, and UV-vis spectra are introduced to measure optical properties of the CZTS QDs for choosing the suitable QDs as the hole-transporting materials (HTM) in the perovskite solar cells (PSCs). The solar cells based on 6.7 nm CZTS QDs with the band gap of 1.81 eV reach the power conversion efficiency (PCE) of 9.5%. The high PCE value is related to the improvement of the Jsc, resulting in the increasement of the external quantum efficiency (EQE) at 680 nm. The devices based on CZTS QDs also show a good stability of PCE value after several days, which may provide a simple and low-cost way for fabricating PSCs.

Resume : Modulated surface photovoltage (SPV) spectroscopy is a powerful tool for ex-situ and quasi in-situ characterization of electronic properties related to the band gap (Eg), tail states (Et), direction of charge separation and diffusion length (L). Furthermore, SPV analysis does not require the preparation of contacts and can be performed after different stages of layer preparation, light soaking etc. Eg, Et and L depend on crystallization, defect formation, solvents, substrates, temperature, additives as well as aging and light soaking. Application of modulated SPV spectroscopy is demonstrated on the examples of the dependence of Eg and Et on the stoichiometry in CH3NH3Pb(I1-XBrX)3 films, on temperature in Mo / CH3NH3PbI3 / PMMA, on different kinds of interfaces and degradation. Furthermore, the degradation of L under light soaking is demonstrated.

Resume : Organic-inorganic hybrid perovskite solar cells evolved to one of the hot topics in the photovoltaic community due to facile processing and high efficiencies. A compact and continuous electron transporting material (ETM) is often used for recombination inhibition. This can be performed by spin coating, spray pyrolysis or atomic layer deposition (ALD). Using ALD, the thickness of these compact layers can be controlled precisely. Due to the self-limiting layer-by-layer process even complex morphologies can be coated with a well-defined homogeneous layer. We present perovskite solar cells with planar geometry using ALD as a crucial method in the manufacturing process of the ETM. ALD allows for a systematic tuning of the layer thickness and chemical composition. Also, two or more different layers can be combined to facilitate the charge extraction. The ALD film is analyzed by spectroscopic ellipsometry. The cell morphology and composition is characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction. The cell performance is measured by cyclic voltammetry and external quantum efficiency measurements. The use of ALD surface treatments paves the way towards three-dimensional perovskite solar cell architectures based on nanoporous 'anodic' aluminum oxide templates.

Resume : Perovskite solar cells have emerged in recent years as a potential low-cost technology for solar energy harvesting. Within only nine years of research the demonstrated performances climbed from 3.8% to above 22% certified efficiency. The deposition of high-quality perovskite thin films by solution processes such as blade and spin coating allows for the utilization of a wide range of planar and structured substrates. Titania nanotubes combine the advantageous high surface area of mesoporous TiO2 nanoparticular films with short, direct carrier pathways, thus facilitating charge extraction. Anodization is a versatile tool for the large-scale fabrication of nanotubes or pores on metal foils or thin films sputtered on FTO glass superstrates. The easy, independent tunability of structure and dimensions enables a conclusive in-depth study of the geometric effects on device performance. In this work, arrays of hexagonally arranged TiO2 nanotubes are employed as electron transport layers for nanostructured perovskite solar cells. The device structure is characterized after each processing step, and the performance of final devices is investigated by J-V curves, external quantum efficiency, and impedance spectroscopy. This system allows for systematic investigation of geometric effects on solar cell performance parameters.

Resume : The film quality of perovskite is very crucial to the performance of perovskite solar cells. Here we present two methods on the growth of high quality perovskite films via a simple and rapid process. Polymer PAN and a series of hydrochlorides with different organic amine cations were introduced into CH3NH3PbI3 solution as an additive to obtain solar cells. Based on the high-quality films, a series of 2D materials and organic molecules as an interfacial layer makes perovskite solar cells with a planar heterojunction have largely enhanced open circiut voltage (Voc) and high power conversion efficiency (PCE) as well as a significant enhancement of stability.

Resume : Diffused exciting around lead halide perovskite photovoltaics is largely justified by skyrocketing increase in power conversion efficiency up to certified 22.7%. The race for record efficiency is far from the end, but research community nowadays is well aware that more important criticality need to be addressed. Unstability and scale-up losses of perovskite PV technology are fundamental limitation toward feasibility of real-world application. Selective contacts quality is a sine qua non and metal oxides, mainly TiO2, SnO2 and NiO, guarantee state of the art efficiency and stability. Processing of metal oxides thin film is at the heart of technology advancement with high interest application including catalysis, energy and electronics. Electrodeposition is one of the most powerful and intriguing deposition technique for metal oxides, due to reproducibility, processing from water, low cost, scalability and the high purity of deposit due to highly selective driving force for film nucleation and growth. We report anodic electrodeposition of NiO on TCO substrates and their implementation as hole selective contact in p-i-n PSC. Detailed investigation through nucleation-growth modelling of electrodeposition mechanism shed light on mass-transport limited process overlapping with oxygen evolution. The effect of electroplating parameters like the concentration and nature of support electrolyte, temperature and applied potential is thoroughly investigated. Deposition efficiency increases at higher temperature while suppor rt electrolyte suppress the deposition process due to nickel cation complexation in solution. Devices based on CH3NH3PbI3 processed in air employing electrodeposited NiO within the architecture ITO|NiO|PSK|PCBM|BCP|Ag deliver a stabilized efficiency up to 16%

Resume : The impressive display market can maintain or even grow its prosperity only by investing in the improvement of the present state-of-the-art technology, lowering the costs and promoting new, environmentally friendly, materials. Here are intervening the OLETs as a possible next generation technology in the display industry. This type of devices can successfully combine the light emission of OLED with the switching of thin film transistors (TFT). The first room-temperature light-emitting diode based on organic-inorganic halide perovskite was reported in 2014 [1,2]. In both these two works, as well in other new studies the CH3NH3PbCl3, CH3NH3PbCl1.5Br1.5, CH3NH3PbBr3, CH3NH3PbBrI2 and CH3NH3PbI3 materials showed electroluminescence in the blue, green, red and near-infrared. But, the optimistic achievements reported these works are shaded by the use of lead in halide perovskite. The research concerning OLET based on halide perovskite is breaking new ground. There are very few published studies regarding the light-emitting TFT and, apparently, no TFT based on lead-free halide perovskite was reported yet. Here we report the fabrication of lead-free (CH3NH3)3Sb2I9-xClx thin films by three methods: i) modified one-step; ii) two-steps starting from spin-coated SbI3 film and iii) two-step starting from thermal evaporated SbI3 layer. Their structural, optical, morphological and electrical properties were analyzed and discussed. The optimized (CH3NH3)3Sb2I9-xClx layer was integrated in a TFT whose photo-electrical response was probed at room temperature. References: 1. Z.K. Tan et al., Nature Nanotechnol. 9 (2014) 687. 2. G. Xing et al., Nature Mater. 13 (2014) 476.

Resume : Vacuum-assisted evaporation of solvents is a promising intermediate step for the controlled deposition of hybrid organic inorganic metal halide perovskite layers from salt solutions. The DMF:DMSO ratio, the components in the salt solutions, the temperature of the substrate during vacuum treatment (Ts) and the time of vacuum treatment (tv) were varied. The band gap and the tail states were characterized by modulated surface photovoltage spectroscopy and correlated with the current-voltage characteristics of solar cells. The band gap increased with decreasing DMF:DMSO ratio and, surprisingly, both Isc and Voc increased with increasing band gap. For CH3NH3PbI3, the optimum DMF:DMSO ratio, Ts and tv were 9:1, 30°C and 10s, respectively, and resulted in an efficiency above 17%. The dependence of the band gap of (CH3NH3)x(CH(NH2)2)1-xPbI3 on x was obtained. For mixed ion systems, a modified vacuum treatment including a purge with a pure gas is needed in order to reach high efficiencies.

Resume : Halides such as CsPbI3, CH3NH3PbI3 and HC(NH2)2PbI3 have attracted many attentions in recent as innovative light absorbing materials in photovoltaic system. It is called halide perovskite solar cell (HPSC) because such halides have AMX3 perovskite crystal structure. Photovoltaic performance of HPSCs strongly depends on quality of the halide perovskite film. In particular, open circuit voltage (VOC) is determined by concentration of non-radiative recombination sites which lead to high voltage deficit that is a difference between optical band gap and qVOC. Therefore, the trap density in halide perovskite film should be minimized to obtain low voltage deficit. In this study, we propose a strategy of rapid crystallization of halide perovskite film to minimize voltage deficit instead of the conventional long annealing process to obtain high crystallinity of the film. We will discuss approach to obtain low voltage deficit by introducing short annealing process through control of crystallization behavior of halides.

Resume : We conduct a systematic study on s, p and d (incomplete) electron atoms in different parts of Wurtzite crystl structure SnO2 for transparent electrode in perovskite solar cells. Next, we will study the bandgap alignment and possibaly 2D electron gas behavior at TiO2 interface at [100], [110] and [111] interface planes.

Resume : Perovskite solar cells have attracted much attention in recent years due to their high efficiencies reaching values over 23% with application in tandem solar cells [1, 2]. However, the problem with perovskite solar cells is instability at high temperature and humidity which pose to be daunting in view of upscaling and application. Therefore, the recent research for improvement in stability has led to the conclusion that perovskite solar cells with mixed dual A-cation (2C) consists of formamidinium and cesium ions in perovskite layer have much better structural stability without loss of efficiency [3]. In the same kind of perovskite with sufficient cesium content, it has been reported that its charge carrier properties are not reduced when increasing bromide concentration [4]. Therefore, the 2C perovskite seems to be a good candidate for application in tandem solar cells with possibility to tune the bandgap by changing bromide and cesium content [5]. However, this raises a question what salt should be used to introduce bromide ions. Here, we investigate the three sources of bromide in the 2C perovskite Cs0.18Fa0.82Pb(I0.94Br0.06)3 absorption layer, meaning lead bromide (PbBr2), formamidinium bromide (FABr) and cesium bromide (CsBr). Using the same ion and mass concentration in the precursor, we have been able to compare the same perovskite composition made from the three different bromide sources. We analyze perovskite layer prepared with gas quenching and antisolvent method in the p-i-n device structure. Results of the following experiment have shown better performance for FABr and CsBr sources of bromide in comparison to regularly used PbBr2. Apart from the efficiency, the cells made with FABr as a bromide source for perovskite layer have also achieved best light stability at continuous MPP tracking compare to other composition. Our study will demonstrate long-term stability study at MPPT. The influence of bromide source on photovoltaic characteristics and long-term stability of the devices will be explained using structural, electrical and optical measurements. [1] D. Zhao, C. Wang, Z. Song, Y. Yu, C. Chen, X. Zhao, K. Zhu and Y. Yan, ACS Energy Lett. 2018, 3(2), 305?306. [2] Solliance Solar Research (2018, 21 March). Solliance and ECN take major step in improving tandem solar cells [Press release]. Retrieved from https://solliance.eu/solliance-and-ecn-take-major-step-in-improving-tandem-solar-cells/ [3] J. W. Lee, D. H. Kim, H. S. Kim, S. W. Seo, S. M. Cho and N. G. Park, Adv. Energy Mater. 2015, 5(20), 1501310. [4] W. Rehman, D. P. McMeekin, J. B. Patel, R. L. Milot, M. B. Johnston, H. J. Snaith, L. M. Herz, Energy Environ. Sci. 2017, 10(1), 361?369. [5] K. A. Bush, K. Frohna, R. Prasanna, R. E. Beal, T. Leijtens, S. A. Swifter, M. D. McGehee, ACS Energy Lett. 2018, 3(2), 428?435.

Resume : Cubic CsPbI3 perovskite nanocrystal is the next generation of materials for optoelectronic application, but this faces serious phase instability and transformed to nonemissive orthorhombic phase. Only a few approaches were reported to stabilize this cubic phase of the nanocrystals. All these approaches were based on heteroatom doping, using spatially designed ligands or following ice cool protocol. Success has been achieved in stabilizing these nanocrystals for few weeks in ambient atmosphere, but the problem of phase transformation during annealing in the reaction flask was not addressed. In contrary to that we have reported high-temperature colloidal synthesis for obtaining thermal, colloidal and phase stable CsPbI3 nanocrystals. The stability was achieved by introducing pre-formed alkylammonium salt and increasing the reaction temperature to 260 ᵒC. The pre-introduced alkylammonium ions at high-temperature passivate the surface firmly and prevented the nanocrystals from phase transformation. These nanocrystals were stable in reaction flask during annealing, in the crude reaction mixture in purified dispersed or in powder form. Details of the temperature gradient, the role of different parameters were investigated. Different spectroscopic analysis was carried out and details of the surface binding of alkylammonium ligands in place of surface Cs in the crystal lattice were investigated. As CsPbI3 is one of the most demanding optical materials, bringing stability by proper surface fictionalization without the use of secondary additives would indeed help in the wide spreading of their applications.

Resume : Doped perovskite nanocrystals have recently emerged as a new class of energy materials for solar concentrators and solid-state lighting device applications. Among these, doping Mn(II) in high band gap CsPbCl3 perovskite host nanostructures has been extensively studied. However, going beyond their optical emissions, herein, the impact of dopant ions on tuning the doped platelet dimensions and retaining the monodispersity is reported. These were performed by designing appropriate compositions of layered perovskites, L2(Pb1–xMnx)Cl4, which on thermal treatment in the presence of Cs(I) ions transformed to Mn-doped CsPbCl3 platelets. Correlating the amount of Mn present in layered perovskites and retained in doped platelets, the role of Mn for the conversion of layered to doped perovskites was established. These doped platelets showed dominated Mn d–d emission and also Mn concentration-dependent emission tuning. Even though several reports of Mn-doped CsPbCl3 have been reported, these findings add new fundamental insight into the design of dimension-tunable doped perovskites from layered perovskites.

Resume : Towards hybridized renewable energy harvesting systems, improvement of suitable materials is needed so that they can be used as self-powered portable electronic systems.1 In this work, methylammonium lead iodide (MAPI) is synthesized via co-precipitation method and used to nucleate -phase in polyvinylidene fluoride (PVDF) films.2 We found that MAPI not only nucleates electroactive phase (up to ~ 91 % ) in PVDF but it also gets stabilized in ambient condition. The piezoelectric energy generation from composite film of MAPI and PVDF is presented under simple human finger touch motion. The hybrid photoactive energy harvester can simultaneously harvest mechanical energy and detect visible light. The fast rising time as well as fast increase in current under non-monochromatic light illumination is detected which is a signature to use as an efficient photoactive device. On account of the photoresponsive and electroactive characteristic of the composite film a new class of stand-alone self-powered flexible hybrid photoactive mechanical energy harvester is made. A significant change of piezoelectric output voltage and current is observed during the visible light detection that promises its futuristic application in piezo-phototronic technology. REFERENCES [1] A. Sultana et al., ACS Appl. Mater. Interfaces 2015, 7, 19091. [2] D. Mandal et al., J. Phys. Chem. B 2011, 115, 10567.

Resume : The characterization of intrinsic defect mechanisms in Al2O3 thin films is essential for their effective use in surface passivation schemes for solar cells. Particularly, perovskite solar cells (PSC) have shown an enhanced long-term stability and an improved protection against ambient conditions when an ALD-Al2O3 layer is introduced into the PSC stack [1]. In this work, we discuss comparatively ALD-Al2O3 films prepared on different substrates (including MAPI: CH3NH3PbI3) and by different process parameters (thermal-ALD, plasma-enhanced-ALD, substrate temperature). These films were analyzed by resonant photoelectron spectroscopy. Intrinsic defect states within the electronic band gap were observed including excitonic, polaronic, and charge-transfer defect states, where their relative abundance depends on the choice of the used ALD parameters and substrate [2,3]. The most pronounced signature of excitonic states is found for the Al2O3 film prepared on MAPI at room temperature. It points to a strong distortion of the octahedral coordinated network with a high number of tetrahedral sites which have a high affinity for OH adsorption and , hence, are responsible for perovskite protection against humidity [2]. [1] M. Kot et al., ChemSusChem 9 (2016) 3401. [2] K. Henkel, M. Kot, D. Schmeißer, J. Vac. Sci. Technol. A 35 (2017) 01B125. [3] K. Henkel, M. Kot, M. Richter, M. Tallarida, D. Schmeißer, in K. Wandelt (Ed.): Encyclo-pedia of Interfacial Chemistry: Surface Science and Electrochemistry, Elsevier, Oxford, 2018, vol. 3.1, pp 18-26.

Resume : Since few years, many efforts have been done to keep the initial efficiency of the perovskite solar cells over a long time. The CH3NH3PbI3-based solar cells have shown an increased long-term stability against ambient conditions while encapsulating them or covering the perovskite film by a thin Al2O3 film. Although the solar cells stability with the Al2O3 film could be successfully improved it has not lasted for a long time. In this paper, we will present that by covering the CH3NH3PbI3 film with a room temperature atomic layer deposited Al2O3 (RT-ALD-Al2O3) [1,2] one cannot only increase the stability, but more interestingly, one can boost the efficiency of solar cells over time. In particular, the Al2O3/CH3NH3PbI3 interface is characterized using the X-ray Photoemission Spectroscopy (XPS) and Field Emission Scanning Electron Microscopy (FESEM). The efficiency of the Au/Spiro-OMeTAD/(RT-ALD-Al2O3)/CH3NH3PbI3/PCBM/TiO2 /ITO/glass solar cells are extracted from the current density-voltage (J-V) characteristics measured under AM1.5G light at 100 Wcm-2. As deduced from the XPS and FESEM studies the ALD precursors only clean the perovskite surface from the –OH groups and do not change its composition. Most importantly, the average efficiency of the solar cells containing 9 RT-ALD-Al2O3 cycles increases from 9.4 to 10.8 % but without the RT-ALD layer it decreases from 13.6 to 9.6 % while storing both in the same environmental conditions for 355 days. [1] M. Kot et al., ChemSusChem 9 (2016) 3401. [2] M. Kot et al., Nucl. Instrum. Methods Phys. Res. B 411 (2017) 49.

Resume : The electronic properties of the perovskites change itself depending on the synthesis methods and the substrates on which they are grown and hence the performance of the device [1 ]. Therefore it is of prime importance to study the electronic properties of the perovskite in order to choose the proper contact materials and optimized solar cell performance. In literature it is discussed that a mixture of the methylammonium (MA) and formamidinium (FA) halide based perovskite are efficient and stable compare to single organic cation based perovskite[ 2]. In the present work we investigated the electronic properties of mixed organic cation perovskite. In particular we conducted the photoemission study on different percentage of the FA and MA in the mixed perovskite and measured the valence band and work function. It is found that the change is electronic structure is minimal in case of FAxMA(1-x)PbI3 upon variation of FA and MA concentration while in FAxMA(1-x)PbBr3 it is larger. In FAxMA(1-x)PbBr3 the value of x=0 to 1 changes the valence band maxima of perovskite form 1.39 eV to 1.97 eV. We believe this finding can be helpful to properly design the perovskite based multijunciton solar cell as well as to choose the proper contact materials for optimum efficiency . 1. S. Olthof and K. Meerholz, Sci. Rep. 2017, 7, 40267. 2.N. Pellet P. Gao, G. Gregori, T-Y. Yang M. K. Nazeeruddin, J. Maier and M. Grätzel, Angew. Chem. Int. Ed. 2014, 53, 3151 ?3157.

Resume : Copper (l) thiocyanate (CuSCN) is an optically transparent, wide bandgap (3.4-3.9 eV), p-type semiconductor used in organic and perovskite solar cells. In comparison to other hole transporting materials, CuSCN is economically favourable and comparable in performance, with efficiencies for hybrid organic-inorganic metal halide perovskite solar cells exceeding 20% [1]. Current research involves several solution-processable methods such as spin coating, spray coating and doctor blading. This study reports the first ever synthesis of CuSCN via aerosol assisted chemical vapour deposition (AACVD). AACVD is a scalable technique with low processing costs, simple reactor design and no vacuum requirement. The study involves deposition of CuSCN using four different solvents; diethyl sulfide, dipropyl sulfide, diethyl methyl sulfide and ammonium hydroxide. For each CuSCN/ solvent combination, deposition temperature, concentration of solution and amount of precursor solution deposited were altered. Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and ultraviolet-visible spectroscopy were used to determine morphological, structural and optical information. [1] N. Arora, M. I. Dar, A. Hinderhofer, N. Pellet, F. Schreiber, S. M. Zakeeruddin and M. Grätzel, Science, 2017.

Resume : We present a facile mechanochemical route for the preparation of organolead halide perovskites, where the chemical reactions are forced by high energy ball mill. Advantages of this methods include fast reaction time, high product purity and avoiding toxic organic solvents (producing less waste). Powder X-ray diffraction measurements demonstrate that mechanosynthesis is a suitable strategy to produce a highly crystalline material showing no detectable amounts of the starting substrates. Here we report a facile mechanochemical route for: (a) preparation of MAPbI3 and other two component materials (b) the stabilization of a structurally stable α-FAPbI3 perovskite by fine and controllable stoichiometry modification in mixed-cation (MA)x(FA)1−xPbI3 perovskites, (c) the first time atomic-level evidence that guanidine cation is directly incorporated into the MAPbI3 and FAPbI3 lattices, forming pure GUAxMA1–xPbI3 or GUAxFA1–xPbI3 phases. The following materials were used to manufacture perovskite solar cells with extraordinary efficiencies reaching 20%. 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, Y. P. Kumar, M. Saliba, M. Saski, S. M. Zakeeruddin, J. Lewiński, M. Grätzel, Sustainable Energy and Fuels, 2017, 1, 689–693 3) D. Prochowicz, P. Yadav, M. Saliba, M. Saski, S. Zakeeruddin, J. Lewinski, M. Grätzel, ACS Applied Materials & Interfaces, 2017, 34, 28418–28425 4) D. Kubicki, D. Prochowicz, A. Hofstetter, M. Saski, P. Yadav, D. Bi, N.Pellet, J. Lewinski, S.M. Zakeeruddin, M. Grätzel, L. Emsley, Journal of the American Chemical Society, 2018, 140, 3345–3351

Resume : It is nearly eight years since the hybrid perovskites have attracted the significant interest of the scientific community, mainly due to the outstanding performance of hybrid perovskite solar cells. A high achievable efficiency of the perovskite-based solar cells, currently reaching 22%, [1] shows that carrier generation efficiency in perovskites may approach 100%, suggesting that perovskites may be promising materials not only for solar cells but also for sensitive photodetectors, which may compete with conventional semiconductor analogs. Despite tremendous interest in solar cell research perovskite photodetectors still lack a basic understanding of their operating principles. Here, we study the performance of planar polycrystalline MAPbI3 perovskite photodetectors produced on interdigitated comb of electrodes made from various metals. We demonstrate that a hole blocking oxide layer between metal electrodes and perovskite may enhance responsivity and photocurrent gain of the planar photodetector based on the polycrystalline film by order of magnitude. Application of the Cr interdigitated comb of electrodes with naturally formed oxide layer enabled to reach resposivity of 152 A/W an external gain of more than 350. We suggest that the gain enhancement originates from the hindered extraction of photogenerated holes and the migration of ions, which creates additional hole traps at interfaces. These effects reduce the barrier for electron injection and enable the passage of a larger number of electrons during the prolonged lifetime of photogenerated holes. The achieved photodetector sensitivity, suggested gain enhancement approach and obtained better understanding of the photocurrent gain mechanism in hybrid metal halide perovskites open a way towards the further development of a cheap and easily producible planar perovskite photodetectors based on interdigitated electrode arrays. Reference [1] National Renewable Energy Laboratory. Best Research-Cell Efficiencies. Available at https://www.nrel.gov/pv/assets/images/efficiency-chart.png. Accessed on 21 May 2018

Resume : Halide perovskites have established themselves as a promising semiconductor material for light-emitting diodes (LEDs), challenging the state-of-the-art organic and quantum dot LEDs. While three-dimensional (3D) perovskites (APbX3 where A = MA or FA and X = I, Br or Cl) suffer from low photoluminescence quantum yield due to small exciton binding energies and non-radiative recombination arising from trap states, quasi-2D perovskite structures derived from intercalating large organic counter-cations within the lead halide networks have recently been found to overcome such problems. However, the organic-based (MA and FA) perovskites are known to have poor thermal stability. This work presents the first study of a quasi-2D perovskite based on the inorganic and more thermally resistant CsPbBr3 compound. The incorporation of a large alkylated molecule – phenylethylammonium bromide (PEABr), into the CsPbBr3 framework is shown to yield huge improvement to the photophysical and morphological properties of the perovskite film. By carefully tuning the molar addition of PEABr, highly compact, smooth and nano-sized grains were achieved with efficient energy funneling through an energy cascade as proven by various spectroscopic analyses. The benefits are well reflected in LEDs made from this quasi-2D mix, giving more than 50-fold increment in luminance and efficiency. The simplicity of this approach allows ample opportunity for large-scale production.

Resume : As a consequence of their quantum- and electronically-confined inorganic lattices, 1D lead-iodide perovskites are ill suited to photovoltaic (PV) applications. In order to circumvent these problems we utilized electron accept-ing viologen (N,N'-dialkyl-4,4'-bipyridinium) dications incorporating hydrogen bond donor functionalities, which served to induce π-π stacking interactions. The resulting 'viologen dimers' display enhanced charge-transfer (CT) interactions with the 1D lead-iodide nan-owires. This manifests in extended light absorption (up to 800 nm), reduced band gaps (1.74 eV to 1.77 eV), and longer photoluminescence lifetimes. More importantly, we demonstrate significant photoconductivity behavior in polycrystalline 1D lead-iodide perovskites for the first time. Thus, this work offers a strategy for inducing the properties required for PV applications in 1D lead-iodide perovskites.

Resume : Organo-lead halide perovskite solar cells (PSCs) have attracted tremendous attention owing to their superior photovoltaic properties. However, despite the excellent power conversion efficiencies (PCEs) that have recently been achieved, the device stabilities are still a challenge for the commercialization of PSCs. Spiro-OMeTAD is a widely used hole transport layer (HTL) in conventional n-i-p PSCs, which has been reported to suffer degradation from the permeate of moisture due to the hygroscopic additive and the presence of pinholes. To fix the relatively low device stability of PSCs based on spiro-OMeTAD, numerous strategies have been developed and applied in PSCs. One approach to diminishing these adverse effects introduced by the moisture permeate and ion migration is to insert a buffer layer. To avoid decreasing the device performance while improving the stability, this p-type semiconductor needs possess high conductivity and superior hole mobility besides hydrophobicity. Lead sulphide (PbS) is a traditional direct bandgap semiconductor with high hole mobility. We found that when inserting a thin layer of PbS between the metal electrode and spiro-OMeTAD, the PSCs with PbS buffer layer exhibited a better photovoltaic performance and significantly enhanced stability with respect to the reference cells. The superior hole mobility of spiro-OMeTAD/PbS bilayer was considered to be the dominated origin of the device performance improvement. And the hydrophobic nature and dense morphology of PbS enable it to provide an efficient permeation barrier against moisture and metal migration. The champion cell with PbS buffer layer displayed a PCE of 19.58% and maintained almost 100% of its initial PCE after 1000 h stored in ambient air. While in this structure the spiro-OMeTAD is still requisite. Metallophthalocyanine (MPc) compounds that consist of an 18-π electron conjugated macrocycle skeletal structure are potential candidates for stable HTLs in PSCs. We reported a novel heavy atom Pc derivative, octamethyl-substituted palladium(II) phthalocyanine (PdMe2Pc), which shows promise as an HTL in PSCs. The introduction of the heavy Pd atom endows the material with a long carrier lifetime and without dramatically reducing its mobility. This PdMe2Pc exhibited a long carrier diffusion length (LD) which is benefit to reducing the charge recombination. The devices based on PdMe2Pc displayed a relatively high PCE of 16.28% and good long-term stability.

Resume : The photovoltaic (PV) market is dominated by silicon solar cells (SCs) delivering high efficiency and long-term stability. However, Si SCs are approaching their efficiency limit and new methodologies are required to overcome it. Tandem SCs, combining Si SCs with a low-cost wide-band-gap absorber material as perovskite, that absorb light in a complementary solar spectrum region, are the most promising option for improving existing PV technologies keeping the production cost low.1 The implementation of highly efficiency semi-transparent perovskite SCs is a critical issue to reach this goal. In this work we present tandem structures using semi-transparent perovskite top-cell and silicon SCs as bottom cell reaching efficiency above 23% in a 4-terminals mechanical stack configuration. Semitransparent perovskite SCs with efficiency above 16.0% efficiency and average transparency >70% in near-infrared, have been realized using indium tin oxide (ITO) as transparent electrode and a thin Ag layer to protect the organic layers and to guarantee a resistant to delamination contact.2 Here we also compare SCs realized both by spincoating and thermal evaporation and we discuss the effects of perovskite bandgap on tandem SCs efficiency. 1. J Werner, et al., Adv. Mater. Interfaces 2018, 5, 731; Y Wu et al., En. Environ. Sci., 2017,10, 2472; K.Bush, et al. Nature Energy volume 2, 17009 (2017); T. Duong, et al. Adv. Energy Mater. 2017, 7, 228. 2. A. Guchhait et al ACS Energy Lett. 2, 4, 807-812

Resume : Perovskite solar cells demonstrated impressive power conversion efficiencies of >22%, while their practical application is hampered by poor stability of the active materials. Functional layers of perovskite solar cells have to sustain electric fields generated by built-in and light-induced potentials. Otherwise, the electrochemical degradation of active or charge transport layer materials would ruin the solar cell 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. We have shown that polarization leads to the appearance of the field-induced gradients in the chemical composition of the films as revealed by PL and Kelvin probe microscopy and ToF-SIMS. The effects of the potentiostatic polarization of the solar cells under forward and reversed bias on their photovoltaic performance will be also discussed and interpreted using results of the light beam induced current (LBIC) mapping. Analysis of the obtained data allowed us to correlate the chemical composition of the perovskite materials with their electrochemical stability, which might guide further design of stable light absorbers for future generation photovoltaics.

Resume : Lead halide based perovskite solar cells have progressed significantly with the efficiency rising from 3.8% to over 22% within a few years. However, the use of toxic Pb derivatives severely limits niches for their application and commercialization. Less harmful tin (Sn), bismuth (Bi) and antimony (Sb) derivatives are considered now as environment friendly alternatives. Here we present a systematic study of thermal and photochemical degradation pathways of a series of complex halides ASnX3, A3Bi2X9 and A3Sb2X9 (X=I, Br) with organic (A+= CH3NH3+, H2NCHNH2+) and inorganic (A+=Cs+) cations under inert anoxic conditions by using a set of complementary techniques: optical spectroscopy, AFM and SEM microscopy, XPS, XRD, EDX microanalysis and mass spectrometry. It has been shown that hybrid perovskite materials incorporating organic cations are intrinsically unstable under the realistic solar cell operation conditions and undergo dramatic changes in their optical, morphological and structural properties. On the contrary, cesium-based all inorganic complex metal halides showed much superior stability. Unfortunately, none of the investigated materials demostarted improved photochemical stability in comparison with the conventional complex lead halides. Nevertheless, the analysis of the comprehensive set of the obtained results creats a solid background for rational design of the next generation light absorbers for efficient and durable perovskite-inspired solar cells.

Resume : Hybrid perovskite solar cells have attracted a considerable attention of researchers due to feasibility of their low-cost production and achieving impressive power conversion efficiencies of >22%. Currently, low operation stability of hybrid perovskite solar cells represents the main obstacle for their practical implementation. Replacing fragile organic moieties with robust inorganic cations represents a promising approach to designing some more stable absorber materials. Inorganic lead-halide based perovskites CsPbI3-xBrx (x=0-3) have demonstrated improved thermal and photochemical stability. Unfortunately, the photovoltaic performances of all-inorganic perovskites remain much infeior in comparison with conventional hybrid materials and suffer from severe phase stability issues (e.g. perovskite phase of CsPbI3 is thermodynamically unstable at room temperature). To overcome these obstacles, more research is needed. Here we report a detailed investigation of all-inorganic perovskite-inspired systems based on CsxPbI2Brx alloys with x ranging from 1 to 4. An optimal non-stoichiometric composition has been identified, which delivered the best optoelectronic, structural and photovoltaic characteristics in combination with the improved stability. The revealed remarkable effect of excessive CsBr on the photothermal stability and photovoltaic performance of CsPbI2Br perovskite films might create a new paradigm in the development of efficient and stable solar cells.

Resume : In just a few years, perovskites solar cells (PSCs) have emerged as one of the most promising solar cell technologies. So far, the PCEs of PSCs above 22% have been reported, which is rivalling values achievable with crystalline silicon solar cells. In spite of the fast growth in the photovoltaic performance, PSCs are severely limited in their large scale applications due to instability issues. These include intrinsic instabilities of the metal halide perovskites, environmental instabilities, and device operation related instability. In this work, the first issue, i.e. intrinsic instability of the metal halide perovskite is studied by combing chemical bonding analysis of DFT calculations and experimental degardation study using Ultraviolet–visible spectroscopy. A comprehensive set of chemical bonding analysis of DFT calculations are done for AMX3 (A=Cs, MA, FA, M=Pb, Sn, X=I, Br, Cl) perovskites. Bond order, net charge, steric repulsion, and Crystal Orbital Hamilton Population (COHP) analysis reveal the relation of covalent, ionic, steric interactions, as well as bonding/antibonding characters with their stability, respectively. As a part of this work, Ultraviolet–visible spectroscopy of AMX3 perovskites film degradation during incremental heating were carried out to probe their thermal stability. A systematic comparison of the theoretical analysis and experimental data points to the most important factors responsible for the trends in the thermal stability of AMX3 perovskites. This sets of chemical bonding analysis also shows promise (from our preliminary study) in explaining the structural instability of metal halide perovskites, i.e. transition of black phase (3D structure) to yellow phase (2D structure).

Resume : Perovskite solar cells (PSCs) has raised hopes for a technological revolution in the field of photovoltaics. However, fabrication parameters should be investigated carefully to achieve optimum PSCs. In this work, the effects of electron transport layer (ETL) thickness, purity of solvents and precursor materials, methods of perovskite synthesis, and type of hole transporting material (HTM) are studied as some of the most important fabrication factors. We found that the performance of the PSC is strongly influenced by the purity of the precursors and solvents. Using precursor materials synthesized in our lab and conventional solvents we could not reach efficiencies more than %1.47, but replacing precursors with high purity commercial ones and using anhydrous solvents easily improved our results up to %4.11. The quality of ETL highly affects the quality and reproducibility of fabricated solar cells. Our experiments show that spin coating of TiO2 as ETL (blocking layer) for two times yields better and more reproducible results. Using a mesopore TiO2 layer is also beneficial. We also tried to use the sputtering method for coating the NiO layer as hole transport material, which showed that even the least sputtering power could damage and degrade the perovskite films. A comparison on two step spin coating and electrodeposition methods for perovskite synthesis were also done which shows morphological superiority of perovskite layer deposited using electrodeposition method.

Resume : Rapid development of technologies and their influence on people's lives is increasingly associated with energy demand. In order to meet this growing energy demand and minimize related costs new and effective energy generation methods are necessary. Among several alternative sources, photovoltaics are among the most promising, as solar energy is free and inexhaustible energy source. Perovskite solar cells often consist of several layers, each having a specific function and made of materials that meet a certain set of requirements. Some of those requirements include the stability issue, which derives from additives that are used in the process of increasing the conductivity of hole transporting materials. Having that in mind triarylamine derivative polymers with different functional groups were synthetized as hole transport materials (HTMs) for perovskite solar cells (PSCs). The novel materials enabled efficient PSCs without the use of chemical doping to enhance the charge transport. Devices employing poly(triarylamine) with methylphenylethenyl functional groups showed better power conversion efficiency then widely used additive-free compound - poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA). Notably, devices with the foremost polymer enabled stable PSCs under 1 sun at maximum power point tracking for ~40 hours and under elevated temperature (85 °C) for more than 140 hours. The results present remarkable progress towards stable PSC under real working conditions, which is crucial for industrial application

Resume : One of the methods to improve perovskite solar cells (PSC) performance is to change the mesoporous titania (TiO2) structure to a more ordered one, e.g. to nanorod or nanotube arrays. Oriented nanorod-like materials on transparent conductive oxide (TCO) are known for their efficient charge separation and transport properties and are thus favourable for achieving good device performance. Recently we have reported successful preparation of vertically aligned TiO2 nanotube (NT) array thin film for photovoltaic application by electrochemical anodization of titanium layer deposited by magnetron sputtering (V. Mandic et al. Solar Energy Materials and Solar Cells 168 (2017) 136–145). If the duration of anodization process is not set optimally, obtained TiO2 NT layer will not have satisfying transparency or will have deep holes that continue in TCO layer underneath which can cause bad performance of PSC. In this work, we propose the use of very thin TiO2 buffer layer in-between TCO and TiO2 NT layer to minimize this problem. TiO2 buffer layer will be deposited by DC reactive magnetron sputtering using titania target and Ar O2 working gas mixture followed by titania layer deposition. Structural, optical and electrical propertis of the improved composite TiO2 NT electrode will be compared to standard one. Influence on PSC performance will be also disused. Acknowledgement to Croatian science foundation (project ZOTONanoPhotovolt 2014-09-9419) for financial support.

Resume : The highest efficiency in perovskite solar cells is currently achieved with mixed-cation hybrid perovskites. The ratio in which the cations are present in the perovskite structure has an important effect on the optical properties and the stability of these materials. The incorporation of formamidinium into hybrid perovskites has proven beneficial in terms of stability as well as efficiency. Indeed, the highest efficiency lab-scale devices currently contain a certain percentage of the formamidinium cation. Therefore this system was chosen for an in-depth NMR study. Formamidinium-methylammonium lead iodide phases (FAxMA1-xPbI3) with different FA/MA ratios were prepared. Powders obtained via a precipitation method were compared with powders obtained by scarping off films. The powders were analyzed using X-ray diffraction and 1H-, 13C- and 207Pb solid–state NMR. The incorporation of formamidine and methylamine in the hybrid perovskite crystal lattice can be derived from differences in relaxation behavior compared to the precursor salts. The 207Pb resonance peak shifts systematically with the FA/MA ratio present in the powders.

Resume : The double perovskites Cs 2 AgBiCl 6 and Cs 2 TlSbCl 6 have been investigated using all-electron full-potential linearized augmented plane wave (FP-LAPW+lo) method within the frame work of the density functional theory. In addition to this, we employed Modified Back-Johnson bandgap correction in order to obtained close value of gap with respect to the experimental value. Our obtained bandgap is 2.66 eV (Cs 2 AgBiCl 6 ) and 1.466 eV (Cs 2 TlSbCl 6 ) with spin orbit coupling. Cs 2 AgBiCl 6 has indirect (L ? X) bandgap while our hypothetical Cs 2 TlSbCl 6 shows direct bandgap at ?-point. We have calculated phonon band structure for both studied compounds and found positive frequencies, which shows the stability. Stable, direct bandgap and value of gap close to MAPI makes Cs 2 TlSbCl 6 a great competitor in Pb-free hybrid perovskites solar cell word.

Resume : Due to its thermal stability, lead-free composition and nearly ideal optical and electronic properties, orthorhombic CsSnI3 perovskite is considered promising as a light absorber for lead-free all-inorganic perovskite solar cells (PSCs). However, the susceptibility of this 3-dimensional perovskite towards oxidation in air has limited the development of solar cells based on this material. Here, we report the findings of a computational study which identifies promising RbyCs1-ySn(BrxI1-x)3 perovskites for solar cell applications, prepared by substituting cations (Rb for Cs) and anions (Br for I) in CsSnI3. We show the evolution of the material’s electronic structure, as well as its thermal and structural stabilities upon gradual substitution. Importantly, we demonstrate how the unwanted yellow phase can be suppressed by substituting Br for I in CsSn(BrxI1-x)3 with x >1/3. We predict that substitution of Rb for Cs results in a highly homogeneous solid solution and therefore an improved film quality and applicability in solar cell devices.

Resume : Self-assembly of organic monolayers (SAM) on solid surfaces is a common technique to modify surface and interfacial properties and are used to alter the wettability, conductivity and biocompatibility of the surface. In this study, we show the self-assembly of perfluorocarbons , free base and metal based fluorinated porphyrins on methylammoniumleadiodide (CH3NH3PbI3) perovskite thin film and demonstrate the interaction of perovskite surface with fluorine by X-ray photoelectron spectroscopy (XPS), Kelvin probe microscopy (KPM) and polarization-modulated infrared reflection absorption spectroscopy (PMIRRAS) measurements. From PMIRRAS measurements, it emerges that weak non-covalent halogen bonding between fluorine from the perfluorocarbon or perfluorinated porphyrin and the perovskite are the driving forces for the formation of SAM on perovskite surface. To the best of our knowledge this is the first report of PMIRRAS measurement on perovskite surface to prove halogen bonding and formation of monolayer. Though the CH3NH3PbI3 perovskite thin film have enormous potential in the area of photovoltaic research, but the perovskite surface is very prone to moisture absorbance which drastically reduces the stability of the device by decomposition of the CH3NH3PbI3 crystal structure to methylamine (CH3NH2). Fluorocarbons are well known for their water repelling properties and generate a strong interfacial dipole moment due to the high electronegativity of fluorine. Thus, the presence of hydrophobic fluorine group on the perovskite surface may improve the stability and efficiency of HOIP-based devices by altering the work function.

Resume : Identified as emerging light absorbers due to their plethora of unique optoelectronic properties, perovskites have also been touted as a promising candidate for light emission. However, despite the effortless transition of perovskites into the current organic light-emitting diodes (OLEDs), misalignment of energy levels at the hole transporting material (HTM) and perovskite interface limits the efficacy of interfacial charge transport. Herein, it is shown that by incorporating a small organic molecule, bathophenanthroline (BPhen), into the CH3NH3PbBr3 emitter via a solvent engineering technique, the energy band levels of the perovskite can be tailored and the energy mismatch at the HTM/perovskite interface can be ameliorated through the formation of a graded emitter layer and accompanying morphological improvements. With a BPhen concentration of 0.500 mg mL−1, more than ten-fold enhancement of device luminance and efficiency was achieved, thus demonstrating a facile and viable approach for fabricating high-performance perovskite light-emitting diodes (PeLEDs).

Resume : Development of solar energy, is presented as one of the most important challenges posed ceramics in the twenty-first century. Realization of this requirement is associated with the searching of materials absorbing electromagnetic waves in the field of ultraviolet and visible light. One of the potential practical applications, using sunlight is photodegradation of organic impurities, in this phenol compounds in aqueous solutions. The most common compounds of the present heterogeneous catalytic systems are TiO 2 based materials. However, in order to reduce its energy gap (3.2eV) and activation also visible light for the mentioned processes the modification of this compound are performed. Another photocatalytic compouds, in this perovskite structure materials, are also studied. The work shows light absorption abilities of KNbO3 and SrTiO3 doped with V and Fe. The pure and doped samples were investigated especially with using UV-VIS spectrometry and additionaly by XRD, SEM and XPS methods. Photocatalytic properties have been tested in practice by examining the degradation of phenol in aqueous solutions. Both dopants caused improving of the absorption as compared to the undoped samples. The results showed also that the ability to absorb light in a wide wavelength range is not a sufficient condition to improvement of photocatalytic activities in practice.

Resume : The water solubility and toxicity of components in lead containing perovskite thin-film solar cells raise concerns over their implementation as feasible commercial products. Substitution of the lead (Pb) in the perovskite with another post-transition metal such as tin (Sn) could result in a wider acceptance. The best-known efficiency using CH3NH3SnI3 was published in 2014 by Noel, N.K., et al., who achieved a record PCE of 6,4% using the spin-coating technique [1]. We intend to build fully vacuum processed perovskite solar cells. In our labs we could produce and analyse lead (Pb) based perovskite solar cells using Glass/FTO/c-TiO2/m-TiO2/CH3NH3PbI3/spiroMeOTAD/Au by means of spin coating with a highest recorded efficiency of 15,6%. Working on the transition to lead-free perovskite solar cells, a Vacuum Flash Evaporation prototype has been built and tested. Analysis methods include XPS, XRD, SEM, PL, microscopy, solar simulator, etc. As upcoming research, tuning the band gap of the material using mixtures of Bromide (Br-) and iodide (I-) in the perovskite and building working devices could be proven useful for constructing tandem solar cell devices. [1] Noel, N.K., et al., Lead-free organic?inorganic tin halide perovskites for photovoltaic applications. Energy & Environmental Science, 2014. 7(9): p.3061-3068

Resume : The performance of perovskite light-emitting diodes (PeLEDs) has been substantially improved over the past few years. However, the related fundamental physics is still far from adequate. A comprehensive analysis of morphology and growth mechanism of the perovskite light-emitting layers, charge injection and suppression of trap density in cesium lead bromide (CsPbBr3)-based PeLEDs was carried out. A hybrid precursor of poly(ethylene oxide) (PEO) and cesium carbonate (Cs2CO3) was used in the formulation of CsPbBr3-PEO-Cs2CO3-based perovskite light-emitting layer. The results show that the use of PEO and Cs2CO3 precursors benefits the operation of the CsPbBr3-PEO-Cs2CO3-based green PeLEDs in the two ways: (1) the presence of PEO in the emitting layer suppresses both the trap density and non-radiative recombination-induced leakage current, realized through passivating the perovskite crystal boundary defects and decreasing CsPbBr3 crystal grain size; (2) the incorporation of Cs2CO3 helps to improve the hole-blocking and electron-transporting in the active layer. As a result, the CsPbBr3-PEO-Cs2CO3-based green PeLEDs with a maximum current efficiency of 1.18 cd/A were obtained, which is about 6 times higher than that of a CsPbBr3-based control PeLED (0.20 cd/A). The CsPbBr3-PEO-Cs2CO3-based all inorganic green PeLEDs also possess a maximum luminance of 10290 cd/m2.

Resume : The variation of anti-solvent is the key factor in controlling the crystallinity and microstructural properties of perovskite films. In this study, we fabricated (FAPbI₃)₁-x(MAPbBr₃)x planar perovskite solar cells (PSCs) employing various anti-solvents, i.e. chlorobenzene, toluene, ethyl acetate and mixed anti-solvent (MAS) for the in-situ crystallization of perovskite film. The photovoltaic performances of PSCs are concisely studied with respect to the morphology and crystallinity changes of the perovskite films. Our findings demonstrated that different anti-solvents has led to different degrees of crystallinity and morphology in perovskite films. Particularly, MAS has been extremely effective in increasing nucleation density of perovskite crystal and suppressing crystal cracks. Among all of the devices, MAS treated PSC exhibited the highest PCE (17.3%) with negligible hysteresis, owing to improved optical absorption and lesser pin-holes in perovskite film. Additionally, MAS treated PSC also displayed remarkably improved air-stability. In summary, our work herein embodies a facile, low-temperature solution route to fabricate high performance PSCs using one-step preparation method.

Resume : Migration of intrinsic ions (e.g., MA+, Pb2+, I-) have received extensive attention due to their influence on the current-voltage hysteresis and stability for organic–inorganic hybrid perovskite films. Here, we demonstrate planar perovskite solar cells (PSCs) with simultaneously improved device efficiency and stability by introducing an N, N’-diphenyl-1, 1’-biphenyl-4, 4’-diamine (NBP) into CH3NH3PbI3 perovskites materials. The inhibition of migration of intrinsic ions is realized by the cation–π interaction between NPB and MA+, it is found that this resulted into reduction of defects in perovskite films and outstanding stability in devices. Several characterization techniques used to understand the effect of addition of NBP ring. The Benzene rich ring of NBP can enhance the crystallization of perovskites and address the issue of low electron extraction efficiency. By employing the cation-immobilized films to fabricate PSCs, a champion efficiency of 19.56% with negligible hysteresis are acquired. In addition, the stability of the devices has also improved significantly.

Resume : The perovskite solar cells (PSCs) have now achieved a rapid development especially for the ever-increasing power conversion efficiency (PCE), while how to solve the long-term operational stability of solar cells has been a tremendous challenge. Here, we investigated that the effects of Ag+ penetration from Ag electrode into the adjacent hole transporting layer (HTL) in the PSCs, magnified by directly doping Ag+ into the Spiro-MeOTAD layer (HTL) and the essential characterization analysis were also applied. The result distinctly demonstrated that the Ag+-ion migration potentially triggers performance degradation in PSCs. The incorporation of Ag+ brought deep-level defects in the Spiro-MeOTAD layer and consequently caused a deterioration of the hole-transporting ability of Spiro-MeOTAD, leading to degradation of solar cell performance. Understanding the impacts of metal electrode penetration in the HTL will promote advancing the overall performance of PSCs.

Resume : New fabrication techniques for solution-processed organic photovoltaics are more and more desirable due to greater demand for affordable method to make large area. Different from traditional spin-coating, here, we report a new fabrication technique inspired by “liquid-chalk”. Solution can be added into liquid-chalk, and is almost all coated on the devices, while spin-coating will waste most of solution. In addition, large area could be easily achieved with it. Meanwhile, the thickness and roughness could be adjusted by optimizing speed, time of annealing and amount of the solution added. We demonstrated solar cells based on liquid-chalk-coated perovskite photoactive layer (CH3NH3PbI3) exhibit comparatively high power conversion efficiency (PCE) of 13.43% on average. A champion PCE of 16.04% was attained by coating speed of 1cm/s, 12min annealing, with the one added 1mL perovskite precursor solution. The results indicate that using liquid-chalk to coat is an effective new way to fabricate perovskite solar cells, and also an ideal method to make large area and plastic devices.

Resume : The perovskite solar cells (PSCs) have drawn a lot of attention recent years on account of their enormous potentials, and still more and more researchers are fascinated by deliberating how to further increase the power conversion efficiencies. Here, we demonstrated a new method, using the CsCO3-doped TiO2 as electron-transporting layer (ETL) to improve the performance of PSCs. Remarkably, compared with the PSCs whose compositions are normal, it is discovered that open-circuit voltages (Voc) of the doped ones are raised with almost-unaffected short-circuit current density (JSC) and fill factors (FF) under certain conditions. To find out the mechanism of the increase of Voc, several characterization technologies are used.