Next generation of earth-abundant materials for solar energy

Resume : Interest in ZnSnN2 as a narrow gap, earth abundant element alternative to conventional compound semiconductors based on indium and gallium has prompted many to investigate its growth and fundamental properties. We have successfully grown single crystal thin films of this ternary heterovalent compound using a plasma-assisted molecular beam epitaxy technique. Resulting films show a distinct variation in cation sublattice ordering, which can be controlled through careful tuning of key growth parameters including substrate temperature and zinc to tin flux ratio. We have observed variation of the band gap energy as it relates to cation ordering through optical transmission measurements corrected for Moss-Burstein and band gap renormalization effects, consistent with density functional theory predictions. Consequently, there is evidence that the material may be tunable to a band gap energy suitable to terrestrial photovoltaic device applications without the need for traditional alloying approaches.

Resume : Zinc tin nitride (ZnSnN2) is a new semiconductor material with earth abundant elements, non-toxic and a low-cost production. Due to its direct bandgap, tuneable from 1.0 to 2.1 eV due to cation disorder[1], ZnSnN2 is a promising candidate for multijunction photovoltaic cells. This work presents the development of ZnSnN2 thin films by reactive co-sputtering using zinc and tin metallic targets. The stoichiometry of the films was controlled by optimizing operating parameters such as the target voltage, the nitrogen partial pressure or the total pressure. The structure of the films was studied by X-ray diffraction. The microstructure was observed by TEM, bright field and dark field images reveals a columnar microstructure of the material with column diameter of approx. 60 nm. The selected area electron diffraction (SAED) pattern shows the growth direction of thin film of ZnSnN2. The measured dhkl are consistent with the X-ray diffraction. More detailed information about the chemical environment of tin atoms has been obtained using advanced characterization techniques such as Mössbauer spectroscopy. Vibration modes were studied with Raman spectrometry and FTIR spectroscopy. The optical band gap has been deduced from UV-Visible spectroscopy measurements, using the Tauc approach we determined the bandgap of ZnSnN2 to be around 1.8 eV. [1] R. Qin et al., “Semiconducting ZnSnN2 thin films for Si/ZnSnN2 p-n junctions,” Appl. Phys. Lett., vol. 108, no. 14, p. 142104, Apr. 2016.

Resume : Implementation of an optically active material on silicon has been a persistent technological challenge. For tandem photovoltaics using a Si bottom cell, as well as for other optoelectronic applications, there has been a longstanding need for optically active, wide band gap materials that can be integrated with Si. ZnSiP2 is a stable, wide band gap (2.1 eV) material that is lattice matched with silicon and comprised of inexpensive elements. From bulk single crystal growth, we have demonstrated the first ZnSiP2 photovoltaic device, and shown that ZnSiP2 has excellent photoresponse and high open circuit voltage of 1.3 V, as measured in a photoelectrochemical configuration. The high voltage and low band gap-voltage offset are on par with much more mature wide band gap III–V materials. Photoluminescence data combined with theoretical defect calculations illuminate the defect physics underlying this high voltage, showing that the intrinsic defects in ZnSiP2 are shallow and the minority carrier lifetime is 7 ns. The favorable results obtained from characterization of bulk material encourage the development of ZnSiP2 as a photovoltaic absorber material. To pursue this development, we have constructed a thin film growth reactor. This reactor employs a combination of chemical vapor deposition, using silane and phosphine as precursor gases, and physical vapor deposition, using an effusion cell to evaporate elemental Zn. We will present a brief review of our bulk characterization work, followed by the results of ZnSiP2 film growth on (100) Si substrates. The composition, structure, and morphology of these films have been characterized by energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy, x-ray diffraction and transmission electron diffraction, and electron microscopy, respectively. These promising results represent significant advancement towards implementing ZnSiP2 as a top cell material on Si-based tandem photovoltaics.

Resume : Organic-inorganic perovskites are quickly overrunning research activities in new materials for cost-effective and high-efficiency photovoltaic technologies. Since the first demonstration from Kojima and co-workers in 2009, several perovskite-based solar cells have been reported and certified with rapidly improving power conversion efficiency. Recent reports demonstrate that perovskites can compete with the most efficient inorganic materials, while they still allow processing from solution as a potential advantage to deliver a cost-effective solar technology. Compare to the impressive progress in power conversion efficiency, stability studies are rather poor and often controversial. An intrinsic complication comes from the fact that the stability of perovskite solar cells is strongly affected by any small difference in the device architecture, preparation procedure, materials composition and testing procedure. In the present talk, we will focus on the stability of perovskite solar cells in working condition. We will discuss a measuring protocol to extract reliable and reproducible ageing data. We will present new materials and preparation procedures which improve the device lifetime without giving up on high power conversion efficiency.

Resume : Two-dimensional (2D) semiconductors are exciting as they allow for atomic scale electronic devices and high performance catalysis. However, these materials are also observed as impurities in photovoltaic (PV) devices, where their unique materials properties can combat desired device behaviour [1]. For example, in certain PV devices, sulfide-based light absorbers are placed in contact with metals to behave as charge extractors. However, these metals can preferentially react with the sulfur to form 2D disulfides at the junction interface. Such sulfide materials typically underperform when compared to other inorganic materials that have similar properties and the formation of these 2D materials could be the cause [2]. As such, we investigate the popular PV sulfide materials SnS, Cu2InS2 and Cu2ZnInS4 in hypothetical contact arrangements in order to discover if this reaction takes place and, if so, if it is performance limiting. A systematic series of first-principles calculations was performed using the projector augmented-wave method [3] and a hybrid functional as implemented in the VASP code [4, 5]. Band alignment between the sulfides and contact metals was determined using surface slab models [6]. Our screening considers thermodynamic stability and electronic properties to ultimately identify optimal device configurations for improved performance in this important field of renewable energy. Acknowledgments: This work is supported by the JSPS Research Fellow program and the MEXT Elements Strategy Initiative to Form Core Research Center. References [1] L. A. Burton, T. J. Whittles, D. Hesp, W. M. Linhart, B. Hou, R. Webster, C. Reece, D. Cherns, D. J. Fermin, T. D. Veal, V. R. Dhanak, A. Walsh, J. Mater. Chem. A, (2016), 1312. [2] L. A. Burton, D. Colombara, R. D. Abellon, F. C. Grozema, L. M. Peter, T. J. Savenije, G. Dennler, A. Walsh, Chem. Mater. 25, (2013) 4908. [3] P. E. Blöchl, Phys. Rev. B 50, (1994) 17953. [4] G. Kresse and J. Furthmüller, Phys. Rev. B 54, (1996) 11169. [5] G. Kresse and D. Joubert, Phys. Rev. B 59, (1999) 1758. [6] L. A. Burton and A. Walsh, Appl. Phys. Lett. 102, (2013) 132111.

Resume : We have studied the effect of dopants such as Na, Sb, and Li in Cu2ZnSnS4 nanoparticle thin films [1]. The as-synthesized CZTS nanoparticles were inherently ligand-free [2], which allows the use of polar solvents, such as water and ethanol. Another advantage of these particles is that the user- and environmentally-friendly chloride salts can be directly dissolved in controllable amounts. This further circumvents the need for later incorporation of dopants, or a ligand-exchange step to functionalize the surface of the nanoparticles. In addition, the homogeneous distribution of additives in the ink allows uniform grain growth within the deposited absorber layer. By including Na in the nanoparticle ink, micron-sized grains throughout the whole absorber are achieved after annealing in a sulfur atmosphere at 600°C. The absorber layer appeared to be of full density, and no closed porosity could be detected. In addition, the photoluminescence signal increased by a factor of 200 after Na-inclusion. Without Na, the grains were very difficult to sinter, the film was porous, and the photoluminescence was low. This suggests that including Na reduces interface recombination in CZTS nanoparticle absorber layers. A concentration of Na/(Cu+Zn+Sn)=30% was necessary for the densification of the absorber, which is significantly higher than that used in other Na-doped CZTS systems. The annealed films were found to be of the desired Cu-poor and Zn-rich composition. [1] Sara Engberg, Andrea Crovetto, Stela Canulescu, Ole Hansen, Yeng Ming Lam, and Jørgen Schou, Na-assisted grain growth in CZTS nanoparticle thin films for solar cell applications, Submitted 2016. [2] Naghmeh Mirbagheri, Sara Engberg, Andrea Crovetto, Søren Simonsen, Ole Hansen, Yeng Ming Lam, and Jørgen Schou, Synthesis of ligand-free CZTS nanoparticles via a facile hot injection route, Nanotechnology, 27 (2016).

Resume : Mg-doped ZnO (ZnMgO) is an intensively researched material for a wide range of applications, mainly due to its tunable band gap depending on the Mg content. This property is especially attractive for buffer layer in thin film photovoltaics to optimize energy band offsets with various absorber materials. So far, ZnMgO films were mainly deposited by vacuum techniques. In order to decrease capital investment costs for deposition equipment and further the fabrication costs of solar cells, non-vacuum techniques such as solution-based deposition approaches are desirable. In this context we present a simple, low-temperature route for the chemical bath deposition of ZnMgO films based on ammonia solutions of Zn and Mg salts. The film growth mechanism and the resulting surface morphology were investigated in detail by scanning electron microscopy and X-Ray diffraction measurements. It was found that different amounts of Mg in the solution cause distinct ZnO surface textures, which were explained by face-selective adsorption of Mg species onto specific ZnO faces. Mg incorporation into the ZnO lattice was confirmed by XRD peak shifts and optical emission spectroscopy measurements. The optical band gap of the films increases with the Mg content, which is however limited to 2.1 mol% due to reasons of thermodynamic solubility. To show the applicability of the deposited ZnMgO films as buffer layers in thin film photovoltaics a Cu2O/ZnMgO type solar cell was fabricated and characterized.

Resume : Hybrid lead iodide based perovskites (e.g. CH3NH3PbI3) have recently demonstrated outstanding electronic properties and delivered impressive power conversion efficiencies in solar cells (>22%). However, practical implementation of this technology is hampered by several issues associated with such intrinsic properties of the complex lead halides as low photostability and high acute toxicity. Therefore, there is a constantly increasing interest to the development of principally new metal halides with non-perovskite crystal structure. Unfortunately, the best efficiencies reached for perovskite-like complexes based on Bi and Sb compounds stay below 1.5%. In the present report, we will highlight the most recent results of our systematic study, which allowed us to design the first non-perovskite type complex metal halides delivering competitive performances in solar cells. In particular, external quantum (incident photon to collected electron) efficiencies of the best systems approached 50-80%, which is comparable to the reference perovskite solar cells. Discovering a new family of the lead-free metal halides with non-perovskite structure enabling efficient operation of solar cells paves a route towards design of a new generation of light absorbers for emerging ?perovskite-inspired? solar cells.

Resume : Our spraying process of water-ethanol based CZTS colloid followed by an annealing in Selenium vapor allowed us recently to reach efficiencies up to 11.5%. This achievement was made possible by a thorough optimization of the stoichiometry of the active layer as well as a proper management of the incoming light. Furthermore, this industrially applicable and easily up-scalable process allowed us to produce 1.1 cm² devices certified at 10%, which entered recently the Table I of the Efficiency Tables published by Progress in Photovoltaics. Besides, preliminary lifetime tests indicated that these solar cells may succeed to pass the standard accelerated lifetime tests without major difficulties: Indeed, non-encapsulated cells left under simulated AM1.5G irradiation and open circuit voltage condition for more than 5000 hours were observed to lose only about 10% of their initial efficiency. In spite of the fact that these achievements appear encouraging, especially in view of the simplicity of our process, these devices still suffer from major losses that reduce severely their photovoltaic performances. During this talk, we will present our latest results, discuss various potential source of losses like the anionic and cationic disorders, and propose some ways forward.

Resume : Despite progress and developments of quaternary Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTS). today solar cells with initial efficiency of about 12.6%, further understanding of CZTS as well as ways for its improvement is necessary. The control of formation/stability of point defects is one of promising approaches for increase of efficiency and stability of CZTS solar cells, because defects are one of major factors, related to phase formation, electrical and optical properties. Because of this, we perform in this work hybrid functional calculations of not only CZTS but also its alloys with Ge or Si. In particular, we analyze trends in electronic properties of CZTS materials with Ge or Si doping at different dopant concentrations. We further analyze optical properties of the materials in term of dielectric function and absorption coefficient. Based on the predicted results, we expand understanding of Cu-based solar cell materials and ways for controlling CZTS properties by doping. C. Persson, R. Chen, H. Zhao, M. Kumar and D. Huang, in Copper Zinc Tin Sulfide-Based Thin-Film Solar Cells, John Wiley & Sons Ltd, 2014, DOI: 10.1002/9781118437865.ch4, pp. 75-105. W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu and D. B. Mitzi, Adv. Energy Mater., 2014, 4, 1301465

Resume : Antimony selenide is an emerging absorber layer for thin-film solar cells. It is of particular interest, not only due to being a binary, stable and sustainable inorganic absorber, but further because of an interesting nanoribbon grain structure which should enable benign grain boundaries [1]. This work reports on the development of a close space sublimation deposition route, widely used for CdTe solar cells, to produce Sb2Se3 devices in a similar superstrate configuration. Cell efficiencies of up to 5.4% have already been achieved by this method close to the leading efficiency of 5.6% for this material [1]. Device and materials characterisation has been used to examine device behavior and comparisons with cells produced via other deposition techniques such as thermal evaporation have been made. Additional discussion will show the high degree of materials variance that can be achieved using this technique from films with large (>30µm) grain size, to nanowires and dendritic nanostructures through subtle variation of the growth conditions. The influence of the choice of n-type partner layer on device performance as well as the influence of the back contact structure and post-growth processing options will also be reported and opportunities to further improve performance discussed. [1] Y. Zhou et al, Thin-film Sb2Se3 photovoltaics with oriented one-dimensional ribbons and benign grain boundaries, Nature Photonics, 9, 409, 2015.

Resume : In this work, we have performed a throughout study on the formation mechanism of copper antimony sulfide nanoparticles, synthesized by hot injection. In a three-neck flask, 0.45 mmol of CuCl and SbCl3, and 7 mL of oleylamine (OLA) are added and heated to the synthesis temperature (ranging from 200 °C to 250 °C). In another flask, 1 mmol of sulfur and 3 mL OLA were added and heated to 60 °C, under stirring. This solution was injected in the three-neck flask and the reaction was kept for 10 minutes. Aliquots were collected after 1, 2 and 5 minutes. The nanoparticles were characterized by UV-VIS-NIR spectroscopy, transmission electron microscopy, Raman spectroscopy, and X-ray diffraction (XRD). XRD patterns and Raman spectra show the initial formation of Cu3SbS4 and, as the reaction progresses, Cu3SbS4 is converted to CuSbS2. The oxidation state of Sb is found to control the stoichiometry of the formed particles; the oxidation of Sb in the Sb2O3 (promptly formed from SbCl3) by sulfur drives the initial formation of Cu3SbS4. Interestingly the transformation from Cu3SbS4 to CuSbS2 is kinetically controlled by Cu3SbS4 reduction by OLA, at high temperatures. Therefore, controlling the oxidation state of antimony is crucial to select which phase will be formed. To the best of our knowledge, there is no previous work in the literature explaining the role of sulfur and OLA and the importance of the Sb oxidation state on the synthesis of copper antimony sulfide nanoparticles.

Resume : The emergence of methyl-ammonium lead halide perovskites (MAPI) motivated investigation of the properties responsible for exceptionally long carrier lifetimes in these material systems. While slow radiative recombination can be attributed in part to the beneficial electronic structure of MAPI (k-splitting of the band edges due to large spinorbit interactions [1]), comparably low non-radiative recombination rates are a consequence of its apparent insensitivity to the presence of defects. This “defect tolerance” is a trait of MAPI and other unconventional semiconductor systems in which the prevalent point defects introduce only shallow electronic levels that do not affect appreciably transport of the photo-generated charge carriers. It has been discussed previously that the defect tolerance of semiconductors emerges from the low valence state of the ions forming the compound and the resulting features in its electronic structure [2]. Based on this analysis we conducted a broad materials search for systems that exhibit such features [3]. The results of our screening revealed several classes of defect tolerant semiconductor materials. Six distinct members of these material classes were subject to the synthesis and measurements of their minority carrier lifetimes all exceeding 1ns, a threshold for promising early-stage PV device performance. These results, while validating the initial screening criteria, also revealed the need for more nuanced model of defect tolerance that in addition to low valence includes good matching of the atomic orbitals of the constituent ions as well as the descriptors of the crystal structure. In this talk I will review our efforts in searching for defect tolerant semiconductors and discuss the physical quantities governing this interesting and important phenomenon. This work was supported as part of the Center for the Next Generation of Materials by Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. [1] P. Azarhoosh, S. McKechnie, J. M. Frost, A. Walsh, and M. van Schilfgaarde, APL Mater. 4, 091501 (2016) [2] A. Zakutayev, C. M. Caskey, A. Fioretti, D. S. Ginley, J. Vidal, V. Stevanovic, E. Tea, and S. Lany, J. Phys. Chem. Lett. 5, 1117 (2014) [3] R. E. Brandt, V. Stevanovic, D. S. Ginley, and T. Buonassisi, MRS Communications 5, 265 (2015)

Resume : Sb2Se3 is of unusual interest as a solar PV absorber as oriented films of its van der Waal’s bonded 1D chain structure offer the possibility that its grain boundaries do not contain dangling bonds and are hence immune to recombination. Devices of this absorber (Eg = 1.17 eV; α > 10 cm-1) have already reached the 5% level. We present film preparation, processing and device results. Thermal evaporation from the binary at RT and 300-550°C yielded discontinuous films at high temperature that had optical band gaps comparable to that expected for amorphous material (1.7 eV), rendering them unsuitable for device use. Hence the RT-deposited films were processed by annealing in an inert atmosphere at 300-450°C for 20 mins. This yielded material having band gaps closer to the expected value which was crystalline having grains in the range 200 – 400 nm. Devices were fabricated from the whole set of annealed films and had the structure: Au contact/Sb2Se3/window layer/TEC5 SnO2:F coated glass. For CdS/ZnO windows, the best performance achieved at the time of writing was for a film annealed at 350°C (η = 3.15%, Voc = 0.38V, Jsc = 18.3 mAcm-2, FF = 45%). Work is ongoing to explore the effects of alternative window layers having favourable band line-ups i.e. ZnS and TiO2. Preliminary work has indicated that sulfur containing window layers diffuse into the absorber changing the band gap and affecting the EQE response. The comparison of window layers is designed to investigate this further.

Resume : The hybrid lead halide perovskites have risen to become some of the highest efficiency single-junction solar absorber materials, with the record cells now matching those of CdTe and CIGS above 20% efficiency.[1,2] Despite their success, the inclusion of lead remains a problem towards manufacture and commercialisation, with encapsulation required to limit environmental damage, and instability of the perovskite layer can lead to performance degradation. One potential avenue for lead-free perovskite materials is to move from the APbX3 (A = MA, Cs) formula to the double perovskite A2MBiX6 (A = Cs, M = Ag, Cu, Au), replacing lead with the much less toxic bismuth. In 2016, multiple groups synthesised and examined members of the novel Cs2AgBiX6 family as stable non-toxic replacements to the lead perovskites.[3–5] Ultimately, however, Cs2AgBiCl6 and Cs2AgBiBr6 both possess high, indirect band gaps, despite the promising results of a long photoluminescence lifetime and apparently robust stability. Recently, however, Cheetham and co-workers synthesised (MA)2AgBiBr6, which retains the indirect gap of its Cs analogue, and (MA)2TlBiCl6,[6] which instead possesses a direct band gap of 2.1eV. In this study, we examine the Cs2MBiX6 (M = Ag, In, Tl; X = Cl, Br, I) family using hybrid Density Functional Theory and fundamentally evaluate their electronic structure, stability and cation disorder. With particular attention to their comparison with the APbX3 perovskites, we establish how inherent orbital interactions can significantly affect their suitability as photoabsorbers.[7] In addition, we examine how the stability of competing phases could affect further synthetic attempts in this area. As such, we hope to provide an outlook for the future of Pb-free double perovskite solar absorbers. References (1) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Prog. Photovoltaics Res. Appl. 2016, 24, 3. (2) Brenner, T. M.; Egger, D. A.; Kronik, L.; Hodes, G.; Cahen, D. Nat. Rev. Mater. 2016, 1 (1), 15007. (3) McClure, E. T.; Ball, M. R.; Windl, W.; Woodward, P. M. Chem. Mater. 2016, 28 (8), 1348. (4) Slavney, A. H.; Hu, T.; Lindenberg, A. M.; Karunadasa, H. I. J. Am. Chem. Soc. 2016, 138 (7), 2138. (5) Volonakis, G.; Filip, M. R.; Haghighirad, A. A.; Sakai, N.; Wenger, B.; Snaith, H. J.; Giustino, F. J. Phys. Chem. Lett. 2016, 4, 1254. (6) Deng, Z.; Wei, F.; Sun, S.; Kieslich, G.; Cheetham, A. K.; Bristowe, P. D. J. Mater. Chem. A 2016, 4, 12025. (7) Savory, C. N.; Walsh, A.; Scanlon, D. O. ACS Energy Lett. 2016, 1, 949.

Resume : We present in this work several materials actively studied as good absorbers for photovoltaic or photocatalytic applications. These materials have been proposed to boost photovoltaic efficiency through coupling the absorption of two low energy photons to achieve a higher energy electron excitation. We have verified with accurate density functional theory (DFT) calculations and beyond, that semiconductors as chalcopyrite CuGaS2, spinel In2S3 or layered SnS2, can provide this situation when a cation in their structure is partially substituted by an element such as transition metal. Experimental work made via wet chemistry methods has been done for several of these materials. These studies verify that new absorption features appear in the optical absorption spectrum which matches the predicted DFT-based theoretical absorption results. Other material actively studied as novel photovoltaic material is the perovskite (CH3NH3PbI3). We have computed its electronic structure and relevant properties with accurate DFT using hybrid functional combining spin-orbit effects. Fast dynamics and large diffusion lengths of the current carriers are keys for the high photovoltaic efficiencies shown by these materials. Computational calculations of lead-free halide double perovskites will be presented at the conference

Resume : The present work reports on photonic sintering of ZnO nano-sheets (ZnO NS) synthesized via a solid state synthesis method. The sintering is performed using flash white light (FWL) combined with deep-UV irradiations under ambient conditions with ultra-high speed as compared to the conventional thermal sintering process. The influences of FWL energy and pulse on sintering are studied using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) technique, and X-ray photoelectron spectroscopy (XPS). The mechanism and effects of photonic sintering on DSSCs is systematically studied, and the advantage of photonic sintering over the conventional thermal sintering is clearly demonstrated by constructing dye-sensitized-solar cells (DSSCs) consisting of photonically and thermally sintered ZnO NS photoanode. A power conversion efficiency (PCE) of 2.9% is obtained when photoanode sintered using an FWL of 20 J/cm2 combined with a deep-UV of 30 mW/cm2 is used. This PCE is nearly two folds higher than that of the pristine ZnO NS (PCE =1.5%). In contrast, a PCE of only 2.0% is obtained when the thermally sintered ZnO NS photoanode is used. The improved PCE suggests that the FWL sintering method is extremely simple and highly effective for improving the performance of photoanodes in DSSCs, and is very advantageous particularly for low-temperature-based solar cells.

Resume : We investigated the characteristics of roll-to-roll sputtered ITO films on 60 μm-thick CPI substrate for use as transparent anodes for high-performance perovskite solar cells. Due to the thermal stability of the CPI substrate, the ITO deposited on CPI and subjected to rapid thermal annealing at 300 °C showed very low sheet resistance of 57.8 Ω/square and high transmittance of 83.6 %, which are better values than those of an ITO/PET sample. Outer and inner bending tests demonstrated that the mechanical flexibility of the ITO/CPI sample was superior to that of a conventional ITO/PET sample due to the thinness of the CPI substrate. In addition, in dynamic fatigue tests with both outer and inner bending, the ITO/CPI films showed no change in resistance after 10,000 cycles due to their good mechanical flexibility. Flexible perovskite solar cells with the structure of Au/PTAA/MAPbI3/ZnO/ITO/CPI showed a high power conversion efficiency of 15.5%. The successful operation of flexible perovskite solar cells on ITO/CPI substrate indicated that the ITO film on thermally stable CPI substrate is a promising of substrate and can advance the commercialization of cost-efficient flexible perovskite solar cells.

Resume : Electrocatalytic conversion of CO2 to value-added hydrocarbons is receiving significant attentions as a promising way to close the broken carbon-cycle. While most metal catalysts produce C1 species, such as carbon monoxide and formate, various hydrocarbons and alcohols comprising more than two carbons have been achieved using copper (Cu)-based catalysts only. Methods for producing specific C2 reduction outcomes with high selectivity, however, are not amenable thus far. In this work, the morphology effect of a Cu mesopore electrode on the selective production of C2 products, ethylene or ethane, is presented. The Cu mesopore electrodes with precisely controlled pore widths and depths were prepared by a sputtering process on anodized alumina oxide. By this simple synthesis method, we demonstrated that C2 chemical selectivity can be tuned by systematically altering the morphology. In detail, ethylene and ethane were exclusively produced with Faradaic efficiencies of 38 % and 46 %, respectively. Compared with the previously reported current efficiency for ethane formation (6 %), this is the highest value ever achieved. Moreover, supported by specific activity analysis and computational simulation, we proved that nanomorphology can change the local pH and additionally, retention time of key intermediates by confining the chemicals inside the pore. This approach exploits new strategy for systematically controlling product selectivity and reaction kinetics of CO2 reduction.

Resume : In order to enhance the conversion efficiency of CZTS solar cells, both large grains with few defects and high carrier mobility are important. Here, Si-doped Cu2ZnSnS4thin films are prepared via sputtering. The influence of the Si doping concentration on the CZTS film macro and microstructures are examined using X-ray diffraction and scanning electron microscopy. In addition, the optical and electrical properties of the CZTS thin films with different Si doping concentrations are examined. The high mobility 771cm2v-1s-1was observed when the doping concentration was 0.8%. Thus, a CZTS thin-film solar cell with a proof of concept power conversion efficiency of 4.6% was fabricated when the doping concentration was 0.5%. CZTS have been widely found to show the p-type conductivity intrinsically but n-type samples still remain as a problem and have not been reported because CuZn antisite acceptor defects are easy to form in these kesterite structure. In this work, the calculation reveals that the formation energy of CuZn acceptors will be higher than after Si doping, which makes it possible to form low concentration donor Cuzn antisites. Which makes it possible to form high concentration donor ZnCu antisites. Finally a solid inversion from p-type to n-type is obtained.

Resume : It is well-known that Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells fabricated by sulfurization after selenization (SAS) of metallic precursor can reach over 22% efficiency. SAS is widened-used in CIGSSe solar cells. The beneficial effects of surface sulfurization were also reported in CIGSSe solar cells. However, the annealing process of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is still based on selenization only. The studys of SAS process and surface sulfurization on CZTSSe solar cells is few so far. In this work, we demonstrate a simple route to fabricate CZTSSe absorber with increased-sulfur content surface by using SAS process of sputtered precursors. The sulfur-modified surface is formed by flowing H2S gas during cooling stage of the selenization process. The effects of surface sulfurization and H2S concentration on composition, microstructure, and electric properties of CZTSSe are thoroughly investigated. By flowing the H2S gas, the open circuit voltage can be increased significantly due to the increment of front surface bandgap and reduction of interface recombination. S/(S+Se) ratio can be controlled simply by increasing the sulfurization time. Increased H2S concentration also promotes the grain growth of CZTSSe absorber. Finally, the CZTSSe solar cell efficiency of 7.8% can be achieved by this method.

Resume : Thin films of Cu2O comprised of wavelike surface characteristic of compact nanoparticles were synthesized using a facile and cost-effective electrodeposition approach. The Cu2O films were synthesized by electrochemically reducing the copper precursors in presence of complexing agents under pH 11 and chronoamperometrically at a fixed potential. The distinct surface morphologies with well-aligned crystal orientation were obtained through the controlled electrodeposition parameters. The high resolution AFM combined with the peak force AFM images mapped the nanomechanical and chemical properties of the Cu2O nanostructured films. The structural, optical, and compositional analyses of the as-deposited thin films show bulk Cu2O material with preferred texture of (111) and trace amounts of CuO on the top layers as a result of the surface oxidation during exposition to atmospheric conditions. The electrodeposition approach could proceed non-intermittently under ambient conditions, and provides a facile and economic way of depositing thin films of Cu2O with wavelike characteristics. The fluorescence lifetime was found be very short in the range of 0.8-1.3 ns for Cu2O films. The Mott-Schottky measurement exhibited p-type conductivity and carrier density was found to be ~2x1018. The observed fluorescence lifetimes, and carrier densities could help implementing the Cu2O films as an efficient hole-conducting, and photoelectrode materials in solar cells and water splitting devices.

Resume : Despite the great achievement of the solution-processed organometal halide perovskite solar cells (PVSCs) during the past few years, the commonly observed large variability of device performance from one-step to two-step process and lab to lab environment has been still elusive. Herein, to elucidate the occurred issue, the processing windows of perovskite films prepared by one-step and two-step solution processes have been investigated in details along with the corresponding electronic properties. It is found that both perovskite films prepared by one-step and two-step process in air ambient exhibit much poor morphology performance than that prepared in well-controlled glovebox with the same process, which make it not suitable for high efficiency device fabrication. More importantly, complete perovskite conversion can be achieved for one-step method processed both in air and in glovebox ambient. While, it is found more difficult to get a uniform perovskite film under air ambient by adopting the two-step method, the residual amount of PbI2 even larger than the obtained perovskite. Besides, a systematical comparison of the photovoltaic performance and electronic properties of the PVSCs prepared by one-step and two-step method in glovebox are also conducted. It is revealed that the worse photovoltaic performance of two-step based PVSCs is ascribed to the stronger trap-assisted recombination process induced by larger trap density in the perovskite film, which is further confirmed by the steady state photoluminescence, transient photovoltage and photocurrent characteristics. Finally, we make a systematic comparison of one-step and two-step perovskite fabrication method by studying the processing windows and electronic properties of the fabrication methods, a high-performance device adopted poly-TPD as the hole transporting material with a power conversion efficiency (PCE) of 17.5% is also achieved.

Resume : Spectral dependent modulated surface photovoltage (SPV) can be a powerful tool to study buried interfaces between lead halide perovskite and charge-selective contact layers. SPV has been applied to the characterization of CH3NH3PbI3, CH3NH3PbBr3 and CH3NH3Pb(I0.75Br0.25)3 layers deposited from solution. The onset energies, amplitudes and exponential tails (given by the energy Et) of SPV signals were investigated for different metal oxide and organic substrates, storage time in N2 and light soaking with blue and red LED light. The highest degree of disorder, i.e. the highest Et, was observed on TiO2. On PEDOT:PSS, the disorder increased with increasing storage time in N2, in contrast to SnO2 substrates showing constant or even decreasing disorder. Furthermore, light soaking was strongly influenced by the storage time in N2. The SPV measurements indicate that the interaction at interfaces between substrates and lead halide perovskite can strongly influence the electronic properties of a lead halide layer and its stability.

Resume : Recent advancement in processing thin organic-inorganic hybrid perovskite films has remarkably improved the performance of perovskite solar cells with their outstanding optoelectronic properties. However, only a few works have focused on the micropattern printing of a perovskite film which is one of the most critical issues in a variety of optoelectronic devices beyond solar cells. Here we demonstrate a simple but robust method of micropatterning a thin perovskite film with controlled crystalline structure which guarantees to preserve its intrinsic photoelectric properties. A variety of micropatterns of a perovskite film are fabricated by either microimprinting or transfer-printing a thin spin-coated precursor film in soft-gel state with a topographically prepatterned elastomeric poly(dimethylsiloxane) (PDMS) mold, followed by thermal treatment for complete conversion of the precursor film to a perovskite one. The key materials development of our solvent assisted gel printing is to prepare a thin precursor film with a high-boiling temperature solvent, dimethyl sulfoxide. The residual solvent in the precursor gel film makes the film moldable upon microprinting with a patterned PDMS mold, leading to various perovskite micropatterns over a large area. Our nondestructive micropatterning process does not harm the intrinsic photoelectric properties of a perovskite film, which allows for realizing arrays of parallel-type photodetectors with micropatterns of a perovskite film with reliable photoconduction performance. The facile transfer of a micropatterned soft-gel precursor film on other substrates including mechanically flexible plastics can further broaden its applications to flexible photoelectric systems.

Resume : The organic/inorganic hybrid solar cells, i.e., dye-sensitized solar cells and perovskite solar cells, have recently attracted much attention because of the relatively high conversion efficiency regardless of the simple fabrication processes. These solar cells have an electron transport layer which is commonly made of TiO2 compact and mesoporous layers. Since the band structure of electron transport layer is one of the parameters to determine the performance of solar cells, the band structure tuning of the electron transport layer is effective to enhance the cell performance. BaTiO3 has similar bandgap to TiO2 but the conduction band locates on the more negative position than that of TiO2. In this study, we have prepared the dye-sensitized solar cells and perovskite solar cells using BaTiO3/TiO2 double mesoporous layer to optimize the band structure of electron transport layer. The solar cells using TiO2 single mesoporous layer was also prepared as a comparison. The mesoporous layers were prepared by spin-coating BaTiO3 or TiO2 pastes. The BaTiO3/TiO2 double mesoporous layer realized a decrease of electron recombination and showed improved cell performance compared with that of TiO2 single mesoporous layer.

Resume : The origin of the impact of mobile ion in perovksite solar cells (PVSCs) has recently become a hot topic under debate. Here, we investigate systematically the structural effect and various recombination pathways in PVSCs with different ion concentrations. By probing the transient ionic current in PVSCs, we extract mobile ion concentrations in a range of 1016 cm-3 to 1017 cm-3 depending on the processing conditions during a two-step process. The PVSC with the lowest ion concentration has both the highest efficiency over 15% and shelf-life over 1300 hours. Interestingly, in contrast to the commonly adopted models in literatures, we find that the crystal size and the bimolecular and trap-assisted recombination are not responsible to the large difference in photovoltaic performance. Instead, by using transient photocurrent and steady-state photoluminescence approaches, we find that the large reduction of short-circuit current (Jsc) in mobile ion populated device is ascribed to the slow decay in photocurrent and the increasing amount of non-radiative recombination. In addition, we also find that the excess mobile ions trigger the deformation of perovskite to PbI2, which severely reduce the device lifetime. The results provide valuable information on the understanding of the role of excess mobile ion on the degradation mechanism of PVSCs.

Resume : In this paper the holes? mobility for the configuration FTO/TiO2/CH3NH3PbI3/Spiro-MeOTAD/Au was measured for the first time by the Photo-CELIV method. The TiO2 dense film was deposited by reactive sputtering at room temperature on FTO glass substrates. High crystalized perovskite films were deposited from solutions in one step by spin coating. Spiro-MeOTAD molecular glass was used as holes transporting layer. The highest holes? mobility from TiO2 thin film through the perovskite and Spiro MeOTAD film to the top gold electrode was of order 8.5 x 10-7 cm2/Vs.

Resume : Due to the discrete band structure of semiconductors, only photons with energies equal or greater than the band gap energy (Eg) will be absorbed and contribute to the electrical photovoltaic solar cell output. Photons having higher energies than Eg, even they are absorbed, their energies are underutilized due to the thermalization of charge carriers. In order to reduce these spectral losses and increase the energy conversion efficacy, many strategies were considered, such as: multi-junction cells (multiple semiconductors stacked cells, intermediate band semiconductors solar cell, up and down converters. Up and down converters are based on rare earth doped materials which may modify the photons energies in order to adapt them to the corresponding value of the band gap of the active material. Hence, the advantage of this concept is that one that it could be applied to all types of solar cells. Since the 50’s different materials and technologies were tested in order to increase the conversion efficiencies and to reduce the fabrication costs. Today world records conversion efficiencies for single junctions solar cells without concentrators are of 28,8% for thin film GaAs, of 25% for single crystal monocrystalline Si, 21.7% for CIGS thin films, 19.3 % for perovskites cells, 13.4% for amorphous silicon thin films solar cells, 11.9% for dye –sensitized cells and of 11.1 % for organic solar cells. In this talk we present the state of art and the new trends in solar cells research.

Resume : Instabilities in perovskite solar cells (PSC), primarily due to water, have hindered their commercialization. Although tremendous efforts have been dedicated to understanding their degradation mechanism, the roles of small polar molecules (MA+ and H2O) on the surface and bulk degradations of PSCs are still far from clear. Herein, the bulk and surface degradation processes of the perovskite were differentiated by employing combinational studies using electrochemical impedance spectroscopy (EIS), Capacitance-Frequance (CF) and Mott-Schottky studies, with particular attention on the roles small polar molecules (MA+ and H2O). It was demonstrated that the absorption of water, which is believed to be the first step of the perovskite degradation, will lead to an initial increase in the Jsc. Similar initial increase in Jsc is also observed upon illumination which induces an electrical field and aligns the MA+ ion at the MAPbI3/C60 interface. On the basis of EIS and XRD analysis, we show that the bulk degradation of PSC involves a lattice expansion process, which facilitates the MA ions diffusion by creating more efficient channels. These results provide a better understanding on the roles of small polar molecules on degradation processes in the bulk and on the surface of the perovskite film.

Resume : In the present work, Cu2ZnSnS4 (CZTS) thin film was deposited onto the glass substrate by spin coating technique. The synthesized thin films have characterized by X-ray diffraction (XRD) and UV visible spectrometer. The X-ray diffraction studies confirms the formation of Kesterite structures with preferential orientation along (112) direction. The absorption spectra of the thin film were characterized using an ultraviolet–visible–near infrared spectrophotometer. And its structural, optical and electrical properties compared with density functional theory based on Wien2K (FP-LAPW) simulation employing a combined Tran–Blaha modified Becke–Johnson (TB–mBJ) potential with scissor corrections. The estimated lattice constant and band gap value is in good agreement with earlier reported results. The optical band gap energy of the thin films CZTS is quite close to the optimum value for solar cell

Resume : In this work, undoped and Fe doped Copper oxide CuO thin films were deposited on glass substrate using a spin-coating sol gel technique, the influence of Fe ions with different dopant concentration and annealed at different temperatures and different times on the structural, optical properties was studied. The crystal structure of films was characterized by X-ray diffraction (XRD). The scanning electron microscopy (SEM) micrographs and UV–vis absorption spectra have also been taken to elucidate the structure and composition of the all films. Absorbance spectra showed that CuO film has absorbance in the visible region. These experimental results were compared by the theoretical study for which we use the density functional theory DFT calculations using the modified Becke–Johnson (mBJ) approximation to study the structural electronic and optical and thermal proprieties of pure and Fe-doped CuO compound. The structural properties, the band gaps, density of states (DOS), the profiles of the optical and absorption spectra, including the real and imaginary parts of dielectric function, reflectivity, refractive index, the electrical and thermal conductivity are obtained. Beside this, our results indicate that CuO is an antiferromagnetic p-type semiconductor material with an indirect gap, and this are showed a good agreement with many experimental data. In fact, this confirms the physical characteristics that can present CuO to be used as suitable absorbent material in solar cells.

Resume : The molecular metal-organic complex on carbon nanotube building-block construction for the carbon nanosystems self-organization investigation is a part of general scientific area cluster-assembled. The aim of tjis research is structural modeling, creation and nano-scale characterization of biohybrid CNT/CdS cluster with modified by biomolecule (cysteine) carbon nanotube and attached by biomolecule to the CdS nanoparticle (quantum dot) as high - photoactive building block for nanosystems in the UV-visible range (photodiodes arrays, photovoltaic structures, solar cells). The control of chemical bonds and molecular structures of CNT’s – COOH and CNT’s –COOH - cysteine before and after CdS’s nanoparticles attaching in suspensions to confirm hybrid complex based on this tube formation by nanoimages in suspensions and adsorbed layers investigation.

Resume : Energy nourished human race, and the seek for more has always been one of the motivations for our civilization. The exploitation of solar energy has been an important source of power, and compared to solar cell, solar fuel system can be more versatile, which stores energy of sunlight into chemical bonds. The most discussed route may be water splitting accelerated by photoelectrodes to produce hydrogen and oxygen. Oxygen generation half reaction is both kinetically and thermodynamically demanding than proton reduction half reaction, thus considered as the bottleneck of the whole water splitting reaction. Water oxidation reaction is a multielectron process, thus water oxidation catalyst is often required. Adopted by Fujishima and Honda, TiO2 has been a favoured photocatalyst for subsequent studies. Nanosized anatase titania comprises high catalytic ability for oxygen evolution and large surface area therefore is of great interest in this project. To fully understand the catalytic mechanism and seek a way to rationally optimize oxygen evolution reaction, extensive studies have been made. The aim of this project is to peek into the working mechanism of water oxidation reaction on the side of TiO2 photo anode by adopting photoInduced absorption spectroscopy, rate law analysis and kinetic isotopic effect, and the results will validate or invalidate the existing mechanisms for the crucial oxygen evolution reaction, and in turns bring insight into optimization of the catalytic cycle of OER.

Resume : This paper presents analytical results for efficiency potential of high-efficiency solar cells such as crystalline Si, GaAs, CIGSe and CdTe solar cells based on external radiative efficiency (ERE), open-circuit voltage loss and fill factor loss. Crystalline Si solar cells have efficiency potential of 28.5% by improvement in ERE from around 1% to 20%. GaAs cells have efficiency potential of 29.7% by improvements in ERE from 22.5% to 30%. CIGSe and CdTe solar cells have potential efficiencies of 26.5% by improvements in ERE from 0.5% to 10% and from around 0.1% to 5%, respectively. Efficiency potential of future generation solar cells such as CZTS and CZTSSe solar cells, MQW and QD solar cells, Perovskite solar cells and ferroelectric solar cells is also discussed based on ERE, open-circuit voltage loss and fill factor loss. In summary, 1) CZTS and CZTSSe solar cells have efficiency potential of more than 20% by improvement in ERE from around about 0.001% to 1%. 2) MQW and QD cells have efficiency potential of 25.8% by improvements in ERE from around 0.1% to 10%, respectively. 3) Perovskite cells have potential efficiencies of 24.9% by improvements in ERE from about 0.02% to 10%, respectively. Further improvements in minority-carrier lifetime based on understanding defect behavior in addition to improvements in front surface, rear surface and interface passivation and decease in series resistance and shunt resistance is suggested in order to realize higher efficiency solar cells.

Resume : Toyota Central Research and Development Laboratories reported the development of a rare metal-free Cu2(Ge,Sn)S3 solar cell with an efficiency of 6.0% [1]. We reported on the crystallographic and optical properties of the narrow band gap Cu2GeSe3 and Cu2(GexSn1-x)Se3 solid solution [2]. However, there are few studies on the crystal structure, optical properties, and band diagram of the Cu2(GexSn1-x)S3 solid solution. In this study, we synthesized Cu2(GexSn1-x)S3 (x=0.0-1.0) powders through planetary ball milling and heating at 600 oC in a H2S gas atmosphere. We determined the band-gap energies from the diffuse reflectance spectra. We estimated the energy levels of the valence band maximum (VBM) from their ionization energies measured by photoemission yield spectroscopy (PYS). The energy levels of the conduction band minimum (CBM) could also be determined by adding the value of the band gap to the VBM level. The band-gap energy linearly increased from 0.87 eV for Cu2SnS3 to 1.53 eV for Cu2GeS3 with increased Ge content. The energy level of the VBM of the Cu2(GexSn1-x)S3 solid solution was almost constant. On the other hand, the energy level of the CBM increased rapidly with increased Ge content. We discuss the band structure of the Cu2(GexSn1-x)S3 solid solution by comparison with those of other absorber materials, such as Cu(In,Ga)Se2 and Cu2Zn(Ge,Sn)S4. [1] M. Umehara et. al., Appl. Phys. Express 6, 045501 (2013). [2] M. Morihama et. al., Jpn. J. Appl. Phys. 53, 05FW06 (2014).

Resume : Morphologies and nanostructure engineering play an important role in developing high performance photoelectrodes of photoelectrochemical water splitting. Large porous three-dimensional nanostructure composites as photoelectrode have exhibited excellent mass transport and charge transport properties. In this work, we prepared three-dimensional nanostructures of semiconducting transition metal oxides, such as TiO2 (anatase phase) and Fe2O3 (hematite) by nanocellulose templating using sol-gel process. We investigated photoelectrochemical performances for nanocellulose-TiO2 and -Fe2O3 composites and thermal treated nanostructures. We found that thermal treatments enable rapid photogenerated electron-hole separation and transport, leading to higher photoelectrochemical efficiency, which are associated with defect formation in oxide system and carbonization of cellulose nanofibers. Nanocelluose-photoactive oxide composites are promising candidate for high efficient photoelectrode for solar water splitting.

Resume : The current challenges like stability, efficiency or hysteresis in hybrid perovskite solar cell are often correlated with surface and interface properties of the used materials. For instance, efficient generation, extraction, and transport of charges with their minimum recombination through interlayer interfaces are crucial to obtain high efficiency solar cell devices. In this work, we use surface sensitive X-ray photoelectron spectroscopy technique to study different hybrid perovskite films, Al2O3 / perovskite interface and the water / (Al2O3) / perovskite system and try to correlate how surfaces and interfaces can impact material properties and device performance.

Resume : The efficiency of crystalline Si (c-Si) solar cells has reached 26.33% and the room for efficiency improvement has become smaller. A straightforward approach to achieve higher efficiency is to fabricate a top cell with the bandgap of 1.7 to 2.3 eV on a c-Si cell. Orthorhombic BaSi2 is one of the candidate materials for solar cells because barium and silicon are abundant in the earth's crust. Since the bandgap of BaSi2 is 1.3 eV, it is required to increase the bandgap of BaSi2 to apply to the top cell material. First-principle calculation predicted that the bandgap of BaSi2 can be increased by substituting carbon for a part of Si(8d). In this study, we attempted to grow BaSi2 on 3C-SiC-on-Si heteroepitaxial wafer to increase the bandgap. Thermal evaporation of BaSi2 powder was employed as an industry-compatible growth method. The 3C-SiC-on-Si wafers provided by AirWater were used and they contain approximately 1-m-thick 3C-SiC film. The substrate temperatures were chosen as 500 to 600 ˚C to permit realization of BaSi2 on Si based on our previous study. As a result, approximately 200-nm-thick BaSi2 films were obtained on 3C-SiC at 550 and 600 ˚C. However, a BaSi2 film was not formed at 500 ˚C. This suggests that higher substrate temperature is required to grow BaSi2 films on 3C-SiC compared with those on Si. Raman shift and EDX suggested that diffusion of C in the films especially in the sample grown at 600 ˚C.

Resume : Recent advances in wet chemical techniques enable the facile synthesis of nanocrystals (NCs) and their assembly into complex solid structures (NC-solids), offering exciting prospects for solar energy conversion, light emission and electronic applications. The properties of these composites are strongly determined by structural details at the NC/matrix interface and the composition of the embedding matrix. We carried out a systematic study of the interaction between InAs NCs and SnSx matrices using a grand canonical ab-initio thermodynamics approach to identify general trends for the stability of structural motifs possibly occurring at the NC/matrix interface. The resulting models have been used as a basis for ab-initio molecular dynamics calculations to investigate the impact of different mass densities and stoichiometries on the internal matrix structure and the NC-solids' electronic properties. We demonstrate that both the NC-matrix interface and the internal regions of the matrix show complex structural features, depending on specific synthesis conditions. Thus to obtain a detailed understanding of experimental data it is necessary to take into account such complex interfacial and matrix-internal structures beyond simplified NC-solid models.

Resume : Organic-inorganic hybrid perovskite solar cells evolved to one of the hot topics in the photovoltaic community due to easy processing and to high efficiencies. A compact titanium oxide layer made by hydrolysis of titanium tetrachloride is often used for recombination reduction. However, the deposition is difficult to control in terms of thickness and homogeneity. Using atomic layer deposition (ALD) the thickness of the titanium dioxide compact layer can be controlled precisely. Due to the self-limiting layer-by-layer process even complicated morphologies can be coated with a well-defined homogeneous layer. We present perovskite solar cells with planar and mesoporous geometry using titanium oxide ALD as a crucial method in the manufacturing process. ALD allows for a systematic tuning of the titanium oxide layer thickness. 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, external quantum efficiency measurements and electrochemical impedance spectroscopy. The use of ALD surface treatments paves the way towards three-dimensional perovskite solar cell architectures based on nanoporous 'anodic' aluminum oxide templates.

Resume : A transparent p-type conductor with measured hole mobility comparable to that of the commercialised n-type equivalents, such as ITO, is still lacking. Realisation of such a compound would prove a major step towards the fabrication of transparent p-n junctions. These being critical components of potential transparent electronic devices, including energy harvesting windows, “invisible” security circuitry and transparent, touch-sensitive display screens. The formation of high conductivity p-type transparent conductors is limited by the electronic make-up of the valence band maximum (VBM). In many wide band gap, optically transparent materials the VBM is composed mainly of localised O 2p states which restrict the mobility and conductivity of the positive hole charge carriers. In contrast, higher hole mobility is achieved for sulfides in which the more diffuse S 3p orbitals dominate at the VBM, although this enhanced mobility is gained at the expense of optical transparency. In an attempt to combine these desired properties, layered oxysulfide compounds, in which wide-gap perovskite layers [AxByOz]2+ are alternated with [Cu2S2]2- sheets have been investigated as potential candidates. Their 2D structure serves to reduce the dimensionality of Cu d-d interactions, maintaining the wide-gap; whilst the overlap between the diffuse S2- 3p and Cu+ 3d orbitals enables higher mobility and conductivity. The attempted powder synthesis of a range of these materials will be discussed here.

Resume : Cu2O is an intrinsic p-type metal oxide semiconductor with direct band gap and high absorption coefficient. Together with low cost, nontoxicity, natural abundance of copper and theoretical conversion efficiency around 20% in a single junction structure it is a good candidate for next generation solar cells. In this study, we report on the effect of target power and O2 flow on DC magnetron sputter deposited CuxO thin films. Copper oxide thin films have been deposited using a broad range of parameters. The film qualities have been structurally, optically and electrically characterized. Target power (Ptar) and oxygen flow (Q(O2)) have been varied in the 80 - 100 W and 12-19 sccm range while keeping all other parameters constant. Single phase thin films with distinct (111) Cu2O XRD peaks, 1,6x1015 cm-3 majority carrier concentration, 26 cm2·V−1·s−1 mobility, 144 Ωcm resistivity and optical band gap of 2,6 eV were achieved at 90 W and 17 sccm O2 flow. Clear correlations between structural, optical and electrical properties were observed. The study shows promising results for Cu2O films deposited by DC magnetron sputtering with high deposition rates.

Resume : In the past few years, the efficiency of solution processed antimony sulfide (Sb2S3) based solar cells has been increased to over 7% by optimizing the hole transport material and the interface properties. However, it is still not clear whether the transport properties of this wet-chemically deposited Sb2S3 thin film will be a limiting factor for higher efficiencies. The transport properties of the Sb2S3 layer in solar cells with a FTO/TiO2/Sb2S3/MoOx/Ag cell stack were investigated by admittance spectroscopy (AS) in the frequency range from 100 Hz to 1MHz. Two characteristic peaks are observed in the differential capacitance spectra (-f dC/df vs f). Curve fitting was conducted to interpret the C-f spectra in order to distinguish the different origins of AS signals. We find that the spectra measured at different temperatures and bias voltages can be well fitted by an equivalent circuit consisting of sub-circuits representing trap states and carrier relaxation. By further evaluating the fitted parameters, a carrier mobility on the order of 10-5-10-4 cm2/Vs is derived for the Sb2S3 layer. Our results indicate that, although the properties at the interface have a strong impact on the cell efficiency, the low carrier mobility in Sb2S3 will be also problematic for the carrier extraction, especially for the thick Sb2S3 layer, which has to be considered for cell design in the future.

Resume : Current commercial thin film photovoltaic (PV) technologies like CdTe and Cu(In,Ga)Se2 rely on the use of scarce (In, Ga, Te) and/or toxic elements (Cd), highlighting the necessity to explore new technologies free of critical raw materials. In this work we report the first Cu3BiS3 based solar cells with proved photovoltaic activity using standard thin films PV substrate configuration. The absorbers were synthesized using a sequential process based on the metallic stack evaporation followed by a reactive annealing under S atmosphere. Through the optimization of composition (Cu/Bi ratio), metallic stack order, annealing parameters and Na doping, we achieve a record conversion efficiency of 0.11%. Combining several characterization techniques we shows that at this stage of the technology development, issues like composition, secondary phases and morphology cannot explain the low efficiencies obtained with this material. Using a deeper characterization of the devices, we found that most probably, this is related to either, a high doping of the absorber, and/or poor transport charge properties. Understanding and solving these issues, can further help to improve the efficiency of Cu3BiS3 based devices towards more competitive conversion efficiencies.

Resume : The achieved improvement for the solar conversion efficiency, linked to the use of perovskites as light harvesters, has expanded the research to pursuit new materials that allow a sustainable energy production based on mixed compounds or tandem solar cell architectures. The hybrid organic–inorganic perovskite materials for photovoltaic application have associated many issues that must be understand and control, to turn useful those materials in the design and development of new solar–based energy technologies. The stability, the cation movement and the intrinsic crystal mobility, the toxicity of the compound or the optimal band gap are some of the problems which have demanded special attention. Because ab initio calculations provide a very efficient way to explore the physical properties, to explain the phenomena of interest, for this work were studied the electronic properties of compounds with the perovskite structure ABX3. Being A = NH4+, CH3NH3+, CH(NH2)2+; B = Pb2+, Sn2+, Ge2+; and X = I–, Br–, Cl–; those compounds were studied to elucidate the role played by the charge distribution, on the photoconductive properties, as a function of the ionic specimen variation. Because it is well known that this methodology underestimate the band gap magnitude, but it is very relevant to identify the correct variation of the band gap, depending on the ion size, special attention was paid on the computational method: scalar relativistic effects and dispersive corrections were included. The two supported corrections were: one proposed by Ortmann, Bechstedt and Schmidt (OBS), and another proposed by Tkatchenko and Scheffler (TS). All calculations, effects and corrections, mentioned above, are implemented in the DMol3 code; those calculations were made using the generalized gradient approximation and the Perdew–Burke–Ernzerhof functional. The obtained results revealed that using the OBS correction allows to describe the band gap variation properly, when the organic cation and the halogen are both gradually increased; otherwise, the results from calculations with the TS correction, only reflect the correct gap increase. However, the TS correction provides a better electronic properties description, if it is compared with those from previous theoretical works; also, our results are contrasted to experiment reported results, to prove the reliability of our methodology for the study of the hybrid organic–inorganic perovskite materials, to explore new compounds of this kind. Acknowledgements: This work was partially supported by COFAA and the projects IPN–SIP–2017: 0451, 0512 and 0559. I. Ornelas and J.J.J. Díaz want to acknowledge the graduate fellowship from CONACYT.

Resume : CZTS exhibits an order-disorder transition at rather low temperature (~270°C) involving the Cu/Zn distribution. The effect of cations disorder is interesting since it is a possible explanation for the potential fluctuations that are responsible for the low Voc values in CZTS solar cells. One of the most evident effects of Cu/Zn disorder is a significant energy gap (Eg) variation. In the most ordered state Eg is about 200 meV higher than in the fully disordered state. In order to understand the influence of stoichiometry on the order-disorder related Eg variation, several samples with different composition have been prepared by changing metal ratios in the precursors. The Eg values have been measured for all samples, in states with different order level obtained after suitable thermal treatments above and below the critical temperature. The Eg value in the completely disordered materials is slightly dependent on the stoichiometry, showing an Eg increase with decreasing Cu/Sn ratio, but seems not to be affected by the Zn/Sn ratio. On the other hand the material stoichiometry influences the ordering kinetic, giving rise to different Eg values in samples with less disorder obtained through the same thermal treatment (annealing followed by a slow cooling step). Vineyard model, generally used to describe the order-disorder transition, has been generalized to non-stoichiometric compounds and an improved model able to describe the relation between stoichiometry and Eg value is proposed.

Resume : The ternary CuSbS2 semiconductor has been currently studied as an alternative absorbed material in thin film photovoltaic applications. It shows a p-type conductivity, exhibit a near-optimal direct band around 1.3 - 1.5 eV, and a strong light absorption (α > 10E4 /cm for hv > 1.3 eV). In addition to the electrical and optical properties, the availability and low cost of its constituent elements make it a suitable material for photovoltaic devices. The reported methods to obtain CuSbS2 thin films include the annealing of a bilayer formed from the corresponding metal sulfides. Mainly, these two layers are deposited by a combination of chemical and physical methods such as chemical bath deposition or electrodeposition with sputtering or thermal evaporation. In this work we propose the use of a cation exchange process between an amorphous antimony sulfide (a-Sb2S3) thin film and a copper solution, to obtain a bilayer of Sb2S3/Cu2S. Followed by a post-annealing treatment to obtain CuSbS2 thin films. The a-Sb2S3 films were deposited from three consecutive chemical baths containing SbCl3, Na2S2O3 solutions at 10 °C for 1 h resulting in films with 340 nm in thickness. The cationic exchange was carried out in a Cu+ solution at room temperature (25 °C). ICP analysis and cross section SEM images confirm the cationic exchange in the solution and the development of a bilayer as a first step for the ternary material formation. After annealing the deposited bilayer at 350 °C for 1 h in a vacuum oven (4 E-6 Torr), polycrystalline CuSbS2 thin films were successfully obtained, without the presence of oxides, copper and antimony sulfides, or other copper-antimony-sulfide phases evidenced by X-ray diffraction. The optical properties of the CuSbS2 thin film obtained shown a direct band gap of 1.5 eV and an absorption coefficient of 10E5 cm-1 (hv > 1.5 eV), values that match with the requirements for the photovoltaic materials. These and other result from characterization techniques demonstrate the viability to obtain CuSbS2 films by the simple chemical methods presented here, with promising properties to be used as absorbed layer in photovoltaic cells.

Resume : Rapid progress on photovoltaic (PV) solar cells requires exploring and identifying the wide range of existing and emerging earth-abundant absorbers. Chalcogenide materials are highly interesting for use as absorber layers in thin film solar cells. In the present study, we explore and compare various emerging compounds, like for examples Cu2XSnS4 (X = transition metals), Cu(Sb,Bi,Te)(S,Se)2, Cu3(Sb,Bi)(S,Se)3, Cu2SnS3, Cu2S, and SnS. We calculate the electronic structure and analyze the optical properties, employing the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, in terms of the dielectric functions and absorption coefficients. By modeling the quantum efficiency of the compounds, we further discuss the optical response. Moreover, the maximum conversion efficiency is calculated with the Shockley-Queisser (SQ) model, using the calculated absorption coefficients and considering different thicknesses of the absorber materials. The results help to understand fundamental physics of then emerging earth-abundant chalcogenide materials in order to design and optimize solar cell devices. [1] R. Chen and C. Persson, J. Appl. Phys. 112, 103708 (2012). [2] R. Chen and C. Persson Thin Solid Films 519, 7503 (2011). [3] S. G. Choi, et al., Appl. Phys. Lett. 101, 261903 (2012). [4] M. Kumar and C. Persson, Appl. Phys. Lett. 102, 062109 (2013). [5] M. Kumar and C. Persson, Energy Procedia 44, 176 (2014). [6] C. Andrea, et al., Sol. Energ. Mat. Sol. Cells 154, 121 (2016).

Resume : Enhancement of Perovskite Solar Cell Efficiency by Optical Engineering Mehmet Koç1,2,3,*, Wiria Soltanpoor1,2, Selçuk Yerci1,2 1 Center for Solar Energy Research and Applications (GUNAM) , Middle East Technical University, Ankara, TURKEY. 2Department of Micro and Nanotechnology, Middle East Technical University, Ankara, TURKEY. 3 Department Electrical and Electronics Engineering, Ankara Yıldırım Beyazıt University, Ankara, TURKEY. *koc.mehmet@metu.edu.tr Perovskite materials are the focus of an enormous research effort in the field of solar cells. The outstanding intrinsic optical properties of perovskites are one of the fundamental keys to their success. Methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) has a bandgap of around 1.55 eV, quite close to the ideal bandgap defined by the Shockley-Queisser limit. Additionally, MAPbI3 has high absorption coefficient and long minority carrier diffusion length (~1000 nm) enabling high quantum and power conversion efficiencies [1]. Thanks to perovskite’s superior optical and electrical properties, perovskite solar cells have reached a record efficiency of 22.1% [2]. Recently, several groups have reported short circuit current (JSC) values of over 18 mA/cm2 [3, 4]. JSC is driven by total light absorbed in the active materials and carrier recombinations in the whole device. While the latter one depends on the material and interface quality, the former one can be boosted by optimizing the layer thicknesses and light trapping. In this study, optical engineering of perovskite solar cells was shown to develop constructive interferences of light within the cell leading to significant improvements in short circuit current. In this regard, a small change within 20 nm in the thicknesses of electron transport, hole transport, transparent conductor oxide and active perovskite layers can yield a JSC enhancement above 2.5 mA/cm2, resulting in an efficiency boost over 1%. Moreover, we showed that performance boosts are governed by spectral interplays. A cell with a poor optical design yielding low JSC can indeed have superior quantum efficiency in the UV region of the spectrum. This study shows that some of the good performing devices reported in the literature are indeed have serious optical losses which could have been improved by small thickness variations. [1] Qianqian Lin, Ardalan Armin, Ravi Chandra Raju Nagiri, Paul L. Burn and Paul Meredith, Nature Photonics, 9, 106–112 (2015) doi:10.1038/nphoton.2014.284 [2] NREL chart, http://www.nrel.gov/ncpv/images/efficiency_chart.jpg, Accessed 13.03.2016, 2016.Chen W, Wu Y, [3] Yue Y, Liu J, Zhang W, Yang X, Chen H, Bi E, Ashraful I, Grätzel M, Han L, Science. 2015 Nov 20;350(6263):944-8. doi: 10.1126/science.aad1015. [4] Cheng-Chiang Chen, Sheng Hsiung Chang, Lung-Chien Chen, Feng-Sheng Kao, Shih-Chieh Yeh, Chin-Ti Chen, Wen-Ti Wu, Zong-Liang Tseng, Chuan Lung Chuang, Chun-Guey Wu, Solar Energy 134 (2016) 445–451, http://dx.doi.org/10.1016/j.solener.2016.05.032

Resume : Organic-inorganic metal halide perovskites have emerged as one of the most promising materials in photovoltaics. The quick rise in perovskite solar cell efficiency is attributed to its excellent optoelectronic properties such as its direct band gap and high charge carrier lifetime. However, in order to commercialize perovskite solar cells, large-scale implementation of such devices with high stability and reproducibility is necessary. In contrast to mesostructured configuration, perovskite solar cells with planar structure are now receiving much attention, mostly due to their potential in various tandem devices as well as flexible structures. Unlike using solution processing to fabricate planar perovskite layers, vapor deposition offers pin-hole free, smoother, more uniform film morphology and a potential to enable the fabrication of larger area photovoltaic devices. Charge carrier recombination within methylammonium lead iodide perovskite, CH3NH3PbI3, can be suppressed by fabricating large crystal grains. Carrier diffusion lengths can exceed 10 µm in the case of large grain perovskite [1]. Therefore, in order to enhance transport properties of perovskite, we used vapor deposition to fabricate large grains. Deposition pressure showed to have a crucial role in producing perovskite grains as large as 1 μm. Furthermore, the annealing ambience can be altered to further obtain large grain perovskite. Thus, the effect of in-situ annealing as well as annealing perovskite under the exposure of different gases such as methanol, water and methylamine vapors was studied. [1] M. I. Saidaminov et al. Nat. Commun. 6, 7586 (2015).

Resume : A fortunate aspect of the material CZTS(Se) is that, apart from being a material made from relatively Earth-abundant and non-toxic elements (unlike CdTe and CIGS, which both face production bottlenecks due to toxicity and scarcity issues respectively), it’s 12.6% efficient champion cell was produced using non-vacuum methods. This combination of material-abundance and preferential-deposition make CZTS an excellent choice for mass-production of solar panels. For non-vacuum processes such as solution processing to be viable and up-scalable, the solvent being used to dissolve precursors cannot be overtly toxic or dangerous to handle, as is the problem currently with the previously mentioned champion CZTSSe cell. Hydrazine's intense reactivity is what makes it simultaneously an excellent choice of solvent and an extreme health hazard. To move away from such a solvent, we have replaced hydrazine with an amine/thiol mixture. The choice of amine and thiol can vary; here we use ethanolamine as an amine source, and cysteamine as a thiol source. The amine/thiol mixture is diluted using DMSO, and is a flexible processing route as it can dissolve elemental metals as well as some binary chalcogenides, and requires minimal safety equipment to handle. Having previously pneumatically deposited (by hand), there was large variability when comparing between cells. To remove this source of error we have assembled an automated ultrasonic deposition system. Ultrasonic systems allow greater control over parameters such as thickness, intrinsically produce a finer droplet size, and also reduce top-surface cooling from excessive airflow. The promise of controllable, repeatable depositions will remove a large quantity of human error from analysis, making the result of experimental variations more prominent, and the conclusions drawn from them more reliable.

Resume : Photovoltaic devices based on hybrid organic-inorganic perovskite absorbers have reached outstanding performance over the past few years, surpassing power conversion efficiency of over 22%. This talk we discuss recent progress and challenges in hybrid perovskite solar cells (HPSCs) with an emphasis on the role of the interface in device performance including stability. This talk will highlight work at NREL to develop scalable HPSCs discussing efforts to controlling the material formation and processing for high-volume manufacturing. In addition to address stability, an examination of different perovskite active layers and their interfacial electronic structure with typical and novel contacts with the HPSCs stack will be presented. Our studies at NREL indicate interface formation of the active layer with different carrier transport materials has direct implications for performance and it evolution over time in the resulting devices. Results of interface studies combined with time resolved spectroscopy, structural studies and device level studies to validate impacts on interface electronics and carrier dynamics will be shown and the technological relevance discussed. In particular work on mini-module devices at the 12cm² will be presented along with performance data for these systems.

Resume : Methylammonium lead iodide perovskites (MAPI) are generally considered direct bandgap semiconductors. However, theoretical calculations have predicted a slight indirect bandgap for MAPI as a consequence of spin-orbit coupling resulting in Rashba-splitting of the conduction band. Currently there is limited experimental evidence to support this theoretical prediction. Using pressure-dependent absorption and emission measurements, we show that a weakly indirect bandgap around 60 meV below the direct bandgap transition is present. Under hydrostatic pressure from ambient to 325 MPa, Rashba splitting is reduced due to a pressure-induced reduction in electric field around the Pb atom. The indirect nature of the bandgap is suppressed, leading to five times faster charge carrier recombination, and a two-fold increase of the radiative efficiency. At hydrostatic pressures above 325 MPa, a reversible phase transition of MAPI occurs, resulting in a purely direct bandgap semiconductor. The finding of an indirect bandgap in MAPI sheds light on the apparent contradiction of strong absorption and long charge carrier lifetime. Novel epitaxial and synthetic routes to higher efficiency optoelectronic devices might be developed based on the pressure-induced changes we observe.

Resume : Inorganic and hybrid halide perovskites, important members of the perovskite family, have shown exceptional photovoltaic performance with efficiencies exceeding 20%. Instability of these materials is a major issue for real world applications including both chemical breakdown and current-voltage hysteresis. Most perovskites adopt cubic crystal structures (Pmm type) at high temperature, and undergo various phase transitions and adopt lower symmetry space groups at lower temperature due to structural instability.[1] This phenomenon is due to a combination of octahedral tilting, molecular rotation and cation disorder within the perovskite unit cell. Depending on the specific transition path, large changes in the electronic and optical properties can take place that can affect photovoltaic performance. Here we present a comprehensive analysis of the vibrational (phonon) structure for CH3NH3PbI3 and a series of 24 inorganic metal halides: AMX3 (A = Cs, Rb; M = Ge, Sn, Pb; X = F, Cl, Br, I). Calculations have been performed using lattice dynamics with forces based on density functional theory building on recent work [2]. We demonstrate, from quantum chemical investigations, that all inorganic halide perovskites exhibit lattice instabilities in their high temperature cubic phase. These instabilities include octahedral titling and second-order Jahn-Teller distortions depending on the chemical identity and radius of the underlying composition. We provide quantitative insights into the thermodynamic driving forces, and how the macroscopic properties will be affected. There are major implications for the applications of these materials. [1] Howard, C. J.; Stokes, H. T. Acta Crystallogr. Sect. B 1998, 54, 782–789. [2] Brivio, F.; Frost, J. M.; Skelton, J. M.; Jackson, A. J.; Weber, O. J.; Weller, M. T.; Goni, A. R.; Leguy, A. M. A.; Barnes, P. R. F.; Walsh, A. Phys. Rev. B 2015, 92, 144308.

Resume : Due to the limited understanding on the specific impact of air upon the formation of CH3NH3PbI3 perovskite crystal from solution phase, developing high-efficiency air-processed perovskite solar cells (PVSCs) is still challenging. Here, we demonstrate a systemic investigation to reveal the original limitations on developing air-processed PVSCs. We found that ambient air would generally lead to uncontinuous PbI2 and low-quality perovskite films. A detailed correlation between the PbI2 solution wetting capability on various substrates and their corresponding film deposition behavior in air has been established. This provides useful information for fabricating perovskite in air with solution deposition method. More importantly, we found that simply preheating the substrate and PbI2 precursor solution can effectively eliminate the detrimental effects of air during the PbI2 film deposition, with a possible work mechanism proposed. Benefiting from the effective elimination of detrimental effect originated from air, along with poly-TPD with well-matched work function as the hole transporting layer, highly efficient air-processed PVSCs, with a champion efficiency of 18.11%, have been achieved, which is the highest efficiency achieved by solution-processed PVSCs in air to date. Our work not only provides a practical strategy for highly efficient air-processed PVSCs, but also provides valuable insights into the thorough understanding of the limitations of fabricating PVSCs in air. References: Sai-Wing Tsang et al, J. Mater. Chem. A, 4, 12748, (2016) Sai-Wing Tsang et al, J. Mater. Chem. A 4, 561 (2016)

Resume : The isolation of stable atomically thin two-dimensional (2D) materials on arbitrary substrates has led to a revolution in solid state physics and semiconductor device research over the past decade. While, graphene is the poster child of 2D materials family, a variety of other 2D materials (including semiconductors) with varying structures and opto-electronic properties have been isolated over the last few years raising the prospects for a new class of devices assembled by van der Waals forces. A fundamental challenge in using 2D materials for opto-electronic devices is enhancing their interaction with light, ultimately responsible for higher performance and efficiency in the devices. In this seminar, I will show our recent work on photovoltaic devices from transition metal dichalcogenides of molybdenum and tungsten (MoS2, WSe2 etc.). We have recently demonstrated near-unity absorption in the visible part of the electromagnetic spectrum in < 15 nm films of these semiconductors by placing them on reflecting metal substrates such as gold and silver. We have further shown that these highly absorbing, ultrathin films can be further used for fabrication of simple Schottky junction photovoltaic devices with microfabricated metallic top contacts. While, this work helps solve the light-absorption problem, the external quantum efficiency (EQE) was < 10% for our Schottky junction devices Very recently, we have extended this early work to fabricate p-n heterojunctions (p-WSe2/n-MoS2) and use graphene as a transparent top contact to amplify our current collection efficiency and push the EQE up to 50%, approaching that of many emerging photovoltaic technologies with active layers in the 100s of nm range. This represents a significant development as both light-absorption and charge collection have been addressed in these devices. Finally, I will present scope for future work using just monolayers of materials to engineer near unity light absorption and collection.

Resume : To build on the success of other mineral systems employed in solar cells, including kesterites (Cu2ZnSnS4) and herzenbergite (SnS), and mineral-inspired systems such as perovskites (CH3NH3PbI3), we have searched for photoactive minerals with the additional constraint that a polar crystal structure is adopted. Electric fields provide a driving force to separate electrons and holes in semiconducting materials. In the context of light-to-electricity conversion in photovoltaic devices, the electric fields that drive photo-carrier separation are typically associated with a ‘p-n’ or ‘p-i-n’ junction. The utility of using materials that contain internal electric fields, arising from spontaneous polarisation of the lattice, has recently become apparent and we further highlight desirable material properties for photovoltaic applications that polar or ferroelectric crystals are likely to possess. We identify enargite (Cu3AsS4), stephanite (Ag5SbS4) and bournonite (CuPbSbS3) as candidate materials and explore their properties using a first-principles quantum chemical approach.

Resume : Metal hydrides can exist in many different forms, and have received special attention as high-density hydrogen storage materials. Few are aware that some of the metal hydrides are actually semiconductors, and therefore might present a potential as solar energy materials in the future. IFE has since 2009 been doing unique research on thin-film metal hydrides. Synthesis of magnesium nickel hydride films with a tuneable band gap from 1.5 to 2.2 eV has been demonstrated. This band gap range is highly relevant for photovoltaics. The synthesis was observed to be facile and the resulting material is relatively stable, even when exposed to oxidation in atmospheric conditions. Yttrium hydride synthesized by reactive sputter deposition has demonstrated photochromic effect, and thereby presents opportunities to integrate in future smart windows for management of solar radiation in buildings. This presentation will entail a short summary of IFE and other's work on thin-film metal hydrides as solar energy materials. It will also explain why the metal hydride system might conceal materials with properties highly relevant for solar energy, which might be the next generation earth-abundant solar energy materials.

Resume : In the last 5 years, hybrid halide perovskites have emerged as a highly efficient class of solar absorbers, with efficiencies reaching 22.4%, quickly surpassing other 3rd generation devices. The highest performing hybrid perovskite is the cubic CH3NH3PbI3 (MAPI), which is made from earth-abundant elements and can be easily solution processed, dramatically reducing manufacturing costs. Unfortunately, chemical stability is a major concern for these materials and much effort has been devoted to increasing the stability of MAPI based devices. Recently, partial substitution of iodine with the pseudohalide ion, SCN-, to form the layered (CH3NH3)2Pb(SCN)2I2 (MAPSI), has been suggested as novel route to increase stability whilst retaining high efficiencies. Here we present density functional theory results which explain why MAPSI can still possess an ideal electronic structure for light absorption, despite the loss in connectivity when moving from a cubic to a layered structure. We also explain, for the first time, why MAPSI is more stable than MAPI. Lastly we explore the possibility of a MAPSI related family of materials whose optoelectronic properties can be fine-tuned for use in photovoltaic applications.

Resume : One potentially attractive family of materials to compete with silicon in terawatt-scale up of photovoltaics (PV) is the ternary copper chalcogenides, taking the general form CunMmChx (M = Sb, Bi; Ch = S, Se), developed as an analogue to CIGS, replacing trivalent In and Ga with Sb or Bi. Here we report comprehensive photoemission studies of CuSbS2 and Cu3BiS3 thin films, including core level analysis, work function and ionization potential determination, comparison of the valence band spectra with calculated density of states in order to elucidate the roles of the Cu d states and Sb and Bi lone pairs. This provides information about band line-ups, enabling the suitability of candidate partner layers in device structures to be determined. The ultimate suitability of these materials for PV applications will also be assessed.

Resume : Zinc tin nitride (ZnSnN2) material was proposed for photovoltaic (PV) applications in the last few years as it was found to be a n-type semiconductor with a direct optical bandgap in the range 1.6 eV to 2.2 eV. Available electrical data on this earth-abundant and non-toxic material are however very scarce in literature. We find conductivity, majority charge carrier density and mobility values in the range 10-2 to >30 S.cm-1, 2E17 to 1.1E21 cm-3 and 0.37 to 10 cm².V-1.s-1 respectively, for layers obtained from various techniques, growth and post-annealing conditions. A first device combining a ZnSnN2 layer with highly doped p-type crystalline silicon was reported recently. It shows a clear rectifying behavior under dark conditions affected by large series resistance. The metallization scheme of this device prevented however measurement under light, essential for PV applications. The goal of the present work is to complete the identity card of ZnSnN2 material in order to further evaluate its potentialities for PV. For this purpose, different set of samples were prepared using sputtering growth technique at room temperature and 250°C with thicknesses below 1µm on silicon and glass substrates. Current-voltage in dark and illumination conditions at various temperatures, Hall Effect, ellipsometry and spectrophotometry were performed. Evidence of current increase under light exposure is reported for the first time for such layers deposited on glass.

Resume : Metal oxides in the recent decades have demonstrated to be materials of great importance for optoelectronic applications. They play key functions for a range of established and emerging technologies and are used for instance as transparent electrodes in thin film transistors (TFT) for flat panel displays, printing electronics and in solar cell devices. Here we report on a general approach to the synthesis of nanoscale metal oxides for photovoltaic applications and present two plasma-based methods capable of delivering the full range of metal oxides (e.g. Cu-, Zn-, W-, Mo-, Co, Ni-oxides) in a simple, single deposition step and directly on devices. We demonstrate for instance the synthesis of cupric oxide (CuO) and zinc oxide (ZnO), the first synthesized via a hybrid plasma electrochemical method while the second through a gas phase plasma approach. Both processes are rapid and produce high purity oxides that can be directly used for solar cell device fabrication. We detail the characteristics of CuO and ZnO produced at specific synthesis conditions. Transmission electron microscopy (TEM) show that spherical CuO nanoparticles can be produced with sizes ranging from 1.5 nm to 3 nm, depending on the processing current. At the synthesis conditions studied, ZnO was found in the form of nano-tetrapod with the arm lengths varying from 19 nm to 42 nm and diameters from 8 nm to 12 nm. We provide the results of the chemical analysis as well as those of the optical characterization (e.g. ultraviolet-visible absorption and photoluminescence spectroscopy). Finally, to demonstrate the simplicity and effectiveness of this synthesis method we have used our CuO nanoparticles and ZnO nano-tetrapod in solar cell device as carrier transport materials. Devices based on methyl ammonium lead iodide were produced and tested demonstrating the applicability of these plasma-based techniques for large scale manufacturing of next generation solar cells.

Resume : The development of In free materials for scalable and cost-effective thin-film photovoltaic technology is one of the major research drivers in solar cell research. Among the various candidates considered for absorber layers in substrate devices, Cu2SnS3 (CTS) exhibits a number of attractive properties: (i) band gap around 1 eV, (ii) absorption constant above 105 cm-1 as well as (iii) Earth abundant and low-cost elements. However, CTS is rather challenging to process due to the large number of stable phases that could be formed, with some being highly conductive. In this contribution, we present a solution based method for generating high quality microcrystalline tetragonal CTS, featuring a high degree of phase purity. The best device with the substrate configuration: glass/Mo/CTS/CdS/i-ZnO/Al:ZnO/Ni-Al (total area of 0.5 cm2) displayed a VOC = 200 mV, ff = 34.5 %, jsc= 27.6 mA/cm2 and power conversion efficiency of 1.9 % under simulated AM1.5 illumination. This the best performance reported for such solar architecture obtained by solution processing, with a dispersion below 20% in 24 devices. Recombination losses were investigated by temperature dependent current-voltage curves and impedance spectroscopy. Two trap states with activation energies of 41 ± 0.4 and 206 ± 7 meV were estimated. The shallower trap is linked to Cu vacancies, while the deeper trap is associated with Sn in Cu antisite defects based on DFT supercell calculations.

Resume : This talk will give an overview of the SWInG project, Action No. 640868 of the H2020-LCE-2014-1 call. The aim of this project is to develop wide band gap thin film solar cells based on kesterite absorbers, for future application in high efficiency and low cost tandem photovoltaic devices. The SWInG working group focuses both on the development of the processes for the synthesis of such solar cells, and on the understanding of the physical and electrical properties of the high band gap absorber. An overview of the main results will be presented.

Resume : The main problem to obtain homogeneous Cu2ZnSnS4 (CZTS) thin films with controlled composition and single kesterite phase is the loss of Sn-related to the high volatility of SnS above 400C. To study in details, the effect of sulphurization on Sn compositions variation in as deposited CZTS, the samples intentionally prepared in Cu-poor and Sn-rich off-stoichiometric ratios of compositions, Cu/(Zn Sn)~0.39 and Zn/Sn~ 0.28 by co-sputtering of Cu, ZnS and SnS and followed by sulphurization at 520C for 10 minutes in RTA furnace (e.g. samples, CZTS_0 as deposited and CZTS_1 sulphurized). The XRD spectra confirms the formation of kesterite phase for CZTS_1 with a secondary phase SnO2 observed, while Raman spectra carried out at 532nm laser excitation observes Raman shifts at 286 and 337 cm-1 and that of recorded at 633nm laser at 337 cm-1 and 370 cm-1, where, all these Raman peaks attributing to CZTS kesterite phase. The optical band gap is found to be 1.5 eV for CZTS_1 obtained by tauc plot. TEM measurements found that the CZTS_0 sample has uniform compositional distribution lying in amorphous phase but upon sulphurization (CZTS_1), it is prone to become non-uniforms, where, regions have been found with Cu/(Zn Sn)~0.61, Zn/Sn~0.41 (in top layer) and other regions Cu/(Zn Sn)~0.2, Zn/Sn~0.31 (in bottom layer). It can be concluded that bottom layers reacting less with sulphur resulted in segregation of Sn and may form SnO2 which can affect photovoltaic device performance, even though thin film confirms the formation of kesterite phase.

Resume : Pure sulphide Cu2ZnSnS4 (CZTS) thin film solar cells have been extensively studied in the last few years as an earth-abundant and environmentally-friendly alternative to well established CIGS technologies. To form phase-pure CZTS material with relatively large grains, a high temperature (>500 °C) sulphurisation annealing step is essential and unavoidable. This would normally lead to the formation of MoS2 between Mo and CZTS, which is detrimental for the solar cell performance. In this work, SixNy thin films with various film thicknesses were introduced as an interfacial layer between absorber and back contact in CZTS thin film solar cells. The SixNy was deposited through plasma enhanced chemical vapour deposition (PECVD) system. The film thickness and stress control of the SixNy film were studied to improve the adhesion of CZTS layer and reduce the chances of cracking the deposited films. Energy dispersive X-Ray spectroscopy (EDS) mapping measurements performed directly on the cross-section of CZTS/SixNy /Mo films indicate that the SixNy intermediate layer can effectively inhibit the formation of MoS2 layer at CZTS/Mo interface region. Due to high resistivity of SixNy, a further very thin layer of Mo (10-30nm) was coated on top of SixNy. It was found this structure would help the adhesion of CZTS on the back surface. The overall improvement leads to significantly reduced series resistance and hence boosts device efficiency by increasing short circuit current density and fill factor of the CZTS solar cells.

Resume : Perovskite solar cells (PSCs) demonstrate high power conversion efficiencies and offer low-cost fabrication routes. This combination makes PSCs a promising technology triggering already today enormous interest in science and business. However, the rather low light stability of PSCs remains a key challenge for their economical breakthrough. In this work, we provide new insights about light-induced degradation of PSCs and propose strategies to improve device stability. We show that light-degradation on typical planar n-i-p PSCs based on methylammonium lead triiodide occurs through the reduction of photocurrent generation, with a 50% reduction of the initial photocurrent in less than 100 hours. The identified degradation is reversible, but in the timescale of months. Therefore, recovery during outdoor day-night cycles will be negligible, and, if not addressed, will lead to a reduction of generated power over deployment time. We found this degradation is influenced by two parameters: the applied bias and the architecture of the device. Conveniently, changes to the architecture of the PSCs can reduce the fast pace of degradation at maximum power point. We report that the dopants added to spiro-MeOTAD, used in PSCs with record efficiencies, are the main cause of photocurrent loss. We propose a degradation mechanism, that we validate through light stability tests of devices with promising hole transport layers. Moreover, we present an approach to mitigate light-induce degradation without any change to the architecture and performance, hence a more general route to improve device stability.

Resume : One of the most relevant family of next generation earth-abundant photovoltaic materials, is the so-called kesterites (Cu2ZnSn(S,Se)4 – CZTSSe). This technology has experienced important progresses, but at the same time it is required to overcome urgently the current efficiency limitations to move towards large scale industrialization. Currently, the record conversion efficiency of kesterites (13.7%) barely exceeds half of the values reported for Cu(In,Ga)Se2 and CdTe. The main challenge for CZTSSe solar cells is the large Voc deficit, which is remarkably higher in comparison to well stablish solar cell technologies. In this presentation, the main conversion efficiency limitations of kesterites will be presented, including: Cu/Zn disorder, formation and characteristics of deep defects, nature and passivation of grain boundaries and surfaces, secondary phases formation and distribution, band-gap and compositional fluctuations, and origin of tail states. In view of these restraints, advanced technological solutions will be presented, in particular the positive effect on the properties of CZTSSe of the incorporation of an ultrathin IV element layer on top of the absorber surface that has been recently discovered by IREC. Finally, the perspectives to catch up with the high efficiencies of other thin film PV technologies will be presented, based in the strategies proposed in the very recently launched European Project STARCELL (H2020-NMBP-03-2016-720907).

Resume : Tin selenide (SnSe), is a binary chalcogenide material known for its superior thermoelectric behavior due to a low intrinsic thermal conductivity which originates from anisotropic bonding of Sn and Se. However, the application of SnSe in the field of photovoltaics has not been widely studied despite its potential as a light absorbering layer. Here, we prepared SnSe thin films using thermal co-evaporation technique and studied its optical and electrical properties with various analysis tools. We investigated the interesting behavior of SnSe thin films that is dramatic changes in carrier concentration upon slight alteration of the stoichiometry, less than a few percentages. This behavior can be explained by variation of relative population of native point defects in SnSe, in particular tin vacancies. Considering estimated depletion width of SnSe when coupled with a n-type CdS, the most common buffer layer for chalcogenide-based thin film solar cells, we found that Sn rich SnSe thin films are more advantageous than pristine SnSe for photovoltaic application. We fabricated photovoltaic devices and analyzed the performance of the devices. Detailed analysis on changes in material properties and photovoltaic performance of SnSe will be presented. Additionally, thin-film type thermoelectric devices based on SnSe films will be also discussed

Resume : A one step growth method for achieving stoichiometric SnS with large grains ? 1 ?m is reported for the first time using the metal organic chemical vapour deposition (MOCVD) process. Tetraethyltin (TET) and ditertiarybutylsulphide (DtBS) were used as precursors with growth temperatures up to 480?C. Hydrogen was used as carrier gas to form radicals to initiate precursor decomposition, below the normal precursor pyrolysis temperatures, for film growth. In situ mass spectroscopy was used to determine the exhaust gas species after injection of the chemical precursors to determine the reaction chemistry. Different chemical precursor concentration ratios were used through partial pressure control by varying the vapour pressure for TET and DtBS for different temperature regimes. The composition of the SnS films was measured with energy dispersive X-ray spectroscopy (EDX) and phase was assessed using X-ray diffraction. S/Sn ratio varied in film composition, as measured by EDX, correlating to input partial pressures of the organometallic precursors. This determined the necessary parameters for achieving 1:1 SnS stoichiometry. At higher growth temperatures, larger S partial pressures were needed for stoichiometry control to balance the higher vapour pressure of S relative to Sn. Higher growth temperatures led to films with larger grains, determined by scanning electron microscopy in plan view.

Resume : We have developed solar cells of chemically deposited thin films of SnS with cubic crystal structure (SnS-CUB) incorporating CdS and (Zn, Mg)O as buffer layers. These cells deposited on commercially available stainless Steel (SS), SS/(SnS-CUB)/CdS/(Zn, Mg)O ZnO/ZnO:Al, showed open circuit voltage (Voc), 0.43- 0.47 mV; short circuit current density (Jsc), 1.87 – 1.97 mA/cm2 with a fill factor of around 0.48- 0.44, depending on the CdS thickness. For depositing the oxides of Zn in these structures, we used sputtering from commercially available targets. The solar cells without incorporating the chemically deposited CdS thin films showed a lower Voc and Jsc.. The cubic SnS absorber films have a crystalline diameter of 24 nm, a band gap (Eg) of 1. 7 eV and show an increase in electrical conductivity ranging from 1E-7 (ohm cm)-1 in the dark to 1E-5 (ohm cm)-1 under 1000 W m-2 illumination. The cubic structure of the films is stable up to 530 ºC under certain conditions of annealing. Efforts are underway to improve the solar cell parameters through increasing the crystal diameter and carrier lifetime in the absorber films as well as by modifying the thickness and composition of CdS and zinc oxide layers. We will present these results in the paper.

Resume : Binary III-N materials and alloys thereof have experienced major success in the blue light-emitting diode industry, yet are not commercially used in thin film photovoltaics (PV). Despite expectations that alloys of InN and GaN could be used to continuously tune the bandgap to span the solar spectrum, low solar conversion efficiencies (< 6%) still persist due to indium segregation in the alloy. Zn-IV-N2 materials derived from III-V nitrides by cation mutation have the potential to overcome the limitations of InGaN for thin film PV. In particular, ZnSnN2 is predicted to have a bandgap well-suited for single junction solar cells (~1.5 eV), which could be tuned as low as 1 eV by controlling cation disorder. However, until recently, development of ZnSnN2 has been hindered by degenerate n-type carrier density (>10^20 cm-3) that obscured the fundamental bandgap and precluded device integration. To unlock the potential of ZnSnN2 for PV, techniques for controlling carrier density (and thus revealing the fundamental gap) are necessary. Here, we present two strategies for controlling carrier density in ZnSnN2: off-stoichiometric (Zn-rich) growth, and hydrogen-assisted post-growth annealing. With these methods, we have achieved doping control over four orders of magnitude from 10^20-10^16 cm-3, which has allowed for spectroscopic identification of the bandgap in zinc-rich ZnSnN2. The results of this work reveal that processing techniques used in development of III-V nitrides can be applied to development of Zn-IV-N2 materials, thus shortening the time between basic material research and solar cell fabrication.

Resume : BaSi2 thin film was synthesized by vacuum evaporation method for solar cell application. The minority-carrier lifetime, τm, of BaSi2 film formed by vacuum evaporation is short and it has been improved by ultrathin AlOx as a passivation layer, which is attributed to chemical and field effect passivation. AlOx layer was prepared by RF sputtering technique. Formation of AlOx is based on deposition of Al with ultrathin layer (3 nm) and exposure this layer to the ambient air. AlOx exhibits a moderate passivation quality although it was deposited at room temperature. The improvement of passivation quality of AlOx was observed after annealing. τm of BaSi2 passivated by 3-nm AlOx is 15 μs. However, independence of lifetime on thickness is observed for higher than 9 nm of AlOx. The result confirmed that AlOx prepared by the current method is promising as a passivation-layer material for lifetime enhancement in BaSi2 film.

Resume : Power drives modern life. There is a great need of sustainable energy in the world due to the growth of global energy consumption and the limited fossil fuel storage on earth. Solar energy is the most likely to be the main source of energy in the future. Design and development of solar fuels materials, especially water-splitting materials is one of the most heated research fields. Advancements in computational technology give us the ability to guide the experiments with computational simulation, data mining and screening. In this study, we use the SMACT code, developed by the Walsh Materials Design group to assess a hierarchy of approaches for virtual screening of materials properties. We apply heuristic rules, machine learning and approximate electronic structure methods, benchmarking the predictions against a database of well-characterised materials. The approaches are then applied for a high-throughput screening of promising compounds for solar energy harvesting. Density functional theory (DFT) calculations are performed on target compounds suggested by the previous screening to give quantitative predictions.

Resume : Organic-inorganic lead halide perovskites have shown promise as high-performance light absorbers in mesoscopic solar cells. Within 4 years of research PCE of 22.1% has been obtained. The hole transport materials (HTM) are currently the bottleneck for the realization of cost effective and stable devices. Spiro-OMeTAD is the most commonly used HTM in perovskite solar cells. However spiro-OMeTAD are generally used in combination of additives in order to increase the mobility which are suspected to cause device degradation via moisture absorption and also the large synthesis and purification is expensive and tedious which encourage the development of new alternative HTM. In this we report a of 2,2’,6,6’-tetraphenyldipyranylidene (DIPO-Ph4), a large quinoid planar π-conjugated heterocycle, as a dopant-free hole-transporting material (HTM) perovskite solar cells (PSCs). Electrical property of DIPO-Ph4 thin film measured in field effect transistors show a high hole transport mobility. CH3NH3PbI3 perovskite solar cells using pristine DIPO-Ph4 showed a higher PCE than the standard non-doped Spiro-OMETAD. The presented results will demonstrate that dipyranylidene could be an excellent building block for high mobility dopant-free HTMs for perovskite solar cells.

Resume : Hybrid inorganic lead halide perovskites have recently been processed in solar cells with remarkable power conversion efficiencies (>20%), challenging those of state-of-the-art silicon devices. Hybrid perovskite solar cells exhibit a peculiar electrical behaviour, whereby their response can be enhanced or reduced depending on their prior optical and electronic conditioning.[1] Understanding this phenomenon is critical for directing future development to either fabricate more reliable solar cells, or exploit it in memory devices.[2] Ionic migration has been proposed as a possible cause of the anomalous photovoltaic current-voltage response in hybrid perovskite solar cells.[3] However, a major objection to this hypothesis is that hysteresis can be reduced by changing the interfacial contact materials, which are unlikely to significantly influence the behaviour of ionic defects within the perovskite phase. Here we use transient optoelectronic measurements, combined with device simulations, to show that the primary effects of ionic migration can in fact be observed both in devices with hysteresis, and with ‘hysteresis free’ type contact materials. The data indicate that electric-field screening, consistent with ionic migration, is similar or identical in both high and low hysteresis CH3NH3PbI3 cells. Simulation of the transients shows that hysteresis requires the combination of both mobile ionic charge and recombination near the contacts. Passivating contact recombination results in higher photogenerated charge concentrations at forward bias, which screen the ionic charge, reducing hysteresis. 1. Snaith, H. J. et al., J. Phys. Chem. Lett., 2014, 5, 1511-1515. 2. Xiao, Z. and Huang, J., Adv. Electron. Mater., 2016, 2, 1600100. 3. Eames, C. et al., Nat. Comm., 2015, 6, 7497.

Resume : Solution-processed organometallic perovskite based solar cells have emerged as a promising thin-film photovoltaic technology. Despite the importance of stability and material degradation, there have been few reports on the photo-stability of operating devices. In a study on large grain perovskite solar cells of the 3D methyl ammonium (MA) lead iodide materials under constant illumination, we investigated the photo-degradation and fast self-healing of the performances.1 A polaronic picture was proposed and related to contributions from both the inorganic and the organic parts of the material.2 Layered perovskites obtained by the same growth procedure recently report a record photovoltaic efficiency of 12.52 % with no hysteresis, more than two times higher than previously reported values. 3 Intrinsic quantum and dielectric carrier confinements,4 and protection afforded by the organic inner barriers in the 2D Ruddlesden-Popper phases, may explain their exceptional photostability under standard illumination as well as humidity resistance over 2000 hours.3 References 1 W. Nie et al, Nature Comm. (2016) 2 J. Even et al, J. Nanoscale (2016); A. Neukirch et al, Nanoletters (2016) 3 H. Tsai et al, Nature (2016) 4 J. Even et al, Phys. Rev. B (2012), ChemPhysChem (2014); D. Sapori et al, Nanoscale (2016)

Resume : Recently, hybrid perovskite solar cells have been intensively investigated as an emerging technology aiming at producing devices of low cost and high performance for sunlight energy conversion. The performance of such devices is found to depend strongly on the quality of the interface between perovskite and the transport layers, especially on the formation of defects, which affect the charge carrier recombination. In this work, we report the results on electrical characterization of solar cells using CH3NH3PbIxCl3-x perovskite as active energy conversion materials and different conjugated polymers (P3HT, spiro-OMeTAD and PBTTTV) as a hole transport material (HTM). The electrical characterization of the devices in the configuration FTO/compact TiO2 /CH3NH3PbIxCl3-x / HTM / Au was performed by current-voltage (I-V) measurements and the defects were investigated by the charge based deep level transient spectroscopy (Q-DLTS). The analysis results show that the efficiency of devices strongly depends on the nature of the conjugated polymer used as HTM. Highest efficiency is obtained for solar cells having spiro-OMeTAD layer. Investigations of defects in devices by the Q-DLTS technique provide trap parameters of perovskite bulk and interfacial layers. Defects in perovskite are present in all cells and are essentially deep traps (of activation energy > 400 meV). Specific interfacial defects of activation energy in the range of 100-360 meV have been determined for each HTM/perovskite contact. The defect formation at the hole transport layer/perovskite interface and the cell performance are discussed from the obtained measurement results.

Resume : Hybrid organic-inorganic metal halide perovskite-based solar cells – particularly those using APbX3 (A = CH3NH3+, HC(NH2)2+, Cs+ and X = I-, Cl-, Br-) as the absorber layer – have demonstrated rapid improvement in power conversion efficiencies in recent years, reaching values in excess of 21% for lab-scale devices.1 A major concern related to this type of absorber is the toxicity of the Pb. Ongoing efforts to replace Pb by Sn have so far yielded relatively low-performing solar cells, likely limited by defect formation in the absorber material due to the tendency of Sn to oxidize from Sn+2 to Sn+4.2 Preparing Sn-based perovskite films under vacuum conditions may be the key to inhibiting Sn oxidation and improving cell performance. We present the first experimental results towards synthesizing Pb-free perovskite thin-films at the Energy Materials In-Situ Laboratory Berlin (EMIL). A detailed x-ray photoelectron and Auger electron spectroscopy study of SnCl2 precursor layers of different thicknesses vacuum-deposited on Mo/glass substrates reveals significant changes in the chemical environment of Sn and Cl along the layer profile. These findings show the effect that substrate conditions can exert on the deposited material. In our contribution, the results of subsequent perovskite conversion treatments of the deposited SnCl2 precursor films will also be discussed. 1 M. Saliba et al., Energy Environ. Sci. 9 (2016) 1989. 2 I. Chung et al., J. Am. Chem. Soc. 134 (2012) 8579.

Resume : Using first-principles calculations, we perform a comprehensive and systematic analysis to establish the role of van der Waals (vdW) interactions in the vibrational properties of the low-temperature orthorhombic phase of the hybrid perovskite CH3NH3PbI3. To this end, we consider the most common approaches for including vdW effects in our phonon calculations: the semiempirical Grimme approximations, the Tkatchenko-Scheffler dispersion corrections, and the vdW density-functional method. The vibrational normal modes are first calculated within the harmonic approximation. We consider the LDA and GGA approximation to the exchange-correlation functional, and include spin-orbit coupling effects. On top of the harmonic calculations, we also evaluate the anharmonicity of the normal modes by solving the nuclear Schrödinger equation via the finite-displacement method. We observe that both the LDA and GGA approximations work remarkably well in describing the vibrational properties of CH3NH3PbI3. We find that vdW effects and relativistic effects do not have any significant impact on the vibrational properties of CH3NH3PbI3.

Resume : Zn(S,O) is one of the most promising candidate to substitute the toxic CdS as buffer layer in thin film solar cells. Zn(S,O) thin films were prepared by annealing in air and sulfur atmospheres the Zn layers deposited by vacuum thermal evaporation method. The samples were analyzed for their optical properties by using UV-Vis-NIR spectroscopy. The optical constants and the dispersion parameters of the films were calculated from the analysis of the transmittance and reflectance data in the spectral range 300-1800 nm. The band gap energy varied from 3.27 to 3.08 eV depending on the sulphur content. The bowing parameter was calculated as 2.45 eV. Swanepoel model was employed to calculate the variation of the refractive index with the wavelength. The lattice dielectric constant εL, the plasma frequency ωp and the ratio of the carrier density to the carrier effective mass N/m* were determined from the analysis of the refractive index. Wemple-Di Domenico single oscillator model was applied to determine the dispersion parameters such as the high-frequency dielectric constant ε∞, the oscillator energy E0 and the dispersion energy Ed of the films.

Resume : In an effort to prepare thin films of novel semiconductor materials that contain earth abundant, low cost and nontoxic materials, Cu2ZnxFe1-xSnS4 (x = 0, 0.25, 0.5, 0.75 and 1) ingots were successfully synthesized by direct fusion method. Crushed powders of these ingots were used as raw materials for the vacuum thermal evaporation. Cu2ZnxFe1-xSnS4 (x = 0, 0.25, 0.5, 0.75 and 1) thin films were deposited on no-heated glass substrates by vacuum evaporation method. The as deposited films were sulfurized for 30 min at Ts = 400°C. The effects of the sulfurization temperature on the structural and optical properties of CZFTS were studied by X-ray diffraction (XRD) and spectrophotometer. X-ray diffraction XRD patterns show that all as-deposited and sulfurized CZFTS films were polycrystalline in nature with a preferential growth along (112) plane. However, CFTS films exhibit a stannite structure while CZTS films had a kesterite structure. Optical measurements show that CZFTS thin films sulfurized at 400 °C exhibited an optical transmittance between 60 and 80 % and all materials have relatively high absorption coefficients in the range of 104–105 cm-1. The band gap energies of sulfurized CZFTS films increased from 1.1 to 1.55 eV with the increase in Zn content. The dispersion of the refractive index is discussed in terms of the single oscillator model proposed by Wemple and DiDomenico and the optical parameters such as refractive index, extinction coefficient, oscillator energy and dispersion energy were calculated. The electrical free carrier susceptibility and the carrier concentration of the effective mass ratio were evaluated according to the model of Spitzer and Fan. The hot probe analysis showed that all sulfurized CZFTS films are p type conductivity.

Resume : CuFeO2 is is a well known p-type semiconductor that belongs to the delafossite family compounds. This work deals with the preparation and characterization of Cu-Fe-O thin films. Cu-Fe-O thin films deposited glass substrate were prepared via sequential thermal vacuum deposition of Cu and Fe or Fe and Cu after what they are heated in vacuum at 200 °C for 1 hour. The obtained binary systems (Cu/Fe or Fe/Cu) were annealed in air atmosphere at 450 °C for 2 hour. Cu-Fe-O thin films were characterized for their structural, morphological, electrical and optical properties by using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), electrical resistivity and UV-Vis spectroscopy techniques. The X-ray diffraction (XRD) patterns revealed the presence of CuO, Fe2O3 and a minor CuFeO2 phase was detected. The absorption coefficient of Cu-Fe-O thin films obtained by annealing the Fe/Cu system is larger than 105 cm-1. Two optical band gaps at 1.7 and 2.1 eV were found. The electrical measurements show a conversion from a metallic phase to the semiconductor phase by a switching in the electrical resistivity values at the air annealing temperature 450 °C.

Resume : In this contribution, we report on indium sulfur fluoride (ISF) thin-films exhibiting semiconducting properties. The films were deposited on fused silica and silicon substrates using a radio-frequency plasma-enhanced reactive thermal evaporation system. The deposition was performed evaporating pure indium in SF6 plasma at a substrate temperature of 423 K. Energy-dispersive X-ray spectroscopy was used to determine the chemical composition of the films. The electrical, optical, and photoelectrical properties of the films were also studied. The conductivity measurements were performed in the temperature range from 280 to 350 K. The synthesized compound is highly-resistive (~500 MΩ-cm at 300 K), and the deduced activation energy of conductivity is about 0.9 eV. The indirect band energy gap of 2.78-2.9 eV is determined from transmittance spectra of the ISF films. The films are photosensitive in the blue - UV range. The photocurrent under 1.5AM illumination is about two orders magnitude higher than the dark current. The spectral-response measurements have also revealed the presence of Urbach’s tail, which is associated with a lack of crystalline long-range order in amorphous/glassy materials. The possible applications in optoelectronics and photovoltaics for ISF films are also discussed.

Resume : Polymer solar cells have been having a growing attention in relation to producing inexpensive, their light weight and flexibility. However in organic solar cells made from P3HT:PCBM materials, electronic interactions between the interlayers and charge separation can be controversial problem and thus the photovoltaic parameters must be enhanced [1, 2]. Herein, this study investigated the role of modification of TiO2 layer with different phenyl boronic acid derivative self-assemble monolayers (SAM) on photovoltaic performances and electrical properties on P3HT:PCBM solar cell. We characterized the modified and unmodified compact TiO2 surfaces by using XRD, AFM and Kelvin Probe Force Microscopy for the determination of structural, morphological and electrical properties. We also characterized P3HT:PCBM layer on top of modified and unmodified TiO2 layer. In photovoltaic characterizations, we determined that the small methoxy phenyl boronic acid SAM molecules improved efficiencies and photovoltaic characteristics. We observed that some SAM molecules improve the photovoltaic parameters and efficiencies.

Resume : Currently, highest reported power conversion efficiency (PCE) of perovskite solar cell is over 20% with TiO2 as electron transport layer (ETL). Anatase TiO2 can achieve optimal electron transport so high temperature annealing is required. As a result, possibility of the using it as ETL in inverted structure or flexible structure is limited as both architectures require low temperature processing. Low temperature atomic layer deposition (ALD) can be used to tackle this issue. Using ALD, we can further reduce metal oxide film thickness and thus reducing resistance of extracting electron. However, amorphous ALD TiO2 cannot form good electronic contact with perovskite so devices exhibit lower efficiency and hysteresis. SnO2 has better band alignment with MAPbI3 and higher carriers mobility than TiO2. Devices with SnO2 via low temperature ALD deposition are able to have less hysteresis and higher Jsc than ALD TiO2. Although SnO2 is obviously a promising candidate for low temperature deposition, further work is still needed for SnO2 devices to reach the efficiency of those with high temperature processed TiO2. In this work, we conducted systematic investigation of the SnO2 films deposited by different methods. Surface treatments such as O2 plasma, UV Ozone and ethanolamine treatment are performed on ETL for optimization of the performance. Relationship between film properties and device performance is discussed in detail.

Resume : CuSbS2 (CAS) appears to be a promising absorber material due to its earth abundant, low cost and low toxic constituent elements, as well as favorable optical and electrical properties. It shows an energy band gap of 1.38 to 1.56 eV and a light absorption co-efficient more than 104 cm-1. Theoretical investigations showed that CAS can be able to exhibit better efficiency than CuInSe2 (CISe). However, it is critical to achieve pure CuSbS2 phase. There are several ternary and binary phases of Cu-Sb-S, and also the melting temperature of CAS is relatively low, therefore it may easily suffer from the decomposition problem or co-deposition of other phases. Since stoichiometric CAS can be formed in small range of Cu/Sb ratio, so it is important to control precise ratio for the formation of pure CuSbS2 phase. In this study, we used non-vacuum methods using hybrid inks to fabricate non-toxic and low cost CuSbS2 solar cells. Non-vacuum hybrid ink method is very useful to control the Cu/Sb ratio by using the appropriate amount of Cu/Sb chemicals to deposite pure CAS phase. We also studied various range of Cu/Sb ratio to understand the behaviour of this material. To study the compositions, crystal structures and morphology of the films, EDS, XRD, SEM analyses were carried out. The effect of the Cu content on the PV parameters of CAS absorber film will be presented.

Resume : Recently, the photovoltaic field has witnessed a unique emergence for a new family of solar cell materials; namely hybrid perovskite solar cells (HPSC) with the composition of [AmH]MX3, where [AmH]+= protonated amine, M= Pb2+, Sn2+, and X= I−, Br−, Cl−. Within a span of few years, an efficiency above 22 % was achieved. Device-wise, to make an efficient solar cell a combination of several material properties is needed. Practical-wise, the wide range of HPSC materials, device designs, and relatively simple fabrication techniques are astonishing and illustrate the rich potential of the field. However, there are some obstacles hindering the commercialization. Many factors contribute to this serious problem. The main one among them is the materials instability which has been recently improved for example in the case of Cesium-containing triple cation perovskite solar cells. Some others including a poor quality of crystals formed by solution processing techniques are extrinsic and can be –in principle – mitigated by proper device handling and protection. However, recently it was theoretically shown that some commonly used HPSCs (i.e., CH3NH3PbI3) are fundamentally unstable reaction and Hull energies around -30 meV/atom. Here we show that replacing CH3NH3+ by other cations that allow stronger electronic coupling between the cation and octahedra (while maintaining the bandgap energy within the suitable range for solar cells), may enhance the stability considerably.

Resume : Thin film is considered as an alternative absorber since it has high absorption coefficient and allows the low cost solar cells with high efficiency. Representatively, Cu(In, Ga)Se2 showed the impressive solar energy conversion efficiency of 20.4 %. However, the scarcity of the In and Ga restricted the terawatt scale deployment. Thus, Cu2(Zn, Sn)S4 containing the only earth-abundant materials has been studied and achieved the energy conversion efficiency of 11.1 %. Nevertheless, the formation of the secondary phases, such as CuxSnySz and SnS, limits the improvement of photovoltaic efficiency. Sb2Se3 having high absorption coefficient (>105 cm-1), band-gap of 1.0-1.3 eV, and high mobility (> 42 cm2/Vs) is an emerging material for photovoltaic applications. Unlike Cu2(Zn, Sn)S4, Sb2Se3 is easy to prepare the uniform composition and large grains due to its single fixed composition and low-melting temperature (~885 K). Recently, J. Tang group employed thermal evaporation method for Sb2Se3 layer and reported energy conversion efficiency of more than 5 %. However, a cost-effective electrochemical approach for the synthesis of Sb2Se3 compounds has not been identified. In this study, Sb-Se precursors were prepared by electrodeposition and conveniently annealed at 573 K to form the Sb2Se3 compounds. After that, to manufacture the photovoltaic device, CdS, ZnO, and ITO layers were deposited. As a result, we simply achieved thin film solar cells having the energy conversion of > 1 %.

Resume : Carrier concentrations and their mobility in the absorber are important parameters that have a strong influence on the efficiency of solar cells. As for many types of thin film solar cells the origin of the p-type doping in wide bandgap CZGS(e) absorber films is still under dispute and the carrier mobility is not straightforward to measure. To electrically characterize them, these CZGS(e) films have been deposited on quartz, glass and on glass with Mo substrates. The films deposited on quartz and glass have been measured with Hall- VanderPauw configuration. On top of the specimen with a conductive Mo back contact an Al schottky barrier was evaporated for capacitance profiling. The films on the quartz substrate were highly resistive while the films on the glass had a resistivity of 34Ω.cm. AC-Hall effect measurements at 1.5Hz showed on the latter specimens that the film was p-type with a doping level 3-5E17cm-3, which correspond with a majority carrier mobility of 0.25cm2/V.s. CV and DLCP measurements on the films with a Mo back contact give a doping level of 5E15cm-3. Based on strong difference in resistivity for the films on quartz and on glass, and the 50-100 times lower carrier concentration in the diodes with a Mo back contact we suggest that the impurities resulting in the p-type doping are introduced from the glass substrate.

Resume : Extremely thin absorber (ETA) solar cells are a promising concept for low-cost photovoltaics. In the ETA community, nanoparticular titanium dioxide films are often used to meet the requirement for large surface area to volume ratios. However, using nanoparticular films as scaffolds relies on the trap-limited diffusion of charge carriers, which restricts device efficiency. Thus, the utilization of highly ordered titania nanotube arrays is a promising approach to provide short, direct carrier pathways. In this work, titanium dioxide nanotubes are grown by anodization of sputtered titanium thin films on indium tin oxide (ITO) glass. An “interrupted anodization” procedure is presented as a novel facile approach to producing open nanotubes without the disordered initiation layer. The n-TiO2 nanotube arrays are subsequently coated with intrinsic, light-absorbing, antimony sulfide via atomic layer deposition (ALD) to realize a uniform absorber thickness over the whole nanotube surface. p-CuSCN is deposited as hole conductor by solution evaporation and a sputtered gold layer is used as a back contact. The device structure is characterized after each processing step, and the performance of final devices is investigated by I-V curves, quantum efficiency, and impedance spectroscopy. This system allows for systematic investigation of geometric effects on ETA solar cell performance parameters.

Resume : Applications of silicon solar cells are limited by their cost and/or efficiency. Amorphous silicon (a-Si) cells provide cost reductions but their planar geometry limits their performance. In the present work, nanopore arrays are used as a template to structure a-Si junctions to address this issue. Nanopore arrays can display excellent antireflection characteristics, efficient light trapping, and improve the efficient charge carrier diffusion and collection in thin coatings due to the short collection length of photogenerated carriers. This offers opportunities for improving solar cell efficiency and reducing material consumption. Here, 'anodic' aluminum oxide is produced as a nanoporous template, then coated with SiO2 by atomic layer deposition. After growing SiO2 doped with either Sb (n) or Al (p) as thin layers on the pore walls, Li is used to reduce SiO2 into Si thermally. Finally, electrical contacts are sputter-coated on both sides of the template to contact each side of the Si homojunction, and fabricate it into a solar cell. This preparative strategy offers a model system in which to investigate systematically how geometry and doping levels can be exploited to optimize the solar cell’s performance.

Resume : CuSbS2 semiconductor is receiving a lot of interest because of its practical application as absorbing layer in solar cell devices. The main advantage of this relatively new material is that is formed exclusively by earth abundant elements. In this work, CuSbS2 thin films were obtained by a sequential process consisting in the thermal evaporation of metal stack precursors, followed by a reactive annealing under S atmosphere. Metallic stack order, Cu/Sb ratio, and reactive annealing conditions were studied and optimized. The films were characterized by XRD, SEM and Raman spectroscopy. Additionally, the absorbers were incorporated into solar cells with the Glass/Mo/CuSbS2/CdS/ZnO/ITO structure and then fully characterized. Fundamental characterization of the CuSbS2 films showed a thickness of 1.4 µm approximately with large grains (~1 m), and well crystalized phases. XRD analysis confirms that the crystalline structure of the annealed films is calcostibite. Additionally, we observe that the metallic stack order and Cu/Sb ratio are crucial for obtaining working devices. In particular, layers prepared from metallic stacks with Cu on top shows poor devices performance, whereas the best working solar cells were obtained with a Cu/Sb ratio close to 1.0. Devices with Cu/Sb ratio below 0.90 results in efficiencies lower than 1%. Through a preliminary optimization of the processes we achieve a record device with Voc = 440 mV, Jsc = 11.4 mA/cm2, FF = 51.2 and η = 2.57 %.

Resume : CZTS (Cu2ZnSnS4) is a promising material for solar cell absorbers and consists of abundant, environmentally friendly elements with a recently obtained efficiency of 9.4 %. For films produced with pulsed laser deposition (PLD) from a quarternary sintered, multi-element target, the composition can deviate significantly from that of the target, depending on the laser fluence. By tuning the laser fluence from a 248-nm excimer laser beam on a sintered CZTS target (2CuS:ZnS:SnS), the metal ratios in the film can be controlled. Films were deposited in high vacuum on a Mo-coated soda lime glass substrate at room temperature. At low fluence below 1 J/cm2 the copper content in the film is low, while at high fluence the films become copper-rich. At a fluence 0.7 J/cm2 we have achieved a ratio Cu/(Sn+Zn) ~ 0.85 which is in the right regime for a cell. After sulfurization and the final deposition of the buffer and window layers the best cell with an effective area of 21 mm2 and only a 400-nm thick CZTS layer had an efficiency of 5.2 % (and Voc = 616 mV, a Jsc = 17.6 mA/cm2 and a fill factor of 48 %). This efficiency is the highest one obtained with an absorber layer of CZTS made by PLD until now. Despite the thin absorber, there are no signs that material and junction quality are significantly lower than that of thicker absorbers: grain size, carrier lifetimes, collection efficiency, shunt resistance, and dark saturation current are all similar to (thicker) benchmark CZTS cells.

Resume : There is an increasing demand to develop higher efficiency photovoltaic materials with desirable properties such as a band gap between 1.1 eV to 1.5 eV, high abundancy and biocompatibility. Although CdTe and CIGS are the most commonly used and commercially available thin film photovoltaic materials in solar cells, there are major drawbacks including the scarcity of tellurium and high toxicity of cadmium would hinder the potential of such materials and impact the environment. In order to overcome such challenge, earth-abundant photovoltaics are of increasing interest to become a better substitute and the new rising star in photovoltaic applications. NaSbS2 has been found to have desirable features for an efficient solar absorber material, such as an experimental band gap value of 1.5-1.8 eV and a large absorption coefficient within the visible light range(1). Recently, Rahayu et al. have demonstrated a strong absorption of light in NaSbS2 coated SSCs (semiconductor-sensitized solar cells) which leads to a high photovoltaic efficiency of 2.30%(2). Employing NaSbS2 in photovoltaics is a relatively new and less explored area, however, non-toxicity and a high earth abundancy, together with the desirable features of NaSbS2 could make it an ideal candidate for photovoltaic applications. In this study, we will investigate NaSbS2 using different characterization techniques such as PXRD, UV-Vis, XPS, EDS and SEM as well as ab initio calculations using hybrid Density Functional Theory to give a detailed analysis of the properties and potential applications of NaSbS2. (1.) V. Bazakutsa, N. Gnidash, A. Kul’chitskaya and A. Salov, J Sov. Physics, 1975, 18, 472-475 (2.) S. Rahayu, C. Chou, N. Suriyawong, B.A. Aragaw, J. Shi and M. Lee, APL Mater. 4, 2016, 116103

Resume : Sb2S3 has been considered as a promising material for solar cell application, owing to its high absorption ability, earth-abundant and low-cost. This study is focused on the preparation of single phase Sb2S3 thin films by chemical spray pyrolysis method as an absorber layer for solar cell applications. Sb2S3 thin films were deposited by ultrasonic chemical spray pyrolysis method onto glass/ITO/TiO2 substrates using the solutions containing SbCl3 (15mM) and thiourea (SC(NH2)2) in molar ratio of 1:3 in methanol. The films were deposited at 220°C in air and subjected to post-deposition thermal annealing in the temperature range of 250-350 °C in air, nitrogen or hydrogen sulfide containing atmospheres. The morphology, chemical composition, structural and optical properties of Sb2S3 thin films were characterized by SEM, XPS, Raman, XRD, UV-vis methods. The results show that Sb2S3 thin films grown at 220 °C and thermally treated in nitrogen atmosphere were dense, continuous, and single phase. A flat ITO/TiO2/Sb2S3/P3HT and structured ITO/ZnO(rod)/TiO2/Sb2S3/P3HT hybrid solar cells showed conversion efficiencies of 1.6 % and 2.3 %, respectively. Solar cells using absorbers annealed in sulfur containing atmosphere are in preparation.

Resume : The recent progress in the use of hybrid organic-inorganic perovskite as light harvester in thin film solar cells yielded power conversion efficiency of >22%, an unprecedented rise from the first value of 3.8% reported few years ago. This recent surge of interest put perovskites based solar cells as strong bet for future PV technology. The main interest is to further optimize the materials, explore novel configuration and architectures in order to further improve the photovoltaic properties as well as make this technology suitable for future commercialization. Spiro-OMeTAD represents the state of the art and currently the best material used in hole transporting materials (HTM) working as electron blocking and p-type material. However, it is too costly due to its multistep synthetic route and complex purification, which raises its price. Due to its relatively poor hole mobility and conductivity, it needs p-type additives in order to efficiently extract and conduct the holes through the device. Recently researchers are focusing in the design of novel small organic molecules as cheaper and more efficient HTMs. Thiophene-based p-type molecule were extensively investigated and are being used in opto-electrical devices, due to their intriguing characteristics. Here we present novel molecules with thiophene-based core, which compromises of different side arms. The opto-electrical properties of the novel molecules were measured, and were tune as such to yield better photovoltaic parameters for efficient device fabrication. The rationally designed molecules can be synthesized in few synthetic steps and gave encouraging results, which are at par with the classical Spiro-OMTAD.

Resume : Iron-disulfide (FeS2) or pyrite is an interesting semiconductor material for solar energy harvesting exhibiting a band gap of about 1 eV and an optical absorption coefficient over two orders of magnitude higher than the one of silicon. So far however, pyrite remains to be only „fools gold“ due to its severe bulk and surface defect states, leading to high charge carrier recombination rates and consequently to low solar cell efficiencies of a few percent only in the best case. Here, we present a straightforward synthesis approach with the aim to convert iron films and sheets as well as bottom-up grown single-crystalline iron-oxide nanowires directly into pyrite at atmospheric pressure using plain low-carbon steel and iron sheets of 25 to 250 µm thickness as well as thin iron films sputtered onto FTO glass slides as substrate material. For the experiments a quartz tube furnace was utilized with attached nitrogen driven di-tert-butyl-disulfide bubbler system. Di-tert-butyl-disulfide acts as the non-toxic sulphur source. The synthesis temperature was in the range of 400°C and besides the substrate materials also the ambient synthesis conditions were altered including oxygen, nitrogen and hydrogen partial pressure. We discuss the material morphology, optical properties as well as their chemical composition. Furthermore, ambient-photoelectron-spectroscopy and measurements of the contact potential difference are shown providing the crucial position of the surface Fermi level.

Resume : This study was intended for solar cell applications to fabricate an alternative p-type semiconductor layer. Similar to the CIGSe and CZTSe, SnSe belongs to the family of selenium-based binary compounds. Moreover, considering the Cd-based layers, the constituent (Sn, Se) elements are non-toxic, and it has a band gap in the range of optimum photovoltaic materials with a high optical absorption characteristics. In this work, the surface characteristics of the SnSe thin films were investigated under the effect of both substrate temperature and post-annealing temperature. Thin films with different substrate temperature (from room temperature to 400 °C) were deposited on well-cleaned soda-lime glass substrates by using rf-magnetron sputtering. Additionally, the obtained thin films were annealed under nitrogen atmosphere up to 400 °C, considering their substrate temperatures during the deposition process. Changing both substrate and annealing temperatures showed that SnSe thin film samples have different structure and composition when compared with the reference as-grown sample deposited under the same condition. The surface characterization of the films were carried out by atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements. In addition, the compositional analysis were done by energy dispersive X-ray analysis (EDXA), their crystalline phases and structure were investigated by X-ray diffraction (XRD) measurements. Transmission spectra and absorption characteristics of the films were studied in the spectral range of 300-1100 nm.

Resume : CZTS nanocrystals used as sensitizers in semiconductor sensitized nanocrystalline solar cells, can be tuned to have an ideal band gap for light absorption and consequently current generation. Although these nanocrystals are known to have high extinction coefficients, solar cell efficiencies remain below 8%. This is partly attributed to the fact that the metal oxide/CZTS nanocrystal/hole transport layer heterojunctions have not yet been optimised for charge extraction. Due to the limited number of photophysical studies on these heterojunctions, there is currently a lack of understanding about the key mechanisms which impact quantum yield. Solution-processed CZTS nanocrystals are prepared by hot injection and passivated with halides for removal of surface states. Time-resolved transient absorption and luminescence spectroscopies are employed to study the mechanisms of the heterojunction interfaces formed by these crystals. Most importantly, the charge separation mechanism is examined by variation of the size of the energetic driving step between the LUMO of CZTS nanocrystals and the metal oxide’s conduction band. This energetic step is controlled by varying the metal oxide in use. This study aims to the establishment of sets of design rules for CZTS nanocrystal sensitised solar cells.

Resume : ZTO is a very interesting material for a wide panel of applications in electronics and optoelectronics, including transparent electrode for solar cells, chemical gas sensors, light-emitting diodes, active layers in thin film transistors, transparent conductive oxide layers for liquid crystal displays, and other emerging display technologies . Thus, Sn and Zn based oxide films (ZTO) were deposited on plane Si(100) substrates by direct liquid injection MOCVD from an original and specially designed heterometallic precursor ZnSn2O(OiPr)5(acac)3. Thermogravimetric and mass spectroscopy analyses were performed on this precursor to understand its behavior in the vaporizer and to verify its thermal stability. Depending on the experimental conditions, the film surfaces are homogeneous, smooth and above all nanostructured. Amorphous or slightly crystallized films were obtained whatever the substrate?s temperature. Chemical and crystallographic structures were controlled using grazing X-ray diffraction (GIXRD), field emission gun-scanning electron microscopy (FEG-SEM) associated to energy dispersive X-ray spectroscopy (EDS), Fourier transformed infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). Results are discussed according to experimental synthesis conditions: the main characteristics of the films (stoichiometry, structure and nanostructure) and electrical/optical properties have been established. In that way, both oxidizing atmosphere during deposition and substrate temperature influences/effects were explored. For peculiar experimental conditions, some films exhibit promising electrical and optical properties.

Resume : Dye-sensitized solar cells (DSSCs) have been intensively studied as an alternative to high cost commercial Si solar cells due to to their simple and inexpensive manufacturing procedures yet relatively high conversion efficiencies. Compared to the conventional TiO2 nanoparticle (NP) photoelectrodes, the TiO2 nanotube (NT) arrays photoelectrodes show faster electron transport and slower charge recombination rates. However, TiO2 NT based DSSCs have not shown efficiencies as high as TiO2 NP based DSSCs due to their lower surface area and reduced dye adsorption. TiCl4 post-treatment of the TiO2 photoelectrode, which has increased the surface area of TiO2 photoelectrode, has been used to improve the cell efficiency. In this study, we have optimized TiCl4 treatment conditions for the trench structured TiO2 NT-DSSCs. Depending on the temperature of the TiCl4 treatment, the TiO2 NP layer formed on the surface of TiO2 NT arrays showed different morphologies. These different morphologies affected the cell efficiency of the trench structured TiO2 NT based DSSCs. The rough surface of the TiO2 NP layer formed by TiCl4 treatment at 50 ℃ was better than the uniform surface formed by TiCl4 treatment at 70 ℃ for charge transfer from the electrolyte to the TiO2 NT arrays. The TiCl4 treatment of the trench structured TiO2 NT arrays was effective at a temperature lower than 70 ℃.

Resume : Perovskite solar cells (PSC) are considered as strong candidates for the next generation of solar cells due to a surge in their efficiency within a very short time span. However, in spite of advances, there are some issues that should be resolved to attain stable and lucrative cells such as the limited stability of perovskite and the high cost of charge transport materials. In this work, we present a full-space material-independent optimization to maximize the efficiency of PSC. The enhancement of PSC performance requires an in-depth analysis of individual components and their cumulative effect on the global performance of the cell. Thus, besides the light absorbing layer and the device, we consider optimizing the electron and hole transport materials (ETM and HTM) as well. Most of the materials used as ETM and HTM and metallic contacts are based understandably on commonly used ones. So, it is reasonable to assume that there is a possibility to identify better matching materials. The effects of various physical parameters on the conversion efficiency have been analyzed. The optimization space identifies the properties of the optimal matching materials by selecting 23 optimum design parameters. The considered optimization parameters are: •Front and back contacts: work function, •ETM and HTM: permittivity, carriers mobilities, dopant concentrations, band gap, absorption, bands densities, affinity energy, and thickness, •Perovskite absorber: thickness The full space optimization identifies the properties of the device layers that can achieve the maximum efficiency of 26.63% without defects and this is reduced to 23.42% if the defects are considered. These results can be used to guide the selection of new materials using screening process.

Resume : Perovskite solar cells (PSCs) have received great attention because of their optimized optical and electrical properties for photovoltaic applications. Recently, a rapid increase in the photovoltaic performance of PSCs with organic hole transport materials (HTMs) has been reported. As the photovoltaic performance of PSCs with organic HTM increased, the possibility of commercialization of PSCs has improved. However, future commercialization can be hampered because the stability of PSCs with organic HTM has not been guaranteed for long periods under real working conditions, including moist conditions. Furthermore, commonly used organic HTMs are high-priced because material synthesis and purification are complicated. We herein report, octadecylamine-capped pyrite nanoparticles (ODA-FeS2 NPs) as a multi-functional layer (charge extraction layer and moisture-proof layer) for PSCs. FeS2 is a promising candidate for the HTM of PSCs because of its high conductivity and suitable energy levels for hole extraction. We synthesized highly dispersed FeS2 NPs that were passivated by ODA, a stabilizing ligand to increase hydrophobicity. A multi-functional layer based on ODA-FeS2 NPs showed outstanding hole transport ability and water-proof performance. Through our approach, the best-performing device with ODA-FeS2 NPs-based multi-functional layer showed a power conversion efficiency of 12.6% and maintained stable photovoltaic performance in 50% relative humidity (RH) for 1000 h. As a result, this study has the potential to break through the barriers for the commercialization of PSCs.

Resume : Earth abundant compounds are of interest as possible alternatives to CIGS technology. CZTS is the most studied material, but also has its own flaws such as easy formation of secondary phases, decomposition effects during synthesis and post process treatments, and formation of Cu-Zn defects that limit device performance. In this context, Cu2SnS3 is an interesting candidate with suitable optoelectronic properties and a wider stable composition range with fewer constituents (avoiding Zn related defects). Currently the technology is still in an early stage of development with a 4.63% record efficiency (6% using Ge doping). The performance is currently limited by the formation and coexistence of different polymorph/stoichiometry phases which indicates the necessity for precise composition control and process optimization. An analysis of Cu2SnS3 films with different stoichiometry, good crystal quality, as well as working solar cells is presented in this work. A comparative study by XRD and Raman allowed to identify the most characteristic Raman bands for the Cu-Sn-S phases: orthorhombic Cu3SnS4 (317 cm-1), monoclinic (292 and 354 cm-1) and cubic (300 and 356 cm-1) Cu2SnS3, and orthorhombic Cu4SnS4 (326 cm-1) compounds. By evaluating the Raman signals of the finished devices, we find a correlation between the cubic contribution and performance. Finally, a non-destructive methodology for fast and precise evaluation of cubic Cu2SnS3 formation at the surface is proposed.

Resume : Tin sulfide (SnS) is an earth-abundant element-based semiconductor material and can be widely used in photovoltaic applications thanks to several advantages such as bandgap in the range of 1.1-1.3 eV which is similar to that of silicon and absorption coefficient higher than 104 cm-1. SnS can be a good absorber layer in solar cells even with very thin films less than 1 um in thickness, and it is reported that, in theory, it could produce a high conversion efficiency of 32% in solar cells. To date, however, the best efficiency of the SnS solar cell is 4.36% and the absorber was fabricated by atomic layer deposition (ALD) technique [1]. SnS thin films were prepared using co-evaporation processes in this study, which is one of the best methods to control the film composition. The films were synthesized with different process conditions such as both Sn and S effusion cell temperatures and deposition time, trying to avoid secondary phase formation in the films. As-synthesized films were annealed at different temperature conditions from 100 to 450oC. When the films were annealed at 400oC, Sn/S ratio got close to 1 and only SnS single phase was formed in the films. These films were characterized using energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, UV-Vis-NIR spectroscopy, and Auger electron spectroscopy (AES), and the process was optimized to get appropriate film properties for the solar cell application.

Resume : An improvement of CZTS solar cells performances after a Post Deposition Annealing (PDA) treatment is often reported in the literature. The physical mechanisms behind this improvements is not completely clear even because the optimum parameters of the PDA process (temperature, time, atmosphere etc.) show a large scattering among different groups [1-3]. For our devices the best PDA process is a 15 minutes long annealing at 300 °C in air on a hot plate, giving an improvement of the device efficiency higher than a factor of 2. One important contribution is the Jsc increase which clearly derives from the gap decrease induced by the Cu-Zn cations disordering. Another contribution comes from the junction improvement as shown by the decrease of J0 and n which leads to an improved fill factor. Several physical mechanisms can be involved in this improvement: sodium or copper diffusion, interface reconstruction, CdS modification. [1] M. Neuschitzer et al. “Complex surface chemistry of kesterites: Cu/Zn Reordering after low temperature postdeposition annealing and its role in high performance devices”, Chem. Mater., 27:5279–5287, 2015. [2] K. Sardashti et al. “Impact of nanoscale elemental distribution in high-performance kesterite solar cells”, Advanced Energy Materials, 5:1402180, 2015. [3] H Xie et al., “Impact of Na dynamics at the Cu2ZnSn(S,Se)4/CdS interface during post low temperature treatment of absorbers”, ACS Appl. Mater. Interfaces, 8:5017–5024, 2016.

Resume : In this work, the structural, morphological and nano-mechanical properties of SnTe thin films were investigated as a comparative study of theoretical and experimental analyses. In theoretical calculations, the crystal structure of the deposited films was constructed by density functional theory (DFT) according to the cubic lattice formation. Some basic physical properties, such as lattice constant, electronic band structures, and optical properties (dielectric functions, refractive index, and energy loss function) were calculated. Also, the elastic constants were predicted using the ?stress?strain? method. We performed numerical estimations of the bulk modulus, shear modulus, Young?s modulus, Poisson?s ratio anisotropy factor, G/B ratio, and hardness. In addition, the XRD characteristics were modeled based on the experimental data and from the optimized structure, lattice parameters were calculated. For experimental step, thin film samples were deposited on glass substrates by using single magnetron sputter system at room temperature. The characteristics of the as-grown samples were detailed by X-ray diffraction (XRD), atomic force microscopy (AFM) and nano-indentation techniques. From XRD spectrum, SnTe films were found in cubic phase with (200) preferred orientation direction. Surface topography imaging was observed by using AFM, and nano-scale topographic features and surface roughness of thin films were also investigated. Moreover, the hardness and Young?s modulus of SnTe thin ?lms determined by means of Berkovich nanoindenter.

Resume : Solar cells based on organic lead halide perovskites have already reached the power conversion efficiency up to 22.1%. However, fundamental recombination mechanisms are not yet fully understood. Photoluminescence (PL) of perovskite thin films can be enhanced by several orders of magnitude upon light exposure [1, 2]. During the illumination defects in the perovskite crystallites can be passivated and thus the PL quantum yield and electrical properties (diffusion length, open-circuit voltage etc.) are enhanced. We studied the enhancement by PL microscopy and compared the results with Fourier-transform photocurrent spectroscopy (FTPS) which was recently used to identify sharp absorption edge as the reason for the high material quality of perovskite films [3]. In this study, we explore the absorption edge (defect density) and photo-current upon illumination. Surprisingly, even if the local electrical quality improves, the photo-current detected by FTPS decreases with illumination time. Furthermore, a microscopic model based on grain boundaries to explain the measured dependences is proposed. [1] S. D. Stranks et al.: Phys. Rev. Applied 2 (2014) 034007. [2] Y. Tian et al.: Phys. Chem. Chem. Phys. 17 (2015) 24978. [3] S. De Wolf et al.: J. Phys. Chem. Lett. 5 (2014) 1035. This research was supported by the Czech Science Foundation project 17-26041Y.

Resume : In this study, we investigate the electrical, structural, and optical properties of band gap front-graded Cu2ZnSn(S,Se)4 (CZTSSe) thin films grown by a modified single-step sulfo-selenization process from copper-poor and zinc-rich precursor metallic stacks prepared by co-evaporation. To investigate how the bandgap was graded in connection with the compositional distribution, we calculated the bandgap energy distribution along the film thickness, based on the transmission electron microscopy and energy-dispersive X-ray spectroscopy composition profile. The band gap of the CZTSSe phase with high S content near the surface layer is determined to be 1.161 eV. From the surface to the bottom, there is a decrease in the S content of the CZTSSe phase, and the band gap subsequently decreases to, 1.029 eV, close to the value of CZTSe. From the results of dimpling-Raman and scanning transmission electron microscopy line scanning, we confirm that the S content drastically increases from the bottom to the top surface of the CZTSSe thin film. The CZTSSe thin-film solar cell exhibits a power conversion efficiency (PCE) of 10.33%, with an open-circuit voltage (Voc) of 0.505 V, short-circuit current density (Jsc) of 31.61 mA / cm2, fill factor (FF) of 64.6%, and Voc deficit of 525 mV. Compared with the performance of the CZTSe solar cell, which had PCE of 7.23%, Voc of 0.424 V, Jsc of 32.83 mA cm-2, FF of 51.9%, and Voc deficit of 576 mV, the Voc and Voc deficit of the CZTSSe cell improved considerably. The high Voc, low Voc deficit, and less loss of Jsc are attributed to the effect of band gap front-grading induced by S grading into the CZTSSe thin film.

Resume : Molybdenum sulfide has been considered a promising low-cost and earth-abundant electrocatalyst alternative to the conventional noble metal catalysts such as platinum (Pt) for use in water splitting. In this presentation, atomic layer deposition (ALD) of molybdenum sulfide (MoS2) will be reported and their application in catalyzing the oxygen evolution reaction (OER) will be investigated. Two examples will be given focusing on electrochemical and photoelectrochemical water splitting, respectively. Firstly, MoS2 is deposited on porous cobalt foam (CF) current collector, forming an integrated electrode that can be directly used for electrochemical water splitting. When used to catalyze the OER, the as-obtained CF@MoSx electrode exhibits a large cathodic shift of ca. 200 mV in onset potential, a low overpotential of only 270 mV to attain an anodic current density of 10 mA cm-2, much smaller charge transfer resistance and substantially improved long-term stability at both low and high current densities. Secondly, ALD of MoS2 is carried out on the silicon nanowire and hematite nanorod array substrates. The introduction of MoS2 electrocatalysts substantially improves photoelectrochemical water splitting performance of these semiconductor electrodes. ALD provides a versatile approach for coupling electrocatalysts with semiconductor photoelectrodes with complex surface topography.

Resume : Organic photocatalysts are attracting more and more attention in the field of solar water splitting owing to desirable properties such as their high synthetic tuneability. In comparison to inorganic materials like metal oxides, HOMO/LUMO energy levels can be adjusted much more easily with respect to the potentials for oxygen/hydrogen evolution. However, only little is known about the underlying processes in organic semiconductors when subjected to conditions that are relevant for sunlight-driven water splitting.[1] Sprick et al. recently published[2] a series of linear polymer photocatalysts with very similar light absorption behaviour, yet a 46-fold difference in hydrogen evolution rate was observed under visible light irradiation and in the presence of a sacrificial electron donor. Consequently, an investigation of this system on a more fundamental level can be expected to allow for the identification of key factors that govern the photocatalytic performance of this class of materials. In this study, we expand the original polymer series towards even higher hydrogen evolution yields and investigate photoinduced processes as a function of photocatalytic activity using transient absorption spectroscopy. Transient absorption spectroscopy is an adequate technique to gain insight into such photoinduced processes, as it yields information about the lifetimes and kinetics of photogenerated charge carriers by probing their evolution over time. In this way, we monitor photogenerated charges on the µs – s timescale, which includes the fraction of charges with a sufficiently long lifetime to perform proton reduction. By relating this long timescale data to photochemical transients observed at ultrafast times, we are able to draw a comprehensive picture of the processes that take place upon illumination. Importantly, we can directly correlate our transient signals to the activity of the respective polymer and are therefore able to rationalise the striking difference in performance. Molecular dynamics simulations complement these optical measurements and, taken together, the presented results lead to an improved understanding of design strategies for more efficient organic photocatalysts. References [1] Guiglion, P.; et al. Macromol. Chem. Phys. 2016, 217 (3), 344–353. [2] Sprick, R. S.; et al. Angew. Chemie Int. Ed. 2016, 55 (5), 1792–1796.

Resume : The development of a high-performance oxygen evolution reaction (OER) catalyst is pivotal for the practical realization of a water-splitting system. Although an extensive search for OER catalysts has been performed in the past decades, cost-effective catalysts with affordable processing costs remain elusive. Herein, we introduce an amorphous cobalt phyllosilicate (ACP) with layered crystalline motif prepared by a room-temperature precipitation as a new OER catalyst; this material exhibits a remarkably low overpotential (η ~ 367 mV for a current density of 10 mA cm-2). A structural investigation based on X-ray absorption spectroscopy reveals that the amorphous structure contains layered motifs similar to the structure of CoOOH, which is demonstrated to be responsible for the OER catalysis based on density functional theory calculations. However, the calculations also unveil that the local environment of the active site in the layered crystalline motif in the ACP is significantly modulated by the silicate, leading to a substantial reduction in the OER η compared with that of CoOOH. This work proposes amorphous phyllosilicates as a new group of efficient OER catalysts and suggests that tuning of the catalytic activity by introducing redox-inert groups may be a new unexplored avenue toward the design of novel high-performance catalysts.

Resume : Although Cu2O is a commonly used narrow band gap semiconductor to fabricate visible response photocatalyst, up to date there is only a few reports on Ag co-catalysed TiO2-Cu2O nanocomposites despite the promising synergetic effect of coupling and plasmonic co-catalysis. Herein we report a simple wet chemical synthesis method coupled with a UV treatment step for TiO2-Ag-Cu2O ternary hybrid nanomaterials. The effect of Ag content and the synthesis sequence of Ag deposition on the photocatalytic performance was investigated. The size distribution, crystal phase, optical and dark adsorption properties of the nanostructures were characterized by SEM, XRD and diffuse reflectance, respectively. Due to the mixed indirect and direct nature of the nanocomposites, the band gap estimation was performed by using both Tauc plot and differential reflectance model. The apparent visible activities followed pseudo-zero order kinetics where TiO2-Ag(3%)-Cu2O catalyst exhibited the highest rate constant, which was ca. two times as high as that of the binary TiO2-Cu2O catalyst. The synthesis sequence of the Ag deposition step significantly altered the material properties resulting in different dark adsorption and apparent visible activities. The improved performance of TiO2-Ag-Cu2O ternary hybrid materials could be related to the level of dark adsorption and could be attributed to the sophisticated charge separation framework between TiO2 and Cu2O and between Cu2O and Ag.

Resume : Using earth abundant metal oxides as photoelectrodes for photoelectrochemical (PEC) water splitting is a clean and sustainable way to produce hydrogen fuel on a large scale. However, solar-to-hydrogen efficiency is low when a metal oxide photoelectrode (e.g. TiO2) is used. This is because of the large band gap (>3 eV) of most metal oxides, primarily due to their low valance band edge position. It has been shown that the band gap of wurtzite phase CdxZn1-xO alloys can be tuned from 3.3 to 1.7 eV by the up-lifting of the valance band edge. In this work, we present the materials properties of ZnO-CdO alloy thin films deposited by radio frequency magnetron sputtering. Wurtzite CdxZn1-xO alloy with x=0.55 has a band gap of 1.95 eV, which is close to the band gap of an ideal PEC photoelectrode. By varying the growth condition, we achieve an effective charge transport in CdZnO thin films with an electron concentration of ~10^18 cm^-3 and mobility of ~10 cm^2/Vs. Since the Si valance band edge is approximately aligned with the conduction band edge of CdZnO, a Si solar cell can be connected to the CdZnO layer to compensate for the lower location of the CdZnO conduction band edge compared to the water reduction potential so that spontaneous water splitting can occur. In this talk we also present preliminary PEC measurements on CdZnO photoanode and CdZnO on p-type Si substrate and propose a tandem structure for efficient PEC water splitting using a CdZnO photoanode.

Resume : Recently, solar absorbers based on metal cations with stable ns2 outer electrons have come to prominence. This is because recent theory suggests that these compounds may replicate the electronic structure of hybrid lead halide perovskites that may be favourable for defect tolerance and good photovoltaic performance [1]. While promising lifetimes have been measured in some materials, efficiencies have been weak. BiOI is one such ns2 compound. While calculations suggest it to replicate the perovskite electronic structure, device efficiencies have typically been <0.1%. Recently, a 1% BiOI photoelectrochemical device was demonstrated, but it remains unclear as to whether there is significant room for improvement. It has also been shown that BiOI is unstable in 0.5 mol/L NaSO4 electrolyte. In this work, we revisit BiOI through theory and experiment. We grow BiOI through chemical vapour transport and show it to be phase-stable in ambient air over a controlled 197-day experiment. Our BiOI exhibited room-temperature photoluminescence, and we find the lifetime to be promising for photovoltaics. Through first-principles calculations, we find BiOI to be tolerant to the vacancy and antisite defects we investigated because of high formation enthalpies and the Fermi level being pinned close to mid-gap. We develop an all-inorganic device structure (ITO/NiOx/BiOI/ZnO/Al) and achieve 1.8% efficiency without hysteresis, almost double previous reports. Our devices achieve up to 80% external quantum efficiency (EQE), which also exceeds previous reports. Through detailed loss analysis using EQE, intensity-dependent current-voltage, and photoemission spectroscopy measurements, as well as optical loss analysis, we identify significant potential for future increases in efficiency by improving carrier collection and interface engineering. Our work therefore shows BiOI to hold strong promise for solar cells. [1] Chem. Commun., 53, 20-44 (2017)

Resume : Trap states and their impact on electronic properties in solution processed opto-electronic materials are decisive for the performance of devices such as organometal halide perovskite solar cells. Especially, the role of grain boundaries and surface defects are of vital importance for optimizing facile fabrication routes toward efficient devices. However, questions concerning a detailed analysis of their effects on the energetic landscape, and the dynamics of photoexcited charge carriers therein, remains widely unanswered. In this study, we investigated methylammonium lead halide perovskite layers, which underwent controlled modification of grain size and crystallinity via methylamine induced defect healing of the formed films. Time-resolved terahertz as well as optical pump-probe and photoluminescence spectroscopy give insights into dynamics and mobilities of photogenerated charge carriers, which in conjunction with characteristics of the film morphologies enable deriving a trap state based model of population and mobility dynamics.

Resume : Recent studies have raised the hypothesis that ion migration is relevant to the working principle of perovskite based solar cells. In addition, ion transport might be the underlying cause of hysteresis effects as well as contributing to the devices' degradation process. One possible route to elucidate the charge density (electronic and ionic) distribution in devices and its effect on energy band alignment within a solar cell is to measure its internal electric field. We use electroabsorption to investigate changes in the internal electric field in perovskite solar cells upon application of an external voltage. Previous studies have assigned the electroabsorption signal of CH3NH3PbI3 perovskite devices to Franz-Keldysh-Aspnes effect. We use this signal as a probe of the internal field within the device, and reconstruct the field evolution due to the hypothesized ionic migration. By performing frequency dependent electroabsorption, we are able to analyse the dynamics of electric field screening within the solar cell's active layer. We therefore investigate the behaviour of perovskite solar cells with different structures and discuss the role of different cations, halide anions and interlayers on the observed hysteresis and device performance. Our approach offers a framework for in situ and small perturbation measurement of internal electric field of perovskite solar cells which can be integrated with other frequency domain techniques to determine charge dynamics in devices.

Resume : Inorganic colloidal nanocrystals (NCs) are attractive materials for their use as active layers in solar cells. They are generally composed of earth-abundant elements, and they can be processed from solutions, allowing an easy and cheap fabrication of devices. Moreover, they present adequate properties for photovoltaic applications, like high absorption coefficients and tuneable bandgaps. In this field, quantum dot solar cells have already achieved efficiencies over 11% although employing materials composed of toxic elements such as lead (Pb). In the search for new photovoltaic materials, fulfilling the desired characteristics of being environmentally friendly and low cost, we have extensively worked on Bi2S3 and AgBiS2 colloidal NCs, composed of earth-abundant, non-toxic elements. We have developed ways to modify the size and shape of the nanocrystals, and we have observed that this affects their optoelectronic properties. We have employed these NCs to fabricate solution-processed solar cells, focusing on completely non-toxic devices. Very recently, we have reported a promising certified efficiency of more than 6%, employing very thin layers (~35 nm) of AgBiS2 NCs. A summary of these results will be presented and future directions, such as the use of these NCs as photocatalysts, will be discussed.

Resume : The recombination dynamics and photoluminescence (PL) efficiencies of lead halide perovskites have been known to display atmosphere sensitivity and instabilities still not completely understood. We combine steady state and time resolved PL and transient absorption measurements to investigate the underlying mechanism of such instabilities. Under illumination in inert environment we observe substantial increase in trap density, reflected in a reduction of PL quantum yields and transformation of recombination dynamics. When the sample is exposed to oxygen, on the other hand, we observe a decrease in trap density, suggesting defect passivation. It was also observed for the first time a broad sub band gap PL at room temperature, originating from long lived trap states. The processes reported were observed on lead halide perovskites of different morphologies and halide and cation compositions, revealing intrinsic instabilities of great relevance for optoelectronic device application. By investigating the factors that influence the dynamics of photo-induced trap formation and passivation and analyzing the interplay of defect population and band-to-band carrier recombination, we propose possible mechanisms for such processes and possible methods to overcome its detrimental effects and improve device performance and stability.