Earth-abundant next generation materials for solar energy - IV

Resume : Hybrid organic-inorganic metal halide perovskites (like CH3NH3PbI3) proved to be particularly successful for applications in energy harvesting, but they suffer from poor long-term stability. One possible solution to this problem is to focus on fully inorganic metal halide perovskites, which exhibit significantly improved long-term stability, keeping the same excellent optoelectronic properties and defect tolerant behavior. In particular, caesium lead halide perovskites (CsPbX3 X = Cl, Br, I) are excellent candidates for the realization of high performance solar cells (CsPbI3) and light emitters (CsPbBr3 and CsPbCl3). In addition, one of the main problems that limits the scalability of these materials comes from the lack of sample homogeneity over large areas. Solution-based techniques, which represent the most common low-cost route used for the deposition of perovskite thin films, do not guarantee the control of the homogeneity over an area of several square centimeters. Moreover, they rely on proper solvents which, however, are not always available or are extremely toxic or expensive, like the ones used for CsPbCl3. In this work we report, for the first time, the successful deposition of thin films of CsPbBr3 and CsPbCl3 by Radio-Frequency magnetron sputtering. Morphological, structural, and optical characteristics of the two materials are compared. A detailed photoluminescence (PL) spectroscopy study was conducted at the macro and micro scale in a wide temperature range (10-300 K) to assess the origin of the inhomogeneous broadening and to quantify the PL quantum yield quenching. Our results prove that this deposition technique allows for the realization of high quality nanometric films. Despite the defect tolerant nature of perovskites, some non-radiative centers are still present, especially at the grain boundaries. Indeed, in recent years, several kinds of treatments have been used for halide perovskites, such as annealing or chemical additives. Hydrogen treatment is an old well-known technique which was proven to be successful to passivate both deep and shallow defects in amorphous silicon and III-V compounds. In this work we present results concerning hydrogen treatment realized for the first time on cesium lead halides (CsPbBr3 and CsPbCl3) thin films obtained by RF magnetron sputtering. Different doses of hydrogen have been used. The samples were characterized mainly by PL in a temperature range from 10 to 300 K. Exciton and carrier recombination dynamics has been studied by time-resolved PL measurements with ps resolution. XRD analysis have been also performed to investigate the hydrogen effect on the lattice crystalline phase. A large enhancement of the PL intensity and an increase of PL lifetime at low temperature were found. Moreover, the comparison of the Arrhenius plots of the PL intensity before and after the treatment indicates a modification of the non-radiative defect properties in the hydrogenated material.

Resume : Perovskite (PK) and perovskite/silicon (PK/Si) tandem solar cells require charge selective layers of high efficiency to promote electrons and holes loss-less extraction from the PK photoabsorbing materials. Tin(IV) oxide (SnO2) is currently one of the main candidates to make obtain highly efficient electron selective layers (ESL) for such solar cells (1). If solution-based processes are mainly reported for SnO2 ESL deposition, several groups investigate Atomic Layer Deposition (ALD) as a more scalable and industry-viable technique (2). Although effective ESL were fabricated using ALD-grown SnO2 (SnO2ALD) for PK and PK/Si tandem solar cells (3, 4), they suffer from interfacial defects with PK. Adding another thin film to SnO2ALD another material layer to form bi-layered ESL has proven to passivate such defects (5, 6). Still, reported performances remain lower than for cells with solution-processed SnO2 ESL (7, 8). Aiming at a better understanding about SnO2ALD limitations when employed as single-layer ESL, this work presents a systematic comparison between a solution-processed SnO2 and SnO2ALD. Chemical, electrical, optical and topographical properties of each type of film were investigated as well as their integration in PK single junction solar cells. Diverse annealing conditions and their impact on SnO2 films properties and device performances were also analyzed. The results highlight strong differences between each film properties. Particularly, SnO2ALD films are much more conductive and do not have the same surface energy than our solution-processed SnO2. As a result, cells employing SnO2ALD single-layer ESL present strong limitations despite the high-quality electronic and optical properties of SnO2ALD. Characterization of PK films properties grown on both type of SnO2 did not rise significant differences and allowed us to assume some hindering factors at the SnO2ALD/PK interface in agreement with other works (6, 9). Ongoing photoelectron spectroscopy studies on energy level alignment and PK/ESL interface chemical environment shall bring further insight about electron extraction mechanisms. 1. C. Altinkaya et al., Advanced Materials. 33, 2005504 (2021). 2. V. Zardetto et al., Sustainable Energy Fuels. 1, 30–55 (2017). 3. J. P. Correa Baena et al., Energy Environ. Sci. 8, 2928–2934 (2015). 4. S. Albrecht et al., Energy Environ. Sci. 9, 81–88 (2016). 5. C. Wang et al., J. Mater. Chem. A. 4, 12080–12087 (2016). 6. A. F. Palmstrom et al., Advanced Energy Materials. 8, 1800591 (2018). 7. Q. Jiang et al., Nature Photonics. 13, 460–466 (2019). 8. J. J. Yoo et al., Nature. 590, 587–593 (2021). 9. Y. Lee et al., Adv. Sci. 5, 1800130 (2018).

Resume : Chalcogenide perovskites have recently emerged as a new class of semiconductors composed of earth abundant, non-toxic elements with large chemical and structural bandgap tunability in the visible-infrared region and desirable opto-electronic properties rendering them suitable candidates for scalable solar energy conversion. Past investigations on single crystals of BaZrS3 (BZS) (band gap = 1.9 eV) and its Ruddlesden-Popper phase Ba3Zr2S7 (BZS-327) (band gap = 1.3 eV) show that these are suitable materials for tandem and single junction solar cells respectively. The demonstration of high-quality epitaxial thin film growth of these chalcogenide perovskites is a critical next step to improve our understanding of their physical properties and realize efficient photovoltaic and other optoelectronic devices. Recently, multiple groups have developed innovative but multi-step approaches to achieve thin films of the prototypical chalcogenide perovskite BZS. Here we report single-step epitaxial growth of BZS and BZS-327 thin films on perovskite oxide substrates by pulsed laser deposition from stoichiometric polycrystalline targets. X-ray diffraction shows that the films are strongly textured out of plane and have a clear in-plane epitaxial relationship with the substrate. Electron microscopy studies confirm the presence of epitaxy, even though away from the interface the films are polycrystalline with a large number of extended defects suggesting the potential for further improvement in growth. X-Ray reflectivity and atomic force microscopy show smooth film surfaces and interfaces between the substrate and the film. The films show strong light absorption near the band edge and photoluminescence in the visible region, and fast and efficient photo response. We further discuss the design, fabrication and testing of various chalcogenide perovskite-based photovoltaic devices. The role of processing parameters such as substrate temperature, background gas composition and pressure, laser fluence and the choice of single crystal substrates on controlling the structure and chemical composition of these films will also be discussed.

Resume : Perovskites have emerged as a new PV material exhibiting remarkable efficiency over the last decade. Apart from strong absorption coefficient and high mobility, their long minority carrier lifetime in the presence of numerous defect states has been considered to the most essential factor leading to the success of perovskite PVs. With this “defect-tolerance” feature, highly efficient perovskite PVs can be fabricated through low-temperature and facile processing, which can hardly apply in silicon or gallium arsenide solar cells. Unfortunately, almost all the efficient perovskites contain lead component, and hence the toxicity of lead and its ready solubility in water still impede the industrialization of this potential PV material. This challenge has motivated researchers to investigate other lead-free alternatives that may preserve the advantages of perovskites. Thus, the investigation of such “perovskite-inspired materials (PIMs)” began to draw the attention in recent years. It is not surprising that one perovskite-inspired material is expected to own the similar defect-tolerance to perovskites, and the origin of this unusual feature has been claimed to be strongly related to its electronic band structure. It has been shown that the defect states tend to be resonant within the bands or “shallow” relative to the band edges when the valence band is mainly composed of anti-bonding orbitals, which can be frequently found in compounds with partially oxidized cations such as Pb2+ or Bi3+. The large Bohr effective charge of these cations also lead to larger dielectric constants that can further help to screen charged defects. The “ns2” electron configuration of cations can be thus regarded as a figure of merit for defect-tolerance. Taking into account of the electron configuration along with band gaps, energy level alignment, and crystal structures etc., several PIMs have been identified. NaBiS2 is one of the potential PIMs with Bi3+ as the partially oxidized cations. We have found that NaBiS2 has a band gap of 1.2 - 1.3 eV along with absorption coefficient over 104 cm-1 in visible region, and this material remains stable when exposed in air for over one month. More importantly, we apply transient-absorption spectroscopy technique to show that NaBiS2 has an extremely long carrier lifetime in the order of micro-seconds, and this lifetime will not decrease significantly when percent-level defects are artificially introduced. This result implies that NaBiS2 may be defect-tolerant at certain extent. We also observe that the carrier lifetime seems to be extended when NaBiS2 is exposed in ambient environment overnight, which may be resulted from the passivation from oxygen. By properly optimizing synthesis and ligand-exchange processes, a preliminary NaBiS2 PV device with an efficiency over 0.7% has been displayed. It is also worth mentioning that NaBiS2 is made from relatively earth-abundant elements and can be thus cost-friendly for industrialization. Both the future prospects and the current challenges of NaBiS2 PVs will be discussed in this work.

Resume : N-type transparent conductors (TCs) are key materials in the modern optoelectronics industry. Despite years of research, the development of a high-performance p-type TC has lagged far behind that of its n-type counterparts, delaying the advent of “transparent electronics” based on transparent p-n junctions.[1] Here, we computationally investigate three layered oxychalcogenide structural motifs to try to predict new p-type TCs, namely the [Cu2Ch2][A3B2O5] (325), [Cu2Ch2][A4B2O6] (426) and [Cu2Ch2][A2BO2] (212) structural motifs. Specifically, we have used a materials informatics approach (SMACT) to screen through the search space using low-cost heuristic tools.[2] This reduces the potential combinations from 1800 to a more computationally tractable 228, which then undergo DFT calculations to assess thermodynamic and dynamic stability. In this talk, I will present an update on how our search has predicted >50 novel semiconductors with potential applications ranging from TCs[3,] to photocatalysts,[4], solar absorbers and thermoelectrics. [1] A. Walsh and J.-S. Park, The Holey Grail of Transparent Electroncs, Matter, 3, 604 (2020) [2] D. W. Davies, K. T. Butler, A. J. Jackson, A. Morris, J. M. Frost, J. M. Skelton and A. Walsh, Computational Screening of All Stoichiometric Inorganic Materials, Chem, 1 617 (2016) [3] B.A.D. Williamson, G.J. Limburn, G.W. Watson, G. Hyett, and D.O. Scanlon. Computationally Driven Discovery of Layered Quinary Oxychalcogenides: Potential p-Type Transparent Conductors? [4] G. J. Limburn, M. J.P. Stephens, B. A. D. Williamson, A. Iborra-Torres, D. O. Scanlon and G. Hyett. Photocatalytic, structural and optical properties of mixed anion solid solutions Ba3Sc2−xInxO5Cu2S2 and Ba3In2O5Cu2S2−ySey, Journal of Materials Chemistry A, 8, 19887 (2020).

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

Resume : The exceptional optoelectronic performance of lead-halide perovskites (LHPs) has motivated enormous research efforts toward the discovery of ‘perovskite-inspired materials’ – compounds which aim to replicate the astonishing performance of LHPs while eliminating issues with stability and toxicity.[1–3] Recently, chalcohalides of group IV/V elements have attracted attention, due to enhanced stability provided by stronger metal-chalcogen bonds, alongside compositional flexibility and ns^2 lone pair cations — a performance-defining feature of halide perovskites.[4] A rigorous description of their atomistic properties and performance potential is lacking, however. Following the first experimental report of solution-grown tin-antimony sulfoiodide (Sn2SbS2I3) solar cells,[5] with power conversion efficiencies above 4% (exceeding the first reported solar efficiency of methylammonium lead-iodide (MAPI)),[6] we assess the structural and electronic properties of this emerging earth-abundant PV material.[7] We find that the experimentally reported centrosymmetric Cmcm crystal structure represents an average over multiple polar Cmc21 configurations. The instability is confirmed through a combination of lattice dynamics and molecular dynamics simulations. We predict a large spontaneous polarisation of 37 μC/cm2 that could be active for electron-hole separation in operating solar cells. The resemblance of this dynamic crystal structure and ferroelectric behavior to that of MAPI begs the question of its importance in high-performance defect-tolerant solar materials. Moreover, using state-of-the-art ab-initio methods (hybrid Density Functional Theory including spin-orbit coupling effects), we rigorously assess the efficiency limits of this material on the basis of its electronic structure and predicted defect behaviour, calculating ηmax > 30 % for film thicknesses t > 0.5 μm. The results shine a spotlight on the largely-unexplored class of A2BCh2X3 mixed-metal chalcohalides. These are candidates for solution-processed ferroelectric and optoelectronic devices, with the substitutional flexibility for engineering band gaps, band energies, and lattice polarisation. Our work provides insight regarding both the potential success of this emerging class of optoelectronic materials and structure-property relationships in perovskite-inspired materials, guiding design strategies and expanding the compositional space of candidate materials. 1 Y.-T. Huang, S. R. Kavanagh, D. O. Scanlon, A. Walsh and R. L. Z. Hoye, Nanotechnology, 2021, 32, 132004. 2 Z. Li, S. R. Kavanagh, M. Napari, R. G. Palgrave, M. Abdi-Jalebi, Z. Andaji-Garmaroudi, D. W. Davies, M. Laitinen, J. Julin, M. A. Isaacs, R. H. Friend, D. O. Scanlon, A. Walsh and R. L. Z. Hoye, J. Mater. Chem. A, 2020, 8, 21780–21788. 3 M. Buchanan, Nature Physics, 2020, 16, 996–996. 4 R. Nie, R. R. Sumukam, S. H. Reddy, M. Banavoth and S. I. Seok, Energy Environ. Sci., 2020, 13, 2363–2385. 5 R. Nie, K. S. Lee, M. Hu, M. J. Paik and S. I. Seok, Matter, 2020, S2590238520304471. 6 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, Journal of the American Chemical Society, 2009, 131, 6050–6051. 7 S. R. Kavanagh, C. N. Savory, D. O. Scanlon and A. Walsh, Materials Horizons, 2021 (Accepted).

Resume : Despite long-standing research efforts to develop p-type transparent conductive materials (TCMs), the current generation of optoelectronic devices still relies exclusively on n-type TCM contacts due to their much better trade-off between conductivity and transparency. An important issue in oxide-based p-type TCMs is the deep energy and localized nature of the 2p oxygen states in the valence band, which has negative consequences on both hole dopability and hole mobility in most oxides. Recent computational work [1,2] has identified several earth-abundant phosphides (particularly BP and CaCuP) that could potentially outperform the existing p-type TCMs. These phosphides have shallow, disperse valence bands that are ideal for high hole dopability and mobility. Although their indirect band gaps are rather low (around 2 eV), their direct band gaps are much wider and could ensure transparency in thin film samples. However, thin films of CaCuP have not been made yet, and previously reported BP thin films have not been thoroughly evaluated as transparent contacts [3]. Using a unique combinatorial sputter chamber with reactive PH3 gas, we have mapped the compositional and thermal phase space in BP and CaCuP. To evaluate their potential as p-type TCMs, we have characterized their application-relevant properties (optical transmission, carrier concentration, and mobility) with high-throughput methods. We have found that the electrical conductivity of CaCuP under optimized growth conditions is almost on par with the conductivity of state-of-the-art n-type TCMs such as ITO and FTO, even in the absence of any extrinsic dopant. However, CaCuP films are only moderately transparent in the visible region due to unexpectedly high absorption strength above the indirect band gap. We have also found that BP can be doped p-type up to remarkably high hole concentrations by using extrinsic dopants combined with off-stoichiometry. However, it is challenging to obtain BP films of high crystal quality by sputter deposition. This issue has so far prevented us from reaching high hole mobilities. Despite the experimental challenges, we confirm that both CaCuP and BP could be outstanding transparent conductors if their growth processes could be further improved. More generally, the field of metal phosphides appears to be a fertile search space for potential p-type transparent conductors. [1] B. Williamson et al., Chem. Mater., 29, 2402–2413 (2017). [2] J. Varley et al., Chem. Mater., 29, 2568–2573 (2017). [3] A. Fioretti, M. Morales-Masis, J. Photonics Energy, 10, 042002 (2020)

Resume : The need to develop sustainable energy solutions is an urgency for the socie- ties, with the additional stringent requirement to limit dependence on critical raw materials and to implement a virtuous circular economy model. In this framework, it is essential to identify new avenues for light-conversion into clean energy and fuels exploiting largely available materials and green production methods. Metal oxide semiconductors (MOSs) emerge among other species for their remarkable environmental, thermal and chemical stability, wide chemical tunability, and interesting optoelectronic properties. MOSs are often key constituents in next generation energy devices, mainly with the role of charge selective layers. Their use as light harvesters is hitherto rather lim- ited, but progressively emerging. One of the key strategies to boost and tailor their properties resorts to doping, that can improve charge mobility, light absorption and can tune band structures to maximize charge carrier separation at heterojunctions. In this presentation, effective methods are identified and critically compared to dope MOSs and to exploit the deriving benefits in relation to performance enhancement in different types of light-conversion devices. It is focused specifically on the best opportunities coming from the use of noncritical raw materials, so as to contribute in defining an economically feasible roadmap for light energy conversion technologies based on these highly stable and largely available compounds.

Resume : Searches for new semiconductors have recently focused on the identification of complex nitrides, seeking to replicate the desirable properties of established binary compounds while opening up additional degrees of freedom in new materials. Substantial interest in ZnGeN2 and ZnSnN2 has been driven by their cation site disorder, which can enable bandgap tuning without commensurate changes in lattice parameter. As these searches continue, there is increasing recognition that metastable nitrides can be uniquely synthesizable compared to compounds containing other anions, potentially affording access to properties not exhibited by ground state compounds or enabling the synthesis of multiple polymorphs within the same chemical space. MgSnN2, like ZnGeN2 and ZnSnN2, is a II-IV-N2 compound with a wurtzite-type ground-state structure analogous to the III-N compounds. The ~2.3 eV bandgap of MgSnN2 makes it an ideal target for solid-state lighting applications, and the additional possibility of bandgap tuning with cation disorder could expand its utility. Unlike the Zn-based compounds, MgSnN2 also displays polymorphism, with the potential to form as a >2.9 eV bandgap, high dielectric constant rocksalt-type metastable phase. Here we present ongoing work on the synthesis of MgSnN2, where we seek to understand and ultimately control formation of the various polymorphs and enable property tuning within each structure. Combinatorial radio-frequency co-sputtering is used to synthesize MgSnN2 at substrate temperatures up to 500 ºC across a range of cation stoichiometries. Our exploratory work found that ground-state wurtzite-type polymorph forms across the entire temperature range, while the metastable rocksalt-type polymorph is found as a secondary phase with high Mg content and substrate temperatures below 200 ºC on Si. Synchrotron x-ray diffraction was used to confirm that both wurtzite- and rocksalt-type phases are cation-disordered, consistent with reduced optical absorption onsets of  2 eV for these mixed phases. We also found that a mixed wurtzite/rocksalt phase of MgSnN¬2 could be grown heteroeptiaxially on GaN substrates at high temperatures, with the close effective lattice match between the GaN (001) and rocksalt MgSnN2 (111) surface promoting the formation of the rocksalt outside of its previously identified temperature range. In our current work, we utilize substrate choice to probe the formation of metastable phases of MgSnN2, seeking to control polymorph formation and cation ordering via epitaxial strain energy. We discuss the factors enabling polymorph control as well as related phenomena in the analogous compounds MgSiN2 and MgGeN2. Our work on MgSnN2 forms the basis for integrating control of polymorph formation and cation lattice site disorder, opening the door for a new set of semiconductors with unprecedented property control which can be integrated with existing materials.

Resume : Zn-IV-N2 semiconductors are promising optoelectronic materials and good candidates for thin film photovoltaic absorbers. Due to their tunable band gap (1.4-3.2 eV) and the choice of earth-abundant and non-toxic elements, they may replace InxGa1-xN alloys materials commonly used for optoelectronics devices [1,2]. Recently, few works investigate the disorder caused by unintentional oxygen incorporation, and the grains boundaries oxygen contamination in ZnSnN2 thin films [3,4]. To reduce oxygen contamination and improve physico-chemical properties, a new approach is investigated by the use of bias during film growth. This work shows the results of ZnSnN2 thin films grown by reactive co-sputtering using zinc and tin metallic targets in a nitrogen reactive atmosphere. The stoichiometry control of the film composition was managed by optimizing the target currents and the nitrogen partial pressure. The composition was measured by electron probe microanalysis (EPMA) to study the evolution of oxygen content under bias conditions. The application of different bias powers (from 0 to 50 W) modified the morphology and the composition of the films by densifying and decreasing significantly the oxygen contamination from 6.7 to 2.0 at. %. The optical band gap has been deduced from UV-visible spectroscopy and electrical properties was investigated by I-V experiments and Hall effect measurements. Ab initio calculations estimate an optical band gap in the order of 1.37 eV (calculated with a hybrid functional mBJ), the practical use of this system has been limited because of the difficulty to reach expected value. Here, we demonstrate that the optical band gap energy can be decreased (from 1.7 to 1.34 eV) to the range of the predicted one by using bias magnetron sputtering at room temperature. UV-visible spectroscopy highlights the reduction of the absorption by free electrons in the IR range responsible for the Burstein-Moss effect. Using first principle calculations, we explore the electronic structure and optical properties to compare with experimental results and we observe a good agreement. The study of bias effect power from 0 to 50 W underlines that an optimal parameter of 20 W bias is a compromise to gain the best structural, electrical and optical properties. Our results provide an interesting method to obtain a potential candidate for photovoltaic application, in an environmental friendly way, for a low-cost industrialization. Keywords: photovoltaic, ZnSnN2, bias effect, thin films, magnetron co-sputtering. [1] Martinez, et al (2017). Journal of Materials Chemistry A, 5 (23), 11418-11435. [2] P. Narang, et al. (2014) Advanced Materials 26.8: 1235-1241. [3] F. Alnjiman, et al. (2018) Solar Energy Materials and Solar Cells 182: 30-36. [4] J. Pan, et al. (2019) Advanced Materials 31(11), 1807406.

Resume : Solid solutions of group 14 nitrides with spinel structure are thermally stable materials very suitable for optoelectronic applications, due to their hardness and oxidation resistance. Specifically, tin-germanium nitride spinel (SnGe)3N4 with a bandgap of 2.2 eV is presented in this work as a perfect candidate semiconductor to host an Intermediate Band (IB), an advanced concept through which theoretical efficiencies way beyond the Shockley-Queisser limit can be achieved with single-junction solar cells. This achievement is possible thanks to the harvesting of sub-bandgap energy photons, in a process involving three different kind of absorption transitions, from the valence band to the intermediate band and from those to the conduction band. This way, materials with an optimal configuration, in terms of well separated and spaced bands, could reach up to 63% efficiencies according to the detailed balance methodology used by Shockley and Queisser. Herein, we propose cobalt doped (SnGe)3N4 spinel as a new IB material with an efficiency limit of 57%. After studying with detail 14 group spinels family via accurate Density Functional Theory (DFT) calculations, several doping materials within transition metals have been tested for the (SnGe)3N4 spinel, that with the most adequate bandgap, to check for the existence of an IB within the gap of the material. PBEsol functional, specifically developed for crystal structures of solids, has been used for the structural relaxations and subsequently, for those elements with promising results, high computational cost sc-GW calculations, based on PBE static calculations used for the materials filtering, have been carried out to obtain precise electronic configurations of the materials. As a final step, Bethe-Saltpeter equation has been used to obtain absorption spectra from the dielectric constants to assess the increase of absorption compared to the host material. Additionally, structural distortion as well as formation enthalpies of the different materials have been obtained to measure in some way the synthesizability of the materials. Theoretical efficiencies are used to quantify the potential of these materials, based on accurate band structures and calculated following detailed-balance methodology.

Resume : III-N materials are commonly used as active layers in LEDs, transistors, solar cells and mechanical devices1, 2. The main spinneret is based on the use of InGaN alloys. However, such layers contain indium and gallium. Significant volatility in their price and supply over the last years has led to considerable concern given their critical roles and their use in a wide range of large-scale electronic devices. Moreover, at present the crystalline III-N materials require the use of epitaxial growth techniques with high cost and high complexity. It is important to study and develop new earth abundant materials with optimized properties for the realization of innovative optoelectronic devices that could be competitive cost for mass production. In this work, we aim at developing a new kind of inexpensive, indium/gallium-free, nitride material that could be the basis of new way for optoelectronic applications. The study of such very innovative material is an ambitious goal but has already started in various countries (Japan, USA). The studies are focusing on MgSnN2 thin films (bandgap energy ≈ 2.3 eV) that is a good candidate for green emitters in LEDs and an absorber material in tandem photovoltaics3. MgSnN2 thin films have been deposited by magnetron co-sputtering at different substrate temperatures (up to 500 °C). The Mg/Sn atomic ratio has been controlled by the current applied to the Mg and Sn targets. The structure of the films has been studied by X-ray diffraction. Whatever the deposition temperature, the films crystallize in a wurtzite-like structure with a strong preferred orientation in the [002] direction. The columnar microstructure of MgSnN2 thin films have been studied by transmission electron microscopy. The optical band gap deduced from UV-visible spectroscopy is ranging in the 2.1 – 2.4 eV range. The electrical resistivity, carrier concentration, type and carrier mobility have been measured by Hall effect. Finally, the chemical environment of the Sn atoms has been investigated using Mössbauer spectrometry. (1) Nakamura, S.; Senoh, M.; Nagahama, S.; Iwasa, N.; Yamada, T.; Matsushita, T.; Kiyoku, H.; Sugimoto, Y.; Kozaki, T.; Umemoto, H.; Sano, M.; Chocho, K. InGaN/GaN/AlGaN-Based Laser Diodes with Modulation-Doped Strained-Layer Superlattices Grown on an Epitaxially Laterally Overgrown GaN Substrate. Appl. Phys. Lett. 1998, 72 (2), 211–213. https://doi.org/10.1063/1.120688. (2) Amano, H.; Sawaki, N.; Akasaki, I.; Toyoda, Y. Metalorganic Vapor Phase Epitaxial Growth of a High Quality GaN Film Using an AlN Buffer Layer. Appl. Phys. Lett. 1986, 48 (5), 353–355. https://doi.org/10.1063/1.96549. (3) Yamada, N.; Matsuura, K.; Imura, M.; Murata, H.; Kawamura, F. Composition-Dependent Properties of Wurtzite-Type Mg1+xSn1–XN2 Epitaxially Grown on GaN(001) Templates. ACS Appl. Electron. Mater. 2021. https://doi.org/10.1021/acsaelm.0c01115.

Resume : To improve the performances and stability in atmospheric conditions of organic photovoltaic (OPV) devices, a critical point is the substitution of chemically unstable and hygroscopic poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) used as Hole Transport Layer (HTL)1. Promising candidates are p-type Transparent Conductive Oxides (TCOs), which combine good optoelectronic properties with a higher chemical stability compared to the organic counterpart. In this work, HTLs of CuCrO2 with various cationic ratios, Cu/(Cu+Cr) varied between 40% and 100%, were synthesized by Aerosol Assisted Chemical Vapor Deposition. These thin films were analyzed to find a good trade-off between transparency, conductivity, level energy, and OPV performances. We obtained a decrease in resistivity and in energy gap in Cu-rich films with an optimal composition for Cu/(Cu+Cr) = 65%. We report here the synthesis of two classes of materials: Cu-rich CuCrO2 films, with low resistivity (<0.1 Ω.cm) and wide Eg of 3.15 eV, and nanocomposite films of CuCrO2 and Cu2O, presenting a measurable Hall-effect p-type mobility of 0.65 cm²V-1s-1 and a Eg below 3.0 eV. To validate the optoelectronic properties of out of stoichiometry CuCrO2 thin films, these materials have been integrated as HTL in direct OPV cells with the architecture glass/ITO(anode)/HTL (PEDOT:PSS or CuCrO2.)/ Active layer/ETL (LiF)/cathode (Al) The effect of the stoichiometry of the CuCrO2 films on the Power Conversion Efficiency (PCE) was studied for OPV fabricated and characterized in anhydrous conditions. We obtain an increasing PCE for Cu-rich CuCrO2 as HTL. The optimal cationic ratio was achieved for Cu/(Cu+Cr)= 65% with a corresponding PCE of 3.1%, through an increase of the short circuit current for this composition, representing the best trade-off between transparency and electrical conductivity among the studied HTLs. The stability of these devices in atmospheric conditions was studied and compared to the one of PEDOT:PSS based-OSC. The integration of oxides as HTL enhances the stability in atmospheric conditions, extending the lifetime of the device. The decrease in PCE, despite the use of CuCrO2 as HTL, was attributed to the degradation of the Active layer. Furthermore, the reusability of the functionalized substrates, glass/ITO/CuCrO2, was tested by the elimination of the top part Aluminum/ETL/Active layer. This was followed by the assembly of new cells above the substrate/ITO/ CuCrO2. The HTL properties were not altered by this procedure, and the recycled device showed similar electrical performances to the original one. This procedure could open many promising routes in prototyping, reducing the manufacturing costs and time, enabling the development of new performant and sustainable OPV. 1. Lee, S. J., Pil Kim, H., Mohd Yusoff, A. R. Bin & Jang, J. Organic photovoltaic with PEDOT:PSS and V2O5 mixture as hole transport layer. Sol. Energy Mater. Sol. Cells 120, 238–243 (2014).

Resume : A newer class of chalcogenides which consist of less toxic and more abundant elements (such as Cu2ZnSn(S,Se)4) or less constituent elements (such as Sb2(S,Se)3) have shown promising potential in recent years. While record efficiencies have increased rather rapidly to ~13% for CZTS and ~10% for Sb2Se3, propelling them as the frontrunner among the emerging inorganic solar cell materials, the efficiencies are still short of the ~20% mark needed to make them commercially attractive. The deficiency is often associated with the relatively lower minority carrier lifetime, tendency to form deep defects and particularly for Sb2(S,Se)3 the difficulty in directional growth. In this talk, I will present our group?s approaches in addressing these issues by using cation substitutions, doping and interface design. In particular, we found that Cd suppressed the formation of Sn-related deep defects in CZTS making CuCdSnS4 as the leading CZTS-inspired solar cell with efficiency ~8% [1]. The enhanced intrinsic properties of Ag and Cd substituted CZTS also makes them a better photocathode for solar to hydrogen conversion in a photoelectrochemical system [2,3]. We also found that directional growth of the favorable [hk1] planes of Sb2S3 can be achieved on ultrathin TiO2/CdS electron transport layer by solution method [4]. Lastly, I will show Al-doped CuS hole transporting layer in p-i-n perovskite solar cell which enhances charge transport and device long term stability [5]. [1] S.Hadke, L.H. Wong* et al, Advanced Energy Materials, 2019, 9 (45), 1902509. [2] YF Tay, LH Wong* et al, Joule, 2018, 2 (3), 537-548 [3] YF Tay, LH Wong* et al, Journal of Materials Chemistry A, 2020, 8 (18), 8862-8867 [4] X. Jin, L. H. Wong* et al, Adv. Funct. Mater., 2020, 30, 2002887. [5] A. Sadhu, LH. Wong* et al, Adv. Funct. Mater 2021, in press. CONTACT/PRESENTING AUTHOR Lydiawong@ntu.edu.sg

Resume : Cation disorder has been often blamed for the poor power conversion efficiency of kesterite solar cells, which remains at a low level compared to other thin-film technologies despite years of optimizations. Recent experiments show that a higher degree of ordering does not necessarily improve the open-circuit voltage, questioning the role of cation disorder in the low efficiency of kesterite solar cells. Through a statistical treatment of disorder in the Cu2ZnSnS4 (CZTS) absorber, we show that extensive Cu-Zn disorder alone cannot be responsible for the large Urbach tails observed in many CZTS solar cells. While cation disorder reduces the band gap as a result of the Gaussian tails formed near the valence-band edge due to Cu clustering, band-gap fluctuations contribute only marginally to the open-circuit voltage deficit, excluding Cu-Zn disorder as the primary source of the low efficiency of CZTS devices. On the other hand, the extensive disorder stabilizes the formation of Sn on Zn antisite and its defect complexes, which as nonradiative recombination centers account for the large open-circuit voltage loss in CZTS. Taking into account the nonradiative recombinations, our analysis predicts a maximum power conversion efficiency at 14% for cation-disordered CZTS. In view of the restricted growth conditions and the remnant degree of cation disorder due to the slow ordering kinetics, we expect the Sn on Zn antisite and its defect complexes remain the fundamental limiting factors towards CZTS devices with higher efficiency.

Resume : Sb2Se3 has recently drawn attraction as an emerging light absorber for photovoltaics due to its earth-abundance and suitable optical properties. In particular, Sb2Se3 possesses a quasi-1-dimensional (Q1D) structure, which creates an anisotropic charge-transporting behavior in which the carrier transport is very efficient along the Q1D direction, which is beneficial for solar cells as long as the absorber is properly aligned along with the preferred orientation. However, Sb2Se3 film is prone to form donor-like defects, such as Se vacancy (VSe), that are detrimental to the performance. Therefore, both growth of Sb2Se3 along the preferred orientations and the suppression of the formation of VSe are crucial in achieving Sb2Se3 solar cells with high efficiency. In this work, we investigated the importance of fine control of the extra supply of Se during the deposition of Sb2Se3 in controlling crystallographic orientations and the population of VSe in the Sb2Se3 films. This control determines the performance of the resulting solar cells in a superstrate configuration with a CdS buffer. The incorporation of Se during the growth resulted in a larger open-circuit voltage due to the passivation of VSe. However, an excess supply of Se disrupted the favorable orientation by selenizing the top region of the CdS, and therefore degraded the short-circuit current. With the optimal supply of extra Se flux during the deposition of Sb2Se3, the power conversion efficiency was improved from 3.7% to 5.2%.

Resume : Since the discovery in 2010 that carbon nitride in the presence of suitable co-catalysts and sacrificial donors could drive both the reduction of protons to molecular hydrogen and the oxidation of water to molecular oxygen, organic materials have received an enormous amount of interest as potential light absorbers for photocatalytic water splitting. At the moment, more than 150 different organic materials, mostly conjugated polymers, are known to be active for hydrogen evolution in the presence of a sacrificial donor and platinum or palladium nanoparticles that act as co-catalyst and site of the hydrogen evolution. However, a much smaller number of organic materials have been reported to be active for the oxygen evolution half-reaction, while the number of organic materials experimentally active for overall water splitting can be counted on the fingers of one hand. Based on this scarcity one could presume that overall water splitting activity is a rare property of organic materials. However, by computational high-throughput screening of thousands of organic polymers we show that this is not the case, although overall water splitting activity combined with a small enough optical gap that a large part of the solar spectrum is absorbed is predicted to be relatively rare for this material class. Finally, we predict which monomers should allow one to most likely realise materials that combine these elusive properties.

Resume : Sulfides and selenides in the perovskite and related crystal structures - chalcogenide perovskites - are an exciting family of semiconductors for optoelectronics and energy conversion applications [1]. Here, we explore the processing and properties of materials in the Ba-Zr-S system to determine their potential for thin-film photovoltaics (PV). We demonstrate making BaZrS3 thin films by gas-source molecular beam epitaxy (MBE) [2]. BaZrS3 forms in the distorted-perovskite structure with corner-sharing ZrS6 octahedra, and has a direct band gap of 1.85 eV. We present a wealth of data on film processing and properties, including reflection high-energy electron diffraction, atomic-force microscopy, X-ray reflectivity and diffraction, scanning transmission electron microscopy, spectroscopic ellipsometry, Hall transport, and photoconductivity. Our single-step MBE growth process produces films that are atomically smooth, single phase, chemically homogeneous, and oxygen free. The films are brightly colored even at 20 nm thick, due to the strong optical absorption typical of chalcogenide perovskites. Films grow via two, competing epitaxial growth modes: (1) buffered epitaxy, with a self-assembled interface layer that relieves the epitaxial strain, and (2) direct epitaxy, with rotated-cube-on-cube growth that accommodates the large lattice constant mismatch between the oxide and the sulfide. The propensity for these two modes can be tuned by adjusting the H2S gas delivery rate. We further show results making thin films of BaZr(S,Se)3 alloys, which provide a path towards tuning the direct band gap in the range 1.3 – 1.9 eV. We also report on time-resolved photoluminescence and photoconductivity studies of excited-state transport in BaZrS3 and the related phase Ba3Zr2S7 (band gap 1.25 eV). We model the data using semiconductor physics simulations, to enable direct determination of parameters key for photovoltaic performance, e.g. surface recombination velocity, Shockley-Read-Hall (SRH) lifetime, and diffusivity. We find that chalcogenide perovskites have SRH lifetime exceeding 100 ns and ambipolar diffusion length on the order of 10 𝜇m. If time allows, we will present further results of advanced characterization including impedance spectroscopy and X-ray absorption spectroscopy. Our high-quality thin-film synthesis and excited-state transport measurements suggest a bright future for chalcogenide perovskites thin-film PV. We will end by briefly summarizing the state of the field, and highlighting key challenges on the road to device demonstrations and commercialization. [1] Jaramillo, R. & Ravichandran, J. In praise and in search of highly-polarizable semiconductors: Technological promise and discovery strategies. APL Materials 7, 100902 (2019). [2] Sadeghi, I., Ye, K., Xu, M., LeBeau, J. M. & Jaramillo, R. Making BaZrS3 chalcogenide perovskite thin films by molecular beam epitaxy. arXiv:2105.10258 [cond-mat] (2021). at

Resume : Indium tin oxide (ITO) is the most commonly used front contact material for a variety of photovoltaic technologies. However, the presence of a high free carrier concentration in ITO thin films results in the well-known phenomenon of free carrier absorption in the near-infrared (NIR) region of the solar spectrum. This causes optical losses especially in those solar cells where the active layer is designed to preferentially absorb NIR photons. Therefore, a combination of high carrier mobility and high NIR transparency is desired for advanced transparent conductive oxides for substituting ITO in solar cells. Following this approach, various transition metals including Mo, Ti, W, Zr, and Nb have been found as potential dopants for inducing high mobility in indium oxide films. However, the doping effect of inner transition metals on the structural, optoelectrical and chemical properties of the host In2O3 films still remain largely unknown. In this work, we explore cerium (Ce), which is a member of the lanthanide group, as a potential dopant in In2O3. It is important to note that Ce has a higher relative abundance in the earth’s upper crust than most other dopants studied before. Accordingly, thin films are prepared on glass by industrially relevant pulsed DC magnetron sputtering with high deposition rates. XRD and Raman measurements revealed the polycrystalline nature of the deposited films, and the corresponding peaks are in close agreement with those of pure bixbyite In2O3. The highest mobility of ICeO achieved in this work is 71 cm2/Vs, for a film deposited at a substrate temperature of 160 oC and oxygen content of 1.4 vol% followed by post-deposition vacuum annealing at 500 oC for 30 minutes. This mobility value is almost four times higher than that of the standard ITO film prepared in this work, and more than two times higher than the value previously reported in the literature (30 cm2/Vs) for DC magnetron sputtered ITO: Ce films on glass. Comparison of the measured mobility values with those calculated using the BHD model and Erginsoy approximation revealed that neutral impurity scattering is the most dominant scattering mechanism in the fabricated ICeO films. However, the density of these neutral defects can be strongly reduced with post-deposition vacuum annealing. In addition, the optical transmission of ICeO is comparable to ITO in the visible region, but the former clearly outperforms the latter in terms of its NIR transparency. Together with a smooth surface (RMS roughness ~ 2-5 nm), pulsed DC magnetron sputtered ICeO/glass substrates therefore demonstrate significant potential to replace the most commonly used ITO/glass substrates for emerging solution processible photovoltaic technologies, such as organic solar cells and perovskite solar cells with low bandgap absorbers. For more information, please refer to: https://doi.org/10.1016/j.ceramint.2020.09.006.

Resume : Exploiting alternative photovoltaic (PV) materials is a particularly appealing hunting ground to overcome the longstanding fundamental limitations of maximum power conversion efficiency (∼33%), materials instability and complex engineered multilayered architectures of the current PV technologies. In this regard, oxides offer versatility of structure and composition; provide thermal, mechanical and chemical stability and can be prepared by cost-effective and scalable synthesis methodologies.[1] In particular, ferroelectric perovskite oxides, have sparked a great deal of interest to be used as versatile and stable light absorbers due to their unconventional PV mechanism, the abnormal PV effect, that could potentially surpass the fundamental efficiency limits of traditional semiconductors.[2] We have recently demonstrated that cation substitution in BiFeO3 system, i.e. BiFe(1-x)CoxO3 (BFCO), can put forward an attractive active material towards visible light absorption while preserving robust ferroelectricity, by means of low-cost solution processing.[3, 4] Nevertheless, the optimal PV device configuration for BFCO is still unknown. Interface engineering, i.e. adding electron and hole transport layers combined with the use of transparent conducting oxide electrodes, are crucial to boost PV performance through improved charge transport and minimized recombination in the device. Here, we present the fabrication and optimization of an all-oxide PV vertical device based on BFCO photo-active layer. First, we study the influence of ZnO as an electrode transport layer. The effect of ZnO film thickness and surface morphology are thoroughly evaluated on light absorption and ferroelectric properties by means of UV-Vis spectroscopy, scanning electron microscopy and macroscopic ferroelectric characterization. Next, the use of In-SnO2 and Al-ZnO as transparent conducting oxides, commonly used in state-of-the-art PV technologies, have been integrated into the BFCO-PV system. Incident photon-to-current conversion efficiency (IPCE) has already been measured at 405 nm and 350 mW/cm2 for the all-oxide BFCO-PV device, displaying values of 2.3% [5], evidencing a significant improvement compared to IPCE of non-optimized Pt/BFCO-PV system (0.4%) and approaching the reference efficiency of 3.3% reported for vacuum-deposited Bi2FeCrO6.[6] In this study, delicate choice and optimization of all the materials integrating the BFCO-PV device have demonstrated to be crucial to start understanding the unexplored relationship between device composition, ferroelectric properties and photoresponse and advance towards higher conversion efficiencies in this innovative concept of all-oxide PV device. [1] M.Coll et al.; Appl. Surf. Sci., 482, 2019 [2] I.Grinberg et al.; Nature Comm., 2, 2011 [3] P.Machado et al.; Chem. Mat., 31(3), 2019 [4] P.Machado et al.; J. Mater. Chem. C, 9, 2021 [5] P. Machado et al.; Manuscript in preparation, 2021 [6] Nechache et al.; Nat. Photonics, 9, 61, 2014

Resume : Halide perovskite solar cells (PSCs) have been the focus of much research in recent years due to their extremely high photoconversion efficiencies and also their ability to be synthesized by solution processing. These materials crystallize in the perovskite, AMX3 crystal structure where A is a monovalent cation (e.g. methyl ammonium, MA, formamidinium, FA, and/or Cs), M is the metal cation and X is the halide anion. While great strides have been made to optimize the device performance of PSCs, there still remain open questions as to the long-term stability of these materials. While constant improvement in stability is being demonstrated a fundamental understanding of degradation mechanisms can still provide key insight into performance improvements. In this talk, we present our methodology for both in-situ and operando X-ray characterization of full PSC device stacks. These methods were developed, in collaboration with researchers at SLAC and NREL, initially to study the device properties of MAPbI3 as a function of temperature. Since then, these methods have been applied to understand the phase stability of mixed A-site PSCs of the form XPbI3 where X = FA, Cs, and/or MA. More recently we have explored tin?lead PSCs devices, to better understand diminished device performance upon thermal treatment. This work showed a stable bulk structure of the perovskite absorber, suggesting that the degradation mechanism is dominated by the surface chemistry. This talk will provide a summary of the operando methods developed as well as a report on these past and more recent results.

Resume : Vacancy-ordered triple perovskites have recently come under the scientific spotlight as promising materials for high-performance next-generation optoelectronic technologies.1?3 Their A3B2X9 stoichiometry facilitates the replacement of the toxic Pb2+ cation with a benign isoelectronic B3+ cation (e.g. Bi3+ or Sb3+) while preserving the perovskite crystal structure. Unfortunately, however, these materials tend to exhibit large bandgaps (> 2 eV), impeding their application in many photo-catalytic/voltaic devices.4,5 In this work, we demonstrate a drastic shift of over 1 eV in the optical absorption onset of Cs3Bi2Br9 (from 2.58 eV to 1.39 eV), upon doping with tin. The origin of this intense visible and near-infrared absorption is identified through a combination of theoretical and experimental characterization. Sn atoms are found to disproportionate in the doped material, inducing a strong intervalence charge transfer (IVCT) transition as well as electronic transitions to and from localised Sn-based states within the band gap, whilst preserving the structural integrity of the perovskite framework. Sn(II) and Sn(IV) ions preferentially occupy neighbouring B-cation sites, forming a double-substitution complex. Unusually for a Sn(II) compound, the material shows minimal changes in optical and structural properties after 12 months storage in air. Our calculations suggest the stabilisation of Sn(II) within the double substitution complex contributes to this unusual stability. Moreover, using hybrid Density Functional Theory, including spin-orbit coupling effects, alongside the Marcus-Hush theory of IVCT behaviour, we comprehensively elucidate the origin of unusual concentration dependence of absorption in the doped perovskite. These results expand upon research on inorganic mixed-valent halides and perovskite-inspired materials, to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structure-property relationships of lead-free vacancy-ordered perovskite structures. 1 Y.-T. Huang, S.R. Kavanagh, D.O. Scanlon, A. Walsh, and R.L.Z. Hoye, Nanotechnology 32, 132004 (2021). 2 Z. Li, S.R. Kavanagh, M. Napari, R.G. Palgrave, M. Abdi-Jalebi, Z. Andaji-Garmaroudi, D.W. Davies, M. Laitinen, J. Julin, M.A. Isaacs, R.H. Friend, D.O. Scanlon, A. Walsh, and R.L.Z. Hoye, J. Mater. Chem. A 8, 21780 (2020). 3 M. Buchanan, Nature Physics 16, 996 (2020). 4 K.K. Bass, L. Estergreen, C.N. Savory, J. Buckeridge, D.O. Scanlon, P.I. Djurovich, S.E. Bradforth, M.E. Thompson, and B.C. Melot, Inorg. Chem. 56, 42 (2017). 5 R. Nie, R.R. Sumukam, S.H. Reddy, M. Banavoth, and S.I. Seok, Energy Environ. Sci. 13, 2363 (2020).

Resume : Lead-free halide perovskites have gained popularity in competition to the lead-based ones which are affected by toxicity and instability over moisture. Among them, double perovskites have been developed to alternatively replace the divalent cations Pb2+ in single perovskite with a combination of a monovalent and trivalent cation, forming structure as A2BB?X6. Here we report the highly stable Cs2Na1-xAgxIn1-yBiyCl6 compounds, with Silver (x) and Bismuth (y) fractions ranging from 10% to 1 ppm, that exhibit efficient and stable broadband, white-light emission ideally suited for lighting applications. Double perovskites were synthesized in small crystals by a super-saturation precipitation method followed by filtration and their photoluminescence quantum yield (PLQY) was measured. The highest PLQY was found to be very close to 100% and the optical properties of the crystals were studied by a combination of time resolved photoluminescence and differential transmission, in order to establish the nature of the light-emitting excited states. The measurements confirmed that emission is produced by self-trapped exciton states; the two key elements to achieve such outstanding emission properties are Bi, whose 6p states enhance the lifetime for electrons in the conduction band, and Ag, with 4d states at the top of the valence band allow efficient radiative recombination of excitons. A systematic study as a function of Bi and Ag content was employed to reveal the diffusion length for optical excitations, localization processes as well as the intrinsic limits for the excited state lifetime. The results provide guidance for rational optimization of such compounds in view of the use as phosphors and active materials for LEDs and displays.

Resume : A general preparative method based on atomic layer deposition (ALD) is presented to overcome the poor chemical compatibility between classical n-type metal oxides and the sulfide or a heavier chalcogenide used as the light absorber layer in thin film solar cells. A sacrificial layer utilized as a scaffold is deposited on top of the Sb2S3 with the purpose of overcoming the well-known material mobility of highly pure Sb2S3 caused by the chemical incompatibility with TiO2. The sacrificial layer is selectively removed once the Sb2S3 has been annealed and converted to stibnite phase while keeping the characteristic conformality of ALD layers. An exhaustive study of the Sb2S3/sacrificial layer (or top-interface) subjected to different physico-chemical treatments is carried out to further enhance the opto-electronic properties and therefore the performance of the solar cells. The photovoltaic characterization reveals that devices in which Sb2S3 interfaces have been treated are superior to the non-treated ones, reaching efficiencies of up to 5.5%. The generality of this approach is demonstrated for different nanostructures and has the potential to be applied in Sb2Se3 to solve the chemical incompatibility in the absence of CdS.

Resume : Antimony selenide (Sb2Se3) is a promising emerging material for use in photovoltaics (PV). It has excellent optical properties, cheap and earth abundant materials and has improved rapidly in the last 7 years, from 2% to over 9% efficient. While device efficiencies have developed rapidly during this time, certain fundamental aspects of Sb2Se3 remain a matter of some debate. One example is the matter of doping. Early studies reported Sb2Se3 to be a natively p-type material and the device architectures used for Sb2Se3 solar cells mostly used it as the p-type material in a planar p-n heterojunction. Theoretical studies have since reported Sb2Se3 to be natively insulating due to compensating native defects, with limited capacity to be doped either p- or n-type through native defects. The discovery by our group that the Sb2Se3 used in our devices was n-type, due to unintentional Cl doping of the source material, and that these devices were competitive with the best reported efficiencies (7.3%), has opened the door to the potential of using extrinsic dopants to control the conductivity of Sb2Se3 [1]. In this study, bulk crystals of both Cl and Sn doped Sb2Se3 were fabricated for investigation of their electronic properties. A combination of ultraviolet, x-ray, and hard x-ray photoemission spectroscopy was used to obtain a depth profile of the energy separation between the Fermi level and the valence band maximum. A solution to Poisson’s equation was then found to fit the data, obtaining an estimate for bulk carrier density as well as a profile of the surface space charge layer. These results are then compared to carrier densities measured by capacitance-voltage and Hall effect measurements. The Cl doped crystal was found to be strongly n-type and the results of the Poisson’s equation solution were consistent with the Hall effect and C-V results (1017-1018 cm-3). 280 meV of surface band bending was observed with a depletion layer extending ~20 nm into the surface. The Sn doped crystal was more weakly doped (1016 cm-3) and had significantly more band bending (500 meV) over a greater depth (~100 nm). The solution for the Sn doped crystal indicates a surface inversion layer, with the near surface region exhibiting slightly n-type conductivity with p-type behaviour further into the bulk. This is in agreement with density functional theory calculations of the defect level formation energies, which predicts self-compensation of the SnSb antisites by Sn interstitials at a relatively low doping level. This study provides an in-depth investigation of the impact of Cl and Sn doping on the electronic properties of Sb2Se3, combining a range of photoemission techniques, electronic characterisation and theoretical calculations on one of PV’s most exciting emerging absorbers. [1] Hobson et al. “Isotype heterojunction solar cells using n-type Sb2Se3 thin films”, Chemistry of Materials (2020) DOI: 10.1021/acs.chemmater.0c00223