Chalcogenide Perovskite Thin Films for Photovoltaics

Chalcogenide perovskites have much to recommend them for photovoltaics (PV). They absorb light strongly, with direct band gap tunable (at least) over the range 1.4 – 1.9 eV. They have limited polymorphism and are stable in air, water, and at high temperature. They are made of Earth-abundant, and (mostly) non-toxic elements, and have isotropic properties. However, there are substantial challenges facing the development of chalcogenide perovskite PV. Synthesis requires aggressive conditions - oxygen-free sulfurization or selenization at high temperature – that severely constrain thin-film growth. The few published reports of transport properties describe <i>n</i>-type material with high electron concentration, undesirable for thin-film PV. Photoluminescence (PL) has been reported, but the quantum yield is often low, and sample-to-sample variability is high. The defects that limit performance are not yet understood, and even less is known about interface and heterojunction design. Clearly, it will be a long road to chalcogenide perovskite PV technology.<br/><br/>I will motivate why, despite these challenges, research on chalcogenide perovskites for PV is worthwhile and exciting. I will then describe our own efforts, which center on the processing and properties of thin films. We have achieved a number of synthesis milestones, including growing thin films of BaZrS₃ and BaZr(S,Se)₃ alloys with tunable band gap, in epitaxial and polycrystalline forms. Selenium alloying can produce films with band gap suitable for single- and dual-junction PV, but the vast majority of synthesis procedures reported to-date focus on pure sulfides. I will discuss our finding of rapid alloying by post-growth selenization of sulfide thin films. This recalls the sulfurization-after-selenization process in CIGS manufacturing, and may make alloy studies more widely accessible. I will also present findings on how variations in cation composition affect crystallization kinetics. These results bolster evidence for BaS₃-liquid-assisted crystal growth, and may be useful for lowering the temperature of thin film synthesis.<br/><br/>I will then discuss our ongoing studies of photoluminescence (PL) and electronic transport. We have previously reported long excited-state PL lifetimes for BaZrS₃ and Ba₃Zr₂S₇, and others have reported band-edge PL even from powder samples. However, PL emission is highly variable, sample-to-sample, and many samples have no measurable band-edge emission. To understand this variability, we carry out a quantitative comparison of temperature-dependent PL of BaZrS₃ and a prototypical halide perovskite, CsPbBr₃. The halide has PL yield between 100 and 10,000 times larger than the chalcogenide. By comparing the vibrational properties of the chalcogenide and the halide, we suggest why defect-assisted recombination may be faster in the chalcogenide. On the other hand, the variability between chalcogenide samples suggests a substantial upside, if the recombination-active defect(s) can be identified and diminished. Our temperature-dependent Hall transport studies find that mobility at room temperature is limited by electron-phonon scattering, even in highly-doped samples; this may be related to our previous finding that chalcogenide perovskites have exceptional dielectric polarizability. At cryogenic temperature, the role of ionized defect scattering varies sample-to-sample. All films are n-type as grown, but with tremendous variability in electron concentration. Studies of post-growth annealing support the hypothesis that the predominant intrinsic shallow donors are sulfur vacancies; we use this understanding to vary electron concentration by over a million-fold.<br/><br/>I will end by highlighting exciting next-steps including alternative methods of thin film deposition to make thicker films at lower temperature, studies of device semi-fabricates including detailed investigation of Mo/BaZrS₃ interfaces, and controlling carrier concentration and type through aliovalent doping.