Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic-Inorganic Solar Absorber.

Methylammonium lead halide (MAPbX3 ) perovskites exhibit exceptional carrier transport properties. But their commercial deployment as solar absorbers is currently limited by their intrinsic instability in the presence of humidity and their lead content. Guided by our theoretical predictions, we explored the potential of methylammonium bismuth iodide (MBI) as a solar absorber through detailed materials characterization. We synthesized phase-pure MBI by solution and vapor processing. In contrast to MAPbX3, MBI is air stable, forming a surface layer that does not increase the recombination rate. We found that MBI luminesces at room temperature, with the vapor-processed films exhibiting superior photoluminescence (PL) decay times that are promising for photovoltaic applications. The thermodynamic, electronic, and structural features of MBI that are amenable to these properties are also present in other hybrid ternary bismuth halide compounds. Through MBI, we demonstrate a lead-free and stable alternative to MAPbX3 that has a similar electronic structure and nanosecond lifetimes.

Open-circuit voltage deficit, radiative sub-bandgap states, and prospects in quantum dot solar cells.

Quantum dot photovoltaics (QDPV) offer the potential for low-cost solar cells. To develop strategies for continued improvement in QDPVs, a better understanding of the factors that limit their performance is essential. Here, we study carrier recombination processes that limit the power conversion efficiency of PbS QDPVs. We demonstrate the presence of radiative sub-bandgap states and sub-bandgap state filling in operating devices by using photoluminescence (PL) and electroluminescence (EL) spectroscopy. These sub-bandgap states are most likely the origin of the high open-circuit-voltage (VOC) deficit and relatively limited carrier collection that have thus far been observed in QDPVs. Combining these results with our perspectives on recent progress in QDPV, we conclude that eliminating sub-bandgap states in PbS QD films has the potential to show a greater gain than may be attainable by optimization of interfaces between QDs and other materials. We suggest possible future directions that could guide the design of high-performance QDPVs.

Strongly Enhanced Photovoltaic Performance and Defect Physics of Air-Stable Bismuth Oxyiodide (BiOI).

Bismuth-based compounds have recently gained increasing attention as potentially nontoxic and defect-tolerant solar absorbers. However, many of the new materials recently investigated show limited photovoltaic performance. Herein, one such compound is explored in detail through theory and experiment: bismuth oxyiodide (BiOI). BiOI thin films are grown by chemical vapor transport and found to maintain the same tetragonal phase in ambient air for at least 197 d. The computations suggest BiOI to be tolerant to antisite and vacancy defects. All-inorganic solar cells (ITO|NiOx |BiOI|ZnO|Al) with negligible hysteresis and up to 80% external quantum efficiency under select monochromatic excitation are demonstrated. The short-circuit current densities and power conversion efficiencies under AM 1.5G illumination are nearly double those of previously reported BiOI solar cells, as well as other bismuth halide and chalcohalide photovoltaics recently explored by many groups. Through a detailed loss analysis using optical characterization, photoemission spectroscopy, and device modeling, direction for future improvements in efficiency is provided. This work demonstrates that BiOI, previously considered to be a poor photocatalyst, is promising for photovoltaics.

A Two-Step Absorber Deposition Approach To Overcome Shunt Losses in Thin-Film Solar Cells: Using Tin Sulfide as a Proof-of-Concept Material System.

As novel absorber materials are developed and screened for their photovoltaic (PV) properties, the challenge remains to reproducibly test promising candidates for high-performing PV devices. Many early-stage devices are prone to device shunting due to pinholes in the absorber layer, producing "false-negative" results. Here, we demonstrate a device engineering solution toward a robust device architecture, using a two-step absorber deposition approach. We use tin sulfide (SnS) as a test absorber material. The SnS bulk is processed at high temperature (400 °C) to stimulate grain growth, followed by a much thinner, low-temperature (200 °C) absorber deposition. At a lower process temperature, the thin absorber overlayer contains significantly smaller, densely packed grains, which are likely to provide a continuous coating and fill pinholes in the underlying absorber bulk. We compare this two-step approach to the more standard approach of using a semi-insulating buffer layer directly on top of the annealed absorber bulk, and we demonstrate a more than 3.5× superior shunt resistance Rsh with smaller standard error σRsh. Electron-beam-induced current (EBIC) measurements indicate a lower density of pinholes in the SnS absorber bulk when using the two-step absorber deposition approach. We correlate those findings to improvements in the device performance and device performance reproducibility.

Investigation of Bismuth Triiodide (BiI3) for Photovoltaic Applications.

Guided by predictive discovery framework, we investigate bismuth triiodide (BiI3) as a candidate thin-film photovoltaic (PV) absorber. BiI3 was chosen for its optical properties and the potential for "defect-tolerant" charge transport properties, which we test experimentally by measuring optical absorption and recombination lifetimes. We synthesize phase-pure BiI3 thin films by physical vapor transport and solution processing and single-crystals by an electrodynamic gradient vertical Bridgman method. The bandgap of these materials is ∼1.8 eV, and they demonstrate room-temperature band-edge photoluminescence. We measure monoexponential recombination lifetimes in the range of 180-240 ps for thin films, and longer, multiexponential dynamics for single crystals, with time constants up to 1.3 to 1.5 ns. We discuss the outstanding challenges to developing BiI3 PVs, including mechanical and electrical properties, which can also inform future selection of candidate PV absorbers.

Atomic layer deposited gallium oxide buffer layer enables 1.2 V open-circuit voltage in cuprous oxide solar cells.

The power conversion efficiency of solar cells based on copper (I) oxide (Cu2 O) is enhanced by atomic layer deposition of a thin gallium oxide (Ga2 O3 ) layer. By improving band-alignment and passivating interface defects, the device exhibits an open-circuit voltage of 1.20 V and an efficiency of 3.97%, showing potential of over 7% efficiency.

3.88% efficient tin sulfide solar cells using congruent thermal evaporation.

Tin sulfide (SnS), as a promising absorber material in thin-film photovoltaic devices, is described. Here, it is confirmed that SnS evaporates congruently, which provides facile composition control akin to cadmium telluride. A SnS heterojunction solar cell is demons trated, which has a power conversion efficiency of 3.88% (certified), and an empirical loss analysis is presented to guide further performance improvements.