A Semitransparent Silver–Bismuth Iodide Solar Cell with Voc above 0.8 V for Indoor Photovoltaics

Because the toxicity of lead (Pb) remains a potential threat to the commercialization of perovskite solar cells, silver–bismuth halides such as AgBiI4, AgBi2I7, Ag2BiI5, and Ag3BiI6 with band gaps of 1.8–1.9 eV and strong absorption are considered to be promising alternatives to Pb-based perovskites, not only for outdoor (1 Sun) but also for indoor photovoltaics because their band gap matches best with the indoor artificial light spectrum. To date, almost all studies have focused on improving the power conversion efficiency (PCE) under 1 Sun using thick absorber films made on mesoporous electron-transport layers. However, in this work, we aimed at developing optically semitransparent Ag2BiI5 solar cells that can be used indoor or on windows. We studied the effect of the precursor composition on the phase evolution, energy band levels, film morphology, and, eventually, solar cell performance. Through compositional engineering and thickness optimization, we accomplished the fabrication of a thin (∼100 nm Ag2BiI5) and semitransparent Ag2BiI5 solar cell that works with a power conversion efficiency of ∼2%, which is greater than many reported with thick and opaque films and slightly lower than the best recorded for Ag2BiI5. The cell shows an open-circuit voltage (Voc) of over 800 mV, the highest reported for Ag2BI5. More interestingly, while a thicker/opaque cell was found to work with a PCE of 5.0% under 1000 lx, the semitransparent cell shows a reasonable performance of 3.3% under the same light intensity.