Dual-source evaporation of silver bismuth iodide films for planar junction solar cells

Non-toxic and air-stable silver bismuth iodide semiconductors are promising light absorber candidates for photovoltaic applications owing to a suitable band gap for multi- or single-junction solar cells. Recently, solution-based film fabrication approaches for several silver bismuth iodide stoichiometries have been investigated. The current work reports on a facile and reproducible two-step coevaporation/annealing approach to deposit compact and pinhole-free films of AgBi2I7, AgBiI4 and Ag2BiI5. X-ray diffraction (XRD) in combination with scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX) analysis reveals formation of pure cubic (Fdm) phase AgBi2I7, cubic (Fdm) or rhombohedra (Rm) phase AgBiI4, each with u003e3 μm average grain size, or the rhombohedral phase (Rm) Ag2BiI5 with u003e200 nm average grain size. A phase transition from rhombohedral to cubic structure is investigated via temperature-dependent X-ray diffraction (TD-XRD). Planar-junction photovoltaic (PV) devices are prepared based on the coevaporated rhombohedral AgBiI4 films, with titanium dioxide (TiO2) and poly(3-hexylthiophene) (P3HT) as electron- and hole-transport layers, respectively. The best-performing device exhibited a power conversion efficiency (PCE) of as high as 0.9% with open-circuit voltage (VOC) u003e 0.8 V in the reverse scan direction (with significant hysteresis).