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

Abstract

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 >3 μm average grain size, or the rhombohedral phase (Rm) Ag₂BiI₅ with >200 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 (V_OC) > 0.8 V in the reverse scan direction (with significant hysteresis).