Performance Improvement of AgBiI4 Solar Cells by a Lewis Base Thiourea Additive

Silver bismuth iodide (Ag–Bi–I) is considered to be a potential photovoltaic material as an eco-friendly semiconductor with a suitable band gap and high stability. However, the reported highest PCE is still far below the ideal value, and the quality of the AgxBiyIx+3y film deserves to be further improved. Herein, we introduce a facile strategy to control the crystallization of AgBiI4 by adding Lewis base additives thiourea (TU) to the AgBiI4 precursor solution. The result indicates that coordination between TU and AgBiI4 precursor solution improves the crystallization, reduces the defect density, and increases the carrier mobility of the AgBiI4 film. The treatment of the TU additive boosts light absorption, reduces charge recombination, and facilitates charge extraction, resulting in improved photovoltaic performance of the AgBiI4 solar cell. Compared to the control device, the treatment of 0.9 mM TU increases the PCE (∼1.87%) of the solar cell by 19.9% and improves the air stability of the solar cell.