High-Efficiency Lead-Free Wide Band Gap Perovskite Solar Cells via Guanidinium Bromide Incorporation

Despite lead halide perovskite­(top)/silicon­(bottom) tandem solar cell achieving >29% efficiency, the toxicity of lead limits the adoption of perovskites in electronic devices. To solve this problem, tin perovskite has been suggested as an alternative candidate to be used as the top cell. However, most of the reports on tin perovskite solar cells are based on narrow band gap materials (band gap: 1.20–1.40 eV), which are not suitable to be used as the tandem top cell. In addition, the efficiency of the previously reported wide band gap lead-free perovskites is not satisfactory, shown to be less than 2.00%. This study explores the fabrication of the wide band gap (1.61 eV) ASnI₂Br perovskite solar cells through the optimization of formamidinium and guanidinium content to improve the efficiency from 1.68 to 7.00%. Substituting the A site with larger cation results in a larger crystal size, which translated into higher device stability. Better energy-level alignment within the device significantly enhanced the charge injection and extraction, which translated into higher open-circuit voltage. This work provides proof that tin-halide perovskite solar cells have the potential in the fabrication of lead-free all-perovskite tandem solar cells.