Influence of the stoichiometry of tin-based 2D/3D perovskite active layers on solar cell performance

Abstract

2D/3D mixed tin perovskites have the advantages of high crystallinity and preferential orientation compared to pure 3D tin perovskite. However, solar cells based on 2D/3D mixed tin perovskites are still limited by low power conversion efficiency (PCE) when compared to their lead-based counterparts. It is essential to gain deeper insight into the factors that limit the performance of these solar cells in order to further improve them. In this work, we demonstrate that the starting stoichiometry of 2D/3D (PEA_0.08FA_xSnI₃) tin perovskite films influences their crystallization and photophysical properties as well as the solar cell performance. The reference 2D/3D film (x = 0.92, where x refers to the stoichiometry of the precursors) is highly crystalline with the 3D phase preferentially oriented and a small amount of 2D phase located at the bottom of the film. The reference solar cell delivers a PCE of about 8.0%. 2D/3D films with even higher FA concentration (x > 1.0) mainly consist of poorly crystalline and randomly oriented 3D phases, with much higher trap density compared to the reference film. The corresponding solar cells therefore suffer from severe trap-assisted charge recombination, and deliver a poor PCE of <1%. FA-deficient 2D/3D films (x ≤ 0.8) form highly crystalline and oriented 3D grains, and at the same time a large quantity of 2D (n ≤ 2) phases throughout the entire film. Furthermore, the FA-deficient films contain excess SnI₂. Consequently, charge transport in FA-deficient films is hindered by both the 2D phases oriented parallel to the substrate and SnI₂, and the corresponding solar cells suffer from the recombination of free holes and electrons, resulting in a lower PCE than the reference devices.