Revealing the impact of the host-salt non-stoichiometry on the performance of perovskite solar cells

Abstract

Non-radiative recombinations via mid-gap energy states are key limiting factors for the bulk electronic quality of ABX₃ type perovskite semiconductors and they are expedited at grain boundaries (GBs), which are the primary location for the growth of “secondary phases” such as unreacted PbI₂ or δ-hexagonal phases. Here, we are able to fine-tune I-rich (AX-rich) or I-poor (Pb-rich) growth conditions for perovskite thin-films, allowing us to generate either highly crystalline “phase-pure” α-FAPbI₃ or films with the presence of “secondary-phases”. Our results demonstrate that the crystallinity and [110] orientation of α-FAPbI₃ can be systematically tailored by adjusting the stoichiometry of the precursor mix, with the addition of only a trace quantity of methylammonium iodide (MAI). Excess MA⁺ cations can lower the phase-transition energy by reacting with FA⁺ cations and by forming the intermediate ion complex FA–MA, which triggers α-phase formation even at room temperature. Under moderate AX-rich conditions, the optoelectronic properties of the thin-films and device performances significantly improved, owing to the suppression of non-radiative recombination (NRR) with complete perovskite conversion and defect passivation at the grain boundaries. With 7 mol% excess MAI, we recorded a photovoltaic efficiency of 19.30% and 19.19% for an aperture area of 0.25 cm² and 1 cm², respectively. Our findings have implications for future investigations on the use of host salts (such as MA⁺ or FA⁺) to enhance the bulk electronic quality of perovskite solids, particularly for their photovoltaic applications.