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

Non-radiative recombinations via mid-gap energy states are key limiting factors for the bulk electronic quality of ABX(3) 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 & delta;-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" & alpha;-FAPbI(3) or films with the presence of "secondary-phases". Our results demonstrate that the crystallinity and [110] orientation of & alpha;-FAPbI(3) 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 & alpha;-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 cm(2) and 1 cm(2), 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.