Understanding the Effect of Different Synthesis Conditions on the Physicochemical Properties of Mixed-Ion Perovskite Solar Cells.

Perovskite solar cells, composed of a mixture of methylammonium (MA) and formamidinium (FA) cations [in the benchmark proportions of (FAPbI3 )0.85 (MAPbBr3 )0.15 ] and titania as an electron-accepting material, are prepared under different conditions, with the objective of finding correlations between the solar cell performance and several important stationary and dynamical parameters of the material. The effects of humidity, oxygen, the use of anti-solvent, and the presence and quality of a mesoporous titania layer are investigated. It is found that an increase in the photocurrent corresponds to a higher content of the desired cubic perovskite phase and to increased long-wavelength absorption of the sample. On the contrary, for poorer-quality cells, additional short-wavelength bands in both absorption and emission spectra are present. Furthermore, a higher photocurrent of the cells is correlated with faster interfacial charge-transfer dynamics. For the highest photocurrent of >20 mA cm-2 , the characteristic times of about 1 μs are observed by electrochemical impedance spectroscopy, and emission half-lifetimes of about 6 ns by time-resolved fluorescence spectroscopy (upon excitation with 420 nm pulses of ≈0.5 mW power). Both first- and second-order rate constants, extracted from the emission measurements, are greater for the cells showing higher photocurrents, probably owing to a more rapid charge injection.