Efficient composition tuning via cation exchange and improved reproducibility of photovoltaic performance in FA(x)MA(1-x)PbI(3) planar heterojunction solar cells fabricated by a two-step dynamic spin-coating process

We synthesized uniform FA(x)MA(1-x)PbI(3) perovskite films with a single a phase by a two-step process combined with a dynamically dispensed spin-coating technique. It uses the continuous dropping of precursor solutions with a constant CH3NH3I (MAI)/HC(NH2)(2)I (FAI) concentration enabling the kinetically controlled grain growth. Dynamic coating cycles are also changed to facilitate a cation-exchange process, control the degree of the mutual inter-mixing between formamidinium lead triiodide (FAPbI(3)) and methylammonium lead triiodide (MAPbI(3)) and examine the formation process and properties of the mixed perovskite films formed under the excess MA/FA cation environment, which has not been clarified so far. Notably, without additional solvent washing steps, FA(x)MA(1-x)PbI(3) films are adjustable in composition, pinhole-free, and have various grain sizes depending on the coating cycles. Perovskite solar cells (PSCs) synthesized from FA(x)MA(1-x)PbI(3) films with two cycles of the dynamic spin coating have achieved a maximum power conversion efficiency (PCE) of 18.50% with an average PCE of 17.06 +/- 0.42%, which shows much-improved performance as well as reproducibility compared with 14.79 +/- 1.82% obtained from the conventional static spin-coating method. In addition, we first found mixed FA(x)MA(1-x)PbI(3) perovskites synthesized under an excess cation environment containing local stoichiometric inhomogeneities as well as excess residual cations (C-N and NH3+) acting as recombination traps, which is supported by the calculation of trap densities.