Atomic Layer Deposition of Electron Selective SnO_x and ZnO Films on Mixed Halide Perovskite: Compatibility and...

The compatibility of atomic layer deposition directly onto the mixed halide perovskite formamidinium lead iodide:methylammonium lead bromide (CH(NH₂)₂, CH₃NH₃)Pb(I,Br)₃ (FAPbI₃:MAPbBr₃) perovskite films is investigated by exposing the perovskite films to the full or partial atomic layer deposition processes for the electron selective layer candidates ZnO and SnO_x. Exposing the samples to the heat, the vacuum, and even the counter reactant of H₂O of the atomic layer deposition processes does not appear to alter the perovskite films in terms of crystallinity, but the choice of metal precursor is found to be critical. The Zn precursor Zn(C₂H₅)₂ either by itself or in combination with H₂O during the ZnO atomic layer deposition (ALD) process is found to enhance the decomposition of the bulk of the perovskite film into PbI₂ without even forming ZnO. In contrast, the Sn precursor Sn(N(CH₃)₂)₄ does not seem to degrade the bulk of the perovskite film, and conformal SnO_x films can successfully be grown on top of it using atomic layer deposition. Using this SnO_x film as the electron selective layer in inverted perovskite solar cells results in a lower power conversion efficiency of 3.4% than the 8.4% for the reference devices using phenyl-C₇₀-butyric acid methyl ester. However, the devices with SnO_x show strong hysteresis and can be pushed to an efficiency of 7.8% after biasing treatments. Still, these cells lacks both open circuit voltage and fill factor compared to the references, especially when thicker SnO_x films are used. Upon further investigation, a possible cause of these losses could be that the perovskite/SnO_x interface is not ideal and more specifically found to be rich in Sn, O, and halides, which is probably a result of the nucleation during the SnO_x growth and which might introduce barriers or alter the band alignment for the transport of charge carriers.