Controlled Humidity Study on the Formation of Higher Efficiency Formamidinium Lead Triiodide-Based Solar Cells

We report on the effect of humidity on the structural, optical, and electrical properties of formamidinium lead halide perovskite (FAPbI3; prepared by a solvent engineering method) and the device characteristics of planar FAPbI3 solar cells. The relative humidity strongly affects the perovskite film morphology, which changes from a uniform, fully covered FAPbI3 film at low relative humidity (e.g., ∼2%) to an inhomogeneous film consisting of many voids (or pinholes) at high humidity (30%–40%). This morphological deterioration with increasing humidity is also accompanied by a reduction of the film crystallinity, decay of optical property, and shorter carrier lifetime. The device based on a planar FAPbI3 film shows the best conversion efficiency of 16.6% (with the stabilized output efficiency of 16.4%) at a low humidity (∼2%). Higher humidity leads to lower device performance, mainly due to the loss of open-circuit voltage and fill factor, which is consistent with the decrease in recombination resistance.