Probing Molecular and Crystalline Orientation in Solution‐Processed Perovskite Solar Cells

The microstructure of solution‐processed organometallic lead halide perovskite thin films prepared by the “gas‐assisted” method is investigated with synchrotron‐based techniques. Using a combination of GIWAXS and NEXAFS spectroscopy the orientational alignment of CH 3 NH 3 PbI 3 crystallites and CH 3 NH 3 + cations are separately probed. The GIWAXS results reveal a lack of preferential orientation of CH 3 NH 3 PbI 3 crystallites in 200–250 nm thick films prepared on both planar TiO 2 and mesoporous TiO 2 . Relatively high efficiencies are observed for device based on such films, with 14.3% achieved for planar devices and 12% for mesoporous devices suggesting that highly oriented crystallites are not crucial for good cell performance. Oriented crystallites however are observed in thinner films (≈60 nm) deposited on planar TiO 2 (but not on mesoporous TiO 2 ) indicating that the formation of oriented crystallites is sensitive to the kinetics of solvent evaporation and the underlying TiO 2 morphology. NEXAFS measurements on all samples found that CH 3 NH 3 + cations exhibit a random molecular orientation with respect to the substrate. The lack of any NEXAFS dichroism for the thin CH 3 NH 3 PbI 3 layer deposited on planar TiO 2 in particular indicates the absence of any preferential orientation of CH 3 NH 3 + cations within the CH 3 NH 3 PbI 3 unit cell for as‐prepared layers, that is, without any electrical poling.