Vacuum Deposited Perovskites with a Controllable Crystal Orientation

The preferential orientation of the perovskite (PVK) is typically accomplished by manipulation of the mixed cation/halide composition of the solution used for wet processing. However, for PVKs grown by thermal evaporation, this has been rarely addressed. It is unclear how variation in crystal orientation affects the optoelectronic properties of thermally evaporated films, including the charge carrier mobility, lifetime, and trap densities. In this study, we use different intermediate annealing temperatures T-inter between two sequential evaporation cycles to control the Cs(0.15)FA(0.85)PbI(2.85)Br(0.15) orientation of the final PVK layer. XRD and 2D-XRD measurements reveal that when using no intermediate annealing primarily the (110) orientation is obtained, while when using T-inter = 100 degrees C a nearly isotropic orientation is found. Most interestingly for T-inter > 130 degrees C a highly oriented PVK (100) is formed. We found that although bulk electronic properties like photoconductivity are independent of the preferential orientation, surface related properties differ substantially. The highly oriented PVK (100) exhibits improved photoluminescence in terms of yield and lifetime. In addition, high spatial resolution mappings of the contact potential difference (CPD) as measured by KPFM for the highly oriented PVK show a more homogeneous surface potential distribution than those of the nonoriented PVK. These observations suggest that a highly oriented growth of thermally evaporated PVK is preferred to improve the charge extraction at the device level.

Automated summary

In this study, the orientation of perovskite films grown by thermal evaporation was controlled by adjusting the intermediate annealing temperature. The highly oriented perovskite films exhibited improved photoluminescence and a more homogeneous surface potential distribution, suggesting that highly oriented growth is preferred for better charge extraction at the device level.