Origin of vertical slab orientation in blade-coated layered hybrid perovskite films revealed with in-situ synchrotron X-ray scattering

Controlling the vertical orientation of perovskite slabs in layered hybrid perovskite films is key for enabling further optimization of photovoltaic device performance. However, the mechanism explaining vertical orientation control in such films remains under debate. Here, we present an in-situ grazing-incidence wide-angle X-ray scattering (GIWAXS) study on the formation of BA2MAn-1PbnI3n+1 perovskite films during blade-coating. The evolution of grazing-incidence transmission wide-angle X-ray scattering (GTWAXS) signal is also monitored to reveal the specific vertically-oriented low-n phases formed in such films. We find that the blade-coating temperature greatly influences the crystallization dynamics of BA2MAn-1PbnI3n+1 perovskite films and perovskite slab orientation via intermediate phase and low-n phase formation. For the perovskite film with targeted dimensionality of n = 4, blade-coating films at higher temperatures suppresses the formation of the 2MAI.3PbI2.2DMF intermediate phase. This in turn suppresses the formation of the n = 2 phase that adopts an undesired horizontal perovskite slab orientation, instead favouring the formation of the n = 3 phase that adopts the desired vertical perovskite slab orientation. Further analysis on the microstructural evolution of films with near-perfect vertical orientation reveals that the formation mechanism proceeds through several stages: (i) sol-gel, (ii) weakly-texture 3D-like perovskite, (iii) n = 3 phase, and finally, (iv) crystallite reorientation into the near-perfect texture. The findings from this in-situ simultaneous GIWAXS and GTWAXS study provide improved understanding of the film formation mechanism for layered hybrid perovskite films with near-perfect vertical orientation.

Automated summary

Controlling the vertical orientation of perovskite slabs in layered hybrid perovskite films is key for optimizing photovoltaic device performance. The study found that blade-coating temperature significantly influences the crystallization dynamics and slab orientation of BA2MAn-1PbnI3n+1 perovskite films, with higher temperatures suppressing the formation of undesired intermediate phases and favoring the formation of desired low-n phases with vertical orientation.