Metal halide perovskite (PVK) solar cells have attracted an extensive amount of attention due to the rapid enhancement of the power conversion efficiency reaching 25% within a decade.^[1] The most explored approach to deposit high quality PVK films is by spin-coating, which has demonstrated excellent performing devices with relatively small areas of about 0.1 cm² .^[2] Recently, it has been claimed that preferential growth of spin-coated PVK layer can lead to suppression of non-radiative recombination.^[3] Thermal evaporation of perovskites does not require the use of harmfull solvents and is compatible with textured substrates and large-area film deposition.^[4] However, control of the PVK crystal orientation in layers prepared by thermal evaporation has harldy been addressed. Here we want to study how we can manipulate the crystal orientation of thermally evaporated PVKs and investigate the effects of crystal orientation on the opto-electronic properties.

We prepared Cs_0.15FA_0.85PbI_2.85Br_0.15 with preferential crystal orientations ranging from (110) to (100) based on the sequential thermal evaporation process. This deposition process starts with the sequential deposition of lead iodide (PbI₂), formamidinium iodide (FAI) and cesium bromide (CsBr) with appropriate thicknesses to obtain a three-layered stack with the desired stoichiometric composition. This process is then repeated a second time to reach the targeted thickness. By applying different annealing temperatures (w/o annealing, 50 °C, 100 °C, 130 °C, and 160 °C) between the first and second deposition, we are able to fabricate 450-nm thick Cs_0.15FA_0.85PbI_2.85Br_0.15 films with a controllable crystal orientation from (110) to (100) , as shown in the TOC. All samples are annealed afterwards to convert the precorsors into the PVK.

We demonstrated by both XRD and 2D XRD that the sample without annealing between the two depositions shows a preferred orientation in the (110) direction, while intermediate annealing at 130 °C or 160 °C leads to a preferred orientation in the (100) direction. This variation in preferential orientation may originate from the variation in surface composition of the first stack as observed from the 2D-XRD, which provides different templates for the crystallisation of the second stack. However, both the (100) and (110) orientated PVK films show comparable opto-electronic properties as observed by photo-luminescence. This result is in contrast with the reported results for solution based PVKs. The comparable PL peak position and intensity indicate that at least part of the opto-electronic properties are not directly affected by the orientation of the PVK crystals. To further support this conclusion, more analysis via photo-conductance measurements will be performed to gain insight into the effect of the crystal orientation on the charge carrier mobility, lifetime, and trap densities.