Relationship between the Annealing Temperature and the Presence of PbI₂ Platelets at the Surfaces of Slot-Die-Coated Triple-Halide Perovskite Thin Films

We investigated triple-halide perovskite (THP) absorber layers with 5 mol % MAPbCl(3) added to the double-halide perovskite (Cs(0.22)FA(0.78))Pb(I0.85Br0.15)(3). As a deposition method, a highly scalable printing technique, slot-die coating, with a subsequent annealing step was used. We found a strong power conversion efficiency (PCE) dependence of the corresponding solar cells on the annealing temperature. The device performance deteriorated when increasing the annealing temperature from 125 to 170 degrees C, mainly via losses in the open-circuit voltage (V-oc) and in the fill factor (FF). To understand the mechanisms behind this performance loss, extensive characterizations were performed on both, the THP thin films and the completed solar-cell stacks, as a function of annealing temperature. Correlative scanning electron microscopy analyses, i.e., electron backscatter diffraction, energy-dispersive X-ray spectroscopy, and cathodoluminescence, in addition to X-ray diffraction and photoluminescence, confirmed the presence of PbI2 platelets on the surface of the THP thin films. Moreover, the area fraction of the PbI2 platelets on the film surface increased with increasing annealing temperature. The deteriorated device performance when the annealing temperature is increased from 125 to 170 degrees C is explained by the increased series resistance and increased interface recombination caused by the PbI2 platelets, leading to decreased V-oc and FF values of the solar-cell devices. Thus, the correlative analyses provided insight into microscopic origins of the efficiency losses.