Relationship between the Annealing Temperature and the Presence of PbI 2 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(I 0.85 Br 0.15 ) 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 °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 PbI 2 platelets on the surface of the THP thin films. Moreover, the area fraction of the PbI 2 platelets on the film surface increased with increasing annealing temperature. The deteriorated device performance when the annealing temperature is increased from 125 to 170 °C is explained by the increased series resistance and increased interface recombination caused by the PbI 2 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.