Relationship Between Annealing Temperature and the Presence of PbI₂ Platelets at the Surfaces of Triple-Halide Perovskite Films

Metal-halide perovskites with a mixture of halides form a promising but complex material system that offers the possibility to vary band gap energies via compositional changes for tandem solar cell applications. We use the so called triple-halide perovskite¹ with the composition (Cs_0.22FA_0.78)Pb(I_0.85Br_0.15)₃+ 5 mol% MAPbCl₃ in slot-die coated solar cells and find a strong dependency of the power conversion efficiency on the annealing temperature. When varying the annealing temperature from 100 to 170 °C an efficiency maximum at 150 °C is found. From 100 to 150 °C, the conversion efficiency increases up to a maximum value of almost 20%, mainly by an increase in the short-circuit current density (<i>j</i>_sc), while the device performance becomes reduced when varying the annealing temperature from 150 to 170 °C, mainly via losses in the open-circuit voltage (<i>V</i>_oc) and the fill factor (<i>FF</i>). Since it is essential to understand the underlying mechanisms of the performance and its limitations for the further improvement of the corresponding solar cell devices, extensive characterizations were performed on both, the triple-halide perovskite thin films and the completed solar cell stacks as function of annealing temperature. We conducted electron backscatter diffractometry (EBSD), energy-dispersive X-ray spectrometry (EDX), and cathodoluminescence (CL) in scanning electron microscopy, in addition to (time-resolved) photoluminescence. Correlative analyses using EBSD, EDX, CL and grazing-incidence X-ray diffraction confirmed the presence of PbI₂ platelets on the surface of the triple-halide perovskite films and growing average grain sizes of both, the perovskite and PbI₂ phases with increasing annealing temperature. Moreover, the area fraction of the PbI₂ platelets on the film surface increased with the annealing temperature. When increasing the annealing temperature further from 150 to 170 °C, the large area fraction of the PbI₂ platelets at the interface between triple-halide perovskite absorber and electron transport layer leads likely to enhanced interface recombination and increased contact resistance, decreasing the <i>V</i>_oc and the <i>FF</i> of the solar cell device. Thus, the correlative analyses provided insight into the microscopic origins of the efficiency maximum for triple-halide perovskite solar cells with varying annealing temperature.<br/>1. Xu, J., et al. Triple-halide wide-band gap perovskites with suppressed phase segregation for efficient tandems. <i>Science </i><b>367</b>, 1097–1104 (2020).