RESUMO
All-perovskite tandem solar cells show great potential to enable the highest performance at reasonable costs for a viable market entry in the near future. In particular, wide-bandgap (WBG) perovskites with higher open-circuit voltage (VOC ) are essential to further improve the tandem solar cells' performance. Here, a new 1.8 eV bandgap triple-halide perovskite composition in conjunction with a piperazinium iodide (PI) surface treatment is developed. With structural analysis, it is found that the PI modifies the surface through a reduction of excess lead iodide in the perovskite and additionally penetrates the bulk. Constant light-induced magneto-transport measurements are applied to separately resolve charge carrier properties of electrons and holes. These measurements reveal a reduced deep trap state density, and improved steady-state carrier lifetime (factor 2.6) and diffusion lengths (factor 1.6). As a result, WBG PSCs achieve 1.36 V VOC , reaching 90% of the radiative limit. Combined with a 1.26 eV narrow bandgap (NBG) perovskite with a rubidium iodide additive, this enables a tandem cell with a certified scan efficiency of 27.5%.
RESUMO
We investigated triple-halide perovskite (THP) absorber layers with 5 mol % MAPbCl3 added to the double-halide perovskite (Cs0.22FA0.78)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 °C, mainly via losses in the open-circuit voltage (Voc) 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 °C is explained by the increased series resistance and increased interface recombination caused by the PbI2 platelets, leading to decreased Voc and FF values of the solar-cell devices. Thus, the correlative analyses provided insight into microscopic origins of the efficiency losses.
