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Appropriate third monovalent A‐site cation incorporation in formamidinium cesium lead iodide for defect passivation and efficiency improvement in perovskite solar cells

The presence of intrinsic defects in the bulk perovskite absorber and recombination of charge carriers hampers the device performance in perovskite solar cells. To overcome the bulk defects in the formamidinium cesium lead iodide (FACsPbI3) perovskite absorber layer, A‐site cation engineering using nitrogen containing heterocyclic aromatic monovalent molecule, glyoxaline iodide/imidazolium iodide (IAI) is carried out successfully in this study. Even though IAI is a relatively larger aromatic cation, an optimized concentration of co‐doping in FACsPbI3 does not destroy the 3D perovskite lattice as evidenced by X‐ray diffraction analysis. Binding energy of Pb‐4f core‐level shifts to lower energy that reveals that the free lone pair of electrons on the nitrogen atom in IAI binds with the undercoordinated Pb2+, which results in in situ defect passivation. As a result, the in situ passivated device shows reduced trap filled limit voltage (VTFL) of 0.09 V (from 0.19), lower trap state density (~1.05 × 1015 cm−3), and reduced diode ideality factor (nid) of 1.36 from 1.48. Due to the reduced nonradiative recombination an increased trap‐assisted lifetime of 410 ns was also observed compared to the bare device (177 ns). The lower theoretical open circuit voltage (VOC) loss with enhanced theoretical fill factor (FF0) reveals that the incorporation of third A‐site cation results in reduced charge carrier transport loss. Finally, the in situ passivated device shows 60 mV higher open‐circuit voltage (VOC) than the bare device (1.01‐1.07 V) with enhanced fill factor, which leads to an improved power conversion efficiency of 16.02%. The findings reported here demonstrate the beneficial role of the third A‐site cation in passivating the inherent defects seen in formamidinium cesium lead iodide absorber.