Synergistic strategy of rubidium chloride regulated SnO2 and 4-tert-butyl-benzylammonium iodide passivated MAxFA1-xPbI3 for efficient mixed-cation perovskite solar cells

Nowadays, the MA-FA mixed-cation perovskite solar cells (PSCs), demonstrating much higher stability than that of single-cation MAPbI3 or FAPbI3, are still encountering a low-efficiency issue. The main reason is that there existing a lot of defects in SnO2 electron transport layers (ETLs) and perovskite absorbers. Herein, we develop a synergistic strategy of Rubidium Chloride (RbCl) doping and 4-tert-butylbenzylammonium iodide (tBBAI) passivation to fabricate high-performance MAxFA1-xPbI3 mixed-cation PSCs. The alkali halide RbCl is first selected as a novel doping additive to regulate the film properties of SnO2 and perovskites. Research shows that the negative Cl- ions can combine with uncoordinated Sn4+ ions due to their strong bonding ability, which can passivate the oxygen-vacancy-related defects and enhance the charge transport of SnO2 ETLs. Meanwhile, the doped Rb+ and Cl- ions may diffuse into perovskite lattices to promote grain growth and reduce the defects of perovskites. Besides, we employ an organic halide salt tBBAI to passivate perovskites and suppress surface defects. Results of deep level transient spectroscopy (DLTS) indicate that SnO2 and SnO2-RbCl based devices have deeper hole traps and large trap densities (ΔE = 0.92 eV, NT = 1.10 × 1016 cm−3; ΔE = 0.93 eV, NT = 2.16 × 1015 cm−3); while tBBAI-treated SnO2-RbCl-based devices own a shallower hole trap and a smaller trap density (ΔE = 0.72 eV, NT = 2.44 × 1013 cm−3); and the corresponding IPb and I(MA/FA) antisite defeats are also identified. Therefore, our synergistic strategy is effectively able to reduce the defect density and suppress the non-radiative recombination. Consequently, the best-performance MA0.85FA0.15PbI3 PSCs achieve an impressive power conversion efficiency (PCE) of 22.54% with a large open circuit voltage (Voc) of 1.16 V, which are by far the highest values of MA0.85FA0.15PbI3 mixed-cation PSCs.