Cation engineering for wide bandgap CH₃NH₃Pb (I_1−xBr_x)₃ perovskite solar cells

Abstract

We intend to demonstrate that the treatment of MA (=CH 3 NH 3 ) Pb (I 1−x Br x ) 3 perovskites with FA (H 2 N-CH=NH 2 ) cations can enhance the perovskites photovoltaic characteristics. Besides, we propose a new route of bandgap engineering employing low-temperature vapor-assisted solution processes (VASP), reducing fabrication time and material usage. Using this proposed method, we synthesized MAPb (I 1−x Br x ) 3 perovskite layers of 0.4≤x≤1 on mesoporous structures in the ambient atmosphere. Then, we fabricated five types of wide-bandgap perovskite solar cells (PSCs), employing five different molar ratios of PbI 2 : PbBr 2 , to tune the bandgaps in the range of 1.78 eV≤ E G ≤2.29 eV. Then, via spin-coating of FAI and FABr, we introduced FA cations into the perovskite samples and obtained higher quality FA 1-y MA y MAPb (I 1−x Br x ) 3 perovskites with smaller trap densities and recombination centers, broader substrate coverage , fewer grain boundaries, a smaller number of pinholes, and hence PSCs with improved photovoltaic performances. The highest efficiencies (11.86% and 12.49%) obtained for untreated and treated samples with x = 0.4 are far from the highest PCEs reported in the literature to date. Nonetheless, the enhancements we observed in the PSCs’ short circuit currents, open-circuit voltages, and PCEs plus the reduction of up to 0.039 in their hysteresis-indices signify the proposed fabrication method and FA treatment can be beneficial to the further development of the perovskite-based solar cells and light emitting diodes.