Management of interfacial energy band alignment in wide-bandgap perovskite solar cells for performance improvement

Abstract

Wide-bandgap perovskite solar cells (PSCs) have been intensively studied owing to their excellent absorption of short-wavelength sunlight. Due to the highly-located lowest unoccupied molecular orbital (LUMO) level of wide-bandgap perovskites, developing a suitable electron transport layer for wide-bandgap PSCs has become a challenge. In this work, we adjusted the interfacial energy band alignment to improve the performance of wide-bandgap PSCs. With the addition of poly(9,9-bis(3′-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene))dibromide (PFN-Br), the conduction band of SnO₂ was tuned from −4.33 to −3.78 eV, which is much closer to the LUMO level (−3.65 eV) of the Cs_0.15FA_0.79MA_0.06Pb(I_0.7Br_0.3)₃ perovskite (with a bandgap of 1.74 eV). Moreover, with the coating of polyethylenimine (PEI), the work function of the ITO cathode shifted from −4.7 to −4.05 eV. With this technique, the energy-level mismatch was significantly reduced at the interface of the cathode side, which facilitated the electron transport in the PSCs. As a result, the average power-conversion-efficiency (PCE) of the wide-bandgap PSCs improved from 17.4% to 19.8% upon the use of PFN-Br and PEI. The best PSC obtained with this strategy exhibits a high PCE of 20.2% and a large open-circuit voltage of 1.29 V, indicating the effectiveness of energy level adjustment in wide-bandgap PSCs.