Overcoming\nthe Open-Circuit Voltage Losses in Narrow\nBandgap Perovskites for All-Perovskite Tandem Solar Cells

Narrow-bandgap (NBG) perovskite solar cells based on\ntin–lead\nmixed perovskite absorbers suffer from significant open-circuit voltage\n(<i>V</i><sub>OC</sub>) losses due primarily to a high defect\ndensity and charge carrier recombination at the device interfaces.\nIn this study, the <i>V</i><sub>OC</sub> losses in NBG perovskite\nsingle junction cells (<i>E</i><sub>g</sub> = 1.21 eV) are\naddressed. The optimized NBG subcell is then used to fabricate highly\nefficient all-perovskite tandem solar cells (TSCs). The improvement\nin the <i>V</i><sub>OC</sub> is achieved via the addition\nof a thin poly(triarylamine) interlayer between the poly(3,4-ethylenedioxythiophene)\npoly(styrenesulfonate) (PEDOT:PSS)-based hole transport layer (HTL)\nand the NBG perovskite. The optimal bilayer HTL results in a champion\npower conversion efficiency (PCE) of 20.3%, compared to 17.8% of the\nPEDOT:PSS-based control device. The <i>V</i><sub>OC</sub> improvement of the single-junction NBG cell is also successfully\ntransferred to all-perovskite TSC, resulting in a high <i>V</i><sub>OC</sub> of 2.00 V and a PCE of 25.1%.