Overcoming the Open-Circuit Voltage Losses in Narrow Bandgap Perovskites for All-Perovskite Tandem Solar Cells

Narrow-bandgap (NBG) perovskite solar cells based on tin-lead mixed perovskite absorbers suffer from significant open-circuit voltage (<i>V</i> _OC) losses due primarily to a high defect density and charge carrier recombination at the device interfaces. In this study, the <i>V</i> _OC losses in NBG perovskite single junction cells (<i>E</i> _g = 1.21 eV) are addressed. The optimized NBG subcell is then used to fabricate highly efficient all-perovskite tandem solar cells (TSCs). The improvement in the <i>V</i> _OC is achieved via the addition of a thin poly(triarylamine) interlayer between the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)-based hole transport layer (HTL) and the NBG perovskite. The optimal bilayer HTL results in a champion power conversion efficiency (PCE) of 20.3%, compared to 17.8% of the PEDOT:PSS-based control device. The <i>V</i> _OC improvement of the single-junction NBG cell is also successfully transferred to all-perovskite TSC, resulting in a high <i>V</i> _OC of 2.00 V and a PCE of 25.1%.