High‐Performance Flexible All‐Perovskite Tandem Solar Cells with Reduced VOC‐Deficit in Wide‐Bandgap Subcell

Abstract Among various types of perovskite‐based tandem solar cells (TSCs), all‐perovskite TSCs are of particular attractiveness for building‐ and vehicle‐integrated photovoltaics, or space energy areas as they can be fabricated on flexible and lightweight substrates with a very high power‐to‐weight ratio. However, the efficiency of flexible all‐perovskite tandems is lagging far behind their rigid counterparts primarily due to the challenges in developing efficient wide‐bandgap (WBG) perovskite solar cells on the flexible substrates as well as their low open‐circuit voltage ( V OC ). Here, it is reported that the use of self‐assembled monolayers as hole‐selective contact effectively suppresses the interfacial recombination and allows the subsequent uniform growth of a 1.77 eV WBG perovskite with superior optoelectronic quality. In addition, a postdeposition treatment with 2‐thiopheneethylammonium chloride is employed to further suppress the bulk and interfacial recombination, boosting the V OC of the WBG top cell to 1.29 V. Based on this, the first proof‐of‐concept four‐terminal all‐perovskite flexible TSC with a power conversion efficiency of 22.6% is presented. When integrating into two‐terminal flexible tandems, 23.8% flexible all‐perovskite TSCs with a superior V OC of 2.1 V is achieved, which is on par with the V OC reported on the 28% all‐perovskite tandems grown on the rigid substrate.