Combining Polymer Zwitterions and Zinc Oxide for High-Performance Inverted Organic Solar Cells

Zinc oxide (ZnO) is a widely used cathode interlayer material in inverted organic solar cells (OSCs). However, there are lots of surfaces or bulk film defects in ZnO layers, which degrade solar cell performance. Here, the typical phosphorylcholine- and sulfobetaine-based polymer zwitterions (PMPC and PDMAPS) are synthesized via reversible addition-fragmentation chain-transfer (RAFT) polymerization to modify ZnO interlayers for inverted OSCs. The polymer zwitterions can efficiently passivate the defects in ZnO films and thus increase the conductivity of the ZnO interlayers. Both PMPC and PDMAPS modified ZnO interlayers show some general advantages in improving the performance of fullerene-based and non-fullerene-based OSCs. The highest efficiency of 16.69% is achieved by using PMPC modified ZnO interlayers in PM6:Y6 based solar cell devices, which is among the best performance in inverted OSCs. Such an improvement in device performance is attributed to the work function reduction of the polymer zwitterions modified ZnO films, which provides an efficient cathode platform to extract and transport electrons from the active layers, to the benefit of suppressing interfacial charge recombination. As a result, the organic-inorganic hybrid composites (ZnO: polymer zwitterions) show efficient interfacial modification to align energy levels at the device interface, which have promising application prospects in organic electronics.