Solution-Processable Nickel Oxide Hole Transport Layer for a Polymer Donor with a Deep HOMO Level

Abstract

The photoactive layer of non-fullerene organic solar cells (OSCs) generally consists of a low band-gap acceptor and conjugated polymer with a deep highest occupied molecular orbital (HOMO) because the hole transfer from the acceptor to the donor as well as the electron transfer from the donor polymer to the acceptor is very important to achieve better performance. Therefore, energy level matching between the HOMO of the donor polymer and the work-function (WF) of the hole transport layer is required. Since the intrinsic p-type NiO has a lower WF value, the efficiency of NiO-based PBDB-T-2F:Y6 OSCs is somewhat suppressed. In this work, the energy barrier between the NiO (5.1 eV) and HOMO of PBDB-T-2F (5.6 eV) by modifying the NiO surface using a 4-(trifluoromethyl)benzoic acid (PTF-BOA) dipole layer (NiO-PTF-BOA) is successfully eliminated. The carboxyl group of PTF-BOA is bonded with Ni atoms on the NiO surface, while the CF3 group attracts electrons, thus resulting in a partial negative charge on the CF3 side. This PTF-BOA dipole layer induces the vacuum level upshift thus enhancing the work function of NiO-PTF-BOA to 5.5 eV. The PBDB-T-2F:Y6 OSCs with NiO-PTF-BOA exhibit an efficiency of 16.36%, which is the highest reported efficiency for NiO-based OSCs.