Development of a conjugated donor-acceptor polyelectrolyte with high work function and conductivity for organic solar cells

To achieve highly efficient organic photovoltaic (OPV) devices, the interface between the photoactive layer and the electrode must be modified to afford the appropriate alignment of the energy levels and to ensure efficient charge extraction at the same time as suppressing charge recombination and accumulation. Recently, p-type conjugated polyelectrolytes (CPEs) have emerged as new hole-transporting materials that can be deposited on electrodes through simple solution processes without additional heat treatment. However, the applications of CPEs have been limited so far because the high electron richness of their conjugated backbones result in low work functions, ∼5.0 eV. Here, by inserting a donor−acceptor (D−A) building block into the CPE backbone, we successfully synthesized a new p-type CPE (PhNa-DTBT), which shows a deep work function above 5.3 eV on several electrodes including Au, Ag, and indium tin oxide. More importantly, PhNa-DTBT produces stable polarons on the polymer backbone and thus achieves a high electrical conductivity of 5.7 × 10−4 S cm−1. As a result, an OPV incorporating PhNa-DTBT as a hole-transporting layer was found to exhibit a high performance with a power conversion efficiency of 9.29%. Also, the OPV device shows improved stability in air due to the neutral characteristics of the CPE which is favorable for stabilizing neighbored active and electrode layers.