An Isomeric Solid Additive Enables High-Efficiency Polymer Solar Cells Developed Using a Benzo-Difuran-Based Donor Polymer.

Currently, nearly all high-efficiency organic photovoltaic devices use donor polymers based on the benzo-dithiophene (BDT) unit. To diversify the choices of building blocks for high-performance donor polymers, the use of benzo-difuran (BDF) units is explored, which can achieve reduced steric hindrance, stronger molecular packing, and tunable energy levels. In previous research, the performance of BDF-based devices lagged behind those of BDT-based devices. In this study, a high efficiency (18.4%) is achieved using a BDF-based polymer donor, which is the highest efficiency reported for BDF donor materials to date. The high efficiency is enabled by a donor polymer (D18-Fu) and the aid of a solid additive (2-chloronaphthalene), which is the isomer of the commonly used additive 1-chloronaphthalene. These results revealed the significant effect of 2-chloronaphthalene in optimizing the morphology and enhancing the device parameters. This work not only provides a new building block that can achieve an efficiency comparable to dominant BDT units but also proposes a new solid additive that can replace the widely used 1-chloronaphthalene additive.