Highly Efficient All-Polymer Solar Cells from a Dithieno[3,2‑f:2′,3′‑h]quinoxaline-Based Wide Band Gap Donor

It is challenging yet appealing for researchers to construct new polymer donors that can work cooperatively with the polymer acceptors and thus realize maximum power conversion efficiencies (PCEs) of all-polymer solar cells (PSCs). We have synthesized two dithieno­[3,2-f:2′,3′-h]­quinoxaline-based wide band gap donor polymers (PBQx-Me-TF and PBQx-H-TF) and a new γ-position based narrow band gap polymer acceptor: PBTIC-γ-TT. The temperature-dependent absorption spectra showed that removal of a weaker electron-donating methyl group in the donor polymer strengthened the aggregation and the absorption coefficients. The crystal structures showed that PBQx-H-TF had a closer π–π stacking distance of 3.33 Å when compared to the PBQx-Me-TF (3.40 Å). The smaller E HOMO offset (0.07 eV) between the donor PBQx-H-TF and acceptor PBTIC-γ-TT than that of PBQx-Me-TF/PBTIC-γ-TT (0.10 eV) provided a better hole transport. The PBQx-H-TF/PBTIC-γ-TT films showed a smaller total energy loss (0.574 eV) than the PBQx-Me-TF/PBTIC-γ-TT film (0.607 eV); hence, this molecular structure adjustment reduced the nonradiative energy loss. PBQx-H-TF also showed better miscibility with PBTIC-γ-TT with a smaller χ value of 0.25. In addition, a bicontinuous interpenetrating microstructure was observed in the active layer blend film (PBQx-H-TF/PBTIC-γ-TT), resulting in a J SC of 22.24 mA cm–2, a FF of 67.80%, and a PCE of 14.21% in the device. These observations revealed the significance of molecular structure adjustment for better device performance, and therefore, PBQx-H-TF can be an excellent candidate for all-PSCs.