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The effect of isomerism in polymer donors is appealing as a means of optimization of molecular configurations in organic solar cells (OSCs) but has not been well explored. Two isomers, PAB-α and PAB-γ, with different orientations of their fused thiophene rings were designed and synthesized to investigate the influence of isomerism in polymer donors on their photovoltaic conversions. It was shown that two polymers with almost identical structures exhibited significant differences in the power conversion efficiency (PCE) of solar devices. The PAB-α-based devices achieve an excellent PCE of 15.05%, while the PAB-γ-based devices only obtain an extremely low PCE of 0.04%. Reasons for such a dramatic performance disparity include first, the absorption spectrum of PAB-γ being markedly blue-shifted and failing to match the absorption spectrum of common high-efficiency acceptors, such as Y6, and second, acceptor Y6 has preferable miscibility with PAB-α for a smaller χ value of 0.067 and smaller root-mean-square value of 0.98 nm. What is more, PAB-α has a closer π–π interaction distance compared to its isomer PAB-γ from grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis, and the order-of-magnitude difference between the hole and electron mobilities of two active layers also made the opposing values of their device efficiencies. Therefore, PAB-α has a superior performance in photovoltaic devices, demonstrating that fine tuning of atomic orientation could bring great changes to the properties of the polymer donors. This provides a new train of thought for the material design and evolution of device performance.

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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.