Isomerism Strategy to Optimize Aggregation and Morphology for Superior Polymer Solar Cells

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-alpha and PAB-gamma, 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-alpha-based devices achieve an excellent PCE of 15.05%, while the PAB-gamma-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-gamma 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-alpha for a smaller chi value of 0.067 and smaller root-mean-square value of 0.98 nm. What is more, PAB-alpha has a closer pi-pi interaction distance compared to its isomer PAB-gamma 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-alpha 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.