Effects of Acyloxy Groups in Anthrabisthiadiazole-Based Semiconducting Polymers on Electronic Properties, Thin-Film Structure, and Solar Cell Performance

Donor-acceptor (D-A) polymers with the anthra[1,2-c:5,6-c']-bis([1,2,5]thiadiazole) (ATz)-based acceptor unit bearing acyloxy groups in the 6,12-positions were synthesized. By incorporating electron-withdrawing acyloxy groups, the synthesized monomers 5a and 5b showed a down-shifted HOMO while maintaining LUMO energy level compared to the alkoxy-substituted ATz monomer ATz2T-o6OD, which we have previously reported. The DFT calculations revealed that the LUMO of the ATz core at 6,12-positions is a nodal plane with negligible changes in LUMO energy levels. In contrast, despite the presence of the acyloxy groups, the polymer PATz4T-a12R (a12R = a12OD and a12DT) synthesized in this study was found to have higher HOMO energy levels than the previously reported alkoxy-substituted polymer PATz4T-o6OD. Such elevation of the HOMO energy levels may be attributed to the unique electronic effects of the acyloxy groups, where the electronic effects of the functional groups are weakened by the lengthening of the pi-electron system in the polymer and the electron-donating mesomeric effects may be dominant. PATz4T-a12R formed unsuitable edge-on orientation and large phase separation in the blended films, resulting in solar cells using it exhibiting a lower power conversion efficiency (PCE) of 3.47% than that using PATz4T-o6OD.

Automated summary

In this study, donor-acceptor (D-A) polymers with anthra[1,2-c:5,6-c']-bis([1,2,5]thiadiazole) (ATz)-based acceptor units bearing acyloxy groups were synthesized. The presence of the acyloxy groups in the synthesized polymers resulted in higher HOMO energy levels compared to previously reported alkoxy-substituted polymers. However, the unsuitable edge-on orientation and large phase separation in blended films led to lower power conversion efficiency in solar cells using the polymers with acyloxy groups.