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

Two types of acyloxyfunctionalized ATz (anthra[1,2-c:5,6-c']-bis([1,2,5]thiadiazole) polymers, PATz4T-a12R, with different alkyl chain lengths were synthesized to investigate their physicochemical properties and solar cell performance in combination with PC61BM. The effect of acyloxy groups on electronic properties, thin film structures, and photovoltaic characteristics was also studied. 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 alkoxysubstituted ATz monomer ATz2T-o6OD. 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. Meanwhile, despite the presence of the acyloxy groups, the synthesized polymer PATz4T-a12R (a12R = a12OD and a12DT) have higher HOMO energy levels than the previously reported alkoxy-substituted polymer PATz4T-o6OD. This elevation of the HOMO energy levels could 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 π-electron system in the polymer and the electron-donating mesomeric effects could 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 of 3.47% than that using PATz4T-o6OD.