Design of ultra-high luminescent polymers for organic photovoltaic cells with low energy loss

Here, new polymers named PBTT-Cn (n = 3, 4, or 5) have been designed based on carboxylate-functionalized thieno[3,2-b]thiophene units. The electroluminescence external quantum efficiencies (EQE_EL) of pristine PBTT-C4 and C5 films are over 1 × 10⁻², which are among the highest results for OPV materials. Through combination with BTP-eC9, the PBTT-C4-based film showed a high EQE_EL of 6 × 10⁻⁴ and the V^non-rad_loss is 0.19 eV. As a result, a high open-circuit voltage of 0.89 V and a satisfactory PCE of 15% were recorded in the PBTT-C4-based OPV cells.