Enhanced photovoltaic effect from naphtho[2,3-c]thiophene-4,9-dione-based polymers through alkyl side chain induced backbone distortion

Abstract

The innovation of photoactive layer materials is crucial for improving the power conversion efficiency (PCE) of polymer solar cells (PSCs). Herein, we report two polymer donors (PBTN-o and PBTN-p), which only differ in alkyl chains substituted at sites (6,7- or 5,8-) of the naphtho[2,3-c]thiophene-4,9-dione (NTD) unit. The single crystals of both NTD monomers demonstrate that NTD with alkyl chains at the 6,7-sites has a planar NTD skeleton, and surprisingly, NTD with alkyl chains at the 5,8-sites produces a bent NTD skeleton. The bent NTD-based polymer (PBTN-p) exhibits a more twisted conjugated backbone than PBTN-o. Our comparative studies show that PBTN-p possesses suitable aggregation properties that can optimize the photoactive layer morphology in NF PSCs. In PSCs, the optimal PBTN-o:BO-4Cl-based device shows a PCE of 11.85% with a V_OC of 0.84 V, J_SC of 22.41 mA cm⁻², and FF of 0.63. In contrast, the optimal PBTN-p:BO-4Cl-based device exhibits a better PCE of 14.10% with the same V_OC of 0.84 V, and an enhanced J_SC of 24.67 mA cm⁻² and FF of 0.68. This work provides a new insight into BHJ morphology optimization by side chain induced polymer main chain twisting in PSCs.