Enhanced Photovoltaic Effect from Naphtho[2,3-c]thiophene-4,9-dione-based Polymer through Alkyl Side Chain Induced Backbone Distortion
The innovation of photoactive layer materials is crucial for improving power conversion efficiency (PCE) of polymer solar cells (PSCs). Herein, we report two polymer donors (PBTN-o and PBTN-p), which only differ on alkyl chains substituted on sites (6,7- or 5,8-) of naphtho[2,3-c]thiophene-4,9-dione (NTD) unit. The single crystals of both NTD monomers demonstrate that the NTD with alkyl chains at 6,7-sites has a planar NTD skeleton, Surpringly, the NTD with alkyl chains at 5, 8-sites produces a bent NTD skeleton. The bent NTD-based polymer (PBTN-p) has a more twisted conjugated backbone than that of PBTN-o. Our comparative studies show that PBTN-p possesses a suitable aggregation property that can optimize photoactive layer morphology in NF PSCs. In PSCs, the optimal PBTN-o:BO-4Cl-based device shows a PCE of 11.85% with a VOC of 0.84 V, JSC of 22.41 mA cm-2, and FF of 0.63, in contrast, the optimal PBTN-p:BO-4Cl-based device gives a better PCE of 14.10% with the same VOC of 0.84 V, and enhanced JSC of 24.67 mA cm-2, and FF of 0.68. This work provides a new insight that BHJ morphology can be optimized by side chian induced polymer main chain twist in PSCs.