Enhancing the power conversion efficiency of polymer solar cells to 9.26% by a synergistic effect of fluoro and carboxylate substitution

Two kinds of new conjugated polymers (P1 and P2) with benzothiadiazole as the acceptor unit and thiophene as the donor unit were designed, synthesized and used as donor materials for polymer solar cells (PSCs). These polymers show a broad absorption in the visible region, a medium band gap of about 1.75 eV, and a low-lying HOMO energy level of about -5.65 eV. The open-circuit voltage (V_oc) of both P1 and P2 was greatly improved to 0.85 V mainly due to the introduction of a carboxylate group at the 3-position of the thiophene spacer. Fluoro substitution on the polymer backbone of P2 can greatly enhance the interchain interaction, leading to a huge increase of short-circuit current density (J_sc). P2-based devices with the active layer spin-coated from 1,2-diclorobenzene (DCB) solutions that contain 1% 1,8-diiodooctane (DIO) and washed with methanol showed a synergistic positive effect, resulting in a significant enhancement of the power conversion efficiency (PCE) up to 8.67%. The PCE could be further improved by constructing inverted devices and the best efficiency of 9.26% was finally obtained. In addition, the mechanism for achieving such a high PCE for P2 based devices was also proposed based on the morphological analysis of the blend films by atomic force microscopy (AFM), transmission electron microscopy (TEM), grazing incident angle X-ray scattering (GIWAXS) and resonant soft X-ray scattering (RSoXS). The improvement can be ascribed to the enhanced molecular packing and proper phase separation of the blend films and the reduced charge recombination.