Side-chain engineering of wide-bandgap copolymers based on two different electron-deficient units for high-performance polymer solar cells†

Incorporation of electron-deficient units into polymer backbones is a promising strategy to enlarge the bandgaps of polymer donor materials. Herein, two wide-bandgap (WBG) polymer donors (P1 and P3) are designed and synthesized by using two different electron-deficient units of 5,10-dihydrodithieno[3,2-c:3′,2′-h][2,6]naphthyridine-4,9-dione (TND) with different side-chains, and 5,6-difluoro-2-(2-hexyldecyl)-2H-benzo[d][1,2,3]triazole. Due to the strong electron-withdrawing ability of both two building blocks, the two copolymers exhibit large optical bandgaps and low-lying highest occupied molecular orbital (HOMO) energy levels, which are matched with the narrow bandgap nonfullerene acceptor of L8-BO. The alkyl chains on the polymer backbones can affect molecular packing and charge transport properties of the copolymers. P3 with 2-butyloctyl side-chains exhibits an enhanced crystallinity and a preferable face-on molecular orientation. When blended with L8-BO, the best P3-based polymer solar cell (PSC) displays a power conversion efficiency (PCE) of 12.56% with an open-circuit voltage (V_OC) of 0.742 V, a short-circuit current density (J_SC) of 22.80 mA cm⁻², and a fill factor (FF) of 74.30%. However, the best-performing P1-based PSC shows a relatively low PCE of 10.37%. This study suggests that the polymer donor materials based on two electron-deficient units have great potential in fabricating efficient nonfullerene PSCs.