Naphthalene diimide and benzothiadiazole copolymer acceptor for all-polymer solar cells with improved open-circuit voltage and morphology

Donor-acceptor (D-A) copolymers PNDIBTH and PNDIBTOC8 based on two strong electron-deficient units, naphthalene diimide and benzothiadiazole, were synthesized and used as acceptors for the fabrication of all-polymer solar cells (all-PSCs). Introduction of the two octyloxy side chains onto the benzothiadiazole in PNDIBTOC8 could not only increase the solubility and molecular weight of the polymer, but also alter its optical and electronic properties. Compared with PNDIBTH, PNDIBTOC8 possesses a much higher molecular weight and raised LUMO level up to -3.72 eV. Investigation of the photovoltaic performance of two polymers in all-PSCs using polymer PBDTTT-C-T as donor materials indicated that PNDIBTOC8 provides an excellent power conversion efficiency (PCE) of 3.14% with a high open-circuit voltage (V-oc) of 0.90 V, much higher than that of the PNDIBTH-based device (PCE of 1.20% with Voc of 0.76 V). Moreover, the dendrite-like phase separation in the PNDIBTOC8-based blend film contributes to the high and more-balanced charge mobilities.