High Voltage All Polymer Solar Cells with a Polymer Acceptor based on NDI and Benzotriazole

All-polymer solar cells (all-PSCs) with a p-type conjugated polymer as a donor and an n-type conjugated polymer as an acceptor have attracted increasing attention recently due to their good film-forming properties, excellent flexibility and high morphological stability. Herein, we synthesized a new n-type conjugated copolymer LA03 based on naphthalene diimide (NDI) and benzotriazole (BTZ) with thiophene π-bridges, which is a structure modification of N2200 by inserting a benzotriazole unit between its two thiophene units. Compared with N2200, LA03 exhibits blue-shifted absorption and a higher lowest unoccupied molecular orbital (LUMO) energy level. The LA03-based all-PSCs with wide-band-gap polymer PBDB-T as a donor, achieved a power conversion efficiency (PCE) of 6.49%, with a high open circuit voltage (Voc) of 0.937 V, which is improved in comparison with the PBDB-T:N2200-based all-PSCs with a PCE of 5.85% and a Voc of 0.812 V. The higher Voc of the LA03-based devices is benefitted from the up-shifted LUMO energy level of the LA03 acceptor. The results indicate that inserting a benzotriazole unit into the main chain of N2200 is an effective way to upshift its LUMO energy level and increase the Voc of the all-PSCs.