Low-Temperature Processable High-Performance D–A-Type Random Copolymers for Nonfullerene Polymer Solar Cells and Application to Flexible Devices

Donor–acceptor random copolymers incorporating 1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) and 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-Th) are developed. Nonfullerene solar cells introducing the random copolymer PBDB-T73 (BDT-Th:BDD=7:3) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene are found to exhibit efficiency of 11.14%, which is superior to that of the PBDB-T alternating copolymer (10.02%). The PBDB-T73 device exhibits high performance due to low bimolecular charge recombination and good charge transport balance even though none of the treatments that are usually necessary for improving crystalline properties of active layers are performed; in contrast, the PBDB-T device requires thermal annealing in order to enhance the polymer's crystallinity and obtain improved fill factor and J_sc values. It is demonstrated that efficient charge transport occurs in the PBDB-T73 blend despite relatively low crystallinity of the polymer. The percolating network structure of the large mixed phase in the PBDB-T73 blend has an important role in its efficient charge transport. Moreover, due to the low-temperature processability of PBDB-T73, flexible PBDB-T73 solar cell devices prepared on polyethylene terephthalate (PET) substrates exhibit high photovoltaic performance, 10.02%, whereas the efficiency of PBDB-T-based flexible devices (7.2%) is significantly lower due to damage to the active layer on indium tin oxide (ITO)/PET that results from thermal annealing.