High-Performance and Uniform 1 cm² Polymer Solar Cells with D₁-A-D₂-A-Type Random Terpolymers

For the commercial development of organic photovoltaics (OPVs), laboratory-scale OPV technology must be translated to large area modules. In particular, it is important to develop high-efficiency polymers that can form thick (>100 nm) bulk heterojunction (BHJ) films over large areas with optimal morphologies for charge generation and transport. Here, D₁-A-D₂-A random terpolymers composed of 2,2′-bithiophene with various proportions of 5,6-difluoro-4,7-bis(thiophen-2-yl)-2,1,3-benzothiadiazole and 5,6-difluoro-2,1,3-benzothiadiazole (FBT) are synthesized. It is found that incorporating small proportions of FBT into the polymer not only conserves the high crystallinity and favorable face-on orientation of the D-A copolymer FBT-Th4 but also improves the nanoscale phase separation of the BHJ film. Consequently, the random terpolymer PDT2fBT-BT10 exhibits a much improved solar cell efficiency of 10.31% when compared to that of the copolymer FBT-Th4 (8.62%). Moreover, due to this polymer's excellent processability and suppressed overaggregation, OPVs with 1 cm² active area based on 351 nm thick PDT2fBT-BT10 BHJs exhibit high photovoltaic performance of 9.42%, whereas rapid efficiency decreases arise for FBT-Th4-based OPVs for film thicknesses above 300 nm. It is demonstrated that this random terpolymer can be used in large area and thick BHJ OPVs, and guidelines for developing polymers that are suitable for large-scale printing technologies are presented.