Cooperatively Tuning Phase Size and Absorption of Near IR Photons in P3HT:Perylene Diimide Solar Cells by Bay-Modifications on the Acceptor

P3HT is a widely used and commercial polymer donor, but it cannot absorb near IR solar photons. Chemical accessibility to tune the frontier molecular orbits of p-conjugated small molecule acceptors increases the possibility to improve their near IR absorption, which is complementary to P3HT. Taking the aggregation tendency of the planar p-system into account, we herein use the traditional n-type organic semiconductor of perylene diimide (PDI) as the model backbone, showing a molecular way to cooperatively tune the aggregation tendency and absorption of the near IR photons. Practically, we replace the 2-methoxylethoxyl units from the mother PDI monomer (OPDI-O), one-by-one, with the 4,8-bis(2-(2-ethylhexylthienyl) benzo[1,2-b':4,5-b']dithiophene (BDT) moieties, giving two other PDI monomers of B-PDI-O and B-PDI-B. Because of the photoinduced intramolecular charge transfer transition from the BDT unit to the PDI core, B-PDI-B exhibits a broad absorption shoulder beyond 600 nm in the dilute solution, and beyond 650 nm in the solid film. This red-shifted absorption enhances usage of the near IR photons of the solar emission when using P3HT as the donor. The steric effects between the PDI and BDT planes produce twisted conformations, which effectively suppress aggregation tendency. The domain size decreases from >0.5 mu m (O-PDI-O) to similar to 100 nm (B-PDI-O) and then similar to 20 nm (B-PDI-B) with using 4% DIO as additive. Consistently, the short-circuit current density, open-circuit voltage, and efficiency of the optimal P3HT:PDI best cells all increase as BDT unit is introduced on the bay-region.