A Solution-Processed Small-Molecule Diketopyrrolopyrrole Dimer for Organic Solar Cells

An easily-accessible, solution-processed diketopyrrolopyrrole (DPP) dimer, namely BDT(TDPP)(2), with benzodithiophene (BDT)-thiophene (T) as the conjugated bridge unit was synthesized. The UV/vis absorption spectrum of the DPP dimer is broad and covers 350-740 nm, and the HOMO and LUMO energies are estimated to be -5.21 eV and -3.54 eV, respectively, by electrochemistry tests, which match well with the energy levels of [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM). Solution-processed, small-molecule solar cells were prepared with BDT(TDPP)(2) as the donor material and PC71BM as the acceptor material. With chloroform used as the processing solvent, the high boiling-point solvent 1,8-diiodooctane (DIO) was used as the co-solvent. Through screening the donor/acceptor ratios (1:1) and the DIO content (0.2% (v/v)), the best device afforded a power conversion efficiency of 3.90%, with a short-circuit current density (J(sc)) of 9.85 mA cm(-2), an open-circuit voltage (V-oc) of 0.71 V, and an fill factor of 0.57.