A diketopyrrolopyrrole-based regular terpolymer bearing two different π-extended donor units and its application in solar cells

Abstract

In this study, to adjust the desired molecular energy levels and bandgap energies of polymers for photovoltaic applications, a regular terpolymer structure was designed. A new regular terpolymer, poly(DPP4T-alt-TBP), containing diketopyrrolopyrrole (DPP), BT, and BP units in the repeating group was successfully synthesized. The DPP-BT monomeric unit in polymer backbone enhanced chain packing and induced a high-lying highest occupied molecular orbital (HOMO) level and the DPP-BP segment induced a deeper HOMO level. The lowest unoccupied molecular orbital (LUMO) level of the terpolymer was also controlled in a similar manner. The HOMO level of the terpolymer was similar to that of poly(DPP-alt-BP), and the energies of the LUMOs were governed by the DPP-BT unit. The polymer chain arrangement of the terpolymer on the substrate was observed to be a mix of face-on and edge-on orientations, which is a different chain arrangement mode to those shown in both poly(DPP-alt-BP) and poly(DPP-alt-BT). A TFT fabricated with poly(DPP4T-alt-TBP) had a charge carrier mobility of 0.59 cm2 V−1 s−1 and a moderately high current on/off ratio. Furthermore, a polymer solar cell containing the terpolymer and PC71BM had a power conversion efficiency of 4.54%, which is significantly higher than those of the PCEs of poly(DPP-alt-BP) and poly(DPP-alt-BT)-based solar cells with identical device configurations.