New conjugated regular terpolymers based on diketopyrrolopyrrole-benzodithiophene and their application to thin film transistors and polymer solar cells

Conjugated binary copolymers containing an electron donor (D) and acceptor (A) in the repeating unit have been extensively studied due to their excellent charge transport properties. Moreover, conjugated terpolymers containing a third monomer in addition to the two D and A monomers have been recognized to exhibit finely tuned properties relative to the binary D-A copolymer. Diketopyrrolopyrrole (DPP)–based conjugated regular terpolymers (CRTs) containing two different electron donor monomers were synthesized herein to exploit the advantages of the terpolymer. Two terpolymers were synthesized by using bithiophene and pyrene as third monomers. Compared to the reference copolymer, poly[3-(5-(benzo[1,2-b:4,5-b']dithiophen-2-yl)thiophen-2-yl)-2,5-bis(2-decyltetradecyl)-6-(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione] (PDPPBDT), the new terpolymers exhibited distinct physical properties, absorption properties, and electronic energy levels. The terpolymers were employed in thin film transistors (TFTs) and polymer solar cells (PSCs) and the device performance was studied. The highest mobility achieved for the TFT devices was close to 1.0 cm² V⁻¹ s⁻¹ and was obtained with the PDPPBTBDT terpolymer film as the active layer, whereas the mobilities of the TFTs employing the PDPPPYBDT and PDPPBDT polymers were relatively low. In contrast with the TFT device results, the PSC fabricated with PDPPPYBDT:PC₇₁BM showed the highest power conversion efficiency (PCE) of 4.46% due to the relatively high V_oc and J_sc. The results of this study confirmed that the use of a regular terpolymer structure permits fine adjustment of the light absorption range, the molecular energy levels, and the film morphology of the polymer, in turn influencing the device performance of TFTs or PSCs.