A bipolar diketopyrrolopyrrole molecule end capped with thiophene-2,3-dicarboxylate used as both electron donor and acceptor for organic solar cells

- Molecule DPP(C2T)2 end-capped with thiophene-2,3-dicarboxylate was synthesized.
- Introducing ester groups to both α and β-position of thiophene ring was achieved.
- Energy levels were manipulated by introducing of extra ester groups.
- DPP(C2T)2 can serve as both donor and acceptor in OSCs.

It is a primary strategy to manipulate the energy levels of organic semiconductor molecules for getting better performances in organic photovoltaic devices. In this paper, we designed and synthesized a diketopyrrolopyrrole (DPP)-based small molecule, 3,6-bis(5-[(diethyl thiophene-2,3-dicarboxylate)-2-yl]thiophene-2-yl)- 2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(C2T)2), by introducing ester groups to both α and β-position of the end-capped thiophene ring. Due to the electron-withdrawing effect of ester groups, the resulting molecule exhibits low-lying the highest occupied molecular orbital (HOMO) energy level of −5.37 eV and the lowest unoccupied molecular orbital (LUMO) energy level of −3.78 eV. Therefore, DPP(C2T)2 can be used as either electron donor or acceptor for solution processed organic solar cells and shows a power conversion efficiency (PCE) of 1.66% when blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and 1.08% with poly(3-hexylthiophene) (P3HT). The crystallinity, carrier mobilities and film morphology are systematically investigated and discussed, in order to study the influence of extra ester group on the properties of the resulting molecule.