An ester-functionalized diketopyrrolopyrrole molecule with appropriate energy levels for application in solution-processed organic solar cells

For highly efficient organic solar cells (OSCs), the electron donor should possess not only a narrow band gap (E-g) but also a low highest occupied molecular orbital (HOMO) energy level. To achieve it, in this paper, we designed and synthesized a diketopyrrolopyrrole (DPP) derivative end capped with an ethyl thiophene-2-carboxylate moiety, 3,6-bis{5-[(ethyl thiophene-2-carboxylate)-2-yl]thiophene-2-yl}-2,5-bis(2-ethylhexyl) pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(CT)(2)). Through UV-vis absorption and cyclic voltammetry (CV) measurements, we demonstrated that the resulting molecule exhibits both a low optical E-g of 1.65 eV and a lower-lying HOMO energy level of -5.33 eV owing to the electronegativity of the ester group and the conjugation effect of the thiophene ring. Therefore, when DPP(CT)(2) is used as the electron donor to blend with [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) for solution processable OSCs, a power conversion efficiency (PCE) of 4.02% combined with an open-circuit voltage (V-OC) as high as 0.94 V and a broad photovoltaic response range extending to around 750 nm is obtained.