A solution-processable diketopyrrolopyrrole dye molecule with (fluoronaphthyl)thienyl endgroups for organic solar cells

Abstract

A solution-processable dye molecule DPP(TFNa)2 that consists of diketopyrrolopyrrole (DPP) as the core and 5-(6-fluoro-2-naphthyl)thienyl as the endgroups is presented for bulk heterojunction organic solar cells. DPP(TFNa)2 is a crystalline solid with a Tm of approximately 216 °C. X-ray diffraction experiments reveal that thermal annealing increases crystallinity of the as-cast film, thus beneficial to the absorption and charge-transport properties. DPP(TFNa)2 exhibits two reversible one-electron oxidation waves at 0.87 and 1.16 V vs. Ag/AgCl reference electrode, respectively. Fitting the space-charge-limited current characteristics in a hole-only device results in a hole mobility of ∼2.7 × 10−4 cm2 V−1 s−1 at low voltages for DPP(TFNa)2. A preliminary characterization of the solar cell (ITO/PEDOT:PSS/DPP(TFNa)2:PC61BM/Al) yields a power conversion efficiency of approximately 3.0% under simulated AM 1.5G illumination (66.4 and 100 mW cm−2, respectively). The fluorine effects on material properties such as morphology, absorption, electrochemistry, charge transport and the resulting device performance are discussed.