Diketopyrrolopyrrole-Based pi-Bridged Donor-Acceptor Polymer for Photovoltaic Applications

Abstract

We report the synthesis, properties, and photovoltalc applications of a new conjugated copolymer (C12DPP-pi-BT) containing a donor group (bithiophene) and an acceptor group (2,5-didodecylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione), bridged by a phenyl group. Using cyclic voltammetry, we found the energy levels of C12DPP-pi-BT are intermediate to common electron donor and acceptor photovoltaic materials, poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), respectively. Whereas P3HT and PCBM are exclusively electron donating or accepting, we predict C12DPP-pi-BT may uniquely serve as either an electron donor or an acceptor when paired with PCBM or P3HT forming junctions with large built-in potentials. We confirmed the ambipolar nature of C12DPP-pi-BT in space charge limited current measurements and in C12DPP-pi-BT:PCBM and C12DPP-pi-BT:P3HT bulk heterojunction solar cells, achieving power conversion efficiencies of 1.67% and 0.84%, respectively, under illumination of AM 1.5G (100 mW/cm(2)). Adding diiodooctane to C12DPP-pi-BT:PCBM improved donor acceptor inter-mixing and film uniformity, and therefore enhanced charge separation and overall device efficiency. Using higher-molecular-weight polymer C12DPP-pi-BT in both C12DPP-pi-BT:PCBM and C12DP