Diketopyrrolopyrrole-based conjugated polymers as additives to optimize morphology for polymer solar cells

Novel random copolymers for optimizing the morphology of the active layer for high performance organic photovoltaic devices have been demonstrated. Three ternary random copolymers PTBDTDPPSiCN(3/7), PTBDTDPPSiCN(5/5), PTBDTDPPSiCN(7/3) were prepared by polymerization of electron-donating thienyl-substituted benzodithiophene (TBDT) with 2,5-bis[8-(1,1,3,3,5,5,5-heptamethyltrisiloxane-3-yl)octly]-pyrrolo[3,4-c]pyrrole-1,4-dione (DPPSi) and 2,5-dio[5-(5-cyano-5,5-dimethyl-pentyl)]-3,6-dithiophen-2-yl-pyrrolo[3,4-c]pyrrole-1,4-dione (DPPCN) of different ratios. The DPPCN block can well-tune the light absorption and molecular packing, while the DPPSi block is in favor of enhancing the charge mobility. And the formation of organic Si-O-Si networks is beneficial to stabilize the morphology of the active layer. These new copolymers have narrow bandgaps and broaden visible light absorption from 500 nm to 1000 nm. Careful balance of the contents of the trimethoxysilyl group and the cyano group can well-tune the surface energy and morphology of the copolymers. Incorporation of these novel copolymers as additives into the blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₀-butyric acid methyl ester (PC₆₁BM) is found to effectively broaden the light absorption, improve the compatibility and morphology of the active layer. As a result, some devices with certain ratios of these copolymers as additives achieve the enhanced efficiency compared with the device based on pristine P3HT:PC₆₁BM.