Isoindigo-based Polymers for High Performance Polymer Solar Cells

Abstract

Isoindigo is a dye molecule with two lactam rings pertaining strong electron-withdrawing characteristic and planar n-conjugated structure, which provides broad optical absorption, high extinction coefficient, and deep HOMO energy level. Therefore, isoindigo is a promising building block for constructing low bandgap conjugated polymers to achieve high performance polymer solar cells (PSCs). We synthesized random conjugated copolymers consisting of DPP and isoindigo as coelectron donor units in donor-acceptor type conjugated copolymer for panchromatic absorption. The copolymer containing equal amount of DPP and isoindigo in the copolymer absorbs wide range of solar spectrum from 600 to 900 nm with low HOMO level. Under the optimized condition, the copolymer-based PSC exhibits a promising PCE of 6.04%, which are superior to the values of the two corressponding homopolymers. We also synthesized highly n-extended conjugated low bandgap polymer composed of isoindigo (il) and thienylvinylene (TVT) and compared the photovoltaic properties of PiiTVT with those of Pii2T, in order to examine the effect of vinylene linkage on the photovoltaic properties of isoindigo-based polymers. The solar cell device fabricated from PiiTVT:PC61BM blend exhibited a PCE of7.09%, which is much higher than the best PCE ofPii2T:PC71BM blend.