The π-extended isoindigo-based low bandgap polymer for high-performance polymer solar cells

Abstract

Recently, low bandgap conjugated polymers have attracted considerable attention for polymer solar cells (PSCs). The desirable design directions for the polymers are that those should have deep HOMO energy levels, broad absorption and balanced crystallinity. Isoindigo, (E)-1H,1H-[3,3]biindolylidene-2,2-dione, which is one of the indigoid natural organic dyes is a symmetrical molecule containing two indolin-2-one units. Those contribute towards strong electron-withdrawing characteristic and planar π-conjugated structure of the molecule. Furthermore, isoindigo-based organic materials have broad absorption spectra, high extinction coefficients, and very deep HOMO energy levels which are closely related to open circuit voltage for PSCs. In this work, we present highly π-extended isoindigo-based alternating conjugated copolymer with 6.5% PCE. To the best of our knowledge, its photovoltaic performance is the best value reported to date for PSCs using isoindigo-based polymers.