Synthesis and Photovoltaic Properties of New Low Bandgap Isoindigo-Based Conjugated Polymers

Abstract

A series of new isoindigo-based low banbap polymers, containing thiophene, thieno[3,2-b]thiophene and benzo[1,2-b:4,5-b′]dithiophene as donors, have been synthesized by Stille cross-coupling reaction. Their photophysical, electrochemical and photovoltaic properties have been investigated. These new polymers exhibit broad and strong absorption between 400 and 800 nm with absorption maxima around 700 nm. The HOMO energy levels of polymers vary between −5.20 and −5.49 eV and the LUMO energy levels range from −3.66 to −3.91 eV. The optical bandgaps of the polymers are optimized for solar cell applications and they are at about 1.5 eV. Polymer solar cells (PSC) based on these new polymers were fabricated with device structures of ITO/PEDOT:PSS/polymers: PC71BM (1:2, w/w)/LiF/Al. The photovoltaic properties of the polymers have been evaluated under AM 1.5G illumination at 100 mW/cm2 with a solar simulator. The combination of broad absorption, optimal bandgap and well matched energy levels with those of PCBMs makes these isoindigo-based low bandbap polymers promising materials for photovoltaic applications.