Thiophene/Phenylene/Thiophene-Based Low-Bandgap Conjugated Polymers for Efficient Near-Infrared Photovoltaic Applications

We have prepared thiophene/phenylene/thiophene (TPT)-based low bandgap conjugated polymers exhibiting tunable energy levels and investigated their application in solar cells. By incorporating various electron-withdrawing comonomers through Stille coupling reactions, we obtained TPT-based donor/acceptor copolymers having bandgaps ranging from 1.0 to 1.8 eV. We compared the absorption spectra, electrochemistry, field effect hole mobility, and photovoltaic properties of these low bandgap TPT derivatives with those of poly(3-hexylthiophene) (P3HT). The absorption coefficients of the thin films fell in the range from 0.77 x 10(5) to 1.4 x 10(5) cm(-1). These materials displayed sufficiently high hole mobilities (> 10(-3) cm(2) V-1 s(-1)) for efficient charge extraction and good fill-factors for organic photovoltaic applications. Electrochemical studies indicated desirable HOMO/LUMO levels, with a good correlation between the HOMO energy levels and the open circuit voltage (V-oc) when the polymers were blended with fullerene derivative as an electron acceptor. Power conversion efficiencies of up to 4.3% were achieved under AM 1.5G simulated solar light (100 mW cm(-2)). Our findings suggest that TPT derivatives presenting suitable electron-withdrawing groups are promising photovoltaic materials.