Synthesis and characterization of three small band gap conjugated polymers for solar cell applications

Abstract

We report on a new series of small band gap conjugated polymers utilizing donor–acceptor–donor substructures in the polymer backbone to broaden and extend the optical absorption to longer wavelengths. Three polymers were prepared by Suzuki polymerization, using the same donor–acceptor–donor segment but with different comonomers. The goal was to investigate how the optical and electronic properties of the polymers were influenced by the different comonomers. Electrochemical spectroscopy, using square-wave voltammetry, shows that increasing the electron-donating strength of the comonomer will raise the HOMO energy level of the polymer, resulting in a decreased band gap. This result is also manifested by comparing open-circuit voltages from the corresponding laboratory fabricated solar cells. The best performing photovoltaic cell, based on APFO-Green15/[60]PCBM (1 : 4 w/w), reached a J_sc of 4.2 mA cm⁻², a V_oc of 0.73 V, and a FF of 0.54, giving a PCE of 1.7%.