Synthesis and Characterization of New Selenophene-Based Donor-Acceptor Low-Bandgap Polymers for Organic Photovoltaic Cells

A series of novel thiophene- and selenophene-based low-bandgap polymers were synthesized using a Stile cross-coupling reaction; these polymers contained quinoxaline and diketopyrrolopyrrole as acceptors. Various acceptors were introduced into the selenophene backbones, and the solubility, absorption spectra, energy levels, charge transport, blend film morphology, and photovoltaic properties of the resulting polymers were investigated. The weight-averaged molecular weights (M-w) of P3TDTQ P3SDTQ, P3TDTDPP, and P3SDTDPP were found to be 12300, 15 500, 13 300, and 17 200, with polydispersity indices of 1.46, 1.85, 1.58, and 1.63, respectively. Photophysical studies revealed low bandgaps of 1.70 eV for P3TDTQ 1.63 eV for P3SDTQ 1.27 eV for P3TDTDPP, and 1.25 eV for P3SDTDPP; the films could harvest a broad solar spectrum, covering the range from 300 to 800 nm (P3TDTQ and P3SDTQ) and from 350 to 950 nm (P3TDTDPP and P3SDTDPP). Solution-processed field-effect transistors fabricated from these polymers had p-type organic thin film transistor characteristics. The field-effect mobilities of P3TDTQ, P3SDTQ, P3TDTDPP, and P3SDTDPP were measured to be 2.8 x 10(-5), 5.0 x 10(-5), 1.0 x 10(-3), and 2.1 x 10(-3) cm(2) V-1 s(-1), respectively. The novel polymers were combined with a PCBM ([6,6]-phenyl C-71-butyric acid methyl ester) acceptor to fabricate bulk heterojunction solar cells, which produced power conversion efficiencies of 0.64-3.18% under AM 1.5 G (100 mW/cm(2)) conditions.