An Alternating Donor-Acceptor Conjugated Polymer Based on Benzodithiophene and [3,4-c]pyrrole-4,6-dione: Synthesis, Characterization, and Application in Photovoltaic Devices

The synthesis is described of a new alternating donor-acceptor semiconducting polymer based on an N-octylthieno[3,4-c] pyrrole-4,6-dione building block together with a newly designed 2,3-bis(2-ethylhexyl)thiophenylethynyl substituted benzodithiophene (BDT). The introduction of electron-rich thiophene units to BDT raises the highest occupied molecular orbital (HOMO) level of the conjugated polymer and the concomitant reduction of the bandgap enhances the harvesting of solar radiation. This modification also introduces less sterically demanding triple bonds, thereby potentially enabling more favourable molecular interactions and an extra dimension of conjugation perpendicular to the main polymer chain. The optoelectronic properties of this new conjugated polymer were evaluated using UV-visible absorption and fluorescence spectroscopy, photoelectron spectroscopy in air, photo-induced charge extraction by linearly increasing voltage (Photo-CELIV), and density functional theory calculations. The polymer absorbs broadly in the wavelength range 300-700 nm in solution and the solid state. The estimated HOMO and LUMO levels of -5.4 and -3.6 eV, respectively, correspond to a bandgap of 1.8 eV. Photovoltaic devices fabricated using the polymer as the active layer displayed power conversion efficiencies (PCEs) of up to 1%. Photo-CELIV results provide evidence that rapid recombination and poor charge mobility are likely contributing factors to the relatively low PCE values observed.