π-Extended Narrow-Bandgap Diketopyrrolopyrrole-Based Oligomers for Solution-Processed Inverted Organic Solar Cells

A series of narrow-bandgap π-conjugated oligomers based on diketopyrrolopyrrole chromophoric units coupled with benzodithiophene, indacenodithiophene, thiophene, and isoindigo cores are designed and synthesized for application as donor materials in solution-processed small-molecule organic solar cells. The impacts of these different central cores on the optoelectronic and morphological properties, carrier mobility, and photovoltaic performance are investigated. These π-extended oligomers possess broad and intense optical absorption covering the range from 550 to 750 nm, narrow optical bandgaps of 1.52-1.69 eV, and relatively low-lying highest occupied molecular orbital (HOMO) energy levels ranging from -5.24 to -5.46 eV in their thin films. A high power conversion efficiency of 5.9% under simulated AM 1.5G illumination is achieved for inverted organic solar cells based on a small-molecule bulk-heterojunction system consisting of a benzodithiophene-diketopyrrolopyrrole-containing oligomer as a donor and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as an acceptor. Transmission electron microscopy and energy-dispersive X-ray spectroscopy reveal that interpenetrating and interconnected donor/acceptor domains with pronounced mesoscopic phase segregation are formed within the photoactive binary blends, which is ideal for efficient exciton dissociation and charge transport in the bulk-heterojunction devices.