Ethynylene-linked benzo[1,2-b:4,5-b′]dithiophene-alt-diketopyrrolopyrrole alternating copolymer: optoelectronic properties, film morphology and photovoltaic applications

An ethynylene-linked benzo[1,2-b:4,5-b′]dithiophene-alt-diketopyrrolopyrrole alternating copolymer, EDPP, was designed and synthesized to improve the open-circuit voltage of organic solar cells. The influence of the ethynylene on optoelectronic properties, energy levels, crystallinity, film morphology, and photovoltaic performance was investigated. Optical and electrochemical tests showed that introduction of ethynylene into the polymer backbone resulted in a larger bandgap, deeper HOMO energy level, and enhanced crystallinity due to the planar conformation and electron-withdrawing properties. Grazing incidence wide-angle X-ray scattering (GIWAXS) showed that the pure EDPP film preferentially adopted a face-on orientation with a π–π stacking distance of 3.65 A. After thermal annealing the face-on and edge-on orientations coexisted and the overall degree of crystallinity increased. Blending with PC71BM did not disrupt the crystallinity of the EDPP. Resonant soft X-ray scattering (RSoXS) showed that the EDPP:PC71BM blend films contained large domains, a few hundred nanometers in size. As a result, EDPP:PC71BM photovoltaic devices exhibited a high open-circuit voltage of 0.88 V, but a low short-circuit current, with a moderate power conversion efficiency of 1.98%.