Chlorination of Side Chains: A Strategy for Achieving a High Open Circuit Voltage Over 1.0 V in Benzo[1,2-b:4,5-b′]dithiophene-Based Non-Fullerene Solar Cells

A benzo[1,2-b:4,5-b′]dithiophene-based donor material with chlorine atoms substituted on its side chains, named PBClT, was designed and developed in order to enhance the open-circuit voltage (Voc) without decreasing charge carrier transfer in the corresponding blend films. The chlorinated polymer, PBClT, was an excellent donor material possessing a blue-shifted absorbance, resulting in desired complementary light absorption with low-band gap acceptor materials, for example, ITIC. In addition, the multiple chlorination dramatically decreased the HOMO energy level of PBClT, and the Voc of the corresponding device increased to 1.01 V, which is much higher than that of the nonchlorine analogous, PTB7-Th, with a Voc of approximately 0.82 V. The GIWAXS experiments displayed that PBClT/ITIC blend films showed a “face-on” orientation, which suggested that the chlorine substituents on the side chains favored π–π stacking in the direction perpendicular to the electron flow in photovoltaic devices. Furthermore, the ...