Enhanced Organic Solar Cell Performance by Lateral Side Chain Engineering on Benzodithiophene-Based Small Molecules

The three novel acceptor-donor-acceptor (A-D-A) conjugated small molecules were synthesized, each featuring a benzodithiophene (BDT) core presenting lateral flexible side chains: TB-BDT6T substituted with 2-ethynyl-5-octylthiophene, TS-BDT6T substituted with 2-(octylthio)-thiophene, and TT-BDT6T substituted with 2-(2-ethylhexyl)-thieno[3,2-b]thiophene groups. The lateral incorporation of functionalized pi-conjugated flexible side chains, without altering the end-capped acceptor (cyanoacetate) moieties, amended the optoelectronic properties of these BDT-based small molecules. X-ray diffraction spectroscopy revealed that these small molecules possess high crystallinity; moreover, the optimized blend film morphologies, recorded using atomic force microscopy, revealed miscibility with PC61BM, and turn out nanoscale phase separations. The energy levels of the highest occupied and lowest unoccupied molecular orbitals of these small molecules were allowed, leading to high open-circuit voltages (V-oc) for their solar cell devices. The bulk heterojunction small-molecule solar cell based on TT-BDT6T:PC61BM blend presented the highest power conversion efficiency (5.80%) with a high value of V-oc of 0.98 V, a short circuit density of 9.49 mA cm(-2), and a fill factor of 62.44% under AM 1.5G irradiation (100 mW cm(-2)).