Enhanced and Tunable Open-Circuit Voltage using Dialkylthio Benzo[1,2-b:4,5-b′]dithiophene in Polymer Solar Cells

In this study, we explore the effect of the dialkoxy and dithioalkoxy side chains on the benzo [1,2-b:4,5-b']dithiophene (BDT) unit by comparing the O-BDT homopolymer (O-PBDT), S-BDT homopolymer (S-PBDT), and S-BDT-alt-O-BDT copolymer (SO-PBDT) by computational calculations and experimental results. The polymers were prepared by Pd-catalyzed Stile coupling. Additionally, hole mobility and film morphology were studied by fabricating organic field effect transistors (OFETs) and using TappingMode AFM, respectively. The photovoltaic properties of the polymers were measured from fabricated PSC devices. The replacement of the alkoxy (-OR) groups with thioalkoxy (-SR) groups lowered the HOMO energy level of the conjugated polymers from 5.31 to 5.41 eV, and consequently enhanced V-oc, while still preserving the excellent properties offered by the BDT-based polymers. Especially, high V-oc of 0.99 V was achieved from S-PBDT polymer based PSC with up to 4.0% of PCE, which is one of the highest efficiencies reported from a homopolymer-based PSC without thermal/solvent annealing or incorporated additives.