Benzodithiophene-based small molecules for vacuum-processed organic photovoltaic devices

Abstract

Benzo[1,2-b:4,5-b’]dithiophene (BDT) derivatives were evaluated as donor materials for the first time in vacuum-processed organic photovoltaic (OPV) devices. Simple BDT derivatives coupled with thiophene (M1), 3-methylthiophene (M2), and 3-(2-ethylhexyl)thiophene (M3) were synthesized and characterized as part of this evaluation study. Similar frontier molecular orbitals were determined for the three molecules by DFT calculations, optical, and electrochemical measurements. Therefore, the effect of the alkyl substituents on the film morphology and molecular order was possible to evaluate as well. All molecules exhibited thermal stability for their co-evaporation with C60 to form bulk heterojunction layers in OPV devices. An increase in short-circuit current density was observed for devices with M2. Microscopy images and hole-transport measurements suggested an induced molecular order by the methyl group of M2 on the active layer. Although low photoconversion efficiency values were obtained (~1.2%), this work demonstrates the feasibility to fabricate fully vacuum-processed OPV devices with BDT derivatives as donor materials. Devices with higher photoconversion efficiencies are expected for BDT derivatives with enhanced optoelectronic properties.