In conjugated molecule based bulk heterojunction organic photovoltaic devices, the thin film morphology plays a crucial role for device performance. While the appropriate energetics is an essential criteria for materials design, it is the film morphology on the different special scales, e.g., orientation, stacking distance, miscibility, and crystalline properties of a molecule that determine how well the OPV device will function. Thus, the ability to proactively control morphology is a key for designing systems that will achieve high device power conversion efficiency (PCE). Although many factors could influence the film morphology, we examine in this work a series of small molecule systems based on benzo[1,2-b:6,5-b’]- dithiophene diketopyrrolopyrole (bBDT(TDPP)¬2) that demonstrates remarkable performance changes based on a morphological tunability with side chain and solvent variations. We investigated 8 side chain combinations, four using combinations of 2-ethylhexyl and 3,7-dimethyloctyl and 4 utilizing combinations of 2-ethylhexyloxy and 3,7-dimthyloctyloxy. All of these molecules show nearly identical basic electronic properties, such as band gap and HOMO and LUMO levels, however in organic photovoltaic devices, they can demonstrate power conversion efficiencies ranging from 1% to 5.5%.We examine the cause of these performance variations gleaned from a variety of morphological characterization techniques but focusing on grazing incidence wide-angle x-ray scattering (GIWAXS), including measurements taken during the spin coating process. Depending on the combination of side chains and the solvent used, the molecules will have different morphological formation processes, resulting in different crystalinities, packing structures and distances, and miscibilities with PCBM. All of these differences result in important changes in the charge separation and transport within the device. Our work with this system offers revelations towards: how to design and optimize future systems, the importance of understanding solvent effects, as well as implications for other organic electronic systems beyond simply OPVs.
This research was supported by the Argonne-Northwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science under Award Number DE-SC0001059.