Effects of non-halogenated solvent on the main properties of a solution-processed polymeric thin film for photovoltaic applications: a computational study

Abstract

Organic photovoltaic (OPV) devices have reached high power conversion efficiencies, but they are usually processed using halogenated toxic solvents. Hence, before OPV devices can be mass-produced by industrial processing, it would be desirable to replace those solvents with eco-friendly ones. Theoretical tools may be then a powerful ally in the search for those new solvents. In order to better understand the mechanisms behind the interaction between solvent and polymer, classical molecular dynamics (MD) calculations were used to produce a thin film of poly(4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl) (PTB7-Th), processed using two different solvents. PTB7-Th is widely applied as a donor material in OPVs. The first solvent is ortho-dichlorobenzene (o-DCB), which is a highly toxic solvent widely used in lab-scale studies. The second solvent is ortho-methylanisole (o-MA), which is an eco-friendly solvent for organic photovoltaic (OPV) manufacturing. Here we use a solvent evaporation protocol to simulate the formation of the PTB7-Th film. We demonstrate that our theoretical MD calculations were able to capture some differences in the macroscopic properties of thin films formed by o-DCB or o-MA evaporation. We found that the interaction of the halogenated solvent with the polymer tends to break the bonds between the lateral thiophenediyl groups and the main chain. We show that those defects may create traps that can affect the charge transport and also can be responsible for a blue shift in the absorption spectrum. Using the Monte Carlo method, we also verified the influence of the resulting MD morphology on the mobility of holes. Our theoretical results showed good agreement with the experimental measurements and both demonstrate that o-MA can be used to make polymer thin films without any loss of key properties for the device performance. The findings here highlight the importance of theoretical results as a guide to the morphological optimization of green processed polymeric films.