Collaborative Research: Design of Zwitterlation Sidechain to Enable Greener Processing of Organic Solar Cells Based On Biomass-Derived Solvents

Organic solar cells play an essential role in enabling the sustainability of our energy supply. One roadblock to the large-scale manufacturing of organic solar cells is the large amount of toxic solvent used in this process. The mature manufacturing protocol uses halogen-containing solvents such as chlorobenzene for organic semiconductor processing. The halogen-containing solvents present a significant threat to the environment and public health. Due to its criticality, extensive efforts have been conducted to develop processes that use the other solvents. These efforts have shown progress in replacing the halogen-containing solvents with other halogen-free organic solvents such as toluene and tetrahydrofuran. However, these organic solvents still negatively impact the environment and public health if used in a large amount. Several biomass-derived greener solvents such as Cyrene and γ-valerolactone have recently emerged as the candidates for making the processing greener. The biomass-derived solvents exhibit an excellent opportunity to develop a greener manufacturing process for organic solar cells. However, deploying these new solvents must overcome one roadblock: limited understanding of molecular thermodynamics about miscibility and assembly of hole-transport semiconducting polymers and electron-transport organic molecules in these new solvents. Overcoming this roadblock will provide principles for engineering semiconducting polymers and organic molecules to optimize the microstructures of photoabsorption-active layers in organic cells. To overcome the roadblock mentioned above, we propose to conduct computational and experimental research to understand molecular thermodynamics about the miscibility and assembly of semiconducting polymers and organic molecules in biomass-derived solvents. Many biomass-derived solvents are more polar than halogen-containing solvents. The zwitterionic sidechain consists of covalently connected cationic and anionic groups. Our previous research has shown that we can control the dipole moment of zwitterionic sidechains by tuning (i) the distance between the zwitterionic motifs and the backbone and (ii) the distance between the cationic and anionic groups in the zwitterionic motifs. We hypothesize that the miscibility and assembly of semiconducting polymers and organic molecules in biomass-derived solvents can be controlled by varying the solute-solvent molecular interactions through the rational design of zwitterionic sidechains in semiconducting polymers and organic molecules. By testing against this hypothesis, we will gain molecular thermodynamics to rationalize the design of zwitterlated semiconducting polymers and organic molecules that can be processed in biomass-derived solvents. We will also deploy the gained knowledge to engineering and processing semiconducting polymers and organic molecules using biomass-derived solvents to optimize the film morphology for solar cells. Driven by the hypothesis, the proposed research will focus on two fundamental questions: (a) What is the effect of zwitterionic sidechains on miscibility and assembly of semiconducting polymers and organic molecules in biomass-derived solvents? The answer will provide molecular thermodynamics understanding of solute-solvent interactions and advance the fundamental knowledge for greener processing. (b) How is the role of the zwitterionic sidechain in the nonequilibrium film formation process? The answer to this question will provide knowledge to rationalize the development of an optimal evaporating process for manufacturing organic solar cells. The proposed research will be conducted using molecular simulations and experiments. The Shao group at the University of Kentucky will conduct density functional theory, all-atom molecular dynamics simulations and well-tempered metadynamics to understand the molecular thermodynamics of polymer/organic molecule solvation, polymer-polymer/organic molecule-organic molecule interactions and the nonequilibrium evaporation process. The Yu group at Cornell University will synthesize the zwitterlated semiconducting polymers and organic molecules and investigate their solubility and morphology in biomass-derived solvents. The Yu group will also fabricate organic solar cells with zwiterlated semiconductor polymers and organic molecules using biomass-derived solvents to provide an understanding of property-process performance.