Molecular solubility and hansen solubility parameters for the analysis of phase separation in bulk heterojunctions

Although the fabrication procedures for bulk heterojunction (BHJ) solar cells are routinely optimized to accommodate new organic materials, the influence of solvent properties and cohesive forces on the film-forming process and the self-assembly of donor and acceptor molecules on the nanoscale are poorly understood. In this study, we measure the solubility of a variety of organic semiconductors in a range of solvents and calculate cohesive forces including dispersion forces, dipole interactions, and hydrogen bonding via Hansen Solubility Parameters (HSPs). HSPs were calculated by measuring the solubilities of various organic semiconductors in 27 solvents and the influence of solvent identity on film morphology of different BHJ mixtures was explored via atomic force microscopy (AFM). The possibility of correlations between HSPs and film morphology was considered; however, it is apparent that the HSP values alone do not play a critical role in determining the morphology of the films of conjugated polymers and molecules. This collection of solubility data constitutes the first of its type for organic semiconducting materials, and may act as a useful reference for the organic semiconductor community to aid in the understanding and selection of solvents for donor-acceptor BHJ mixtures.