A Systematic Approach to Solvent Selection Based on Cohesive Energy Densities in a Molecular Bulk Heterojunction System

The solubilities of 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethyl- hexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBFu)(2)) and [6,6]-phenyl-C(71)- butyric acid methyl ester (PC(71)BM) in a series of solvents are measured, and this data is used to calculate the Hansen solubility parameters of the two materials. The dispersion, polar, and H-bonding parameters of DPP(TBFu)(2) and PC(71)BM were found to be (19.3, 4.8, 6.3) and (20.2, 5.4, 4.5) MPa(1/2), respectively, with an error of +/- 0.8 MPa(1/2). Based on the solubility properties of the two materials, three new solvents (thiophene, trichloroethylene and carbon disulfide) were utilized for the DPP(TBFu)(2):PC(71)BM system which, after device optimization, led to power conversion efficiencies up to 4.3%.