Surface treatment by binary solvents induces the crystallization of a small molecular donor for enhanced photovoltaic performance

The surface treatment of the active layer with binary solvents composed of methanol (MeOH) and 1-chloronaphthalene (CN), was demonstrated to effectively improve the power conversion efficiency (PCE) from 2.4% to 6.5% for p-DTS(FBTTh₂)₂:PC₇₁BM based small molecular solar cells. The optical properties and morphology of the p-DTS(FBTTh₂)₂:PC₇₁BM films were carefully investigated. The results indicate that treatment with MeOH:CN binary solvents could significantly enhance the absorption of the active layer, due to the formation of more p-DTS(FBTTh₂)₂ nanofibrils associated with higher crystallinity as revealed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The two-dimensional grazing incidence wide-angle X-ray scattering (GIWAXS) results further demonstrate that the molecular packing of p-DTS(FBTTh₂)₂ molecules could be strongly enhanced after treatment with the binary solvents. In contrast, pristine methanol shows no significant influence on the crystalline structure, phase separation or the photovoltaic properties of the p-DTS(FBTTh₂)₂:PC₇₁BM system, showing that the CN solvent plays the main role in inducing the crystallization of p-DTS(FBTTh₂)₂ molecules.