Transition from Solution to the Solid State in Polymer Solar Cells Cast from Mixed Solvents

Certain solvent additives significantly affect the morphology of the active layer in bulk heterojunction (BHJ) conjugated polymer solar cells and improve the device performance. Previous examinations of the BHJ films have shown that the best additives are characterized by higher boiling points than the host solvent and are poorer solvents for the conjugated host polymer than for the fullerene acceptor; however, little in the way of a mechanistic explanation has been presented, particularly on the dynamics of the transition from solution to the bulk material. This article combines spectroscopic analysis in various solvent mixtures and during solvent evaporation to show that a key feature of the film growth concerns aggregation of polymer chains into more ordered supramolecular structures prior to complete drying. The driving force for aggregation occurs in a medium that (a) is more fluid and allows chains to find optimal registry or conformations and (b) can solvate the fullerenes. We propose that when a single solvent that is good for the two components of the BHJ blend is used, the chains remain solvated up to a point where viscosity inhibits their motion and they are unable to attain similar packing characteristics.