Manipulation and Direct Characterization of Polymer/Small-Molecule Interface Morphology in Bulk-Heterojunction Solar Cell.

To investigate the effect of miscibility between conjugated polymers (CPs) and Y6 on bulk-heterojunction (BHJ) type morphology, we propose three different CPs with similar chemical structures but different miscibility with Y6. After selectively removing Y6 from the CP/Y6 blend films, their interface morphology and interlocked dimensions are quantitatively compared using a square-wave model. As CP-Y6 miscibility increases, a higher intermixed interface is formed, providing an enlarged CP-Y6 interface area. Conversely, as the miscibility between CP and Y6 decreases, the height and width of the interlocked dimensions formed by phase separation gradually decrease and increase, respectively. Additionally, when the CP-Y6 interface morphology and electrical properties of the corresponding organic photovoltaic (OPV) device are correlated, as the highly intermixed CP-Y6 interface develops, the exciton dissociation efficiency increases owing to the reduced exciton diffusion length to be dissociated, but the bimolecular recombination tends to deteriorate simultaneously. Furthermore, if the miscibility between CP and Y6 is excessive, the formation of a charge transport pathway through phase separation is interrupted, deteriorating the charge transport capability in BHJ-type OPVs. However, it was confirmed that introducing F atoms into the conjugated backbone of CP can reduce the bimolecular recombination, providing ameliorated light-harvesting efficiency.