Solubility of [6,6]-Phenyl-C61-butyric Acid Methyl Ester and Optimal Blending Ratio of Bulk Heterojunction Polymer Solar Cells

The nanoscale mechanism determining the optimal electron donor/acceptor blending ratios is not yet clear. In this study, we used coarse-grained molecular simulations to simulate the thermal annealing process of poly-2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]-thiophene (PBTTT):[6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) blends to reveal the correlation between solubility of PCBM in electron donor materials and the optimal electron donor/acceptor blending ratio of the bulk heterojunction polymer solar cells. Substantial intercalation of PCBM into PBTTT is observed, and an interpenetrating network comprising two phases-PBTTT-plus-PCBM and pure PCBM-is formed when the blending ratio is beyond 1:1. By comparing morphological properties of PBTTT:PCBM blends with those of the poly(3-hexylthiophene) (P3HT) :PCBM blend, a blend which we investigated earlier, we reveal that, in addition to specific interfacial area and percolation probabilities, the solubility of PCBM in PBTTT also has significant effects in determining the optimal blending ratio. Due to PCBM intercalation into PBTTT, more PCBM must be inserted for the precipitation of pure PCBM domains for electron transport. Herein, we provide insight into the effects of PCBM solubility and the nanoscale mechanisms determining the optimal blending ratios and demonstrate how multiscale simulation can potentially aid the development of novel bulk heterojunction blends.