Multiscale molecular simulations of the nanoscale morphologies of P3HT:PCBM blends for bulk heterojunction organic photovoltaic cells

In this study, we developed a multiscale molecular simulation framework including coarse-grained (CG) molecular simulation, reverse-mapping, and morphology evaluation schemes to investigate the nanoscale morphologies of bulk heterojunction (BHJ) blend films comprising poly(3-hexylthiophene) (P3HT) and the methanofullerene derivative PCBM. A stable and phase-separated blend film with the fibrillar P3HT structure was observed after CG simulation of the thermal annealing process, and by the reverse-mapping technique the atomistic details-showing strong [small pi]-[small pi] interaction between thiophene rings-were retrieved. To evaluate the morphologies of P3HT:PCBM blends, a spatial-discretization scheme was developed. With such a scheme, we estimated the average domain sizes, interface-to-volume ratios, and percolation ratios of the blends at different P3HT:PCBM weight ratios. The average domain sizes determined through these simulations were in excellent agreement with those reported experimentally. Moreover, our simulations indicated that blend films having weight ratios close to 1 : 1 would have the highest interface-to-volume ratio and the most balanced charge carrier transport in both the P3HT and PCBM phases, consistent with the experimental observation that a 1 : 1 weight ratio is optimal for P3HT:PCBM blends. The multiscale molecular simulation framework proposed herein can be extended to investigating the morphologies of other photoactive layers of organic photovoltaic cells.