Modelling of P3HT:PCBM interface using coarse-grained forcefield derived from accurate atomistic forcefield.

To understand the morphological evolution of P3HT:PCBM bulk heterojunction during thermal treatment process, we employed coarse-grained Molecular Dynamics (MD) simulations with a forcefield derived from atomistic model and experimental data such as crystal structure and melting temperature. The current study focuses on the differences between interfaces that PCBM forms with various P3HT orientations. Crystallinity analysis suggests that more ordered P3HT is observed near the interface for face-on and amorphous case, while no such trend is observed for edge-on and end-on configurations due to weaker interactions at the interface as evident from the considerably less negative interfacial energy. An analysis of pathways for C60 diffusion into P3HT using both an energy-based and solvent surface approach for amorphous P3HT reveals continuous chain motion-assisted pathways while for crystalline P3HT diffusion pathways remain restricted to grain boundaries. Based on these calculations, we propose a morphological evolution process for P3HT:PCBM bulk-heterojunction, which starts with nucleation crystallisation at the P3HT:PCBM interface, followed by PCBM diffusion along the grain boundaries and amorphous P3HT regions towards PCBM-rich domains.