Continuous transformation paths for the molecular crystals of the PCBM fullerene derivative

We use first-principles calculations to probe various crystalline structures for phenyl-C61-butyric acid methyl ester (PCBM), an organic semiconductor with remarkable performance in photo-conversion systems. In particular, we investigate continuous transformations that can change PCBM crystals from a body-centered cubic (BCC) to a face-centered cubic lattice, or from a BCC to a simple cubic configuration. We thus identify several local energy-minimum structures that differ significantly in terms of molecular conformations and electronic properties. The results suggest the possibility of hysteresis during PCBM crystal phase transitions and the importance of substrate interactions or bi-axial stress on the photo-conversion efficiency of PCBM-based systems.