Influence of molecular shape on solid-state packing in disordered PC₆₁BM and PC₇₁BM fullerenes

Molecular and polymer packings in pure and mixed domains and at interfacial regions play an important role in the photoconversion processes occurring within bulk heterojunction organic solar cells (OSCs). Here, molecular dynamics simulations are used to investigate molecular packing in disordered (amorphous) phenyl-C₇₀-butyric acid-methyl ester (PC₇₁BM) and its C₆₀ analogue (PC₆₁BM), the two most widely used molecular-based electron-accepting materials in OSCs. The more ellipsoidal character of PC₇₁BM leads to different molecular packings and phase transitions when compared to the more spherical PC₆₁BM. Though electronic structure calculations indicate that the average intermolecular electronic couplings are comparable for the two systems, the electronic couplings as a function of orientation reveal important variations. Overall, this work highlights a series of intrinsic differences between PC₇₁BM and PC₆₁BM that should be considered for a detailed interpretation and modeling of the photoconversion process in OSCs where these materials are used.