Endohedral vs exohedral boron in C60: Bonding nature and impact on hot-electron relaxation dynamics

Endohedral and exohedral fullerenes have both been employed as electron acceptors in polymer solar cells (PSCs). However, their differences in hot-electron relaxation dynamics remain unclear. Previous studies have shown that the location of a single atom, whether inside or outside the fullerene cage, results in significant differences in charge distribution. In this work, the hot-electron relaxations of endohedral B@C60+ and exohedral C60B+ are investigated using nonadiabatic molecular dynamics simulations. Our results reveal that the location of the boron atom—inside or outside the fullerene—significantly impacts the bonding interactions between boron and C60. Compared to C60B+, the weaker interactions in B@C60+ reduce the orbital overlap between LUMO+3 and LUMO+2 and increase the energy gap between them. This, in turn, slows hot-electron relaxation by weak nonadiabatic coupling, making B@C60+ more suitable for PSC applications. This study provides valuable insights into how atomic positioning affects the electronic properties in fullerene-based materials, contributing to the design of more efficient electron acceptors for photovoltaic devices.