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.