Increasing impact ionization rates in Si nanoparticles through surface engineering: A density functional study

We propose design pathways to improve the efficiency of multiple exciton generation in nanoparticle-based solar cells by carrying out ab initio calculations of impact ionization (II) rates in semiconducting nanoparticles (NPs). In NPs with unreconstructed surfaces, quantum confinement has two competing effects: it enhances the effective Coulomb interaction and thus the II rates, but it also blue-shifts the gap, which tends to reduce the II rates. The competition of these effects determines the utility of NP-based solar cells. We report that surface reconstruction of NPs can tip the balance towards the enhancement of II by creating a substantial density of states at lower energies. Our results suggest that manipulating the surfaces of NPs, e.g., by engineering the ligands and embedding structures, may lead to an efficient multiexciton generation within the solar spectrum. DOI: 10.1103/PhysRevB.87.155402