Multiple Exciton Generation in Small Si Clusters: A High-Level, Ab Initio Study

Abstract

The electron-hole excitonic nature of high energy states is investi- gated in neutral and charged Si clusters, motivated by the multiple exciton generation (MEG) process that is highly debated in photovoltaic literature. Silicon forms the basis for much of the photovoltaic industry, and our high-level, first principles calculations show that at 2-3 times the lowest excitation energy, the majority of optically excited states in neutral Si7 and Si10 take on multiple exciton (ME) character. The transition from single excitons (SEs) to MEs is not as sharp in Si as in PbSe clusters, but it is much more pronounced than in CdSe. The closer similarityofSitoPbSethanCdSeisunexpected,sinceSiclustersarelesssymmetric than PbSe clusters. Charging suppresses MEG in Si clusters; however, the suppres- sion is less pronounced than in PbSe. A strong ME signal is seen already at 5 Eg uponcharging.ThelowMEthresholdsandnearlycompleteswitchfromSEstoMEs create a good possibility for efficient MEG in neutral Si nanoclusters and reveal hope that reasonable quantum yields can still be obtained despite charging. SECTION Dynamics, Clusters, Excited States