Carrier multiplication in semiconductor nanocrystals via intraband optical transitions involving virtual biexciton states

Abstract

We propose and analyze a physical mechanism for photogeneration of multiexcitons by single photons ͑carrier multiplication͒ in semiconductor nanocrystals, which involves intraband optical transitions within the manifold of biexciton states. In this mechanism, a virtual biexciton is generated from nanocrystal vacuum by the Coulomb interaction between two valence-band electrons, which results in their transfer to the conduction band. The virtual biexciton is then converted into a real, energy-conserving biexciton by photon absorption on an intraband optical transition. The proposed mechanism is inactive in bulk semiconductors as momentum conservation suppresses intraband transitions. However, it becomes highly efficient in the case of zerodimensional nanocrystals, where quantum confinement results in relaxation of momentum conservation, which is accompanied by the development of strong intraband absorption. Our calculations show that the efficiency of the carrier multiplication channel mediated by intraband optical transitions can be comparable to or even greater than that for impact-ionization-like processes mediated by interband transitions.

Multiple exciton generation in semiconductor nanocrystals: Toward efficient solar energy conversion

“…In addition, the results reported by Ellingson et al demonstrated that measuring the exciton population by probing the photoinduced absorption associated with intraband transitions yielded QYs equivalent to those measured using the interband bleach approach introduced by Schaller. The 3E g onset initially claimed by Schaller and Klimov was later revised slightly to ∼ 2.85E g [72]. MEG measurements by Murphy and Beard et al on ∼ 5.4 nm diameter PbTe NCs also showed MEG [15], with a threshold for strong MEG at ∼ 3E g QYs similar to those reported for PbSe and PbS [14].…”

“…In general there are three theories that have been proposed to explain efficient MEG in semiconductor NCs; (1) a coherent superposition of single and multiple exciton states [14,55]; (2) excitation of a virtual multiexciton state that is strongly coupled to a real multiexciton state [72,76] and (3) incoherent impact ionization, analogous to bulk impact ionization with proper modification of the electronic states [77,78].…”

Numerical Study of Carrier Multiplication Pathways in Photoexcited Nanocrystal and Bulk Forms of PbSe

“…Previous studies of photogeneration processes [5,14], implicitly assumed constructive interference within the second and the third pathways. The assumption led to significant overestimation of photogeneration QE.…”

“…From theory point of view, multiexciton photogeneration [5,14], coherent multiexciton production [15,16], and incoherent impact ionization (II) [17][18][19], are currently under debate as primary mechanisms of CM. The ab initio calculations on small clusters ( 1 nm) demonstrated the role of strong Coulomb correlations in multiexciton photogeneration and emphasized the role of fast exciton-biexciton dephasing [20].…”

Multiple exciton generation in quantum dot-based solar cells

“…is an accepted mechanism, it is not yet clear if the same process is responsible for MEG in quantum-confined materials. The increased efficiency of MEG in these structures has not yet been fully explained and different models have been proposed where the multi-excitonic state is created through an excited virtual multi-excitonic state [42,43] or via an instantly generated superposition of single-and multi-excitonic state [41,44].…”