Impact Ionization Can Explain Carrier Multiplication in PbSe Quantum Dots

Abstract

The efficiency of conventional solar cells is limited because the excess energy of absorbed photons converts to heat instead of producing electron-hole pairs. Recently, efficient carrier multiplication has been observed in semiconductor quantum dots. In this process, a single, high-energy photon generates multiple electron-hole pairs. Rather exotic mechanisms have been proposed to explain the efficiency of carrier multiplication in PbSe quantum dots. Using atomistic pseudopotential calculations, we show here that the more conventional impact ionization mechanism, whereby a photogenerated electron-hole pair decays into a biexciton in a process driven by Coulomb interactions between the carriers, can explain both the rate (<<1 ps) and the energy threshold ( approximately 2.2 times the band gap) of carrier multiplication, without the need to invoke alternative mechanisms.

Distribution of Multiexciton Generation Rates in CdSe and InAs Nanocrystals

“…The onset of CM in PbSe at around 2.2E g has been attributed to this crossing [17]. However, it still remains an open question whether this crossing is indeed relevant for efficient CM.…”

“…The lighter particle of the pair takes most of the kinetic energy and eventually looses part of this energy by creating additional charge carriers. For NCs, several theoretical approaches for describing CM have been proposed [4,6,[16][17][18][19]. Efros, Nozik and their co-workers [4,16] developed a time-dependent density matrix formalism taking into account the populations and coherences of single exciton coupled to a single biexciton state.…”

“…This approach treats a single trion neglecting the fact that the charged particle decays into a dense manifold of trions. Under such circumstance a rate approach might be more appropriate, describing the decay as an incoherent impact ionization process [17], similar to the treatment of the process in bulk. …”

“…This approach treats a single trion neglecting the fact that the charged particle decays into a dense manifold of trions. Under such circumstance a rate approach might be more appropriate, describing the decay as an incoherent impact ionization process [17], similar to the treatment of the process in bulk. This point of view was elaborated by Allan and Delerue [18] using a tight binding model and Franceschetti et al [17] based on a semiempirical pseudopotential density of states calculation.…”

“…The previous theoretical work established general conditions for efficient CM, that the frequency (in a coherent model [16]) or rate (in an incoherent model [17]) of bi-exciton generation must be faster than the rate [19] however, the relevance for NCs is not immediately obvious.…”