Comparing Multiple Exciton Generation in Quantum Dots To Impact Ionization in Bulk Semiconductors: Implications for Enhancement of Solar Energy Conversion

Abstract

Multiple exciton generation (MEG) in quantum dots (QDs) and impact ionization (II) in bulk semiconductors are processes that describe producing more than one electron-hole pair per absorbed photon. We derive expressions for the proper way to compare MEG in QDs with II in bulk semiconductors and argue that there are important differences in the photophysics between bulk semiconductors and QDs. Our analysis demonstrates that the fundamental unit of energy required to produce each electron-hole pair in a given QD is the band gap energy. We find that the efficiency of the multiplication process increases by at least 2 in PbSe QDs compared to bulk PbSe, while the competition between cooling and multiplication favors multiplication by a factor of 3 in QDs. We also demonstrate that power conversion efficiencies in QD solar cells exhibiting MEG can greatly exceed conversion efficiencies of their bulk counterparts, especially if the MEG threshold energy can be reduced toward twice the QD band gap energy, which requires a further increase in the MEG efficiency. Finally, we discuss the research challenges associated with achieving the maximum benefit of MEG in solar energy conversion since we show the threshold and efficiency are mathematically related.

Multiple exciton generation in quantum dot-based solar cells

“…Interestingly, in QDs, recent spectroscopic findings are indicative of MEG thresholds MEG th ( ) E being closer to the energy conservation limit (2E g ) than in the parental bulk material [30,31,45,46]. In these reports, the authors also find a close correlation between MEG th E and the MEG efficiency (η MEG ).…”

“…For MEG in particular, recent spectroscopic findings suggest a combination of high MEG threshold energies and photon energy-dependent MEG efficiency (i.e. MEG efficiency increases slowly after the threshold energy rather than as a step function) as the predominant reasons for the modest contribution to PV power conversion [29,30].…”

Optimization schemes for efficient multiple exciton generation and extraction in colloidal quantum dots

“…15 In addition, confinement leads to the absence of conservation of momentum, modified carriercooling rates, and reduced dielectric screening, all of which account for enhanced MEG in QDs. 15,16 Besides this basic understanding, a detailed microscopic description of MEG in QDs is needed for an efficient design and optimization of QD solar cells. Since the first demonstration of efficient MEG in PbSe QDs by Schaller and Klimov in 2004, 5 significant attention has been paid to the study of QD based systems for efficient MEG.…”

“…Several groups have been studying MEG efficiency in colloidal semiconductor QDs where they showed a production of multiple electron hole pairs upon absorption of a single photon. 12,[16][17][18] Lead chalcogenide QDs (PbS and PbSe), cadmium chalcogenide QDs, indium based QDs (InAs and InP) and silicon QDs are some of the intensively studied QD systems for exploring the efficiency of MEG. 2,[19][20][21][22][23][24] MEG is commonly measured by ultrafast transient absorption spectroscopy, 25 which allows one to capture the rapid processes of bi-exciton formation and Auger recombinations on the ps time scale.…”

Origins of improved carrier multiplication efficiency in elongated semiconductor nanostructures

“…This leads to an increase in both single-and bi-exciton densities of states, with the latter increasing at a faster rate, though, causing a decrease in the energy E 0 at which the two curves cross (which is closely related to the energy position of the CM onset). This effect is important as a lowering of the CM onset is associated with increased CM efficiency 12,13 and enhanced solar energy conversion 14,15 . In 0D confined structures, CM is typically described in terms of impact ionisation (II) 1,3,[16][17][18] , a term originally used to describe this process in bulk materials: the photon energy in excess of the bandgap, rather than being lost to the lattice in the form of phonons, can be transferred by one of the two photogenerated carriers to an electron in the valence band, exciting it to the conduction band, and creating a further exciton (for a total of two excitons generated per absorbed photon, see Fig.…”