High-efficiency carrier multiplication through direct photogeneration of multi-excitons via virtual single-exciton states

Schaller, Richard D.; Agranovich, V.M.; Klimov, Victor I.

doi:10.1038/nphys151

Cited by 461 publications

(574 citation statements)

References 22 publications

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“…[24] Finally, another group proposed the instantaneous formation of multi-excitons through weak coupling with a virtual singleexciton state to explain MEG, in which case the electron/ hole cooling does not play a role. [7] The threshold for MEG in PbSe QDs of 6.8-nm diameter (E g = 0.64 eV) was determined at 1.8 eV. [9] Here, we find that PbSe 6.8-nm QDs have transition h at 1.9 eV.…”

Section: Resultsmentioning

confidence: 61%

“…[7][8][9][10] The mechanism for MEG, however, is still under debate. Two theoretical papers used impact ionization (II) to explain the ultrafast carrier multiplication.…”

Section: Resultsmentioning

confidence: 99%

“…[3][4][5][6] For example, PbSe QDs hold great promise in the field of photovoltaics because of the recently observed multiple exciton generation (MEG) in these nanocrystals. [7][8][9][10] Here we report on detailed investigations of the optical absorption spectra of PbSe QDs. First, the assignment of the multiple features (up to 11) in the absorption spectrum of highly monodisperse PbSe QDs is discussed and compared to calculated spectra of PbSe QDs that were recently published.…”

Section: Introductionmentioning

confidence: 99%

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Optical Investigation of Quantum Confinement in PbSe Nanocrystals at Different Points in the Brillouin Zone

Koole

Allan

Delerue

et al. 2008

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We present detailed investigations on the optical properties of PbSe nanocrystals. The absorption spectra of monodisperse, quasispherical nanocrystals exhibit sharp features as a result of distinct optical transitions. To study the size dependence, absorption spectra of nanocrystals ranging from 3.4 to 10.9 nm in diameter are analysed and a total of 11 distinct optical transitions are identified. The assignment of the various optical transitions is discussed and compared to theoretically calculated transition energies. By plotting all transitions as a function of nanocrystal size (D) we find that the energy (E) changes with the following relationship E / D À1.5 for the lowest energy transitions. The transition energy extrapolates to approximately 0.3 eV for infinite crystal size, in agreement with the bandgap of bulk PbSe at the L-point in the Brillouin zone. In addition, high-energy transitions are observed, which extrapolate to 1.6 eV for infinite crystal size, which is in good agreement with the bulk bandgap of PbSe at the S-point in the Brillouin zone. Tight-binding calculations confirm that the high-energy transitions originate from the S-point in the Brillouin zone. The S-character of the high-energy transitions may be of importance to explain the mechanism behind multiple exciton generation in PbSe nanocrystals.

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Section: Resultsmentioning

confidence: 61%

“…[7][8][9][10] The mechanism for MEG, however, is still under debate. Two theoretical papers used impact ionization (II) to explain the ultrafast carrier multiplication.…”

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Optical Investigation of Quantum Confinement in PbSe Nanocrystals at Different Points in the Brillouin Zone

Koole

Allan

Delerue

et al. 2008

Small

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“…Both processes are governed by the same transition matrix elements. This identity is indeed the reason why impact-ionisation had been previously dismissed 19,20 as the source of CM in NCs, based on the then perceived incompatibility between the ∼ 10 ps AR lifetimes and the observed < 400 fs CM decay. The crucial difference between the two processes, however, is that the initial states in the former represent the final states in the latter and vice versa.…”

Section: Introductionmentioning

confidence: 99%

“…An alternative model proposed by Klimov explains CM in terms of direct generation of multiexcitons via Coulomb coupling to virtual single-exciton states 19 . In this model the CM efficiency depends on the ratio of the biexciton and single exciton generation rates, and therefore on R(E).…”

Section: Introductionmentioning

confidence: 99%

Origins of improved carrier multiplication efficiency in elongated semiconductor nanostructures

Sills

Califano

2015

Phys. Chem. Chem. Phys.

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Nanorod solar cells have been attracting a lot of attention recently, as they have been shown to exhibit a lower carrier multiplication onset and a higher quantum efficiency than quantum dots with similar bandgaps. The underpinning theory for this phenomenon is not yet completely understood, and is still the subject of ongoing study. Here we conduct a theoretical investigation into CM efficiency in elongated semiconductor nanostructures with square cross section made of different materials (GaAs, GaSb, InAs, InP, InSb, CdSe, Ge, Si and PbSe), using a single-band effective mass model. Following Luo, Franceschetti and Zunger we adopt the CM figure of merit (the ratio between biexciton and single-exciton density of states) as a measure of CM efficiency and investigate its dependence on the aspect ratio for both (a) constant cross section (i.e. varying the volume) and (b) constant volume (i.e., varying the cross section), by decoupling electronic structure effects from surface-related effects, increased absorption and Coulomb coupling effects. The results show that in both (a) and (b) cases elongation causes an increase in both single-and bi-exciton density of states, with the latter, however, growing much faster with increasing energy. This leads to the availability of more bi-exciton states than single-exciton states for photon energies just above the bi-exciton ground state and therefore suggests a higher probability of CM at these energies for elongated structures. Our results therefore show that the origin of the observed decrease of the CM threshold in elongated structures can be attributed purely to electronic structure effects, paving the way to the implementation of CM-efficiency-boosting strategies in nanostructures based on the lowering of the CM onset.

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Modeling and Simulation of Photocatalytic Reactions at TiO2 Surfaces

Kamisaka

Yamashita

On Solar Hydrogen &Amp; Nanotechnology

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Since the discovery of the Fujishima-Honda effect [1], the surface chemistry of TiO 2 has been expected to play a central role in solar hydrogen generation processes. In addition, strong photocatalytic activity [2] and photoinduced hydrophilic conversion phenomena [3] have been observed on TiO 2 surfaces. A dye-sensitized TiO 2 surface can be used as a building block for solar-cell devices [4]. Introduction of some impurities in TiO 2 thin films induces significant changes in their magnetic [5] and conducting properties [6]. All these characteristic features show the large number of possible industrial applications that are yet to materialize; for example, the fabrication of new catalysis and optoelectronic devices and solar cells. Some of these features have already been utilized in industrial applications [7]. Because TiO 2 is nontoxic and has a low cost, it will improve such processes. Most of the applications of TiO 2 are related to clean chemical processes and reusable-energy resources. From these favorable features, the importance of TiO 2 in the next-generation chemical industry cannot be overemphasized.However, despite intensive research into TiO 2 , from both experimental and theoretical aspects, the understanding of the fundamentals of TiO 2 chemistry is still limited. Most of the microscopic mechanisms of the properties mentioned above have not been clarified. Even some basic quantities, such as the effective mass of the electrons and the stoichiometry of bulk TiO 2 ,

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High-efficiency carrier multiplication through direct photogeneration of multi-excitons via virtual single-exciton states

Cited by 461 publications

References 22 publications

Optical Investigation of Quantum Confinement in PbSe Nanocrystals at Different Points in the Brillouin Zone

Optical Investigation of Quantum Confinement in PbSe Nanocrystals at Different Points in the Brillouin Zone

Origins of improved carrier multiplication efficiency in elongated semiconductor nanostructures

Modeling and Simulation of Photocatalytic Reactions at TiO2 Surfaces

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