Interband and Intraband Optical Studies of PbSe Colloidal Quantum Dots

Wehrenberg, Brian L.; Wang, and Congjun; Guyot‐Sionnest, Philippe

doi:10.1021/jp021187e

Cited by 640 publications

(700 citation statements)

References 28 publications

(49 reference statements)

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“…[15] or Figure 4 in ref. [5]. This feature may be the 1S h -1P e (or 1P h -1S e ) transition, implying that transition b is the 1P h -1P e transition.…”

Section: Resultsmentioning

confidence: 96%

“…Therefore, lead chalcogenide quantum dots are promising building blocks for a wide number of optoelectronic applications in the near infrared. [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.…”

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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“…[15] or Figure 4 in ref. [5]. This feature may be the 1S h -1P e (or 1P h -1S e ) transition, implying that transition b is the 1P h -1P e transition.…”

Section: Resultsmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

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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“…Hence, the ability to synthesize nanocrystals of different sizes leads to mate rials with different state energies. Additionally, when the nanocrys tal size series is composed of the same inorganic semiconductor and organic ligands, the state tunability (including the energy of the lowest energy absorption) is largely decoupled from the bulk dielectric and chemical properties of the materials 39,40 . Thus, by incorporating nanocrystals with different sizes and hence different state energies into working devices, it is possible to assess energetic parameters of device performance.…”

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confidence: 99%

In situ measurement of exciton energy in hybrid singlet-fission solar cells

et al. 2012

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singlet exciton fission-sensitized solar cells have the potential to exceed the shockley-Queisser limit by generating additional photocurrent from high-energy photons. Pentacene is an organic semiconductor that undergoes efficient singlet fission-the conversion of singlet excitons into pairs of triplets. However, the pentacene triplet is non-emissive, and uncertainty regarding its energy has hindered device design. Here we present an in situ measurement of the pentacene triplet energy by fabricating a series of bilayer solar cells with infrared-absorbing nanocrystals of varying bandgaps. We show that the pentacene triplet energy is at least 0.85 eV and at most 1.00 eV in operating devices. our devices generate photocurrent from triplets, and achieve external quantum efficiencies up to 80%, and power conversion efficiencies of 4.7%. This establishes the general use of nanocrystal size series to measure the energy of non-emissive excited states, and suggests that fission-sensitized solar cells are a favourable candidate for third-generation photovoltaics.

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“…The slow relaxation is also reminiscent of the long excited-state lifetime that is observed in studies of PbSe NQDs. 36,38 Rationalization of the slow relaxation in PbSe NQDs has drawn upon consideration of the large dielectric screening correction of the optically active exciton radiative recombination rate. 36 However, the same calculation applied to InAs NQDs does not provide for a reduced rate of radiative relaxation.…”

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confidence: 99%

Carrier Multiplication in InAs Nanocrystal Quantum Dots with an Onset Defined by the Energy Conservation Limit

2007

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Carrier multiplication (CM) is a process in which absorption of a single photon produces not just one but multiple electron−hole pairs (excitons). This effect is a potential enabler of next-generation, high-efficiency photovoltaic and photocatalytic systems. On the basis of energy conservation, the minimal photon energy required to activate CM is two energy gaps (2E g ). Here, we analyze CM onsets for nanocrystal quantum dots (NQDs) based upon combined requirements imposed by optical selection rules and energy conservation and conclude that materials with a significant difference between electron and hole effective masses such as III−V semiconductors should exhibit a CM threshold near the apparent 2E g limit. Further, we discuss the possibility of achieving sub-2E g CM thresholds through strong exciton−exciton attraction, which is feasible in NQDs. We report experimental studies of exciton dynamics (Auger recombination, intraband relaxation, radiative recombination, multiexciton generation, and biexciton shift) in InAs NQDs and show that they exhibit a CM threshold near 2E g .Carrier Multiplication Thresholds in Bulk and Nanocrystalline Semiconductors. It has recently been established that carrier multiplication (CM), the production of multiple excitons following absorption of a single photon of sufficient energy, is efficient within the range of solar photon energies in semiconductor nanocrystal quantum dots (NQDs). [1][2][3][4][5][6][7][8][9] This finding is important because CM can potentially provide increased power conversion efficiency in low-cost, singlejunction photovoltaics via an enhanced photocurrent. 10,11 To obtain optimum photovoltaic power conversion efficiency using a single semiconductor material with energy gap E g , it is desirable that the CM onset energy is minimal and that the CM efficiency above the onset is as large as is allowed by energy conservation.According to energy conservation, the apparent minimal photon energy that is required to activate CM (pω CM ) is simply 2E g . However, the CM thresholds observed in bulk materials are significantly larger than this value. 12,13 In bulk semiconductors, the generally accepted mechanism for multiexciton generation is impact ionization in which a highenergy electron (or a hole) transfers its energy to a valenceband electron that is excited across the energy gap. 14 The CM threshold in this case is determined not only by conservation of energy but also by conservation of translational momentum, which increases the threshold above 2E g . For example, using a free-particle approximation 15,16 and applying both energy and momentum conservation laws, one can obtain that the CM threshold is approximately 4E g (Figure 1a).In NQDs, translational momentum conservation is relaxed. Therefore, one might expect that the CM threshold could reach the 2E g limit as defined by energy conservation. However, available experimental data indicate that pω CM is still greater than twice the energy gap in such materials. For example, in PbSe and PbS NQDs, it ...

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Interband and Intraband Optical Studies of PbSe Colloidal Quantum Dots

Cited by 640 publications

References 28 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

In situ measurement of exciton energy in hybrid singlet-fission solar cells

Carrier Multiplication in InAs Nanocrystal Quantum Dots with an Onset Defined by the Energy Conservation Limit

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