High-Efficiency Carrier Multiplication and Ultrafast Charge Separation in Semiconductor Nanocrystals Studied via Time-Resolved Photoluminescence

Schaller, Richard D.; Sýkora, Milan; Jeong, Sohee; Klimov, Victor I.

doi:10.1021/jp065282p

Cited by 184 publications

(198 citation statements)

References 33 publications

(86 reference statements)

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“…36 As illustrated in Figure 1a, we generated MEs using the latter method. Since the behavior of MEs generated by either method has been shown to be the same, 28 the results of this study are valid also for the MEs generated by CM. The assemblies of CdSe/CdS QDs used in the experiments feature 88% photoluminescence quantum yield, which results from a very high degree of surface passivation and small density of defect sites in the QDs.…”

Section: Resultsmentioning

confidence: 54%

Observation of Biexcitons in Nanocrystal Solids in the Presence of Photocharging

et al. 2013

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In nanocrystal quantum dots (NQDs), generating multiexcitons offers an enabling tool for enhancing NQD-based devices. However, the photocharging effect makes understanding multiexciton kinetics in NQD solids fundamentally challenging, which is critically important for solid-state devices. To date, this lack of understanding and the spectralÀtemporal aspects of the multiexciton recombination still remain unresolved in solid NQD ensembles, which is mainly due to the confusion with recombination of carriers in charged NQDs. In this work, we reveal the spectralÀtemporal behavior of biexcitons (BXs) in the presence of photocharging using near-unity quantum yield CdSe/CdS NQDs exhibiting substantial suppression of Auger recombination. Here, recombinations of biexcitons and single excitons (Xs) are successfully resolved in the presence of trions in the ensemble measurements of time-correlated single-photon counting at variable excitation intensities and varying emission wavelengths. The spectral behaviors of BXs and Xs are obtained for three NQD samples with different core sizes, revealing the strength tunability of the XÀX interaction energy in these NQDs. The extraction of spectrally resolved X, BX, and trion kinetics, which are otherwise spectrally unresolved, is enabled by our approach introducing integrated time-resolved fluorescence. The results are further experimentally verified by cross-checking excitation intensity and exposure time dependencies as well as the temporal evolutions of the photoluminescence spectra, all of which prove to be consistent. The BX and X energies are also confirmed by theoretical calculations. These findings fill an important gap in understanding the spectral dynamics of multiexcitons in such NQD solids under the influence of photocharging effects, paving the way to engineering of multiexciton kinetics in nanocrystal optoelectronics, including NQD-based lasing, photovoltaics, and photodetection.

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Section: Resultsmentioning

confidence: 54%

Observation of Biexcitons in Nanocrystal Solids in the Presence of Photocharging

et al. 2013

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“…This triggered a wide and deep study [68] on multiple exciton effect in semiconductor quantum dot structure. So far, significant MEG was observed in PbSe, PbS [69], PbTe [70], CdSe [71], Si [72] and InAs [73], and other semiconductor nanostructures, respectively. It was found that in the 0.5 to 3.5 eV surface solar spectral regions, silicon nanoscale crystals have very strong multi-exciton effect.…”

Section: Multi-exciton Effect Photovoltaic Cellsmentioning

confidence: 99%

Advances in Conceptual Electronic Nanodevices based on 0D and 1D Nanomaterials

et al. 2013

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Nanoelectronic devices are being extensively developed in these years with a large variety of potential applications. In this article, some recent developments in nanoelectronic devices, including their principles, structures and potential applications are reviewed. As nanodevices work in nanometer dimensions, they consume much less power and function much faster than conventional microelectronic devices. Nanoelectronic devices can operate in different principles so that they can be further grouped into field emission devices, molecular devices, quantum devices, etc. Nanodevices can function as sensors, diodes, transistors, photovoltaic and light emitting devices, etc. Recent advances in both theoretical simulation and fabrication technologies expedite the development process from device design to prototype demonstration. Practical applications with a great market value from nanoelectronic devices are expected in near future.

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“…2,11 Transient absorption (TA) spectroscopy is a convenient technique with which to measure exciton population dynamics. Other time-resolved spectroscopies have also been employed to study MEG, including time-resolved photoluminescence (TRPL) 12,13 and time-resolved terahertz spectroscopy (TRTS), 14 which measures the intraband photoinduced absorption. 15 These experimental probes provide complementary information.…”

mentioning

confidence: 99%

Variations in the Quantum Efficiency of Multiple Exciton Generation for a Series of Chemically Treated PbSe Nanocrystal Films

et al. 2009

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We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (NC) films. We find that the average number of excitons produced per absorbed photon varies between 1.0 and 2.4 ((0.2) at a photon energy of ∼4E g for films consisting of 3.7 nm NCs and between 1.1 and 1.6 ((0.1) at hν ∼ 5E g for films consisting of 7.4 nm NCs. The variations in MEG depend upon the chemical treatment used to electronically couple the NCs in each film. The single and multiexciton lifetimes also change with the chemical treatment: biexciton lifetimes increase with stronger inter-NC electronic coupling and exciton delocalization, while single exciton lifetimes decrease after most treatments relative to the same NCs in solution. Single exciton lifetimes are particularly affected by surface treatments that dope the films n-type, which we tentatively attribute to an Auger recombination process between a single exciton and an electron produced by ionization of the dopant donor. These results imply that a better understanding of the effects of surface chemistry on film doping, NC carrier dynamics, and inter-NC interactions is necessary to build solar energy conversion devices that can harvest the multiple carriers produced by MEG. Our results show that the MEG efficiency is very sensitive to the condition of the NC surface and suggest that the wide range of MEG efficiencies reported in the recent literature may be a result of uncontrolled differences in NC surface chemistry.Multiple exciton generation (MEG) in semiconductor nanocrystals (NCs) (also called quantum dots (QDs)) can produce n excitons for each absorbed photon possessing an energy of at least n multiples of the band gap energy (E g ), where n is an integer.1-3 If multiexciton formation, dissociation, and charge collection are simultaneously efficient, the resulting enhanced photocurrent can increase solar energy conversion efficiencies.4,5 Recently, we reported a Schottky-junction photovoltaic device based on a thin film of colloidal PbSe NCs that demonstrated a power conversion efficiency of >2% and a short-circuit current density, J SC , greater than 20 mA cm -2 . 6 The NC film in this device was treated with 1,2-ethanedithiol (EDT) in acetonitrile in a layer-by-layer procedure to produce a conductive NC film.7 Excitons are created, separated, and transported all within the singlecomponent NC film. The internal quantum efficiency (IQE), defined as the fraction of photons absorbed by the NCs that produce carriers in the external circuit, was found to be as high as 0.8, indicating efficient charge separation and transport. 8 However, no evidence was found in the IQE spectra to suggest that multiple charge carriers were collected per absorbed photon. To harvest MEG excitons from a NC film, the inter-NC charge transfer event that produces free electrons and holes must be faster than Auger recombination (nonradiative exciton annihilation), which typically occurs in 10-100 ps. In addition, the chemical treatments that are used to produce the c...

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High-Efficiency Carrier Multiplication and Ultrafast Charge Separation in Semiconductor Nanocrystals Studied via Time-Resolved Photoluminescence

Cited by 184 publications

References 33 publications

Observation of Biexcitons in Nanocrystal Solids in the Presence of Photocharging

Observation of Biexcitons in Nanocrystal Solids in the Presence of Photocharging

Advances in Conceptual Electronic Nanodevices based on 0D and 1D Nanomaterials

Variations in the Quantum Efficiency of Multiple Exciton Generation for a Series of Chemically Treated PbSe Nanocrystal Films

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