Photoconductivity studies of treated CdSe quantum dot films exhibiting increased exciton ionization efficiency

Jarosz, M. V.; Porter, Venda J.; Fisher, Brent; Kastner, M. A.; Bawendi, Moungi G.

doi:10.1103/physrevb.70.195327

Cited by 177 publications

(216 citation statements)

References 39 publications

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“…The inset indicates the times at which the QD PL intensity I has decreased from its initial value of I 0 so that I=I 0 ¼ e À1 and I=I 0 ¼ e À2 ( e À1 and e À2 , respectively). Reduction of QD PL efficiency has previously been attributed to a decrease in radiative exciton recombination rate (e.g., reduced electron-hole wave function overlap [15,16]), an increase in nonradiative exciton recombination rate (e.g., exciton dissociation [17,18]), or a decrease in the probability of forming thermalized excitons (e.g., hot charge carrier trapping by QD surface traps [19,20]). Because our QD film is 8% PL efficient, PL lifetime is dominated by the nonradiative rate.…”

Section: Figs 2(c) and 2(d)mentioning

confidence: 99%

Origin of Efficiency Roll-Off in Colloidal Quantum-Dot Light-Emitting Diodes

et al. 2013

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We study the origin of efficiency roll-off (also called ''efficiency droop'') in colloidal quantum-dot light-emitting diodes through the comparison of quantum-dot (QD) electroluminescence and photoluminescence. We find that an electric-field-induced decrease in QD luminescence efficiency-and not charge leakage or QD charging (Auger recombination)-is responsible for the roll-off behavior, and use the quantum confined Stark effect to accurately predict the external quantum efficiency roll-off of QD light-emitting diodes. DOI: 10.1103/PhysRevLett.110.217403 PACS numbers: 85.60.Jb, 85.35.Be, 85.60.Bt Quantum-dot light-emitting diodes (QD-LEDs), which capitalize on the excellent color saturation and high photoluminescence efficiency of colloidal QDs, offer the prospect of a new generation of display technologies [1]. However, these devices suffer from decreasing efficiency [measured as the ratio of the number of photons emitted out of the QD-LED to the number of electrons injected into the device, per unit time, known as external quantum efficiency (EQE)] at high current densities. This behavior, termed efficiency roll-off or efficiency droop, is a problem that affects most types of LEDs [2][3][4]. The origin of the efficiency roll-off continues to be a topic of debate and understanding its cause is essential to developing highbrightness, high current density QD-LEDs. In this Letter, we investigate the origins of the roll-off behavior in QD-LEDs by performing simultaneous measurements of electroluminescence (EL) and photoluminescence (PL) intensities of a QD-LED, which pinpoint the cause to be a decrease in QD luminescence efficiency. Comparison of EL and PL spectra reveals that strong electric fields are responsible for the reduced QD luminescence, and the quantum confined Stark effect (QCSE) and transient PL measurements consistently explain the observed phenomena.The device structure investigated was a QD-LED with organic-inorganic hybrid charge transport layers that has recently attracted attention owing to its record high EQE and brightness [2]. The device was fabricated on a glass substrate coated with indium tin oxide and has the structure: ITO ð150nmÞ=ZnO ð50 nmÞ=QDs ð30 nmÞ=4, 4-bis(carbazole-9-yl) biphenyl (CBP) ð100 nmÞ=MoO 3 ð10 nmÞ=Al (100 nm). ZnO was radio-frequency sputtered, QDs were spin-cast out of chloroform, and CBP, MoO 3 , and Al were thermally evaporated. We used CdSe-ZnCdS core-shell QDs with a peak PL wavelength of 610 nm, provided by QD Vision, Inc. Current density and normalized EQE for a typical device are shown in Fig. 1(a). The EQE peaks at 2% for 4 V applied bias and rolls-off by 50% by 8 V. The energy band diagram of the device is shown in the inset and is based on literature values [4][5][6][7].The decrease in EQE at high biases may be a result of either charge carriers leaking out of the QD layer or a reduction in QD luminescence efficiency. To identify which of these two mechanisms dominates, we perform a simultaneous EL-PL experiment and monitor the relative PL efficiency of t...

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Section: Figs 2(c) and 2(d)mentioning

confidence: 99%

Origin of Efficiency Roll-Off in Colloidal Quantum-Dot Light-Emitting Diodes

et al. 2013

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“…Since the as-deposited films are insulating due to the bulky ligands on the nanocrystal surfaces, we first treated the films with methanolic sodium hydroxide to remove the ligands and improve conduction. 45,46 The resulting films are n-type even without Ag doping. Before testing the influence of Ag, we first verified that the NaOH treatment did not affect the number of Ag per nanocrystal in the films ( Figure S10 in the Supporting Information).…”

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confidence: 99%

Electronic Impurity Doping in CdSe Nanocrystals

et al. 2012

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ABSTRACT:We dope CdSe nanocrystals with Ag impurities and investigate their optical and electrical properties. Doping leads not only to dramatic changes but surprising complexity. The addition of just a few Ag atoms per nanocrystal causes a large enhancement in the fluorescence, reaching efficiencies comparable to core−shell nanocrystals. While Ag was expected to be a substitutional acceptor, nonmonotonic trends in the fluorescence and Fermi level suggest that Ag changes from an interstitial (ntype) to a substitutional (p-type) impurity with increased doping.

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“…The prevailing deposition method for colloidal QD systems is spin casting, 4 which introduces limitations such as solvent incompatibility with underlying films and the inability to pattern side-by-side pixels for multispectral photodetector arrays. The alternative deposition method of drop casting is applicable to the fabrication of lateral QD devices (such as photoconductors 5 and transistors 6 ), but resulting films are generally of nonuniform thickness and unsuitable for vertical heterojunction structures.…”

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Heterojunction Photovoltaics Using Printed Colloidal Quantum Dots as a Photosensitive Layer

Arango

Oertel

et al. 2009

Nano Lett.

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We demonstrate a bilayer photovoltaic device consisting of a heterojunction between colloidal cadmium selenide (CdSe) quantum dots (QDs) and a wide band gap organic hole-transporting thin film of N,N′-diphenyl-N,N′-bis(3-methylphenyl)[1,1′-biphenyl]-4,4′-diamine (TPD) molecules. The active light-absorbing film of QDs is nondestructively printed onto TPD using microcontact stamping. Indium−tin−oxide (ITO) provides the top contact. The resulting device structure can accommodate different size QDs, produces an exceptionally large open circuit voltage (0.8 V) for an architecture with symmetric electrodes, and yields an internal quantum efficiency of 10% at the first QD absorption peak.

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Photoconductivity studies of treated CdSe quantum dot films exhibiting increased exciton ionization efficiency

Cited by 177 publications

References 39 publications

Origin of Efficiency Roll-Off in Colloidal Quantum-Dot Light-Emitting Diodes

Origin of Efficiency Roll-Off in Colloidal Quantum-Dot Light-Emitting Diodes

Electronic Impurity Doping in CdSe Nanocrystals

Heterojunction Photovoltaics Using Printed Colloidal Quantum Dots as a Photosensitive Layer

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