Direct measurement of the internal electric field distribution in a multilayer organic light-emitting diode

Campbell, I. H.; Joswick, M. D.; Parker, Ian D.

doi:10.1063/1.115152

Cited by 79 publications

(38 citation statements)

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“…QDs placed adjacent to the ZnO are expected to exhibit energy levels that differ from that of QDs placed adjacent to the CBP, which has a lower dielectric constant [13]. The difference between EL and PL spectra (even when the peaks are matched) can be explained by the fact that the electricfield distribution is generally different between forward and reverse biased diodes [14], thus affecting the two populations differently.…”

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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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confidence: 99%

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

et al. 2013

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“…This technique has been used to measure the workfunction difference of the electrodes, 7 the electric-field distribution in multilayer devices, 8 and poling of multilayer polymer films. 9 We find that the applied bias generates a counter electric field that partially offsets it.…”

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confidence: 99%

Optical studies of electric fields in poly(2-methoxy-5-ethyl (2′-hexyloxy) para-phenylene vinylene) light-emitting diodes

Giebeler

Whitelegg

Campbell

et al. 1999

Applied Physics Letters

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“…1. We note that quite similar ideas were used recently by Campbell, et al (1995)¸ to interpret electroabsorption measurements on electroluminescent organic heterostructure diodes.…”

Section: Introductionmentioning

confidence: 88%

“…The built-in electrostatic potential V bi established by these fields is thus one of a cell's important device parameters. One promising approach to illustrating and estimating V bi exploits electroabsorption measurements: by measuring the transmittance or reflectance of the cell as a function of an external potential V, an inference of the built-in potential can be made (Nonomura, Okamoto, and Hamakawa, 1983;Wang, Schiff, and Hegedus, 1994;Campbell, Jowick, and Parker, 1995;Jiang, Wang, Schiff, Guha, Yang, and Deng, 1996).…”

Section: Introductionmentioning

confidence: 99%

Research on High-Bandgap Materials and Amorphous Silicon-Based Solar Cells, Final Technical Report, 15 May 1994-15 January 1998

Schiff¹,

Gu²,

Lin³

et al. 1998

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or DOE Information Bridge http://www.doe.gov/bridge/home.html Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste 1 Preface This research project had four broad objectives involving a variety of technical approaches:• We sought a deeper understanding of the open circuit voltage V OC in amorphous silicon based solar cells, and in particular for cells employing wide bandgap modification of amorphous silicon as their absorber layer. Our technical approach emphasized the development of electroabsorption measurements as a tool for probing the built-in potential of a-Si:H based solar cells.• We sought a better understanding of the optical properties of amorphous silicon and of microcrystalline silicon materials such as used in the p + layer of some advanced a-Si:H based solar cells. Electroabsorption spectroscopy appears to be useful for this purpose, and was a natural extension of the built-in potential research.• We sought to improve our understanding of the fundamental electron and hole photocarrier transport processes in the materials used for amorphous silicon based solar cells. Our technical approach was been to develop photocarrier time-of-flight measurements in solar cells.• We aimed to improve V OC in wide bandgap cells by searching for superior materials for the p-type "window layer" of the cell, and explored thin-film boron phosphide for this purpose.In addition to the authors of this report, we have reported work which includes the contributions of several collaborators, in particular Reinhard Schwarz, Stefan Grebner, and We have also benefitted from the cooperation and interest of several other scientists and organizations. In particular we thank Christopher Wronski at Pennsylvania State University, Steve Hegedus at the Institute of Energy Conversion, University of Delaware, and Murray Bennett at Solarex Corp. Thin Films Division. Summary• The built-in potential is a crucial device parameter for solar cells, and one which is only roughly known and understood for a-Si:H based cells. We have developed a technique based on electroabsorption measurements for obtaining quantitative estimates of the built-in potential in aSi:H based heterostructure solar cells incorporating microcrystalline or a-SiC:H p layers. This heterostructure problem has been a major limitation in application of the electroabsorption technique. The new technique only utilizes measurements from a particular solar cell, and is thus a significant improvement on earlier techniques requiring measurements on auxiliary films.• Using this new electroabsorption technique, we confirmed previous estimates of V bi ≈ 1.0 V in a-Si:H solar cells with "conventional" intrinsic layers and either microcrystalline or a-SiC:H p layers. Interestingly, our measurements on high V oc cells grown with "high hydrogen dilution" intrinsic layers yield a much larger value for V bi ≈ 1.3 V. We speculate that these results are evidence for a 2 significant interface dipole at the p/i heterostructure interface. Although we believe that i...

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Direct measurement of the internal electric field distribution in a multilayer organic light-emitting diode

Cited by 79 publications

References 0 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

Optical studies of electric fields in poly(2-methoxy-5-ethyl (2′-hexyloxy) para-phenylene vinylene) light-emitting diodes

Research on High-Bandgap Materials and Amorphous Silicon-Based Solar Cells, Final Technical Report, 15 May 1994-15 January 1998

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