The Schottky energy barrier dependence of charge injection in organic light-emitting diodes

Campbell, I. H.; Davids, P. S.; Smith, D. L.; Barashkov, N. N.; Ferraris, John P.

doi:10.1063/1.121208

Cited by 103 publications

(63 citation statements)

References 15 publications

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“…The formation of Schottky barrier between metal electrode and organic electronics materials has been extensively studied both theoretically [71][72][73] and experimentally [74][75][76][77] in recent years. In addition, the application of SAMs (Self Assembled Monolayers) of π-conjugated thiols for single-molecule electronics have been extensively studied [78].…”

Section: Metal-molecule Interfacementioning

confidence: 99%

Metal−Molecule Schottky Junction Effects in Surface Enhanced Raman Scattering

2010

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We propose a complementary interpretation of the mechanism responsible for the strong enhancement observed in Surface Enhanced Raman Scattering (SERS). The effect of a strong static local electric field due to Schottky barrier at the metal-molecule junction on SERS is systematically investigated. The study provides a viable explanation to the low repeatability of SERS experiments as well as the Raman peak shifts as observed in SERS and raw Raman spectra. It was found that strong electrostatic build-in field at the metal-molecule junction along specific orientations can result in 2-4 more orders of enhancement in SERS.

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Section: Metal-molecule Interfacementioning

confidence: 99%

Metal−Molecule Schottky Junction Effects in Surface Enhanced Raman Scattering

2010

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“…These assumptions can be relaxed; we plan to do so in future work. For example, the mobility can be adjusted to account for the electric field through the functional formμ =μ 0 exp Ê /Ê 0 , whereμ 0 andÊ 0 are material parameters [7].…”

Section: Discussionmentioning

confidence: 99%

“…The energy barrier to injection across the MEH-PPV/Ca interface is approximately 0.1 eV, which indicates a space-charge-limited OLED [7]. Specifically, Parker reports current-voltage curves for OLEDs for varying widths of the polymer thin film,L.…”

Section: Comparison Between Asymptotic Analysis and Numerical Solutiomentioning

confidence: 99%

“…The current in an organic device is limited by charge transport across the organic thin film [6] when the energy barrier to injection is lower than 0.3-0.4 eV [7], as opposed to being limited by the electrode-film contacts that provide the majority of the resistance in inorganic devices [8]. At the electrode-organic film interface, 'space-charge layers' exist where the charge carrier (electron or hole) density varies rapidly [9].…”

Section: Introductionmentioning

confidence: 99%

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Asymptotic analysis of double-carrier, space-charge-limited transport in organic light-emitting diodes

2013

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We analyse electron and hole transport in organic light-emitting diodes (OLEDs) via the drift-diffusion equations. We focus on space-charge-limited transport, in which rapid variations in charge carrier density occur near the injecting electrodes, and in which the electric field is highly non-uniform. This motivates our application of singular asymptotic analysis to the drift-diffusion equations. In the absence of electronhole recombination, our analysis reveals three regions within the OLED: (i) 'space-charge layers' near each electrode whose widthλ s is much smaller than the device widthL, wherein carrier densities decay rapidly and the electric field is intense; (ii) a 'bulk' region whose width is on the scale ofL, where carrier densities are small; and (iii) intermediate regions bridging (i) and (ii). Our analysis shows that the currentĴ scales asĴ ∝εμV 2 /L 2λ s , whereV is the applied voltage,ε is the permittivity andμ is the electric mobility, in contrast to the familiar diffusionfree scalingĴ ∝εμV 2 /L 3 . Thus, diffusion is seen to lead to a large O(L/λ s ) increase in current. Finally, we derive an asymptotic recombination-voltage relation for a kinetically limited OLED, in which charge recombination occurs on a much longer time scale than diffusion and drift.

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“…In both cases, a static field of 10 kV/cm is applied. At such low field charge injection effects are negligible, 14 and only photocurrent is observed. Photoexcitation is accomplished with 400 nm ͑3 eV͒ laser pulses with durations of 180 fs [full width at half maximum (FWHM)], allowing above band gap excitation of both GaAs and MEH-PPV [1.6 (Ref.…”

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confidence: 99%

Ultrafast charge generation in a semiconducting polymer studied withTHzemission spectroscopy

et al. 2004

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We study the ultrafast charge generation in a semiconducting polymer (MEH-PPV) by measuring the radiated THz field after photoexciting the biased polymer with a femtosecond visible pulse. The subpicosecond temporal characteristics of the emitted wave reflects the ultrafast photoconductivity dynamics and sets an upper limit for charge generation of 200 fs following photoexcitation, and reveals the dispersive nature of charge transport in MEH-PPV. A comparison of the fields radiated from MEH-PPV and the well-characterized model semiconductor system ͑GaAs͒ allows for an accurate estimate of the quantum efficiency for charge generation in the polymer, found to be less than 1%. Both observations are consistent with ultrafast charge generation in semiconducting polymers through hot exciton dissociation. Semiconducting conjugated polymers have recently received much interest owing to their potential in technological applications, particularly in electronics. 1 Despite this fact, the fundamentals of the photoexcitation physics in these materials have remained controversial: there is ongoing debate whether a band structure description is appropriate to these materials. 2,3 In this picture, interband excitations directly generate free charges, which may relax to lower energy bound states. This strongly contrasts the exciton model, in which molecular excitons are formed upon photoexcitation, so that secondary processes such as exciton-exciton annihilation 4 or hot exciton dissociation 5,6 are required to generate free charges.Although the observation of significant photoconductivity 7 demonstrates the presence of free charges, it is unclear whether carriers are generated directly on ultrafast time scales or produced on longer time scales through, for example, exciton-exciton annihilation. Unfortunately, conventional transient photoconductivity measurements 8,9 lack the resolution to resolve the ultrafast excitation process. Using time-domain THz spectroscopy, 10 we have recently shown that photoexcitation of the polymer poly(2-methoxy-5-(2Ј-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) leads to the generation of charges within 300 fs. 11 Additional information about the time scale of charge carrier generation and nature of the transport can be obtained by probing the radiation dispersed by the accelerating charges. 12,13 When changes in photocurrent occur on (sub) picosecond time scales, THz radiation is emitted. The shape of the THz pulse is determined by the rise and decay of the photocurrent, and thus provides a direct probe of the ultrafast transient photoconductivity. In this manner, information regarding the rate of charge carrier generation and time-dependent mobility can be obtained. This technique is ideally suited to the study of charge generation and cooling on sub-picosecond time scales, and has been successfully applied to study the photoexcitation dynamics in materials such as low-temperature grown GaAs. 13 Previous measurements on semiconducting polymer systems have also proven useful, 12 but suffered from...

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The Schottky energy barrier dependence of charge injection in organic light-emitting diodes

Abstract: Analysis of metal-oxide-based charge generation layers used in stacked organic light-emitting diodes

Cited by 103 publications

References 15 publications

Metal−Molecule Schottky Junction Effects in Surface Enhanced Raman Scattering

Metal−Molecule Schottky Junction Effects in Surface Enhanced Raman Scattering

Asymptotic analysis of double-carrier, space-charge-limited transport in organic light-emitting diodes

Ultrafast charge generation in a semiconducting polymer studied withTHzemission spectroscopy

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