Charge injection in organic light-emitting diodes: Tunneling into low mobility materials

Davids, P. S.; Kogan, Shulim; Parker, Ian D.; Smith, D. L.

doi:10.1063/1.117530

Cited by 115 publications

(59 citation statements)

References 8 publications

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“…(41). Note also that (D6) implies that x ζ ≈ L/(1 + µ h /µ e ), which is also valid in the high-injection regime.…”

Section: Bipolar Regime Fluxmentioning

confidence: 91%

“…The importance of back-flux and its effect on carrier injection in organic semiconductors with low carrier mobility has been recognized, but prior treatments have been primarily phenomenological and without the level of rigor afforded to the forward flux. 29,41 It is important to note that we have treated the forward and backward flux equivalently, using the same physics, and in a way that is thermodynamically consistent.…”

Section: A Injection Mechanisms and The Notion Of Quasi-equilibriummentioning

confidence: 99%

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Charge injection and transport in low-mobility mixed ionic/electronic conducting systems: Regimes of behavior and limiting cases

Mills

Lonergan

2012

Phys. Rev. B

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A comprehensive analysis of a model describing charge carrier injection and transport in lightemitting electrochemical cells (LECs) and related mixed ionic electronic conductors (MIECs) is given. Ions are treated using a modified drift-diffusion transport equation that accounts for volume exclusion effects, and electronic injection is treated using a spatially dependent tunneling mechanism that explicitly accounts for both forward and backward fluxes. Systems containing both one and two mobile ionic species are treated and compared. The unique physics of LECs stem from ionic polarization processes that can lead to field screening and narrowed injection barriers, producing increased electrode exchange currents via tunneling. The latter process promotes the establishment of electronic quasi-equilibrium throughout the double-layer regions and hence promotes bulk-limited conduction. Explicit expressions are given describing the conditions necessary to assume field screening and bulk-limited conduction, which determine the applicability of either traditional semiconductor device models such as Fowler-Nordheim or electrochemical models such as the Nernst equation. Having established these conditions, several distinct regimes of bulk-limited LEC behavior are described. Explicit formulae for the biases delineating these regimes are given as well as formulae for the current in each regime. At low biases, the current generally increases exponentially with bias; the bulk remains field-free, and the transport is predominantly unipolar and diffusive. At high biases the current rises much less rapidly, and bulk transport is bipolar, occurring through a combination of drift and diffusion. The nature of the bulk region in the high-bias regime is markedly different in systems with one and two mobile ionic species. At intermediate biases, space charge effects preferentially drive injection of the minority carrier causing a transition from unipolar to bipolar injection. It is demonstrated that many of the models proposed to describe LECs exist upon a common continuum, and that the major factor separating them is simply the magnitude of the applied bias. This work allows one to estimate at what biases an idealized LEC with particular equilibrium concentrations of ionic and electronic carriers will transition from one mechanism to another. It also aids in conceptually mapping mechanisms and internal details of the system onto each regime of behavior.

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“…(41). Note also that (D6) implies that x ζ ≈ L/(1 + µ h /µ e ), which is also valid in the high-injection regime.…”

Section: Bipolar Regime Fluxmentioning

confidence: 91%

Section: A Injection Mechanisms and The Notion Of Quasi-equilibriummentioning

confidence: 99%

Charge injection and transport in low-mobility mixed ionic/electronic conducting systems: Regimes of behavior and limiting cases

Mills

Lonergan

2012

Phys. Rev. B

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“…Furthermore, at low fields the tunneling model was found not to be applicable to the experimental J±V characteristics, which has been attributed to space-charge effects [4], the contribution of thermionic emission to the current [5] and to band-bending effects at the interface [6]. Recently, the quantitative discrepancy at high fields and the qualitative discrepancy at low fields have been attributed to the low mobility of the organic materials, which leads to a large backflow of injected carriers into the injecting contacts [7].…”

Section: à3mentioning

confidence: 99%

Device physics of polymer light-emitting diodes

Blom

Jong

Liedenbaum

1998

Polym. Adv. Technol.

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This article reviews a device model for the current and light generation of polymer light‐emitting diodes (PLEDs). The model is based on experiments carried out on poly(dialkoxy‐p‐phenylene vinylene) (PPV) devices. The transport properties of holes in PPV have been investigated with indium tin oxide (ITO)/PPV/Au hole‐only devices. The hole current is dominated by bulk conduction properties of the PPV, in contrast to previous reports. As the hole current is space‐charge limited, the hole mobility as a function of electric field E and temperature T can be directly determined. The hole mobility exhibits a field dependence ln(μ) ∼ ✓E as also has been observed from time‐of‐flight experiments in many molecularly doped polymers and amorphous glasses. For the zero‐field hole mobility an activation energy of 0.48 eV is obtained. The electron conduction in PPV has been studied by using Ca/PPV/Ca electron‐only devices. It appears that the electron current is strongly reduced by the presence of traps with a total density of 1018 cm−3. Combining the results of electron‐ and hole‐only devices a device model for PLEDs is proposed in which the light generation is due to bimolecular recombination between the injected electrons and holes. It is calculated that the unbalanced electron and hole transport gives rise to a bias‐dependent efficiency. By comparison with experiment it is found that the recombination process in PPV is for 95% nonradiative. Furthermore, the experiments reveal that the bimolecular recombination process is thermally activated with an identical activation energy as measured for the charge carrier mobility. This demonstrates that the recombination process is of the Langevin‐type, in which the rate‐limiting step is the diffusion of electrons and holes towards each other. The occurrence of Langevin recombination explains why the conversion efficiency (photon/carrier) of a PLED is temperature independent. © 1998 John Wiley & Sons, Ltd.

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“…Besides the characteristics of light emitting organic layer, interface interactions (P.S. Davids et al, 1996) of this layer with other layers in OLED play important role in defining the characteristics of the display. There have been innumerable studies on different aspects of PEDOT: PSS (L.S.…”

Section: Device Structure Principlementioning

confidence: 99%

Organic Light Emitting Diodes: Device Physics and Effect of Ambience on Performance Parameters

Hameed¹,

Predeep²,

Baiju³

2011

Optoelectronics - Devices and Applications

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Charge injection in organic light-emitting diodes: Tunneling into low mobility materials

Cited by 115 publications

References 8 publications

Charge injection and transport in low-mobility mixed ionic/electronic conducting systems: Regimes of behavior and limiting cases

Charge injection and transport in low-mobility mixed ionic/electronic conducting systems: Regimes of behavior and limiting cases

Device physics of polymer light-emitting diodes

Organic Light Emitting Diodes: Device Physics and Effect of Ambience on Performance Parameters

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