Bipolar charge and current distributions in organic light-emitting diodes

Abstract

The electron and hole charge distributions and the luminance profile in organic light-emitting diodes (OLEDs) depend upon the bulk properties of the emissive layer, as well as on the injection characteristics at the anode and cathode interfaces. We address the problem of separating the relative contributions of hole injection, electron injection, and recombination to the overall performance of single layer OLED devices. Using the approach of Parmenter and Ruppel [J. Appl. Phys. 30, 1548 (1959)], and including Langevin recombination, expressions are derived for the current–voltage and radiance–current dependencies in terms of electron and hole mobility, luminescence yield, and a “current balance” factor. When one carrier dominates the current flow, as in many practical cases, it is possible to obtain a simple asymptotic relationship which permits a test of the assumptions required to obtain the analytic solution. Experimental data from poly(2-methoxy-5(2′-ethyl)hexoxy-phenylenevinylene) diodes fabricated with various anode and cathode materials are evaluated in the context of this analytical approach.

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

“…Asx → ∞, it is clear thatẼ 1 → V , becauseẼ 0 decays exponentially, thereby matching the bulk field, but the behaviour asx → 0 needs to be examined carefully. A power series expansion aroundx = 0 reveals two homogeneous solutions to (4.18): 20) and…”

“…Equation (1.1) is known as the Mott-Gurney law [12]. For single-carrier injection, the Mott-Gurney law has been extended to include the effects of charge traps [15][16][17][18][19], uneven injection barriers [8,20] and intrinsic charge [17]. The latter is responsible for an ohmic regime (Ĵ ∝V) that precedes the space-charge-limited regime described by the Mott-Gurney law [12].…”

Spectrally narrowed leaky waveguide edge emission and transient electrluminescent dynamics of OLEDs

“…By using the multilayer structures, charge blocking layers can be introduced at the organic layers interfaces to improve the efficiency of the charge recombination and, in turn, the charge balance factor. 66,67 Production efficiency of emissive excitation β is the branching ratio for singlets and triplets. Based upon simple spin statistics, assuming weak intermolecular interactions, the upper limit for the internal QE in OLEDs is 25%.…”

The use of charge extraction by linearly increasing voltage in polar organic light-emitting diodes

“…In organic lightemitting diodes (OLEDs) where electrons and holes are injected by electrodes and recombine in the active layer, leading to light emission, one of the key factors determining the (current-to-luminance) efficiency is the charge balance factor. [1][2][3] This factor can be maximized by the stack design, e.g., using blocking layers, as well as optimizing the carrier mobilities in the charge transport layers. Bimolecular carrier recombination is directly proportional to carrier mobility; 4 thus, a high carrier mobility translates into a high recombination efficiency which in turn is linked to a high charge balance factor, too.…”