Charge injection into light-emitting diodes: Theory and experiment

Abstract

An analytic theory is presented for dark injection from a metallic electrode into a random hopping system, e.g., a conjugated polymer or a molecularly doped polymer. It encompasses injection of a charge carrier from the Fermi level of the electrode into tail states of the distribution of hopping states of the dielectric followed by either return of the charge carrier to the electrode or diffusive escape from the attractive image potential. The latter process resembles Onsager-type geminate pair dissociation in one dimension. The theory yields the injection current as a function of electric field, temperature and energetic width of the distribution of hopping states. At high electric fields it resembles that the current calculated from Fowler-Nordheim tunneling theory although tunneling transitions are not included in the theory. Good agreement with experimental data obtained for diode structures with conjugated polymers as a dielectric is found.

Monte Carlo study of charge transport in organic sandwich-type single-carrier devices: Effects of Coulomb interactions

“…The effect of these image dipoles is to reduce the disorder close to the electrodes. 34 This leads to a reduced current, because the effect of enhanced injection into energetically low-lying states in the organic material 35 is less. This reduction only occurs for high injection barriers.…”

Injection-limited electron current in a methanofullerene

“…A Monte Carlo simulation shows that the initial jump is essential and determines the temperature and field dependence of the injection process. [9][10][11] The injection-limited current density J ͑ILC͒ can then be analytically approximated by averaging the product of the frequency of the initial jumps of the charge carrier across the metal-polymer interface and the probability to escape recombination, over all site energies E and the initial jump distances x. Arkhipov et al have shown that the ILC can be described by [9][10][11] …”

“…It has been recently demonstrated 8 that the hole injection into OC 1 C 10 ϪPPV is consistently described by a hopping based model, which takes full account of both positional and energetic disorder of the system and image force effect. [9][10][11] According to this model a charge carrier will make an initial jump from the Fermi level of the metal into localized states close to the metal-semiconductor interface, followed by either a diffusive escape from the interface or a backflow to the metal. A Monte Carlo simulation shows that the initial jump is essential and determines the temperature and field dependence of the injection process.…”

“…5 The inverse localization radius equals ␥ ϭ10/nm. 9 The electrostatic potential U is represented by the sum of the barrier for charge injection, the image potential, and the external potential. As a result, U is subject to two material-dependent parameters, the dielectric constant and b , the energy distance from the Fermi level of the electrode to the center of the Gaussian density of states of the PCBM transport states.…”

Electro-optical properties of a polymer light-emitting diode with an injection-limited hole contact

“…The field and temperature dependence of the injection-limited ͑IL͒ current is consistently explained by an injection model based on injection into tail states of the energetic distribution of hopping sites. 8 In this letter, the role of an injection barrier on the performance of a PLED is investigated by incorporating the hopping-based injection mechanism in the device model. The current and light output of an IL PLED have experimentally been investigated for a PPV-based PLED with limited hole injection from a Ag anode.…”