We present a combined quantum-chemical and Monte Carlo approach for calculating exciton transport properties in
disordered organic materials starting from the molecular scale. We show that traps and energetic disorder are the main
limitations for exciton diffusion in conjugated polymers. An analytical model for exciton hopping in a medium of sites
with uncorellated energetic disorder gives a quantitative description on the dependence of the diffusion length to both the
energetic disorder strength and temperature. We demonstrate how traps and energetic disorder can pin down the
diffusion length in conjugated polymers to values below 10 nm.
Changes in the external quantum efficiency of bilayer organic light emitting devices with layer length have been measured for devices of configuration ITO\TPD\Alq\Mg:Ag with the Alq length varying between 25-200nm. It has been independently concluded for similar devices that the thickness of the Alq layer can be optimised with regard to the external quantum efficiency. However, our simulations of the internal quantum efficiency of this structure with an electrical transport model predict that the internal quantum efficiency is invariant with respect to the Alq layer thickness. We deduce that optical microcavity effects cause the variation in external quantum efficiency. These microcavity effects alter the external efficiency through optical interference and through altering the singlet exciton density profile. A combined electrical-optical model based on our electrical transport model and an optical model has been used to calculate the external efficiency for these devices. We find a clear variation in efficiency with layer thickness, matching the experimentally observed trends.
We present a study of electrical transport in organic light emitting diodes using a 1D drift diffusion model. This model includes bipolar transport, charge injection and electron trapping on the same footing. As input we have mobilities, doping densities typical of organic semiconducting devices, and barrier heights taken from internal photoemission measurements. Charge density, trap filling, field, potential and recombination profiles in addition to current-voltage characteristics are provided by the mode. We have obtained result for two-layer organic devices, examining the influence of contacts and of traps on the current-voltage characteristic. The density of filled traps is determined by the position of the quasiFermi level with respect to the trap energy levels, and this changes with position and applied bias. The quasiFermi level profile is sensitive to both the type of contact and the doping density. Traps at a single energy level, and with exponential distributions with respect to discrete energy levels have been considered. We see an injection limited current at low biases and bulk limited transport at higher biases with a trap limited current contribution.
We present a structure which is capable of being fabricated into two distinct devices, both with considerable potential in the field of optical communications in particular with reference to wavelength domain multiplexing. The structure is based on two back to back p-i-n Ga<SUB>x</SUB>Al<SUB>1-x</SUB>As structures with a single quantum well of GaAs in each intrinsic region. The light emitter device operates by forward biasing either of the p-i-n elements. In forward bias holes flood into the quantum well in the intrinsic region. Electrons are prevented from doing so by a potential barrier. A longitudinal electric field applied along the central n-doped region heats the electrons in this region and gives them sufficient energy to overcome the barrier and flood into the quantum well and hence recombine with holes which are already present. The wavelength converter device operates with one p-i-n structure forward biased and one reverse biased. The forward biased element has a quantum well positioned near the p-doped region. Light of the appropriate wavelength is absorbed in this quantum well. The holes scatter out of the quantum well and drift into the p- doped region. The electrons are scattered out of the quantum well and drift towards the n-doped region, creating additional carriers through impact ionization, thereby creating gain. The electrons flooding over the n-doped region, must overcome a potential barrier to enter the forward biased element, therefore cold electrons are prevented from entering this region. Electrons which are able to overcome the barrier fall into a quantum well positioned near the barrier, where holes are already waiting, as in the light emitting device.