Organic Light-emitting Device Performance Optimization And Exciton For Nonproliferation Studies

The thesis is focused on improving the performance of OLEDs and investigating the exciton diffusion process in organic thin films. The four problems discussed are listed below:1．High-contrast organic light-emitting devices. High-contrast OLEDs with low-reflection cathodes are fabricated. The cathode consists of a semi-transparent metal layer, a phase-changing (PC) layer, and a reflective metal layer. With Al doped Alq3 as PC layer, devices exhibit the average reflectivity of the ambient light as low as about 13%. And its electrical characteristics are almost identical to that of a conventional device, although the thickness is increased by 70%. The improvement in conductivity could be attributed to the conductive Al cluster distributed in the organic matrix.2．Optimize the thickness of hole transport layer in doped OLEDs. Current-voltage (Ⅰ-Ⅴ) and electroluminescence (EL) characteristics of OLEDs with NPB of various thicknesses as hole transport layer and Alq3 selectively doped with DCM as electron transport layer have been investigated. A trapped charge induced band bend model is proposed to understand theⅠ-Ⅴcharacteristics. It is suggested that space charge changes the injection barrier and therefore influences the electron injection process in addition to the carrier transport process. Enhanced external quantum efficiency of the devices due to the electron blocking effect of an inserted NPB layer is observed. The optimal thickness of NPB layer is experimentally determined to be 12±3 nm in doped devices, a value different from that of undoped devices, which is attributed to the electron trap effect of DCM molecules. This is consistent with the result that the proportion of Alq3 luminescence in total EL spectra increases with NPB thickness up to 12 nm under a fixed bias. 3．Singlet exciton diffusion in organic thin films. Limitations of the analytical method for calculating the exciton distribution in organic thin films, attributed to the improper boundary conditions when the organic film approaches the exciton diffusion length, were analyzed by comparison with an exciton random walk simulation. The random walk simulation results are in better agreement with in situ photoluminescence (PL) measurements than predictions based on the one-dimensional (1D) diffusion equation, especially for thin films (< 15 nm). The three-dimensional (3D) exciton diffusion length in Alq3 is determined to be 26 nm, equivalent to 15 nm upon projection to 1D. The result is not sensitive to the molecular size, a parameter arbitrarily set in the simulation. In addition, the exciton distribution in operating organic light emitting devices (OLEDs) was also simulated.4．Triplet exciton diffusion in organic thin films. Measuring the luminescence of inserted sensing layers with high phosphorescent efficiency is an effective way to investigate the triplet exciton diffusion in non-emitting host materials. Researchers used to adopt simple models without taking the influence of doped layers into account. In this letter, to include the dopant effects, a "double-layer" model is proposed for describing the exciton diffusion in doped and pure layers separately. The triplet diffusion length in Ir(ppy)3-doped and pure CBP films are calculated to be 6．8 nm and 61．4 nm, respectively.