Analysis of device performance and thin-film properties of thermally damaged organic light-emitting diodes

Abstract

This paper reports the variation in the optical and geometrical properties of individual organic layers to be used for thermally damaged top-emission organic light-emitting diodes (TEOLEDs). The copper deposited on the back of TEOLEDs is employed as a thermal facilitator, and a certain thermal damage occurs to the organic layers and devices. The phosphorescent host material 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) is rapidly damaged to a significant extent owing to the low glass transition temperature (T-g), which also changes its optical and geometrical surface properties. Although the optical properties of the hole transport layer, N,N'-di(1-naphthyl)-N,N-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB) were changed slightly, the surface morphology was changed significantly. Despite having a higher T-g, the exciton blocking layer, tris(4-carbazoyl-9-ylphenyl)amine (TCTA), shows notable variations in optical properties and surface morphology due to heat exposure. Surprisingly, the electroluminescence spectra and micro-cavity are affected by increasing temperature without any considerable changes in device performance. Hence, this study reveals that besides T-g, the surface morphologies and thicknesses of the organic layers are also important factors in the annealing process and play a vital role in causing thermal damage to TEOLEDs.