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. • Surface morphology variations with respect to temperature in the TEOLEDs. • The variation in optical and geometrical properties of organic layers depending on the thickness. • CBP, TCTA and NPB showing notable damage to the properties by thermal exposure. • The color purity of TEOLEDs was affected by micro-cavity after thermal exposure.