Degradation Analysis\nof Exciplex-Based Organic Light-Emitting\nDevices Using Carbazole-Based Materials

A spectral shift and new emission bands in the green\nand red regions\nhave been observed in deep blue exciplex-based organic light-emitting\ndiodes (OLEDs) using carbazole-based materials, namely, tris(4-carbazoyl-9-ylphenyl)amine\n(TCTA). To deeply understand the origin of these new bands, single-layer\nand bilayer TCTA-based OLEDs subjected to electrical and optical (ultraviolet\n(UV)) stresses were investigated by using various optical, electrical,\nmorphological, and chemical measurements. The results showed that\nthe stress-induced emission bands primarily originate from morphological\nchanges rather than chemical changes. The accumulation of excitons\nin the TCTA layer induces molecular aggregation, leading to the formation\nof electrically active electronic states, namely, electroplexes and\nelectromers, which lead to the appearance of additional emission bands\nin green and red regions. Impedance spectroscopy measurements on single-layer\nOLEDs complemented this study. The results showed that TCTA degradation\naffects charge injection and transport. It was concluded that the\nstress-induced emission bands are caused by aggregate domain formation\nand are closely linked to the formation of electrically active defects,\nwhich act as trap states for charge carriers in the TCTA band gap.