Host Structure Engineering in Organic Light-Emitting Diodes for Improved Stability and Efficiency

Abstract

Organic light-emitting Diodes have been adopted multi-layer structure to improve performances. All the organic materials in each layer need to be chosen appropriately to achieve high charge balance factor, especially for the host material in emissive layer. The host material and structure plays an important role because it controls charge balance and recombination distribution, energy transfer to dopant. Herein, the effect of exciplex forming host structure on organic light-emitting .`diodes (OLEDs) has investigated. The exciplex formation was observed by mixing two organic materials with strong hole transporting and strong electron transporting properties. Two different types of emitters are used to confirm the effect of exciplex type host structure, the one is green phosphorescent dopant of iridium(III)tris(2-phenylpyridine) (Ir(ppy)3) and the other one is green thermally activated delayed fluorescent dopant of (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN). Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and 3',5'-Di(carbazol-9-yl)-[1,1'-biphenyl]-3,5-dicarbonitrile (DCzDCN) are used for exciplex forming host structure in mixing ratio of 50:50. For the 4CzIPN emitter, additional exciplex type host structure with 1,3-Bis(N-carbazolyl)benzene (mCP) and DCzDCN is carried out. The green phosphorescent device with exciplex forming host structure exhibited improved carrier injection property and charge balance, leading to higher power efficiency and longer lifetime. The green thermally activated delayed fluorescent device with exciplex type host structure also showed longer lifetime.