Novel aromatic extended carbazoles as a chemical platform of bipolar hosts for improved lifetime in phosphorescent organic light-emitting diodes

Abstract

Novel aromatic extended carbazoles, 9H-fluoreno[9,1-bc]carbazole, 9H-dibenzo[a,c]carbazole, and 15H-phenanthro[9,10-a]carbazole, were developed as donor moieties constructing bipolar host materials for phosphorescent organic light-emitting diodes. The 9H-fluoreno[9,1-bc]carbazole, 9H-dibenzo[a,c]carbazole, and 15H-phenanthro[9,10-a]carbazole implemented hosts showed high glass transition temperature, bipolar charge transport character, and triplet energy for energy transfer to red phosphorescent emitters. The hosts built on the aromatic extended carbazoles and quinazoline greatly improved the lifetime of red phosphorescent organic light-emitting diodes while enhancing the quantum efficiency. The material characterization data and device analysis results confirmed that the aromatic extended carbazole based host materials are effective to extend the lifetime of the red phosphorescent devices by thermal stability and polaron stability.

Introduction

Carbazole has been dominantly used as the hole transport type building unit of host materials because of high triplet energy, good thermal stability, good electrochemical stability and good hole transport properties. A lot of hole transport type host and bipolar host materials have been designed to have carbazole as a building unit [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Furthermore, many hole transport materials were also developed using the carbazole units [16], [17], [18]. The carbazole derived host or hole transport materials were effective to get high external quantum efficiency (EQE) and stable lifetime in the organic light-emitting diodes (OLEDs) [5], [6], [8], [19], [20], [21].

Although the carbazole based compounds showed high EQE and reasonable lifetime in the OLEDs, carbazole may not be the best hole transport unit for high EQE and lifetime. The device performances can be upgraded if the carbazole unit can be stabilized by chemical modification. One effective way of stabilizing the carbazole unit by chemical change is to extend the aromatic structure of carbazole [22], [23]. For example, benzocarbazole, a derivative of carbazole, which has one additional aromatic unit conjugated to the carbazole unit outperformed carbazole when it was used as the hole transport unit of host materials [24], [25], [26], [27]. It was proven that the extended aromatic structure is helpful to extend the lifetime of OLEDs possessing the carbazole derived hole transport units [24], [26], [27], [28], [29]. Until now, several carbazole derived building units have been reported as aromatic extended carbazole units. However, further investigation of the aromatic extended carbazole units is needed to enhance the lifetime of OLEDs.

In this work, three compounds with different carbazole units as the hole transport unit of bipolar host materials were synthesized and the effect of the carbazole unit on the EQE and lifetime of phosphorescent OLEDs was investigated. 9H-fluoreno[9,1-bc]carbazole (FRC), 9H-dibenzo[a,c]carbazole (DBC), and 15H-phenanthro[9,10-a]carbazole (PNC) were carbazole derived hole transport units and they were merged with a phenylquinazoline unit to produce bipolar host materials. Comparison of the EQE and lifetime revealed that the DBC unit performs better than FRC and PNC units and extension of the aromatic structure is a good approach to reach long lifetime in the OLEDs.