Combined host–guest doping and host-free systems for high-efficiency white organic light-emitting devices

Abstract

Highly efficient white organic light-emitting devices (WOLEDs) with a four-layer structure were realized by utilizing phosphorescent blue and yellow emitters. The key concept of device construction is to combine host–guest doping system of the blue emitting layer (EML) and the host-free system of yellow EML. Two kinds of WOLEDs incorporated with distinct host materials, namely N,N'-dicarbazolyl-3,5-benzene (mCP) and p-bis(triphenylsilyly)benzene (UGH2), were fabricated. Without using light out-coupling technology, a maximum current efficiency (η_C) of 58.8 cd/A and a maximum external quantum efficiency (η_EQE) of 18.77% were obtained for the mCP-based WOLED; while a maximum η_C of 65.3 cd/A and a maximum η_EQE of 19.04% were achieved for the UGH2-based WOLED. Meanwhile, both WOLEDs presented higher performance than that of conventionally full-doping WOLEDs. Furthermore, systematic studies of the high-efficiency WOLEDs were progressed.

Introduction

White organic light-emitting devices (WOLEDs) have attracted a large volume of attention because of their wide applications in displays and lighting sources. To generate white emission, the most efficient approach is to utilize host–guest doping system by mixing three primary colors or two complementary colors into a single- or multiple- stacked emitting layer (EML). As for electroluminescent materials, phosphorescent emitters which can achieve the much desired 100% internal quantum efficiency are widely used, due to the capability of harvesting both singlet and triplet excitons [1], [2]. Nowadays, WOLEDs with high performance have already been achieved, such as high efficiency exceeding 100 lm/W [3], wide color temperature ranging from 2300 K to 8200 K as sunlight-style illumination [4], high color rendering index of 90 together with a long lifetime of 100,000 h [5], and high contrast ratio for full color active matrix displays [6]. Even so, for the development of WOLEDs in the near future, many challenges still lay ahead while no fundamental obstacles are in the way. Among various techniques for fabricating WOLEDs, the most efficient and used way is to adopt the host–guest doping system with a multilayer structure, as the WOLEDs aforementioned. However, it suffers from problems of manufacturing complexity and difficulty. For example, the doping concentration of each emitter in EMLs should be precisely controlled so as to obtain white emission, since natural energy transfer from higher-energy emitters to lower ones occurs. Therefore, the specified doping concentration of red, yellow or even green emitters is usually required to be very low, which makes it tough to guarantee the accuracy of the doping concentration, the usage efficiency of the materials and the reproducibility of the devices. To simplify the fabrication process, non-doped WOLEDs made of host-free EMLs have been investigated [7], [8], and co-evaporation of multiple emitters and hosts can be avoided. Whereas, device performance is strongly susceptible to the thickness of host-free EMLs due to the aggregation-induced quenching effect, resulting from strong intermolecular interaction that is caused by the short distance between the emitter molecules [9].

Taking their own advantages into account, a concept of combining the host–guest doping system with the host-free system into one device has been introduced to construct WOLEDs, which is similar to the delta-doping method reported by Zhao et al. [10]. As a result, this method can yield a modified device that renders simplified structure and high efficiency. So far, the strategy has been mainly employed in fluorescent or fluorescent/phosphorescent hybrid devices [11], [12], where the fluorescent emitters greatly influence the device efficiency. Recently, Lee et al. applied a similar method and reported an all-phosphorescent WOLED with high efficiency [13], by dispersing host-free yellow emitter between two triple-doped blue EMLs. Nevertheless, the fabrication process is still very complex. Hence, it is necessary and promising to perform more studies on simplified WOLEDs with high efficiency.

In this work, we propose an easily realizable strategy to fabricate highly efficient WOLEDs consisting of phosphorescent blue–yellow emitters, by combining the host–guest doping system of blue EML with the host-free system of yellow EML into one device. Two host materials, N,N'-dicarbazolyl-3,5-benzene (mCP) and p-bis(triphenylsilyly)benzene (UGH2), were selected as p-host and n-host for the blue EML, respectively. Additionally, conventional WOLEDs using the full-doping system were also constructed for comparison. Furthermore, the reasons for high-efficiency of WOLEDs which employed the host-free yellow EML were also analyzed.