Very-High Color Rendering Index Hybrid White Organic Light-Emitting Diodes with Double Emitting Nanolayers
A very-high color rendering index white organic light-emitting diode (WOLED) based on a simple structure was successfully fabricated. The optimized device exhibits a maximum total efficiency of 13.1 and 5.4 lm/W at 1,000 cd/m2. A peak color rendering index of 90 and a relatively stable color during a wide range of luminance were obtained. In addition, it was demonstrated that the 4,4′,4″-tri(9-carbazoyl) triphenylamine host influenced strongly the performance of this WOLED. These results may be beneficial to the design of both material and device architecture for high-performance WOLED.
KeywordsWhite light Hybrid Color rendering index Organic light-emitting diodes Double emitting nanolayers
White organic light-emitting diodes (WOLEDs) are considered to be one of the most stormily developing technologies nowadays owing to their potential applications in the display markets and next-generation lighting sources . In terms of the practical use, whereas the color rendering index (CRI) is not a critical parameter for OLEDs in full-color displays, it must be considered in addition to Commission International de l′Eclairage (CIE) when designing WOLEDs for lighting since the CRI gives an indication of how well the light source will render colors of objects it illuminates .
For a source to be human eye-friendly, a WOLED with a CRI above 80 is required . Since D′Andrade et al. took the first step to demonstrate that the CRI of WOLEDs can be greatly enhanced from 50 to 83 , a great deal of attention has been attracted to the improvement of CRI, which is expected to meet the commercial requirements. For instance, Sun et al. developed a hybrid WOLED which combines fluorescent blue emitters with phosphorescent green–red emitters, achieving a CRI of 85 . Park et al. synthesized a yellowish-green dopant to fabricate a three-color phosphorescent WOLED with a CRI of 86.8 . In recent years, in order to satisfy the demand of high-quality lighting systems, such as surgery, photography, and exhibition of museums, WOLEDs with very-high CRI (≥90) are extremely required .
As a matter of fact, a few efforts have been explored to the pursuit of very-high CRI WOLEDs. For example, Wang et al. designed a WOLED using exciplex emission from mixed acceptors, achieving a CRI of 90.4 at 11 V (a maximum brightness of 425 cd/m2) . Chang et al. constructed a phosphorescent WOLED using a four-emitting-layer and double-confining-layer structure, achieving a CRI of 94 and a forward-viewing power efficiency (PE) of 3.8 lm/W at 1,000 cd/m2 . Chen et al. inserted a spacer between the fluorescent emitter and the phosphorescent emitters to realize a hybrid WOLED, achieving a CRI of 91.2 (no efficiency data are reported) . Hao et al. fabricated a WOLED combining emission from excitons and interface-formed electroplex, achieving a CRI of 90.2 at 8 V (a maximum efficiency of 1.39 cd/A) . Jou et al. demonstrated an efficient WOLED using five emitters, achieving a CRI of 96 with 5.2 lm/W at 1,000 cd/m2 . Li et al. proposed a WOLED using a four-emitting-layer architecture, achieving a CRI of 97 with 8.19 lm/W at 1,000 cd/m2 .
Based on these facts [7, 8, 9, 10, 11], it can be concluded that various approaches can be used to develop very-high CRI WOLEDs. However, it is easily noted that these very-high CRI devices show either rather poor luminance/efficiency [7, 10] or somewhat complicated structures [6, 8, 9, 11], which limit their further applications. Therefore, to greatly lower the cost and vastly simplify fabrication processes, very-high CRI WOLEDs with simple structures are urgently needed to meet the future commercial demand. Moreover, negligible report has been documented to illustrate the role of the host in the performance of very-high CRI WOLEDs.
In this paper, we developed a very-high CRI WOLED with a hybrid structure which comprises with only two emitting layers. The resulting device exhibits a maximum total PE of 13.1 and 5.4 lm/W at 1,000 cd/m2. A peak CRI of 90 is obtained, which represents the highest value in hybrid WOLEDs based on the dual-emitting-layer structure, to the best of our knowledge. Besides, a relatively stable color can be obtained during a large range of luminance (1,000–12,000 cd/m2). Moreover, it is demonstrated that the 4,4′,4″-tri(9-carbazoyl) triphenylamine (TCTA) host plays a critical role in realizing such high performance. Such presented results demonstrate that efficient very-high CRI WOLEDs can be effectively realized by using simple structures.
3 Results and Discussion
More remarkably, a very-high CRI of 90 is obtained at both a low luminance (10 cd/m2) and high luminances (4,000–12,000 cd/m2). The device exhibits Duvs of 0.0283, 0.0198, 0.0235, 0.0245, and 0.0238 at 10, 1,000, 4,000, 8,000, and 12,000 cd/m2, respectively. In addition, the combination of high CRI and the special CRI R9, which quantifies the color reproduction of saturated red and should be positive, is very interesting for commercial applications . Our device shows R9 s of 81, 69, 84, 91, and 92 at 10, 1,000, 4,000, 8,000, and 12,000 cd/m2, respectively. Besides, similar to other WOLEDs which show relatively large color-shift at low luminances and relatively slight color-shift at high luminances [12, 20, 21, 22], our device exhibits only a little CIE coordinates variation of Δ(x, y) ≤ (0.03, 0.03) during a wide range of luminance (1,000–12,000 cd/m2), revealing that the device exhibits a relatively stable color .
It is clearly noted that previous reports are usually focused on the effect of dopants in the very-CRI WOLEDs [6, 8, 9, 11], negligible attention is paid to the influence of hosts. Herein, to explore the working mechanism of the simple device and to illustrate the significance of the TCTA host guaranteeing the high performance, we used a well-known bipolar material 4,4-N, N-dicarbazolebiphenyl (CBP) to replace TCTA as the host. The configuration of CBP-based device (W2) is ITO/MeO-TPD: F4-TCNQ (100 nm, 4 %)/NPB (15 nm)/TCTA (5 nm)/CBP: Ir (ppy)3 : Ir(piq)3 (25 nm, 1:9:0.8 %)/MADN: DSA-ph (20 nm, 1 %)/Bepp2 (250 nm)/LiF (1 nm)/Al (200 nm).
On the other hand, it can be easily inferred that a narrow recombination zone of device W1 is formed at the phosphorescent emitting layer/flourescent emitting layer interface due to the existed energy barriers  together with the fact that TCTA is typical hole-type materials [15, 20] and MADN preferentially transports electrons . The narrow recombination zone can cause intensified triplet-triplet and triplet-polaron interactions at high luminances , which not only reduces the efficiency of phosphor-based devices (W1), but also results in an efficiency roll-off of device W1 with increasing luminance [29, 30].
Finally, it should be pointed out that spacers are usually needed to be inserted between the fluorescent- and phosphorescent-emitting layers to realize hybrid WOLEDs, otherwise, no white light can be generated [4, 9, 15, 30, 31]. Herein, we, for the first time, report a very-high CRI hybrid WOLED without a spacer to separate the fluorescent and phosphorescent emitting layers. We hope more efficient very-high CRI devices can be developed in this simple and new way.
In summary, we have successfully demonstrated a very-high CRI WOLED based on double emissive nanolayers. The resulting device exhibits a maximum total efficiency of 13.1 lm/W and an efficiency of 5.4 lm/W at 1,000 cd/m2. A peak CRI of this simple device is as high as 90 and a relatively stable color during a wide range of luminance can be obtained. In addition, it is demonstrated that the TCTA host plays a key role in the performance of this WOLED. Undoubtedly, such achieved results would provide an instructive guide for the rational design of ultra high-performance OLEDs, which will be very advantageous for the commercialization of solid-state lighting in the near future.
The authors are grateful to the National Natural Science Foundation of China (Grant Nos.61204087, 61306099), the Guangdong Natural Science Foundation (Grant No. S2012040007003), China Postdoctoral Science Foundation (2013M531841), the Fundamental Research Funds for the Central Universities (2014ZM0003, 2014ZM0034, 2014ZM0037, 2014ZZ0028) and the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20120172120008).
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