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The development of white organic light emitting diodes (WOLEDs) holds great promise since they combine the potentialities of high efficiency and inexpensive production with the appealing features of high-color-quality white light. Nevertheless, reliability and color quality still need to be optimized to make WOLEDs commercially competitive as alternative lighting sources. Development of stable and high color-rendering index emitters plays a key role in the progress of WOLED manufacturing. In the paper, we have investigated several approaches to achieving high color-rendering WOLEDs for solid-state lighting. A light-emitting layer (EML) of WOLED, consisting of 8 wt% Pt-4-doped 1,3-bis(9-carbazolyl) benzene (mCP), was very stable and shifted slightly from (0.312, 0.398) at 1000 cd m to (0.320, 0.395) at 9000 cd m. Introduction The concept of organic electroluminescent (EL) can be traced back to its history since 1987, when Dr. C. W. Tang fabricated the first efficient organic devices from aluminum tris(8-quinolinolato) (Alq3) [1]. Since then, more and more organic compounds were adopted for lighting applications [2, 3]. One device on the cusp of widespread use is the organic light-emitting diodes (OLEDs) as they are low-cost, easily processable, lightweight and compatible with flexible substrates [4, 5]. Research over the last decade has paved the way for the implementation of efficient red, green, and blue OLEDs in passive and active matrix displays [6-8]. At the meanwhile, many efforts have been paid to the influencing mechanism of the light emission property, since the band structure could be modulated by interface strain or phase transition[9-10]. Interest in the application of white organic light-emitting diode (WOLED) technology are of great importance, not only because WOLED are highly efficient and have longer lifetime than incandescent bulbs, but particularly because they have the possession of high-quality white-light illumination [11-13]. There are two critical parameters that define the color quality of a white light source: the Commision Internationale de l’Éclairage (CIE) chromaticity coordinates [14], and the color-rendering index (CRI) [15]. In order for light to be perceived as white by the human eye, two important CIE standard illuminants are “colorless” white light [corresponding to CIE coordinates of (0.33, 0.33)] and warm light [corresponding to CIE coordinates of (0.448, 0.408)], respectively [16]. Equally important as its chromaticity is the ability of a light source to reproduce the color of objects. The so-called CRI, which is a dimensionless value ranging from 0 to 100, usually serves as a measure for this property. A CRI ≥ 80 is required for indoor-lighting applications [16, 17]. To meet these requirements, a lot of efforts have been devoted to fabricate WOLED. For example, Kido et al. reported white light-emitting organic electroluminescent devices using the poly (N-vinylcarbazole) emitter layer doped with three fluorescent dyes. White emission covers a wide range of the visible region [18]. Wang et al. reported a WOLED with CIE coordinates of (0.38, 0.43) by co-doping blue 0546 4th Annual International Conference on Material Science and Engineering (ICMSE 2016) © 2016. The authors – Published by Atlantis Press and orange dyes into a single host material [19]. Jabbour et al. reported an excimer-based WOLED with a CRI of 81 and CIE coordinates of (0.40, 0.43) at 500 cd m by doping a platinum (II) complex into a novel host material [20]. In the following sections we review some current developments in the exciting field of WOLEDs, with a special focus on approaches to the emission color and high CRI WOLED manufacturing. This review begins by discussing different color-conversion approaches to generate white light with organic semiconductors (Section 2). We then discuss recent trends in the development of excimer-based WOLEDs (Section 3). Different device configurations, in particular three primary colors (R, G, and B) based techniques, are introduced and some new concepts developed will be briefly reviewed (Section 4). The review closes with a brief overview of the high CRI of multiple peak emissions for WOLEDs. 1 Advanced Color Conversion Method As a source of white light, an ideal WOLED should emit a continuous spectrum covering the entire visible range (400-800 nm) and should suppress the color shift. To date, extensive studies have been carried out on the fabrication of WOLEDs by various approaches such as a single emissive layer structure doped with different fluorescent material [21, 22], stacked multi-emissive layers structure in which each layer emits different colors to generate white emission [23, 24], a stacked structure WOLED [25] and a tandem cell structure [26]. To avoid chromatic instability from the emissive layers, a simple method for fabricating WOLEDs has been developed by using color conversion materials (CCMs) on the inverse glass side to a blue-emitting device [27]. Unfortunately, this approach sets limits in high-efficiency and high-purity WOLEDs, particularly for the application in full-color flat-panel displays. Very recently, a simple solution process has been used for the fabrication of a chromatic-stable WOLED, in which a red fluorescent polymer poly(2-methoxy-5-[2′-ethylhexyloxy]-pphenylenevinylene (MEH-PPV) was developed to function as both a color conversion layer (CCL) and a hole injection layer (HIL) into the device [28]. Fig. 1. (a) EL spectra of down-conversion CC-HIL WOLED and CCM-WOLED. (b) Current density-voltage characteristics of down-conversion CC-HIL WOLED and CCM-WOLED. The EL spectra of CC-HIL device and CCM device at a current density of 100 mA cm are illustrated in figure 1, corresponding to CIE coordinates of (0.329, 0.347) and (0.297, 0.304), respectively. The total output emission spectrum is a mixture of the blue EL and red PL. Compared with CCM device, the red EL intensity of CC-HIL intensity is slightly higher. It is apparent that the CC-HIL device exhibits a better performance, which should be ascribed to the smoother hole injection and enhanced light absorption efficiency. The color conversion HIL can effectively absorb the ITO/organic-guided modes and convert into low-energy PL emission. Regardless of the specific scheme, CC-HIL approach is generally considered to offer a higher chromatic-stable and simpler device fabrication than other approaches described below. Nevertheless, fabricating organic layer from solution is easily dissolved when the subsequent layer is deposited. As 0547 4th Annual International Conference on Material Science and Engineering (ICMSE 2016) © 2016. The authors – Published by Atlantis Press discussed below, if all components required to generate white light are mixed into one material, one can fabricate a WOLED with a single active layer that is sandwiched between electrodes. Thus reduces the number of processing steps for device and allows straightforward deposition of the active layer using conventional vacuum thermal evaporation processes. 2 Excimer-Based Single Molecular The major disadvantage associated with the single-layer concept is that one has to compromise on certain material characteristics in order to be able to combine different functionalities in a single material [15]. One promising approach to reducing the number of dopants and structural heterogeneities inherent in the multiple layer architecture is to employ a lumophore that forms a broadly emitting exciplex [10]. The excimer is only bound in the excited state and has no ground absorption, hence providing a unique solution to achieving efficient energy transfer from the host to the light-emitting centers. [29]. For example, platinum [1,3-difluoro-4,6-di(2-pyridinyl) benzene] chloride (Pt-4), produces light blue-monomer and red-excimer emission in both PL and EL. However, square-planar Pt complexes are known to form excimers when positioned in close proximity in concentrated solutions and thin films [30-32]. Fig. 2. Changes in CIE coordinates (X, Y) of the WOLED with luminance. The inset is the normalized EL spectra of the WOLED at different current densities. A key feature of the device in Fig. 2 is the light-emitting layer (EML), consisting of 8 wt% Pt-4-doped 1,3-bis(9-carbazolyl) benzene (mCP). It was quite stable and shifted slightly from (0.312, 0.398) at 1000 cd m to (0.320, 0.395) at 9000 cd m. This indicated that the energy transfer process between the monomer and excimer molecules was highly prevented as there was no ground state absorption in the excimer molecules [33]. The inset made the normalized EL spectra of WOLED at different current densities. The excimer emission was almost unchanged with respect to the monomer emission with increasing current intensity. 3 Standard RGB Configuration Given the problem of luminance efficiency (LE) and other challenges such as charge balance and efficiency roll off, CRI of excimer-based single molecular WOLED architectures remains below values required for solid-state lighting, i.e., below 80. Currently, intense research interest is still focused on the classic approach to fabricating WOLED by employing multi-emission layer structures in which two complementary colors or three primary colors (blue, green, and red) are emitted from different organic layers. Recent results have shown some WOLEDs employing ultra-thin EMLs [34-36]. They have successfully reduced efficiency roll-off based on a carrier and exciton-confining structure, and their 0548 4th Annual International Conference on Material Science and Engineering (ICMSE 2016) © 2016. The authors – Published by Atlantis Press contribution to the efficiency and stability of WOLEDs. However, due to the sensitive nature of ultra-thin layers, a slight variation in them can bring about obvious effect on the performance of WOLED. Therefore, Yu et al. varied the thickness ratio between ultra-thin red and green EMLs, keeping total thickness of two

High Color-Rendering White Organic Light-Emitting Diodes for the Application of Lightening