Wide Color Gamut Deep-Blue OLED Architecture for Display Application

Over the last 75 years, there have been four display technologies that have been scientifically advanced and frequently used in the production of direct-view televisions (TVs). They are, namely, cathode ray tube (CRT), liquid crystal display (LCD), Plasma, and organic light emitting diode (OLED), in historical order [1]. Amongst, OLED has made a melodramatic entrance for smartphone and TVs displays. For customers, the new OLED TVs are the natural beneficiaries to substitute their earlier or aging Plasma and CRT TVs because they offer even better Black Levels and Infinite Contrast Ratios for truthfully replicating dark picture content [1]. Additionally, with the goal of enhancing the realism, a wider color gamut is of vital importance to all the display devices. OLEDs with a high color-saturation emerge as full-color displays, having the advantage of a high yield in mass production for large-size displays. International Telecommunication Union (ITU) proposed several standards such as the Adobe RGB [2], National Television System Committee (NTSC) [3], SMPTE RP 431-2 for digital cameras [4], High Definition Television (HDTV) ITU-R BT.709-5 [5], and Rec. 2020 for ultra-high definition (UHD) TVs [6]. An essential requirement for display devices is an accurate representation of color. For an emissive OLED display, it can be achieved by optimizing the device architecture with an engineered molecules to generate more saturated red, green and blue (RGB) primary colors, hence a wider color gamut display. Here, we demonstrate a wide color gamut, deep-blue OLED for the next generation high quality full-panel displays based on the lateral introduction of the cyano group on a local excited (LE) chromophoric axis, which significantly affects the nature of the excited state and leads to the optimal hybridization local and charge transfer states (HCLT) [7]. Most notably, OLED based on the HLCT molecule (C4) showed more than two-fold increment in the external quantum efficiency (EQE) and uncompromised color purity as compared to molecule (C1), which lacks cyano group and possesses LE state. The engineered molecular design features cross-conjugated localized and charge-transfer chromophores which result in a hybridized local charge transfer (HLCT) excited state. An OLED using this material presented high an external quantum efficiency (6.5%) with excellent color saturation (CIEy ~ 0.058) and small full-width at half maximum (48 nm). Moreover, the resultant deep blue emission achieved 99, 101, 101, 100, and 96 % color saturation, respectively, compared with the sRGB, NTSC, Adobe RGB, HDTV ITU-R BT.709-5, and Rec. 2020 standards. To the best of our knowledge, this is the first example of phenathroimidazole free HLCT-based deep-blue emitter exhibiting efficient performance comparable to the deep-blue emitting dopants known in the literature [8-9]. Furthermore, we anticipate that the proposed deep-blue emitter and OLED based on it could serve as a promising candidate in high-quality, wide color gamut, energy-saving, display and solid-state lighting panel fabrication, coupled with the cost-effective solution process. Reference [1] http://www.lg.com/sa_en/lgoled/sub/brandstory/brandstory.html [2] National Television system committee, Proceedings of the IRE, https://doi.org/10.1109/JRPROC.1954.274601 [3] Adobe Systems Inc., “Adobe RGB (1998) color image encoding,” 2005. [4] SMPTE RP 431–2, “D-cinema quality — reference projector and environment,” 2011. [5] ITU-R Recommendation BT.709–5, “Parameter values for the HDTV standards for production and internationalprogramme exchange,” 2002. [6] ITU-R Recommendation BT.2020, “Parameter values for ultra-high definition television systems for productionand international programme exchange,” 2012. [7] K. Konidena, K. R. J. Thomas, D. K. Dubey, S. Sahoo, and J. H. Jou, Chem. Commun., 2017, 53, 11802--11805 [8] C. Chen, X. Yuan, S. F. Ni, Q. X. Tong, F. L. Wong and C.S. Lee, Chem. Sci., 2017, 8, 3599. [9] Liu, Z. Yu, D. He, Z. Zhu, Zheng, Y. Yu, W. Xie, Q. X. Tong and C. S. Lee, J. Mater. Chem. C, 2017, 5, 5402. Figure 1. NTSC color saturation of the fabricated deep-blue OLEDs having device structure of ITO/PEDOT:PSS/CBP:Blue dye/TPBi/LiF/Al. (a) Neat film, (b) 1 wt. %, (c) 3 wt.%, (d) 5 wt.% blue dye having a cyano group on an its local excited chromophoric axis doped in CBP host. Figure 1