Jou et al., J Nanomater Mol Nanotechnol 2018, 7:1 DOI: 10.4172/2324-8777.1000234 Journal of Nanomaterials & Molecular Nanotechnology Research Article a SciTechnol journal high contrast, high response speed, low power consumption, simple Nano-Structures Enabling fabrication, ultra-thin structure, light-weight, flexibility, and low Sunlight and Candlelight-Style cost. [5]. Numerous architectural and fabrication approaches have been introduced to develop a high energy-efficiency and long lifetime OLEDs OLED devices [3,6]. Over the past few years, OLEDs started to replace the conventional and power consuming light sources, incandescent Jwo-Huei Jou*, Sujith Sudheendran Swayamprabha, Rohit bulbs and fluorescent lights which are non eco-friendly, contain Ashok Kumar Yadav and Deepak Kumar Dubey harmful mercury, and non-disposable. While, OLEDs are self- illuminating, power efficient, and disposable. Color and color temperature (CT) of light show a strong influence Abstract on human physiology and psychology [7]. Long time exposure to light Nano structures enable organic light-emitting diode (OLED) devic- with high color temperature and short wavelength leads to different es to be fabricated with relatively high efficiency and brightness, types of cancer [8]. A good lighting improves alertness, psychomotor opening up a new era for high quality displays and lighting, wherein vigilance, task performance, morning cortisol level, and sleep quantity devising a pseudo-natural light is always a must. The uses of in- [9]. Jou’s group contributed remarkable effort in the designing and candescent bulbs are the most friendly, electricity-driven lighting sources, lighting measure from the perspectives of human eye pro- development of human-friendly light [10-19]. The same group tection, melatonin generation, artifacts, ecosystems, the environ- reported both candlelight- and sunlight-style OLED with color- ment, and the night skies due to their intrinsically low blue emission. temperature and chromaticity similar to that of the natural lighting However, they are phasing out because of the energy wasting. To source [10-19]. OLED succeeds to make light free from hazardous overcome these difficulties, researchers are focusing on develop- violet and ultraviolet emission, scorching infrared, unpleasant glare, ing a new light with high efficiency, whose emission spectra would toxic mercury, and importantly blue hazard [16-19], also succeed to also match with those of the natural lights. In 2009, Jou’s group in- fabricate OLED devices with sunlight style chromaticity and color vented the world’s first electrically powered sunlight-style OLED that temperature [10-15]. Combined with high energy-efficiency and high yielded a sunlight-style illumination with various daylight chroma- ticities, whose color temperature ranges between 2,300 and 8,200 light-quality, all these justifies OLED to be a healthy and good light K, fully covering those of the entire daylight at different times and for lighting to safeguard human health and minimize light pollution. regions, and contributed a noteworthy incentive to OLED technol- Types of Natural Light ogy in general lighting. By putting more efforts on this technology, a blue hazard free, low color temperature candlelight-style OLED Natural light includes the light from the sun, stars, fires, bugs, was developed by employing candlelight complementary emitters, etc. Even though light from the moon is reflected light from the namely orange-red, yellow, green, and sky-blue. The resultant can- sun, which provides a significant luminance at the full moon. Some dlelight OLED that exhibits a 1,900 K color temperature is exactly glow worms, special kinds of fish, and mushrooms produce light like candles or oil lamps, which is friendly to human eyes, physi- ologies, ecosystems, artifacts, and night-skies. Specifically, it is at via bio-luminescence [20-27]. Sunlight, light from wood or fires least 10 times safer from the retina protection perspective or 5 times generate a smooth and continuous spectrum over the entire visible better for melatonin to naturally occur after dusk, as compared with range, perfectly matching that of the blackbody radiation at the same the blue light-enriched white OLED, LED and CFL counterparts. In corresponding color temperature. this article, we discuss the device structure, physics, and engineer- ing behind the serendipity of the pseudo-natural light-style OLEDs. Nature and importance of sunlight Keywords Color temperature of sunlight varies with the time, region, and Candlelight-style; OLED; Sunlight-style weather. It is 2,500 K at dawn, 3,250 K at dusk, 5,500 K at noon, or 8,000 K at noon in high-latitude country, and 6,500 K on a cloudy day as shown in Figure 1. High CT induce the production of cortisol, Introduction which can enhance the alertness and productive work. The low CT, The new era of an organic light-emitting diodes (OLEDs) begin dusk hue emission is physiologically friendly with the most attractive with the invention by Tang and Van Slyke in 1987 [1]. OLEDs are chromaticity. It influences the migration of birds, the bird’s singing at progressively attracting much attention because of their potential dusk and the mating of crabs and moths [28-34]. The peoples living in applications and superior performance in the high-quality flat- northern countries face melancholy or even suicide due to the lack of panel displays, lighting, and optoelectronics [1-4]. In comparison sunlight during the long winter [7]. to general lighting and display sources, OLEDs have prominent Nature and importance of candlelight features like high color purity, high luminance, wide viewing angle, Candles are physiologically friendly light source due to the low color temperature and less blue emission. Candlelight exhibits a CT *Corresponding author: Jwo-Huei Jou, Department of Materials Science around 1,900 K, which can create a romantic atmosphere during and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, Tel: +886(3)574-2618; E-mail: [email protected] dinner time. This pleasant sensation may be due to the production of melatonin, which regulates the sleep, wakefulness and helps people Received: November 01, 2017 Accepted: January 06, 2018 Published: January relax. The main drawbacks of conventional candlelight are energy 11, 2018 All articles published in Journal of Nanomaterials & Molecular Nanotechnology are the property of SciTechnol, and is protected International Publisher of Science, by copyright laws. Copyright © 2018, SciTechnol, All Rights Reserved. Technology and Medicine Citation: Jou J, Swayamprabha SS, Yadav RAK, Dubey DK (2018) Nano-Structures Enabling Sunlight and Candlelight-Style OLEDs. J Nanomater Mol Nanotechnol 7:1. doi: 10.4172/2324-8777.1000234 Melatonin suppression sensitivity (MSS) One method of expression of melatonin suppression sensitivity is in photon quanta [39] (λλrC− )/ SPQ =10 (3) Where, SPQ suppression power per photon quanta of a given monochromatic light, λ relative to that of the reference light, λr, and C is a fitting constant. Another method of expression of melatonin suppression sensitivity in lux (lx), for general illumination purpose. Therefore the above formula becomes [39], λλS () PQ (4) Figure 1: Color temperature of sunlight at different regions, times, and SL ()λ = weathers [11]. V ()λ Where, SL(λ) is the action spectrum of MLT suppression per lux, V(λ) is the photopic luminosity function, which converts photo quanta into illuminance in terms of lx. For polychromatic light, the following expression is used. λSPQ() λλ Sd I () λ λ S ()λ = ∫ (5) LC λλ λ λ ∫V() SdI () Where, SLC (λ) is the correlated suppression power per lux and SI(λ) is the power spectrum of the studied light. Sunlight OLEDs Figure 2: Sunlight-style OLED with a) single carrier modulation layer and b) double carrier modulation layers [10,11]. The color temperature of light influences the human health and work performance. Drowsiness has been observed under low color waste (0.1-0.3 lm/W), flickering, glare, obtrusiveness, burning problems, temperature lighting. To have high-quality lighting, emission with a potential fire hazards, high heat due to intens infrared radiation, and daylight chromaticity and a color temperature span covering that of unpleasant smoke due to incomplete burning. Indoor applications of sunlight is essential, especially considering its strong effect on human candles may consume oxygen and generate greenhouse gases [35]. psychology. To have a high-quality sunlight-style illumination, Theoretical emission with daylight chromaticity and a wide color-temperature span from 2,000 to 6,000 K is necessary. A good lighting can improve Spectral resemblance index (SRI) alertness, psychomotor vigilance, task performance, morning cortisol level and sleep quantity. In 2009, Jou’s group reported the first single Jou’s group introduced that the spectral resemblance index for device with sunlight-style emission [10]. calculating the light quality of a given light [36]. The calculation is based on the direct comparison of a light source’s lumen spectrum Device architecture with the natural light counterpart having the same color temperature. The sunlight-style OLED has a relatively simple design, consisting L(,λλ Td ) of various color-emitting materials, electron transporting layer, carrier SRI = ∫ ×100% (1) λλ modulation layer, and electron injection layer with few nanometers of ∫ LBR (, Td ) thickness. Jou’s group examined the influence of both single and double where