Rohit Ashok Kumar Yadav

Nano-Structures Enabling Sunlight and Candlelight-Style OLEDs

Nano structures enable organic light-emitting diode (OLED) devices to be fabricated with relatively high efficiency and brightness, opening up a new era for high quality displays and lighting, wherein devising a pseudo-natural light is always a must. The uses of incandescent bulbs are the most friendly, electricity-driven lighting sources, lighting measure from the perspectives of human eye protection, melatonin generation, artifacts, ecosystems, the environment, and the night skies due to their intrinsically low blue emission. However, they are phasing out because of the energy wasting. To overcome these difficulties, researchers are focusing on developing a new light with high efficiency, whose emission spectra would also match with those of the natural lights. In 2009, Jou’s group invented the world’s first electrically powered sunlight-style OLED that yielded a sunlight-style illumination with various daylight chromaticities, whose color temperature ranges between 2,300 and 8,200 K, fully covering those of the entire daylight at different times and regions, and contributed a noteworthy incentive to OLED technology in general lighting. By putting more efforts on this technology, a blue hazard free, low color temperature candlelight-style OLED was developed by employing candlelight complementary emitters, namely orange-red, yellow, green, and sky-blue. The resultant candlelight OLED that exhibits a 1,900 K color temperature is exactly like candles or oil lamps, which is friendly to human eyes, physiologies, ecosystems, artifacts, and night-skies. Specifically, it is at least 10 times safer from the retina protection perspective or 5 times better for melatonin to naturally occur after dusk, as compared with the blue light-enriched white OLED, LED and CFL counterparts. In this article, we discuss the device structure, physics, and engineering behind the serendipity of the pseudo-natural light-style OLEDs.

An Approach for Measuring the Dielectric Strength of OLED Materials

Surface roughness of electrodes plays a key role in the dielectric breakdown of thin-film organic devices. The rate of breakdown will increase when there are stochastic sharp spikes on the surface of electrodes. Additionally, surface having spiking morphology makes the determination of dielectric strength very challenging, specifically when the layer is relatively thin. We demonstrate here a new approach to investigate the dielectric strength of organic thin films for organic light-emitting diodes (OLEDs). The thin films were deposited on a substrate using physical vapor deposition (PVD) under high vacuum. The device architectures used were glass substrate/indium tin oxide (ITO)/organic material/aluminum (Al) and glass substrate/Al/organic material/Al. The dielectric strength of the OLED materials was evaluated from the measured breakdown voltage and layer thickness.

Molecule based monochromatic and polychromatic OLEDs with wet process feasibility

Wet-process enables organic light-emitting diodes (OLEDs) to be made cost-effectively via a continuous process, such as roll-to-roll manufacturing. Initially, wet-process based OLEDs used to be fabricated with polymer-based emitters and/or host materials. However, small molecules that can be processed using a wet-process are more promising for commercialization due to easier control over their molecular weight, purification, solution viscosity, layer thickness, and uniformity. We have hence reviewed herein (i) what constitutes the essential materials for wet-process feasible OLEDs, (ii) applicable wet-process technologies, and (iii) reported wet-processed mono- and polychromatic OLED devices with sound efficiency performance. And last, but not least, we have pointed out the most critical challenges that wet-processed electronics including OLED must face before gaining sufficient ground for competition and turning into disruptive technology. These include issues such as multiple-layer feasibility, film integrity, film forming capability, and device lifetime.