Samsung Advanced Institute of Technology

Ever since the quantum confinement effect of semiconductor materials in nanoscale dimension was discovered, so called quantum dot (QD) materials have attracted much attention from academia as well as industries for potential practical applications such as displays, bio sensors, solar cells, photo sensitive absorbing layers, and multi-level channel materials for semiconductor devices. Especially, QD has evident advantages as an emitter for display applications compared to conventional technologies; 1) flexible band gap engineering by tuning size and composition, 2) direct energy transition at band edge with high density, 3) excellent color gamut based on narrow spectral width, 4) cost efficient and scalable synthetic process, 5) minimum light scattering effect at visible region because of the small size, and 6) compatibility with conventional device fabrication processes. Based on these merits, two types of QD displays - color converting and direct electrical driving - have been developed in general. A color converting QD device using its photoluminescent (PL) properties already appeared in various forms, and the prototype of QD backlight proved its potential with much improved efficiency and stability. After the first commercial QD TV products released in 2013, vast line-ups have been manufactured by various companies adopting QD color converting films for better color gamut and high efficiency. Recently, a patterned QD color converting layer to replace the conventional color filter has been newly employed to further enhance the contrast ratio and viewing angle of displays.

QD light emitting diode (QD-LED or QLED) is another class of display using electroluminescence (EL) of QD through the recombination of electrons and holes at the QD emitter. QD-LED has been known as an ultimate ideal display because of 1) high efficiency (no separate backlight), 2) perfect contrast ratio fulfilled by pixel operation, 3) very thin (sub-micron) device structure to be a slim module, 4) low cost solution process, 5) scalability for large size screen, 6) potential use of flexible substrate, as well as the original advantages of QD as a light emitter. These values make researchers and engineers continue to pay attention to QD-LED technology. In this review article, important progresses in the development of QD-LEDs are summarized and the fundamental understandings are disclosed to predict the direction of the technology and further practical advancements in the future. In addition, the strength of QD-LED is re-examined by comparing competing technologies, and basic elements - emitters and other functional layers - for the improvement of the device performance are investigated in detail. Finally, the currently facing technological limits and diverse approaches to address these issues are discussed.