Introduction
editLEDs (Light Emitting Diodes) are semiconductor light sources that give off light in the form of photons when a current is passed through them. They are far smaller and more energy efficient than their LCD (Liquid Crystal Display) counterparts. In the past two decades, advancements have been made in developing OLEDs (Organic Light Emitting Diodes) which utilize organic compounds that allow for increased energy efficiency as well as device flexibility. A Blue OLED is simply an OLED that is able to emit blue light.
Blue OLEDs emit light via phosphorescent or fluorescent materials, and each type carries its own advantages and disadvantages. The most efficient lighting comes from phosphorescent material, but these materials tend to have a short lifespan. Fluorescent materials experience greater longevity but are typically less efficient.[1][2]
Basic Principle
editOLEDs typically consist of five layers (respectively): the substrate, the anode, the conductive layer, the emissive layer, and the cathode. As electrical current is applied, electrons flow from the cathode (electron injector) to the anode (electron remover) through the emissive layer and conductive layer. The emissive layer, made of organic plastic molecules, is responsible for transporting electrons from the cathode while the conductive layer, made of some different organic plastic than the emissive layer, is responsible for transporting electron holes from the anode. When the electrons and electron holes meet up at the boundary between the emissive and conductive layers, energy is released in the form of photons (light). The color of the light is dependent upon the type of molecule used in the emissive layer.[3][4]
For blue OLEDs, Polyfluorene derivatives are most commonly found in the emissive layer. Compared to other polymers, polyfluorenes offer high thermal stability, good solubility, and the capacity to emit light across the entire visible spectrum. This makes polyfluorenes one of the only conjugated polymers capable of emitting blue light.[5]
Challenges Associated with Blue OLEDs
editBlue OLEDs are significantly outperformed by their red and green equivalents. The majority of current blue OLEDs have issues with lifetime, efficiency, and color purity. Blue Polyfluorene emitters degrade at a faster rate than that of any other color polyfluorene emitter resulting in a color balance issue. The degradation is accelerated because of the low luminous efficacy associated with blue light. In order to produce blue light at an intensity similar to that of red or green, a higher current is required. This higher current initiates higher levels of excimer and aggregate formation in the polyfluorene, diminishing the polyfuorene’s ability to complete charge carrier recombination while concurrently causing it to emit longer wavelengths. The process described above similarly describes why blue OLEDs are inefficient, but it’s also important to note that the low luminous efficacy of blue light also contributes to its poor efficacy. For example, putting equivalent amounts of energy into a blue and red LED will result in a red LED that is perceived to be brighter.[6][7]
Active Work
editThe majority of blue OLED research is focused on improving the efficiency of polyfluorene emitters as well as discovering novel phosphorescent materials that exhibit increased stability; however, the development of better organic emitters is not the only way to improve blue OLEDs. Recently, researchers have achieved a 100% increase in efficiency (surpassing the 5% theoretical limit) using a common blue fluorescent emitter. This was accomplished by carefully optimizing the OLED device to best accommodate the electron-hole recombination in the emissive layer.[8]
Significance of Blue OLEDs
editBlue OLEDs play an important role in producing the colors seen on OLED screens. Manufacturers such as Samsung, Sony, HTC, Motorola and DuPont use OLED screens in a wide variety of consumer electronics. Many of these displays use RGB color, meaning that all colors seen on the screen are created with varying combinations of red, green, and blue. When all three colors are combined, white is displayed. Without blue OLEDs, many colors, including white, would not be possible.
References
edit- ^ True blue light emission could make oled displays the next big thing in consumer electronics. (2011, June 24). Retrieved from http://phys.org/news/2011-06-true-blue-emission-oled-big.html
- ^ Chang-Gua, Z. et al. (2009). Fluorene-based oligomers for highly efficient and. Advanced Materials, 21, 2425-2429.
- ^ Sasabe, H., & Kido, J. (2010). Multifunctional materials in high-performance oleds: challenges for solid-state lighting. Chemistry of Materials, 23, 621-630.
- ^ Freudenrich, C. (2005, March 24). How oleds work. Retrieved from http://electronics.howstuffworks.com/oled6.htm
- ^ Leclerc, Mario (2001). Polyfluorenes: Twenty Years of Progress. J. Polym. Sci. Part A: Polym. Chem, 39, 2867–2873.
- ^ Nayak, P. et al. (2010). Blue and white light electroluminescence in a multilayer oled using a new aluminuim complex. J Chem. Sci, 122(6), 847-855.
- ^ Do-Hoon, Hwang; Bon-Won Koo, In-Nam Kang, Sung-Hyun Kim, and Taehyoung Zyung (2004, November). Conjugated Polymers Based on Phenothiazine and Fluorene in Light-Emitting Diodes and Field Effect Transistors. Chemical Matter, 7.
- ^ Chang-Gua, Z. et al (2011). Achieving highly efficient fluorescent blue organic light-. Advanced Functional Materials, 21, 699-707.