Materials used in optoelectronic devices such as light emitting diodes (LEDs) are chosen both for the way they conduct electrons and for the way they transmit light. Researchers continue to experiment with new materials and manufacturing techniques to find materials with low electrical resistivity and high transparency that are also environmentally safe and low-cost. Recently a team from South Korea tested the performance of aluminum-doped zinc oxide (AZO), an alternative electrode material for optoelectronic devices. The team tested films that had been manufactured by atomic layer deposition (ALD), and concluded that the AZO films hold promise as an electrode material for organic light emitting diodes. The researchers published the results of their study in the Journal of Vacuum Science and Technology A.
Currently, indium tin oxide (ITO) is widely used as an electrode material for optoelectronic devices. Indium has many desirable electronic and phonic properties, but it also has downsides, including its chemical instability, potential toxicity, and high cost. The drawbacks have prompted researchers to search for alternative materials, including aluminum-doped oxide (AZO). The way that AZO electrodes are manufactured significantly affects the material's properties.
AZO films made by sputtering and pulsed laser deposition techniques have demonstrated properties of low resistivity and high transmittance that are comparable to ITO films. However, the relatively new atomic layer deposition (ALD) manufacturing technique offers the possibility of more exact control of the doping concentration and film thickness, and is cost efficient. The South Korean research team decided to investigate the optoelectronic properties of AZO films made using this new technique.
The researchers created AZO films in an ALD reactor at temperatures ranging from 200 to 260 degrees Celsius. The researchers examined the electrical properties of the AZO films in relation to the influence of crystallinity, surface morphology, and optical properties as a function of deposition temperature. The AZO films were also incorporated into an organic LED (OLED) device and tested. The scientists found that OLEDs made with the AZO film deposited at the highest temperature of 260 degrees C demonstrated the lowest electrical resistivity and highest transmittance.
The successful results means that researchers will likely explore how other zinc-based electrodes made with ALD perform, notes Hyung-Ho Park, one of the authors of the JVST A paper. He says that scientists will continue to try to enhance the electrical properties of these materials to meet and exceed the performance of the current indium-based electrodes. However, Park notes that a property of the films called the surface work function, as well as the characteristics of the interface between the oxide film and the organic layer in an OLED also affect OLED performance. These characteristics can be precisely controlled by ALD techniques, says Park. His team's next steps will be to study an AZO anode with a buffer layer created using ALD and investigate the OLED characteristics as a function of the buffer layer's properties.