The demand for high performance transparent conductor materials has significantly increased because the unique combination of electrical and optical properties—allowing light to effectively cross the path of electric conduction—is needed for top electrodes used in light emitting diodes (LEDs), photovoltaic cells and optical detectors.
Up until now, the focus has been on high optical transparency in the visible spectrum while maintaining a high electrical conductivity. Many applications require shorter wavelength light, such as solar blind detection in the range of 240–280 nm, ultraviolet (UV) curing (260–320 nm), biomolecule sensing (250–400 nm), UV germicidal irradiation in the upper UV range (260–280 nm), UV lithography (248 nm), UV phototherapy (ultraviolet B (UVB), 280–315 nm), photochemotherapy (ultraviolet A (UVA), 315–400 nm), and light sources for plant growth stimulating the secondary metabolism by exposure to radiation in UVB.
This wide application space has driven research in the area of UV LEDs as environmentally benign alternative over conventional UV light sources, such as low-pressure mercury lamps, offering higher efficiency, longer lifetime, and fast switching.
In contrast to the high external quantum efficiencies (EQEs) of 45–96% achieved for LEDs emitting in the visible (400–700 nm) and UVA spectrum, LEDs emitting deeper in the UV have significantly lower EQEs of only around 1%, attributed to poor hole injection, and high defect densities of the wide band gap semiconductor in the active region.
Another roadblock towards a higher EQE is the lack of a transparent electrode material with high electrical conductivity and high optical transparency in the deeper UV range that has similar performance to transparent conductors in the visible. Indium–tin oxide (ITO), which is the most widely used transparent electrode, has a strong absorption edge near 360 nm rendering it unsuitable as a UV transparent electrode.
To read the full article, click here.