Self-powered UV photo detector charges energy storage devices

•  Researchers from the Indian Institute of Science (IISc), Bangalore, have developed a cost-effective, high-performance, self-powered UV photodetector that can use the harvested optical energy for direct self-charging of energy storage devices such as supercapacitor.

About Process

• Zinc oxide (ZnO), the base material for UV detection, can be doped with vanadium to produce photodetectors that are self-powered. When doped with vanadium, the microstructure of ZnO changes from nanorods to closely-packed nanoflakes, causing an increase in the surface area to the volume of the material.
• Doping ZnO with vanadium also creates surface defects within the band gap (between the conduction and valence bands) of ZnO, which helps in trapping the UV radiation that falls on the nanoflakes.
• The nanoflakes are 80 per cent more porous than nanorods.
• The nanorods are one-dimensional and so the possibility of light reflection from the top surface is more. But nanoflakes are two-dimensional and the light penetration is more
• The UV light that gets into the pores undergoes multiple reflections and finally gets absorbed.
• The vanadium-doped zinc oxide nanoflake structure has 98 per cent light-harvesting efficiency, which is much higher than the 84 per cent seen in zinc oxide nanorods
• The VZnO nanoflakes were annealed (heated and allowed to cool slowly) in the presence of hydrogen gas at 350 degree C (hydrogenated) to increase the conductivity and reduce the recombination of photo-generated charge carriers.

Analysis

• It can be used for operating electronic devices in the absence of external power source.
• This photo detector can use the harvested optical energy for direct self-charging of energy storage devices such as super capacitor. It can also be used for operating electronic devices in the absence of external power source..
• Compared with ZnO, which generates only 40 nA photocurrent, the nanoflakes (VZnO) produced five times more photocurrent. Once the nanoflakes were hydrogenated, the current generation capacity further increased to 1,000 nA, said Boruah.
• If the increased optically active surface area of the nanoflakes enhanced the generation of electron-hole pairs (photo response), resulting in increased current generation, hydrogenation brought about a further enhancement in the electron-hole pair generation as well as increased free electron density, leading to more current generation.
• When exposed to UV light, the device, after hydrogenation, was able to detect photo signal within milliseconds, which is nearly 100 times faster than conventional UV photodetectors.

By: Vishal