IISc’s self-powered UV photodetector can power electronic devices

buddha
The UV light that gets into the pores of the nanoflakes undergoes multiple reflections and finally gets absorbed, says Buddha Deka Boruah.

In a novel approach, 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. It can also be used for operating electronic devices in the absence of external power source.

The researchers developed the photodetector by integrating semiconducting vanadium doped zinc oxide (VZnO) nanoflakes with a conducting polymer. The photodetector has superior performance in terms of faster detection of photo signals in the order of milliseconds even when UV light intensity is low.

The Vanadium Zinc Oxide material has 98 per cent light harvesting efficiency.The results were published in the journal ACS Applied Materials & Interfaces.

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 volume ratio 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,” says Buddha Deka Boruah from the Department of Instrumentation and Applied Physics at IISc and the lead author of the paper. The UV light that gets into the pores undergoes multiple reflections and finally gets absorbed.

detector-optimized
Once hydrogenated, the  nanoflakes’ photocurrent generation capacity increased to 1,000 nA.

“The vanadium-doped zinc oxide nanoflake material has 98 per cent light-harvesting efficiency, which is much higher than the 84 per cent seen in zinc oxide nanorods,” says Prof. Abha Misra from the department who is the corresponding author of the paper.

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.

More photocurrent

Compared with ZnO, which generates only 40 nA photocurrent, the VZnO nanoflakes produced five times more photocurrent. Once the nanoflakes were hydrogenated, the current generation capacity further increased to 1,000 nA, says 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, says Prof. Misra.

Published in The Hindu on October 15, 2016

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