UV Treatment Dramatically Improves Photodetector Performance
Michael Alba posted on November 09, 2016 |
A depiction of the photodetector UV radiation treatment. (Image courtesy of Moscow Institute of Physics and Technology.)
A depiction of the photodetector UV radiation treatment. (Image courtesy of Moscow Institute of Physics and Technology.)
Researchers recently discovered a way to increase the bandwidth of polymer-based photodetectors. Typically, these devices are sensitive only to a narrow bandwidth of light. However, with a simple ultraviolet (UV) treatment, the researchers were able to increase the photodetector bandwidth and achieve a spectral response from UV to near-infrared light.


30 Seconds of UV

The researchers began with an organic, polymer-based photodetector using zinc oxide (ZnO) nanoparticles as the anode interfacial layer. Then, they exposed the photodetector to 350 nm, 30 μW/cm2 UV light for just 30 seconds. They found that this simple UV treatment dramatically increased the photoresponse of the device.

A performance metric of photodetectors called external quantum efficiency (EQE) measures the percentage of incident photons required to produce an electron. Before the UV treatment, the polymer-based photodetector had an EQE of 30 percent, meaning it took roughly three photons to produce one electron. After the UV treatment, the photodetector had an incredible 140,000 percent EQE–so a single photon produces 14,000 electrons.

The researchers attribute the extreme improvement to the ZnO nanoparticles. In the typical manufacture of the organic photodetectors, oxygen atoms from the ZnO molecules detach and capture electrons, meaning they can’t be used as charge carriers. The researchers posit that the UV treatment frees up some of these electrons, resulting in the vastly improved performance of the photodetector.

The EQE over a range of incident wavelengths is shown for: left, the pristine device at a bias voltage of -2 V; right, the UV treated device at a variety of bias voltages. Note the extreme difference in the EQE scale – the left scale is linear and the right is logarithmic. (Image courtesy of Advanced Functional Materials.)
The EQE over a range of incident wavelengths is shown for: left, the pristine device at a bias voltage of -2 V; right, the UV treated device at a variety of bias voltages. Note the extreme difference in the EQE scale – the left scale is linear and the right is logarithmic. (Image courtesy of Advanced Functional Materials.)
The downside of the UV treatment is an increase in dark current, the current produced even in the absence of light. This means an increase in the amount of noise experienced in the photodetector. However, the researchers hope to tackle this side effect in future studies.

In spite of this drawback, the research is a big step towards the design of more effective photodetectors, which are used in a plethora of applications from your smartphone to the space station.

"There is a lot of demand for photodetectors that are sensitive to a wide range of frequencies, but they are difficult to design,” said researcher Vadim Agafonov. “It's hard to find the right materials, because the substances that permit ultraviolet light tend to be nontransparent to infrared radiation, and vice versa. We found a way to 'broaden' the spectral response of photodetectors…You can thus convert a polymer-based photodetector into a highly sensitive broadband device. The process itself is quick, cheap, and efficient, which is important for practical applications.”

To learn more, you can read the research paper in Advanced Functional Materials. Or read about a practical application of light power in Can This Student-Built Car Break the Solar-Powered Land Speed Record?

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