From left: Lee Han-eol, professor of materials science and engineering at GIST; Park Gwang-uk, professor of materials science and engineering at Jeonbuk National University; and Kim Jeong-hyeon and Hwang Ju-chan, doctoral students at Jeonbuk National University. [Provided by GIST]
From left: Lee Han-eol, professor of materials science and engineering at GIST; Park Gwang-uk, professor of materials science and engineering at Jeonbuk National University; and Kim Jeong-hyeon and Hwang Ju-chan, doctoral students at Jeonbuk National University. [Provided by GIST]

A next-generation photodetector that generates its own electrical signals using ambient light — without a battery or any external power source — has been developed by South Korean researchers.

The Gwangju Institute of Science and Technology (GIST) announced Tuesday that a research team led by Lee Han-eol, a professor in the Department of Materials Science and Engineering, jointly developed a self-powered photodetector with researchers at Jeonbuk National University. The device combines different semiconductors to create a strong internal electric field, enabling operation without an external power supply.

Photodetectors convert light into electrical signals and are used across a wide range of applications, including the Internet of Things (IoT), wearable devices and drones.

A key challenge in the field has been separating and transporting the electrons and holes generated by incoming light quickly and without applying an external voltage. Conventional approaches have relied on "heterojunction" structures that bond different semiconductors together, but defects arising during the bonding process, weak internal electric fields and low light-absorption rates have limited device performance.

To overcome these limitations, the research team focused on zinc tin nitride (ZnSnN₂, or ZTN), a next-generation semiconductor composed of zinc, tin and nitrogen.

During fabrication of the ZTN thin film, the team precisely controlled the material's compositional ratio and charge-carrier concentration. They then bonded the optimized ZTN with gallium nitride (GaN) to form a strong internal electric field at the interface between the two semiconductors.

The internal electric field reached 88 kilovolts per centimeter (kV/cm). That provided the foundation for effectively separating the electrons and holes generated by light and converting them into electrical signals — without applying any external voltage.

The team also modified the surface of the ZTN layer to further improve the photodetector's performance.

By etching regular micro-holes into the surface, the researchers caused incoming light to linger longer inside the material.

Low-voltage and low-illuminance operating characteristics of the self-powered photodetector, and a demonstration of its use as a supplementary power source for a commercial temperature and humidity sensor. [Provided by GIST]
Low-voltage and low-illuminance operating characteristics of the self-powered photodetector, and a demonstration of its use as a supplementary power source for a commercial temperature and humidity sensor. [Provided by GIST]

On a flat surface, some light is reflected or escapes outward, but in a micro-hole structure, light scatters and reflects repeatedly off the sides and bottoms of the holes. This extends the time light spends inside the material and allows more charge carriers to be generated.

As a result, the lifetime of light-generated charge carriers increased from 3.5 nanoseconds in the conventional flat structure to a maximum of 6.2 nanoseconds — roughly 1.8 times longer.

By simultaneously controlling the material composition and the light propagation path, the team built a self-powered photodetector capable of stably sensing low light intensities without an external voltage.

The team also verified the device's practical potential. They connected the photodetector to a commercial temperature and humidity sensor and tested its operation. Even under an external voltage of just 2.2 volts — insufficient to drive the commercial sensor on its own — the photodetector generated a photovoltage of about 0.6 volts when exposed to light, and the team successfully used that output alongside the external supply to power the sensor.

"We demonstrated that self-powered performance can only be improved by designing not just the junction between different semiconductors, but also the light propagation path from the very stage of material fabrication," Lee said. "Going forward, we will expand the application of this technology to ultra-low-power sensors that reduce battery dependence and to energy-conversion components for compact electronic devices."

The findings were published in Small Structures, an international journal in the field of materials science.


nbgkoo@heraldcorp.com