"20 Times Higher Light Sensitivity": KAIST Develops World's Best-Performing Optical Sensor

A "battery-free optical sensor" that enables precise sensing for wearable devices, biometric signal monitoring, and autonomous vehicles using only a light source-without the need for a battery-has been developed in South Korea.

Traditional silicon semiconductors used in optical sensors have low light responsiveness, and two-dimensional semiconductor molybdenum disulfide (MoS₂) is so thin that doping processes to control its electrical properties are difficult. This has limited the realization of high-performance optical sensors.

However, the battery-free optical sensor developed with domestic technology overcomes these limitations. It operates without power in environments where a light source is present, and has attracted attention for demonstrating the world’s highest performance.

The research team is conducting an experiment on the battery-free optical sensor function. Provided by KAIST

KAIST announced on August 14 that a research team led by Professor Lee Gayoung from the School of Electrical Engineering has developed a battery-free optical sensor that operates without an external power supply.

This sensor boasts up to 20 times higher sensitivity compared to existing products, delivering top-tier performance among technologies of its class disclosed to date.

The battery-free optical sensor was fabricated by applying a "Van der Waals bottom electrode"-which makes the semiconductor highly responsive to electrical signals-to a PN junction structure optical sensor, without doping.

The PN junction structure refers to a semiconductor configuration where p-type (hole-rich) and n-type (electron-rich) materials are joined. This allows the device to generate electrical signals on its own in the presence of light, without any supply of electrical energy. The characteristic of allowing current to flow in one direction when exposed to light is a key element in optical sensors and solar cells.

However, for a proper PN junction to form, a process called "doping" is required. Doping involves intentionally introducing impurities into a semiconductor to alter its electrical properties. Yet, for two-dimensional semiconductors such as molybdenum disulfide, which are atomically thin, doping as done in conventional semiconductors can damage the structure or degrade performance. This is why it has been challenging to create ideal PN junctions in two-dimensional semiconductors.

The research team overcame these limitations and maximized device performance by devising a novel device structure that incorporates two key technologies: the "Van der Waals electrode" and the "partial gate."

Comparison data of photoelectric characteristics between the previously reported battery-free optical sensor and the newly developed battery-free optical sensor. Provided by KAIST

The partial gate structure sends electrical signals to only part of the two-dimensional semiconductor, allowing one side to behave like a p-type and the other like an n-type, thus enabling the device to function as a PN junction electrically without doping.

The team also ensured that the original structure of the two-dimensional semiconductor was preserved, while creating an environment for smooth electrical signal transmission. This was achieved by inducing the metal electrode to gently adhere to the Van der Waals bottom electrode through Van der Waals forces, rather than forming a strong chemical bond with the semiconductor, which could otherwise damage its lattice structure.

This means that even in ultrathin two-dimensional semiconductors, it is possible to realize a fully functional PN junction without structural damage. The research presents an innovative approach that simultaneously secures both structural stability and electrical performance in two-dimensional semiconductor devices.

Through this, the research team emphasized that they have achieved a battery-free optical sensor with performance more than 20 times higher than conventional powered sensors, over 10 times higher than silicon-based battery-free sensors, and more than twice as high as existing molybdenum disulfide sensors. In particular, the high sensitivity of the battery-free optical sensor indicates its potential application as a high-precision sensor for biometric signal detection or operation in low-light environments.

Professor Lee Gayoung stated, "Our team has succeeded in realizing a PN junction in a two-dimensional semiconductor that can control electrical flow without the doping process. This technology can be applied not only to sensors but also to core components that regulate electricity inside smartphones and electronic devices, serving as a catalyst for advancing miniaturization and battery-free operation in next-generation electronics."

This research was supported by the National Research Foundation of Korea, the Korea Basic Science Institute, Samsung Electronics, and the Korea Institute for Advancement of Technology.

Doctoral students Hwang Jaeha and Song Junki from the School of Electrical Engineering participated as co-first authors in this study, which was also introduced in the materials science journal "Advanced Functional Materials" on July 26.

This content was produced with the assistance of AI translation services.