Connecting the (Quantum) Dots: Chung-Ang University Researchers Develop an Innovative Design for Color Image Sensors

Vertically stacked quantum dots offer unprecedented pixel density for ultra-compact, flexible, and sensitive color image sensors

Scientists have developed a new type of color image sensor using vertically stacked quantum dots—nanoparticles tailored to be sensitive to specific light frequencies. Their innovative pixel structure uses much less area per pixel than conventional image sensors, allowing for much higher integration. Coupled with a simple fabrication procedure, high photosensitivity, and great durability, their design could pave the way to the next generation of high-resolution color image sensors for a wide variety of applications.

Figure 1. Quantum dots as a tool for next-generation color image sensors

Quantum dots are nanoparticles tailored to be sensitive to specific light frequencies, thanks to quantum effects mainly arising from their small size. Their durability, sensitivity, and ease of use in manufacturing processes make them uniquely attractive for developing revolutionary color image sensors.

Photo credit: Shutterstock

Color imaging has evolved very rapidly over the past few decades. Today, commonplace devices such as smartphones contain CMOS image sensors that could put older professional cameras to shame. However, despite the advantages brought by CMOS technology, the conventional design of image sensors is starting to show its limitations as our needs become more and more refined.

Many rising fields of application, such as self-driving cars, flexible electronics, and healthcare and medical imaging, demand even higher resolutions and levels of integration. This is difficult to achieve because of the way each pixel of a color image is captured. In most image sensors, the red, green, and blue components of a given pixel are captured independently using a dedicated photodetector ‘cell’ for each color. While the three cells of each pixel are arranged laterally and as close to each other as possible to use the available area efficiently, this design takes at least thrice as much space as each individual cell. In addition, the manufacture and processing costs for these photodetector arrays can be high due to their complexity.

To address this problem, a team of scientists, including Professor Sung Kyu Park of Chung-Ang University, Korea, delved into stacked quantum dot (QD)-based sensors. In their paper—published in Advanced Materials—they present a newly developed type of photodetector and its integration into a dense sensor array for high-resolution multispectral (color) imaging.
This paper was made available online on 11 October 2021 and was published in Volume 34 Issue 2 of the journal on 13 January 2022.

QDs are nanoparticles less than 10 nanometers in diameter whose size causes them to manifest certain quantum effects, including photon absorption and their conversion into electric carriers. By precisely engineering their size and composition, QDs can be tailored to respond only to light of a specific color(s). The advantage of QDs over the traditional lateral pixel arrangement is that QDs can be stacked vertically in each pixel. Though one would think that the QDs in the lower positions would be occluded by those above, the reality is that photons not absorbed by the upper levels of QDs do penetrate and reach the bottom ones. In this way, photodetectors for each color in each pixel can be accommodated into a much tighter area.

Using a low-temperature fabrication procedure, the scientists managed to squeeze in an astoundingly high number of pixels in a small area, as Prof. Park highlights: “The device density of our photodetector array is 5500 devices per square centimeter, which is remarkably larger than that reported for previous solution-processed flexible photodetectors, which reaches up to 1600 devices.”

In addition to these remarkable enhancements the vertically stacked QD pixels achieved a great color selectivity and photosensitivity. In the long term, the team believes future improvements could make vertically stacked QDs replace existing CMOS image sensors in many applications thanks to their simple fabrication, low power consumption, durability, and capabilities.

Satisfied with the results of their work, Prof. Park comments: “We think our design is a great advancement towards establishing a low-cost, high-resolution and integrated image sensor system that goes beyond conventional ones. It should be widely applicable in fields such as wearable sensory systems, biomedicine, and autonomous driving.”

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About Chung-Ang University

Chung-Ang University is a private comprehensive research university located in Seoul, South Korea. It was started as a kindergarten in 1918 and attained university status in 1953. It is fully accredited by the Ministry of Education of Korea. Chung-Ang University conducts research activities under the slogan of “Justice and Truth.” Its new vision for completing 100 years is “The Global Creative Leader.” Chung-Ang University offers undergraduate, postgraduate, and doctoral programs, which encompass a law school, management program, and medical school; it has 16 undergraduate and graduate schools each. Chung-Ang University’s culture and arts programs are considered the best in Korea.

Website: https://neweng.cau.ac.kr/index.do

About Professor Sung Kyu Park

Sung Kyu Park is a Professor at the School of Electrical and Electronics Engineering at Chung-Ang University, Seoul, Korea. He received his Ph.D. from The Pennsylvania State University Park, PA, in 2007, and his advisor was Prof. Thomas N. Jackson. Park’s group is currently developing high-performance and stretchable metal–oxide transistors for skin electronics and smart sensory applications including photonic/image sensors, neuromorphic photonic synapses, gas sensors, and mechanical pressure sensors.