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

Color imaging has evolved very rapidly over the past few decades. 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.

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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.

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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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