Quantum dot photodetector offers new type of color image sensor
Currently, say the researchers, conventional CMOS image sensor technology used to provide color imaging in commonplace devices such as smartphones is starting to show its limitations as application needs become more and more refined. For example, 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, say the researchers. 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 three times 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, the researchers used stacked quantum dot (QD)-based sensors to develop a new type of photodetector and demonstrated its integration into a dense sensor array for high-resolution multispectral (color) imaging. 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, say the researchers, 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,” say the researchers, “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 researchers report that they managed to squeeze in an astoundingly high number of pixels in a small area.
“The device density of our photodetector array is 5500 devices per square centimeter,” says Professor Sung Kyu Park, “which is remarkably larger than that reported for previous solution-processed flexible photodetectors, which reaches up to 1600 devices.”
In addition, the vertically stacked QD pixels achieved a great color selectivity and photosensitivity. In the long term, say the researchers, they believe 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.
“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,” says Park. “It should be widely applicable in fields such as wearable sensory systems, biomedicine, and autonomous driving.”
For more, see “Vertically Stacked Full Color Quantum Dots Phototransistor Arrays for High-Resolution and Enhanced Color-Selective Imaging.”
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