This 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, this design could help to pave the way to the next generation of high-resolution colour image sensors for a wide variety of applications.
While devices such as smartphones contain CMOS image sensors, the conventional design of image sensors is starting to show its limitations at a time when applications, such as self-driving cars, flexible electronics, and healthcare and medical imaging, are demanding higher resolutions and levels of integration.
At present, most image sensors, the red, green, and blue components of a given pixel are captured independently using a dedicated photodetector ‘cell’ for each colour. 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 up a lot of space. In addition, the manufacture and processing costs for these photodetector arrays can be high due to their complexity.
To address this the scientists involved in the project, including Professor Sung Kyu Park, looked at stacked quantum dot (QD)-based sensors.
In a paper - published in Advanced Materials - they have presented a newly developed type of photodetector and its integration into a dense sensor array for high-resolution multispectral (colour) 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 colour(s). The advantage of QDs is that they can be stacked vertically in each pixel and that photons are not absorbed by the upper levels of QDs and so don’t penetrate and reach the bottom ones. Consequently, photodetectors for each colour in each pixel can be accommodated into a much tighter area.
Using a low-temperature fabrication procedure, the scientists have been able to squeeze in an astoundingly high number of pixels in a small area, as Prof. Park explained. “The device density of our photodetector array is 5500 devices per square centimetre, 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 colour 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.
According to Prof. Park, “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.”