Transparent Image Sensors: The Future with Graphene & Quantum Dots

Researchers from The Barcelona Institute of Science and Technology and Qurv Technologies have recently demonstrated a flexible and transparent image sensor that could potentially be used in numerous applications, including mirrors, windows, and glasses. An image sensor is a device that captures visual information and converts it into electronic signals for devices like cameras. 

What challenges do image sensors present, what did the researchers demonstrate, and how practical is the technology?

An almost invisible 8x8 array of photodetectors utilising graphene and quantum dots could reshape eye-tracking technology. Instead of the current side placement in devices, these sensors have the potential to be positioned directly in front of the eye. This innovation was featured in ACS Photonics.  

According to a detailed study published in ACS Photonics, the combination of graphene and quantum dots in the image sensor design is revolutionary. Graphene is a single layer of carbon atoms known for its strength, flexibility, and conductivity. It pairs well with quantum dots, tiny semiconductor particles that can absorb and emit light, often used in display technologies. This combination results in a sensor that is not only transparent but also efficient in converting light to electrical signals.

What challenges do image sensors present?

Of all sensor technologies developed in the field of electronics, image sensors have arguably been one of the most transformative thanks to their ability to provide analog and digital visual information. The ability to record images as a data stream has allowed spacecraft to send back photos from the deepest parts of space, transmit television across entire nations, and even allow individuals to take selfies in the most absurd of places.

For all the benefits that image sensors provide, there are numerous challenges that they face. One such challenge is the need for a high degree of sensitivity so that high-quality images can be generated in low-light conditions. While amplifiers can be used to increase the brightness of images, this also amplifies noise, resulting in more grainy images (which is why professional photographers try to maximise light and minimise amplifier settings, known as ISO). 

Another challenge faced by image sensors is that an image sensor alone cannot produce images; it requires lenses to focus light onto the sensor and bring subjects into focus. A sensor with no lens will receive light in all directions, resulting in a complete blur where nothing can be resolved. For large cameras, adding lenses isn’t an issue, but for smartphones, the need for incredibly small and advanced lenses is paramount. 

But, one emerging technology is introducing new challenges for image sensors: flexible electronics. Simply put, there is a strong incentive for engineers to develop flexible electronics for numerous applications, including wearable electronics, medical devices, and advanced HMIs. While a substantial proportion of electronics have been made to be flexible, image sensors still remain in the realm of ridged electronics. 

The reason for this comes down to how image sensors are designed. To produce the highest quality images, image sensors need to use photodiodes that provide exceptional sensitivity. Photodiodes are devices that detect light and convert it into an electrical signal. This usually comes from crystalline structures based on silicon, a material commonly used in electronics, especially in chips and sensors, due to its excellent semiconductor properties.

Of course, cameras can be integrated into flexible devices, but only as a module that itself doesn’t flex. This means that portions of a flexible design are ridged and, therefore, prone to damage over time (as repeated flexing introduces weaknesses into connectors and substrates). 

Researchers demonstrate transparent image sensor

Recognising the challenges faced with image sensors in emerging technologies, researchers from The Barcelona Institute of Science and Technology (ICFO) and start-up Qurv Technologies have published their findings on a new image sensor that is both flexible and semi-transparent, paving the way for new applications in the field of image sensors.

"Due to their ability to capture vast amounts of information, traditional image sensors play a pivotal role in today’s society. However, the opaque nature of both their pixels and stacked read-out electronics can be a limiting factor in applications such as human–computer interfaces, smart displays, and both augmented and virtual reality. In this paper, we present the development and analysis of the first semitransparent image sensor and its applicability as an eye-tracking device. Consisting of an 8 × 8 array of semitransparent photodetectors and electrodes deposited on a fully transparent substrate, the sensor’s pixels achieve an optical transparency of 85–95% and high sensitivity, with more than 90% of these pixels demonstrating a noise equivalent irradiance <10–4 W/m2 for wavelengths of 637 nm. The fabrication of such sensors represents a fundamental shift in how we think about cameras and imaging as these devices can be concealed in plain sight." Source: ACS Photonics, "Semitransparent Image Sensors for Eye-Tracking Applications"

To create the new sensor, the researchers turned to a number of materials that have been making headlines over the past few years: graphene and quantum dots. Graphene was chosen as it exhibits excellent conductivity, flexibility, and transparency, but it is also excellent at converting photons into electric charge. A photon is a tiny particle of light. When it hits certain materials, it can generate an electric charge. However, as graphene absorbs little light, the researchers then decided to also integrate a quantum dot layer above the graphene.

However, as graphene absorbs little light, the researchers then decided to also integrate a quantum dot layer above the graphene. Quantum dots are able to absorb photons extremely well and can pass these photons to the graphene layer, resulting in improved efficiencies (around 60% photon-to-electricity conversion).

As both layers are extremely thin, they are mostly transparent, with the final image sensor having a transparency of around 85%. Additionally, to keep electrical contacts transparent, the researchers turned to indium tin oxide, which is a commonly used transparent electrode material. Indium tin oxide (ITO) is a transparent material that's often used in electronics because it can conduct electricity.

The researchers demonstrated their new sensor by creating an 8x8 array, with each pixel being around 60 x 140 microns in size. While the sensor is only able to produce black-and-white images (due to the lack of colour filters), the researchers were able to demonstrate that images can be made with the new transparent image sensor. 

In a deeper dive into the technology, as highlighted by

Even though typical silicon photodiodes can see efficiencies of around 90% and increased levels of sensitivity, the fact that the transparent image sensor was able to demonstrate 60% efficiency with an 85% transparency shows that this sensor has real potential in future applications. 

How practical is this technology?

While the sensor array produced by the researchers was small, it only takes a few multiplications of this array to obtain a practical image sensor. But while the image sensor was made transparent and flexible, one needs to ask, what could it be used in?

As image sensors need lenses to make them suitable for producing images, it goes without saying that these sensors would not be used in such applications. Instead, it is more likely that these sensors will be integrated into monitors, mirrors, and glasses to provide advanced sensing capabilities such as glaze detection and eye tracking. 

It is also possible that such a sensor could be fitted into viewfinders that can be used in feedback loops with a camera sensor to account for variations in what a user is seeing. For example, it is possible that an advanced viewfinder will detect what the eye is trying to focus on and, therefore, bring that object into focus. 

Currently, this technology remains in the lab, but the fact that an 8x8 array was manufactured and demonstrated to work shows that it could be readily manufactured using modern production processes. Thus, it may not be that far in the distant future when all manner of everyday items have integrated image sensors invisible to the naked eye.  

Further insights from Graphene-Info shed light on the unique properties of graphene and quantum dots. While graphene is an excellent conductor and adept at converting photons into electrons, it doesn't absorb much light. Quantum dots, on the other hand, are brilliant light absorbers. This synergy between the two materials is what makes the transparent image sensor so promising. The future of this technology not only lies in its transparency but also in its adaptability to various applications, making it a game-changer in the realm of electronics.

As highlighted by IEEE Spectrum, the potential applications of this technology are vast. From virtual-reality and augmented-reality devices to systems enhancing driving safety, the possibilities are endless. Augmented reality (AR) overlays digital content on the real world, while virtual reality (VR) immerses the user in a completely digital environment. Frank Koppens, a key figure in the research, envisions a future where devices like phones or laptops have screens that double as sensors, detecting hand movements. Even shop windows could integrate these smart sensors, responding to human gestures. How practical do you think this technology can be?