Red-sensitive, blue-sensitive and green-sensitive colour sensors stacked on top of each other instead of being lined up in a mosaic pattern - this principle could allow image sensors with unprecedented resolution and sensitivity to light to be created. However, up to now, the reality hasn't quite met expectations. Researchers from Empa and ETH Zurich have now developed a sensor prototype that absorbs light almost optimally - and which is also cheap to produce.
The human eye has three different types of sensory cells for the perception of colour: cells that are respectively sensitive to red, green and blue alternate in the eye and combine their information to create an overall coloured image. Image sensors, for example in mobile phone cameras, work in a similar way: blue, green and red sensors alternate in a mosaic-like pattern. Intelligent software algorithms calculate a high-resolution colour image from the individual colour pixels.
However, the principle also has some inherent limitations: as each individual pixel can only absorb a small part of the light spectrum that hits it, a large part of the light is lost. In addition, the sensors have basically reached the limits of miniaturisation, and unwanted image disturbances can occur; these are known as colour moiré effects and have to be laboriously removed from the finished image.
Transparent only for certain colours
Researchers have therefore been working for a number of years on the idea of stacking the three sensors instead of placing them next to each other. Of course, this requires that the sensors on top let through the light frequencies that they do not absorb to the sensors underneath. At the end of the 1990s, this type of sensor was successfully produced for the first time. It consisted of three stacked silicon layers, each of which absorbed only one colour.
This actually resulted in a commercially available image sensor. However, this was not successful on the market because the absorption spectra of the different layers were not distinct enough, so part of the green and red light was absorbed by the blue-sensitive layer. The colours therefore blurred and the light sensitivity was thus lower than for ordinary light sensors. In addition, the production of the absorbing silicon layers required a complex and expensive manufacturing process.
Empa researchers have now succeeded in developing a sensor prototype that circumvents these problems. It consists of three different types of perovskites - a semiconducting material that has become increasingly important during the last few years, for example in the development of new solar cells, due to its outstanding electrical properties and good optical absorption capacity.
Depending on the composition of these perovskites, they can, for example, absorb part of the light spectrum, but remain transparent for the rest of the spectrum. The researchers in Maksym Kovalenko's group at Empa and ETH Zurich used this principle to create a colour sensor with a size of just one pixel. The researchers were able to reproduce both simple one-dimensional and more realistic two-dimensional images with an extremely high colour fidelity.
Accurate recognition of colours
The advantages of this new approach are clear: the absorption spectra are clearly differentiated and the colour recognition is thus much more precise than with silicon. In addition, the absorption coefficients, especially for the light components with higher wavelengths (green and red), are considerably higher in the perovskites than in silicon.
As a result, the layers can be made significantly smaller, which in turn allows smaller pixel sizes. This is not crucial in the case of ordinary camera sensors; however, for other analysis technologies, such as spectroscopy, this could permit significantly higher spatial resolution. The perovskites can also be produced using a comparatively cheap process.
However, more work is still needed in order to further develop this prototype into a commercially usable image sensor. Key areas include the miniaturisation of pixels and the development of methods for producing an entire matrix of such pixels in one step. According to Kovalenko, this should be possible with existing technologies.
Perovskites are such a promising material in research that the prestigious journal Science has published a special edition about them. It includes a review article by the Empa/ETH research group led by Maksym Kovalenko about the current state of research and potential uses of lead halide perovskites nanocrystals.
These have properties that make them a promising candidate for the development of semiconductor nanocrystals for various optoelectronic applications such as television screens, LEDs and solar cells: they are inexpensive to manufacture, have a high tolerance to defects and can be tuned precisely to emit light in a specific colour spectrum.
The paper was published on 10 November.
M Kovalenko, L Protesescu, MI Bodnarchuk
"Properties and potential optoelectronic applications of lead halide perovskites nanocrystals" (Science, 2017)
Rainer Klose | Empa - Eidgenössische Materialprüfungs- und Forschungsanstalt
Proteins imaged in graphene liquid cell have higher radiation tolerance
10.12.2018 | INM - Leibniz-Institut für Neue Materialien gGmbH
High-temperature electronics? That's hot
07.12.2018 | Purdue University
What if a sensor sensing a thing could be part of the thing itself? Rice University engineers believe they have a two-dimensional solution to do just that.
Rice engineers led by materials scientists Pulickel Ajayan and Jun Lou have developed a method to make atom-flat sensors that seamlessly integrate with devices...
Scientists at the University of Stuttgart and the Karlsruhe Institute of Technology (KIT) succeed in important further development on the way to quantum Computers.
Quantum computers one day should be able to solve certain computing problems much faster than a classical computer. One of the most promising approaches is...
New Project SNAPSTER: Novel luminescent materials by encapsulating phosphorescent metal clusters with organic liquid crystals
Nowadays energy conversion in lighting and optoelectronic devices requires the use of rare earth oxides.
Scientists have discovered the first synthetic material that becomes thicker - at the molecular level - as it is stretched.
Researchers led by Dr Devesh Mistry from the University of Leeds discovered a new non-porous material that has unique and inherent "auxetic" stretching...
Scientists from the Theory Department of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science (CFEL) in Hamburg have shown through theoretical calculations and computer simulations that the force between electrons and lattice distortions in an atomically thin two-dimensional superconductor can be controlled with virtual photons. This could aid the development of new superconductors for energy-saving devices and many other technical applications.
The vacuum is not empty. It may sound like magic to laypeople but it has occupied physicists since the birth of quantum mechanics.
10.12.2018 | Event News
06.12.2018 | Event News
03.12.2018 | Event News
10.12.2018 | Life Sciences
10.12.2018 | Physics and Astronomy
10.12.2018 | Life Sciences