A Stacked Color Sensor

Color Sensor: Original image (left) and corresponding portrayal of the red, green and blue regions, and a composite image. (Source: Empa)

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 color image from the individual color 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.

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 color. 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 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 color 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 color fidelity.

The advantages of this new approach are clear: the absorp­tion spectra are clearly differen­tiated and the color recog­nition is thus much more precise than with silicon. In addition, the absorp­tion coeffi­cients, especially for the light components with higher wavelengths, 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. (Source: Empa)

Reference: M. Kovalenko et al.: Properties and potential optoelectronic applications of lead halide perovskites nanocrystals, Science 358, 745 (2017); DOI: 10.1126/science.aam7093

Link: Thin Films and Photovoltaics, Swiss Federal Laboratories for Materials Science and Technology Empa, Dübendorf, Switzerland

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