Unprecedented resolution with a stacked color image sensor

The human eye has three different types of sensory cells for the perception of color: cells that are respectively sensitive to red, green and blue alternate in the eye and combine their information to create an overall colored 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 miniaturization, and unwanted image disturbances can occur; these are known as color moiré effects and have to be laboriously removed from the finished image.

Stack instead of mosaic: the perovskite layers each absorb only part of the light spectrum. Image: Empa

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 colors, 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 (see “Further reading”). 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.