Empa and ETH Zurich create a perovskite-based stacked image sensor

Researchers from Empa and ETH Zurich have developed a perovskite-based sensor prototype that absorbs light almost optimally and is also cheap to produce.

The team explains that the working mechanism of the human eye, not very different than various image sensors, is based on 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. However, this mechanism has 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 used in various applications have basically reached the limits of miniaturization, and unwanted image disturbances can occur; these are known as color moiré effects and have to be 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. This requires that the sensors on top let through the light frequencies that they do not absorb to the sensors underneath. Empa researchers have now succeeded in developing a sensor prototype that avoids these problems. It consists of three different types of perovskites. 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 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 significant: 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 (green and red), are considerably higher in the perovskites than in silicon. As a result, the layers can be made much 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, the team states that more work is needed in order to further develop this prototype into a commercially usable image sensor. Key areas include the miniaturization of pixels and the development of methods for producing an entire matrix of such pixels in one step. According to the researchers, this should be possible with existing technologies.