Team develops 'cheaper and more sensitive' image sensors

2 min read

Researchers from Technische Universität München (TUM) have developed image sensors claimed to be cheap to produce and more sensitive to light than conventional silicon versions.

They are said to consist of electrically conductive plastics, which are sprayed on to the sensor surface in an ultra-thin layer.

According to TUM, the chemical composition of the polymer spray coating can be altered so that the invisible range of the light spectrum can be captured, opening up new development possibilities for low-cost infrared sensors aimed at compact cameras and smartphones.

Image sensors are at the core of digital cameras. Before an image appears on the display, the sensors first convert the light from the lens to electrical signals. The image processor then uses these to create the final photo.

Many compact and mobile phone cameras contain silicon-based image sensors produced using CMOS (complementary metal oxide semiconductor) technology.

During tests, the organic sensors proved to be up to three times more sensitive to light than conventional CMOS sensors
During tests, the organic sensors proved to be up to three times more sensitive to light than conventional CMOS sensors

Prof Paolo Lugli and Dr Daniela Baierl from TUM have developed a cost-effective process to improve the performance of these CMOS sensors and their approach revolves around an ultra-thin film made of organic compounds.

The challenge lay in applying the plastic solution to the surface of the image sensors.

The researchers tested spin- and spray-coating methods to apply the plastic in its liquid, solution form as precisely and cost-effectively as possible. They were looking for a smooth plastic film no more than a few hundred nanometres thick — spray coating was found to be the best method, using either a simple spray gun or a spray robot.

In tests, organic sensors are said to have proven their worth by showing they are up to three times more sensitive to light than conventional CMOS sensors, whose electronic components conceal some of the pixels and therefore the photoactive silicon surface.

Similarly, organic sensors can be manufactured without the expensive post-processing step typically required for CMOS sensors, which involves applying micro-lenses to increase the amount of captured light.

Every part of every single pixel, including the electronics, is sprayed with the liquid polymer solution, giving a surface that is totally light-sensitive. The low-noise and high-frame-rate properties of the organic sensors also make them a good fit for cameras.

Another advantage of the plastic sensors is that different chemical compounds can be used to capture different parts of the light spectrum. The PCBM and P3HT polymers are ideal for the detection of visible light while other organic compounds, such as squaraine dyes, are sensitive to light in the near-infrared region.

‘By choosing the right organic compounds, we are able to develop new applications that were too costly up until now,’ said Lugli in a statement. ‘The future uses of organic infrared sensors include driver assistance systems for night vision and regular compact and cellphone cameras. Yet, the lack of suitable polymers is the main hurdle.’

A paper describing the research — A hybrid CMOS-imager with a solution-processable polymer as photoactive layer — has been published in Nature Communications.