An eye for the dye: Novel IR sensors

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  • Published: Feb 1, 2013
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: An eye for the dye: Novel IR sensors

So NIR...

A hybrid CMOS-imager with a solution-processable polymer as photoactive layer

Image sensors developed by researchers at the Technische Universität München (TUM) are more sensitive to light than the conventional silicon devices. The sensors are simple and inexpensive to produce and could easily be modified by switching dye molecules to use them for infrared applications. The team makes their new sensors by spraying an ultra- thin layer of conducting polymer on to the sensor surface in an ultra-thin layer.

Changing the precise composition of the polymer coating adjust the specific wavelengths that the sensor detects, which the researchers suggests opens up the possibility of developing low-cost infrared sensors for compact cameras and smart phones

Writing in Nature Communications Paolo Lugli and Daniela Baierl of TUM and their colleagues allude to how image sensors are at the heart of every digital camera and that commonly these are based on silicon devices, such as the well-known CMOS (complementary metal oxide semiconductor) technology. Going organic might reduce costs as well as come with additional benefits, the team reasoned. The challenge lay in how to coat an image sensor with a suitable polymer layer from solution. Spin- and spray-coating approaches were tested in an effort to find a simple and cost-effective approach to adding a smooth and ultra-thin polymer layer just a few hundred nanometres deep.

...and not...

Sensors based on organic polymers can achieve sensitivity to light three times that of standard CMOS devices; part of the improvement is that the sensitive layer is not partially concealed by electronic components that hide some of the pixels on a silicon surface. Moreover, organic precludes the need for costly post-processing steps needed in the manufacture of CMOS sensors where micro-lenses have to be applied to improve light capture. The team points out that every part of every single pixel, including the electronics, is covered with the liquid polymer solution spray so that it is 100 percent light-sensitive in terms of area. Organic films also offer low noise and potentially high frame rate properties making them useful for cameras and potentially spectrometric applications.

...too far

The team explains how different polymers offer different sensitivity at different wavelengths. For instance, the PCBM (the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester currently being investigated in photovoltaic applications) with P3HT (a polythiophene) are highly sensitive to visible light. By using squaraine dyes it is possible to shift the range into the near-infrared region of the spectrum.

“By choosing the right organic compounds, we are able to develop new applications that were too costly up until now,” explains Lugli. “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.”

Related Links

Nature Commun, 2012, 3, online: "A hybrid CMOS-imager with a solution-processable polymer as photoactive layer"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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