Sensors Mag

Leveraging Monolithic Integration Benefits

May 1, 2006 By: Peter Adrian

This content is excerpted from Sensor Technology Alert and Newsletter, a sensor intelligence service published by the Technical Insights unit of Frost & Sullivan.

It has been well known for sometime that organic semiconductor technology provides vital benefits such as simplicity in fabrication process and most of all, monolithic integration, which provides cost-effective and reliable integration of all components of an optoelectronic device on a single substrate for a wide range of applications. But, in spite of these advantages there has been a dearth of devices that have actually implemented monolithically integrated organic components in organic optoelectronic devices.

Researchers at Swiss Center for Electronics and Microtechnology Inc. (CSEM), Switzerland, have developed an optical touch and proximity sensor based on polymer light-emitting diodes (PLEDs) and polymer photodiodes (PPDs). It is reportedly the first functional device combining solution-processed PLEDs and photodiodes. The developed sensing technology targets applications such as light emitting signs with touch switch functionality, (flexible and/or large-area) position-sensitive proximity detectors and others.

Lukas Burgi, project manager, CSEM says, "The sensor's thin-film light sources and detectors are monolithically integrated in the same plane of a common substrate and are processed from the liquid phase at room-temperature. These key features make the sensor potentially attractive for applications; for instance simple information displays with touch switch functionality, where low cost, small device thickness, tight space requirements or mechanical flexibility matters the most. Additionally, we tested this fabrication technique for a static light-emitting sign with integrated touch switch."

A monolithically integrated optical sensor was fabricated by deposition of polymeric semiconductor materials from solution. The sensor system, which is based on the combination of PLEDs and PPDs on a common substrate using a sequential disposition technique, has been termed as SENSoLED, and demonstrates the feasibility of integrated organic optoelectronic systems. The material used for PLEDs and bulk hetero-junction PPDs is a blend of polyfluorene materials.

With two operating modes--proximity mode or touch sensor, the device exhibits considerable versatility.When in the former mode, signal enhancement takes place when the light emitted by the PLED is reflected from nearby objects and falls back onto the photodiode pixels. On the other hand, the functioning of the touch sensor relies on change of refractive index with the slightest touch, which consequently, influences the angle of total reflection and therefore alters the guided modes in the substrate, resulting in a change (generally a decrease) in the photodiode signals. Furthermore, small molecule organic photodiodes have also been integrated on top of OLEDs to form an optical bistable switch.

The research has also identified situations where the performance of the developed technology may be hampered. For any practical application of the sensor, the presence of unwanted light from the surrounding is unavoidable. A solution for this problem is use of a sample and hold (s/h) circuit to assist the device in operating under normal indoors lighting conditions. Signal detection and background subtraction is achieved by modulation of the PLED driving current and a corresponding pixel-wise demodulation of the photodiode signals by virtue of a sample and hold stage followed by a low-pass filter.

CSEM is hopeful that commercialization of this technology with possible applications such as simple signs with integrated touch switch on glass substrates can be seen within the next 2 years whereas, more complex applications, especially those on flexible polymeric substrates, will take at least 5 years.

As far as collaborations are concerned Burgi told Sensor Technology, "We have a strong collaboration with Ciba Specialty Chemicals in the domain of electroluminescent polymers and printable electronics. We are actively looking for collaborations to advance our sensor technology and are in touch with potential collaborators." The researchers aim at developing a fabrication process entirely based on print processes (that is, gravure printing); optimization of sensor components, layout and electronics; incorporation of micro-optical components to improve the sensor's response for specific applications and improvement of operational lifetime.

Possible future applications for this potentially cost effective technology range from the aforementioned simple information displays with integrated touch-screen, to flexible, disposable sensor heads in medical instrumentation. The technology offers long term development potential toward future applications such as biochemical sensing and artificial skin.

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