Photodetectors and sensors designed to be flexible
Over the last ten years, major players in the organic chemistry industry have significantly invested in R&D to create novel materials for companies attracted by the exceptional properties of printed electronics: large area, thin, light weight, flexible or conformable, and soon to be made transparent. A start-up of CEA Liten, Isorg is developing photo detectors and image sensors based on organic materials to be printed on plastic substrates. The company plans to industrialize these products by the end of next year. This new generation of electronic materials brings disruptive innovation compared to the silicon industry because it is based on solutions of organic materials and plastic substrates or glass. It is a low carbon footprint industry; the manufacturing process takes place at ambient temperature and only requires a 10000 class clean room for the printing techniques, compared to the costly vacuum and high temperature environments required for traditional semiconductor materials. Most of these properties are already largely used for photovoltaic devices, but until now they had not been developed for photo detection.
In order to adapt these manufacturing capabilities to new market needs and to enable the fast prototyping of custom devices, various applications including a sheet-to-sheet manufacturing process have been selected by Isorg - see figure 1. In CEA Grenoble, Isorg has set up a new technology platform named PICTIC (French acronym for "Plateforme d'Impression de Composants pour les Technologies de l'Information et de la Communication et les Capteurs" which stands for printed components platform dedicated to information technologies and sensors). The 450m2 of clean room facility is dedicated to the development of printing techniques for organic electronics on plastic and glass substrates. Printed light emitting diodes (PLED), organic thin film transistors (OTFT) and organic photo detectors (OPD) will be prototyped for pre industrialization on this newlyequipped line and it is expected that by 2013, Grenoble's facilities will be capable of producing thousands of 320x380mm foils or glass plates. Layered photo detector approach The technology structure of the organic photo detector (OPD) is based on a multi-layer stack as shown in figure 2. Various organic semiconductors are being tested to achieve a large bandwidth for applications in the near infra-red, close to 950nm. The robustness of the device to humidity is linked to the device's structure. There are also a number of new developments ongoing in partnership with the chemicals supplier's equipment to reduce the photodiode size as well as to enable transparent electrodes.
Principle of operation
Each photo detector cell can be read measuring the photo generated current, using a resistor-based Trans Impedance Amplifier (TIA) as shown on figure 3, or collecting the photo-generated charges using an integrator as shown on figure 4. From an electrical point of view, running an organic photo detector in an application doesn't add any design complexity compared to using silicon-based devices. All the "usual design tricks" to get rid of noise or speed-up reading can be implemented. Considering power consumption, it is expected that the image sensor applications will run at a power lower than traditional silicon-based alternatives. Energy harvesting techniques may also be combined to further reduce overall power consumption. Designing reading circuitry for new application based on organic photo detectors is quite similar to what is required for CCD or CMOS image sensors, hence design efforts are minimal with of-the-shelves standard products. Only for high volumes, an application specific integrated circuit (ASIC) may be worth designing. Printing techniques allow for very fast prototyping and only three days are required to fabricate brand new organic photo detectors, with custom shapes that adapt the sensors to specific applications. This compares favorably with the lengthy and costly development and prototyping steps for silicon-based devices. This means that even small and medium size volume applications could get a dedicated sensor product design. The current-voltage characteristic (shown in figure 5), sensitivity and dark current levels are quite similar or fare better compared to some silicon counterparts. One potential technical limitation could be due to a higher capacitance versus photo diode's area. This point may be partially overcome using a larger biasing operating point for a specific application
Currently, only passive pixels may be designed because only the photo diode is present on the plastic foil, so Isorg is working on several industrial and consumer applications where passive pixels are more than enough. But CEA Liten and Isorg are working on a hybrid structure combining organic photo detectors with organic thin film transistors, which may be available for prototyping within a year. To promote its large area, thin, lightweight and flexible image sensors, Isorg has developed an attractive demonstrator, the Magic Pad. The prototype platform is able to sense 3D activity in a range of 30cm, with a resolution of 100 pixels on a 80x80mm active area. The Magic Pad is providing exciting and new functionalities such as multimedia 3D navigation without contact (for instance to browse music tracks on a computer).
Through optical sensing, the Magic Pad's surface is able to detect the user's hand position or motion. The photo sensors enable 3D functionality by measuring the height of the hand over the Magic Pad's surface. Overlaid on a display, the integrated optical sensors can not only provide a convenient user interface but also offer scanning capabilities, with character recognition. Like other optical sensors, the Magic Pad is sensitive to ambient light conditions and it is able to auto calibrate when needed. The development platform features a FPGA from Altera, on which different sensor's topologies can be tested with the associated processing software. Additional features, like backlighting with white or infrared LED and video outputs (VGA or LCD color screen display) are also available. Isorg's target for the near future is to design photo sensors with a 80%u03BCm pitch for a 300dpi scanning resolution.