Photodetectors and sensors designed to be flexible Posté le Mercredi 14 septembre 2011

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

Passive pixels

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 80μm pitch for a
300dpi scanning resolution.

EETimes Europe
September 2011

http://www.electronics-eetimes.com/en/magazine/eete-2010.html








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