Page 276 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 253

Finally we note that the above discussion has focused on passive
pixel sensors (PPSs), in which the transistor is used merely as a means
of controlling the connection of the photodiode to an external ampli-
fier. This kind of approach is appropriate for large-area applications
where cost considerations mean it is impractical to provide each pixel
with its own dedicated amplifier circuitry. However, PPS systems do
not provide in situ amplification and can suffer from excessive noise
arising from the capacitance of the data lines. In active pixel sensors
(APSs), every photodiode has three or four dedicated transistors that
play the dual role of pixel selection and in situ amplification. Tedde
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et al. have shown that APS panels provide superior signal-to-noise
performance and improved scalability to large-area panels, albeit at
the expense of increased circuit complexity. Using a pixel circuit com-
prising an OPD on top of three a-Si TFTs, they were able to achieve a
2
2
detection limit of 1 μW/cm compared to 6 μW/cm for a comparable
PPS system. They reported good signal linearity and an in situ ampli-
fication of up to 10 using the APS pixel circuitry.

6.7.3 Diagnostics
The low-cost nature of organic semiconductor devices raises exciting
opportunities in the areas of point-of-care and self-test diagnostics.
These markets are notoriously price-sensitive and have historically
been dominated by products with manufacturing costs in the cents to
low dollars range. The current state of the art in this price bracket is
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the lateral-flow test, which is the technology behind home fertility
and pregnancy tests and involves a simple color change in response
to an analyte of interest. Lateral flow tests are qualitative (or at best
semiquantitative), typically offering results of the yes/no or high/
medium/low variety. In many situations, however, quantitative data
are required. The medical practitioner, for instance, may need to
monitor the progression of a disease or the efficacy of a pharmaceuti-
cal treatment by determining the precise concentration of a specific
biomarker. This currently requires that a blood or urine sample be
sent to a remote laboratory where a quantitative assay is carried out
using conventional analytical chemistry procedures. The ability to
carry out such tests directly at the point of care would bring signifi-
cant benefits: in particular, it would eliminate the need for patients to
make repeat visits, thereby freeing up clinician’s time, bringing for-
ward the initiation of treatment, improving recovery prospects, and
lowering treatment costs. Importantly such tests could also be
deployed in developing countries that lack the centralized infrastruc-
ture needed for conventional testing, and hence would help address
serious deficiencies in current health care provision.
There are relatively few technologies that offer a viable low-cost
solution to quantitative point-of-care chemical analysis, but one prom-
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ising approach is the use of microfluidic devices. The microfluidics