Page 346 - Organic Electronics in Sensors and Biotechnology

Organic Electronics in Memories and Sensing Applications 323

which depends on the quantum efficiency for charge carrier genera-
tion η, the generation rate G, and the average lifetime of the PCC τ.
The generation rate G is given by the product of absorption coef-
ficient α(λ) and the photon flux density φ per unit area per unit time.
If we assume that charge generation takes place within the active
region, i.e., η≠ 0 for 0 ≤ x ≤ d and η = 0 for d ≤ x ≤ t and I (λ) is pri-
ph
marily controlled by the generation process in the regime above the
absorption edge, then I (λ) can be written in terms of
ph

−
⎧ ⎪ Ke − α ( td) − e − α λ t ) for light frrom top
(
λ
λ
I () ∼ n () = ⎨ λ (8.5)
ph ph K (1 − e − α λ ) for light from bottom
d
⎩ ⎩ ⎪ λ
Where K = ( αητλ P )/ hc.
λ
λ
0
Equation (8.5) can be used to estimate the depletion zone d in a
phototransistor. 103
The photocurrent spectral response of photoresponsive P3HT OFETs
depending on the gate voltage is shown in Fig. 8.18. The aforemen-
tioned dependencies are well studied in multilayer thin-film diode
structures; however, studies of the photogeneration processes in OFETs
are scarce.
8.3 Large-Area Pressure and Temperature Sensors
Most advanced large-area, flexible sensor skins have been reported by
57
Someya et al. By using pressure-sensitive rubber and temperature-
sensitive organic diodes, large-area pressure and temperature sensor
arrays capable of capturing images of pressure and temperature
distributions have been demonstrated. As an example, images of
pressure distributions from a kiss mark are shown in Fig. 8.19. This
example outlines the huge potential of organic electronics in large-
area electronic surfaces.

8.4 Summary
In this chapter we provided a brief overview of the state of the art in
organic semiconductors, in polymeric gate dielectrics, in functional
gate dielectrics, and in OFETs derived from such materials. We have
outlined practical applications of OFETs in nonvolatile memories, as
well as in sensors for recording changes in ambient conditions, such
as changes in temperature and pressure. Although there has been tre-
mendous progress in these fields of organic electronics, there is still a
vast amount of research necessary to bring the technology to matu-
rity. Hence there is still plenty of room for exciting new developments
in memories and sensor applications, and organic electronics has a
bright future in research and development.