Page 268 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 245

the inkjet approach. In addition, a range of conventional contact-
based printing and coating techniques are under development, e.g.,
gravure, flexographic, screen, and offset printing. These techniques
have not been widely used for photodiode applications, although they
are all being intensively investigated for solar energy applications.
Contact methods, although still at an early stage of development, show
considerable promise: Tobjork et al., for instance, fabricated a flexible
solar cell with 0.74% power conversion efficiency using a gravure
64
coating technique, and Sakai et al. used screen printing to fabricate
65
a bulk heterojunction solar cell reaching 2.35%. The possibility of
using printing methods to fabricate large continuous and pixelated
photodetectors offers exciting opportunities. The largest single-
element detector currently available is a 20 in photomultiplier tube,
2
and the largest two-dimensional sensors are approximately 2000 cm .
Printed OPDs offer the prospect of creating much larger devices at
significantly lower cost.
The best OPV devices are currently fabricated on rigid glass sub-
strates coated with ITO, and so they do not make full use of the
processing advantages of organic materials. In particular, they do not
exploit the potential for low-cost large-area reel-to-reel manufactur-
ing, which requires the use of a conformable substrate. Accordingly,
in recent years, many researchers have sought to fabricate organic
devices on flexible ITO-coated plastic substrates. 66–69 Brabec and co-
workers at Linz, for instance, have reported 6 cm × 6 cm PV devices
on ITO-coated plastic substrates with solar power conversion effi-
70
ciencies of 3%. The use of such substrates, however, presents two
significant technological problems. The first problem is that plastics,
unlike glass, are highly permeable to oxygen and water, so additional
barrier coatings must be incorporated into the substrates to prevent
contamination (and associated degradation) of the active layer. These
barrier coatings must balance the need for extremely low permeabil-
ity (which, generally speaking, calls for thick and dense barrier coat-
ings) with the conflicting needs for high transparency and flexibility
(which require that the barrier layers be as thin as possible). One
option is to use a single layer of material such as silicon oxide, which
exhibits an extremely low permeability to oxygen and water. In prac-
tice, however, this approach is ineffective since unavoidable pinhole
defects in the oxide layer provide pathways for moisture to reach the
active layer. The most successful barrier coatings to date use a stack
of different materials, e.g., alternating thin layers of polymer and
ceramics, to create a highly impermeable film. Pinhole defects are still
present in these composite films, but they generally extend across a
single layer only and hence do not create continuous moisture path-
ways to the active layer. Today’s barrier coatings still lack the degree
of transparency and flexibility one ideally requires, and further devel-
opment is needed in this regard.