Page 225 - Organic Electronics in Sensors and Biotechnology

202 Cha pte r S i x

OPV devices may in the future have an advantage over other tech-
nologies is in spectral range. OPDs with a spectral range 350 to 1000 nm
have already been reported, which is better than typical multialkali-
5
†
based PMTs (300 to 850 nm) and approaches that of UV-enhanced
silicon photodiodes (190 to 1100 nm). We are confident that OPDs
with greatly enhanced spectral ranges will be developed in due
course. Finally––and this is a point that we will consider in some
depth in the following pages––organic photodiodes may one day
become the solution of choice for low bandwidth low light-level opti-
cal detection, offering comparable performance to the silicon photo-
diode at a price point close to that of the photoresistor.

6.3 OPV Devices
The vast majority of organic photodetectors can be classified as pho-
todiodes, in that they (1) generate a photocurrent even in the absence
of an applied bias and (2) exhibit no internal gain (i.e., at most one elec-
tron is extracted into the external circuit for each absorbed photon). In
addition, there have been a few reports of organic phototransistors
6, 7
(OPTs) which, like their inorganic counterparts, combine photosen-
sitivity with current amplification. OPTs have been less well studied
than even organic photodiodes and will not concern us here, although
they may turn out to have important applications in image sensors
due to their ease of fabrication.

6.3.1 Device Architectures
There are two main device architectures for organic photodiodes: the
discrete heterojunction (Fig. 6.6b) and the bulk heterojunction (Fig. 6.6c).
The simplest discrete heterojunction devices consist of a bilayer of a
hole transporting material (the donor) and an electron transporting
material (the acceptor) sandwiched between two electrodes. The sim-
plest bulk heterojunction devices consist of a single blended layer of
the donor and acceptor, again sandwiched between two electrodes.
The key requirement in both cases is that the frontier orbitals of the
two organic materials be substantially offset to encourage electron
transfer from the donor to the acceptor or hole transfer in the oppo-
site direction (Fig. 6.6a). The result is a partitioning of the holes and
electrons into the donor and acceptor phases, respectively, from
where they can be transported to the relevant electrodes.
The charge separation step does not in itself guarantee exciton
dissociation since the electron-hole pairs may remain coulombically

† Note that Ag-O-Cs photocathodes are sensitive from 400 to 1200 nm, but they have
low peak quantum efficiencies of just 0.36% compared to ~20% for alkali-based
photocathodes. However, PMTs based on Ag-O-Cs can still be a viable choice for
low level light detection due to the low-noise nature of the gain process.