Page 218 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 195

application, it is important to take into account both the optical
demands of that application and wider issues relating to the circum-
stances in which the detector will be used. In data comms, the inten-
sity of the optical signal is usually quite high, and the speed of
response tends to be the overriding consideration. In sensing applica-
tions, other issues such as sensitivity, linearity, dynamic range, and
spectral range are often more important. In some cases, tolerance to
harsh operating conditions may be required, including resilience to
corrosive chemicals or extremes of temperature. In others, issues such
as the size, weight, power consumption, or cost of the photodetectors
may be important. It is rarely the case that a single technology will
meet all of an engineer’s design criteria perfectly, and selecting a suit-
able detector frequently comes down to finding an acceptable com-
promise between many competing criteria.
It is for the above reasons that the emergence of a new detection
technology is important. OPDs will offer their own mix of advantages
and disadvantages, making them superior for some applications and
inferior for others. It is likely that new applications will arise for which
conventional detectors are either technically or commercially unsuited,
opening up completely new technological opportunities. We shall
speculate in later sections about what these opportunities might be, but
we start by reviewing the main photodetectors in current use.

6.2 Conventional Photodetectors
Photomultiplier tubes (PMTs) are vacuum tube devices that use the
photoelectric effect to convert photons to electrons (Fig. 6.1). 1, 2

Photocathode
Electrons Anode
Incident Electrical
photon connectors
Scintillator

Light Focusing
photon Dynode
electrode Photomultiplier tube (PMT)
(b)

(a)

FIGURE 6.1 (a) Typical photomultiplier tube in vacuum housing. (b) Schematic of
PMT, indicating current gain via secondary electron emission at dynodes. (Picture
courtesy of wikipedia.)