Page 257 - Organic Electronics in Sensors and Biotechnology

234 Cha pte r S i x

increases linearly with the measurement time τ. Error ε is minimized
by ensuring that (1) the shunt resistance is as high as possible; (2) the
input bias current and dc offset voltage are as small as possible; and
(3) the integration period is as short as possible, extending only over
the time during which the device is illuminated.
To measure small currents, a small feedback capacitance should
be used: a 1 fA input, a 1 pF feedback capacitance, and a 1 s measure-
ment time will, for instance, result in a 1 mV output. In use the feed-
back capacitance is discharged immediately prior to performing the
measurement. As we will see later, current sensing via charge integra-
tion is especially useful for photodiode arrays.

6.6 The State of the Art
The message from the above discussion is clear: for fast low-noise
amplification, we require photodiodes with low capacitances, high
shunt resistances, and high quantum efficiencies over the spectral
range of interest. In this section, we consider what is currently achiev-
able using organic photodiodes, and we speculate what might be
possible in the future through improved device engineering and
materials design.

6.6.1 Capacitance
As noted above, the capacitance densities of organic devices are relatively
2
high, typically of the order of 300 pF/mm compared with around
2
2
30 pF/mm for standard Si devices and 3 pF/mm for PIN devices.
We are aware of no research that has specifically addressed the issue of
minimizing the capacitance density of organic devices (while main-
taining good overall device performance). However, since the
capacitance density is inversely proportional to both the electrode
separation and the relative permittivity of the intervening medium,
the obvious solution is to increase either the thickness d or the relative
permittivity ε of the active layers. (The latter could be achieved by
r
grafting easily polarized side-chain groups onto existing OPV mate-
rials, assuming it is possible to do so without unduly compromising
†
their charge transport properties). The principal challenge in both
cases is to maintain a high quantum efficiency in spite of the reduced
internal field strength, e.g., through the use of high carrier mobility
materials. At the time of this writing, an order of magnitude reduc-
2
tion in capacitance densities to ~ 30 pF/mm seems quite realistic,

† Conjugated macromolecules with permittivities as high as 900 were studied and
52
reported in the 1960s, but we are not aware of any recent work that has sought to
incorporate such materials into devices.