Page 265 - Organic Electronics in Sensors and Biotechnology

242 Cha pte r S i x

one electron-hole pair and so can contribute at most one electron to
the external circuit. The absence of gain limits sensitivity and means
that OPV devices are best compared to conventional (gain-free) pho-
todiodes. The question arises, whether it might be possible to achieve
gain in organic photodiodes under any circumstances. There have in
fact been several reports of OPV devices that, when operated under
reverse bias, show anomalously high photocurrents (see Refs. 61 and
62 and references therein). In these devices, the change in current ΔI
under illumination satisfies

V −
V
ΔIV() I light () I dark ()
= ℜ (6.68)
e e

where ℜ is the rate at which photons strike the device. In other words,
under illumination, the current changes by an amount that massively
exceeds eℜ, seemingly implying that more than one electron is gener-
ated for each absorbed photon. This is shown in Fig. 6.25 for an

10 4

10 3
Multiplication (%) 10 2 1

10
N N
CH 3 CH 3 10 0
Bathocuproine
10 –1
(a)
400 500 600 700 800
Wavelength (nm)
0 V –0.5 V –1 V
–1.5 V –3 V –4 V
Absorption

(b)

FIGURE 6.25 (a) The chemical structure of bathocuproine. (b) The apparent gain vs.
emission wavelength in an ITO/PEDOT:PSS/pentacene/C60/bathocuproine/Al
device under various reverse DC biases. (Reprinted with permission from Ref. 61.
Copyright 2007, American Institute of Physics.) (See also color insert.)