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An Intr oduction to Or ganic Photodetectors 217
the geometrical capacitance density a value of ~300 pF/mm . This
2
compares with about 30 pF/mm for a typical silicon photodetector
2
2
(Hamamatsu S4797-01) and about 3 pF/mm for a PIN-type silicon
photodetector (Hamamatsu S5821). There is some scope for lowering
the capacitance of OPV devices by using thicker films, but this prob-
ably allows for a 5- to 10-fold reduction at best (since the use of exces-
sively thick films frustrates the extraction of charge carriers and tends
to reduce device efficiencies). Organic photodiodes are therefore rela-
tively high-capacitance devices, which have important implications
for their noise characteristics and speed of response.
6.4 Device Characteristics
6.4.1 Spectral Response
The spectral response of a photodetector is normally characterized in
terms of either its “photosensitivity” or its quantum efficiency, both
of which are ordinarily measured under short-circuit conditions. The
photosensitivity S(λ) at an illumination wavelength λ is defined as the
ratio of the photocurrent I(λ) to the incident power P(λ):
λ
I()
S() = (6.17)
λ
λ
P()
In situations where the photodiode is illuminated by a broadband
excitation source, the resultant photocurrent I is given by
λ
max
I = ∫ P()λ S()λ dλ (6.18)
λ
min
The quantum efficiency η(λ) is the fraction of incident photons that
are successfully converted to a photocurrent in the external circuit
I λ () e
/
ηλ() = (6.19)
ℜ λ ()
where ℜ(λ) is the rate at which photons of wavelength λ impinge on
the device. Clearly, ℜ(λ) = P(λ)/(hc/λ). The quantum efficiency and
the photosensitivity are therefore related by the following identity:
/
/
ηλ() = I λ () e = I λ () e = hc I λ () = hc S () (6.20)
λ
ℜ λ () P λ () (hc/ λ) λ e P(()λ λ e
/