Page 241 - Organic Electronics in Sensors and Biotechnology

218 Cha pte r S i x

0.6 0.25
OPV OPV
0.5
0.20
0.4
EQE (*100%) 0.3 Responsivity (A/W) 0.15
0.10
0.2
0.1 Si PV 0.05 Si PV
0.0 0.00
400 450 500 550 600 650 700 750 400 450 500 550 600 650 700 750
Wavelength (nm) Wavelength (nm)
(a) (b)
FIGURE 6.13 (a) External quantum effi ciency spectral response curves and
(b) photosensitivity spectral response curves for a Si device and an ITO/PEDOT:
PSS/P3HT:PCBM/Al device.

Usually S(λ) and η(λ) are determined by reference to a calibrated
photodetector: if, at the illumination wavelength λ, the calibrated
photodetector has a photosensitivity S (λ) and a quantum efficiency
cal
η (λ) and the currents from the test device and calibrated detector
cal
are I(λ) and I (λ), respectively, then:
cal
λ
λ
λ
()
S() = I() ηλ = I()
λ
λ
λ
S () I () η () I (()λ (6.21)
cal cal cal cal
The spectral response characteristics of a typical ITO/PEDOT:
PSS/P3HT:PCBM/Al device are shown in Fig. 6.13 in terms of both
photosensitivity and quantum efficiency. Also shown are the spectral
response characteristics of a typical Si photodiode (OSRAM, SFH2430)
for comparison. The peak efficiencies of the organic device (0.24 A/W,
55.7%) are comparable to those of the silicon device (0.18 A/W,
41.0%), although the active range is much narrower due to the nar-
row absorption range of the P3HT:PCBM system. (Note that although
the silicon photodiode we have chosen is fairly typical, superior
devices with substantially higher quantum efficiencies are available;
likewise, superior organic devices can also be fabricated.)

6.4.2 Rise Time and Cutoff Frequency
The speed of response is a critical consideration in data communica-
tions and time-resolved sensing applications, and it is usually char-
acterized in terms of either the rise time or cutoff frequency. The rise
time of a photodiode is conventionally defined as the time required
for the output to change from 10 to 90% of its final level in response
to a step increase in the intensity (Fig. 6.14). The fall time is similarly