Page 270 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 247

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0.1 Dark current 50
Illuminated current
Current density (mA/cm 2 ) 1E–3 EQE(%) 40
0.01
1E–4
30
1E–5
20
1E–6
1E–7 10
1E–8 0
–1.0 –0.5 0.0 –0.5 1.0 300 400 500 600 700 800
Voltage (V) Wavelength (nm)
Low dark current: 0.46 pA EQE >55% el/ph@ 0 V
(a) (b)

(c)

FIGURE 6.27 (a) Current-voltage curves, (b) spectral response curve, and
(c) photograph of a fl exible ITO-free photodiode fabricated on a PET plastic
substrate. The structure of the device was PEDOT:PSS/P3HT:PCBM/Al. (From
Ref. 83—reproduced by permission of the Royal Society of Chemistry, 2007.)

6.7.2 X-Ray Imaging
In X-ray imaging (XRI), a uniform source of X-rays is passed through
an obstacle––often human tissue––and the transmitted light is
detected using a two-dimensional panel of photodiodes. X-rays are
difficult to focus, and the XRI panels must therefore be at least as
large as the objects they are used to image. Crystalline silicon wafers
are available only with diameters up to 300 mm, and for substantially
larger applications, amorphous semiconductors must be used. The
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most widely used material for XRI is amorphous silicon (a-Si). How-
ever, due to its low atomic mass, a-Si is unable to detect X-rays directly
and must be used in conjunction with a scintillator screen that emits
visible photons when struck by X-rays. Higher atomic mass materials
such as amorphous selenium (a-Se) can detect X-rays directly and
hence do not require a scintillator. Both a-Si and a-Se can be deposited
onto large-area substrates by chemical vapor deposition, and panels
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of up to about 1600 cm are currently available. However, due to the
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