Page 274 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 251

physical configuration, choice of substrate material, and frequency of use.
X-ray exposure can cause a number of chemical changes in polymeric
materials––cross-linking, chain scission, ionic desorption, etc.––and
hence might be expected to degrade optoelectronic properties. Keiva-
89
nidis and coworkers investigated the aging of a variety of OPV
devices under cumulative radiation doses of up to 500 Gy––typical of
the total exposure experienced by a diode array inside a chest X-ray
system over its full operational lifetime. The researchers considered
three separate bulk heterojunction systems based on three commonly
used donor-acceptor complexes: P3HT:PCBM, F8BT:PDI, and TFB:
PDI (Fig. 6.30). The devices were fabricated by spin-coating a 70 nm
layer of PEDOT:PSS onto ITO-coated glass substrates, followed by a
100 to 200 nm layer of the blended active material, and finally a 70 nm
layer of evaporated aluminum. The spectral response curves were
measured before and after exposure to 500 Gy X-ray radiation through
the glass substrate. The P3HT:PCBM device showed a 17% reduction
in quantum efficiency due to X-ray exposure, compared to 2% for
F8BT:PDI and 3% for TFB:PDI. The dark currents of all three devices
were unaffected by X-ray exposure. The reduction in the quantum
efficiency of the P3HT:PCBM device was tentatively attributed to
the effects of secondary electrons, which are released when X-rays

N S
N

H C C 8 17 n H C C H N n
8 17
H
17 8
17 8
F8BT TFB
C H
6 13
O
S n MeO
PCBM
P3HT

O O

N N PDI
O O

FIGURE 6.30 Chemical structures of three donor/acceptor complexes tested by
Keivanidis and coworkers for stability against X-ray-induced degradation. (Reprinted
with permission from Ref. 89. Copyright 2008, American Institute of Physics.)