Environmental Engineering Reference
In-Depth Information
both a single exponential mode and a linear mode in which the latter one is pre-
ferred [
37
]. However, Krebs et al. found that such a linear model could represent
degradation well only at the beginning of the device life [
15
], therefore it is sug-
gested that a single exponential could be a good fitting in a few examples and
several exponentials could fit better. It is found that the decay curves could be fitted
with a two-term exponential function: an initial fast decay and a second slower
decay. The fast decay is independent on atmospheric conditions such as device
operation and various barrier layers at the active layer/Al interface. The slow decay
depends on the atmosphere due to the reaction with oxygen [
15
].
Logdlund et al. demonstrated a theoretical investigation on the interaction of Al
with PPV and its derivatives [
38
]. It is found that Al prefers the reaction with
vinylene linkages in methoxy groups based PPV materials via forming covalent
bonds, Al-C. This is actually useful for cathode in OPVs due to the LWF of Al.
However, when methoxy groups are replaced with carbonyl groups, new reactive
sites are induced, resulting in Al-O bonds and exhibiting comparable stability.
Moreover, another explanation is proposed to be the existence of electron charge
transfer from the Al atoms to the polymer chains. Karst et al. studied the small-
molecule OPVs (ITO/PEDOT:PSS/CuPc/PTCDA(or DAAQ)/Al) with upper Al
electrode exposed to air [
39
]. The results show that the device with no exposure to
air yields a small V
oc
(0.125 V), and meanwhile, the V
oc
(0.5 V) increases largely
when the device is exposed to air as a result of an oxide layer (probably Al
2
O
3
)
present at the Al/organic interface. It is explained that the direct contact of organic
acceptor with Al or Ag induces Fermi level pinning, resulting in small V
oc
.
Formation of such an oxide layer separates the organic and Al layers, leading to no
more Fermi level pinning and high V
oc
.
Jeranko et al. demonstrated decay processes by using two-dimensional mapping
of PV response on the active area, concluding that a major degradation is attributed
to the path along the edges of the active area and the variance in the photocurrent
over the area of the device caused by the process of electrodes [
12
,
15
].
A cathode buffer layer is essentially necessary for good charge extraction by
cathode. Usually, it is thin enough to possibly prevent the opposite charge from
being injected or collected due to the existence of energy barrier at the organic/
metal interface.
Melzer et al. discussed the effect of inserting a C
60
layer in between active layer
and cathode by I-V characteristics and impedance spectroscopy, suggesting the
presence of a strong dipole layer at the C
60
/metal interface [
40
]. Krebs et al. also
inserted a C
60
layer at the MEH-PPV/Al interface [
15
], therefore, a significant
improvement on device stability is observed on the first decay parameter due to the
formation of an efficient barrier layer that conducts the electrons well and facili-
tates the exciton dissociation. Moreover, the authors claimed that high current
density due to high illumination intensity could accelerate the degradation, indi-
cating that the weight of the first exponential increases with the illumination
intensity. This could be attributed to chemical reactions of photoinduced radical
anions and radical cations at the Al interface. The concentration of charge carriers
at the Al surface increases with the current density increasing, resulting in the
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