Environmental Engineering Reference
In-Depth Information
shelf lifetime (devices are kept in the dark), which is 912 h for inverted devices
and 256 h for normal ones; meanwhile the other one is that the devices are kept
under 100 mW/cm
2
, exhibiting a negligible degradation for inverted devices and a
faster degradation for normal devices due to continuous illumination. The stability
tests imply that the degradation rate for inverted devices is slower than that for
normal devices.
Such stability improvement could also be achieved by using TiO
x
as the
modification layer for ITO. Kuwabara et al. compared the stability of
P3HT:PCBM-based inverted and normal devices with a structure of ITO/TiO
x
/
P3HT:PCBM/PEDOT:PSS/Au and ITO/PEDOT:PSS/P3HT:PCBM/Al, respec-
tively. The devices were measured in ambient atmosphere under 100 mW/cm
2
[
85
]. As shown in Fig.
6.15
, the PCE of the normal device drops to 50 % of its
initial value after continuous illumination for 10 h, and the inverted device without
encapsulation keeps its initial efficiency for 20 h under continuous illumination,
indicating good stability either in ambient environment or under continuous
illumination for inverted devices.
As an alternative, modifying ITO by a LWF metal would be an effective and
simple approach to lower the work function of ITO. On the other hand, the contact
between anode and active layer should be significantly improved by a buffer layer
such as PEDOT:PSS [
86
], V
2
O
5
[
67
,
73
], and WO
3
[
87
] in order to reduce the
exciton quenching and then increase the charge collection efficiency. Except for
the roles of hole transport and extraction, such an anode buffer layer also acts as an
optical spacer, adjusting the effective optical field distribution [
88
]. More impor-
tantly, the interface stability could be enhanced due to the prevention of oxygen
and moisture from diffusing to active layer by using transition metal oxides (MoO
3
and V
2
O
5
).
Zimmermann et al. [
89
] systematically investigated the long-term stability of
P3HT:PCBM-based OPVs with an inverted structure, as shown in Fig.
6.16
, where
Fig. 6.15 The variation in g
(PCE) of inverted device
(ITO/TiOx/P3HT:PCBM/
PEDOT:PSS/Au) and
conventional device (ITO/
PEDOT:PSS/P3HT:PCBM/
LiF/Al) with irradiation time
[
85
]. (Reprinted from [
85
],
with permission from
Elsevier) (
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