Environmental Engineering Reference
In-Depth Information
Fig. 6.18 Air-stability of the
inverted and forward cells
[
93
]. (Reprinted from [
93
],
with permission from
Elsevier) (
http://
improved as compared with that of the normal device [
93
]. Figure
6.18
shows the
change in the normalized PCE against the air-exposure time for inverted and
normal OPVs. It is observed clearly that the shelf lifetime of the inverted device F
is *15 days, however the shelf lifetime of the normal cell G is only *2 days. The
explanation for the improvement of air stability for the inverted cell can be as
follows. In the normal device G (ITO/MoO
3
(10 nm)/P3HT:PCBM/Ca(10 nm)/
Ag), the LWF metal (Ca) cathode on top of the active layer would easily react with
the water and oxygen in air although a layer of Ag is deposited on top of the Ca
layer. On the contrary, in the inverted device F (ITO/Ca(1 nm)/P3HT:PCBM/
MoO
3
(10 nm)/Au), air-stable Au is used as the top anode, however the reactive Ca
incorporated between ITO and P3HT:PCBM layer has been prevented from its
direct contact with air, resulting in the improvement of air stability. Figure
6.18
also shows the degradation of a normal PEDOT-Al device (ITO/PEDOT/
P3HT:PCBM/Al) with time for comparison, which presents the poorest air sta-
bility with a short shelf lifetime of *0.5 day. The rapid degradation of the PE-
DOT-Al cell can be ascribed to the degradations of both organic/Al and ITO/
PEDOT interfaces [
28
,
98
]. Therefore, the results reveal that LWF metals are also
suitable to be adopted as electron selective interfacial layer for inverted cell and
good for stability improvement.
6.4.3 Device Encapsulation
As discussed before, OLEDs suffer from different modes of degradations as (i)
dark-spot degradation, (ii) catastrophic failure, and (iii) intrinsic degradation. The
first two modes of degradation could be effectively controlled by means of proper
device encapsulation and adequate control over device fabrication conditions;
meanwhile, the intrinsic degradation mode has been far more challenging and
continues to be an issue for OLED commercialization [
20
,
31
,
32
].
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