Environmental Engineering Reference
In-Depth Information
Fig. 6.19 Schematics of the cross-section of the PV devices: a Encapsulated by a barrier film
with complete coverage over all device surface and b Encapsulated by a barrier film that has poor
surface coverage over the P3HT/PCBM layer [
108
]. (Reprinted from [
108
], with permission from
Elsevier) (
http://www.sciencedirect.com/science/journal/15661199
)
encapsulating films to disintegrate as the devices aged; (3) susceptibility to
mechanical damages during device characterization and handling. The Al
2
O
3
/
HfO
2
nanolaminated structure can overcome the issue of hydrolysis-induced aging
for only Al
2
O
3
films, owing to the hydrophobicity of the HfO
2
layers. However, a
UV-curable epoxy resin film as a capping layer is required for the Al
2
O
3
/HfO
2
encapsulation of the devices in order to reach the control of the degradation profile.
The aims to use this capping layer are (1) to reduce the accidental damages during
characterization and handling due to its thin thickness and also to enhance the
mechanical protection; (2) to diminish the small defects in the ALD film due to the
hydrophobicity of the P3HT:PCBM surface. This technique can prevent device
degradation induced by ambient atmosphere and also play a role in annealing step
beneficial to the increase of the initial efficiency. Figure
6.19
shows the compar-
ison of two devices encapsulated by a barrier film with complete coverage over all
device surface and poor coverage over the P3HT:PCBM layer, indicating the
importance of complete coverage surface for stability improvement. When the
ALD temperature is fixed at 140 C and the deposition time is 1 h, the devices
achieve a PCE of 3.66 % due to optimal annealing during encapsulation.
As shown in Fig.
6.20
, Potscavage et al. developed a 200 nm Al
2
O
3
barrier
layer grown by ALD as the encapsulation layer in pentacene/C
60
based small-
molecule OPVs [
107
]. The encapsulated device maintains a stable efficiency when
exposed to ambient atmosphere for over 6000 h, much longer than those without
encapsulation having a fast degradation upon exposure to air for 10 h. Moreover,
the thermal annealing during ALD deposition improves the V
oc
and then the
overall PCE of the devices.
Figure
6.21
compares the change of main parameters of OPVs with various
encapsulations from the initial values after exposure to ambient atmosphere, i.e.,
no encapsulation, UV epoxy, Al
2
O
3
, and Al
2
O
3
/UV epoxy. The device with no
encapsulation degrades very fast, whose J
sc
drops to \20 % of its initial value.
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