Environmental Engineering Reference
In-Depth Information
In OPV devices, PPV and its derivatives have been used, however, these
polymers based devices exhibit low stability under illumination in air. Hence, an
alternative is to use the polythiophenes, such as P3HT, which has given a PCE of
5 % or higher [
99
-
101
]. At the same time, the lifetime is reported in the range of
1000 h [
102
]. Very long-lifetime devices based on P3HT have also been presented
with accelerated lifetime testing for 4000 h or even more than 1 year [
103
,
104
]. It
is thought that accelerated testing for 1 year corresponds to an operational lifetime
of 3-10 years [
37
,
104
].
Such a long lifetime would bring the technology into an expectation that this
should be commercialized in the near future. However, one of the significant
concerns or big challenges about commercialization is the stable encapsulation
scheme for OPV devices. It is known that OPVs exhibit a rather poor property of
air stability due to the chemical and physical and even mechanical degradations,
primarily resulting from the oxygen and moisture in the atmosphere. Except for the
stability improvements in essential materials and device structures, it is necessary
and important to develop the technology and approaches of the device encapsu-
lation benefiting to the long lifetime of OPV operation. Therefore, adequate
encapsulation from ambient oxygen and water is an essential requirement for the
commercial viability of OPV cells. The encapsulating layer in turn should be thin,
defect-free, light weight, and offer ease of processing [
105
].
In this section, we will discuss a few methods on encapsulation techniques and
their effect on the device stability.
Krebs et al. presented P3HT:PCBM-based device with an operational stability
for more than a year after an efficient and proper method for the encapsulation,
exhibiting a decrease in the efficiency to 65 % compared to the initial value [
104
].
This method follows the process of (1) device preparation on glass substrates; (2)
sealing by using glass fiber reinforced thermosetting epoxy against a back plate.
Such an encapsulation makes it possible to transport oxygen and moisture sensitive
OPV devices outside a glove box. The author emphasized that once the substrates,
backplate, prepreg, and jig are available in laboratory, this fast and simple sealing
procedure requires only 10 min of work besides the thermosetting time of 12 h.
Atomic layer deposition (ALD) is a chemical vapor deposition technique
involving the cycling of alternate precursor gasses into a chamber to react on the
surface of samples and grow high-quality and conformal films [
106
,
107
].
Ultrathin barrier layers have become promising candidates for device encapsula-
tion [
105
,
107
,
108
], which are deposited by ALD at low temperature, particularly
suitable for organic and flexible electronics as passivation layers with excellent
film conformity [
105
].
Chang et al. presented a thin-film encapsulation of Al
2
O
3
/HfO
2
for OPVs by
using ALD [
108
], in which 26 nm Al
2
O
3
/HfO
2
nanolaminated films consist of 52
pairs of alternating 2 Al
2
O
3
and 3 HfO
2
layers, and the deposition temperature is
140 or 150 C, and the chamber pressure is 0.1 Torr. Encapsulating the devices
with ALD films brings three problems of ALD films: (1) poor nucleation on the
surface of P3HT:PCBM layer exposed at the device edges, resulting in incomplete
coverage of the ALD films (Fig.
6.19
b); (2) rapid hydrolysis in air, causing the
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