Environmental Engineering Reference
In-Depth Information
form bilayer cathodes, such as Ca/Al and Ba/Al, the development of new inter-
facial materials to use as cathode interlayer is still required.
Inorganic fluorides, such as lithium fluoride (LiF), are promising electron
extraction materials for BHJ-PSCs. A bilayer electrode of LiF/Al is usually chosen
to replace a pristine Al cathode, because a thin layer of LiF can guarantee a good
Ohmic contact between the metal and the organic active layer [
23
]. The formation
of a favorable dipole moment or a LiF buffer layer at the organic active layer/LiF/
Al interface is usually considered to be the reason why LiF could improve the
performance of BHJ-PSCs. It was considered that a monolayer covered LiF can be
decomposed upon Al deposition and resulting in Li-doping of the organic layer to
deliver a low work-function contact; while a thick LiF usually generates a dipole
layer to lower the work function of electrode [
24
-
26
].
Caesium carbonate (Cs
2
CO
3
) is another efficient electron extraction material
used to fabricate bilayer cathode for BHJ-PSCs [
27
,
28
]. Interestingly, the Cs
2
CO
3
layer can be deposited by either thermal evaporation or solution spin coating.
The implementation of Cs
2
CO
3
is beneficial for reducing the electron extraction
barrier and series resistance (R
s
). Compared to bare Al device, the device with
inserted Cs
2
CO
3
exhibited enhanced V
oc
and fill factor (FF) [
29
,
30
]. However,
whether the actual product of thermally evaporated Cs
2
CO
3
is Cs
2
OorCs
2
CO
3
is
still uncertain[
27
,
31
-
33
]. Anyway, it is widely accepted that the formation of
Al-O-Cs complex yields the low work-function contact and thus facilitates the
electron extraction [
27
].
n-Type semiconducting metal oxides such as titanium suboxide (TiO
x
), which
is a well-known photocatalyst and is widely used in dye-sensitized solar cells
[
34
,
35
], had attracted considerable attention recently due to its solution pro-
cessability from sol to gel process [
36
], nontoxicity, and transparency. TiO
x
possesses a conduction band edge and valence band edge of -4.4 and -8.1 eV,
respectively, which would endow TiO
x
good electron extraction ability from active
layer and outstanding hole-blocking ability [
36
]. Hence, TiO
x
is an ideal cathode
modification material for BHJ-PSCs application [
36
]. The TiO
x
layer can also
resist the permeation of oxygen and moisture into the organic active layer, because
the formation of robust titanium oxide film via annealing the titanium oxide film
deposited by sol-gel process at 150 C. Moreover, the TiO
x
layer can also function
as an optical spacer via spatially redistributing the incident light intensity to further
improve the light harvesting property of BHJ-PSCs [
36
]. Recently, Park et al.
reported a promising PCE of 6.1 % for BHJ-PSCs with poly[N-9
0
-heptadecanyl-
2,7-carbazole-alt-5,5-(4
0
,7
0
-di-2-thienyl-2
0
,1
0
,3
0
-benzothiadiazole)] (PCDTBT, see
Scheme
3.1
):PC
71
BM blend as active layer by inserting TiO
x
as an electron
extraction layer and optical spacer. Notably, the device exhibited internal quantum
efficiency (IQE) closing to 100 % around 450 nm, which indicates the high effi-
cient conversion of incident photon into charge carrier [
37
]. However, the intrinsic
electrical properties of the TiO
x
film are dominated by the processing conditions.
Additionally, the electron mobility of TiO
x
is 1.7 9 10
-4
cm
2
V
-1
s
-1
, which is
almost two orders of magnitude lower than that of PC
61
BM, and this may
Search WWH ::
Custom Search