Environmental Engineering Reference
In-Depth Information
3.3.1 Cathode Interlayer
In principle, ITO is capable of collecting either electrons or holes since its work
function (4.5-4.7 eV) lies between the typical HOMOs and LUMOs of common
OPV materials. Therefore, the polarity of ITO can be modified to efficiently collect
either electrons or holes by coating functional interlayers of different work func-
tions onto its surface [
29
]. For inverted solar cell application, electron transporting
layers (ETL, or electron selective layer) with low work function are formed on top
of ITO to modify the ITO interface for efficient electron extraction and collection
from active layer. Such ETL materials typically include inorganic ones like
Cs
2
CO
3
[
29
,
33
], Ca [
94
], Al
2
O
3
[
95
], ZnO [
96
-
98
], and TiO
x
[
99
,
100
]. Since
light needs to pass through the ETL to reach the active layer to generate photo-
current, the layers are typically very thin to maintain high optical transmittance.
The electron mobility and work function of the ETL layer should also be opti-
mized in order to realize efficient electron collection. The summary of perfor-
mance of inverted PSCs using different ETL designs is listed in Table
3.3
and the
chemical structures of organic materials used as ETL in inverted PSCs are shown
in Scheme
3.4
(WPF-6-oxy-F is shown in Scheme
3.2
).
3.3.1.1 Metal Oxide Thin Film as ETL
ZnO and TiO
x
are the most commonly utilized ETL materials for inverted solar
cells due to the high optical transparency in the visible and near infrared region,
high carrier mobility, and its solution processibility. Many demonstrations of using
these n-type metal oxide layers as the electron selective layer for inverted solar
cells have been reported in literatures.
An efficient P3HT:PC
61
BM BHJ inverted solar cell from a high-temperature
processed sol-gel ZnO interlayer on ITO and an Ag electrode as the top hole-
collecting contact was first demonstrated by White et al. [
96
]. The zinc acetate
(ZnAc) sol-gel precursor was directly spin cast onto ITO and then thermally
annealed at 300 C for 5 min to hydrolyze and crystallize into amorphous ZnO
thin film. Improved conductivity and mobility after annealing led to conversion
efficiencies of 2.97 % of the inverted device. Interestingly, it was found that when
these devices were exposed to air, the performance gradually improved. They
attribute the improvement to the oxidation of Ag which formed Ohmic contact
with P3HT. State-of-the-art inverted solar cell using ZnO as ETL and a low-band-
gap donor material PCDTBT can yield PCE as high as 6.33 % [
97
]. To further
improve the electronic properties of ZnO, Al doping was explored and inverted
devices with Al-doped ZnO (AZO) as ETL were thus fabricated [
101
,
102
].
Although the AZO device performance is not greatly improved compared to
undoped ZnO devices, AZO layer can be made much thicker ([100 nm) than ZnO
layer without hampering the solar cell performance; thus, it is more robust and
easier to process.
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