Environmental Engineering Reference
In-Depth Information
heterojunction can be used instead of a BHJ, so that only PC
61
BM contacts the
metal layer [
139
]. Another approach is to have additional hole-blocking or
electron-blocking layers in the ICL. Heavily doped p-n junction as ICL was
proposed to ensure Ohmic contact between the subcells [
140
,
141
]. This approach
allowed the addition of the V
OC
of individual subcells in tandem architectures with
minimal loss due to absorption or reflection in the interlayer. More importantly,
the good electrical contact of the ICL does not depend on the choice of photoactive
materials.
More commonly used ICL design for tandem cells is the bilayer composite
consisting of a low work-function (n-type) layer and a high work-function (p-type)
layer [
142
]. Typical materials used for p-type layer are high work-function metal
oxides such as MoO
3
or WO
3
. These metal oxides can easily form Ohmic contact
with organic active layers and they are also highly transparent. Zhao et al. used
MoO
3
as high work-function contact for the top cell and a bilayer of LiF and Al
forming a low work-function contact with the bottom cell [
143
]. Negligible optical
loss in the ICL was observed because of almost 100 % transmittance of the
interlayer and no potential losses were observed either.
A major challenge in fabricating polymer tandem cells is the processing of the
top cell without damaging the underlying layers. Thus, the criterion for choosing
the materials for ICL, besides Ohmic contact forming ability, is the robustness of
the thin film so that it can withstand any solution process and protect the bottom
cell layers. Kawano et al. demonstrated a polymer tandem cell using ITO/PE-
DOT:PSS as ICL [
144
]. The ITO layer in the ICL was sputtered onto bottom cell
active layer in 1 Pa of argon gas so that the damage in active layer can be
prevented during sputtering. It was found that 20 nm of ITO was sufficient to
protect the underlying active layer from solution process of the top cell. Hadipour
et al. used an efficient multilayer ICL consisting of n-type and p-type layers [
145
].
The n-type layers were LiF/Al and the p-type layers were Au/PEDOT:PSS.
Au layer was deposited to protect the bottom LiF/Al contact from the solution
process of the top cell. V
OC
of the tandem cell was equal to the sum of V
OC
of the
component cells which shows the effectiveness of the ICL. However, the disad-
vantage of using metal layers is the loss of incident photons for the top cell.
3.4.2 Solution-Processed ICL
The real advantage of polymer solar cells is the ease of fabrication using solution
process, thus the development of solution-processed ICL for tandem cells is
necessary in order not to compromise this advantage. There were many reports on
solution-processed electron transport layers such as ZnO and TiO
2
(or TiO
x
), that
can be used as n-type layers in the ICL. These electron transport layers are wide-
band-gap semiconductors and are highly transparent to sunlight. PEDOT:PSS,
being a high work-function conducting polymer, is predominantly used to form
Ohmic contact for holes with the polymer BHJ and is an alternative to thermally
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