Environmental Engineering Reference
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evaporated high work-function metal oxides. The first demonstration of all solu-
tion-processed polymer tandem cells was using ZnO NPs dissolved in acetone to
deposit the n-type layer, and aqueous-based neutral pH PEDOT:PSS as the p-type
layer [
146
]. ZnO has been shown to form an efficient contact for electron
extraction from polymer BHJ because of its matching energy level with PC
61
BM.
The alcohol dispersed ZnO NPs does not damage the bottom polymer layer and
aqueous-based neutral PEDOT:PSS layer does not affect the ZnO layer. Such an
ICL is robust enough to protect the bottom polymer cell from any subsequent
solution process. It was thus guaranteed that during the entire fabrication process,
none of the solution process steps damaged the previously formed layers. Using
these solution-processed ICLs, double and triple junction tandem cells were shown
with only minor losses in V
OC
.
Sol-gel derived TiO
2
is another potential candidate as an n-type contact. Kim
et al. and Sista et al. used sol-gel based TiO
2
layer for efficient electron extraction
from the bottom cell and PEDOT:PSS as a p-type layer [
12
,
147
]. Though the
synthesis approaches of TiO
2
employed by the two groups are totally different,
highly efficient tandem cells were both demonstrated without V
OC
losses. Kim
et al. used a TiO
x
precursor solution to coat 20-30 nm thick dense films on
polymer layer that was followed by baking in air for hydrolysis of precursor into
solid state TiO
x
. The purpose of the TiO
x
layer was threefold: being an electron
transport layer, a hole-blocking layer, and an optical spacer. On the other hand,
Sista et al. used crystalline NPs of TiO
2
dispersed in alcohol solvent to form an
electron transport layer [
148
]. Several other reports on solution-processed tandem
cells
have
used
ZnO
or
TiO
2
via
various
processes
as
n-type
contact
and
PEDOT:PSS for p-type contact, yielding efficient tandem cells.
3.4.3 Efficient Recombination Inside ICL
The charge recombination rate inside the ICL should match the charge extraction
rate from the two sub cells, otherwise it will lead to accumulation of charges in the
ICL, increasing the R
s
and even induces significant loss in the photocurrent near
V
OC
, resulting in low efficiency devices with S-shaped J-V curves.
Gilot et al. observed such S-shaped J-V curves in their tandem solar cell
devices and they attributed the problem to the non-Ohmic contact between ZnO
and PEDOT:PSS in the ICL, which would form a counterdiode, and thus hinders
charge recombination [
146
]. To restore efficient charge recombination in the ICL,
a thin Ag layer was inserted between ZnO and PEDOT layer, and the S-shaped
curve disappeared. Alternatively, an Ohmic contact between n-type and p-type
layers inside ICL can be formed by heavily doping the two layers. In the case of
PEDOT:PSS/ZnO ICL, further doping of PEDOT:PSS is not necessary, and the
doping level of ZnO was increased by UV irradiation. Gilot et al. observed that the
S-shaped curve disappears soon after UV irradiation and the V
OC
of the tandem
cell increases. Similar cases were reported by Sista et al. in tandem solar cell
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