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devices using PEDOT:PSS/TiO
2
ICL [
147
]. They observed S-shaped curves in
such tandem devices, and when the devices were irradiated with UV light of
wavelength below 400 nm, the J-V characteristics returned to normal and the PCE
was thus greatly improved.
The UV activation phenomenon described above is useful to study the interface
interaction between transitional metal oxides and heavily-doped conjugated
polymers. A similar behavior was observed for single junction devices with TiO
2
/
PEDOT:PSS/Al as the composite cathode, confirming the theory that this transi-
tion from a high resistance state to a low resistance state is the result of Schottky-
to-Ohmic transition of PEDOT:PSS/TiO
2
contact [
147
]. The p-type PEDOT:PSS
and n-type TiO
2
form a metal-semiconductor contact with a triangular barrier at
the TiO
2
/PEDOT:PSS interface. At a low doping level of TiO
2
, the triangular
barrier width is large and blocks electrons in TiO
2
to recombine with holes from
PEDOT:PSS. After irradiating with UV light, the free carrier concentration in
TiO
2
increases significantly and the barrier width decreases to an extent that
electrons can tunnel through the barrier. However, this transition to Ohmic contact
is not permanent, as the device reverts back to the high resistance state after
prolonged storage in the dark. Thus, methods that can realize stable doping of
metal oxides are desired. One approach is the chemical doping using electron-
donating species. Park et al. reported the doping of TiO
2
NPs by Cs
2
CO
3
[
148
],
causing a significant energy level shift of TiO
2
. It was observed that Ti ions were
partially reduced by Cs ion through charge transfer, thus increasing the n-type
doping.
3.4.4 ICL for Inverted Tandem Solar Cells
As reported by Chou et al., the advantages of the tandem and inverted structure can
be combined by employing a metal oxide-only interlayer (MoO
3
/Al/ZnO) to
connect two inverted BHJs devices [
149
]. An inverted tandem solar cell with
P3HT:PC
61
BM and poly[(4,4
0
-bis(2-ethylhexyl)dithieno[3,2-b:2
0
,3
0
-d] silole)-2,6-
diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT, see Scheme
3.1
):PC
71
BM as
bottom cell and top cell respectively can reach a PCE of 5.1 %. A low temperature
hydrolysis process was developed to form a dense and smooth amorphous ZnO
layer with excellent diode properties, while the crystalline ZnO appears rougher
and more porous. In addition, the MoO
3
provides a much robust resilience against
the sol-gel process compared to V
2
O
5
or Al. As a result, the materials and con-
figurations of the interlayer for the tandem architecture were no longer limited by
the acidic PEDOT:PSS. Compared to PEDOT:PSS, the absorption of the metal
oxide-based interlayer is also smaller, resulting in higher photocurrent for both
single and tandem cells. Sun et al. also reported a multilayered ICL in inverted
tandem cells [
150
]. MoO
3
/Ag/Al/Ca interlayer was vacuum deposited, and
followed by deposition of the rear cell via spin-coating process. Such an interlayer
structure features high transparency and low R
s
, as well as effective charge
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