Environmental Engineering Reference
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fact that high PCE can be achieved in a device without ETL poses the question of
how and to what extent an ETL can improve the electron-collecting ability of ITO
in inverted devices.
3.3.2 Anode Interlayer
Direct contact of the top high work-function metal to the active layer can lead to
degradation of the solar cell performance, therefore, a hole-transporting/electron-
blocking layer (HTL/EBL) is generally deposited between these two layers to
improve charge selectivity and collection of holes. The materials that have been
utilized as HTL are various high work-function transition metal oxides such as
MoO
3
, or denoted as MoO
x
because of oxygen vacancy formed during thermo-
evaporation [
97
,
120
,
121
], WO
3
[
122
,
123
] and V
2
O
5
[
29
,
124
], and solution-
processed organic materials including PEDOT:PSS [
99
,
112
,
113
], SPDPA [
125
],
dextran-doped poly(allylamine hydrochloride) (PAH-D) [
126
], and cross-linkable
molecule N,N
0
-diphenyl-N,N
0
-bis-(3-methylphenyl)-(1,1
0
)-biphenyl -4,4
0
- dia-
mine- bis (vinyl benzyl ether) (TPD-BVB) [
127
]. The chemical structures of the
organic HTL materials are shown in Scheme
3.5
(PEDOT:PSS and SPDPA are
shown in Scheme
3.3
).
3.3.2.1 PEDOT:PSS as HTL or Anode
The literature on HTL design of inverted solar cells is relatively scarce compared to
ETL. For solution-processed HTL PEDOT: PSS is often used. The challenge of
using PEDOT:PSS as HTL in an inverted device is that PEDOT:PSS is an aqueous
dispersion and it is difficult to coat it onto an organic active layer due to its
hydrophobic property. Cosolvent [
99
] or surfactants [
112
] are added to improve
interfacial compatibility of PEDOT: PSS and active layer. However, the confor-
mation of PEDOT:PSS using these methods was different and consequently its
work function and conductivity would be changed. Another way to uniformly
O
NH
3
+
O
O
O
N
Cl
-
O
HO
HO
N
O
N
N
HO
OH
x
+
H
3
N
y
Cl
-
PAH-D
TPD-BVB
Scheme 3.5
Chemical structures of organic HTL materials for inverted polymer solar cells
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