Environmental Engineering Reference
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enhanced PCE of 4.7 % of P3HT:PC
61
BM solar cell when a layer of ethylene
glycol was spin coated over PEDOT:PSS. The enhancement was achieved by
the increase of PEDOT:PSS conductivity that improves charge extraction, and
enhanced PEDOT:PSS transparency that contributes to enhanced P3HT:PC
61
BM
absorption [
68
]. Soon after, similar work was also reported by Peng et al., who
treated PEDOT:PSS layer by ethanol and 2-propanol. Similar enhancement of PCE
was also observed when using this treated PEDOT:PSS as anode buffer layer, and
this enhancement was attributed to higher conductivity and optimized surface
morphology of the PEDOT:PSS [
69
].
3.2.2.2 Semiconducting Oxides as Anode Interlayer
Aside from the modification of PEDOT:PSS, the development of new materials for
hole-collecting/transporting is also vigorously conducted. In principle, an efficient
anodic interlayer to substitute PEDOT:PSS should first possess the capability to
withstand the organic solvent erosion. Second, a good transparency is desired for
the anodic interlayer to guarantee the efficient incident light absorption in organic
active layer. Moreover, it should be taken into account as well that the surface
properties of the anodic interlayer can significantly influence the phase separation
process and morphology of the BHJ active layer.
p-Type semiconducting transition metal oxides such as molybdenum oxide
(MoO
3
), vanadium oxide (V
2
O
5
), nickel oxide (NiO), and tungsten oxide (WO
3
)
were hence widely employed as anodic buffer layers to modify the interface
between ITO and organic active layer in the conventional BHJ-PSCs. These
oxides have relatively large band gap, which guarantee the good optical trans-
parency in visible and near infrared light region of the anode, and consequently
allow incident solar photons to reach the organic active layer. More importantly,
the Fermi level of these oxides usually positions in the range between 5.0 and
5.4 eV, which align well with the HOMO energy level of most donor conjugated
polymers to form ideal Ohmic contact for efficient hole extraction and transporting.
Furthermore, the lowest energy level of the conduction band of these p-type
oxides usually located above 2.5 eV, which is much higher than the LUMO energy
level of most organic photovoltaic (OPV) materials (including both acceptors and
donors), indicating the good electron-blocking ability of these oxides.
Shrotriya et al. first reported the use of thermally-evaporated MoO
3
as the
anodic interlayer for BHJ-PSCs to replace PEDOT:PSS [
70
]. The BHJ-PSCs with
5 nm of MoO
3
as anodic interlayer exhibited slightly better performance (3.36 %
PCE) than that of PEDOT:PSS device (3.10 % PCE). They revealed as well that an
optimized thickness is critical to obtain ideal device performance: a thinner layer
of MoO
3
results in a smaller V
oc
and leakage current because of the incomplete
coverage, whereas a thicker layer of MoO
3
increases the R
s
and in turn leads to a
smaller J
sc
and FF [
70
]. Similar solar cell performance enhancement was also
observed by Kim et al. when MoO
3
was inserted as anodic buffer layer between
ITO and organic active layer [
71
]. Most recently, Sun et al. reported encouraging
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