Environmental Engineering Reference
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attractive in other optoelectronic applications such as flat panel displays and
inorganic solar cells. In PSCs, their application has been limited to buffer layers
that are solution processed or to the use of acceptor material in hybrid solar cells.
3.3 Ultrathin Metal Films
Prior to the discovery and the subsequent dominance of ITO as the material of
choice for transparent conductors over the last four decades [
73
], very thin metals
usually evaporated were used as semitransparent conductors in optoelectronics.
With the advent of ITO that exhibited far superior properties, these thin semi-
transparent metals were rapidly replaced by ITO. However, the economic and
physical incompatibility of ITO in low-cost applications particularly has led to
revisiting ultrathin metals as a replacement to ITO.
In thin metal films, the surface scattering of free charge carriers causes an
inverse relationship between film resistivity and thickness. As a result, there is a
threshold below which further reduction in thickness leads to dramatic increases in
R
sh
of the metal film. This critical threshold thickness is observed to lie between 5
and 10 nm in commonly employed electrode metals such as Al, Au, and Ag as
predicted by Fuchs-Sondheimer (FS)-Mayadas-Shatzes (MS) model and also
experimentally observed [
74
,
75
]. The FS-MS model accounts for surface scat-
tering as well as scattering from the grain boundaries. At a thickness of 10 nm, a
transmission of the Ag film is observed to be *60 % (Fig.
15
)[
74
,
75
]. Metals
also exhibit very high reflection. Calculations have shown that while absorption of
10 nm thick silver is merely 3 %, reflections can amount to 63 % [
76
]. A dielectric
capping layer, similar to antireflection coating, can suppress this reflection and
induce a desired interference pattern enhancing the amount of photons reaching
the photoactive layer in a solar cell. Such a capping layer can be either organic or
inorganic dielectric compounds materials [
77
]. Coating of a metal electrode with
aluminum hydroxiquinoline (Alq
3
) in small molecular-based solar cell is reported
to cause an improvement in photocurrent up to 50 % [
77
,
78
].
A very few scientific reports are available on thin metal films as transparent
conductor in the fabrication of PSCs [
74
,
79
,
80
]. Nonetheless, ultrathin metal
layer has been adopted as transparent conductors in PSCs in both top-and bottom-
illuminated inverted architectures. In bottom illuminated solar cell with the
structure substrate/ultrathin Au /ZnO/P3HT:PCBM/PEDOT:PSS/100 nm Au, a
PCE of 2.52 % is observed. Such a PCE is nonetheless lower than the ITO-based
reference devices (PCE: 3.53 %). The lower PCE is mainly attributed to the lower
J
sc
which in turn is caused by the reduced incoupling of light into the device. In the
same structure when top illuminated realized by using 10 nm thick Au top elec-
trode and 100 nm Au bottom electrode, the lowest PCE 1.75 % is observed and
was attributed due to optical transmission loss due to the presence of PEDOT:PSS
layer [
79
]. The top illuminated inverted structure has been explored in other
studies and has resulted in poor performance in comparison to that of reference
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