Environmental Engineering Reference
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Fig. 15 Thickness dependence of sheet resistance in ultra-thin gold films determined by
simulation based on FS-MS model (solid line) and experimentally verified (dots). Inset shows
visible
transmission
of
ultra-thin
Au
films
with
various
thicknesses.
The
dashed
line
is
absorbance of P3HT:PCBM. 2013 Elsevier. Reprinted, with permission from Ref. [ 74 ]
devices. In such a structure, controlling the metal film properties on different
substrates is not an easy task. Film morphology is affected by the type of substrate
and its roughness, the surface treatment of the substrate, and the deposition con-
ditions [ 75 ]. Without any morphological control, it is unlikely that small amounts
of Au or Ag can be deposited on PEDOT:PSS can result in continuous films.
Several authors have noticed that Ag has a tendency to coalesce when deposited on
organic layers. Incorporating processing strategies to achieve continuous film
formation of Ag or Au on organic layer may reduce the loss in performance. Such
strategies involve deposition of a work function compatible low-surface energy
metals. For example, MgAg can form very flat semitransparent metal films
at \5 nm thickness while Ag forms island [ 81 , 82 ].
Ultrathin metal films have been deposited by evaporation. However, recently a
solution processed semitransparent Ag electrode was reported having a R sh of 5 X
! -1 and a corresponding transmission of 30 % at 550 nm along with a roughness
of 2 nm. When incorporated in the fabrication of a P3HT: PCBM-based solar cell,
a PCE: 1.6 %; FF: * 60 %; V oc : 0.51 V; and J sc :5.3 mA/cm 2 was observed. The
low J sc is attributed to lower incoupling of light [ 83 ].
3.4 Polymers
The most important advantage of polymeric transparent conductors is that they can
be solution processed and are therefore readily processed in an R2R setup using
the plethora of coating and printing techniques available [ 39 ]. Poly(3,4-ethyl-
enedioxythiophene):poly(styrenesulfonate) or PEDOT:PSS is the most widely
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