Indium Tin Oxide-Free Polymer Solar Cells: Toward Commercial Reality - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 195

submicrometer to ensure uniformity of charge collection in any device [ 14 ]. Such a
periodic array of metal grid lines enables light scattering and coupling, therefore,
enhancing photon flux to the photoactive material [ 15 ]. Unlike in ITO where
transparency and conductivity are competing parameters, conductivity in nano
metal grids can be improved by increasing the line height without increasing the
width of the grid lines and therefore avoiding decrease in transmittance due to
shading from the grid lines. The relationship of the R sh of a periodic array of metal
grid to physical attributes of the grid lines can be deducted from Kirchoff's rule.
Assuming a square wire network with N 9 N wires, R sh is given by;

N
N þ 1

qL
wh

R sh ¼ ð 3 Þ

where q is the wire resistivity, L is the wire length, w is the wire width, and h is the
wire height [ 16 ].
Optical simulations based on a finite-difference frequency-domain (FDFD)
method have indicated that a Ag metal nanogrid structure having a period of
400 nm, grid height of 100 nm, and a grid width of 40 nm, an optical transmission
of [80 % is achievable in both 1- and 2-dimensional network structure with a R sh
of 1.6 X! -1 [ 17 ]. Experimentally, metal nanopatterns based on Ag, Cu, and Au
with a line width of 70 nm, height of 80 nm, and a period of 700 nm, a trans-
mittance above 70 % is observed over the entire visible spectrum at a R sh of *10
X! -1 [ 18 ]. These metal nanopatterns based on Ag and Cu when incorporated in
laboratory scale PSCs have resulted in similar performance to ITO-based reference
devices [ 18 , 19 ]. Figure 8 shows an example of reported properties of metal
nanogrids.
Although metal nanowire grids show comparable properties to ITO, its up-
scaling is not cost feasible in the processing of large-area PSCs. Processing of
nanopattern can be carried out using a variety of lithography techniques [ 21 ]. One
such method based on nanoimprinting lithography (NIL) is utilized the printing of
metal nanogrids for application in organic solar cells and OLEDs [ 18 , 22 ].
However, such a technique requires multiple processing steps including evapo-
ration in the preparation of resist template as well as in the subsequent processing
of the electrode metal grid. Figure 9 shows an example of the processing steps
utilized in the preparation of metal grids for application in PSCs and OLEDs [ 19 ,
20 ]. More information on NIL can be found elsewhere [ 23 ]. Recently, an R2R line
and vacuum-free methods have been successfully applied in preparation of the,
however, evaporation of the final metal is still needed (Fig. 10 )[ 24 , 25 ].

3.1.2 Metal Nanowires

The low-cost processing limitations of metal nanogrids led to the development of
metal nanowires (NW) as a potential transparent electrode. Metal NWs offer the
possibility of solution processing and a dispersed random network of metal NWs