Indium Tin Oxide-Free Polymer Solar Cells: Toward Commercial Reality - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 199

optimizing transparency and R sh of a random of NWs. Processing methods are
sought to produce nanowires of longer length. The use of electro spinning, for
example, could produce ultra-long metal wires (nanofibers). Such nanofibers of Cu
films have been demonstrated to exhibit similar R sh and transmittance as ITO films:

2

N c ¼ 1
p

4 : 236
L

ð 4 Þ

where N c is the percolation threshold and L is length of the sticks [ 29 ].
Nanowire films, however, have a drawback. When incorporated in solar cells in
the normal device architecture (Fig. 3 ), the high roughness leads to interpenetra-
tion of the nanowires to the counter electrodes leading to large dark current
leakage or even short circuit. The roughness of the NW film can exceed the layer
thickness of the overlying photoactive materials (50-200 nm). Several processing
strategies have been explored to passivate the surface roughness of NW films.
First, an alternate inverted structure, the top illuminated inverted device in Fig. 3 ,
is often employed in which NWs are laminated onto PEDOT:PSS layer. Such a
structure alleviates the problem of shunting due to roughness although complete
elimination is seldom accomplished. With the use of a short pulse high voltage to
burn the remaining shunts, a working module with the structure substrate/Ag film/
Cs 2 O3/P3HT:PCBM/PEDOT:PSS/AgNW has shown a PCE of 2.5 %; J sc :
10.59 mA/cm 2 ; V oc : 0.51 V, and FF: 46 % on a device area of 2 mm 2 [ 30 ]. In
contrast, AgNW transparent conductors in a P3HT:PCBM-based solar cell in a
normal architecture showed a PCE of 1.1 % with significantly lower V oc than ITO-
based reference devices [ 31 ]. Second, in a bottom illuminated inverted device, the
problem of roughness can be circumvented by increasing the thickness of the
buffer layer. Increasing TiO 2 thickness in a device structure substrate/AgNW/
TiO 2 /P3HT:PCBM/Mo 2 O 3 /Ag has shown a PCE of 3.42 %; J sc: 10.1 mA/cm 2 ; V oc
of 0.56 V; and a FF: 61.1 % [ 32 ]. Alternatively, by using a composite electrode of
AgNW, sol gel TiO 2, and PEDOT:PSS in a normal structure AgNW/TiO 2 /PE-
DOT:PSS/P3HT:PCBM/Ca/Al, JV properties of the cell were found similar to
cells on commercial ITO substrates (Fig. 12 ) as well as it was found that TiO 2 and
PEDOT:PSS bind the AgNWs and also result in strong adhesion to the substrate
[ 33 ]. Such buffer layers function as a smoothening layer on top of rough AgNW
film, improve packing of AgNW and their adhesion to substrate, tune work
function of the electrode, and provide charge selectivity.
AgNW films provide superior mechanical flexibility than its ITO counterparts
and can withstand severe bending under numerous bending cycles. For example,
tests have shown that a AgNW (L avg .6.5 lm; diameter 85 nm) film at a nanowire
density of 79 mg m -2 can withstand 1,000 bend cycles without any change in R sh
whereas an ITO substrate catastrophically fails after a 160 bend cycles [ 34 ]. Sim-
ilarly, AgNW wire films on PET could withstand a bending angle of 160 without
significant change in the R sh of the film while ITO-PET when subjected to bending
angle of 60 showed a third order of magnitude decrease in the R sh (Fig. 12 )[ 35 ].