Indium Tin Oxide-Free Polymer Solar Cells: Toward Commercial Reality - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 202

3.1.3 Carbon Nanotubes
It was not too long after the first report on carbon nanotubes in 1991 by Sumio
IIjima that the excellent mechanical, electrical, and optical properties of carbon
nanotubes were identified. Theoretically, single-wall carbon nanotubes (SWCNTs)
can have a current conductivity up to 1-3 10 6 Sm -1 and mobility of 100,000 cm 2
V -1 s -1 [ 42 , 43 ]. In films, a random network of CNTs possess far lower con-
ductivity and mobility than a single tube primarily due to the presence of high
junction resistance as well as due to a large number of parameters that affect CNT
conductivity and mobility such as purity, lattice perfection, bundle size, wall
number, metal/semiconductor ratio, diameter, length, and doping level. Nonethe-
less, conductivity up to 6,600 SCM -1 and mobility up to 10 cm 2 V -2 is experi-
mentally observed in films [ 42 ]. As a result, a wide variety of advanced
applications have been envisioned for CNTs; transparent conductors being one of
them.
An early report noted that a 50 nm thin film of p-doped SWNT film has a R sh of
30 X! -1 with a transmission of [70 % over visible region of the light spectrum
[ 44 ]. However, accomplishing such results in subsequent reports has not been easy
due to the large number of parameters involved. Much like nanowires, the R sh of
CNT films is dominated by junction resistance. The use of doping by acid treat-
ments has been shown to cause a threefold decrease in junction resistance and a
30 % increase in the nanotube conductivity when compared to pristine untreated
samples [ 45 ] (Fig. 14 ). For example, acid treatment of CNTs has resulted in
improvement in R sh of spray-coated SWCNT film from 110 X! -1 (pristine
SWCNT) to 37 X! -1 (doped SWCNT) with transmittance of 78 and 76 %
(550 nm) respectively (Fig. 14 )[ 46 ].
Apart from the large junction resistance, the roughness of the CNTs thin films
and their adhesion to the substrates has been equally impeding factors to their
efficacy in organic solar cells. Such issues are similar to those observed in
nanowire-based transparent conductors as well. Shunts due to roughness of the
SWCNT surface is circumvented, for example, by either using thick active layer
(0.5-1 lm thick) or using a planarization layer such as PEDOT:PSS, or both [ 48 ,
49 ]. Other approaches include, for example, the application of pressure upon
transferring of dispersed named with a PDMS stamp [ 50 ]. Using such a method, a
low roughness value of 10 nm over a scan area of 25 lm 2 was observed in an
untreated SWCNT-based film. When the film is used in the fabrication of PSCs, a
PCE of 2.5 % was attained. With the use of doped SWCNT, such a transfer
method will likely yield similar performance to ITO (PCE 3 %).
The earlier reports were generally proof of principle studies wherein upscaling
compatibility of processing was not the central concern. With the promising results
shown by these proof of principle studies, recently, reports on large-scale com-
patible processing have emerged. Spray coating is recently investigated in the
preparation of large-area CNT transparent film on PET substrate [ 46 , 47 , 51 , 52 ].
Generally, CNTs are processed from suspensions in a solvent. High van der Waals
forces render the nanotubes very susceptible to bundling. De-bundling is achieved