Environmental Engineering Reference
In-Depth Information
carboxymethylcellulose (CMC)—as a dispersing agent and an ultrasonic spraying
is used to deposit the film over large area (6 9 6 inch). Once deposited, the
SWCNTs are doped by exposing the film to nitric acid which simultaneously
eliminates CMC. Such a film is found to be highly homogenous (rms roughness of
3 nm scanned over 100 lm
2
area) and have resulted in a R
sh
of 60 X!
-1
at a
transmission of 60-70 % (at 550 nm). PSCs with the structure SWCNT/PE-
DOT:PSS/P3HT:PCBM/Ca/Al fabricated on such a film have resulted in a PCE of
3.1 % which is close to reference device with ITO substrate that demonstrated a
PCE of 3.6 % [
46
]. Unlike in other devices where the main function of PE-
DOT:PSS is providing charge selective transportation as a buffer layer, it has been
observed that in SWCNT-based PSC devices does not require a charge selective
PEDOT:PSS buffer layer in normal devices. In fact, the elimination of PEDOT:
PSS results in superior PCE because of improvement in optical transmission to the
photoactive material (Fig.
14
). Nonetheless, PEDOT: PSS provides a planarization
layer suppressing the intercalation of nanotubes into the counter electrodes. The
elimination of PEDOT: PSS requires increasing thickness of the active layer in
order to prevent electrical shorts due to roughness in the surface of CNT film [
47
].
The transparent films of CNTs have higher transmission beyond the visible region
and are suggested to generate higher current density than theoretical predictions
that are based on transmission in the visible range (Fig.
14
). So far, the highest
reported PCE of P3HT:PCBM-based PSCs with SWCNTs transparent conductors
is 3.6 and 2.6 % on glass and PET substrates, respectively [
53
].
The adhesion of nanotubes on surface of a substrate is yet another processing
challenge. Recent advancement in this regard includes the application of 1 %
solution of 3-aminopropyltriethoxy silane on the substrate which has been found to
improve the adhesion of SWCNTs onto the substrates due to the formation of
cross-linked siloxane on the surface of substrates.
It has been a decade since the application of CNTs as transparent conductors
were recognized, however progress has been slow primarily owing to the chal-
lenges in processing. There is a long way before SWCNTs transparent conductors
can make their way into R2R processing. Reports thus far have demonstrated proof
of principle and the task ahead is to demonstrate a robust processing technique(s)
that is feasible for application in the processing of PSCs and is easily scalable.
3.1.4 Graphene
Graphene is a two-dimensional material consisting of a monolayer of sp
2
-
hybridized carbon atoms resulting in a hexagonal arrangement. A monolayer of
graphene theoretically would exhibit a reflectance of less than 0.1 % and an
absorbance of 2.3 %, therefore resulting in a theoretical transmission limit of
97.7 %. However, graphene is a zero band gap semiconductor and undoped
graphene sheet has high R
sh
values of 6 kX!
-1
[
54
,
55
]. Doping is an efficient
method that allows tailoring of various properties of graphene and can be carried
out following various chemical and electrical routes [
56
]. So far, a R
sh
of 30 X!
-1
Search WWH ::
Custom Search