Low-Cost Pt-Free Counter Electrode Catalysts in Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 76

diamond structure, as a CE catalyst in DSCs [ 1 ]. The photovoltaic performance is
positively correlated with roughness factor of the Ca CE. In contrast, the roughness
factor is negatively correlated with the charge-transfer resistance (R ct ) in the CE/
electrolyte interface. The DSC using Ca CE showed power conversion efficiency
(PCE) of 3.89 %, slightly lower than that of the DSC using Pt CE (Pt-DSC) which
produced a PCE of 4.30 %. Grätzel and Kay introduced graphite and carbon black
(Cb) into a monolithic DSC as the CE catalyst, and the DSC produced a PCE of
6.7 % [ 2 ]. In this kind of carbon CE, graphite improved the lateral conductivity of
the CE and Cb granted the CE a large surface area, resulting in high catalytic
activity. They also found that the thickness of Cb film affected the fill factor (FF)
and PCE significantly, while open-circuit voltage (V oc ) and short-circuit current
density (J sc ) varied very little as the Cb film thickness increased [ 3 ]. The DSC
using Cb CE with 14.47 lm thickness produced the highest PCE of 9.1 %.
Zou et al. fabricated fiber-shaped DSCs using carbon fiber (Cf) as CE and the
DSC gave a PCE of 2.7 % [ 4 ]. Wang et al. introduced mesoporous carbon (Cm)
into DSCs as CE, yielding a PCE of 6.18 % [ 5 ]. Ramasamy et al. prepared fer-
rocene-derivatized large pore size mesocellular carbon foam (Fe-MCF-C) used as
CE catalyst in a DSC which gave a PCE of 7.89 % [ 6 ]. Carbon nanotubes (CNTs)
can be divided into single-wall CNTs (SNTs), double-wall CNTs (DNTs), and
multi-wall CNTs (MNTs) and they can be designed as a semiconductor or metallic
material according to the varied chiralities [ 7 ]. Using CNTs to replace Pt can
endow with CE the following advantages: nanoscaled transfer channels, large
specific surface area, low-cost, high catalytic activity, and light weight. The DSC
using SNTs as CE yielded a PCE of 4.5 % [ 8 ]. Lee et al. applied MNTs as CE in
the DSCs. The DSCs using MNTs and Pt CEs showed PCE of 7.67 % (MNTs) and
7.83 % (Pt) [ 9 ]. The high density of the defect-rich edge planes of MNTs guar-
antees its high catalytic activity. The technique adopted to prepare CNTs CEs is
crucial for obtaining high catalytic activity. Kim et al. prepared CNTs CEs with
screen printing (SP) technique and chemical vapor deposition (CVD) technique.
The CNTs (SP) were randomly oriented and woven into each other, whereas the
CNTs (CVD) were grown directly on the substrate. The DSC using the CNTs (SP)
CE gave a CPE of 8.03 %, while the well aligned CNTs CE made the PCE value
reach to 10.04 % [ 10 ]. The advantages of CNTs (CVD) CE can be attributed to
high conductivity because of the well-aligned arrangement. Besides SP and CVD
techniques, spraying technique can be used to fabricate CNTs CE [ 11 ]. The CNTs
film thickness was regulated by spraying time and the impact of CNTs film
thickness (or spray time) on the performance of DSCs was also investigated. As
the time increased from 5 to 30 s, the J sc and FF increased rapidly and the PCE
value improved from 0.68 to 3.39 %. The highest PCE of 7.59 % was obtained at
spraying time of 200 s. In their work, the PCE increased continuously with
spraying time (0-200 s). According to our experiences, there should exist an
optimal spraying time (i.e., thickness).
Zhu et al. attempted to find the differences between SNTs, DNTs, and MNTs as
CEs under the same conditions [ 12 ]. The DSC using SNTs CE performed best
(1.46 %), compared with the other two DSCs based on DNTs (0.45 %) and MNTs