Low-Cost Pt-Free Counter Electrode Catalysts in Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 81

As we know, WO 3 ,Nb 2 O 5 , SnO 2, and other oxides are widely used as pho-
toanode semiconductor. Now, it has been proved that some oxides can be used as
CE catalysts. If the oxide is used in the photoanode, the direct contact of oxide and
I 3 - /I - redox couples may cause the I 3 - to be reduced by the electrons injected in
the conductive band of the oxide due to autocatalytic activity. This is to say, a
number of the electrons injected in the conductive band cannot be collected by the
substrate and, thus, flow into the external circuit. The autocatalytic activity of
oxide can result in a large dark current density. This may be a key reason for the
poor performance of the DSCs using TiO 2 —free oxides as photoanodes [ 40 ]. In
addition, Ma et al. found that the catalytic activity can be enhanced significantly by
sintering the oxide CEs in N 2 atmosphere [ 41 ]. The fundamental reason for the
high catalytic activity of some oxides is still unclear and requires further study.
3.4 Sulfides Selenides and Phosphides
The aforementioned carbides, nitrides, and oxides all show catalytic activity for
the regeneration of the redox couples in DSCs. Similarly, transition metal sulfides
are expected to behave in a similar fashion. Grätzel et al. prepared CoS CE on
flexible substrate and the DSC yielded a PCE of 6.5 % [ 42 ]. Co and Ni both belong
to Group VIII A metals. Meng et al. prepared NiS CEs by periodic potential
reversal (PR) and potentiostatic (PS) techniques. The DSCs showed PCE of
6.83 % (PR-NiS) and 3.22 % (PS-NiS) [ 43 ]. Lin et al. introduced copper zinc tin
sulfide (CZTS) to DSCs system as CE. CZTS performed well for the regeneration
of iodide from triiodide. The DSC using the CZTSSe CE gave a PCE of 7.37 %
higher than the corresponding photovoltaic of the Pt-DSC [ 44 ]. Ma et al. intro-
duced MoS 2 and WS 2 into DSCs as CEs [ 45 ]. MoS 2 and WS 2 showed high
catalytic activity compared to Mo (or W) carbides, nitrides, and oxides and the
DSCs gave high PCE of 7.59 % (MoS 2 ) and 7.73 % (WS 2 ) which can match the
performance of the Pt-DSC (7.64 %). Besides sulfides, Ma et al. used Ni 5 P 4 and
MoP as CEs in DSCs, which showed PCE of 5.71 % (Ni 5 P 4 ) and 4.92 % (MoP).
Meanwhile, Gao et al. introduced Ni 12 P 5 as CE in DSCs, a PCE of 3.94 % was
achieved [ 46 ]. Recently, selenides of Co 0.85 Se and Ni 0.85 Se were also proposed as
CEs in DSCs, and both of them showed high catalytic activity [ 47 ].
4 Composites
As the name indicates, composite CE catalysts commonly comprise two or more
components such as TiN/CNTs, Pt/Carbon, Carbon/PEDOT/PSS, Carbon/TiO 2 ,
CoS/PEDOT/PSS, and so forth. The advantage of this CE is, naturally, the com-
bination of the best qualities of all components into one composite.