Low-Cost Pt-Free Counter Electrode Catalysts in Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 78

the main disadvantage of carbon CEs is the poor bonding strength between the
carbon films and the substrate. Adding TiO 2 can improve bonding strength between
carbon films and the substrate, but it was found that adding too much TiO 2 can lower
the catalytic activity, because of the bad conductivity of TiO 2 . Meanwhile, the
impact of carbon film thickness on the catalytic activity has been investigated. An
excessively thin carbon film can lead to insufficient catalytic activity while an
excessively thick carbon film can crack and detach from the substrate. To achieve
high catalytic activity, carbon film with 25 lm thickness is appropriate. The
thickness of the carbon films affects FF significantly.
Lee et al. studied the impact of carbon particle size on the catalytic activity [ 17 ].
The DSC using the nanocarbon (surface area, 100 m 2 g -1 ) CE gave a PCE of 6.73 %,
much higher than the DSC using microcarbon (surface area, 0.4 m 2 g -1 ) CE. The
different behavior between the two kinds of carbon materials can be attributed to the
surface area and conductivity. Meng et al. prepared flexible carbon CE using Ca as
the catalyst and graphite sheet as the substrate [ 18 ]. Low (series resistance) R s and R ct
were obtained for this carbon CE due to high electrical conductivity of graphite and
high catalytic activity of Ca. The DSC using this CE showed a PCE of 6.46 % which
can match the performance of the DSC using Pt CE (6.37 %).
Carbon is indeed a qualified CEs catalyst in DSCs. However, the main disad-
vantage of carbon CE is still the poor bonding strength between carbon film and
the substrate. This may be a potential unstable factor for long-term use. Deposited
carbon film on the substrate by in situ technique could resolve this problem.
Furthermore, fabricating incorporated carbon CE (carbon catalysts and carbon
substrate) may be another solution. Opacity is another disadvantage of carbon CEs.
We believe developing transparent carbon CEs for DSCs will become a promising
research topic in the future.
2 Conductive Polymers
Transparency and flexibility are two merits for DSCs which require transparent
flexible photoanode, transparent electrolyte, and transparent flexible CE. Gener-
ally, transparent flexible substrate (ITO-PET, ITO-PEN) deposited with Pt is
widely used as CE. Besides Pt, conductive polymers like poly (3, 4-ethylenedi-
oxythiophene) (PEDOT) or its derivatives can be used as CE catalysts.
PEDOT-Polystyrenesulfonate (PSS) was used as CE catalyst in quasi-solid
DSCs in which this PEDOT performed better than Pt for ionic liquid electrolyte
(ILE) [ 19 ]. In the EIS test, the R ct value for the PEDOT-PSS/ILE was much lower
than Pt/ILE. However, in the organic liquid electrolyte (OLE), the result was
opposite. Yanagida et al. gave a detailed explanation for the aforementioned
phenomenon [ 20 ]. ILE needs high I 2 concentration due to high viscosity and low
conductivity. Therefore, the porous PEDOT CE performed better than Pt in ILE.
Gîr t u et al. indicated that there existed charge transfer between PEDOT and iodide
redox couple by X-ray photoelectron spectroscopy (XPS) [ 21 ]. This phenomenon