296
J. B. Liyanage et al.
One of the drawbacks of using metals as composites is that I
−
/I 3
− , the most
commonly used redox electrolyte, is corrosive in the presence of metals, which
would affect the durability of the system (Chou et al. 2009).
15.3.2 Metal Oxides
Metal oxides can be considered as the most commonly used material in preparing
composites with TiO 2 , for DSCs. Many have been studied so as to provide the
electrons an easier transport with a stepwise conduction band edge. Among all metal
oxides, Al 2 O 3 , ZnO, NiO, SiO 2 and SnO 2 have been studied extensively.
Zhang et al. studied the use of a blocking layer of Al 2 O 3 , so as to avoid recombination reactions, leading to a better V OC and FF. Although V OC and FF increase with
greater thickness of the Al 2 O 3 layer, confirming the function of the blocking layer,
J SC decreases. This may be due to the fact that the probability of electrons tunneling
through the barrier has an exponential relationship with the tunneling length; or due
to the decrease in dye adsorption due to the blocking layer. At an optimal coating
the highest efficiency was obtained was 2.59% (Zhang et al. 2003).
Similar studies have been done by coating Al 2 O 3 with reactive direct current
magnetron sputtering (Wu et al. 2008), by stepwise condensation (Choi et al. 2008),
using highly ordered TiO 2 /Al 2 O 3 (Kim et al. 2010) and also using highly ordered,
vertically oriented TiO 2 /Al 2 O 3 nanotubes (Kim et al. 2014), where they have also
made a modification to the structure.
Another popular material in preparing composites with TiO 2 is ZnO, due to its
high electron mobility and band gap quite similar to that of TiO 2 . However, using pure
is thought to cause degradation of the dye forming Zn
2+ -dye complexes. Manthina
et al., studied the use of ZnO containing composites using 1D nanostructures. Overall
dye uptake was lowered, but even taking this into account, it was observed that
the performance of the pure TiO 2 system was greater than that of the ZnO-TiO 2
containing. The authors hypothesise that the transfer of electrons from TiO 2 to ZnO
is attenuated as the CB edge of ZnO lies at a slightly more negative potential than
that of TiO 2 , and more work is needed to remove this barrier (Manthina et al. 2012).
Self-organized nanotubular metal oxides have attracted much interest in the recent
past. But there is no simple synthesis method to obtain these structures other than
anodization of the metal (Beranek et al. 2005), also leading to many grain boundaries,
defects, and trap sites, becoming a factor that retards the electron transport time. Kang
et al. tried to suppress the charge recombination by applying a wide band gap metal
oxide, such as ZnO, to coat the TiO 2 film. The formation of an energy barrier by
coating as a shell at the electrode/electrolyte interface was found to be essential for
increasing the physical separation of the injected electrons from the cations of the
redox electrolyte, thereby decreasing the rate of charge recombination. Here, in order
to improve the FF, an H 2 O 2 surface treatment was done, which reduced the thickness
of the TiO 2 barrier layer. The ZnO coating was attributed to suppress electron flow
J. B. Liyanage et al.
One of the drawbacks of using metals as composites is that I
−
/I 3
− , the most
commonly used redox electrolyte, is corrosive in the presence of metals, which
would affect the durability of the system (Chou et al. 2009).
15.3.2 Metal Oxides
Metal oxides can be considered as the most commonly used material in preparing
composites with TiO 2 , for DSCs. Many have been studied so as to provide the
electrons an easier transport with a stepwise conduction band edge. Among all metal
oxides, Al 2 O 3 , ZnO, NiO, SiO 2 and SnO 2 have been studied extensively.
Zhang et al. studied the use of a blocking layer of Al 2 O 3 , so as to avoid recombination reactions, leading to a better V OC and FF. Although V OC and FF increase with
greater thickness of the Al 2 O 3 layer, confirming the function of the blocking layer,
J SC decreases. This may be due to the fact that the probability of electrons tunneling
through the barrier has an exponential relationship with the tunneling length; or due
to the decrease in dye adsorption due to the blocking layer. At an optimal coating
the highest efficiency was obtained was 2.59% (Zhang et al. 2003).
Similar studies have been done by coating Al 2 O 3 with reactive direct current
magnetron sputtering (Wu et al. 2008), by stepwise condensation (Choi et al. 2008),
using highly ordered TiO 2 /Al 2 O 3 (Kim et al. 2010) and also using highly ordered,
vertically oriented TiO 2 /Al 2 O 3 nanotubes (Kim et al. 2014), where they have also
made a modification to the structure.
Another popular material in preparing composites with TiO 2 is ZnO, due to its
high electron mobility and band gap quite similar to that of TiO 2 . However, using pure
is thought to cause degradation of the dye forming Zn
2+ -dye complexes. Manthina
et al., studied the use of ZnO containing composites using 1D nanostructures. Overall
dye uptake was lowered, but even taking this into account, it was observed that
the performance of the pure TiO 2 system was greater than that of the ZnO-TiO 2
containing. The authors hypothesise that the transfer of electrons from TiO 2 to ZnO
is attenuated as the CB edge of ZnO lies at a slightly more negative potential than
that of TiO 2 , and more work is needed to remove this barrier (Manthina et al. 2012).
Self-organized nanotubular metal oxides have attracted much interest in the recent
past. But there is no simple synthesis method to obtain these structures other than
anodization of the metal (Beranek et al. 2005), also leading to many grain boundaries,
defects, and trap sites, becoming a factor that retards the electron transport time. Kang
et al. tried to suppress the charge recombination by applying a wide band gap metal
oxide, such as ZnO, to coat the TiO 2 film. The formation of an energy barrier by
coating as a shell at the electrode/electrolyte interface was found to be essential for
increasing the physical separation of the injected electrons from the cations of the
redox electrolyte, thereby decreasing the rate of charge recombination. Here, in order
to improve the FF, an H 2 O 2 surface treatment was done, which reduced the thickness
of the TiO 2 barrier layer. The ZnO coating was attributed to suppress electron flow