302
J. B. Liyanage et al.
layer by making an ohmic contact between FTO and the TiO 2 film. This system gave
an efficiency of 6.58%, which is about 21.2% enhancement when compared with
bare FTO-based DSCs (Lee et al. 2009).
In 2010, Chandiran et al. studied doping of TiO 2 with Nb
5+ to create donor levels
below the conduction band and promote charge transport over recombination and
improving the transparency. Under optimal conditions, doped with 0.5% Nb and
post treatment with TiCl 4 , an overall efficiency of 8.7% was obtained. Here, Nb
5+
would replace the Ti
4+ in the lattice structure as they have similar radii and form
strong hybridization with 4d and 3d orbitals. Due to the down-shift of the conduction
band edge, doping with Nb
5+ allows to drastically slow the dynamics of electron
recombination with I 3
− , resulting in an increase of electron lifetime. The TiCl 4 posttreatment also acts to increase the electron lifetime. On the other hand, the gain
achieved is partially compensated by a loss of electron transport (Chandiran et al.
2010). Lu et al. have performed a similar study, with the use of a water-soluble
precursor, and have obtained a positive shift in the flat band potential along with
an increase in the conductivity. An overall efficiency of 7.8% was obtained, and an
increase of 18.2% compared to pure TiO 2 (Lü et al. 2010).
Another widely used transition metal used in doping is silver. Because the Nernstian potential of the conduction band in TiO 2 is lower than the standard electrode
potential of Ag
+ /Ag, the difference of potential is formed. Thus, the electron will
be transferred from the conduction band of TiO 2 –Ag
+ which was absorbed on the
surface of TiO 2 nanospheres, and silver ions are reduced to silver atoms (Han et al.
2012).
Peng et al., studied the change in solar cell efficiency when silver is coated by
photo-deposition. At the deposition time of 10 min, the conversion efficiency was
improved from 5.97 to 6.86%. The Ag layer formed can reflect incident light and
lengthen the optical path in electrodes, and in this case lead to an increase in J sc
up to 13.55 mA cm
−2 . Furthermore, electron recombination was slowed down by
Ag deposition, which led to a larger V OC of 0.735 V. However, past the optimal
deposition time, there was an increase in the series resistance due to the formation
of the Ag barrier layer (Peng et al. 2013).
Wei and coworkers too have studied the use of the noble metal silver nanoparticles
in TiO 2 nanotubes, using electrophoretic deposition. It was observed that the total
dye adsorption was reduced with the deposition time, as silver nanoparticles on
the surface of TiO 2 , reduce adsorption capacity. The holes in the electrolyte and
photoelectrons in TiO 2 recombine easily through the TiO 2 –Ag-electrolyte interface
(Wei et al. 2017), which lead to an overall efficiency of 5.01%, under optimum
conditions of 30 min of deposition.
Similar doping methods have been done using zinc (Ghanbari Niaki et al. 2014),
copper (Navas et al. 2012; Wijayarathna et al. 2008), nickel (Archana et al. 2013)
and iron (Liu 2014), in order to improve photovoltaic properties.
Yttrium too has been used as a dopant as it is known to not change the band gap
of TiO 2 (Li et al. 2009), even though the problems arise when considering the longterm stability of the DSC. With 1% Y-TiO 2 , an efficiency of 9.1% was observed.
J. B. Liyanage et al.
layer by making an ohmic contact between FTO and the TiO 2 film. This system gave
an efficiency of 6.58%, which is about 21.2% enhancement when compared with
bare FTO-based DSCs (Lee et al. 2009).
In 2010, Chandiran et al. studied doping of TiO 2 with Nb
5+ to create donor levels
below the conduction band and promote charge transport over recombination and
improving the transparency. Under optimal conditions, doped with 0.5% Nb and
post treatment with TiCl 4 , an overall efficiency of 8.7% was obtained. Here, Nb
5+
would replace the Ti
4+ in the lattice structure as they have similar radii and form
strong hybridization with 4d and 3d orbitals. Due to the down-shift of the conduction
band edge, doping with Nb
5+ allows to drastically slow the dynamics of electron
recombination with I 3
− , resulting in an increase of electron lifetime. The TiCl 4 posttreatment also acts to increase the electron lifetime. On the other hand, the gain
achieved is partially compensated by a loss of electron transport (Chandiran et al.
2010). Lu et al. have performed a similar study, with the use of a water-soluble
precursor, and have obtained a positive shift in the flat band potential along with
an increase in the conductivity. An overall efficiency of 7.8% was obtained, and an
increase of 18.2% compared to pure TiO 2 (Lü et al. 2010).
Another widely used transition metal used in doping is silver. Because the Nernstian potential of the conduction band in TiO 2 is lower than the standard electrode
potential of Ag
+ /Ag, the difference of potential is formed. Thus, the electron will
be transferred from the conduction band of TiO 2 –Ag
+ which was absorbed on the
surface of TiO 2 nanospheres, and silver ions are reduced to silver atoms (Han et al.
2012).
Peng et al., studied the change in solar cell efficiency when silver is coated by
photo-deposition. At the deposition time of 10 min, the conversion efficiency was
improved from 5.97 to 6.86%. The Ag layer formed can reflect incident light and
lengthen the optical path in electrodes, and in this case lead to an increase in J sc
up to 13.55 mA cm
−2 . Furthermore, electron recombination was slowed down by
Ag deposition, which led to a larger V OC of 0.735 V. However, past the optimal
deposition time, there was an increase in the series resistance due to the formation
of the Ag barrier layer (Peng et al. 2013).
Wei and coworkers too have studied the use of the noble metal silver nanoparticles
in TiO 2 nanotubes, using electrophoretic deposition. It was observed that the total
dye adsorption was reduced with the deposition time, as silver nanoparticles on
the surface of TiO 2 , reduce adsorption capacity. The holes in the electrolyte and
photoelectrons in TiO 2 recombine easily through the TiO 2 –Ag-electrolyte interface
(Wei et al. 2017), which lead to an overall efficiency of 5.01%, under optimum
conditions of 30 min of deposition.
Similar doping methods have been done using zinc (Ghanbari Niaki et al. 2014),
copper (Navas et al. 2012; Wijayarathna et al. 2008), nickel (Archana et al. 2013)
and iron (Liu 2014), in order to improve photovoltaic properties.
Yttrium too has been used as a dopant as it is known to not change the band gap
of TiO 2 (Li et al. 2009), even though the problems arise when considering the longterm stability of the DSC. With 1% Y-TiO 2 , an efficiency of 9.1% was observed.