15 Yet to Be Challenged: TiO 2 as the Photo-Anode Material …
303
The improvement in performance can be attributed to the increase of electron lifetime
in the new photoanode, which affords close to unity charge collection efficiency
(Chandiran et al. 2011).
15.4.3 Post-transition Metal
As another part of the study done by Chandrian et al. in (2011), gallium too was doped
to TiO 2 , to improve the charge collection efficiency and the electrode transparency.
The highest efficiency obtained with 1% gallium was 8.1%. With the increase in
percentage of the Ga dopant, an increase in the open circuit voltage from 732 to
768 mV was observed. In turn, lower short circuit current led to a smaller overall
efficiency. As the impurity levels increase the band gap of TiO 2 would also increase,
making it favorable for electron transport. But having more positive conduction band
potential would make it unfavorable for the electron to transport. Then again, having a
more negative conduction band would also be unfavorable, which is why an optimum
amount of the dopant must be used (Chandiran et al. 2011).
Another widely used post transitional metal as a dopant in TiO 2 is tin. Duan et al.
studied tin as a dopant so as to improve the charge collection and electron transport
while maintaining optimum J SC . The Sn-doped TiO 2 nanoparticles showed a high
V OC of 722 mV, due to the negative shift of the flat band potential, and an enhanced
J SC of 16.01 mA cm
−2 , due to the faster electron transport in the Sn-doped TiO 2
films (Duan et al. 2012).
Despite the lack of agreement on aluminium doped TiO 2 , the substitution of
Al into the TiO 2 lattice was confirmed by Pathak et al. It was suggested that Al
doping reduces the number of sub-bandgap states, increasing the V OC and electron
conductivity, which led to an overall improvement in device performance of DSC
(Pathak et al. 2014).
15.4.4 Lanthanides
Ytterbium and holmium, have been used, together with fluorine, as an upconversion
material of TiO 2 by Yu et al. Upconversion materials have the ability to convert
lower energy (near-infrared or infrared) radiation into high energy radiation (ultraviolet or visible) via multiphoton absorption and energy transfer (ET) processes (Zou
et al. 2012). The improved DSCs conversion efficiency of 8.93% observed is associated with closer attachment of the upconversion process, enhanced light harvesting,
and photogenerated electron–hole pair separation, as well as elevated Fermi level.
Ho
3+ –Yb
3+ –F
− tridoped TiO 2 had excellent NIR-to-green upconversion ability helping dye sensitized solar cells to utilize more NIR light (Yu et al. 2014).
Erbium and ytterbium have also been used as co-dopants, in an upconversion
material in the work of Shan et al. (Shan and Demopoulos 2010).
303
The improvement in performance can be attributed to the increase of electron lifetime
in the new photoanode, which affords close to unity charge collection efficiency
(Chandiran et al. 2011).
15.4.3 Post-transition Metal
As another part of the study done by Chandrian et al. in (2011), gallium too was doped
to TiO 2 , to improve the charge collection efficiency and the electrode transparency.
The highest efficiency obtained with 1% gallium was 8.1%. With the increase in
percentage of the Ga dopant, an increase in the open circuit voltage from 732 to
768 mV was observed. In turn, lower short circuit current led to a smaller overall
efficiency. As the impurity levels increase the band gap of TiO 2 would also increase,
making it favorable for electron transport. But having more positive conduction band
potential would make it unfavorable for the electron to transport. Then again, having a
more negative conduction band would also be unfavorable, which is why an optimum
amount of the dopant must be used (Chandiran et al. 2011).
Another widely used post transitional metal as a dopant in TiO 2 is tin. Duan et al.
studied tin as a dopant so as to improve the charge collection and electron transport
while maintaining optimum J SC . The Sn-doped TiO 2 nanoparticles showed a high
V OC of 722 mV, due to the negative shift of the flat band potential, and an enhanced
J SC of 16.01 mA cm
−2 , due to the faster electron transport in the Sn-doped TiO 2
films (Duan et al. 2012).
Despite the lack of agreement on aluminium doped TiO 2 , the substitution of
Al into the TiO 2 lattice was confirmed by Pathak et al. It was suggested that Al
doping reduces the number of sub-bandgap states, increasing the V OC and electron
conductivity, which led to an overall improvement in device performance of DSC
(Pathak et al. 2014).
15.4.4 Lanthanides
Ytterbium and holmium, have been used, together with fluorine, as an upconversion
material of TiO 2 by Yu et al. Upconversion materials have the ability to convert
lower energy (near-infrared or infrared) radiation into high energy radiation (ultraviolet or visible) via multiphoton absorption and energy transfer (ET) processes (Zou
et al. 2012). The improved DSCs conversion efficiency of 8.93% observed is associated with closer attachment of the upconversion process, enhanced light harvesting,
and photogenerated electron–hole pair separation, as well as elevated Fermi level.
Ho
3+ –Yb
3+ –F
− tridoped TiO 2 had excellent NIR-to-green upconversion ability helping dye sensitized solar cells to utilize more NIR light (Yu et al. 2014).
Erbium and ytterbium have also been used as co-dopants, in an upconversion
material in the work of Shan et al. (Shan and Demopoulos 2010).