Nanostructured Nitrogen Doping TiO Nanomaterials for Photoanodes of Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 64

Fig. 6 a Effect of surface area and absorbance intensity (520 nm) on g [ 56 ]; b Effect of N/Ti
molar ratio on J SC and g [ 42 ]
Sung et al. also reported an enhanced photocurrent and efficiency in N-doped
DSCs [ 58 ]; they attributed the enhancement to the increase of N-doped TiO 2 in the
near-vis absorbance by nitrogen doping and partially to the morphological prop-
erties of the N-doped TiO 2 film. However, the visible light response of N-doped
TiO 2 can only contribute photocurrent in tens of microamperes, which are far from
enough to fill the gap caused by dye-sensitized films [ 55 ]. The efficient electron
transport and retarded electron recombination can also lead to an increase in J SC
which will be discussed in the following section.
3.3 Effect of N-Doping TiO 2 on the Open-Circuit
Voltage (V OC )
The increased V OC of N-doped DSCs also enhanced the overall energy conversion
efficiency. In theory, the V OC of DSCs is determined by the difference between the
Fermi level (E F ) of semiconductor and potential of redox couples [ 60 ]. It is helpful
to get information about whether N-doping would cause a shift of E F and thus the
V OC .
Dai et al. measured the V fb of N-doped and undoped TiO 2 films [ 44 ]. They
found that V fb of N-doped TiO 2 shifts to the negative by 0.06 and 0.1 eV compared
with that in the pure TiO 2 electrode (Fig. 7 ).
In the previous literature, Hashimoto et al. reported that the flatband potentials
of N-doped TiO 2 tend to shift to a positive direction [ 61 ]. Kisch et al. observed that
the quasi-Fermi level of electrons is anodically shifted by 0.07-0.16 eV [ 62 ].
Higashimoto et al. reported that the flatband potential of N-doped TiO 2 is not
influenced by small amounts of nitrogen species doped into TiO 2 [ 63 ]. Therefore,
there is still no conclusion about the change in E F of TiO 2 after N-doping.
Our group used surface photovoltage spectroscopy (SPS) to measure the energy
levels of bare N-doped TiO 2 films and dye-sensitized N-doped TiO 2 electrodes
[ 55 ]. In Fig. 8 a, we can see that an impurity level exists from where photoexcited
electrons are injected into the conduction band, indicating that nitrogen is doped