Nanostructured Nitrogen Doping TiO Nanomaterials for Photoanodes of Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 65

Fig. 7 Absorbance measured
at 780 nm as a function of
applied potential for Undoped
and N-doped TiO 2 electrode
[ 44 ]
into the TiO 2 lattice, where it forms nitrogen-induced states. In Fig. 8 b, the
observed signal starting at around 740 nm (scanning from long to short wave-
length) for N-doped and undoped TiO 2 is due to electron injection from the N719
dye into the conduction band of TiO 2 . We observed that the signal for dye-
sensitized N-doped ST-01 with respect to the undoped ST-01 is blueshifted at
about 20-40 nm. This result may indicate the shift in the electron quasi-Fermi
level in N-doped TiO 2 .
We further investigated the relationship between voltage and charge using a
charge extraction technique [ 64 ]. The similar slope of voltage-charge plots of the
N-doped and undoped TiO 2 solar cells (N-A and TiO 2 DSCs) indicate a similar
trap distribution (Fig. 9 ). However, if sustaining a certain voltage, more charge
needs to be present in the TiO 2 -based DSCs. The relationship between charge and
voltage revealed that less charge is needed to get a high V OC in N-doped DSCs.
We also noticed that the increase in V OC cannot always be observed, which is
also dependent on the synthesis method of N-doped TiO 2 . The synthesis method
may influence the type of N-doping, such as lattice perfection, interstitial doping,
or just physical adsorption. Therefore, much effort would be made to further
understand the doping mechanism in energy level of N-doped TiO 2 .
3.4 Long-Term Stability
In DSCs system, photoanodes made of TiO 2 nanomaterial are not photoexcited
instead of dyes. However, N-doping would cause TiO 2 visible light absorption
response, which is widely applied to photocatalysis area. Whether N-doped TiO 2 can
possibly accelerate the deterioration of the dye or DSC system is always a concern.
The stability test for N-doped DSCs was conducted in our previous work
(Fig. 10 ), N-doped DSCs were examined during irradiation for 2000 h under white
light illumination (100 mW/cm 2 )at25C[ 23 ]. The N-doped DSCs possessed
good stability with efficiency and photovoltage maintaining its initial values above
90 %. Dai group also reported great stability of N-doped DSCs, the efficiency of