Nanostructured Nitrogen Doping TiO Nanomaterials for Photoanodes of Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 63

N-doped and undoped TiO 2 prepared under the same dry method conditions,
a 12.3 % enhancement of energy conversion efficiency was reached by N-doping.
Then, we found that the nitrogen dopant type and amount influence the perfor-
mance of N-doped TiO 2 photoanodes (Fig. 6 )[ 56 ]. The different N dopants and
wet methods affected the N-doping amount, the surface area of the N-doped TiO 2 ,
and thereby the photovoltaic performance of the DSCs. By using the same nitrogen
source (ammonia) and N-A wet method, it was found that the energy conversion
efficiency of N-doped DSCs showed much dependence on the N dopant amount.
A series of N-doped DSCs with different N dopant amounts showed the energy
conversion efficiency of 5.01-7.27 %. Meanwhile, the pristine TiO 2 -based DSCs
showed an efficiency of 4.32 % only. Our work also showed that the fiber-type
multiwall carbon nanotubes incorporated into N-doped TiO 2 electrode can

enhance the electron collection efficiency of DSCs [ 57 ].
Yang et al. also reported the effect of N-doped amount on the performance of
DSCs. Interestingly; they obtained three folds higher conversion efficiency for the
optimum N-doped DSCs than the undoped ones, both J SC and V OC were improved
[ 36 ]. In 2010, Sung et al. also reported the improvement of N-doped DSCs, which
is due to the enhanced J SC [ 58 ]. However, this N-doping effect is related to the
synthesis method of N-doped TiO 2 . In 2010, Dai et al. reported that through
solvothermal treatment of N-doping process, the DSCs showed similar photo-
voltaic performance. However, the N-doped TiO 2 led to a more stable long-term
stability and retarded electron recombination [ 44 ].
Overall, the N-doping modifying TiO 2 photoanodes contribute to the
enhancement performance of DSCs either in J SC or V OC . We give a detailed
discussion below.

3.2 Effect of N-Doping TiO 2 on the Short-Circuit
Current (J SC )

A significant enhancement of J SC was achieved for N-doped DSCs [ 23 , 55 , 56 , 58 ].
Our work suggested that the significantly enhanced photocurrent of the devices
was found to be related to the N dopant amount and the change in surface property,
which affects dye uptake amount in N-doped TiO 2 electrodes. We investigated the
amount of dye adsorbed on the electrodes. The J SC of the N-A and N-U solar cells
were higher than that of pure TiO 2 solar cells, although the N-A electrodes pos-
sessed almost the same amount of dye as pure TiO 2 electrodes did, while the N-U
electrodes obtained a lower dye uptake than that of pure TiO 2 electrodes. On the
other hand, the isoelectric points of TiO 2 have an effect on the dye-loading.
Surfaces with higher isoelectric points are preferable for the attachment of dye
with acidic carboxyl groups [ 59 ]. During the wet method synthesis of N-doped
TiO 2 precursor, hydrolysis of TTIP was conducted in solvent containing nitrogen
sources with weak alkaline. Therefore, we can speculate that the pH-dependent
zeta potential and the isoelectric points of N-doped TiO 2 were changed.