Nanostructured Nitrogen Doping TiO Nanomaterials for Photoanodes of Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 68

Fig. 12 Schematic
electronic band structure of
3.5 nm CdSe with an
effective band gap of 2.17 eV
and nanocrystalline TiO 2 /N
with a 3.2 eV band gap [ 65 ]
4 Electron Kinetic Behaviors in DSCs Based on N-doped
TiO 2 Photoanodes
4.1 Charge Transport and Electron Lifetime
Understanding electron transport in the N-doped DSCs is helpful to further
improve the performance of semiconductor photoanodes. We investigated the
charge transport by intensity-modulated photocurrent and photovoltage spectros-
copy (IMPS/IMVS) to study the effect of N-doping treatment [ 55 ]. Usually, IMPS/
IMVS measurements were affected by particle size in correlation with surface area
and morphology, resulting in differences in the number of particles and the
quantity of the charge associated with that electrode. When comparing the doped
samples with pure samples, we needed to assure they possessed similar particle
sizes and surface area to obtain comparability results. Therefore, we studied the
N-doping effect by using the N-doped DSCs (N-A) solar cells and pure TiO 2 solar
cells, which had similar particle sizes (22.74 and 23.13 nm, respectively).
The electron transport time (s tr ) and electron lifetime (s e ) were deduced from
IMPS and IMVS results (Fig. 13 )[ 55 ]. All of the time constants followed a
general trend; they decreased as the light intensity increased. Fast electron
transport and short electron lifetime existed for the N-A. Commonly, fast electron
transport can improve the charge-collection efficiency and thus increase the J SC .In
general, N-doping would cause some defects in TiO 2 lattice, which would cause
more traps in the doped films. However, we observed similar slopes, suggesting
similar trap distributions.
V OC decay measurements were performed to further clarify the electron life-
time. As shown in Fig. 14 a, the V OC decay is slightly different in the N-A DSCs,
which follows the sequence of N-A [ pure TiO 2 . Additionally, the decay is faster
in the N-A solar cell, which is also in agreement with the shorter lifetime found in