Nanostructured Nitrogen Doping TiO Nanomaterials for Photoanodes of Dye-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 69

Fig. 13 IMPS time constants as functions of light intensity for N-doped and TiO 2 DSCs [ 55 ]

the N-A DSCs (Fig. 14 b). The electron lifetime can be calculated from the voltage
transients using Eq. ( 1 )[ 56 ]:

1

s e ¼ kT
e

dVoc
dt

ð 1 Þ

where k is the Boltzmann constant, T the absolute temperature, and e the positive
elementary charge. The calculated electron lifetimes are shown in Fig. 14 bas
functions of the open-circuit potential. Specifically, the lifetimes increased expo-
nentially when the voltage decreased. However, the lifetime of the N-A solar cell
was shorter than in the pure TiO 2 solar cells at U \ 0.6 V, but it was longer when
U [ 0.6 V.
The electron lifetime can also be deduced from EIS spectra. The electron
lifetime can be estimated by the maximum frequency in the EIS spectra as
described: s e = (2pf max ) -1 [ 47 ]. We conducted a linear fit of electron lifetime,
which tended to decrease as the N/Ti molar ratio increased (Fig. 14 c).
Therefore, the N-doping treatment can improve the electron transport but
decrease the electron lifetime in DSCs.

4.2 Electron Recombination

The difference between N-doped DSCs and TiO 2 DSCs with respect to their charge
transfer properties was also studied by EIS analysis [ 47 - 49 ]. The Nyquist diagram
typically features three semicircles in order of increasing frequency. These three
semicircles correspond to the following: the Nernst diffusion within the electrolyte,
electron transport at the oxide/electrolyte interface, and redox reaction at the
platinum counter electrode.
The main concerns in N-doped DSCs are the following: (1) the impedance due
to electron transfer from the conduction band of the mesoscopic film to triiodide