Environmental Engineering Reference
In-Depth Information
Generally, nanocrystalline mesoporous metal oxide semiconductor (typically
TiO
2
) films, which adsorb dye molecules and transport photogenerated electrons to
the outer circuit, serve as electron conductors and dictate the efficiency of electron
transport and collection [
4
]. Therefore, the oxide semiconductor of photoanodes
plays a key role in the performance of DSCs. Specifically, an excellent photoanode
should include: (1) a large surface area and an appropriate isoelectric point (IEP)
which can guarantee a high amount and quality of dye uptake; (2) a perfect lattice
and low electron trap distribution to reduce the photogenerated electron losses; and
(3) a good neck-connection between nanoparticles, which facilitate the electron
transport during collection to the conductive substrate.
In recent years, nanocrystalline metal semiconductor materials are extensively
studied for the mesoporous photoanodes of DSCs. Nanocrystalline mesoporous
photoanodes made of TiO
2
materials, as one of the most widely used semicon-
ductors, show an excellent performance in the DSCs. Some researchers also study
other types of semiconductors, such as ZnO [
5
], Zn
2
SnO
4
[
6
], WO
3
[
7
], SrTiO
3
[
8
], Nb
2
O
5
[
9
], SnO
2
[
10
], CeO
2
[
11
], FeS [
12
], and NiO [
13
]. However, the
photovoltaic performance of DSCs based on these semiconductor materials
remains low because some of these materials are not stable in dye solution, such as
ZnO and so on. On the other hand, some materials have a low isoelectric point
(IEP), such as SnO
2
, which is not suitable for dye molecular linking. However,
these non-TiO
2
materials need to be further studied and developed to get a good
photovoltaic performance for DSCs. Up till now, TiO
2
is still the best choice for
photoanodes in DSCs. Aiming at the further improvement for DSCs, on the one
hand, we can develop more efficient and diverse structures of TiO
2
materials. On
the other hand, we can modify TiO
2
to enhance its performance, such as chemical
doping. As oxygen deficiencies exist in pure TiO
2
crystal structures [
14
-
16
] these
oxygen deficiencies can induce TiO
2
to a visible light absorption response pro-
ducing electron-hole pairs. The photoexcited TiO
2
will lead to the oxidation of
iodide or dye by photogenerated holes. Such deficiencies are possible causes for
the shortened lifetime of DSCs. Element doping is an effective way to improve the
performance of TiO
2
. We can choose metal or nonmetal to proceed with TiO
2
doping. Recently, some studies reported the modifying of pure TiO
2
with metal
doping, i.e., Zn-, La-, Ta-, and Nb-doped TiO
2
[
17
-
20
]. The performance of DSCs
can be improved by adjusting doping metals. However, metal doping can affect the
position of conduction band (CB) of TiO
2
contributing to the change in photo-
voltage. Besides, metal doping also introduces more recombination sites for
electron [
21
]. Nonmetal doping of TiO
2
materials is another good choice for fine-
tuning of TiO
2
. Nonmetal elements, such as N [
22
,
23
], C [
24
], B [
25
], I [
26
] etc.,
are used to dope TiO
2
. Especially for DSCs, nitrogen seems to be the most
effective element to enhance the photovoltaic performance of DSCs.
This review summarizes the recent works on the N-doped TiO
2
materials and their
application into photoanodes of DSCs. Herein, the synthesis methods and optical
properties of N-doped TiO
2
are introduced briefly. Then the effect of N-doped
TiO
2
photoanodes on the performance of DSCs is described in detail. Finally, we
discuss the charge transport in DSCs based on N-doped TiO
2
photoanodes.
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