Page 185 - Multifunctional Photocatalytic Materials for Energy

Metal-based semiconductor nanomaterials for thin-film solar cells 171

In this photoelectrochemical cell, the mesoporous semiconductor layer plays a
major role in how the DSSC functions. An ideal photoanode material must satisfy
the following characteristics: (A) the semiconductor layers should be uniform and
mesoporous and should have a very high surface area to accommodate maximum dye
adsorption, (B) the layers should be so transparent that they do not affect the dye’s
light absorption, (C) the layers should be “highways” for fast transport of electrons;
(D) the quality of the semiconductor layers should be high enough that the electron
diffusion length will increase, resulting in increasing the thickness of the absorber
layer; and (E) there should be a porous photoanode that is completely accessible to
the electrolyte [59].

8.4.1 NPs

Because of the photoanode materials’ limited specific surface, the early DSSCs’ PCEs
were very low until O’Regan and Gratzel introduced mesoporous TiO 2 film consisting
of small NPs (20 nm in size) as scaffold for light harvesters in 1991. The dramatically
increased specific surface boosted the PCEs up to 7%. Since then, DSSCs based on
TiO 2 nanostructures have attracted a great deal of attention, and different strategies
have been proposed to optimize the TiO 2 structure and morphology, with the aim
of reducing charge losses during charge collection and increasing light harnessing.
Among the different strategies, the TiCl 4 treatment of TiO 2 photoanodes introduced by
O’Regan and colleagues demonstrated the easiest and most effective way to increase
the specific area and significantly decrease the electron/electrolyte recombination rate
constant (up to 20-fold) [60]. Therefore the combination of these two effects greatly
increased the photocurrent and the consequent boost of PCE. Although 12% PCE
based on mesoporous TiO 2 NPs was achieved, two obstacles affect the development
of DSSCs: (a) electron drift, which is an essential electron transport mechanism in
most photovoltaic cells, is prevented in DSSCs by ions in the electrolyte that screen
macroscopic electric fields and couple strongly with the moving electrons, and (b)
the electron lifetime and the diffusion length are short because of the numerous grain
boundaries that exist in TiO 2 NPs [61].

8.4.2 1D nanomaterials
The dynamic competition between the electron transport in the TiO 2 and the interfa-
cial recombination of electrons is the rate-limiting factor of the DSSCs’ photoanodes.
Because of defects, surface states, grain boundaries, and so on, that act as electron
trapping sites within the TiO 2 NPs, the electron transport time constants in TiO 2 NPs
are relatively slow [62–65]. To enhance the electron transport speed and decrease the
hole-electron recombination, 1D nanostructures such as NTs [66–71], NRs [72–75],
and NWs [76,77] are used to replace NPs. As shown in Fig. 8.15, the 1D nanostruc-
tures can significantly optimize the electron transport path and decrease intercrystal-
line contacts, thereby accelerating electron transport and lowering the recombination
probability.