Biomedical Engineering Reference
In-Depth Information
[43]. However, the absorption efficiency is rather low and its
development was further abandoned for commercial purposes.
In the 1990s, dye-sensitized photoelectrochemical cells proposed
combining the better absorption properties of dyes with TiO
2
semiconductivity [44, 45], even though efficiency was limited by the
deposition of thick dye films deposited on titania compact layers. In
this way, photon absorption was enhanced, but recombination was
also promoted.
A notorious improvement was achieved by using nanostructured
providing large surface area for the deposition of dye thin
films. Thus, an enhanced absorption was allowed without being
penalized by recombination. Parellel TiO
2
TiO
2
nanotube arrays prepared
by anodization of metallic titanium layer not only provide a large
surface area but also ordered pore geometry. The nanotubes disposal
seems to be more suitable for pore geometry of the nanotubes for
solid-state cells manufacturing.
An improved solar harvesting efficiency compared with
conventional sol-gel-derived TiO
has been demonstrated [46]. In fact,
the electron diffusion length is consistent with a collection efficiency
for electrons close to 100%, even for thick nanotube films [47].
The efficiency of dye-sensitized solar cell has reached as much
as 11% [48], but values higher than 15% are required to make them
competitive with Si-based solar cells. Electrons transport time is
delayed in the nanotube by the presence of grain boundaries, defects,
and trap sites. This is an important limitation in manufacturing
long TiO
2
nanotube for efficient dye-sensitized solar cells. For this
reason, several solutions have been proposed, including ZnO thin
film coating [49], substitutional doping (C, N, etc.) [50, 51], and
combining TiO
2
2
with quantum dots (QDs) [52].
Charge storage application:
charge can be directly store in
capacitors and batteries. In capacitors, charge is electrostatically
attracted and retain at the electrode surface, the capacitance
being related to the amount of stored charge and proportional to
area. Capacitors are able to deliver high power pulses with a very
long cycle life, but the energy density is somewhat poor. In turn,
electrochemical batteries rely in a reversible redox reaction to
accumulate charge. The limited reaction rate is responsible for lower
charge and discharge rates though allowing higher energy densities.
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