Civil Engineering Reference
In-Depth Information
generations. This, however, is not the only reason for the use of QDs: the
QD also enables material moulding into different forms as well as being
cheaper due to the fact that they use basic chemical reactions (MIT, 2007)
and they also reduce wasteful heat seen in the previous generations, in
addition to maximizing the amount of light to electricity conversion.
Issues with non-utilization of PVs extra electron energy ('hot carrier' or
'hot excitation') have been previously highlighted. Normally it is kinetic
free energy which is lost in pico or subpicoseconds through a process of
electron-phonon scattering creating heat from the kinetic energy. Some of
these issues have been previously solved by tandem PVs, but not all. A
positive aspect is that the quantum dot holds on to the hot carrier for a
longer time, thus enabling an extended period for it to cool, extending the
lifetime of the hot electrons by as much as 1,000 times. Part of this is the
three-dimensional array of the QD, which enables strong electronic cou-
pling producing an extended lifetime for excitons. This provides more
movement of hot carriers and the possibility of more electric generation,
enabling more charge from one photon (Manna and Mahajan, 2007).
Quantum dots are very interesting and exciting as they have optical pro-
perties that are not seen in typical materials and enable a good spectrum
control of emitted light, and these properties come from confi nement of the
electron-hole pairs. The QD offers a varied emission and absorption spec-
trum which is linked to the various particle sizes, although it is important
to remember that the QDs will only stay in a confi ned space if their
wavelengths are associated.
The relationship between the size of dot with wavelength and energy can
be summarized as a smaller dot equalling shorter wavelength fi t, also equal-
ling higher energy of the electron, with the larger dots having the opposite
relationship. Also different from traditional PVs is the QDs' photovoltaics'
ability to obtain three electrons from one high energy photon of sunlight
where normally it was only a maximum of one (Nozik, 2001; Ellingson
et al.
, 2005; ISIS Press Release, 2006). This is referred to as multiple exciton
generation, or MEG for short.
Extended research is ongoing with regard to progression in QDs. Coni-
beer (2007) discusses a crystalline material tandem thin fi lm QD which
obviously enables the wider band gap (BG) and is constructed of silicon
placed between layers of Si-based dielectric compound with a QD diameter
of 2 nm and a BG of 1-7 eV. The process to produce this QD is as follows:
Thin fi lm is created by using either sputtering or chemical vapour depo-
sition (CVD) methods.
High temperature toughening of the crystallized QD.
This method means that issues with lattice mismatching are not suffered
as the framework is lacking a defi nite shape. However, Conibeer (2007)
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