Design, Fabrication, and Modification of Cost-Effective Nanostructured TiO for Solar Energy Applications - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 10

with electricity produced from coal. But the production cost of these first-genera-
tion solar cells is presently around $3.5/Wp, which is highly dependent on the price
of the silicon material [ 10 ].
Second-generation solar cells, including amorphous (or nano-, micro-, poly-)
silicon, CuInGaSe 2 (CIGS), and CdTe, are based on thin film technologies. These
thin film solar cells possess some advantages, such as relatively simple manu-
facturing processes which reduce the production cost to about $1/Wp, multiple
choices of applied materials, and the possibility of flexible substrates. However,
second-generation solar cells face several shortcomings as well such as the toxicity
(e.g., Cd) and low abundance (e.g., In and Se) of the component materials.
Moreover, it is necessary to further increase their efficiency for practical utilization
[ 11 ]. Both the first- and second-generation solar cells are based on single junction
devices which must obey the Shockley-Queisser limit with a maximum thermo-
dynamic efficiency of 31-33 % when the optimum band gaps fall between about
1.1 and 1.4 eV [ 12 ].
Significantly, the third-generation solar cells, including tandem cells, hot carrier
cells, dye-sensitized solar cells, and organic solar cells can overcome this ther-
modynamic efficiency limit. The most important thing is that these third-generation
solar cells are promising to convert solar energy into electricity at a highly com-
petitive price, that is, less than $0.5/Wp [ 2 , 10 ].
Among these third-generation solar cells, DSSCs offer many attractive features
that facilitate market entry. They afford low production cost (i.e., inexpensive to
manufacture, possibility of roll-to-roll processing and low embodied energy), low-
toxicity, earth-abundant materials (except Pt and Ru), good performance in diverse
light conditions (i.e., high angle of incidence, low intensity and partial shadowing),
lightweight, flexible, and design feasibility (i.e., transparent, bifacial and selected
colors) [ 2 , 13 ]. Since the first DSSC was reported with efficiency of 7.1 % in 1991
[ 14 ], much attention has been devoted to this promising electrochemical device.
After two decades of concentrated efforts, DSSCs have developed into a powerful
photovoltaic technology with a recorded efficiency as high as 12.3 % [ 15 ].
Typically, a DSSC is made of five components: a conductive mechanical
support (e.g., transparent conductive glass or Ti foil), a semiconductor film (e.g.,
TiO 2 ), a sensitizer (e.g., ruthenium dye N719), an electrolyte (e.g., iodide/triiodide
couple), and a counter electrode (e.g., Pt-coated electrode). The operating prin-
ciples of DSSCs are further described in Fig. 1 a[ 16 ]. In DSSCs, electricity is
created at the semiconductor film on which a monolayer of visible light absorbing
dye is chemisorbed. Photo-excitation of the absorbed dye molecules generates
excited electrons which are further injected into the conduction band of the
semiconductor and quickly migrated to the external circuit through the conductive
substrate. The original state of the dye is subsequently restored by electron
donation from the electrolyte, usually an organic solvent containing a redox sys-
tem, such as the iodide/triiodide (I - /I 3 - ) couple. The regeneration of the sensitizer
by I - prevents the recapture of the conduction band electron by the oxidized dye
while I - is regenerated in turn by the reduction of I 3 - at the counter electrode. The
counter electrode returns charge from the external circuit back to the cycling