The Renaissance of Iron Pyrite Photovoltaics: Progress, Challenges, and Perspectives - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 135

absorption, requiring thicker layers of photoactive material. These drawbacks have
kept C-Si solar cell energy more expensive than other methods of conventional
energy generation. Other semiconducting materials have been proposed to avoid
the high material consumption such as CdS, CdSe CdTe, CuInSe 2 due to their
better absorbance. While these materials exhibit better absorbing properties, they
contain toxic elements that propose environmental problems both in mass pro-
duction and in widespread use. A material that exhibits high absorption while
retaining low material cost and toxicity will be necessary to drive energy costs
down.
Iron Pyrite (FeS 2 , Fool's Gold, Iron Disulfide) exhibits promising properties for
use in solar cell devices. It boasts an indirect band gap of 0.95 eV and an
absorption coefficient of greater than 10 5 cm -1 , which is unusual for an indirect
band gap [ 3 , 4 ]. In comparison to silicon, another indirect band gap photovoltaic
(PV) material, FeS 2 shows two magnitudes greater absorption coefficient [ 5 ]. This
higher absorption coefficient means that thinner films, and thus less material, can
be used while creating devices.
Pyrite is a cheaper material than many other inorganic solar materials. Since
silicon is the dominant material for commercial PVs, it is logical to use for com-
parison. Silicon is the second most abundant material in the earth's crust, while iron
trails in fourth [ 6 ]. Even so, silicon production still trails in extraction costs which is
*$1.70/kg [ 7 ]. This is 57 times higher extraction costs to that of iron ($0.03/kg).
The huge difference between the price stems from thermodynamics of converting
raw material into final elemental forms. It requires 24 kWh/kg to purify silicon
from its feedstock of silica (SiO 2 ) while it only takes 2 kWh/kg to achieve iron from
hematite (Fe 2 O 3 )[ 8 ]. This natural barrier will always exist for silicon solar cells and
will limit its use in a future where PVs need to be cost-effective.
Much research has been conducted to increase the efficiency of standard silicon
solar cells to combat this natural cost of production. Yet, studies show that a
crystalline silicon cell with efficiency of 19 % will still produce 10-100 times less
energy than the annual global consumption [ 7 ]. When compared to FeS 2 , creating
a solar cell with 4 % efficiency and three times less material consumption could
produce the same amount of energy. This example shows that it may be best to put
aside the mentality of trying to achieve the best efficiency of silicon cells and look
into mass production of cheap, less efficient cells.
It is obvious from the above that being able to use less material is conducive to
keeping solar cell costs down. Thin film solar cells have been extensively
researched in the past and show great promise to help improve, and have put to use
both organic and inorganic active layers [ 9 - 14 ]. Recently, with the explosion of
nanotechnology research, nanocrystals have been utilized in creating films of
material that are less than 300 nm that show greater than 3 % efficiencies [ 15 - 18 ].
Combining the promising aspects of nanomaterial and iron pyrite's notable
properties could unlock the door to creating low-cost solar cells with minimal
material usage, which could drive down the cost of energy and decrease our
dependence on other, less green, sources of energy.