(174g) Synthetic Iron Pyrite (FeS2) Nanocrystals for Use In Hybrid Organic-Inorganic Solar Cells

One of the
greatest sources of energy comes from solar radiation but the high cost
associated with fabricating and maintaining silicon solar cells prevents its
spread in popularity. Low-cost and highly efficient photovoltaic devices have
to be developed in order to significantly increase the proportion of solar
energy to the total energy used.  A material that is promising in this aspect
is iron pyrite (FeS₂, commonly known as fool's gold), which is abundant
as well as environmentally benign with a useful band gap of ~0.95 eV and a high
optical absorption coefficient (α
> 10⁵ cm^-1, 2 orders of magnitude greater than
crystalline silicon).  Because of this high optical absorption coefficient,
iron pyrite can absorb most incident light with much less material than silicon-based
photovoltaic devices.  Here we report a simple method of synthesizing
crystalline FeS₂ (pyrite) nanocrystals via a hydrothermal procedure. 
In particular, we aim to control the size and morphology of nanocrystals in
order to investigate the surface and interfacial properties on the performance
of hybrid organic-inorganic photovoltaic devices.

Iron pyrite
nanocrystals were synthesized using a hydrothermal method.  Typically, poly(vinylpyrrolidone)
(PVP) solution was mixed with poly(vinyl alcohol) (PVA) solution and FeCl₂·4H₂O followed by the addition of NaOH and sulfur
powder.  After stirring the mixture for 30 minutes, the Teflon liner containing
the reaction mixture was placed in a stainless steel autoclave and reacted at
170-220°C for 12-24 hours.  The reactor was cooled
naturally to room temperature.  Once cooled, the products were washed with
deionized water and absolute ethanol several times, and dried under vacuum.  The
pyrite phase was confirmed by powder XRD as well as high resolution TEM and
selected area electron diffraction (SAED).  The size and morphology were
determined by SEM.  Results indicate that the molecular weight of surfactants
(PVP and PVA), their concentration and ratio, as well as the concentration of
NaOH play important roles in controlling the morphology, i.e., cubic or
octahedral shape, of the resulting nanocrystals.  The reaction mechanisms,
especially the interaction of surfactants with different crystalline facets guiding
the final morphology will be discussed.  Furthermore, the synthesized
nanocrystals were blended with P3HT polymer and hybrid solar cells were made.  The
relationship between the size and morphology of nanocrystals and the solar cell
power conversion efficiency were obtained.  The effect of crystalline facets on
the charge generation and separation at the polymer-nanocrystal interface will
be discussed.