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.