Ebin Bastola

Application of composition controlled nickel-alloyed iron sulfide pyrite nanocrystal thin films as the hole transport layer in cadmium telluride solar cells

Here, we report hot-injection colloidal synthesis, characterization, and control of electronic conductivity of nickel-alloyed iron sulfide (NixFe1−xS2) pyrite nanocrystals (NCs). The Ni-alloyed iron pyrite NCs were synthesized using iron (Fe) and nickel (Ni) bromides as Fe and Ni sources, and elemental sulfur (S) as a sulfur source. As Ni is incorporated into the iron pyrite (FeS2) NCs, the X-ray diffraction (XRD) peaks shift towards lower diffraction angles indicating higher lattice constants of the alloyed NCs in accord with Vegards law. Scherrer-analysis and scanning electron microscopy (SEM) imaging indicate that the average particle sizes of alloyed NCs are smaller compared to pure FeS2 NCs. In UV-Vis-NIR spectra, the alloyed NCs have higher absorbance in the infrared (IR) region than pure FeS2 NCs indicating Ni-alloyed NCs have higher densities of mid-band gap defect states. Based on thermal probe and Hall-effect measurements, the majority charge carriers in these alloyed NCs depend upon the material composition. Pure iron pyrite (FeS2) and Ni0.1Fe0.9S2 NCs show p-type conductivity while Ni0.2Fe0.8S2 and higher Ni concentration alloys exhibit n-type conductivity. Application of these alloyed NC thin films as the hole transport layer for CdTe solar cells revealed that Ni0.05Fe0.95S2 NCs perform best with the average increase in efficiency of ∼5%, with the best cell performing up to 8% better than the laboratory standard copper/gold (Cu/Au) cell.

Elemental anion thermal injection synthesis of nanocrystalline marcasite iron dichalcogenide FeSe2 and FeTe2

We report a hot-injection colloidal method for the synthesis of nanocrystalline (NC) iron diselenide (FeSe2), and iron ditelluride (FeTe2) derived from iron(II) bromide as the iron (Fe) precursor. These highly crystalline iron chalcogenides are synthesized by injecting elemental selenium (Se) or tellurium (Te), complexed with oleylamine, into a reaction flask containing the iron precursor. NC FeSe2 and FeTe2 films have been characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. Both FeSe2 and FeTe2 NC particles exhibit marcasite phase with orthorhombic crystal structure. The as-synthesized NC FeSe2 and FeTe2 show irregular shapes with consistent stoichiometric ratio. In addition, we discuss these iron chalcogenides with regard to their thin film electronic properties including sheet resistance, resistivity, majority carrier type, and their possible device applications.