Hematite nanorods for use in solar cells

Hematite (α-Fe2O3) nanorods were grown on tin oxide coated glass substrates from an aqueous solution of FeCl3 and NaNO3 with varying growth parameters (growth time, growth temperature, reactant concentration, pH of precursor solution, substrate type, and substrate cleaning procedure).
Hematite has a bandgap of ~2.2 eV which enables it to absorb a large part of the visible spectrum, but it has poor conductive properties, especially a short diffusion length for electron holes. The geometry of the nanorods enables light to be absorbed along the long axis of the rod, ensuring a high absorption, while the holes only need to diffuse a short distance in the radial direction to reach the hole carrying phase. Furthermore the path of the photo excited electron is straight and does not cross any grain boundaries, which should present an advantage over multicrystalline colloidal films. The position of the valence band of hematite makes it a possible candidate material for use as the O2 evolving anode in an optically driven water splitting process.
The morphology and crystal structure of the synthesized nanorods was characterized by SEM and TEM, and their chemical composition was investigated using energy dispersive spectrometry (EDS). It was found that the rods consisted of smaller fibers 7~nm in diameter. The crystal structure of the rods was found to resemble that of α-Fe2O3, and a crystal direction corresponding to the axial direction of the rods was suggested.
The optical absorption and photo electrochemical properties of the rod arrays along with its efficiency as a water splitting anode was investigated and compared to results from the literature. Practically no photoresponse was observed from the nanorod samples, which is attributed to the inherently poor conductive properties of α-Fe2O3.

Supervisors: Kristine Kilså and Jesper Nygård
Censor: Krisitian Sylvester-Hvid, Risø DTU