P-Type Metal Oxides for Thin Film Solar Cells
Nanocrystalline CuMO2 (M = BIII, AlIII, GaIII, InIII, ScIII, CrIII) metal oxides are an attractive family of wide band gap p-type semiconductors with high demand for applications in organic photovoltaics, perovskite solar cells, and dye-sensitized solar cells. In these devices, CuMO2 materials act as hole transport layers whereby they facilitate the transfer/transport of electron vacancies (i.e. holes) from the photoactive layer to the external circuit. Currently, NiO is the most well studied p-type metal oxide within this field; however, serious challenges exist with regard to its low hole mobility and lack of transparency in the visible region.
CuMO2 materials have been reported to have very large hole mobilities (10 cm2 V-1 s-1 for single crystalline CuAlO2) with band gaps greater than 3.1 eV. Large hole mobility in metal oxide materials is rare due to the large effective masses of holes in the O 2p valence band. In CuMO2 materials, rapid hole transport is attributed to delocalization of the valence band over Cu 3d and O 2p orbitals as well as the layered delafossite crystal structure which features of planar sheet of CuI atoms stabilized by edge-sharing MIIIO6 octahedra.
Research in our group focuses on the synthesis and fundamental characterization of structural and electronic properties of these high surface area, nanocrystalline materials. We are specifically interested in understanding the nature and density of defects states and how they may contribute to their overall function. We are also highly interested in the studying the kinetics and efficiency of hole transfer reactions at the solid-solution interface.
Inorganic Redox Storage Molecules
The storage of electrochemical energy in molecular species is a critical challenge to many energy conversion strategies; from dye-sensitized solar cells to photo/electrocatalytic water splitting to redox flow batteries. Arguably, the best method to store electrochemical energy within molecules is by the formation of chemical bonds coupled to multi-electron oxidation/reduction reactions. For example, water splitting results in the 2e- reduction of 2H+ to H2 to form an H-H bond and the 4e- oxidation of 2H2O to O2 to form an O=O double bond.
A general strategy for achieving multi-electron oxidative chemical bond formation is to force intermediate oxidation states to be unstable with respect to disproportionation. This is exhibited in the I3-/I- redox couple where 2e- oxidation of I- to I3- is highly favored due to the dispropotionation of an I2.- intermediate (2I2.- --> I- + I3-). A similar situation exists for the O2/H2O redox couple mentioned above. Here, the instability of intermediate H2O2 results in a higher favorability of for the direct 4e- oxidation of H2O to O2.
Research in our group targets the development of first-row inorganic coordination compounds that undergo similar redox-cycles. In particular, we are interested in small coordination compounds of nickel and cobalt that undergo changes in coordination environment upon oxidation of Ni(II) or Co(I) (four coordinate) to Ni(IV) or Co(III) (six coordinate). By fine tuning the association constants for for metal-ligand bond formation, we can to force the disproportionation of Ni(III) or Co(II) intermediates and thus generate a strongly favored 2e- redox couple that is controlled by metal-ligand bond formation. With this chemistry we seek to ask fundamental questions about multi-electron transfer reactions. Such as, Are 2e- transfer at the same time or are there transient 1e- intermediates involved?