Zn(1-x)CuxO and Zn(1-x)FexO and ZnO nanostructures for photovoltaic applications / Muhd Firdaus Kasim, Norlida Kamarulzaman and Roshidah Rusdi

Global research interest is focussed on wide band gap semiconductor materials. ZnO is one of the wide band gap semiconductors attracting great interests from the scientific community. ZnO is considred one of the safest metal oxides. This is due to its special and unique properties amongst which are a direct and wide band gap of 3.37 eV, a large exciton binding energy of 60 meV, high electron mobility, etc. This has made ZnO suitable for a wide range of applications in optoelectronics devices, LEDs, varistors, solar cell, lasers, chemical sensors and others. There are many techniques used to synthesize ZnO nanostructures such as hydrothermal, combustion, spray pyrolisis, sol-gel method and etc. Most of the synthesis methods to produce nano ZnO require a catalyst and some require substrates. This yields very little end product. In this research work, nanostructured ZnO and doped Zn(1-x)MxO (M=Cu and Fe) are prepared with no catalyst or substrates used. The advantages of this method are direct and relatively easy of getting large amounts of different morphologies of nanostructured ZnO and doped Zn(1-x)MxO nanostructured materials. Another advantage is the formation of the nanorods at a relatively lower temperature than most other methods. This presented a promising way for commercialization of the ZnO and Zn(1-x)MxO doped nanomaterials. The materials were characterized using simultaneous thermogravimetric analysis, X-ray diffraction and transmission electron microscopy. Structures such as nanotubes, nanorods, and spherically shaped crystals were formed at certain annealing temperatures. It is found that the ZnO and doped Zn(1-x)MxO nanorods grew in the direction of c-axis. For comparisons of size and morphology, the ZnO and doped Zn(1-x)MxO precursors were annealed at different temperatures. The lattice parameter of the ZnO and Zn(1-x)MxO nanomaterials are determined and found related to their light absorption properties. It is also found that the band gap energies of the ZnO nanomaterilas are dependent on the size and aspect ratios of the nanostructures. The band gap is wider when the aspect ratio is higher for rod-like shapes and for smaller diameters for spherical shapes which is in accordance with the quantum size effect theory. The conductivity studies of nano ZnO showed that the conductivity decreases when the annealing temperature increases. The conductivity studies were also carried out at various temperatures. When the temperature increases the conductivity also increases. The results obtained obeyed the Arrhenius model. The band gaps of the ZnO and doped Zn(1-x)MxO nanomaterilas are dependent on morphology and dopant atoms. These results mean that it is possible to engineer the band gap of the materials with the right choice of dopant. The different band gap values of the modified compounds can make them suitable for semiconductor applications either as a diode or metal oxide semiconductor component. Therefore, modifying materials in terms of size of naostructures or doping with other transition metal can change their fundamental properties of band gap. This is turn affects their electrical behaviour and makes them suitable for various applications in semiconductor technology.