Study of Cu2O/Cu Schottky barrier junction

Grace Padmini Thambirasa

The research described in this thesis was carried out in the context of an attempt to develop a new inexpensive material based on simple technology to produce a low cost solar cell. CU2O semiconductor was chosen for this purpose and Schottky barrier CuaO/Cu cells were prepared. The procedures to produce the cells were optimized to get the best performance. The cell was treated chemically to reduce the series resistance and to increase the shunt resistance, which were the main limiting factors found by electrical characterization of the cells. A low barrier height was obtained for the cells which may be due to poor interfaces and mechanical stress at the junction. The optical characterization showed that the spectral photoresponse for these cells had a much narrower band width than for Si solar cells and it also demonstrated the importance of thinner Cu2O films. Minority carrier lifetimes from voltage decay measurements were found to lie between 2 - 4µs. The minority carrier diffusion length was found to be less than 1µm, by comparing experimental and theoretical predictions of the dependence of the spectral photoresponse on the thickness of Cu2O films. The theoretical analysis of efficiency showed that to improve the cell's performance the series resistance should be < 1Ω, with shunt resistance greater than 2kΩ and the saturation current density as low as possible. X-ray diffraction analysis indicated a uniform distribution of copper and oxygen in the Cu2O film. X-ray diffraction using powder methods on the cell showed that the film was preferentially-oriented with the majority of the crystal planes of Cu2O being (110) and (111). This analysis also showed that there were no traces of copper metal inclusions in the Cu2O film. From SIMS analysis it was found that the Cu2O film had some impurities which may or may not affect the cell's performance. The copper - to - oxygen ratio was uniform throughout the film, for the cell, before any chemical treatment. The cells which are treated with Na2S had a sulphur-rich surface which improved the light absorption at the surface of the CU2O and thus improved the current output. The sulphide treatment also reduced the sheet resistance of the films. This analysis shows that Cu2O/Cu solar cells have intrinsic difficulties which limit their potential for low-cost applications. The high resistivity and low barrier heights are fundamental properties of the material and will not be easily overcome with simple technology.

Study of Cu2O/Cu Schottky barrier junction

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