The Photovoltaic Property Of ZnO/Si Heterojunction By Direct Current Sputtering

ZnO is a wide band semiconductor material with a band gap of 3.37eV at room temperature, it is considered to be a new transparent conductive oxide instead of ITO and SnO₂ because of its high transmittance in visible light region. Otherwise, native ZnO films demonstrate n type conduction resulting from its intrinsic defects (such as V_o and O_i), the electron concentration of n type ZnO can easily arrive over 10²⁰/cm³ by doping. Considering these research background, the main work of this thesis is to study photovoltaic effect of ZnO/Si heterojunction. In this case, the contents of the dissertation are listed as follows:In chapter one, a comprehensive review was given on ZnO in the crystal structure, basic physical and chemical properties, optical and electrical properties; secondly introduce the method of preparation ZnO films briefly, such as sputtering, pulse laser deposition, chemical vapor deposition et al. Finally, introduction of the application of ZnO films in solar cells was given, indicating the core of the thesis.In chapter two, ZnO film has been prepared by DC reactive sputtering on n type Si(100) substrate. X-ray diffraction and morphology studies revealed that the films demonstrated a strain stress in c axis, which is consistent with the result obtained from Raman scattering. A strong ultraviolet emission with no distinct visible emissions was observed in room temperature photo luminescence (PL) spectrum.In chapter three, influence of growth conditions on the photovoltaic property of ZnO/Si heteroj unction had been investigated, including substrate temperature, content of oxygen, growth time. The results demonstrated that the photo current and photo voltage are increase rapidly with the growth temperature, which was connected closely with the crystal quality of ZnO films; the effect of Ar/O₂ ratio on the photovoltaic property was also discussed related to the concentration of intrinsic defects in ZnO films. The photo voltage maintained 350mv with the change of oxygen partial pressure, and photo current initially increased with it, and then decreased sharply. When the Ar/O₂ ratio was 7/2, the short circuit current reached its maximum as 700μA. Open circuit voltage and short circuit current with the ZnO films thickness were investigated. Open circuit voltage remained almost constant at～320 mv with the thickness of ZnO films. Short circuit current reached its maximum as 626μA when the film thickness is 150nm. The mechanism of photovoltaic effect was also discussed.In chapter four, the photo current response spectrum of ZnO films and photo voltage response spectrum of ZnO: Al/Si heterojunction were investigated to analyzed the mechanism of photovoltaic effect. The result demonstrated that the direction of photovoltage generated in ZnO and Si was converse, this was the possible reason for the low photovoltaic efficiency of this simple structure solar cell. We also put forward the method to solve the problem,In chapter five, influence of annealing condition on the photovoltaic effect of ZnO/Si heterojunction was investigated, including annealing temperature, annealing atmosphere, annealing time. Photovoltaic effect was improved when the heterojunction was annealed in N₂ 400℃, the photocurrent increased by two magnitudes than that of unannealed. Conversly, when it was annealed over 500℃, the photovoltage and photocurrent decreased sharply, to Zero at last. For annealing atmosphere, the photovoltaic effect was improved greatly in N₂, subsequently air, finally O₂. Influence of photovoltaic effect by annealing time was not obvious.