Preparation Of CdS/CdTe Thin Films For Solar Cell Applications

High-efficiency and low-cost thin film solar cells have attracted extensive interests in the past decades. Of all the candidates in the photovoltaic field, CdS/CdTe thin film solar cells are generally considered to be highly promising, because they are cheaper, much easier to produce and stable to work.So far, several deposition techniques of CdS or CdTe thin films have been developed, including physical and chemical methods. Compared with chemical methods, physical methods are more environment-friendly. Also, in high-efficiency thin film solar cells, CdS layers are normally deposited by chemical bath deposition while CdTe layers by physical methods such as sputtering or sublimation. All-physically deposited CdS/CdTe thin film solar cells would help to achieve flow production, thus reducing cost and increasing yield.In this thesis, CdS and CdTe polycrystalline thin films are prepared by two kinds of physical methods:electron beam evaporation (EBE) and vacuum evaporation (VE).Further, CdS/CdTe thin film solar cells are fabricated successfully. The characteristics of thin films and solar cells are closely investigated to evaluate the feasibility of these two methods.Main contents are listed as follows:1. Cadmium sulphide (CdS) thin films are deposited by EBE method at different substrate temperatures. The structural characteristics are studied by the X-ray diffraction (XRD) pattern. The CdS films are polycrystalline, exhibiting a preferred orientation along the (002) plane. Atomic force microscopy (AFM) reveals that the root mean square (RMS) roughness of the films increases as the substrate temperature increases. The optical band gap values of CdS films increase slightly with the increasing substrate temperature, in the range of2.42-2.48eV. The electrical characteristics of CdS/p-Si heterojunction are investigated in dark condition and under illumination. Excellent rectifying and photovoltaic properties are observed.2. VE technique is employed to deposition CdS thin films. The impact of deposition temperature and atmosphere pressure on the properties of CdS films is studied. XRD spectra indicates that CdS films are polycrystalline films with a preferential growth along the (002) plane. AFM study shows the surfaces of the films are smooth and uniform. The grain size, surface roughness and the values of the optical band gap increase with the increasing deposition temperature and decrease as the pressure increases. Hall effect measurement shows that increasing the deposition temperature or atmosphere pressure both increase the resistivity of CdS thin films.3. CdTe thin films with different thicknesses are prepared by EBE method. The films are annealed at different temperatures in nitrogen atmosphere. XRD study shows that the CdTe films are polycrystalline films with a cubic structure, showing a preferential growth along the (111) plane. After being annealed, the intensity of the (111) peak decreases compared with those of (220) and (311), indicating the significant growth of (220) and (311) oriented grains at the expense of (111) oriented grains. Annealing treatment is proved to have promoted the crystallinity of CdTe films by both XRD spectral and AFM study. The absorption edge of CdTe films is found to shift towards a longer wavelength and the optical band gap decreases as the film thickness increases. CdTe films annealed at300℃possess the largest value of refractive index, indicating the largest packing density.4. CdS/CdTe thin film solar cells are successfully fabricated by two kinds of physical methods, EBE and VE. The performance of different-structured solar cells deposited by different evaporation methods is analyzed to study the impact of back contact. The parameters of the solar cells reveals that the CuxTe layer reduces the resistivity between p-type CdTe film and bottom electrode, leading to a higher current density when the cell is illuminated. Meanwhile, all-electron-evaporated solar cells are found to have higher values of open-circuit voltage and efficiency, despite of their larger resistivity and smaller current.