| As an important electronic material in energy-information industry, poly-silicon thin film is widely used in LSI and semiconductor discrete devices because of its excellent photoelectric characteristics and low-cost of preparation. Poly-silicon thin film solar cells with high efficiency, stability and low-cost would replace the a-Si thin film solar cells as a new generation of non- pollution civil solar cells.In order to better understand the optical performance of polysilicon thin films, identify the current transport mechanisms of the material, we have studied the photoelectric properties of the polycrystalline silicon thin films which are prepared by PECVD technology under the following conditions: substrate temperature 300 oC; SiCl4/H2 ratio 8/40sccm; gas pressure 80 Pa; RF powers 80 W to 160 W. And the results are presented as follows:(1) The conductivity and activation energy at high temperature (room temperature to 150 oC) and the low temperature electrical transport properties of the polysilicon thin films have been studied, which were prepared by PECVD technology using SiCl4 and H2. We have discussed the influence of annealing on activation energy measured in temperature increasing and decreasing processes. And the dark conductivity changes with temperature in polysilicon thin film were also measured. In different temperature range, the temperature dependence of dark conductivity is difference which can be classified as different conductive mechanism. The influence of the crystallization rate on the conductivity has also been discussed. Thermionic emission theory and electronic tunneling theory have also been applied to discuss the current transport of the films with different crystallization rate.(2) Space-charge-limited current (SCLC) theory was used to study the current-voltage characteristics of polysilicon thin films. The experimental results showed that the current was completely controlled by the local state near the quasi-Fermi level. The state density distribution above the Fermi level was calculated using step by step method and the linear least squared fit method respectively. The state density of the polysilicon samples obtained was in the range of 1016to1017cm-3eV-1. The distribution of the gap states density varied due to the deposition conditions. There had a minimum states density near the Fermi level for all the samples.(3) Based on the study of the illumination stability of polycrystalline silicon material with different crystallization rate, the relationship between the optoelectronic properties and the microstructure of the material was analyzed. The results showed that though the polysilicon film with lower crystallization rate (Xc = 45%) still had light soaking degradation effect, it had better illumination stability than general amorphous silicon materials. Polysilicon material with high crystallization rate (Xc= 61% and Xc=75%) showed persistent photoconductivity effect. For all the samples, their conductivity increased when they were in light soaking. Their Fermi levels moved upward as the defect states were filled by light generated carriers. As a result, their activation energies decreased. Finally, two-phase structure barrier model was applied to study the time dependence of dark conductivity of the samples under different DC bias. |
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