Growth Of Polycrystalline Silicon Thin Films On Flexible Substrate

With the shortage of conventional energy, the renewable energy especially the solar power has seen a rapid growth in recent years. The thin film cells have drawn great interests due to their low cost and high efficiency. Among them the polycrystalline silicon thin film has the advantages such as abundant raw materials, low production and energy cost, stable performance, extensive application and so on, which enable it promising in replacing crystalline silicon solar cells and becoming the next generation solar cells.In this thesis, polycrystalline silicon thin films are fabricated on the flexible substrate and the properties of the films are investigated. Comparing with amorphous silicon thin films, polycrystalline silicon thin films are much more stable and lack of photodegratdation. On the other hand, the flexible substrate gives much more possibilities for the development of flexible solar cells in the future, due to its flexibility, low weight, and envisaged cost-effective roll-to-roll manufacturing. Therefore, it has vital values to fabricate polycrystalline silicon thin films on flexible substrate in terms of scientific research and industry application.Several methods are used to fabricate polycrystalline silicon thin films on flexible substrate-polyimide material as well as glass substrate, including Hot-Wire Chemical Vapor Deposition (HWCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD) and aluminium induced crystallization. Surface morphology of the thin films is characterized by scanning electron microscopy (SEM), and the crystallinity is investigated by X-ray diffraction (XRD) and Raman spectroscopy (Raman). And spectrophotometer is applied to research the optical properties of thin films. The results are as following:1 Using the conventional method to fabricate silicon thin films on the polyimide (PI) and glass substrate by HWCVD, it is found that the films on the glass substrate are polycrystalline silicon, and the quality of the films are influenced by the parameters such as the ratio of gas flow, the temperature of substrate, the pressure and so on. However, it is difficult to fabricate polycrystalline silicon thin films on the PI substrate and it is proven to be amorphous.2 There are some processes before and after the deposition of the thin films, which play an important role to control the structure and crystallinity of the thin films. The pre-H₂ thermal process can deal with the possible impurity on the surface of the substrate. The after-H2 thermal process can lead the amorphous phase to crystallize. The relationshipbetween the thin film deposition rate and time is also investigated.3 The method of intermittent supply of silane is applied to fabricate polycrystalline silicon thin films on PI substrate by HWCVD. The structure of thin films and grain size are affected by the period of intermittent supply. It is found that the films have larger grain size and good crystallinity at the short period of intermittent supply, but low deposition rate. Furthermore, the optical absorption of thin films on the PI substrate and glass substrate is extremely different.4 For the first time, the intermittent-conventional two-step-deposition method is induced to fabricate polycrystalline silicon thin films on the PI substrate. During the first step of intermittent supply of silane, it is found that two or more intermittent times can cause the nucleation and crystalline.5 The diffusion model and etching model were combined to explain the different results between conventional methond and intermittent supply of silane. The main difference between conventional method and intermittent method is the process of supplying H₂ solely. On one hand, H₂ destrories the amorphous phases of the thin films; on the other hand, it accelerates the surface diffusion of Si atoms, and helps them find the suitable loctions to form silicon nucleus and then grow up.6 The thin films grown by PECVD are amorphous on the glass and PI substrates, and they are also not crystallized by the annealing process, which may be related with the substrate materials, the annealing temperature and time and so on.7 By Al induced method to crystallize amorphous silicon thin films, it is necessary to form Al layer on the substrate firstly and then fabricate thin film by PECVD. Then there are two kinds of annealing methods: Conven-tional Furnace Annealing (CFA) and Rapid Thermal Annealing (RTP). It is found that CFA can not obtain the crystalline films, while RTP is useful for the formation of polycrystalline silicon.