| Energy crisis and environmental pollution have become the primary problems faced by the world economy development. As a consequence, more and more countries begin to turn on the clean and renewable power. As an inexhaustibility and green energy, solar energy provides a promising prospect.At present, solar cells are undergoing the transition from Generation I (crystalline Si) solar cells to Generation II (thin film) solar cells. On the basis of Generation I and Generation II solar cells, the concept of Generation III approaches to photovoltaics is proposed and assumed to have advantages of low cost, high efficiency, long lifetime, abundant raw materials, nontoxicity, and so forth. Thin film solar cells based on microcrystalline silicon are of particular interest owing to the absence of Staebler-Wronski effect, simple preparation and large scale production in international photovoltaic markets. Silicon nanowires (SiNWs) based solar cells are a promising class of photovoltaic devices due to several performance and processing benefits they enable, including the strong broadband optical absorption and a direct path for charge transport by the geometry of such nanostructures.In this thesis, we systematically performed the study on the preparation and growth mechanisms of highly crystallized silicon films on glass and plastic. Meanwhile, we successfully synthesized SiNWs on Si substrates by the self-designed CVD system. The growth mechanism, microstructure and optical properties were analyzed in details. The outline of the thesis is shown as follows:â—The effects of the distance between the inductance coil and substrates (dIS) on thecrystallinity and optical band gap of silicon films were studied by inductivelycoupled plasma (ICP-) CVD. We proposed a gas-phase transport process for thedeposition of silicon films in the ICP-CVD system with a mixture of SiH4/H2 asthe source gas. On the basis of the experimental results, it is observed thatlow-temperature silicon films possessed highly crystallized quality with the dIS of 5 cm;â—Combining the unique properties of ICP-CVD, we developed a new approach, AICG (Aluminum-Induced Crystallization Growth), to prepare highly-crystallized silicon films at low temperatures. In the process, highly crystallized silicon films were prepared on Al-coated glass and polyethylene napthalate (PEN) substrates using ICP-CVD at low temperature with a mixture of SiH4/H2 as the source gas. Moreover, the phenomenon of layer exchange occurring in the AIAC (Aluminum-Induced Annealing Crystallization) process was not observed in AICG. Based on the experimental results, a modified growth mechanism was suggested in terms of gas phase reactions and surface reactions for the crystallization process of silicon films on Al-coated substrates at low temperatures;â—Using Au as catalyst, fine and straight SiNWs with diameters of 8～30 nm and lengths of several micrometers were synthesized by self-designed CVD system at 480℃and atmosphere pressure. The growth of SiNWs follows the VLS growth mechanism. SiNWs consist of crystalline silicon cores covered by amorphous silicon oxide sheaths (～3 nm). Raman spectroscopy shows an asymmetric peak around 517 cm-1, with a down-shift of 4 cm-1 with respect to the first-order Raman peak of single-crystalline silicon caused by the size confinement effect.
|
PDF Full Text Request