Sontheimer, Tobias: Design of electron beam evaporated Si thin films for solid phase crystallized solar cells on glass. , Technische Universität Berlin, 2011
10.5442/d0017
Abstract:
The solid phase crystallization of high-rate electron beam evaporated amorphous Si layers on glass is a promising low-cost approach to fabricate high-quality polycrystalline thin film solar cells. This thesis presents a comprehensive structural analysis of high-rate electron beam evaporated Si layers on various SiN- and ZnO-coated and nanostructured sol-gel coated glass substrates. Derived design parameters are used to develop e-beam evaporated Si thin film solar cells and explore novel cell concepts embedded in a transparent conductive ZnO front contact and light harvesting architectures. A systematic analysis of the solid phase crystallization kinetics of evaporated Si on SiN and ZnO-coated glass substrates with different topographic features reveals an essential correlation between the surface roughness of the substrate, the deposition temperature of the Si and the crystal nucleation rate. The crystal growth velocity, however, is not influenced by the substrate topography. In addition, the time-lag decreases with increasing deposition temperature, being consistent with a declining structural disorder in the amorphous material. These results define design rules that allow for the systematic control of the microstructure of the resulting poly-Si film and give rise to the development of poly-Si solar cells on glass. A conversion efficiency of 6.8% is achieved for poly-Si mini-modules on planar SiN-coated glass deposited at a deposition rate of 600 nm/min by systematically varying the deposition parameters. This result is equivalent to the highest efficiencies in an established low-rate PECVD-based reference system. The implementation of a rear-side light trapping structure into the device leads to a record efficiency of 8%. A working device on a ZnO:Al front contact layer, however, requires implementing a diffusion barrier that separates the ZnO layer from the poly-Si film. Texture-etched ZnO and self-organizing spheres embedded in sol-gel are found to be e-beam compatible statistical light trapping approaches, providing a sufficiently flat template to grow compact poly-Si in accordance to the distinct growth characteristics of e-beam evaporated Si. Advanced photon management demands a controlled design of nanostructured Si. An easily scalable fabrication process for periodic arrays of equidistant free-standing Si crystals is developed on nano-imprinted sol-gel coated glass substrates by selectively etching e-beam evaporated Si. The design is extremely versatile and can simply be controlled by varying the feature size of the used imprint stamp. Crystalline Si arrays on a sol-gel template with a 2μm period exhibit a tremendous absorption, inspiring the development of periodic three-dimensional solar cell architectures.