Silicon Epitaxial Layers Grown on Buried Porous Silicon Templates for solar cells: Detailed Electrical and Chemical Understanding

摘要

In order to place solar power prominently in the global energy mix in the long term future, there needs to be continued cost reductions in the photovoltaic industry. In silicon photovoltaics, which forms 85-90% of the market, the cost of the silicon material itself forms a major fraction of the final solar module cost. Thus, the international technology roadmap for photovoltaic (ITRPV) forecasts a continuous reduction in the thickness of the wafer used to fabricate the solar cells.With reduction in consumption of high quality expensive silicon as the motivation, two silicon solar cell concepts have been envisaged, namely the wafer-equivalent epitaxial silicon solar cell (WE-epicell) and the layer-transferred epitaxial silicon solar cell (LT-epicell). In both cell concepts, the entire photovoltaic power conversion occurs in a thin (20-50 µm) epitaxially-grown layer. In comparison, the thickness of a standard silicon solar cell is ~170 µm.In WE-epicells, the epitaxial layer is grown on a low-cost and often low-purity native multi-crystalline silicon substrate, on which it remains attached in the final solar cell. In LT-epicells, the epitaxial layer is grown on a high quality mono-crystalline silicon substrate and then transferred to a low-cost carrier such as glass. The parent substrate is then re-used for the next cycle of epitaxial silicon layer transfer. In both cell concepts, porous silicon plays important functions. Porous silicon is formed by electrochemically etching a p+ silicon substrate in an acidic electrolyte and sintering it at 1130 oC. Firstly, in WE-epicells, porous silicon acts as an embedded Bragg reflector at the interface between the epitaxial layer and the low-cost substrate in order to reflect long-wavelength photons reaching the interface thereby reducing optical losses through transmission of light into the substrate, where any absorption does not contribute to the photo-generated current. In this way, the short-circuit current density of the WE-epicell is enhanced by the porous silicon Bragg reflector.Secondly, low-cost substrates used in WE-epicells often contain significant concentration of efficiency-killing metal impurities which can diffuse into the epitaxial layer and contaminate it. In addition to its optical function, porous silicon also acts as a gettering layer to trap metal impurities at its void surfaces, effectively maintaining a relatively “clean” epitaxial layer in its proximity. This allows higher efficiency WE-epicells to be made on low-cost, low-purity silicon substrates.Thirdly, in LT-epicells, porous silicon is the enabling technology for the layer transfer process, whereby the porosity of the porous silicon is tuned such that an elongated empty space forms within the substrate where the porous silicon is etched. This acts as the detachment layer for the layer transfer of the epitaxial layer that is grown on top.Finally, in both cell concepts, the epitaxial layer is grown on top of annealed and sintered porous silicon. Although this is the case by design rather than choice, porous silicon also functions as a template for the epitaxial growth of high quality silicon.The work of this thesis focuses on the in-depth study of two of these functions: (1) porous silicon as a gettering layer in the context of WE-epicells and (2) porous silicon as a template for epitaxial growth in the context of both WE-epicells and LT-epicells. Based on the theoretical and experimental understanding from these studies, suggestions for improvement of porous silicon as a gettering layer and as a template for epitaxy are proposed and implemented.