Fabrication and characterization of ultra-thin semiconductors for solar energy conversion devices using spalling process

Abstract

Recently, there has been an increasing demand for environmentally friendly renewable energy as an alternative to the depletion of fossil fuels and climate change. Among the various renewable energy sources, energy generated by solar energy conversion is considered a realistic alternative. However, due to the higher cost of solar energy conversion systems, such as photovoltaic modules, than the conventional fossil fuels, it has been difficult to implement solar energy conversion systems. Among the costs of photovoltaic modules, about 40% is for the silicon wafer that is used as a light absorbing material. Thus, it is crucial to reduce the cost of the silicon wafer in order to lower the cost of the solar energy conversion system. The reason for the high cost of silicon wafers is that the conventional wafering process has a high material loss and it is difficult to produce silicon wafers with appropriate thicknesses. In this dissertation, we introduce a new wafering process using a spalling process to overcome the limitation of the conventional wafering process and demonstrate a sub-50 $\mu$m-thick Si wafer without material loss. In addition, we developed a cost-effective and efficient solar energy conversion device using the ultra-thin silicon wafer fabricated by the spalling process.

In this dissertation, chapter 3 describes a new spalling technique that can control the thickness of an ultra-thin semiconductor by decoupling the crack initiation and propagation. The advanced method shows that the exfoliated Si thickness can be adjusted to sub- 5 – 50 $\mu$m. In chapter 4, we describe the systematically analyzed results of the material properties on fabricated ultra-thin silicon. In particular, we inspect the defects on the fabricated ultra-thin silicon that occurred in the spalling process and improve the minority carrier lifetime by appropriate removal of the analyzed defects. Chapter 5 demonstrates a cost-effective and efficient photoelectrochemical cell using the fabricated ultra-thin silicon as a light absorber. We analyzed the effect of the silicon light absorber thickness on the photoelectrochemical properties. In addition, the performance improvement strategy for producing the ultra-thin silicon-based photoelectrochemical cell is proposed and demonstrated.

The spalling process developed through this study shows that it is possible to fabricate an ultra-thin silicon substrate with a sub-50 $\mu$m thickness by overcoming the limitations of conventional wafering technology. The ultra-thin semiconductor fabrication technology using the spalling process can be applied not only to silicon but also to other high-cost semiconductor substrates, such as GaAs, GaN, InP, and Ge, and the manufactured ultra-thin semiconductor substrate will be used as a cost-effective and efficient solar energy conversion device in the future. Furthermore, it can be used in various fields, such as flexible devices and transmission type optical devices.