The challenge of crystalline thin film silicon solar cells

The high production costs of thick high-efficiency crystalline Si solar cells are inhibiting widespread application of photovoltaic devices, amorphous Si suffers from inherent instability. Thus, crystalline thin film Si may offer a chance for low cost and high efficiency cells. The present contribution reviews the status of thin film Si photovoltaics, which have reached efficiencies of above 17 % with single crystalline films of 50 mu m thickness using high-efficiency techniques. We discuss the basic problems which have to be solved in the development of a polycrystalline thin film Si cell. The most challenging problem is to control the nucleation and growth of crystalline Si on foreign substrates. While there are some promising results, mainly based on recrystallization techniques for Si deposition on high temperature substrates such as graphite, deposition at low temperatures is still in a very early stage of investigation. Thin film cells need light trapping; we discuss here the principles and compare experiments with results from our simulation program SUNRAYS. Polycrystalline cells contain grain boundaries which have to be passivated in order to achieve high efficiencies. It seems that liquid phase epitaxy opens a new road to intrinsic physical grain boundary passivation. In the last part of our paper, we demonstrate that the technologies of amorphous Si may be useful for the formation of low-temperature charge separating junctions as well as for surface passivation for efficient cells based on thin film crystalline Si.