1. Field of the Invention
The disclosed technology relates to a method of forming a doped region in a semiconductor layer of a substrate and the use of such method.
2. Description of the Related Technology
Methods for forming a doped region in a semiconductor layer of a substrate are already known to the person skilled in the art.
For example for n-type silicon photovoltaic cells an emitter can be realized by using p-type doping elements such as boron or aluminum. Good solar cell results have been obtained by using boron diffusion for the emitter formation process. However, belt firing of screen printed aluminum paste to form the emitter by Al alloying can be a good alternative, since it is a much simpler approach compared to boron diffusion in a tube furnace. Moreover a large knowledge base is available for this kind of processes since screen printing and firing of an aluminum paste is used industrially for the formation of a back surface field (BSF) for p-type photovoltaic cells.
Al alloying by screen printing and firing may however lead to discontinuities in the doped region. This is related to the alloying process, wherein the alloying action of the aluminum from the paste particles with the silicon wafer starts locally at points where the Al and the silicon are in intimate contact. For p-type cells, it has been reported (F. Huster, “Investigation of the alloying process of screen printed aluminum pastes for the BSF formation on silicon solar cells”, 20th European Photovoltaic Solar Energy Conference, 2005, page 1466) that in order to achieve a closed BSF without discontinuities there is a need for a closed liquid Al—Si layer on the surface at the peak firing temperature. Therefore, a minimum of 6 mg/cm2 deposited amount of aluminum paste may be required. When using an Al alloying process comprising screen printing and firing for the formation of a BSF in p-type cells, discontinuities in the doped region may lead to higher back surface recombination velocities.
However, when using an aluminum screen printing and firing process for the formation of an emitter of an n-type silicon photovoltaic cell, the process needs to be further optimized. Process parameters such as the quantity of paste and the firing conditions are more critical when this process is used to form an Al-alloyed emitter region on n-type cells. Any discontinuity in the junction can lead to shunt paths in the cells, thereby significantly reducing the conversion efficiency of the cells.
Studies related to the formation of an Al-alloyed emitter by screen printing and firing on n-type photovoltaic cells show that the problem of shunt paths (caused by discontinuities in the junction) exist even when using an amount of screen printing Al paste up to 7.5 mg/cm2. Since the cost of the screen printing paste is an important part of the total cost of a cell process, a higher quantity of paste required will result in an increased cell cost. Furthermore, higher quantities of screen printing paste lead to more bowing of the cells. This will be even more critical when making cells on very thin substrates.
Recently the junction discontinuities in n-type silicon photovoltaic cells with an Al alloyed emitter formed by firing a screen printed Al paste have been reduced by optimizing the firing conditions (Ly Mai et al, “Improved process for the formation of Al-alloyed emitter in n-type solar cells”, 18th Int. Photovoltaic Science Engineering Conf., India, January 2009). The optimized process involves a low temperature solid phase epitaxial growth process after the conventional standard spike firing to minimize the impact of junction discontinuities.