BSF (Back Surface Field) has been used as a way to increase solar cell performance by reduction of the surface recombination velocity (SRV). There are many methods for BSF production, one method for production is to introduce highly doped layer on the rear surface of the wafer. Depending on the dopant type in the wafer, the BSF layer could be either n+ or p+.
Explanation of Aluminum Back Surface Field Solar Cells
Printing a full aluminum layer on the rear of the cell, with ensuing alloying through firing, produces a BSF and improves the cell bulk. But aluminum is expensive and a second print of Al/Ag is needed for solder able contact. Generally in production, the rear contact is just made by single step printing of an Al/Ag grid printed.
For reducing the effect of the rear surface recombination velocity on the voltage and current, a similar effect is used at the rear surface when it is closer than the diffusion length to the junction. A back surface field (BSF) contains a region which is higher doped at the solar cell’s rear surface. The interface between the high and low doped regions act similar to ap-n junction and electric field is formed at the interface that presents a barrier to flow of the minority carrier to the rear surface. Thus, the minority carrier concentration is maintained in the bulk of the device at the higher levels and the BSF has a passivating net effect on the rear surface.
Aluminum back surface field solar cells efficiency
By examination of standard solar cell device presenting a full area aluminum back surface field (BSF), front contact formed by light induced nickel and silver plating, research has shown that the conversion efficiency can reach up to 20.1%.
Screen printed Aluminum and rapid thermal alloying are used together in order to get an Al back surface field (Al-BSF) that can lower the effective back surface recombination velocity. This process has been combined into a high efficiency, laboratory fabrication and a high throughput, industrial process to achieve such solar cell efficiencies in excess of 19.0% and 17.0%.
The critical process requirements for optimal formation of Al-BSF are:
Using of a fast ramp rate to reach the alloying temperature
Thick film Al deposition preceding the alloying.
The common approach for providing the p-contact for industrial p-type silicon solar cells is using Aluminum alloyed screen printed and fired rear contact. This is due to the cost effective process sequence and also its reliability and simplicity.