Correlation of effective recombination velocity of a backsurface field with a boron concentration profile

The efficiency of silicon solar cells can be increased by reducing the recombination of minority carriers at the backsurface of a cell. This can be achieved by application of a low-high junction, commonly called a backsurface field (BSF). The dependence of the effective backsurface recombination velocity S_eff of a BSF on the BSF doping profile and the base doping concentration was studied experimentally. The doping profiles were analyzed using sheet resistance measurements, stripping Hall measurements, and secondary ion mass spectrometry. The effective recombination velocity was obtained from photoconductivity decay probed with microwaves on symmetrical p⁺pp⁺ structures, and from photocurrent decay measurements on n⁺pp⁺ solar cells. The measured values of S_eff were compared with calculated values, based on the measured doping profiles. Both from photoconductivity and photocurrent decay measurements, S_eff was found to decrease with an increasing difference in acceptor concentration between the BSF and the base of the solar cell. The measured values of S_eff, obtained with the two experimental techniques, are in agreement with each other.