Novel Recombination Lifetime Mapping Technique through Kelvin Probe Studies

Nicholas Alderman1,2, Lefteris Danos1, Martin C Grossel2, Tom Markvart1 1Solar Energy Laboratory, University of Southampton, Southampton, United Kingdom /2School of Chemistry, University of Southampton, Southampton, United Kingdom

The Kelvin probe has been shown to be a very powerful and versatile tool, allowing the extraction of data such as diffusion length, surface photovoltage and impurity concentrations. In this paper, we investigate the extraction of the surface recombination velocity (and by assuming a bulk lifetime, the recombination lifetime) from the I-V type dependence of the sample from a model of the currents involved at the silicon surface. By using an X-Y stage, the recombination lifetime can be imaged for entire wafers, instead of obtaining an average value of lifetime similar to that obtained from the Sinton WCT-120 lifetime tool. This is useful in determining where further improvements in the surface passivation can be obtained, by observing problem areas in the passivation layer. As  few lifetime imaging techniques are available, such as microwave reflectance, luminescence and infrared lifetime mapping (and even fewer with multi-purpose equipment), this technique is valuable in the characterisation of high-performance solar cell passivation layers. The Kelvin probe measurements were illustrated on the example of silicon (111) samples with varying surface recombination lifetimes passivated by the attachment of organic monolayers. By comparison with traditional recombination lifetime determination methods (Sinton WCT-120 wafer lifetime tool) our technique was shown to produce accurate, reliable measurements of the recombination lifetime.