Superacid-derived surface passivation for measurement of ultra-long lifetimes in silicon photovoltaic materials

Accurate measurements of bulk minority carrier lifetime are essential in order to determine the true limit of silicon's performance and to improve solar cell production processes. The thin film which forms when silicon wafers are dipped in solutions containing superacids such as bis(trifluoromethane)sulfonimide (TFSI) has recently been found to be effective at electronically passivating the silicon surface. In this paper we first study the role of the solvent in which TFSI is dissolved for the passivation process. We study ten solvents with a wide range of relative polarities, finding TFSI dissolved in hexane provides improved temporal stability, marginally better passivation and improved solution longevity compared to dichloroethane which has been used previously. Sample storage conditions, particularly humidity, can strongly influence the passivation stability. The optimised TFSI-hexane passivation scheme is then applied to a set of 3 Ω cm n-type wafers cut from the same float-zone ingot to have different thicknesses. This enables the reproducibility of the scheme to be systematically evaluated. At 10¹⁵ cm⁻³ injection the best case effective surface recombination velocity is 0.69 ± 0.04 cm/s, with bulk lifetimes measured up to the intrinsic lifetime limit at high injection and >43 ms at lower injection. Immersion of silicon in superacid-based ionic solutions therefore provides excellent surface passivation, and, as it is applied at room temperature, the effects on true bulk lifetime are minimal.