Abstract
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.
Highlights
The highest efficiency silicon solar cells require substrates with bulk minority carrier lifetimes well into the millisecond range
This was done by routinely cross-checking quarter samples taken from the same wafer with different passivation schemes
The different solvents could change the pathway in which TFSI breaks apart and a variety of species could be responsible for the surface passivation in each case
Summary
The highest efficiency silicon solar cells require substrates with bulk minority carrier lifetimes well into the millisecond range. [7] for a review) can provide excellent stable passivation, the passivation process itself can affect the bulk lifetime due to bulk passivation, external gettering [8] or thermal effects These artefacts can often be avoided if room temperature temporary surface passivation is used instead, and this has been recently reviewed by Grant and Murphy [9]. The level of passivation is similar to some of the best dielectric-schemes, but possible changes in bulk lifetime during the passivation step are avoided due to the low temperatures used This passivation scheme can be used to measure true bulk lifetimes resulting from cell processing, as recently done in the fabrication of interdigitated back contact (IBC) solar cells [11]
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