Superacid Passivation of Crystalline Silicon Surfaces

Abstract

The reduction of parasitic recombination processes commonly occurring within the silicon crystal and at its surfaces is of primary importance in crystalline silicon devices, particularly in photovoltaics. Here we explore a simple, room temperature treatment, involving a nonaqueous solution of the superacid bis(trifluoromethane)sulfonimide, to temporarily deactivate recombination centers at the surface. We show that this treatment leads to a significant enhancement in optoelectronic properties of the silicon wafer, attaining a level of surface passivation in line with state-of-the-art dielectric passivation films. Finally, we demonstrate its advantage as a bulk lifetime and process cleanliness monitor, establishing its compatibility with large area photoluminescence imaging in the process.

Degradation of Surface Passivation on Crystalline Silicon and Its Impact on Light-Induced Degradation Experiments

“…As can be seen, J 0s reflects changes at the surface pretty well and changes only little during changes of τ b in the first hours of treatment. The changes in the bulk and at the surface are also revealed in an injection-dependent visualization of lifetime data as introduced in [25] and shown in Fig. 3.…”

“…• C. After another dip in 2% HF, samples were immersed in a nonaqueous solution of bis(trifluoromethane)sulfonimide dissolved in dichloroethane (2 mg/ml) for ∼ 60 s. This procedure leads to very good passivation of sample surfaces as described in [25] and [26] while only subjecting a sample to moderate temperatures, therefore leaving its defect properties rather unchanged. Calculating values of J 0s after repassivation as described in the previous section results in values ∼ 2 fA/cm 2 on a P-doped sample and values ranging from 5 to 12 fA/cm 2 on B-doped samples in this study.…”

Superacid-Treated Silicon Surfaces: Extending the Limit of Carrier Lifetime for Photovoltaic Applications

“…To prepare the solution, 100 mg of bis(trifluoromethane)sulfonimide (Sigma-Aldrich, 95%) was measured out and then dissolved in 50 ml of anhydrous 1, 2-dichloroethane (Sigma-Aldrich, 99.8%), in accordance with the optimum recipe described in [17]. Once prepared, the solution was stored in a glass container with an air-tight cap.…”

“…By this procedure, an upper limit S of 3 and 13 cm/s on n-and p-type silicon are achieved, respectively [17]. From all these recent studies, it is evident that organic films can provide exceptional silicon surface passivation at very low temperatures (< 200°C), which could open up other opportunities for new and innovative low-temperature gettering and hydrogenation techniques to improve low-quality silicon for their inclusion as the base material in highly efficient solar cells [18]- [21].…”

“…The procedure incorporates the previously described SA method of Bullock et al [17] and enables us to reduce S below 1 cm/s. We demonstrate the surface passivation achieved by immersion in a TFSI-containing solution is sensitive to the pre-treatment cleaning/etching procedures and also to humidity, yet can be sufficiently stable for short term (∼ 3 h) photoconductance (PC) and photoluminescence (PL) imaging measurements.…”