Experimental determination of base resistance contribution for point-like contacted c-Si solar cells using impedance spectroscopy analysis

• Impedance spectroscopy (IS) analysis is applied to c-Si high-efficiency solar cells.
• Rear dielectric passivation and point contacts lead to a base resistance term Rbase.
• A device model is developed to explain IS data allowing the determination of Rbase.
• Dependence of Rbase on substrate resistivity and rear pitch is reproduced.
• The proposed method is successfully applied to experimental devices.

One of the most common strategies in high-efficiency crystalline silicon (c-Si) solar cells for the rear surface is the combination of a dielectric passivation with a point-like contact to the base. In such devices, the trade-off between surface passivation and ohmic losses determines the optimum distance between contacts or pitch. Given a certain pitch, the series resistance related to majority carrier flow through the base and the rear point-like contact (Rbase) is commonly calculated a-priori and not crosschecked in finished devices, since typical techniques to measure series resistance lead to an unique value that includes all ohmic losses. In this work, we present a novel method to measure Rbase using impedance spectroscopy (IS) analysis. The IS data at high frequencies allow to determine Rbase due to the presence of the capacitor formed by the metal/dielectric/semiconductor structure that covers most of the rear surface. The method is validated by device simulations where the dependence of Rbase on carrier injection, base resistivity and pitch are reproduced. Finally, Rbase is measured on finished devices. As a result, a more accurate value of the contacted area is deduced which is a valuable information for further device optimization.

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