Design of CdZnTe and Crystalline Silicon Tandem Junction Solar Cells

Abstract

A tandem photovoltaic system consisting of cadmium zinc telluride/crystalline silicon (CZT/c-Si) combines the successful technologies of cadmium telluride and silicon into a single platform and offers the potential efficiencies up to 46% in theory. However, the highest efficiency fabricated CZT/c-Si tandem cell is only 16.8% today. In this paper, we develop a detailed model for single-junction CZT and tandem CZT/c-Si PV cells, which is verified with experimental data. Based on this model, we propose three hypotheses to explain the anomalously low Voc observed in tandem cells: a lowquality tunnel junction, a Schottky barrier, and through-thickness shunting path. We then suggest a simple experiment to distinguish these hypotheses. After that, we provide a physics-based analysis of the magnitude of all the loss mechanisms present in the cell and an experimental strategy to mitigate each one. Ultimately, we predict that the ideal efficiency of CZT/c-Si tandem cell could reach 34.1%, if all these loss mechanisms were mitigated, and the CZT bandgap were adjusted to 1.8 eV, without requiring any improvement in bulk or surface recombination rates.