Setting the Baseline for the Modelling of Kesterite Solar Cells—The Case Study of Tandem Application

The research on Kesterite-based solar cells is reaching a turning point. After a strong effort and record efficiencies in the early 2010s, this promising thin film photovoltaic absorber, free of toxic and critical raw materials, has seen a decade of relative stalling in terms of performance despite the continuous progress of competing technologies. Recent advances in the understanding of the bottlenecks hampering Kesterite solar cells have permitted <b>new efficiency records</b> within the past two years, finally breaking the 13% threshold. <b>Device modelling is bound to be instrumental</b> going forward and in spite of an extensive usage throughout the past decade, no realistic and quantitative representation of a Kesterite solar cell has been reported. This often leads to models relying on a partial knowledge of material and device parameters extracted from different sources, and this lack of consistency severely limits the accuracy of those models both quantitatively and qualitatively while rendering results’ comparison between different laboratories almost impossible. In the meantime, low-cost tandem devices combining either inorganic thin film, perovskite and crystalline silicon cells have emerged as the next frontier for future efficient large scale photovoltaic deployment, and accurate quantitative device modelling combining electrical and optical simulation is more than ever needed to assess existing devices and predict possible improvements in future architectures.<br/>This work reports on the modelling of Kesterite solar cells by combining SCAPS for the electrical part and the transfer matrix method for the optical part, using an in-lab developed modelling program. The objective is twofold. Firstly, <b>accurate and quantitative baselines based on the systematic characterization of Kesterite solar cells</b> fabricated at our group using a consistent experimental process are proposed. Our focus is on narrow bandgap Kesterite Cu₂ZnSnSe₄ (CZTSe) and wide bandgap Kesterite Cu₂ZnSnS₄ (CZTS). Specifically, we demonstrate in both cases the <b>simultaneous reproduction of all four photovoltaic figures of merit (voltage, current density, fill factor and efficiency) with less than 1% of relative discrepancy between the model and the experimental devices.</b><br/>Secondly, and using the aforementioned accurate modelling baselines and transfer matrix optical modelling, a case study on the potential of Kesterite solar cells in tandem devices is performed, namely for a narrow bandgap CZTSe subcell in tandem with a Perovskite top cell, and for a wide bandgap CZTS top cell in tandem with a crystalline silicon bottom cell. In each case, several material and optical improvements to the Kesterite cell are proposed and evaluated by the model with a high degree of quantitative accuracy, including bulk defect density, interface defects, bandgap engineering, doping gradient and selective contacts. It is found <b>that a moderate reduction in the density of Sn_Zn antisite defect permits to significantly enhance the efficiency of CZTSe solar cells up to 16%, which in combination with a 21% efficient Perovskite top cell allows to reach a total tandem efficiency of 30%</b>. In contrast, even ambitious optimizations to the wide bandgap CZTS solar cell do not permit to overcome the 26% efficiency threshold in tandem with a silicon subcell. The results from our simulation are compared with experimental values reported in the literature using Kesterite in tandem devices, allowing to assess the excellent robustness of the baseline parameters determined in this work. The complete set of material, optical and device input parameters are <b>openly shared</b>, which not only allows various research groups working on Kesterite solar cells to assess their validity, but also permits for the community to use common baselines for the numerical modelling of devices for <b>more consistent comparisons between research groups</b>.