Optical approaches to improving perovskite/Si tandem cells
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Optical approaches to improving perovskite/Si tandem cells 1
Haejun Chung1, Xingshu Sun1, and Peter Bermel1 Birck Nanotechnology Center, Electrical and Computer Engineering, 1205 W. State St, West Lafayette, IN 47906, U.S.A.
ABSTRACT Recently, metal-halide perovskites have demonstrated an extraordinarily rapid advance in single junction cell efficiency to over 20%, while still offering potentially low costs. Since the bandgap is larger than the ideal single-junction value, perovskite-based tandem cells can theoretically offer even higher efficiencies. Instead, however, the record tandem cell performance in experiments to date has come in slightly below that of record single junctions, although slightly higher than the same single junctions. In this work, we consider both how this disconnect can be explained quantitatively, and then devise experimentally feasible, variance-aware approaches to address them. The first stage of our approach is based on reconfiguring dielectric front coatings to help reduce net reflected power and balance junction currents by reshaping the reflection peaks. This method could be applied to post-fabrication stage of perovskite/c-Si tandem cells, and also applicable to cell and module level structures. In the second stage of our approach, we can almost entirely eliminate Fresnel reflection by applying a conformal periodic light trapping structure. In the best case, a short circuit current (Jsc) of 18.0 mA/cm2 was achieved, after accounting for 4.8 mA/cm2 of parasitic loss and 1.6 mA/cm2 reflection loss. Further improvements may require a change in the baseline materials used in perovskite cells. INTRODUCTION Metal-halide perovskites have gained a great deal of attention for their extraordinarily rapid increase in single junction efficiencies, rising from below 1% to now exceeding 20 % [1,2]. Furthermore, perovskites are solution-processable materials that appear resilient in the presence of defects [1,2]; thus, they have a potential to serve as ultra-low cost solar cells. Although longterm stability and reliability is a major potential challenge, recent work has suggested these problems could be addressed successfully [3]. Furthermore, metal-halide perovskites have a
Fig. 1: Schematic diagram representing average reflection and parasitic absorption losses in perovskite/silicon tandem cells. The average reflection was calculated by Fresnel’s law (without accounting for multiple reflections here).
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large and tunable bandgap, which raises the prospects of even higher efficiencies as a top cell in a tandem solar cell. Recently, perovskite-crystalline silicon tandem cells were proposed, with theoretical efficiencies expected to exceed 30 % [4–6]. However, measured efficiencies more typically fall between 13% and 18 %, which is below that [5,7–9]. Reaching
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