Abstract
Perovskite/silicon 2-terminal tandem cells have made significant advances towards >25% efficiency. Despite this, there is limited understanding of how the optical properties of the materials affect the optical losses within the tandem cell. Using an accurate optical model, we investigate, identify and propose solutions to the optical loss mechanisms inherent in a typical perovskite/silicon 2-terminal tandem cell. The results highlight, firstly, the requirement for low absorption in all layers above the perovskite film, and secondly, the importance of the proper choice of refractive index and thickness of charge transport layers of the perovskite cell, in order to minimize reflection at the interfaces formed by these layers. We demonstrate that the proper choice of these parameters is based on, and can be guided by, basic optics principles which serve as design guidelines. With careful selection of charge transport materials, optimization of the perovskite absorber thickness and the introduction of light trapping within the silicon cell, a matched current of over 20 mA/cm2 can be realized, enabling efficiencies greater than 30% using currently available cell processing methods and materials.
Highlights
Organo-metallic halide perovskite materials have made extremely rapid advances in solar cell applications; progressing from 3.5% efficiency in 2009 [1] to 22.1% in 2016 [2]. This rapid improvement can be attributed to their ability to be solution-processed at low temperatures, high material quality, and simple architectures that allow for flexibility with regards to the design of perovskite solar cells in both stand-alone and tandem applications
Development of tandem perovskite solar cells has progressed steadily for both monolithic 2-terminal [3,4,5,6] and stacked 4-terminal [6–8] architectures, with the most promising results occurring when utilized with a silicon bottom cell due to its low bandgap (1.1 eV) allowing for higher potential current
We demonstrate here that polycrystalline silicon (poly-Si) only provides a minor optical benefit over ITO as a tunnel junction material, even in the presence of light trapping within the crystalline silicon (c-Si) sub-cell
Summary
Organo-metallic halide perovskite materials have made extremely rapid advances in solar cell applications; progressing from 3.5% efficiency in 2009 [1] to 22.1% in 2016 [2]. To understand how different charge transport materials affect the optical properties of a perovskite/silicon 2-terminal tandem solar cell, we first analyze a p-i-n orientation cell with layer composition MgF2/ unannealed IZO/ spiro-OMeTAD/ MAPbI3/ compact (cp)-TiO2/ annealed ITO/ c-Si/ Ag with layer thicknesses 105 nm/ 44 nm/ 160 nm/ 300 nm/ 44 nm/ 30 nm/ 200 μm/ 200 nm, respectively. Minimizing reflection loss via appropriate charge transport material selection The largest light loss mechanism for the 2-terminal perovskite/silicon tandem cell is reflection The majority of this reflection can be attributed to two sets of interfaces, the IZO/spiro-OMeTAD/Perovskite layer interfaces and the cp-TiO2/ITO/c-Si layer interfaces, as the large index contrasts between the adjacent layers results in high Fresnel reflection. From these light trapping results, it is clear that rear-side light trapping can provide increased silicon absorption, which in turn allows for thicker perovskite cells for current matching, and significant tandem cell efficiency gains
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