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  Saliba, M.; Unger, E.L.: Cation-Alloying as a Pathway to Reproducible Solution-Based Preparation of Efficient Metal Halide Perovskites Solar Cells with Increased Stability. In: DPG, AK Energie [Ed.] : Tagungsband DPG Frühjahrstagung 2017, Arbeitskreis EnergieDPG, 2017, p. 87-97
  https://www.dpg-physik.de/veroeffentlichung/ake-tagungsband.html

Open Access Version

Abstract:
With a certain amount of serendipity, research on dye-sensitized solar cells led to the discovery of metal halide perovskite semiconductors as a solar energy conversion material1,2 that has inspired world-wide research activities leading to efficiency increases from about 10% in 20123,4 to now 22%.5–8 Apart from their use as a single junction solar cell technology, metal halide perovskites can be processed as an add-on layer onto other solar cells to realize efficient and low-cost tandem architectures. The perovskite band gap can be tuned continuously from 1.1 to 3.0 eV by chemical engineering which makes them particularly relevant for multi-junction devices.9–12 Experimental demonstrations of efficient tandem devices comprising metal-halide perovskites as one13–15 or both10,16 components in a tandem stack highlight the potential for scalable and low-cost multi-junction devices17 for which efficiencies approaching 30% are considered feasible.12,18–20 To reach this performance in a perovskite/silicon tandem, the ideal band gap of the absorber material in the top device should be about 1.7 eV12,18,21 (sine silicon has a band gap of 1.1 eV).11,22,23 In this review, we dedicate section I to the discussion of the chemical tunability of metal halide perovskite with a particular focus on absorber materials with absorption onset around 1.6 eV. Section II reflects on the benefit and role of including cesium (Cs) highlighting the work by Saliba et al.5,6 as this approach demonstrates a reliable pathway to obtain efficient solution-processed metal halide perovskite absorbers with high reproducibility and extended operational device stability. Section III focuses on the state-of-the art of perovskite photovoltaics with relation to the absorption onset of the absorber layer and emphasizing materials with band gaps between 1.6 eV-1.75 eV as these are of great importance to the development of efficient tandem solar cells.12,18–20 In the concluding section IV, we reflect on more general trends in metal halide alloys partially discussed also elsewhere23 highlighting cation-alloying as an approach to obtain highly efficient devices in the band gap range between 1.6 and 1.7 eV. The data shown and discussed here is dominantly based on three recent publications and the interested reader is kindly referred to Unger et al.23 and Saliba et al.5,6 for more in-depth discussions and experimental details.