Making Efficient and Stable Mixed Tin-Lead Perovskite Solar Cells

A bilayer interdiffusion growth (BIG) method was used to prepare low-bandgap tin-lead perovskites with reduced methylammonium content. Solar cells fabricated by this method show simultaneous improvement in efficiency and stability.

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All-perovskite thin-film tandem solar cells comprising wide-bandgap mixed halide perovskite top cells and low-bandgap mixed tin (Sn)-lead (Pb) perovskite bottom cells are promising to deliver high power conversion efficiencies at low manufacturing costs. However, as a key component of all-perovskite tandem solar cells, low-bandgap mixed Sn-Pb perovskites have shown a shortcoming in simultaneously achieving high efficiency and stability, limiting the development of this emerging photovoltaic technology. 

To overcome this challenge, we use a two-step bilayer interdiffusion growth (BIG) method to simultaneously improve the efficiency and stability in formamidinium (FA)-based low-bandgap mixed Sn-Pb perovskite solar cells. Using the BIG process, we produced high-quality and large-grained perovskite films with only 10 mol% volatile methylammonium (MA) content, enabling single-junction mixed tin-lead all-perovskite tandem solar cells with improved efficiency and stability.

The conventional two-step sequential deposition method developed for high-efficiency lead halide perovskite solar cells has not demonstrated success in preparing mixed Sn-Pb perovskites with high (>50%) Sn contents, mainly due to the rapid crystallization of Sn-based perovskites. The fast formation of small-sized perovskite grains block the ion exchange of the organic salts and underneath metal halide layers. In this work, we designed the two-step BIG process with a prolonged ion diffusion and exchange stage to enable the full conversion of metal halide into the perovskite phase, which allows for an increased grain size of mixed Sn–Pb perovskites with a low MA concentration (Fig. 1). The MA-less mixed Sn-Pb perovskites show significantly improved intrinsic stability than state-of-the-art FA/MA mixed Sn-Pb perovskites produced by the single-step antisolvent method. Furthermore, we applied one-dimensional pyrrolidinium perovskite to passivate surface defects in the perovskite films and improve the devices' junction quality, resulting in an increased minority carrier lifetime and a high open-circuit voltage.  

Figure 1. Top-view and cross-sectional SEM images of mixed Sn-Pb films prepared by the BIG process.

The Sn-Pb perovskite solar cells produced by the BIG method delivered a power-conversion efficiency of 20.4% under AM 1.5G illumination. More importantly, the reduced MA content and improved perovskite film quality enabled excellent material stability against light and heat. An encapsulated mixed Sn-Pb solar cell retained 92% of its initial efficiency after 450 h of continuous 1 sun illumination. Our work shows that the BIG process is an effective way to simultaneously maximize performance and stability for mixed Sn–Pb perovskite solar cells.

If you are interested in our work, full details are in a paper published in Nature Energy:  Li, C., Song, Z., Chen, C. et al. Low-bandgap mixed tin-lead iodide perovskites with reduced methylammonium for simultaneous enhancement of solar cell efficiency and stability. Nat Energy 5, 768–776 (2020). https://doi.org/10.1038/s41560-020-00692-7

Also, you can check our previous progress on developing high-efficiency mixed Sn-Pb and all-perovskite solar cells. 

  • Tong, J., Song, Z., Kim, D.H., et al. Carrier lifetimes of >1 μs in Sn-Pb perovskites enable efficient all-perovskite tandem solar cells. Science 364, 475-479 (2019). DOI: 10.1126/science.aav7911  
  • Zhao, D., Chen, C., Wang, C. et al. Efficient two-terminal all-perovskite tandem solar cells enabled by high-quality low-bandgap absorber layers. Nat Energy 3, 1093–1100 (2018). https://doi.org/10.1038/s41560-018-0278-x
  • Zhao, D., Yu, Y., Wang, C. et al. Low-bandgap mixed tin–lead iodide perovskite absorbers with long carrier lifetimes for all-perovskite tandem solar cells. Nat Energy 2, 17018 (2017). https://doi.org/10.1038/nenergy.2017.18 

Zhaoning Song

Research Assistant Professor, University of Toledo

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