Band Structure Engineering of Cs2AgBiBr6 Perovskite through Order−Disordered Transition
Update: 2018-02-24 10:29:31      Author: yangjuan@csrc.ac.cn

Given the low cost, suitable bandgap, high optical absorption and long carrier lifetime, the family of organic-inorganic perovskite halide AIBIIX3, especially CH3NH3PbI3, has become the most investigated light-harvest material for solar cells in the last few years. Although solar cells based on CH3NH3PbI3 thin films have approached a high power conversion efficiency of more than 22%, two serious problems including toxicity of the water soluble Pb2+ ions and thermodynamic instability of CH3NH3PbI3 in air against decomposition have presented major barriers to its commercial application. Cs2AgBiBr6 was proposed as one of the inorganic, stable, and non-toxic replacement of CH3NH3PbI3. However, the wide indirect band gap of Cs2AgBiBr6 suggests that its application in photovoltaics is limited.

Recently, Su-Huai Wei’s group in CSRC [1-2] show that by introducing disorder to the cation occupancy of Ag and Bi in Cs2AgBiBr6 they could engineering the band structure of Cs2AgBiBr6. Using the Monte Carlo and the first principle calculation, they predicted that disordered Cs2AgBiBr6 with the band structures changing from indirect band gap of 1.46 eV to pseudo-direct band gap of 0.44 eV could be synthesized by quenching from temperature beyond the phase transition temperature. Introducing n-type dopants such as Ba2+ and La3+ into the alloy can significantly reduce the energy difference and thus the transition temperature. Depending on the extent of disorder, the light absorption in the visible and the near infrared region for the disordered Cs2AgBiBr6 alloys could be considerably enhanced, which has broaden the application of the compound. This work shows that introducing cation disorder is a useful way for band structure engineering of the perovskite materials for various applications.

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FIG. 1: The left-hand side show schematically the changes from the ordered double perovskite cell to the disordered perovskite cell and the corresponding changes of the band gap from ordered, partially disordered (at phase transition) to random atomic configurations. The right-hand side show the calculated optical absorption coefficients (α) of the fully ordered (black line), partial disordered (blue line), and fully disordered (red line) Cs2AgBiBr6.


References:

[1]     J. Yang, P. Zhang, and S.-H. Wei*, J. Phys. Chem. Lett. 9, 31 (2018).

[2]     P. Zhang, J. Yang, and S.-H. Wei*, J. Mater. Chem. A (in press).


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