Highly stable and Pb-free bismuth-based perovskites for photodetector applications

Abstract

Herein, we report the synthesis of highly stable, Pb-free bismuth iodide (BiI₃ or BI), stoichiometric methylammonium bismuth iodide [(CH₃NH₃)₃Bi₂I₉ or MA₃Bi₂I₉ or s-MBI] and non-stoichiometric methylammonium bismuth iodide [(CH₃NH₃)₂BiI₅ or MA₂BiI₅ or Ns-MBI] perovskite thin films for photodetector applications. These films are synthesized at room temperature by a single step solution process spin coating method. The structural, optical, and morphological properties of these films were investigated using different characterization techniques such as XRD, Raman spectroscopy, FE-SEM, UV-Visible spectroscopy, etc. Formation of BI, s-MBI and Ns-MBI thin films is confirmed by XRD and Raman spectroscopy measurements. XRD analysis reveals that BI has a hexagonal crystal structure and a P6₃/mmc hexagonal space group for s-MBI and Ns-MBI. The optical properties of BI thin films show a high absorption coefficient (∼10⁴ cm⁻¹) and a band gap of ∼1.74 eV. Similarly, s-MBI films have a high absorption coefficient (∼10³ cm⁻¹) and an indirect band gap of ∼1.8 eV. Moving from s-MBI to Ns-MBI, the value of absorption coefficient is ∼10³ cm⁻¹ and the band gap corresponds to ∼2 eV. Finally, photodetectors based on the synthesized BI, s-MBI and Ns-MBI perovskites have been directly fabricated on FTO substrates. All photodetectors exhibited highly stable photo-switching behaviour along with excellent photoresponsivity and detectivity, with a fast response and recovery time. Our work demonstrates that Pb-free BI, s-MBI and Ns-MBI perovskites have great potential in the future for realizing stable photodetectors.