Bismuth-Based Perovskite-Inspired Materials for Energy Harvesting

Lead-halide perovskites (LHPs) have emerged as one of the leading thin film absorbers over the past decade, demonstrating the ability to achieve high efficiencies in photovoltaic devices when processed using low-temperature, simple fabrication methods. However, the toxicity of the water-soluble lead content may present limitations to their widescale adoption. This talk examines bismuth-based “perovskite-inspired” materials, focussing on Cs₂AgBiBr₆ double perovskite (replicating the crystal structure of LHPs) and BiOI (replicating important features of the electronic structure of LHPs). Bismuth-based compounds have the advantage of demonstrating no evidence for toxicity [1].<br/>The first part of this talk discusses Cs₂AgBiBr₆. We show from transient absorption spectroscopy measurements that the charge-carrier lifetime exceeds a microsecond. However, the bandgap of 2.25 eV is too wide for photovoltaic or photoelectrochemical applications. We show how the bandgap can be reduced through alloying with Sb, and that the mixed alloys have lower bandgaps than the pure Bi- or Sb-based double perovskites owing to nonlinear mixing between the Bi³⁺ and Sb³⁺ orbitals [2]. Furthermore, we fabricate field-effect transistors to examine the transport properties of Cs₂AgBiBr₆ in detail, and identify the mobility to be limited by a thermally-activated hopping process. This is consistent with the formation of small polarons recently identified through spectroscopic measurements, and we discuss the implications of carrier-phonon coupling on the ultimate potential of this material for photovoltaic applications [3].<br/>Finally, we discuss our work on BiOI thin film photovoltaics. We develop an all-inorganic device structure that leads to photovoltaic devices achieving up to 80% external quantum efficiency at 450 nm wavelength. These devices are demonstrated to be effective for indoor light harvesting, with the capability of powering carbon nanotube inverters. Furthermore, the BiOI indoor photovoltaics are shown to be stable for several months in an environment with &gt;1000 ppm O₂ and &gt;500 ppm H₂O. This talk finishes with a discussion of the potential of the wider family of perovskite-inspired materials for harvesting the energy from indoor lighting to sustainably power the Internet of Things [4].<br/>[1] Nat. Chem., 2010, 2, 336<br/>[2] J. Mater. Chem. A, 2020, 8, 21780<br/>[3] Adv. Funct. Mater., 2021, 2104981. DOI: 10.1002/adfm.202104<br/>[4] Adv. Energy Mater., 2021, 11, 2002761