Bismuth: An Infrared-Emitting, Deep-Level Trap in CsPbBr₃

Halide perovskites have already demonstrated impressive optoelectronic performance in applications ranging from photovoltaics, to light-emitting diodes and lasers, to X-ray and gamma-ray detectors and scintillators. Understanding how extrinsic dopants behave in this class of materials is a critical next step to gaining greater control over the stability and performance of this versatile class of materials.<br/><br/>While Bi³⁺ was originally proposed as a potential n-type dopant in lead halide perovskites, experiments and theory clearly demonstrate that it is a deep-level trap that gives rise to broad near-infrared emission. We synthesized a series of crystals of CsPbBr₃ doped with Bi content ranging from 0.01 to 0.32 atomic percent, as quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES). The absorption edge shifts redder with Bi incorporation, but density function theory (DFT) calculations show that this increased absorption comes from the electronic structure of Bi as a deep-level trap, not from a shift in the CsPbBr₃ band edge. Indeed, all doped crystals emit both narrow band-edge (2.37 eV) and broad defect-level (1.12 eV) photoluminescence at room temperature, consistent with our theoretical prediction. We also apply STEM-EDS to the most heavily doped samples to characterize the spatial uniformity of the Bi incorporation within the CsPbBr₃ crystals. Our combined results demonstrate the power of current theoretical approaches to predict defect behavior in halide perovskites, which will be invaluable in guiding the development of these materials.