On the origins of the green luminescence in the “zero-dimensional perovskite” Cs4PbBr6: conclusive results from cathodoluminescence imaging

Abstract

There is great interest in the use of highly-efficient all-inorganic halide perovskites Cs_nPbBr_2+n for optoelectronic applications. There however remains considerable debate as to the origins of the green luminescence in the zero-dimensional phase of the perovskite Cs₄PbBr₆, with theories suggesting it originates either from defects in the Cs₄PbBr₆ lattice or CsPbBr₃ impurities/inclusions. The confusion has arisen due to the two phases being miscible and typically co-existing. Moreover, low impurity levels of CsPbBr₃ in Cs₄PbBr₆ are difficult to detect by XRD measurements, yet have much stronger photoluminescence than bulk CsPbBr₃ that exhibits quenching, further contributing to the confusion as to the origins of the green photoluminescence. With the rise of significant debate and misconceptions, we provide conclusive evidence that the green emission from Cs₄PbBr₆ is indeed due to nanocrystalline CsPbBr₃ impurities. This is demonstrated by undertaking cathodoluminescence and EDX measurements on samples prepared mechanochemically by ball-milling. Cathodoluminescence imaging clearly shows the presence of small crystals embedded in/or between larger crystallites of Cs₄PbBr₆ and they emit around 520 nm. EDX shows that the smaller crystal inclusions have a Pb : Br ratio that is approximately 2 times higher, confirming the CsPbBr₃ phase, which has the expected size-dependent shift to shorter wavelengths (about 528 to 515 nm). These studies make significant inroads into understanding these lead halide perovskites for their use in a variety of optoelectronic and photovoltaic applications.