"Tuning the optical absorption and emission of vacancy-ordered double perovskites"

A. Brumberg^a,b, O. Kuklinski^b, E. E. Morgan^a,b, G. T. Kent^b, T. Amanda Strom^a, A. Mikhailovsky^c, M. L. Chabinyc^b, and R. Seshadri^a,b,c

^aMaterials Research Laboratory; ^bMaterials Department; ^cChemistry and Biochemistry Department / University of California, Santa Barbara

Vacancy-ordered double perovskites (VODPs) of the formula A₂MX₆, where M is a tetravalent metal ion, offer improved ambient stability and compared to traditional divalent lead halide perovskites. These so-called “vacancy” ordered double perovskites are comprised of fully isolated metal halide octahedra rather than the typical corner-connected octahedra, greatly altering their charge carrier and exciton behavior. However, if VODPs are to be incorporated into device architectures, we must understand their structure-property relationships and photophysics as they relate to this altered atomic structure.

While VODPs can easily be formed through a variety of synthetic routes, very little research has explored whether the synthetic route affects the resulting perovskite structure in any way. This is despite extensive literature in other perovskite and inorganic classes demonstrating that synthesis and processing conditions, such as precursor selection, solvent system, and annealing temperature greatly affect both material structure and optoelectronic properties. Here, we investigate the role of metal oxide vs. halide precursor and organic solvent on the crystal lattice, optical absorption, and emission of tellurium-based VODPs. We find that while the organic solvent critically affects the reactivity of tellurium bromide, the solvent is less influential in reactions performed using tellurium oxide. We observe that the choice of metal precursor and solvent affect the powder color and absorption band edge, and we attribute this variation to a particle size effect. Notably, the emission energy remains unchanged, consistent with the idea that emission in these zero-dimensional structures arises from a second-order Jahn-Teller distortion in the [TeBr₆]^2- octahedra. However, emission lifetimes range from 7 to 86 ns, and upon particle grinding to reduce the particle size increase to 870 ns. Overall, our work highlights how even minor changes in synthetic procedures can lead to variability in metrics such as the optical band gap and emission lifetime and sheds light into how to manipulate the optical properties of semiconductor systems of light-emitting applications.