Abstract
Lead-free double perovskites have great potential as stable and nontoxic optoelectronic materials. Recently, Cs2 AgBiBr6 has emerged as a promising material, with suboptimal photon-to-charge carrier conversion efficiency, yet well suited for high-energy photon-detection applications. Here, the optoelectronic and structural properties of pure Cs2 AgBiBr6 and alkali-metal-substituted (Cs1- x Yx )2 AgBiBr6 (Y: Rb+ , K+ , Na+ ; x= 0.02) single crystals are investigated. Strikingly, alkali-substitution entails a tunability to the material system in its response to X-rays and structural properties that is most strongly revealed in Rb-substituted compounds whose X-ray sensitivity outperforms other double-perovskite-based devices reported. While the fundamental nature and magnitude of the bandgap remains unchanged, the alkali-substituted materials exhibit a threefold boost in their fundamental carrier recombination lifetime at room temperature. Moreover, an enhanced electron-acoustic phonon scattering is found compared to Cs2 AgBiBr6 . The study thus paves the way for employing cation substitution to tune the properties of double perovskites toward a new material platform for optoelectronics.
Highlights
Lead-free double perovskites have great potential as stable and nontoxic optoelectronic materials
We demonstrate that 2% alkali substitution alters the structural phase transition temperatures between the hightemperature cubic and low-temperature tetragonal phase by a few degrees kelvin
While the low-temperature regime is governed by acoustic phonon scattering, the high-temperature regime is dominated by longitudinal optical (LO) phonon scattering as is the case for lead-halide perovskites and double perovskites.[9,46]
Summary
Masoumeh Keshavarz,* Elke Debroye,* Martin Ottesen, Cristina Martin, Heng Zhang, Eduard Fron, Robert Küchler, Julian A. While the low-temperature regime is governed by acoustic phonon scattering, the high-temperature regime is dominated by longitudinal optical (LO) phonon scattering as is the case for lead-halide perovskites and double perovskites.[9,46] We find that γLO remains constant within the error bars for all materials while γA shows an increase for the alkali-substituted materials hinting to a “softer” crystal structure as demonstrated through the octahedral distortion parameter and instability factor in the XRD analysis (see Supporting Information) To assess their structural properties upon atomic substitution, we look into the temperature evolution of the compounds by means of high-resolution PXRD and thermal expansion. Our results suggest the possibility to employ cation substitution as tuning parameter of the intrinsic material properties toward a new material platform for optoelectronic devices
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