ABSTRACT
Bismuth-based halide perovskites are nontoxic alternatives to widely studied lead-based perovskites for optoelectronic applications. Here, we synthesized Cs2NaBiCl6 thin films and attempted to synthesize Cs2NaBiBr6 using physical vapor deposition. While Cs2NaBiCl6 forms a stable cubic structure with a 3.4 eV band gap and could be synthesized successfully, Cs2NaBiBr6 does not form and is unstable with respect to dissociation into Cs3-xNaxBi2Br9 and Cs3-xNaxBiBr6. Furthermore, the close X-ray diffraction patterns of Cs3-xNaxBi2Br9 and Cs2NaBiBr6 raise doubts about the previous reports of the latter's formation based on X-ray diffraction alone.
ABSTRACT
Cs2AgInBr6 is among the lead-free halide perovskites of interest, predicted by first-principles calculations to be stable with a direct band gap, but there has been only one report of its synthesis. Herein we report the formation of Cs2AgInBr6 thin films through thermal evaporation of CsBr, AgBr, and InBr3 and subsequent annealing between 130 °C and 250 °C. Cs2AgInBr6 appears stable in this temperature range. However, Cs2AgInBr6 thin films are thermodynamically unstable at room temperature, remaining cubic only long enough to be characterized but not long enough to be useful for practical devices. Cs2AgInBr6 decomposed into Cs2AgBr3, Cs3In2Br9, AgBr, and InBr3 upon cooling from 130 °C to 250 °C to room temperature. This conclusion did not depend on illumination, film thickness, annealing environment, or details of the film formation, pointing to an intrinsic thermodynamic instability of the material. Optical absorption measurements may be interpreted as Cs2AgInBr6 having a direct band gap of 1.57 ± 0.1 eV.
ABSTRACT
When excited by photons with energies greater than 2.2 eV, the bandgap energy, Yb-doped Cs2AgBiBr6 thin films synthesized via physical vapor deposition emit strong near-infrared luminescence centered at â¼1.24 eV via the Yb3+ 2F5/2 â 2F7/2 electronic transition. Robust, reproducible, and stable photoluminescence quantum yields (PLQY) as high as 82.5% are achieved with Cs2AgBiBr6 films doped with 8% Yb. This high PLQY indicates facile and efficient energy transfer from the perovskite host, Cs2AgBiBr6, to Yb, making Cs2AgBiBr6 the most promising lead-free down-conversion material.
ABSTRACT
Bismuth-based halide perovskites have been proposed as a potential nontoxic alternative to lead halide perovskites; however, they have not realized suitable performance. Their poor performance has been attributed to substandard film morphologies and too wide of a band gap for many applications. Herein we used a two-step deposition procedure to convert BiI3 thin films into A3Bi2I9 (A = FA+, MA+, Cs+, or Rb+), which resulted in a substantial improvement in film morphology, a larger band gap, and greater compositional tunability compared toresults when using aconventional single-step deposition technique. Additionally, we attempted to reduce the undesirably wide band gap in Rb3Bi2I9 thin films by inducing chemical pressures through cation-size mismatch, with an underlying hypothesis that cation-size mismatch could induce compressive strain within the 2D Rb3Bi2I9 lattice. However, we found that all A xRb3- xBi2I9 compositions with x > 0 adopted the 0D structure, and no changes to the band gap were observed with alloy. These results imply that the band gap of A xRb3- xBi2I9 is insensitive to A-site alloying.
