One-fifth of global electricity consumption is based on lighting; efficient and stable white-light emission with single materials is ideal for applications. Photon emission that covers the entire visible spectrum is, however, difficult to attain with a single material. Metal halide perovskites, for instance, have outstanding emission properties but contain lead, and therefore yield unsatisfactory stability. A new report published by Jiajun Luo and co-workers details a lead-free double perovskite that exhibited stable and efficient white light emission. In its mechanism of action, the material produced self-trapped excitons (STEs) due to Jahn-Teller distortion of the AgCl6 octahedron in the excited state of the complex, observed when investigating exciton-phonon coupling in the crystal lattice. The results are now published in Nature.
White light emission from a single emitter layer is of interest in lighting applications due to its simplicity compared to multiple emitters. Typically, broadband and white light emissions originate from self-trapped excitons (STEs) existing in semiconductors with localized carriers and a soft lattice. The authors focused on the double perovskite Cs2AgInCl6 as a promising material that emits warm white light due to its broad spectrum and all-inorganic, lead-free nature. The study optimized the alloy to form Cs2(Ag0.6Na0.4)InCl6 with a small percentage of bismuth doping to emit warm white light with increased quantum efficiency for more than 1000 hours. Materials for lighting applications can be defined as those emitting a “warm” white light for indoor applications and “cold” white light that approximates the visible region of the solar spectrum. In the study, Luo et al first sought to understand the origins of broadband emissions in Cs2AgInCl6 using mathematical modeling and computational studies to relax the lattice and represent self-trapped excitons (STEs) to investigate exciton-phonon coupling. Such systems will be fundamental to engineer the next generation of energy-efficient and cost-effective lighting and display technologies.
A self-trapped exciton (STE) is defined as a bound electron-hole pair carrier that can dramatically enhance luminescence, energy transport and lattice defect formation in the crystal. The researchers found that STEs in the double perovskite Cs2AgInCl6, arose from strong Jahn-Teller distortion of the integral AgCl6 octahedron complex. The trapped excitons had a similar orbital character to the free exciton, indicating parity-forbidden transition (arising due to disruption of the centre of symmetry). The theoretical analysis showed an extremely low photoluminescence quantum yield (PLQY) for pure Cs2AgInCl6. To enhance the PLQY for practical applications as broadband materials, the system had to be modified, specifically by breaking parity-forbidden transition to manipulate the symmetry of the STE wavefunction.
White light emission from Cs2Ag1−xNaxInCl6. a) Luminosity function (dashed line) and photoluminescence spectra (solid lines) of Cs2Ag0.6Na0.4InCl6 measured at different temperatures from 233 K to 343 K. b) Photoluminescence stability of Cs2Ag0.60Na0.40InCl6 against continuous heating on a hotplate, measured after cooling to room temperature. c) Operational stability of Cs2Ag0.6Na0.4InCl6 down-conversion devices measured in air without any encapsulation. The boxplot showed the results for the different samples measured separately with the box edges representing quartiles and band in the box representing the mean and maximum data. d) XRD patterns of a Cs2Ag0.6Na0.4InCl6 film (black line) and powder (red line). The inset shows a 300 nm thick quartz substrate and 500 nm thick Cs2Ag0.6Na0.4InCl6 films under 254 nm UV illumination.
A practical approach to achieve this was via partial substitution of Ag with an element that could sustain the double perovskite structure. The substitute required a distinctively different electronic configuration to Ag, such as a group-IA element or alkali metal. The scientists therefore explored alloying Na into Cs2AgInCl6 to form pure Cs2NaInCl6, which demonstrated broadband emission on substitution but with very low efficiency due to strong phonon emission, requiring optimization of the Na content in the complex.
Since lattice mismatch between the two perovskites (Cs2AgInCl6 andCs2NaInCl6) was very low (0.3 percent) the scientists anticipated Na+ incorporation would occur without detrimental defects or phase separation. For the synthesis, CsCl, NaCl, AgCl and InCl3 precursors were mixed into an HCl solution in a hydrothermal autoclave. The mixture was heated for a defined period of time and cooled down to result in a final white precipitate product (90 percent yield). The pure double perovskite phase was confirmed using X-ray diffraction patterns (XRD) of a series of product compositions. The results agreed with plasma optical emission spectrometry (ICP-OES). The results were also in agreement with similar alloying experiments that were previously conducted with lithium (Li). The study thus suggested a general trend for alkali-metal-induced photoluminescent enhancement in double perovskites.
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