Solid-state light-emitting diodes (LEDs) have revolutionized the lighting industry due to their higher luminous efficiency and low power consumption. Most of the present-day white LEDs are based on a blue-LED coated with yellow phosphors. It is ideal to develop a single material that can emit white light. Conventionally, doping oxide materials with a combination of rare earth ions are shown to emit an entire visible spectrum of light. Quantum dots, small molecule organic dyes, and polymers with fine-tuned chemical structures also show white emission. Lately, halide perovskite materials have attracted attention in optoelectronic applications. These materials offer high carrier lifetime and excellent tunability in properties which motivate the materials community to explore them in light-emitting applications.
In our recent work, we have provided a clear design strategy for creating intense white light emission in halide double perovskites (HDP) by creating ‘asymmetry’. Two key questions are answered: (i) how to create asymmetry? and (ii) what the role of asymmetry in emission is?
How to create asymmetry in halide double perovskites?
The crystal structure of HDP consists of series of two distinct octahedra where the center of Octahedron-1 has a monovalent ion and Octahedron-2 has a trivalent cation. Halide ions occupy the corners of the octahedra. The HDP is formed by the network of Octahedron-1 and Octahedron-2 alternating with each other in all three dimensions. Here, we experimented with mixing trivalent cations with different ionic radii to get the octahedron-1 distorted. We have selected Cs2AgB’Cl6 HDP and systematically distorted the AgCl6 octahedra (Octahedron-1) by using a mixture of Bi/In, Bi/Sb, and In/Sb.
The interesting properties are observed with the mixture of Bi3+ and In3+. The pristine Cs2AgInCl6 is a direct bandgap semiconductor but shows weaker emission as the primary optical transition is forbidden with valence band maximum and conduction band minimum sharing the same parity. On the other hand, the indirect bandgap system Cs2AgBiCl6, shows weaker emission. In the mixed system Cs2AgInxBi1-xCl6, at a narrow range of x values between 0.8 and 0.9, the photoluminescence emission enhanced nearly 144 times compared to the parent systems. Under the same illumination conditions, the compound with x=0.85 shows at least ten times higher light emission compared to a standard Ce:YAG emitter.
Did the photoluminescence arise from defects? The emission measurements and optical power-dependent studies reveal the following. Single crystals of mixed trivalent compounds show broad and intense white light emission, and the power-dependent photoluminescence shows linear dependence of emission as a function of optical power. These results ruled out the role of defects in the emission. Essentially, it is the distorted crystal structure that was responsible for such an intense white light emission.
What role might this distortion have played?
We unearthed the answers by a combination of simulation and experiments. We find that the transition is changing from forbidden to allowed as the x values are decreased to 0.9 while maintaining the direct bandgap. As a consequence, the emission yield increased significantly. To further confirm the role of distortion, we intentionally distorted the end members (x=0 and 1) using DC electric bias and observed an increase in the photoluminescence.
Apart from intense emission, we also noticed several orders of magnitude increase in the excited state fluorescence lifetime. The pristine double perovskite shows a lifetime of few hundreds of picoseconds, whereas at x between 0.8 and 0.9, the lifetime has increased by around six orders of magnitude reaching few microseconds. The distortion has created local polarization leading to enhanced charge separation in these mixed trivalent cationic HDPs.
So, there is a double benefit in creating distortion: one is the increase in the photoluminescence yield, and the other is the enhanced photoluminescence lifetime. The former property is essential to create bright white light emitters with high luminous efficiency, whereas the latter is important to reduce charge recombination in photocatalytic or photoelectrocatalytic reactions. We anticipate that our work will attract a lot of interest among the light-emitting diode community to create bright white light-emitting materials by distorting the crystal structure. The Photocatalysis community can immensely benefit by creating asymmetry in materials to reduce recombination and enhance conversion efficiency.
For more information, please check our article in Communications Materials: "Manipulation of parity and polarization through structural distortion in light-emitting halide double perovskites"
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