Xiaoming Wang (left) and Yanfa Yan from the University of Toledo are part of an international team that discovered a lead-free halide double perovskite that emits warm white light. [Image: Dan Miller, University of Toledo]
Phosphor-coated LEDs have become a low-energy alternative to incandescent lighting. The blue-tinged light they give off, however, is not ideal for indoor living spaces, where consumers prefer warmer tones. Now, an international research team has discovered a material that the researchers believe could replace the LEDs’ phosphor coating and produce a warm white light while consuming even less energy (Nature, 10.1038/s41586-018-0691-0).
The material is a lead-free halide double perovskite. Metal halide perovskites are used in optoelectronic devices because they are tolerant to defects and are inexpensive to process. They also exhibit tunable emission across the visible spectrum. But many of those that perform best contain lead, an environmentally hazardous substance, and can be unstable.
The new material combines a lead-free double perovskite that already emits a warm white light, Cs2AgInCl6, with sodium. Adding sodium to the compound dramatically increases its emission efficiency.
It works in theory
The research team included scientists from Huazhong University of Science and Technology in China, who conducted most of the experiments with the material, and from the University of Toledo, USA, who performed theoretical calculations to find out why the material produces white light so efficiently.
The work of the Toledo group suggests that the broad emission of Cs2AgInCl6 comes via self-trapped excitons and originates from the Jahn-Teller distortion of the molecular structure. Alloying with sodium partly substitutes the silver with an element that maintains the double perovskite structure but that has a very different electronic configuration. The sodium breaks the parity-forbidden transition and reduces the electronic dimensionality of the material. This leads to efficient white-light emission via radiative recombination of the self-trapped excitons.
All of this, the authors found, results in a three-orders-of-magnitude efficiency gain relative to the sodium-free compound. Too much sodium, however, and the efficiency drops again. The work suggests that the efficiency peaks at 86 percent if the sodium concentration stays below 40 percent.
Building an LED
To create a white-light-emitting LED, the researchers simply pressed the perovskite powder onto a commercially available ultraviolet LED. They did not use epoxy, or encapsulate the device in silicon to protect it.
The color temperature of the emitted light was 4054 K, which is acceptable for bright indoor lighting in home offices or kitchens, for example. Daylight has a color temperature of greater than 5000 K, while soft indoor lighting, more common for dining or relaxing, tends to be below 3000 K. The team operated the LED for more than 1,000 hours without seeing any significant degradation.
Single-material advantage
Broad photon emission across the visible spectrum is unusual from a single material. It is of particular interest for lighting applications though, since a single material makes for a simpler device structure and avoids any self-absorption or color instability that can result from mixing multiple materials.
The authors’ revealed the emission mechanism in this perovskite material with their theoretical simulations and showed that the performance of lead-free perovskites could surpass that of lead-based equivalents.
Future promise
Although getting the material ready for commercial applications will take more work, the authors hope that their results will lead to new diodes for next-generation lighting and displays. Their future research will focus on optimizing the quality of the emitting layer and device configuration to enhance performance. “The ability to reduce the power that bulbs consume and improve the color quality of light that the bulbs emit,” Yanfa Yan of the University of Toledo said in a press release accompanying the work, “is a positive step to making the future more environmentally friendly.”