Co-doping double perovskites for single-component white LEDs

Chinese researcher uses Eu3+-Bi3+ co-doping double perovskites for single-component white-light-emitting diodes

Paper author Hongwei Song, professor at the College of Electronic Science and Engineering, Jilin University, said: “With lead-halide perovskites reaching a mature research stage approaching product marketing, concerns remain about the material’s stability and the toxicity of lead-based salts.”

Double perovskites with Cs₂AgInCl₆ composition, often doped with various elements, have been in the spotlight owing to their intriguing optical properties, namely, self-trapped exciton (STEs) emission and dopant-induced photoluminescence.

This interest has sparked different synthesis approaches towards both crystals and nanocrystals, and the exploration of many alloy compositions with mono- and trivalent cations other than Ag⁺ and In³⁺.

Song explained that, in the development of lead-free perovskite materials, people’s first thought is to replace the Pb element with a non-toxic element.

In order to replace Pb in halide perovskite, researchers chose several low-toxic cations in the same period closest to it, such as Sn, Ge, Bi, Sb, In, etc., because they have similar inactive shell s-orbital. This is the key to the unique photoelectric properties of perovskite materials.

The emergence of double perovskite just solved the above problems, lead-based perovskite materials have attracted great attention in the solid-state lighting area due to their high efficiency, high colour rendering and tuneable luminescence performance.

This is both an opportunity and a challenge for the overall development of the photoelectric industry.

• Additive to improve efficiency and stability of perovskite solar cells
• First single-crystal perovskite optical fibres developed
• Solar panels could be revolutionised by perovskites

White light emission

Song said: “Since the pioneering work on Cs₂AgInCl₆ in 2017 reported by Giustino et al. and Zhou et al. nearly simultaneously, many efforts have been devoted to its synthesis, modification of its composition, study of its electronic structure, optoelectronic properties, and applications.

“Recently, a record of white light emission with 86% PLQY was achieved by Luo et al. via simultaneous alloying of Ag⁺ with Na⁺ and Bi³⁺ doping, marking an important milestone in the development of Cs2AgInCl6 related materials.

“Despite several advantages, major issues with these lead halide perovskites remain their poor stability and toxicity.

“In order to solve such problems, various attempts have been made to reduce the toxicity of perovskites while still maintaining their efficient optical properties.”

The existence of Bi3+ ions decrease the excitation (absorption) energy, provides a new absorption channel and increases the energy transfer rate to Eu³⁺ ions.

Through adjusting the Bi³⁺ and Eu³⁺ concentrations, a maximum photoluminescence efficiency (PLQY) of 80.1% is obtained in 6% Eu³⁺ and 0.5% Bi³⁺ co-doped Cs₂AgInCl₆ DPs.

Optimum energy transfer rate

Song added: “The energy transfer efficiency can be fitted with the decay rates under different Bi³⁺ doping concentrations. It can be seen that the energy transfer rate improves as a whole with the increase of the doping concentration of Bi³⁺, and the optimum energy transfer rate corresponding to the Bi³⁺ concentration is 0.5%.

“Next, we conducted PLQY test on the materials. For the undoped Cs2AgInCl₆ DPs, PLQY is only 0.5%, which dramatically increases to 20.1% after the addition of Bi³⁺. After co-doped with Eu³⁺ and Bi³⁺ ions, PLQY continues to increase, and reaches the maximum of 80.1% when the Eu concentration reaches 6%.

“Here, we propose a possible mechanism to describe Eu³⁺ emission in Bi/Eu3+: Cs₂AgInCl₆. Cs2AgInCl6 DP is a direct bandgap semiconductor. Bi³⁺ doping provides a new absorption channel for the material, which may be caused by the contribution of the Bi³⁺ orbital in the band edge, breaking the STE-state compatibility ban transition, generating a new light absorption channel at a lower energy, and promoting the PLQY emitted by STE.

“For the Eu³⁺emission, we think there are two pathways. First, the energy transfer from STE to Eu³⁺ ions is possible as we have observed the Eu³⁺ emission in the Eu³⁺ doped Cs2AgInCl6 DPs.

“Second, the Eu3+ emission may mainly come from the energy transfer from Bi3+ ions to Eu3+ ions. The Bi3+ ions absorb the excitation light and transfer the energy from ¹P₁, ³P₂, ³P₁, ³P₀ levels of Bi³⁺ ions to ⁵D₃, ⁵D₂, ⁵D₁ and ⁵D₀ levels of Eu³⁺ ions. The characteristic emission of Eu³⁺ ions is then formed through ⁵D₀→⁷F_j(j=0,1,2,3) transitions.”

Song concluded: “Finally, we prepared the white light-emitting diodes based on Bi³⁺ and Eu³⁺ co-doped Cs₂AgInCl₆ DPs were fabricated with the optimum colour rendering index of 89, the optimal luminous efficiency of 88.1 lm/W and a half-lifetime of 1493 h.

“This strategy of imparting optical functions to metal halide DPs may lead to future applications, such as optical fibre communications, daily lighting, military industry, displays, and other fields.”

The research is published in Energy Material Advances.

Image: Song Hongwei’s research team at Jilin University in China developed the new materials. Credit: Song Hongwei/ Jilin University.

Research Aether / Technology Uncovered