Highly efficient LEDs based on 2D perovskite films
The quest for better light-emitting diodes (LEDs), offering both lower costs and brighter colours, has recently drawn scientists to a material called perovskite. Researchers from China and Hong Kong have now developed a 2D perovskite material for more efficient LEDs, with their results published in the journal Nature Communications.
LEDs have been used in our everyday lives for many decades, yet current high-quality LEDs still need to be processed at high temperatures and using elaborated deposition technologies — which make their production cost expensive. Scientists have recently realised that metal halide perovskites — semiconductor materials with the same structure as calcium titanate mineral, but with another elemental composition — are extremely promising candidates for next-generation LEDs. These perovskites can be processed into LEDs from solution at room temperature, thus largely reducing their production cost. But the electro-luminescence performance of perovskites in LEDs still has room for improvements.
Led by Professor Yang Xuyong from Shanghai University and Professor Andrey Rogach from City University of Hong Kong (CityU), researchers have found that they can turn up the light emission from perovskites to a brighter level, like a dimmer switch. The key to the powerful change lies in the addition of around 10% of a simple organic molecule called methanesulfonate (MeS).
In this study, the 2D perovskites used by the team have a nanometre-level thickness. The MeS reconstructs the structure of the 2D perovskite nanosheets, while at the same time enhancing exciton energy transfer between sheets of different thicknesses. Both of these changes have greatly enhanced the electro-luminescence of the thicker, green-emitting perovskite sheets within the 2D structure.
The MeS is also useful in reducing the number of defects in the 2D perovskite structure. During the process of light production, where radiative recombination took place, part of the excitons required for the process will be ‘wasted’ in the non-radiative recombination which produces no light. MeS reduces the number of uncoordinated Pb2+ cations, the cause for excitons to undergo the non-radiative recombination, making sure more excitons are participating in light production.
The team succeeded in realising efficient and bright LEDs, achieving what is claimed to be the best-reported performance in both current efficiency and external quantum efficiency for devices based on this kind of perovskites. They achieved brightness of 13,400 candela/m2 at a low applied voltage of 5.5 V, and external quantum efficiency of 20.5% — close to the maximum that many existing LED technologies can achieve, and almost doubling the external quantum efficiency level of 10.5% reported in their previous study two years ago. Their work has therefore put perovskite LEDs hot on the heels of current commercial display technologies.
“The achieved high brightness, excellent colour purity and commercial-grade operating efficiency mark 2D perovskites as extremely attractive materials for future commercial LEDs, and potentially also display technology,” Prof Rogach said. “It’s a tangible outcome from both fundamental and applied research into novel nanoscale materials.”
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