|Highly efficient quantum dot near-infrared light-emitting diodes
|Xiwen Gong, Zhenyu Yang, Grant Walters, Riccardo Comin, Zhijun Ning, Eric Beauregard, Valerio Adinolfi, Oleksandr Voznyy and Edward H. Sargent
|Nature Photonics, 2016, 10 (4), pp 253-257 DOI: 10.1038/nphoton.2016.11
Light-emitting diodes (LEDs) harness the chemical properties of electroluminescence to bring cheaper, more efficient, and more versatile lighting to people across the world. LEDs based on nanoparticles called quantum dots are even more promising. They can be processed in solution (so they’re cheap to manufacture), they are highly efficient, and their light emission can be tuned to different colors using the same material (so they’re versatile). Quantum dots, which are nano-sized particles of a crystal, are ideal for this purpose: they are so small that they fall into the regime of quantum confinement, where their size determines their optical properties. Other materials often require more energy-intensive solid-state processing, are less efficient, and lack the flexibility of light output that quantum dots’ tunability enables. However, existing LEDs made from quantum dots face a major challenge in balancing the efficiency of light emission against their power consumption.
Light emission in quantum dot LEDs requires the charges generated in the quantum dot – the excited electron and the positive charge it leaves behind – to stay separated and not recombine. However, strategies used to prevent recombination, like capping the quantum dot with organic ligands or embedding the quantum dot in a polymer matrix, also insulate the quantum dot in the LED. Then, the LED’s power consumption increases as a higher voltage is needed to inject charges into the quantum dot and create electroluminescence. Obviously, this is not ideal for a light-emitting device that is supposed to be cheap and efficient! The question that this work targets is how power consumption of quantum dot LEDs can be minimized.
The composite material the authors created combines the highly efficient electroluminescence of the quantum dots with a matrix of another inorganic material called a perovskite (Figure 1). The kind of perovskite used in the authors’ work is an organohalide perovskite, where one cation of the perovskite crystal is an organic molecule and the anion is a halide. The advantage of using an organohalide perovskite matrix over another material, such as a polymer, is that charge transport inside these types of perovskites is highly efficient. This prevents the quantum dot from becoming insulated, so the power consumption of the device doesn’t increase. As a bonus, the interaction between the perovskite matrix and the quantum dot actually improves the quantum dot’s electroluminescence – which is also the mechanism of light production in LEDs. Generally, sites on the surface of the quantum dot can collect the electric charges travelling through the device, preventing them from contributing to the electroluminescence of the quantum dot. However, the perovskite matrix helps eliminate these sites, so the amount of light the LED produces goes up, improving efficiency. Ultimately, the novel composite quantum dot-perovskite material reported in this paper represents a significant step towards the development of efficient, cheap, and versatile LEDs.