Efficient and bright warm-white electroluminescence from lead-free metal halides

Abstract

Solution-processed metal-halide perovskites are emerging as one of the most promising materials for displays, lighting and energy generation. Currently, the best-performing perovskite optoelectronic devices are based on lead halides and the lead toxicity severely restricts their practical applications. Moreover, efficient white electroluminescence from broadband-emission metal halides remains a challenge. Here we demonstrate efficient and bright lead-free LEDs based on cesium copper halides enabled by introducing an organic additive (Tween, polyethylene glycol sorbitan monooleate) into the precursor solutions. We find the additive can reduce the trap states, enhancing the photoluminescence quantum efficiency of the metal halide films, and increase the surface potential, facilitating the hole injection and transport in the LEDs. Consequently, we achieve warm-white LEDs reaching an external quantum efficiency of 3.1% and a luminance of 1570 cd m−2 at a low voltage of 5.4 V, showing great promise of lead-free metal halides for solution-processed white LED applications.

Research progress of full electroluminescent white light-emitting diodes based on a single emissive layer

“…For example, in some organic molecules 16 , 38 such as triphenylamine derivatives, thermally activated rotation in the molecular structure leads to the enhancement of π–π stacking and electronic coupling, which thereby results in a redshift in some molecules. In some crystals, the lattice distortion-induced self-trapping state can also promote the coexistence of multicolour centres and has attracted much attention for some perovskite emitters in recent years, such as Cs 2 Ag 0.60 Na 0.40 InCl 6 , δ -CsPbI 3 , and CsCu 2 I 3 26 , 28 , 29 . In addition to structural control, the introduction of new elements can often achieve new luminescence centres owing to their newly formed energy level.…”

“…Many PeLEDs have been demonstrated with EQEs higher than 20% in the green 75 – 77 , red 78 , 79 , and near-infrared 80 – 82 regions. In addition to these exciting results, WPeLEDs, especially SEL–WPeLEDs, have also been developed rapidly 26 – 29 , 83 – 87 . In 2016, just after some of the earliest reports of PeLEDs, a SEL-WPeLED (Fig.…”

“…In addition to the monochromatic emitting characteristics of perovskites, as generally considered, their broadband-spectrum luminescence performance has also attracted the attention of researchers. As we proposed above, a single layer with broadband-light emission is considered to be an ideal strategy to achieve SEL–WLEDs, e.g., perovskite materials with the characteristics of self-trapped excitons (STEs) 26 , 28 , 29 , 84 , 86 . STEs are often generated in organic molecular crystals, condensed rare gases, and halide crystals owing to lattice distortion 88 , 89 .…”

“…8t, u ). Some perovskite-like materials such as caesium copper halides (CsCu 2 I 3 /Cs 3 Cu 2 I 5 ) have also been developed in WPeLEDs owing to their STE-based broadband light emission from an SEL 29 , 84 , 86 . In a very recent work, Chen H. et al introduced an organic additive into CsCu 2 I 3 /Cs 3 Cu 2 I 5 , resulting in trap-state reduction and PLQY promotion from 18% to 30% 29 .…”

“…Some R/G/B QDs can also be mixed in an SEL 21 – 25 , showing potential in lighting applications, especially considering the advantage of their low-cost solution-process capability. Some very recent studies have confirmed that some perovskites can emit broadband white light 26 – 29 , and they have great potential for imitating sunlight, the ideal light source, and are expected to create future lighting technology.

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