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Bright infrared quantum-dot light-emitting diodes through inter-dot spacing control

Abstract

Infrared light-emitting diodes are currently fabricated from direct-gap semiconductors using epitaxy, which makes them expensive and difficult to integrate with other materials. Light-emitting diodes based on colloidal semiconductor quantum dots, on the other hand, can be solution-processed at low cost, and can be directly integrated with silicon1. However, so far, exciton dissociation and recombination have not been well controlled in these devices, and this has limited their performance2,3,4,5,6,7,8. Here, by tuning the distance between adjacent PbS quantum dots, we fabricate thin-film quantum-dot light-emitting diodes that operate at infrared wavelengths with radiances (6.4 W sr−1 m−2) eight times higher and external quantum efficiencies (2.0%) two times higher than the highest values previously reported. The distance between adjacent dots is tuned over a range of 1.3 nm by varying the lengths of the linker molecules from three to eight CH2 groups, which allows us to achieve the optimum balance between charge injection and radiative exciton recombination. The electroluminescent powers of the best devices are comparable to those produced by commercial InGaAsP light-emitting diodes. By varying the size of the quantum dots, we can tune the emission wavelengths between 800 and 1,850 nm.

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Figure 1: Physical and electronic structure of the LEDs.
Figure 2: Relationship between LED performance and inter-dot distance.
Figure 3: Current density–voltage characteristic of a device made of MOA-capped quantum dots with diameters of 4.5 nm.
Figure 4: Emission spectra and infrared image of LEDs.

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Acknowledgements

This material is based on work supported by the National Science Foundation (NSF, grant no. EEC-0646547) and by the New York State Foundation for Science, Technology and Innovation (NYSTAR). J.J.C. and D.S. acknowledge support from the Cornell Center for Materials Research with funding from IGERT: a Graduate Traineeship in Nanoscale Control of Surfaces and Interfaces (DGE-0654193) of the NSF. This publication is based on work supported in part by an award (no. KUS-C1-018-02) made by King Abdullah University of Science and Technology (KAUST). GISAXS measurements were conducted at Cornell High Energy Synchrotron Source (CHESS) and the authors thank D.-M. Smilgies for calibration of the beam line set-up.

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L.S. and J.J.C. conceived and designed the experiments. L.S. and D.S. performed device characterization and optical measurements. J.J.C. synthesized the materials, fabricated the devices, and performed GISAXS and optical measurements. A.C.B. calculated the energy levels of the quantum dots. L.S. and F.W.W. co-wrote the paper. F.W.W., T.H. and G.G.M. (now at Ecole Nationale Supérieure des Mines, France) supervised the project. All authors discussed the work, commented on the manuscript and contributed to revision of the manuscript.

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Correspondence to Liangfeng Sun or Frank W. Wise.

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The authors declare no competing financial interests.

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Sun, L., Choi, J., Stachnik, D. et al. Bright infrared quantum-dot light-emitting diodes through inter-dot spacing control. Nature Nanotech 7, 369–373 (2012). https://doi.org/10.1038/nnano.2012.63

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