Parth Vashishtha1 3 Jonathan Halpert2 1 3

1, Victoria University of Wellington, Wellington, , New Zealand
3, MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, , New Zealand
2, Hong Kong University of Science and Technology (HKUST), Hong Kong, , Hong Kong

Semiconductor quantum dots have proven to be promising materials for optoelectronic devices, such as light emitting devices (LEDs) and solar cells, due to their thin linewidth emission spectra, high photoluminescence quantum yield and high absorption coefficient.1-3 By controlling the reaction temperature, and reactants concentrations, researchers are also able to synthesise pure inorganic perovskite quantum dots, CsPbX3 (X = Cl, Br, I) with precise colour tuneability and high quantum-yield which are soluble in non-polar solvents.4 All of these properties make them promising materials for fabricating nanocrystal LEDs (NC-LEDs). Here, we synthesize high quantum yield (50- 80%) monodispersed quantum dots, with tuneable emission spectra over the entire visible region, by a colloidal synthesis method and successfully process them produce thin films as the emitting layer in an organic LED-type device architecture. The electroluminescence emission wavelength is tuned by varying the temperature of the growth solution and the halide ratio in the reactants. We demonstrate the feasibility of producing LEDs with electroluminescence from blue to red using solution processing techniques for the charge transport layers. Most importantly, we demonstrate the field induced evidence of halide separation in mixed halide CsPb(Br/I)3 NCs which causes color instability in the NC-LEDs. Moderate iodide content mixed halide devices seemed to have greater compositional stability.

1. Yang, Y.; Zheng, Y.; Cao, W.; Titov, A.; Hyvonen, J.; Manders, J. R.; Xue, J.; Holloway, P. H.; Qian, L., High-efficiency light-emitting devices based on quantum dots with tailored nanostructures. Nat. Photonics 2015.
2. Anikeeva, P. O.; Halpert, J. E.; Bawendi, M. G.; Bulovic, V., Quantum dot light-emitting devices with electroluminescence tunable over the entire visible spectrum. Nano Lett. 2009, 9 (7), 2532-2536.
3. Chuang, C.-H. M.; Brown, P. R.; Bulović, V.; Bawendi, M. G., Improved performance and stability in quantum dot solar cells through band alignment engineering. Nat. Mater. 2014, 13 (8), 796.
4. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V., Nanocrystals of cesium lead halide perovskites (CsPbX3, X= Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut. Nano Lett. 2015, 15 (6), 3692-3696.