Halide perovskite nanocrystal-based light emitting diodes

Lead halide perovskite (LHP)-based optoelectronic device has been a hot research topic in the last years, owing to the versatile optoelectronic properties of this class of materials, such as large light absorption coefficient, long-range charge carrier mobility, high defect tolerance, direct bandgap, and to the facile synthesis process. Since their synthesis by Kovalenko group in 2015, LHP nanocrystals (NCs) gained increasing attention also for application in light-emitting diodes (LEDs), owing to the possibility to easily tune their emission wavelength across the whole range of visible light and their high color purity. A detailed introduction on the chemistry and physics of LHP NCs, some basic knowledge about LEDs and the development of LEDs based on the LHP NCs are provided in Chapter 1. In Chapter 2, the chemicals, the basic experiments and the characterization techniques adopted in the thesis are introduced. In Chapter 3, I identified four issues that need to be solved before the colloidal LHP NCs could be efficiently applied into the fabrication of high efficient and stable LEDs. Given that long-chain and insulating ligands are used during the NC synthesis to ensure their good solubility and stability in solution, conductivity of LHP NCs films is too low to transport carriers efficiently. Secondly, post-treatments on the LHP NCs, often aimed at improving the aforementioned transport properties, are easy to introduce new defects and compromise the optical properties of the LHP NCs. Thirdly, since LHP NCs are ionic crystals and sensitive to humid air as well as to post-treatments, their long-term stability is still a big issue. Finally, it is difficult to obtain smooth and compact LHP NCs films, which are necessary for low current leakage and high-efficient devices. I addressed the above four issues step-by-step and obtained highly stable LHP NCs films. A combination of benzoyl bromide, ammonium thiocyanate and ethyl acetate was used to treat the pristine LHP NC solution. The method demonstrated to be able to improve the conductivity of the LHP NCs films and well retain the optical properties and morphology even after ten months. In Chapter 4, I employed the obtained NCs as emissive layer in green LEDs based on the proposed treatment method in Chapter 3. The champion device showed high efficiency of 1.2% at 518 nm with a maximum brightness near 3000 cd/m2 and high stability during operation with a half-lifetime of 27 min at a constant bias of 5 V as well as during storage (23 days in air). Furthermore, I conducted a mechanism study on the efficiency roll-off of the NCs-based LEDs using conductive atomic force microscopy (c-AFM). Morphology and current distribution of the NCs films under increasing bias were collected and a new insight about the efficiency roll-off was proposed. In Chapter 5, I further focused on the improvement of the efficiency of blue LEDs based on LHP NCs, which is still lower than that of green and red ones. In this context, I studied the effect of addition of various metal halides during the synthesis of LHP NCs on their optical properties. I found that the post-synthesis addition of CuCl2 leads to the formation of NCs with sky-blue emission and high stability in air. I applied the obtained NCs in the fabrication of sky-blue LEDs. The champion device, based on NCs with further optimized ligands, produces the up-to-date highest external quantum efficiency (EQE) of 5.02% and the highest luminance of 130 cd/m2 at the maximum EQE. In summary, this thesis firstly provides a promising route and proposes a possible mechanism to achieve high stable LHP NCs film. Secondly, efficient green LHP LEDs were obtained. Thirdly, a possible mechanism of the efficiency roll-off in LHP NC LED was proposed and may give guides for the design of NC LEDs with suppressed efficiency roll-off in the future. Last but not least, high efficient and stable sky-blue LED was fabricated based on CuCl2-treated LHP NCs, paving a promising way towards the fabrication of highly efficient LHP NC-based blue LEDs.