Light-emitting diodes based on perovskite thin films and nanocrystals

Abstract

Metal halide perovskites (MHPs) are crystalline materials featuring unique and attractive properties, such as excellent carrier mobilities, wide absorption range that extended to near-infrared wavelength, long carrier diffusion length, and solution processability. By engineering the chemical structure and exploring new synthesis routes, researchers have boosted the photoluminescence quantum yield of MHP to near unity with narrow emissions. Therefore, MHPs are considered as one of the most promising candidates for next-generation light-emitting diodes (LEDs). In this thesis, optical and electrical properties of different dimensional MHPs are investigated to meet the requirements of efficient and stable lighting and summarized as follows: (1) A simple electroluminescent device, namely, light-emitting electrochemical cell (LEC), is demonstrated with 3D methylammonium lead iodide perovskite (MAPbI3) as the light emitters. Room temperature emission can be facilely achieved from the LEC at both forward and reverse voltages. The shift of the emission profile in the mixed halide perovskite (MAPbI3-xBrx) LEC is also investigated, implying that iodine ions are migrating in the emissive layer under operation. (2) By introducing the dielectric and non-ionic poly(2-ethyl-2- oxazoline) polymer into the MAPbI3, nanocomposite films are obtained with controllable perovskite grain sizes (20–30 nm). The perovskite nanograins are wrapped by the polymer matrix thus the exciton diffusion within the film is suppressed and grain boundaries are also been passivated. As a result, the near-infrared emitting LED with perovskite-polymer nanocomposite light emitter exhibited high radiance of 12.31 W sr-1 m-2 and high external quantum efficiency (EQE) of 0.76%. (3) Through changing the formation temperature of the (C3H7NH3)2CsPb2I7 perovskite, tunable emission wavelengths from 654 to 691 nm are achieved in LEDs. With the analysis on the crystallinity and thermodynamic stability, the obtained perovskites are confirmed to be pure phase quasi-2D perovskites that the emission colour is stable against the bias voltages. The formation temperature induced colour tunability is also demonstrated in a series of quasi-2D perovskites with different alkylammonium cations. (4) By introducing an ultra-thin layer of perfluorinated ionomer (PFI), the hole injection efficiency has been significantly enhanced in the CsPbBr3 based perovskite LEDs. In addition, the PFI interlayer prevented charging of the CsPbBr3 nanocrystals thus their superior emissive property was preserved, which led to the 3-times increase in the luminance reaching 1377 cd m-2. Further device engineering with a polyhedral oligomeric silsesquioxane (POSS) blocking layer increased the luminance of the CsPbBr3 based LED to 2983 cd m-2. Also, the device operation lifetime was 3 times longer when compared to the LED without a POSS blocking layer, which can be ascribed to the confined carriers by the POSS layer that greatly improved the radiative recombination. (5) With additional polymer ligand coating, high-efficient and stable red emitting CsPbI3 perovskite LED is demonstrated with peak EQE of 19.6% and long operational lifetime of 316.5 hours. The polymer ligand prevented the direct attacking of nanocrystal surface by polar molecules and prevented the original oleylammonium oleate ligands from peeling-off during the purification procedures. Thus, the colloidal and phase stability is greatly improved, and the luminescence properties are well preserved.