Organometal halide perovskites (OHPs) are emerging as promising materials for light-emitting diodes (LEDs) due to their high photoluminescence (PL) efficiency, high color purity (full-width at half-maximum, FWHM approximately 20nm), facile solution processibility, and tunable bandgap. Unfortunately, three-dimensional (3D) OHP displays a small Eb (tens meV), leading to slow electron-hole capture rates. A 2D Ruddlesden-Popper OHP (L2An-1PbnX3n+1) comprises different layers of [PbX6] octahedra sandwiched between ammonium halide (L) barrier layers (A=Cs, CH3NH3+ or HC(NH2)2+, X=Cl, Br, I). The quantum confinement gradually enhances as the thickness of the L2An-1PbnX3n+1 layer reducing. Besides quantum confinement, there is also dielectric confinement arising from the large difference in the dielectric constants of the organic ligands (Ɛorg) and the inorganic phase (Ɛinorg). This leads to exciton wave functions being tightly confined in the 2D inorganic layers with a large Eb (up to hundreds of meV). However, the energy funnel is likely to contain inefficient pathways as the size distribution of nanocrystals, the phase separation between the OHP and the organic phase, resulting a poor film morphology and low LED performance. Controlling the OHP crystallization and phase separation has become an essential issue for achieving high performance 2D OHP Light-emitting diodes.
Recently, the research group led by Prof. Baoquan Sun, collaborated with Prof. Henning Sirringhaus and Felix Deschler from Cavendish Laboratory of University of Cambridge has developed a facile method to efficiently suppress crystallization of the organic phase, and a OHP light-emitting diodes with an external quantum efficiency of 15.5% is achieved. Their work has been published in Nature Communications (Nat. Commun., 2018, 9, 3892), titled as ‘Solution-processed perovskite light emitting diodes with efficiency exceeding 15% through additive-controlled nanostructure tailoring’. This results in a significant improvement in the photoluminescence quantum yield (PLQY) up to approximately 70% and achieving an EQE of 15.5%, which is one of the highest performance reported for an OHP LED to date.
They reported the photo- and device physics that is responsible for this significant improvement in device performance. The strategy can also benefit other perovskite systems, such as the recent mixed-dimensionality OHP solar cells, and allow better control of phase separation and crystallization through the incorporation of judiciously selected organic molecules.
Link to Paper: https://www.nature.com/articles/s41467-018-06425-5
Link to Prof. Sun’s Group: http://web.suda.edu.cn/bqsun/index.html
Editor:Danting Xiang