Bidentate chelating ligands as effective passivating materials for improving performance of organic-inorganic hybrid perovskite light-emitting diodes
Organic-inorganic hybrid perovskites are recognized as a promising candidate for photovoltaic applications. More recently, this emerging type of perovskite materials also becomes highly attractive as active materials for other optoelectronic devices, including lasers, light-emitting diodes (LEDs) and photodetectors. The aim of this thesis is to develop high-performance LEDs based on solution processes. We have found the efficiency of photoluminescence (PL) of CH3NH3PbBr3 thin films can be enhanced after post-deposition surface treatments with bidentate chelating ligands, including 1,10-phenanthroline, 4,4'-bipyridine and 4,7-diphenyl-1,10-phenanthroline (Bphen). The PL intensities of were improved significantly after we spin-coated 1,10-phenanthroline and 4,4'-bipyridine on the thin film surfaces. Meanwhile, the treatments also resulted prolonged PL lifetimes, suggesting the passivation of the defects in the perovskite thin films. The unsaturated or under-coordination Pb ions, which are also Lewis acids, has been considered as one of the origins of the electronic traps in perovskite thin films. Therefore, the chelating ligands, which behaved as Lewis bases, could effectively react with the Lewis acids and passivate the defects. The morphologies of the perovskite films were also examined using X-ray diffraction, atomic force microscopy, and scanning electron microscopy; the results indicated that the surface treatments did not significantly affect the films. Moreover, the lower defect densities, which were deduced from the current-voltage curves of the hole-only devices, after the treatments supported the functions of the above ligands. Finally, perovskite LEDs were fabricated and the device passivated with 1,10-phenanthroline exhibited a nearly doubled quantum efficiency. We anticipate that this approach proposed in this thesis could lead to a general method for improving the PL efficiencies and the device performance.
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