| Metal halide perovskite nanocrystals(PeNCs)have become one of promising alternative to conventional semiconductor nanostructures,especially because of their superior characteristics,such as tunable and narrow emission,extraordinary photoluminescence quantum yield(PLQY),wide color gamut,high tolerance to defects,and facile synthesis.Based on these excellent optoelectronic properties,PeNCs have risen to prominence in optoelectronic fields,such as scintillators,light-emitting diodes(LEDs),lasers,solar cells,et.al.Despite the impressive progress in achieving PeNCs-based optoelectronic device,whereas there are still challenges in property optimization and application of PeNCs,including poor stability owing to their highly dynamic ligand binding as well as ionic crystal structure of perovskites,inferior spectral stability because of halogen ions-exchange,and poor charge transport originating from the insulating nature of the long-chain organic ligands commonly used for the PeNCs limiting the achievement of LEDs with integrated high efficiencies and high brightness.To tackle the above problems,this thesis aims to achieve high external quantum efficiency(EQE)and high brightness PeNCs-based LEDs.From the perspective of ligands regulating the optical properties,charge transport properties and stability of PeNCs,this thesis developed the ligand engineering strategies.As a result,PeNCs with excellent stability and charge transport properties and PeNCs-based LEDs with integrated high efficiencies and high brightness have been achieved.The main contents and results of this thesis are summarized as follows:A facile and robust approach was developed to synthesize Cs Pb X3 PeNCs ensembles with highly uniform size and cubic shape,in which particles were wrapped with thin silicone layers(Cs Pb X3@silicone)by controlling hydrolysis of the precursor APTES.Such silicone wrapping effectively passivates the surface-defects and restrains charge carrier losses from nanocrystals,thereby leading to an enhancement in PLQY.Cs Pb X3@silicone PeNCs show great stability in polar solvents while maintaining cubic phase for longer than 3000 hours.The silicone wrappings effectively inhibit exchange of halide anions in the multi-halide-component system.Based on the excellent properties of the Cs Pb X3@silicone PeNCs,the bright white LEDs with CIE color coordinates of(0.32,0.30)were fabricated.The electron transfer(ET)rates from inorganic Cs Pb Br3 PeNCs to mesoporous titanium dioxide films were modulated by using different surface ligands including long-chain alkyl oleic acid and oleylamine,cross-linked insulating APTES and aromatic naphthoic acid molecules as the ligand-bridge.The ET process was systematically investigated using time-resolved photoluminescence spectroscopy.Calculations results verified the ligand-bridge barrier effect of the three species upon the ET process.Transient absorption measurements excluded carrier-delocalization effect of the naphthoic acid ligand and confirmed the conclusion that ligands with low barrier help to enhance charge transport properties.A novel strategy was developed by employing a dual-purpose organic lead source(Lead naphthenate,Pb(NA)2)with low barrier for the synthesis of highly luminescent PeNCs with enhanced charge transport property.For Pb(NA)2,the metal ions works as lead sources while the naphthenate can function as the surface ligands afterward.The influence of Pb(NA)2 on the photonic properties of obtained PeNCs under different synthesis conditions is comprehensively investigated.The monodispersed Cs Pb Br3PeNCs with controllable size and excellent electrical properties were obtained,which showed superior PLQY up to 80%.Based on the enhanced electrical properties of the Pb(NA)2-derived PeNCs,the resultant LEDs exhibit a high peak EQE of 8.44%and a superior maximum luminance of 31759 cd/cm2.Formamidinium-lead-bromide PeNCs with highly luminance and enhanced charge transport were synthesized,which were capped with rationally designed short aromatic ligands of 2-naphthalenesulfonic acid(NSA)with low barrier.Compared with commonly used oleic acid ligands,the NSA molecules not only preserve the surface properties of the PeNCs during the purification but also notably improve the electrical properties of the assembled emissive layers,thereby ensuring efficient charge injection/transport in the devices.The resulting champion LEDs with a CIE coordinate of(0.19,0.77)approaching the Rec.2020 green primary color demonstrates a high brightness of 67115 cd/cm2 and a peak EQE of 19.2%.More importantly,the device shows negligibly efficiency droop at an elevated brightness of 20000 cd/cm2 and a well-retained efficiency of over 10%at around 65000 cd/cm2,presenting a breakthrough in LEDs based on PeNCs. |
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