Synthesis Regulation For High-Quality Metal Halide Perovskite Quantum Dots And Their Application In Electroluminescence Devices

In the current information society,display technology,as an important port for information interaction,has become an indispensable part of people’s lives.Lightemitting diode(LEDs),as an important part of display technology,need to have high color purity,high luminescence efficiency,high resolution and long-term stability.In order to meet the growing display market,people have been committed to the development of high-performance and cost-effective display technology.In recent years,metal halide perovskite quantum dots(QDs)have attracted much attention due to their excellent optoelectronic properties,such as high photoluminescence quantum yield(PLQY),wide spectral coverage,easily adjusted band gaps,and high carrier mobility.In addition,solution-processable,flexible and cost-effective features make perovskite materials more attractive to industry.Therefore,metal halide perovskite QDs are the ideal emitters that can fully meet the requirements of ultra-high definition displays and flexible displays at a low cost.The development of perovskite quantum dot LEDs(QLEDs)has been very rapid in recent years,and breakthrough progress has been made.However,in the face of actual display applications,there are still many challenges,such as poor device stability and low efficiency of blue perovskite QLEDs,which are mainly caused by the defect vacancies and ion migration in perovskite QDs.Therefore,the synthesis of high-quality perovskite QDs is the key to achieve efficient and stable perovskite QLEDs.In view of the above problems,this thesis aims to develop high-quality all-inorganic perovskite QDs and high-performance LEDs by regulating the synthesis process of perovskite QDs,synthesizing high-quality perovskite QDs by means of phase-conversion engineering,halogen-source substitution,and molecular additives,and at the same time,revealing the influence of the type of reactive precursor on the structural morphology and luminescent properties of QDs,so as to develop high-performance LED devices.Based on this,highly efficient and mechanically stable flexible perovskite QLEDs were further developed for future flexible display applications.The specific research contents are as follows:1.For the traditional hot-injection method to synthesize perovskite QDs,there are usually excessive Pb2+ions in the synthesis process,resulting in a large number of halogen vacancies,which seriously affects the PLQY.Cs4PbBr6 with low Pb content was converted into highly luminescent Zn-doped CsPbBr3/Cs4PbBr6 QDs by ZnBr2induced phase conversion engineering.In this process,ZnBr2 can be integrated into the crystal structure of Cs4PbBr6 by insertion reaction,achieving the transition to tightly packed Zn-doped CsPbBr3 QDs.Due to the similar ionic radius of Zn2+and Pb2+,both ions can occupy the B-site of perovskite structure at the same time.Meanwhile,the extra Br-ions provided by ZnBr2 not only can facilitate the transformation process,but also can passivate the surface defects of the QDs,resulting in high PLQY of 77.5%,which is higher than that of the unmodified CsPbBr3 QDs(～50%).In addition,doping of Zn2+ions can increase the concentration of electrons in QDs,thereby improving the conductivity of QDs,which is conducive to the fabrication of electroluminescent devices.Furthermore,the as-synthesized Zn-doped CsPbBr3/Cs4PbBr6 QDs were used as emitters to fabricate LEDs.The device achieved the maximum current efficiency of 9.8 cd/A and a peak external quantum efficiency(EQE)of 3.2%,which is the best performance of the CsPbBr3/Cs4PbBr6 based LEDs reported at the time.This reveals the great application potential of CsPbBr3/Cs4PbBr6 QDs as electroluminescent emitters in LEDs.2.Mixing halogen components is an effective way for perovskite QDs to achieve wavelength regulation to meet the display requirements.However,in mixed halogen QDs,in addition to the aforementioned low PLQY caused by a large number of halogen vacancies,there also exists ion migration that leads to spectral instability.To suppress the deep trap state Cl vacancy and ion migration in CsPb(Br/Cl)3 QDs,trichloro(1H,1H,2H,2H-tridecafluoro-n-octyl)silane(PFTS),of which the Cl-ions function as the Cl-precursors,while the F-ions could work as the surface passivator was explored to replace PbCl2 as reactant,and the obtained products exhibited a high PLQY of 87%.In addition,the hydrophobic C-F bonds in PFTS can improve the stability of QDs in air.As a result,the LEDs based on PFTS-synthesized QDs showed a 3.8-fold increase in maximum brightness and a 5.6-fold increase in peak EQE compared to LEDs based on PbCl2-synthesized QDs.At the same time,the passivated halogen vacancies in QDs can block the ion migration channels and inhibit the occurrence of phase separation,thus improving the stability of the device under the operating voltage and realizing spectrally stable pure-blue perovskite QLEDs.3.High-quality perovskite QDs should not only have high PLQY and good stability,but also have uniform particle distribution to form high-quality perovskite QD films to achieve high-performance QLED devices.Since the nucleation and growth process of perovskite QDs fundamentally determines their crystal quality,zeolite was utilized to prepare homogeneous and high-quality CsPb(Br/I)3 QDs via promoting the homogeneous nucleation and growth,and suppressing the Ostwald ripening.In addition,the zeolite underwent decomposition during the reaction and interacted strongly with perovskite QDs through Pb-O bonds and hydrogen bonds,which inhabited the defect formation and suppressed halide ion migration.As a result,the QDs achieved high PLQY of 92%and significantly improved stability.Moreover,the strong binding affinity of the decomposed zeolite to QDs restrained the particle aggregation and fluorescence quenching during film formation,resulting in homogeneous perovskite films with high PLQY.As a result,perovskite QLEDs using this high-quality CsPb(Br/I)3 QDs as emissive layer were fabricated,which showed Commission International de I’Eclairage(CIE)color coordinates of(0.705,0.291),approaching to the Rec.2020 red primary color.The devices achieved a peak external quantum efficiency of 23.0%and outstanding spectral stability.4.For next-generation flexible display applications,high-performance flexible perovskite QLED is an essential optical component,which requires that high-quality perovskite emissive layers should also have excellent flexibility,but there is a paucity of related research In this regard,it is proposed to introduce a low-cost and environmentally friendly highly flexible biomass material,ethyl cellulose(EC),to cross-link with the perovskite QDs to improve the flexibility of the perovskite emissive layers,which showed stable photoluminescence and negligible cracks after repeatedly bending.Theoretical analyses and experimental tests confirmed that the cross-linking was achieved by strong interactions between the two through the simultaneous formation of hydrogen bonds and Pb-O coordination bonds.Additionally,this crosslinking effect can reduce the defect densities of QDs and thereby improving their PLQY.Simultaneously,the synergistic effect of efficient defect passivation and hydrophobic ether groups of EC significantly improved the environment stability of QDs.Consequently,flexible LEDs based on the EC treated perovskite QDs achieved a EQE of 12.1%,which is the highest efficiency reported for flexible perovskite QLEDs at the time.At the same time,the mechanical stability of the device was significantly improved,maintaining high device performance even after repeated bending at an ultrasmall bending radius of 1 mm.The innovations of this thesis are as follows:1.Aiming at the problem of low PLQY due to the presence of excess Pb2+ions in the conventional hot-injection method for the synthesis of perovskite QDs,a ZnBr2induced phase transition engineering strategy was proposed to obtain high-efficiency QDs with a PLQY of 77.5%through the phase transformation of low Pb-content Cs4PbBr6,and to improve the electrical conductivity of the QDs.High-efficiency electroluminescent device was then developed.2.To address the problems of deep trap state halogen vacancies and ion migration in mixed-halogen perovskite QDs,PFTS molecule that can act as both halogen source and surface passivator instead of traditional lead halide as halogen source was used to synthesize high-quality QDs with high crystallinity and PLQY of 87%,while suppressing ion migration.And spectrally stable perovskite QLEDs were achieved,which showed 3.8-fold increase in the maximum luminance and 5.8-fold increase in the peak EQE.3.In order to fundamentally solve the problem of poor crystalline quality of perovskite QDs,zeolite was introduced to regulate the nucleation and growth kinetics of QDs,and at the same time inhibit the formation of defects and the migration of halogen ions,resulting in homogeneous and high-efficiency high-quality QDs as well as QD thin films.Finally,spectrally stable perovskite QLEDs meeting the Rec.2020 standard and showing a peak EQE of up to 23%were achieved.4.In order to meet the demand for highly flexible LEDs for flexible display applications,cross-linking through the formation of hydrogen and Pb-O coordination bonds between the highly flexible material ethyl cellulose and perovskite QDs was utilized to simultaneously improve the flexibility,optical properties and stability of perovskite emissive layer.Furthermore,flexible perovskite QLEDs with excellent mechanical stability and a peak EQE of 12.1%were achieved,which was the highest efficiency of flexible perovskite QLEDs reported at the time.