Synthesis Of Red InP-based Quantum Dots And Their Application In QLED

Semiconductor quantum dots（QDs）have attracted extensive attention of researchers due to their advantages of tunable emission wavelength,good stability,high color purity,solution preparation,and low preparation cost,etc.At present,high-performance and high-stability electroluminescent devices mainly use Cd/Pb-based QDs as luminescent materials,but the high toxicity of Cd/Pb elements seriously threatens the environment and human health,which is the main obstacle to the realization of industrial applications.Therefore,heavy metal-free QDs luminescent materials have become the target of extensive attention from researchers in recent years.Among them,the InP-based luminescent material not only has a large exciton Bohr radius（10 nm）,but also has a band gap（1.35 eV）close to Cd Se（1.74 eV）,which can be emitted by adjusting the size（1.5-3.0 nm）.The wavelength covers the range of blue light to near-infrared light（480-750nm）,so it is considered to be one of the most potential non-Cd/Pb luminescent materials,and the related quantum dot light-emitting diodes（QLED）have gradually attracted extensive attention in the field of research and application.However,the optical properties of current InP-based QDs are far behind that of Cd-based QDs,making it difficult to meet the fabrication requirements of high-performance QLED.It is mainly caused by the following aspects:（1）For InP nucleation,during nucleation synthesis,at this high temperature,phosphorus source precursors（such as tris（trimethylsilyl）phosphine（P（TMS）₃））will be rapidly dissipated,and monomers cannot continue to be produced,so that the growth of QDs enters the Ostwald ripening stage prematurely,resulting in severe uneven particle size distribution.At present,the synthesis of high-quality InP-based QDs mainly uses P（TMS）₃ as the phosphorus source,which exists some disadvantages such as expensive phosphorus source materials,incapable of large-scale production,and unsuitable precursors for storage.Inaddition,when P（TMS）₃ comes into contact with air,it will produce toxic gas phosphine（H₃P）,which is extremely harmful to the human heart and respiratory system.Although relatively stable and environmentally friendly phosphorus sources such as tris（dimethylamino）phosphine（P（NMe₂）₃）are used to prepare InP-based QDs,their optical properties are poor,and the performance of the related QLED devices is also far behind that of P（TMS）₃ as a phosphorus source.Therefore,the development of non-P（TMS）₃ method to prepare high-quality InP-based QDs has become a key problem that needs to be solved for this system to be applied.（2）Interms of the growth of InP-based core-shell QD shells,the choice of QD shell materials and the growth of thick shells are also an important challenge.Due to the large lattice mismatch between the InP core and the ZnS shell,the thick ZnS shell is difficult to grow,resulting in unsatisfactory photoluminescence quantum yield（PLQY）and full-width at half-maximun of InP/ZnS core-shell QDs.Therefore,it is necessary to develop gradient shell or alloy shell core-shell QDs to alleviate the lattice mismatch between core-shell materials and reduce the generation of interface defects during the growth of shell.Inaddition,the thickness of the shell layer is also an important factor in affecting the optoelectronic properties of QDs.When the shell layer is thin,the QDs are unstable,and the probability of F（?）rster resonance energy transfer（FRET）between QD particles increases after film formation,so that the quantum yield of the thin film decreases rapidly compared with the solution.When the shell layer is thick,it is not conducive to the charge injection and transport when it is used as a light-emitting layer,thereby affecting the performance of the device.Based on this,in this paper,P（TMS）₃ and P（NMe₂）₃ were used as phosphorus sources to synthesize high-quality InP cores,and the optical properties of the two material systems were compared by adjusting the shell structure and thickness.Finally,the advantages and disadvantages of InP-based QDs prepared by the two precursors were summarized.Furthermore,the luminescence properties of red QLED constructed from InP-based QDs prepared from two precursors were compared,and the advantages and disadvantages of P（NMe₂）₃ as a phosphorus source were summarized,leading to the further improvement of InP-QLED prepared from this phosphorus source.The main content is the following two parts.（1）Comparison of red InP-based QDs based on P（TMS）₃ and P（NMe₂）₃ and QLED performance:First,P（TMS）₃ and P（NMe₂）₃ were used as phosphorus source precursors to synthesize high-quality InP cores with the same peak position.Second,the same ZnSe and ZnS shells were coated on the basis of the crystal core with the same peak position,and the quantum dots and their corresponding QLED properties were compared.The results showed that compared with P（TMS）₃,although the quality of the cores prepared by P（NMe₂）₃ as phosphorus source is poor,resulting in the overall performance of core-shell QDs and performance of related QLED prepared by P（NMe₂）₃ phosphorus source method and QLED performance is relatively low.However,when P（NMe₂）₃ is used as the phosphorus source,the synthesis process is simple and does not require multiple growth and purification of the nucleus.And the core-shell structure QDs have good stability and are easy to store for a long time.Inaddition,the PLQY decreased when the InP cores prepared by P（TMS）₃ as the phosphorus source were coated with 6 layers of ZnSe,while the PLQY still increased when P（NMe₂）₃was used as the phosphorus source after coating 6 layers of ZnSe.Based on this luminescent material,the maximum brightness of InP-QLED prepared from P（TMS）₃ is 27,125 cd/m²,and the maximum external quantum efficiency（EQE）exceeds 10%.However,the maximum EQE of InP-QLED prepared by P（NMe₂）₃exceeds 8%,and the maximum brightness exceeds 10,000 cd/m²,which is not far behind the InP-QLED prepared by P（TMS）₃.（2）Preparation of red InP-based QDs by P（NMe₂）₃ phosphorous source method and its application in QLED:Base on of the first work,P（NMe₂）₃ was used as the precursor of phosphorus,the quality of the InP core was improved by adjusting the molar ratio of indium to phosphorus during nucleation.And based on the6 layers of ZnSe,the InP/10ZnSe/6ZnS core-shell QDs with higher quality were obtained by further adjusting the thickness of the ZnSe and ZnS shell layers.After that,we introduced ZnSeS shell to optimize the shell structure of QDs,which not only alleviated the lattice mismatch between ZnSeand ZnS shells,but also effectively reduced lattice defects and surface defects during the growth of QD shells.Finally,red InP/ZnSe/ZnSeS/ZnS core-shell QDs with a full-width at half-maximun of 41 nm and a PLQY of over 95%were obtained.The QLED based on this luminescent material has a maximum EQE of over 12%and a brightness of 44,160 cd/m².