All Inorganic Lead Halide Perovskite Nanocrystals For Efficient Light-Emitting Diodes

Lead halide perovskites have recently been established as a new type of direct bandgap semiconductors that exhibit excellent optoelectronic properties such as high photoluminescence quantum efficiency(PLQY),narrow emission linewidth,simple bandgap tunability,high charge-carrier mobility,and solution processability.Therefore,lead halide perovskites have been considered as promising candidates for efficient light-emitting diode(LED),which are in high demand for next-generation flexible displays and highly efficient solid state lighting.In the design of lead halide perovskite luminescent layer in LED device,there are three types:three-dimensional(3D)polycrystalline films,quasi-two-dimensional(2D)polycrystalline films and colloidal perovskite nanocrystals(NCs).Among them,the exciton binding energy of the 3D polycrystalline film is low,and the surface defects of perovskite layer are easily formed at the grain boundary,so their light emission efficiency is poor.The reported optimization strategies for 3D polycrystalline films are to use organic molecules to limit the growth of perovskite crystals,reduce the grain size of the 3D polycrystalline film and passivate the defects at the grain boundary.The quasi-2D film is a quantum-well structure formed by organic molecules and inorganic units,and its exciton binding energy is high,and the PLQY is largely enhanced in comparison to 3D polycrystalline film.However,the number of inorganic layers(n value)of the quasi-2D polycrystalline film is difficult to control,and organic molecules will hinder the transport of carriers,which will be unfavorable for adjusting the emission spectrum and the performance of device.Perovskite colloidal NCs usually have high exciton binding energy and PLQY,and are easy to control the uniformity of size and the formation of smooth film,which are suitable for the highly efficient LED devices via solution method.However,there are still some challenges in perovskite NCs,such as the instability of the lattice structure,the insulating property of long-chain organic ligands,and Auger recombination effect in the size confined NCs.Therefore,we take the realization of highly efficient and stable all-inorganic lead halide perovskite NC LEDs as the research goal.Starting from the crystal structure and surface ligands of the perovskite NCs,we developed the strategies of metal ion doping/substitution and short-chain ligand processing engineering to improve the quality of lead halide perovskite NC film.Furthermore,we carried out a series of structure and interface optimization to improve the performance of LED devices.This thesis mainly includes the following four aspects:1.We proposed doping strategy of heterovalent Ce3+ions to enhance the photo/electroluminescence efficiency of CsPbBr3 NCs via a facile hot-injection method.The ultrafast transient absorption and time-resolved photoluminescence spectroscopy revealed that the PL kinetics of CsPbBr3 host were accordingly modulated due to the doping-induced near bandedge states,which provided more emissive channels and accelerated the kinetics of electron-hole recombination,thereby leading to the improvement of PLQY of CsPbBr3 NCs.As a result,the LED device fabricated by adopting Ce3+-doped CsPbBr3 NCs as the emitting layers exhibited a pronounced improvement of electroluminescence with external quantum efficiency(EQE)from 1.6 to 4.4%via Ce3+-doping.Our study blazes a new trail for rare-earth ions doping into perovskites NCs to further enable a wider property regulation for this new class of semiconducting NCs.2.We introduced Sr2+ cations into CsPbI3 hosts to stabilize the few-nanometer-sized a-CsPbI3 quantum dots with simultaneously improved PLQY and stability of as-synthesized quantum dots.By controlling the synthesis conditions,we achieved the synthesis of a series of sizes of a-CsPbI3 quantum dots with the size tunability(diameters from 15 down to 4.7 nm).Our first-principles calculations confirmed that the substitution of Pb2+ by Sr2+ can enhance the formation energy of a-CsPbI3 quantum dots and reduce the structural distortion,making the cubic perovskite structure more stable.The femtosecond transient absorption further indicates that the PLQY enhancement of α-CsPbI3 quantum dots in our synthesis comes from the synergistic effect of iodide passivation and strontium substitution.Furthermore,the few-nanometer-sized α-CsPbI3 quantum dots turned out to retain high photoluminescence and highly close packing in solid state thin films,and the fabricated red light emitting diodes exhibited high brightness(1250 cd m-2 at 9.2 V)and good operational stability(L50>2 h driven by 6 V).This study shows that the metal ion substitution strategy can improve the stability of the perovskite lattice without affecting the luminescence performance of the perovskite,thereby preparing efficient and stable perovskite LED devices.3.We developed a metal-ligand complex surface passivation strategy to eliminate trap states and replace the long-chain ligands,enabling high PLQY(90%)and low charge-transport barrier of blue perovskite CsPbCl3-xBrx NCs.On the basis of elaborate surface engineering of CsPbCl3-xBrx NCs by calcium-tributylphosphine oxide(Ca-TBPO)complex,we achieve a record EQE of 3.3%and high brightness of 569 cd m-2 for pure-blue PeLED with an electroluminescence peak at 463 nm.In contrast,the device of the PeLED based on OAm-NCs exhibited low performance with 0.72%EQE and 68 cd m12 luminance.This study proves that Ca-TBPO can improve the luminous efficiency and carrier transport performance of blue perovskite nanocrystals to prepare high-performance blue LED devices.4.Based on the new short-chain ligand tributyl phosphine oxide passivating perovskite nanocrystals,we introduced a surface-engineering process to exchange the original long-chain oleic acid/oleylamine ligands by the cerium-tributylphosphine oxide hybrid ligands to increase the PLQY CsPbBr3 NCs exceed 90%and improve the carrier transport capability in the CsPbBr3 NC film.Using ultrafast transient absorption and time-resolved photoluminescence spectroscopy,we demonstrate that the hybrid ligand passivation can efficiently remove surface trap states to enhance radiative recombination and homogenize the exciton concentration to suppress nonradiative Auger recombination in the CsPbBr3 NC thin film.Consequently,we fabricate a light-emitting diode with efficient charge injection into the CsPbBr3 NC emitting layer,achieving a pronounced improvement of electroluminescence with an EQE from 5.5%to 9.1%.More importantly,the efficiency roll-off characteristics of high-brightness light-emitting diodes is effectively mitigated.The study shows that Ce-TBPO ligand can passivate the surface defects of perovskite NCs and improve its carrier transport performance to inhibit the Auger recombination process and reduce the efficiency roll-off in perovskite LED devices.