Highly Efficient Metal Halide Perovskite Light-Emitting Diodes:Materials Synthesis And Device Optimization

Metal halide perovskites have emerged as a new family of semiconducting materials in the applications of highly efficient light-emitting diodes(LEDs).However,the efficient and stable metal halide perovskite LED could be achieved only if the low photoluminescent(PL)efficiency and instability issues of metal halide perovskite materials are addressed.It has been reported that the metal halide perovskite bulk materials process low exciton binding energy and long electron-hole diffusion distance and therefore the excitons are easy to be dissociated,leading to low light-emission efficiency.In order to increase the exciton binding energy of the metal halide perovskite for light emitting,the nanosizing perovskite is an effective way.In addition,compared to perovskite bulk materials,the stability of perovskite nanocrystals is higher due to the passivation of organic ligands and the strong van der Waals forces between organic molecules,making nano-sizing fabrication highly attractive for the emitting layer of perovskite LED.Though the perovskite-based LEDs in the green and red light spectral range have been demonstrated in high brightness and good efficiency,their stability still cannot satisfy the requirements of practical applications.More importantly,the performance of metal halide perovskite blue LED is still limited by the low light emission efficiency of present metal halide perovskites.Therefore,the fabrication of highly efficient and stable perovskite LEDs,especially the blue perovskite LED,is the most challenging issue for realizing the practical application of perovskite LEDs.Based on the above mentioned research background,our project focused on the synthesis of perovskite nanomaterials with high photoluminescence and the fabrication of blue perovskite LEDs with high efficiency and good stability.My thesis included the following four aspects:1.We reported a new type of highly photoluminescent perovskite-related CsPb2Br5 nanoplatelets synthesized by a facile ligand assisted precipitation method via selecting appropriate ligand and the ratio of reactants in the precursor solution.The PL emission spectrum of the CsPb2Br5 nanosheets exhibited a highly symmetric band centered at 512 nm with narrow full-width-at-half-maximum(12 nm).The absolute photoluminescent quantum yield(PLQY)of CsPb2Br5 nanoplatelets in the toluene is about 87%and could theoretically be used as an efficient emitting layer in perovskite LED.Moreover,the fast anion exchange has also been demonstrated in the as-synthesized CsPb2Br5 nanoplatelets to extend their photoluminescence spectra to the entire visible spectral region,which greatly enriches nanosized perovskite photoluminescent materials.2.On the basis of high quality perovskite nanocrystals,we further studied the assembly characteristics of highly photoluminescent perovskite nanocubes.We report a new type of metal halide perovskite supercrystals via the gold-bromide complex triggered assembly of newly emerged attractive CsPbBr3 nanocubes.Through introducing gold-bromide(Au-Br)complexes into CsPbBr3 nanocubes suspension,the self-assembly process of CsPbBr3 nanocubes to form supercrystals was investigated with the different amount of Au-Br complexes added to the suspensions,which indicates that the driven force of the formation of CsPbBr3 supercrystals included the van der Waals interactions among carbon chains and electrostatic interactions between Au-Br complexes and surfactants.Accordingly,the optical properties change with the assembly of CsPbBr3 nanocubes and the variation of mesoscale structures of supercrystals with heating treatment was revealed as well,demonstrating the ionic characteristics of CsPbBr3 nanocrystals.The fabricated CsPbBr3 supercrystal presents a novel type of semiconducting supercrystals that will open an avenue for the assembly of ionic nanocrystals.3.Based on above synthesis of perovskite nanomaterials,we modified the synthesis strategy and directly prepared a series of quasi-2D perovskite nanosheet blue-emissive films with high PLQYs by a one-step spinning coating method.By compositional engineering via increasing the content of chloride,we achieved the tunable light emission wavelength of quasi-2D CsPbBrxCl3-x perovskite nanosheet films from green(504 nm)to blue(470 nm)with a high PLQY up to 42%at 486 nm.In addition,we developed a novel NiOx/LiF hole transport layer with high affinity to the precursor solution for improving the film quality of quasi-2D CsPbBrxCl3-x perovskite while reducing the quenching effect of hole transport layer to as-formed perovskite nanosheet films.As a result,the blue PeLEDs based on quasi-2D CsPbBrxCl3-x perovskite nanosheet films show color-tunable emissions and the external quantul efficiency of 0.52%as well as high brightness of 1446 cd m-2 are achieved at 490 nm.4.We have developed a one-step spin-coating method to prepare high-quality CsPbl3-xBrxthin film with a high PLQY of?40%by adding PVDF-HFP and L-Arg into the perovskite precursor solution.Using the optimized CsPbl3-xBrx thin film with 10 mol%of L-Arg and 4 wt%of PVDF-HFP as an emitter layer,we fabricated pure red perovskite LED emitting at 637 nm with a high brightness of 3100 cd m-2 and high EQE of 4.5%.We unraveled that PVDF-HFP played a crucial role on the formation of CsPbI3-xBrx nanocrystals and the small L-Arg molecule acted as passivation agent on the surface of nanocrystals to reduce the trap-state density in the fabricated high-quality CsPbI3-xBrx thin film.Our proposed mixed halide anion strategy in solution-processable CsPbX3 perovskite films with polymer and amino acid synergetic regulation showed a great potential to fabricate efficient pure color perovskite LEDs.