Preparation Of Copper-Based Thin Films With High Hole Conductivity And Its Application Of QLED

Quantum Dot Light-Emitting Diodes（QLEDs）have gained much attention in the lighting and display fields due to their high color saturation,spectral stability and solution processability.At present,QLEDs usually use organic hole injection layer（HIL）,such as poly（3,4-ethylenedioxythiophene）:polystyrene sulfonate（PEDOT:PSS）.However,PEDOT:PSS currently has the following three shortcomings:First,the intrinsic thermal stability of organic materials is relatively poor;the acidity of PSS may lead to the degradation of ITO anode;the complexity of the organic synthesis process leads to expensive preparation.Inorganic conductive transition metal oxides（TMOs）,such as Ni O_x,Mo O₃,V₂O₅,WO₃,etc.,are expected to serve as an alternative to PEDOT:PSS due to their high material stability,low material cost and needlessness for acid additives.However,the transition metal oxides studied so far have a strong local effect on electrons due to their high electronegativity of oxygen,resulting in relatively low overall conductivity of the films,and thus the current QLEDs based on TMOs hole injection layers have low efficiency and luminance.In order to further improve the performance of QLEDs based on inorganic hole injection layer,this paper attempts to introduce copper-based CuI and Cu₂SnS₃ materials with low cost,high work function and high hole mobility as hole injection layers to construct red QLEDs.The CuI films with different concentrations were characterized and processed for optimization,and 4-trifluoromethylbenzoic acid（TFBA）ligand modification was used to further modulate the energy levels to achieve energy level matching in the devices.As well as the two-step precursor approach was used to synthesize[Cu Sn CS（NH₂）_n]Cl_m ionic crystal precursors and optimize the reaction temperature for in situ ablation to prepare highly pure and dense and smooth covalent crystalline Cu₂SnS₃ films.In this way,the hole injection efficiency was improved and the hole-electron injection balance was promoted to achieve the comprehensive device performance.The main work of this thesis consists of the following two parts:（1）CuI as hole injection layer for constructing QLEDsThe CuI film is dense,smooth and has a high transmittance of more than 85%in the visible range of390-780 nm,and its work function is about 4.95 e V,which balances the energy gradient between ITO（4.70e V）and TFB（5.40 e V）very well.CuI is an excellent p-type semiconductor material that is well suited for use in QLEDs.In this thesis,CuI solution was prepared using acetonitrile as the solvent,and CuI films were prepared by spin coating.The effect of CuI films at different concentrations on the performance of QLEDs was investigated,and it was found that the films prepared at a concentration of 10 mg/m L of CuI were the smoothest and flattest,at which time the QLEDs obtained a maximum external quantum efficiency（EQE）of17.14%and a maximum current efficiency of 24.38 cd/A.The maximum luminance of the device exceeded70,000 cd/m².Next,the energy level gradient of the device was further optimized by modifying the CuI film with 4-trifluoromethylbenzoic acid（TFBA）ligand,and this process promoted the hole injection efficiency and enhanced the effective hole-electron complex,and the optoelectronic performance of the device was further improved.The maximum EQE of QLEDs modified with TFBA ligands reached 20.4%,and the luminance reached 25220 cd/m² at this time.And at the maximum brightness of 135100 cd/m²,the EQE and current efficiency of the device can still be maintained above 16%and 24 cd/A,while QLEDs had a low efficiency roll-off.（2）Cu₂SnS₃ thin film as hole injection layer for constructing QLEDsAnhydrous copper chloride,anhydrous stannous chloride,and thiourea were added to ethylene glycol methyl ether to spontaneously complex and precipitate white snowflake[Cu Sn CS（NH₂）_n]Cl_m ionic nanocrystals,which were further centrifuged to purify the[Cu Sn CS（NH₂）_n]Cl_m ionic nanocrystals dissolved in N,N-dimethylformamide as the final[Cu Sn CS（NH₂）_n]Cl_m ionic crystal precursor solution.Compared with the traditional commonly used one-step precursor method,this thesis used a two-step method to prepare[Cu Sn CS（NH₂）_n]Cl_m ionic crystal precursors,which can effectively enhance the purity of Cu₂SnS₃ films by avoiding the influence of metal ion and thiourea that were not involved in the reaction.The Cu₂SnS₃ films prepared by this method also possessed a highly flat morphology and a mobility of up to 9.83 cm²/（V·s）with excellent electrical properties.The complexation mechanism of[Cu Sn CS（NH₂）_n]Cl_m ionic crystals was also investigated during the precursor synthesis,and the phase transition temperature for the conversion of[Cu Sn CS（NH₂）_n]Cl_m ionic crystals to Cu₂SnS₃ covalent crystals was determined during the in situ ablation.In constructing the device,the thickness of Cu₂SnS₃ film was optimized by adjusting the spin-coating speed,at which time the maximum current efficiency of the prepared QLEDs was 11.12 cd/A and the maximum EQE was 13.02%.In addition,the performance of the constructed QLEDs was further optimized by adjusting the ablation temperature of the[Cu Sn CS（NH₂）_n]Cl_m ionic crystal precursor.The Cu₂SnS₃ film generated at an ablation temperature of 200°C was the densest and smoothest,and the obtained QLEDs showed the best performance with a maximum brightness of more than 70,000 cd/m²,and its maximum EQE and maximum current efficiency were 21.09%and 19.89 cd/A,respectively,which were better than the performance of inorganic hole injection layer materials commonly used to construct QLEDs today.