Structure Design Of Active Layer And Device Performance Study Of Light-emitting Electrochemical Cells

Light-emitting electrochemical cells（LECs）are considered to be the simplest thin-film illuminated display devices due to their low turn-on voltage,single-layer architecture,low fabrication cost,and air-stable metal electrodes,it has become a research hotspot in the field of electroluminescence in recent years.LECs possess a device structure with a single active emitting layer,in which the film formation quality of the active emitting layer is critical for obtaining excellent electrochemical performance and operational stability of the device.The quality of the active layer films can be improved by changing the film formation method,controlling the deposition conditions,and modifying the surface properties of the substrate to obtain efficient and stable LECs devices.In our work,considering the internal factors of active layer film formation,we design and modify the active layer structure by using various emitter materials,metal salt ion additives and electrolytes to fabricate LECs devices based on different active layers and study their performance as follows:（1）The Poly[2-methoxy-5-（2-ethylhexyloxy）-1,4-phenylenevinylene]（MEH-PPV）was used as the emitter material in the active layer and the orange-red light LECs with the structure of ITO/MEH-PPV:PEO:Li CF₃SO₃/AL were fabricated.The optical properties of active layer films doped polyethylene oxide（PEO）with various molecular weights were investigated,and the results indicated that the fluorescence intensity of the films increased significantly and the emission peak was red-shifted from 600 nm to605 nm as the molecular weight of doped PEO increased from 100,000 to 5 million,however,with no significant difference in the fluorescence decay lifetime,all of them were 0.52 ns.The devices were measured for electroluminescence performance,and the peak luminance L_max was 47 cd/m² based on the 100,000 molecular weight PEO device,while the L_max of the 5 million molecular weight PEO device was 479 cd/m²,with significant luminance improvement.The L_max of the thin film LECs devices prepared by using 5 million molecular weight PEO with the lower spin-coating speed increased to 697 cd/m²,indicating that changing the spin-coating process and modulating the components of the active layer can help improve the electrochemical performance of LECs devices.（2）The 2,3-diphenylbenzo[b]thiophene S,S-dioxide（DP-BTO）and 2,3-bis（[1,1’-biphenyl]-4-yl）benzo[b]thiophene S,S-dioxide（DBP-BTO）organic small molecules with aggregation-induced emission（AIE）properties were used as emitters of the active layer to fabricate ITO/DP-BTO:PEO:Li CF₃SO₃/Al blue LECs devices and ITO/DBP-BTO:PEO:Li CF₃SO₃/Al cyan LECs devices were constructed,respectively.The morphology of the active layer of the DP-BTO-based device was characterized by SEM,and no significant pinhole defects and phase separation were observed on its surface,indicating the good quality of DP-BTO-based films.The blue emission peak of the DP-BTO-based film is positioned at 473 nm,and the EL blue emission peak of the device is at 463 nm at 7 V and 461 nm at 12 V,reaching a peak brightness of 36.58 cd/m² at15 V.The cyan emission peak of the DBP-BTO-based film is at 490 nm.SEM measurements of DBP-BTO-based films with different doping ratios demonstrated that the films with DBP-BTO:PEO:Li CF3SO3 doping ratio of 8:8:1 had fewer hole defects,and the peak luminance of LECs devices based on them was as high as 166 cd/m².The successful fabrication of DP-BTO-based blue LECs and DBP-BTO-based cyan LECs revealed the potential of AIE materials as emitters for the application of novel LECs devices.（3）Perovskite films doped with different PEO concentrations were prepared and annealed at various temperatures to fabricate LECs devices with the structure of ITO/PEDOT:PSS/Cs Pb Br₃:PEO:Li PF₆/AL.The fluorescence properties and SEM morphology of the PEO-doped films with different PEO concentrations were investigated.The films with PEO concentration of 5 mg/ml under the annealing temperature of 50°C showed a slower decay of fluorescence intensity and a average fluorescence lifetime of 2.41 ns;the surface holes of the films were few,smooth and dense,which were beneficial to improve the electroluminescence performance of the devices.Electrochemical tests were applied to the LECs devices,and the devices fabricated at a PEO-doped concentration of 5 mg/ml and annealed at 50°C were found to exhibit the peak brightness of 70.78 cd/m².In the research on the effect of the evaporation rate of aluminum electrode on device properties,it is observed that high evaporation rate of 3-4（?）will destroy the active layer to the degree that it affects the interfacial charge transfer and decreases the device performance,while the low evaporation rate of 0-1（?）enables the aluminum electrode to be deposited more uniformly,which is beneficial to obtain devices with stable performance.