Controllable Fabrications Of Highly Fluorescent Electrochemical Polymerization Films And Their Electroluminescent Devices

In recent years, the investigations of electrochemical polymerization (EP) methodsand films have been paid great attention in the field of organic optoelectronic materials. InEP method, the monomers are electrochemically oxidized or reduced at the interfacebetween electrolyte solution and electrode, and then the coupling reaction occurs amongthe radical monomers with deposition of the polymer film onto the electrode. This methodcan efficiently adjust the properties of resultant polymer films such as chain structures,doping levels and morphologies, and obtain various conductive polymer films withdifferent structures and properties. EP occurring at the interface between electrolytesolution and electrode provides a model for us to investigate the basic interface chemistry,and the mechanisms and the processes of conducting polymers.Generally EP films exhibit high conductivity and good charge transport property,while the luminescent property of EP film is very poor. Up to now, the EP films are mainlyapplied in organic light-emitting diodes (OLEDs) as the hole injection and transport layer.Few researchers studied the used of EP films as light-emitting layer for preparing OLEDs.The structural defects, doped electrolytes and rough morphology presenting in the EP filmsare the main factors that limit the application of EP films in OLEDs. To most of the EPsystems, it is very difficult to control the molecular weights of products and the structures of films, which fundamentally limited the fabrication of EP films with high qualities andcontrollable structures.Compared to the common EP precursors such as pyrrole and thiophene, EP ofN-alkylcarbazole can generate only dimer instead of polymer with long chain in certainpotential ranges. Thus the EP product exhibits well-defined and controllable structure,which benefits the precise control of the structures of EP films. By linkingN-alkylcarbazole with highly fluorescent units, our group designed and synthesized a kindof novel EP precursors with high fluorescence and well-defined structures. The separationof the fluorescent units from the electro-active carbazole group by alkyl chains ensures thatthe EP process has no impact on fluorescence. The doping level of EP films could becontrolled to1%by controlling the reduction potentials, by using electrolytes with largeanion, and by washing the films with suitable solvents. Such low doping level significantlyimproved the fluorescent efficiencies of EP films to more than50%. The EP films exhibitedsmooth and compact morphology with RMS roughness less than3nm by utilizing fast scanrates in cyclic voltammetry and by employing electrolytes with large anion. The optimizedLEDs where EP films acted as emitting layer presented high luminance efficiency up to5.8cd A^-1(red),10.4cd A^-1(green) and4.4cd A^-1(blue), which demonstrated the precisecontrol of the structures of EP films greatly improved the quality of EP films, especially theluminescent property of EP films.We fabricated highly cross-linked polymer laminated films with controllablestructures by utilizing electrochemical copolymerization (ECP) and multilayerelectrochemical polymerization (MEP), which expanded the controllability of EP methodin the film structures and properties. ECP and MEP methods can precisely and efficientlycontrol the composition and interface of resultant films, and can obtain EP films withcontrollable composition and clear interface. By employing ECP films and MEP films asemitting layer, we first realized highly cross-linked white OLEDs (WOLEDs). All the ECPand MEP devices exhibited excellent spectroscopic stability. The EL spectroscopic shapesremained almost unchanged in the working voltage ranges of devices. The reasons of thebias independent EL spectra is because the cross-linked structures in EP films fix the position of the molecules and avoid the phase separation in EP films, thus the EP filmsexhibit excellent structural and phase stability. The optimized ECP and MEP devicesexhibited high luminance efficiency up to6.7cd A^-1with CIE coordinates of (0.33,0.35)and5.5cd A^-1with CIE coordinates of (0.35,0.35), respectively.Usually the EP of conjugated polymers (CPs) cannot efficiently proceed because ofthe low solubility of CPs in highly-polar electrolyte solution, which made the efficientdiffusion and electron-transfer of precursors to electrode is hard to occur. Thus the EP filmsof CPs showed very rough morphology and weak fluorescence. Based on above analysis,we designed a novel multi-functional CP precursor that combines high solubility inelectrolyte solution, high electroactivity, high fluorescence and efficient electron-transportability. After EP under low oxidation potential, the resultant EP film exhibited highthickness of larger than200nm, smooth morphology of RMS roughness less than4nm,and high fluorescent efficiencies of more than50%. The CP precursor presented highelectrode reactivity, which is the essence to realize an efficient film-growth and thick filmon electrode during the EP process under low oxidation potential. The optimized EPsingle-layer device exhibited high luminance efficiency up to3.8cd A^-1, which reached atop level in the single-layer blue-emitting PLEDs, which demonstrated this strategyincluded the molecular design and low potential EP for film-forming is a breakthrough forpolymer EP from uncontrollable to controllable.We developed controllable EP and electrochemical doping for the fabrication ofmodified electrode. By utilizing suitable EP precursors and methods, we obtainedhigh-quality EP films with defined structures. Further electrochemical doping caneffectively adjust the optical and electrical properties of EP films, such as work functionand conductivity. PLEDs where EP films as hole-transport layer showed34%higherefficiency than PLED where PEDOT:PSS as hole-transport layer. Further researchdemonstrated this neutral EP film significantly improved the stability of neighbouringorganic layer. The inverted OPV devices where EP films as electron-transport layerexhibited high PCE%up to6.31%.