Dynamics In Both Monomolecular And Bimolecular Fluorescent Organic Light Emitting Diodes And The Device Optimization

Organic light emitting devices(OLEDs)have been leading the research and development in organic semiconductors,and representing a primary driving force in information display as well as solid-state lighting innovations.However,even as they now enter the marketplace,outstanding challenges in the efficiency and long-term degradation process of OLEDs remain.These are intimately tied to the dynamics of the basic excitation in these materials and devices,namely singlet excitons,triplet excitons,and polarons,to which electrons and holes,respectively,relax as they are injected from the electrode into the organic layer of the OLEDs.This dissertation focus on high efficient heavy-metal-free thermally activated delayed fluorescence(TADF)OLEDs and polymer light emitting devices(PLEDs),aiming at understanding the electronic properties of these materials,the nature of the organic semiconductor excitations and the fundamental physics that controls the properties of these devices.The first part of this dissertation studies the TADF based on exciplex type OLEDs.TADF is a promising pathway to achieve 100%internal quantum efficiency.In this process,light emission is realized as delayed fluorescence after efficient reverse intersystem crossing(RISC)from the lowest triplet excited state(Ti)to the lowest singlet excited state(S1).We begin by identifying the efficient exciplex emission in the mixed 4,4',4"-tris[3-methylphenyl(phenyl)amino]triphenylamine(m-MTDATA):tris-[3-(3-pyridyl)mesityl]borane(3TPYMB)(1:1)system,and then propose a highly efficient OLEDs based on these two materials.The exciplex-based TADF OLEDs achieve a very high performance OLEDs including a high current efficiency and a low efficiency roll-off.Further,as exciplex states can be utilized as highly efficient exciplex emitter or exciplex host for fluorescent or phosphorescence emitter molecule guests,understanding the CT mechanism and exciplex excited states in OLEDs is of crucial importance to enable optimization of device efficiencies.Thus the detailed recombination mechanism of the prepared exciplex type OLEDs is investigated by the transient electroluminescence method to explore the dynamic mechanism of exciplex state and exciton interaction in the devices.The results clarify the efficiency roll-off following high peak EQE and offer in-depth insight to the exciplex motion including its transport and emission mechanism and give a clear understanding of the dynamic properties of the tightly bound,long-lived fluorescent CT states in finely mixed film.The second part of this work focuses on polymer light emitting devices(PLEDs).The potential for commercialization is perceived to be high for these semiconductor devices because they are seen to compete in application areas where the market can bear the costs of development.The principal interest in the use of polymers lies in the scope for low-cost manufacturing,using solution-processing of film-forming polymers.However,outstanding challenges in capablilty for stacking an arbitrary number of functional layers and achiving balanced charges remain.Therefore,we introduce an bilayer of ZnO/PEIE to adjust the electron injection and incorporates an PMMA layer to tune the hole injection.This strategy enables all-solution processed PLEDs and results in a super balanced electrons and holes in the EML and thus a higher efficiency and a significantly extended operational lifetime.Further,one key challenge of high-performance flexible OLEDs is the availability of flexible transparent electrodes with superior optical,electrical,and mechanical properties.Herein,we demonstrate a new type of large-scale flexible transparent electrode based on an ultra-thin silver film,which is achieved by co-depositing a proper amount of nickel with silver deposition.Centimeter-size,flexible OLEDs are fabricated using this electrode,which show a 34%enhanced current efficiency compared to their ITO counterparts.Also,the ultra-thin Ag based OLEDs show close-to-identical emission spectra at different viewing angles and bending stability over 1000 circles.Our work demonstrates the great potential of ultra-thin doped Ag based transparent electrode for use in a wide variety of high-performance flexible organic optoelectronic devices.