Research On Organic-inorganic Hybrid Inverted Eletroluminescent Devices And The Stability

In the past thirty years,organic light-emitting devices(OLEDs)have been developed rapidly.This kind of thin film devices with"sandwich structure"have huge potential in lighting and display applications,and therefore has been widely studied.As the material basis of OLEDs,organic semiconductors offer advantages compared with inorganic semiconductors,such as easy fabrication and modification,mechanical flexibility and low cost.However,the device performance is still affected by the intrinsic properties of organic semiconductors such as charge imbalance,low carrier mobility,and low glass transition temperature which may cause morphological changes and deteriorate device performance.In addition,there is usually an energy level mismatch at the interface between the organic semiconductor and the metal or metal oxide electrode.Inorganic semiconductors,especially metal oxides,are expected to solve the problems faced by organic materials due to their significant differences in electronic structure,charge transport mechanisms and optoelectronic properties.Therefore,organic-inorganic hybrid heterojunctions are often introduced into devices to enhance carrier injection/transport,device stability,and light outcoupling efficiency,etc.Considering the superiorities of organic-inorganic hybrid heterojunction in improving the performance and stability of OLEDs,the application of organic-inorganic hybrid heterostructures in inverted OLEDs(IOLEDs)and its effect on device stability were studied here.As their bottom electrodes can be directly connected with the drain line of n-channel thin film transistor,high-performance IOLEDs have practical significance for the development of active-matrix OLED(AMOLED)display technology,which can greatly reduce driving voltages and avoid image sticking of the AMOLED displays.However,till now,insufficient electron injection from bottom electrode to organic semiconductors still restricts the realization of high-performance IOLEDs.Therefore,in this work,organic-inorganic hybrid heterostructures are studied to achieve effective carrier injection,transport and exciton recombination in IOLEDs.The work mainly includes the following three parts:(1)Organic-inorganic bulk heterostructure was achieved by doping inorganic material into the organic material polyethyleneimine(PEI),and its effects on the device were studied.Through forming molecular dipoles and interface dipoles,the organic alkylamine-based polymer PEI can reduce the work function of various electrodes[such as ITO,Au,Ag,Al and poly(3,4-ethylenedioxythiophene):poly(styrenesurfonate)(PEDOT:PSS)]by approximately 1 e V.However,PEI shows serious quenching effect,and the insulating properties of PEI also limit the performances of devices.To address the issue,PEI was modified by adding zinc acetate dihydrate,and the modified PEI-Zn films were prepared.Through the chelation effect,the electronegativity of amine was weakened by electron transfer from N atom of PEI to Zn²⁺.It thus passivates the PEI,leading to an increase of exciton lifetime of emitting layer from 0.78?s on the PEI to0.98?s on the PEI-Zn film.Besides,both the PEI and PEI-Zn film prepared by spin coating were very smooth,the root mean square surface roughness(RMS)of the two films was below 2 nm,which indicates that the introduction of zinc acetate dihydrate will not induce a significant change on surface morphology.High-performance IOLEDs were then successfully prepared employing the modified PEI-Zn film as the electron transport layer,whose brightness and efficiency were significantly higher than those of the devices with PEI.On the base of type I hybrid OLED device with PEI-Zn as the electron transport layer,type II hybrid OLEDs with organic-inorganic hybrid heterojunction on both sides were prepared by introducing inorganic metal oxide MoO₃as hole transport layer.It is found that the thickness of MoO₃ has a significant influence on the efficiency and spectrum of the device.As the thickness of MoO₃ increases from3 nm to 130 nm,the performance of the device shows a trend of first decreasing,then increasing and then decreasing,and when the thickness of MoO₃ was 120 nm,the maximum external quantum efficiency of the device could reach 15.55%.Finally,the device stability was studied,and it is found that the operating lifetime of the devices with PEI-Zn was nearly 5 times of that in the device with PEI,indicating that the stability of the device PEI-Zn is also improved compared to the PEI device.In addition,the thick MoO₃ hole transport layer also further improves the stability of the device.(2)Organic-inorganic planar hybrid structure was constructed by using TiO₂/PEI,and its influences on device performances of IOLEDs were also investigated.The metal oxide TiO₂ is widely used in organic solar cells,but it still faces some problems before using TiO₂as an electron transport layer in IOLEDs.For example,TiO₂ has stronger exciton dissociation ability,which reduces the probability of exciton recombination.In addition,there is a huge electron injection barrier from TiO₂ to the organic small molecule emitting layer,which limits electron injection and carrier balances.To address the issue,organic interlayers were inserted between TiO₂ and organic emitting layer to alleviate the electron injection barrier and exciton quenching.With a designed tri-layered lnterlayer of PEI/DMAC-BPP/TPBI,IOLEDs with TiO₂ as electron transport layer were successfully perpared.By analyzing the energy levels,exciton lifetime and exciton recombination region location,it was found that the designed interlayers could form a stepped energy level for efficient electron injection from TiO₂ to emitting layer and thereby improve the carrier injection balance.Meanwhile,the exciton quenching effect was alleviated and a better exciton restriction was achieved.As a result,high-performance IOLEDs were achieved with a maximum external quantum efficiency of17.68%.Besides,an efficient white IOLED was also prepared with a maximum external quantum efficiency of 12.02%.In addition,the influences of thick inorganic hole transport layer on the performances and stabilities of devices were also studied.A 120nm-thick MoO₃ was used as hole transport layer in an IOLED with TiO₂as electron transport layer.The performances of device with 120 nm-thick MoO₃ layer were comparable to that of devices with conventional 3 nm-thick MoO₃ layer,and it shows much enhanced device stability.(3)Top-emitting IOLEDs were prepared with ZnO/PEI/DMAC-BPP/TPBI as electron transport layer.And the influences of organic-inorganic hybrid structure on microcavity effect were also investigated.Due to 100%aperture ratio,top-emitting devices have huge potential in display application.Here,the effects of top-emitting structure on the emitting materials with different radiative mechanisms,including phosphorescence,fluorescence,and thermally activated delayed fluorescence(TADF)materials,were studied first.It was found that under the effects of multi-beam and wide-angle interferences,the emission spectra of TADF material-based devices became narrower,which was close to half of the full width at half maximum(FWHM)of traditional devices.In addition,it also showed that in the absence of substrate and waveguide mode,a higher outcoupling efficiency can still be achieved for these top-emitting TADF devices by using a TADF emitter with a higher horizontal dipole ratio.By combining the faster radiative recombination rate,higher outcoupling efficiency and improved radiative efficiency,the external quantum efficiency of the top-emitting TADF OLEDs was enhanced by over 60%comparing with the conventional TADF devices.Based on the research above,TADF material 9,10-bis(4-(9H-carbazol-9-yl)-2,6-dimethylphenyl)-9,10-diboraanthracene(CzDBA)and phosphorescent material tris(2-phenylpyridine)iridium(III)[Ir(ppy)₃]were used as emitters to prepare inverted top-emitting devices.With inorganic metal oxide ZnO as the electron injection and transport layer,organic interlayers PEI/DMAC-BPP/TPBI were also used to reduce electron injection barrier and enhance electron injection in the inverted top-emitting device.According to theoretical analysis with classical electromagnetic theory,it is found that the introduction of inorganic transport material has an important influence on the optical loss distribution in the device.Compared with the top-emitting device with conventional electron injection material Li F,inverted top-emitting devices with ZnO show much better device efficiency and stability,which indicates that the introduction of the inorganic transport layer has a significant and positive impact on the optical process and the carrier transport process of the device.