Optoelectronic Properties Of Solution-Processed OLED And All-inorganic Perovskite Light-emitting Diodes

In recent years,organic light-emitting diodes(OLEDs)have been successfully commercialized due to the self-luminous,fast response,ultra-thin,and flexiblecharacteristics.At present,organic light-emitting diodes play an important role in both flat panel display and solid-state lighting.Typically,the emissive layer of OLEDs can be obtained by either thermal evaporation under high vacuum or by a solution-processing technique.Despite the fact that the former can offer high efficiency and has yielded commercial products,the overall cost of this technique is much higher than the latter,since high temperature is required for processing and the material utilization is typically low.In contrast,the solution-processed method has the advantages of high material utilization rate,simple processing method,and is suitable for largearea fabrication.This paper is focused on solution-processed light-emitting diodes based on polymer TADF materials and perovskite materials.The polymer TADF(Thermally activated delayed fluorescence)material can break the limitation of the 25% exciton utilization rate owning to the potential upconversion of 75% of the triplet(T1)to singlet(S1)excitons via the thermally assisted reverse intersystem crossing(RISC)approach.Also,since the polymer material has good solubility in organic solvents,it can be suitable for the fabrication of solution-processed light-emitting materials with high efficiency.Perovskites also have brilliant prospects for light-emitting diodes(LEDs)application because of their high color purity,tunable bandgap,low non-radiative recombination rates,and also high and balanced electron/hole mobility.In addition,the low material cost and solution pocessability of perovskite LEDs(Pe LEDs)make them suitable for large-scale fabrication.Over the past several years,substantial progress has been made in the development of near-infrared and green perovskite LEDs,which showed a maximum external quantum efficiency(EQE)of more than 20%.Typically,perovskites with mixed halide have been used to fabricate pure red and blue Pe LEDs,in most cases,these devices have exhibited inferior performance.Therefore,the development of efficient and stable perovskite LEDs with pure red and blue emissions is an urgent target in this field.1.In chapter 3,a novel and efficient blue light-emitting polymer P3-SO2 with a relatively small DEST value was synthesized via a C–N coupling condensation approach.In addition,P1–P4 involving the incorporation of a narrow-bandgap unitexhibited higher efficiencies.The maximum EQEs of 5.3%,6.1%,and 8.7% were achieved for pure blue,sky blue,and bluish-green PLEDs by employing P3-SO2,P1,and P3,respectively.In addition,the yellow light emitting diode with m CP as the host,the polymer TADF material as assisted host and the fluorescent material TBRb as emitter was fabricated.The maximum current efficiency is 18.4 cd/A with TBRb at anextremely low doping concentration of 1%,and the efficiency roll-off of the TADF device is suppressed at the same time.2.In chapter 4,we demonstrate an efficient light-emitting diode with pure red emission based on polymer-assisted in-situ growth of high quality all-inorganic Cs Pb Br0.6I2.4 perovskite nanocrystal filmwith homogenous distribution of nanocrystals in size of 20 ~ 30 nm.With this method,we can also dramatically reduce the formation temperature of Cs Pb Br0.6I2.4 and stabilize its perovskite phase.Eventually,we successfully demonstrate a pure-red-emission perovskite light-emitting diode with a highexternal quantum efficiency of 6.55% and luminance of 338 cd/m2.Furthermore,the device obtains an ultra-low turn-on voltage of 1.5 V and a half-lifetime of over 0.5 hours at a high initial luminance of 300 cd/m2.3.In chapter 5,we demonstrated an approach to form high-quality Cs Pb Br3 perovskite film through polymer additive treatment.The addition of Poly(2-methyl-2-oxazoline)can modify the morphology of perovskite film,reduce crystal size,passivate crystal surface and finally improve the PLQY of perovskite film.In the optimized condition of adding 20% PMOXA in Cs Pb Br3 film,highly-efficient perovskite LED with maximum EQE of 3.0% and luminance of 16648 cd/m2 was achieved.Also,the LED showed a low turnon voltage of 3.0 V and CIE of(0.08,0.77),reflecting the achievement of an excellent green-emissive LED.More importantly,the Cs Pb Br3 LED with 20% PMOXA presented an excellent spectrum stability and a half-lifetime of 2.4 hour with initial luminance of 1000 cd/m2,presenting the potential of Cs Pb Br3 LED in the future application like white-light illuminations and full-color displays.