Study On Control Of Energy And Charge Carriers In Novel Organic Light-emitting Devices

In this doctoral thesis,we have firstly introduced photoelectric properties of carbon-based organic semiconductor materials and functional devices,charge transport and energy transfer process of organic semiconductors,and working mechanisms and development of OLEDs.Then,we review the three types of OLEDs,such as ultrathin emissive layer OLEDs,tandem OLEDs and exciplex OLEDs.The issues(i.e.,charges injection and transport,excitons generation and diffusion,excitons harvesting and decay)of designed OLEDs have been deeply explored and investigated,achieving a series of OLEDs with simplified structure and enhanced device performance.The novelty of this thesis can be summarized as follows.1.A novel wide color-range tunable,highly efficient and low efficiency roll-off fluorescent organic light-emitting diode(OLED)is fabricated using two undoped ultrathin emitters having complementary colors and an interlayer between them.With a threshold voltage of 2.82 V,the color tunable organic light emitting diode(CT-OLED)shows a maximum luminance of 39,810 cd/m²,a peak external quantum efficiency(EQE)of 6%and the efficiency roll-off as low as 11.1%at the luminance from 500 cd/m² to 5000 cd/m².The fabricated CT-OLEDs are tunable from cold white(0.30,0.36)to warm white(0.43,0.42)with correlated color temperature(CCT)6932 K and 3072 K,respectively,demonstrating that our proposed approach helps to meet the need for lighting with various CCTs.2.A novel design philosophy of tandem OLEDs incorporating a doping-free green phosphorescent bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)-iridium(III)(Ir(ppy)₂(acac))as an ultrathin emissive layer(UEML)into a novel interface-exciplex-forming structure is proposed.Particularly,relatively low working voltage and remarkable efficiency are achieved and the designed tandem OLEDs exhibit a peak current efficiency of 135.74 cd/A(EQE=36.85%)which is two times higher than 66.2 cd/A(EQE=17.97%)of the device with a single emitter unit.This might be one of the highest efficiencies of OLEDs applying ultrathin emitters without light extraction.3.We successfully fabricate a novel simplified efficient WOLED structure using energy transfer from interface exciplexes to complementary color dyes as undoped ultrathin emissive layers(UEMLs)based on tandem OLEDs structure.Particularly,the designed WOLED with Commission Internationale De L'Eclairage chromaticity coordinate(0.36,0.41)obtaining a peak illumination of 41.5 cd/A(EQE=18.59%)without light out-coupling technology,which is one of the highest current efficiencies among reported WOLEDs applying undoped UEMLs,demonstrating the superiority of the novel concept.Moreover,the designed white OLEDs generally emit warm white spectrum with low correlative color temperature of 3686K?4700K,which show great potential for their application in healthy illumination.4.A novel exciplex-forming host is applied so as to design highly simplified reddish orange light-emitting diodes(OLEDs).The reddish orange OLEDs achieve a strikingly high power efficiency(PE)of 31.80 lm/W with an ultralow threshold voltage of 2.24 V which is almost equal to the triplet energy level of the phosphorescent reddish orange emitting dopant.The power efficiency of the device with the exciplex-forming host is enhanced,achieving36.2%mainly owing to the lower operating voltage by the novel exciplex forming cohost,compared with the reference device(23.54 lm/W).Moreover,the fabricated OLEDs show extraordinarily low current efficiency(CE)roll-off up to 1.41%at the brightness from 500 to5000 cd/m² with a maximal CE of 32.87 cd/A(EQEmax=11.01%).