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Study Of The Properties And Mechanism Of Organic Thin Film Electroluminescent Devices

Posted on:2005-06-21Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y Z WuFull Text:PDF
GTID:1118360122496219Subject:Materials science
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Organic thin film electroluminescence (EL), as new research field, has been a hot subject in flat panel displays and attracted more and more people. Higher and higher technology for information display is required as a result of rapid development of information technology. Display panel with rich color, low cost, environmentally protection, lightweight, even flexibility, has become a goal of modern people. Organic thin film electroluminescence would be an ideal technology for information display of modern times due to its low driving voltage, highly efficient luminescence, rich color, fast response, wide viewing angle, portability, etc. However, organic electroluminescence is by itself a subject relate to chemistry, material, physics, electronics etc., and need people to investigate it from material design and synthesis, device fabrication and its performance measurement, luminescence process and mechanism, etc.In this dissertation, main study is on device structure, performance and luminescence mechanism etc. of organic light emitting diodes (OLEDs). Firstly, several blue OLEDs are investigated and highly efficient blue OLEDs with good color purity are obtained; secondly, electron injection materials and its effect on the OLED performance are systematically investigated, and a proper model is in an attempt to explain the underlying mechanism; thirdly, highly efficient red OLEDs with high color purity are fabricated based on phosphorescent material and luminescent mechanism is studied.(1) In the study of blue OLEDs, firstly, device using lithium tetra-(8-hydroxy-quinolinato) boron (LiBq4) as an active layer was prepared and effects of electron transporting layer tris-(8-hydroxy-quinolinato) aluminum (Alq) on EL spectra and current-voltage characteristics are investigated. Experimental results show that operating voltage of the device may be lowered without emergence of luminescence of Alq as the thickness of Alq equals 5 nm. The mechanism of Alq's effect on the device performance is also discussed. The following experiment is fabrication of blue OLEDs with good color purity using l,4-bis[2-(3-N-ethylcarbazoryl)vinyl]benzene (BCzVB) doped into 4,4'-N,N'-dicarbazole-biphyenyl (CBP). Maximum luminance and externalefficiency are 8500 cd/m2 and 2.6%. Commission Internationale de 1'Eclairage (CIE) co-ordinates are x=0.15, y=0.16. These values are further improved by inserting bis(2-methy]-8-quinolinato)4-phenylphenolate aluminum (BAlq) between CBP:BCzVB and Alq layers to be 11000 cd/m2, and 3.3%, with little variance of CIE co-ordinates. The analysis of photoluminescence spectra reveals that the emission originates from energy transfer and carrier trapping, while the latter dominates. Finally, single hetero structure blue OLED using TPD as a hole transporting layer, DPVBi as an emissive layer is realized and compared with the conventional device using Alq as an electron transporting layer. Maximum external efficiency and the CIE co-ordinates are 1.5%, x=0.16, y=0.16, respectively.(2) In the study of electron injection material, composite cathodes using organic material, 8-hydroxy-quinolinato lithium (Liq) and inorganic material, LiF and stable metal Al, were used in Alq based devices, and effects of the thicknesses of Liq and LiF on the luminescent efficiency and current-voltage characteristics of the.device are investigated. Experimental results show that the device performance is optimal when the thickness of Liq or LiF equals about 0.5 nm, and the device performance of 0.5-nm-thick Liq device is similar to that of 0.5-nm-thick LiF device, but their luminance and efficiency is several times higher than that of the device with an Al only cathode. As the thickness of Liq or LiF layer increases, the current density-voltage curves shift to higher voltage range and the efficiencies are lowed in both cases, resulting in deterioration of the device performance. But performance of the device using Liq as an electron injection layer is less sensitive in efficiency to Liq thickness than that using LiF. A dipole model is...
Keywords/Search Tags:organic thin film electroluminescence, luminescent device, electron injection, dipole layer, blue, red, color purity, phosphorescence, luminescent efficiency, energy transfer, carrier trapping
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