| Organic semiconductors are a fascinating class of materials, with a wide range of properties. Electronic devices based on organic semiconductors, such as Organic Light-Emitting Diodes (OLEDs) and field-effect transistors (FETs) have attracted much interest as possible inexpensive and flexible alternatives to inorganic devices. In recent years, the investigation of organic light emitting diode based on small organic molecules has attracted growing interest, due to their attractive characteristics and potential applications to flat panel displays. The primary reason is that large numbers of organic materials are known to have high fluorescence quantum efficiencies in the visible spectrum. Hence, they are ideally suited for multicolor display applications.The light emission is the result of radiative decay of singlet excitons formed by recombination of charge carriers. However, not all charge carriers which recombine contribute to the generation of light:a part of the excitons may decay non-radiatively or may be quenched in the bulk or at interfaces. Because the ratio of singlet exciton formation to triplet exciton formation under electrical excitation is approximately 1:3 due to spin statistics, organic fluorescent emitters are limited to 25% internal quantum efficiency whereas phosphorescent emitters can in principle reach a quantum efficiency of 100% if the intersystem crossing process is efficient. In this regards, doping is an important technology for electronic imaging and optoelectronics devices based on molecular materials. Emissive doping is usually used for tuning emission colors and enhancing luminescence efficiency. Therefore, efficient organic devices could be demonstrated by using highly fluorescent dye molecules as the emissive dopant in the emitting layer of OLED that based on energy transfer from host to guest molecules. Hereby, the emission wavelength can be tuned in the desired way and the efficiency has been improved by optimization of the properties of the guest and host molecules. In order to achieve low driving voltage and high efficiency in OLED devices, it is necessary to facilitate the injection of charges. One effective approach to enhance carrier injection and transport is to conductively dope the organic layer. This conductive and control doping in the transport layers plays a crucial role for the OLED. The work performed in this dissertation is mainly based on the fabrication and characterization of various types of organic light emitting diodes, focusing on the improvement and charge transport phenomena.Here, red organic light-emitting devices were constructed that based on a wide band gap host emitting system of 9,10-di(2-naphthyl)anthracene (ADN) co-doped with 4-(dicyano-methylene)-2-t-butyle-6-(1,1,7,7-tetramethyl-julolidyl-9-enyl)-4H-pyran (DCJTB) as a red dopant and 2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,1 1H-10(2-benzothiazolyl)-quinolizine-[9,9a,lgh] coumarin (C545T) as an assistant dopant. The typical device structure was glass substrate/ITO/4,4',4"-tris(N-3-methylphenyl-N-phenylamino) triphenylamine(m-MTDATA)/N-bis-(naphthalene-1-yl)-N,N diphenylbenzidine(NPB)/[ADN:DCJTB:C545T/Alq3/LiF/Al]. It was found that C545T dopant did not emit by itself but did assist the energy transfer from the host (ADN) to the red emitting dopant. The red OLEDs realized by this approach not only enhanced the emission color, but also significantly improved the EL efficiency. The EL efficiency reached 3.5 cd/A at a current density of 20 mA cm-2, which is enhanced by three times compared with devices where the emissive layer is composed of the DCJTB, doped ADN. The saturated red emission was obtained with Commission International De L'Eclairage (CIE) coordinates of (x=0.618, y=0.373) at 620 nm, and the device driving voltage is decreased as much as 38%. We attribute these improvements to the assistant dopant (C545T), which leads to the more efficient energy transfer from ADN to DCJTB. These results indicate that the co-doped system is a promising method for obtaining high-efficiency red OLEDs.In this work, we demonstrated blue organic light-emitting devices based on wide band gap host material,2-(t-butyl)-9,10-di-(2-naphthyl)anthracene (TBADN), blue fluorescent styrylamine dopant, p-bis(p-N,N-diphenyl-amino-styryl)benzene (DSA-Ph) have been realized by using molybdenum oxide (MoO3) as a buffer layer and 4,7-diphenyl-1,10-phenanthroline(BPhen) as the ETL. The typical device structure used was glass substrate/ITO/MoO3(5 nm)/NPB(30 nm)/[TBADN:DSA-Ph(3wt%)](35 nm)/BPhen(12 nm)/LiF(0.8 nm)/Al(100 nm). It was found that the Cell-Mo//B-based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA cm-2, its driving voltage and power efficiency are 5.4 V and 4.7 Lm/W, respectively, which is independently reduced 46%, and improved 74% compared with those the m-MTDATA//Alq3 is based on, respectively. The J-V curves of'hole-only'devices reveal that a small hole injection barrier between MoO3//NPB leads to a strong hole injection, resulting low driving voltage and high power efficiency. The results strongly indicate that carrier injection ability and balance shows a key significance in OLED performance.Charge carriers in organics transport via hopping mechanism, which is quite different from that in inorganic crystalline semiconductors. This makes the charge carrier mobility in organics rather low and strongly dependent on electrical field and temperature, leading to the measurement of the mobility in organics much difficult. The conventional methods to determine the mobility in organics include surface charge decay, transient photocurrent or electroluminescence measurement, and time-of-flight measurement. However, all these methods require determination of transit time of charge carriers, which is complicated, and need intricate instruments. The current through organic devices is space charge limited and governed by the dc mobility. In principle, the information of charge carrier mobility should be reflected in their current-voltage characteristics, i.e., the mobility could be derived from processing or transforming their current-voltage characteristics. One method relating to this idea is space charge limited current (SCLC) technique.In this work, the electron mobility of 4,7-diphenyl-1,10-phenanthroline (BPhen) doped 8-hydroxyquinolinato-lithium (Liq) at various thicknesses (50-300 nm) has been estimated by using space-charge-limited currents measurements. It has been observed that the electron mobility of doped BPhen (33 wt.% Liq:BPhen) approaches its true value when the thickness is more than 200 nm. The electron mobility of 33 wt.% Liq: BPhen at 300 nm is found to be-5.2×10-3 cm2/(V·s) (at 0.3 MV/cm) with weak dependence on electric field, which is about one order of magnitude higher than that of pristine BPhen (3.4×10-4 cm2/(V·s)) measured by space-charge-limited currents. For thickness typical of organic light-emitting devices, the electron mobility of doped BPhen is also investigated. KeywordsOrganic Light Emitting Diodes, Co-doped, BPhen. Electron mobility, Space Charge Limited Current (SCLC),8-hydroxyquinolinonate-lithium (Liq)...
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