| In recent years, the phosphorescent organic light-emitting diodes (PHOLEDs), employing phosphors as emitters, have attracted great attention due to their potential to achieve nearly unity internal quantum efficiency by utilizing both single and triplet excitons for emission. The researches on PHOLEDs almost focus on how to effectively improve the luminous efficiency, suppress the efficiency roll-off at high current density, and prolong the operational lifetime of the device. In order to achieve a high efficiency PHOLED, it is necessary to enhance the charge carrier injection/transport abilities and build a balanced charge carrier transport system for reducing the leakage current and increasing the probability of exciton formation. Besides, it is also very important to limit the long lived triplet exciton in intended recombination zone. So a typical design of efficient OLEDs is the introduction of heterojunctions (HJ) with matched energy level to achieve high efficiency. However, the heterojunction interface could generally limit the device reliability and result in high driving voltage. The accumulated charges in these interfaces are prone to form chemically unstable cationic species which would accelerate the formation of nonradiative trap centers and the device degradation. Moreover, the high working voltage of the HJ-OLED could lead to high Joule heating, which would deteriorate the organic materials. In order to prolong the operational lifetime of the devices, the sharp heterojunction interface should be eliminated to prevent the charge accumulation in these interfaces and reduce the driving voltage. Such PHOLEDs could, however, not control effectively the charge carrier transport, reducing the probability of carrier recombination and resulting in exciton quenching because of exciton diffusion into the carrier transport layer. In this thesis, based on a typical multi-heterojunction electroluminescent device A with the structure of ITO/NPB/CBP:Ir(ppy)3/Bphen/Alq3/LiF/Al, we carried a series of design and optimization on its carrier transport layer and emitting layer (EML) so as to improve simultaneously the efficiency and operational lifetime of the devices. The main works in our study are accomplished as follows:(1). Finding a suitable organic material severed as both electron transport layer(ETL) and hole/exciton blocking layer(HBL) in place of Alq3, eliminating the HBL/ETL interface to improve device performance. So this material should possess high electron mobility and could prevent the hole/exciton to penetrate into ETL. Based on this point, we chose some common hole blocking materials including BCP, TPBi, BAlq and Bphen by reviewing a large number of literatures. In order to compare their electron transport ability, we designed a serial of single carrier devices based on these materials. As a result Bphen demonstrated the greatest property. So Bphen was eventually used ETL and the first optimized device B with the structure of ITO/NPB/CBP:Ir(ppy)3/Bphen/LiF/Al were fabricated. This device harvested the maximum luminance of50002cd/m2, peak current efficiency of25.0cd/A corresponding to peak power efficiency of10.6lm/W. The efficiency of device B was not improved substantially compared with device A, but luminance and operation lifetime of the device were enhanced effectively. The lifetime of device with encapsulation was measured under constant current at the initial brightness of500cd/m2.The lifetime of device B is340h, a remarkable increasing compared with76h of the referential device A.(2). Searching an excellent P-CBP severed as hole transport layer in replace of NPB on basis of the device B, eliminating the HTL/EML interface to further improve device performance. We studied the hole transport property of MoO3doped CBP and FeCl3doped CBP with different weight doping concentration. As a consequence CBP:MoO3(15%) demonstrated the best hole transport property, so this system was eventually used as HTL. The second optimized device C with the structure of ITO/CBP:MoO3(15%)/CBP/CBP:Ir(ppy)3/Bphen/LiF/Al is manufactured. The performance of this device was improved effectively due to eliminate the HTL/EML interface and enhance hole transport ability. It harvest the maximum luminance of69000cd/m2, peak current efficiency of29.2cd/A corresponding to peak power efficiency of17.6lm/W, which is about1.3and1.7times that of the referential device A. Besides, the lifetime of device C is prolonged to836h, which is higher11times over the device A.(3). Exploring an excellent N-Bphen severed as electorn transport layer on basis of the device C, to enhance further the electron transport ability and construct the balanced hole and electron transport system and then improve device performance further. We studied the electron transport property of the doped Bphen by Alq3, LiF and CsF under different weight doping concentration respectively. The results demonstrated that the electron transport property of the doped Bphen system became worse when utilizing Alq3or LiF as dopant. In contrast, the electron transport ability of the doped Bphen employing CsF as dopant could be enhanced effectively. Especially, the electron transport ability of the CsF doped Bphen and the hole transport ability of CBP:MoO3(15%) was perfectly balanced when the doping ratio is33%. So Bphen:CsF(33%) was eventually used as ETL and then the third optimized device D with the structure of ITO/CBP:MoO3(15%)/CBP/CBP:Ir(ppy)3/Bphen:CsF(33%)/LiF/Al was fabricated. It harvest the maximum luminance of109004cd/m2, peak current efficiency of40.9cd/A corresponding to peak power efficiency of32.1lm/W, which is about1.8and3times that of the referential device A. Besides, the lifetime of device C is prolonged to1184h, which is about16times of that of the device A.(4). Introducing double emitting layer and mixed host into EML structure on basis of the device C, regulating effectively carriers transport and recombination to improve device performance. We fabricated a double emitting layer device E utilizing CBP and Bphen as double host, with the structure of ITO/CBP:MoO3(15%)/CBP/CBP:Ir(ppy)3(20nm)/Bphen:Ir(ppy)3(10nm)/LiF/Al. This device obtained the peak current efficiency of32.5cd/A corresponding to peak power efficiency of32.7lm/W. And the lifetime of device E was2196.5h, which is about2.5times that of the single heterojunction device C. Subsequently, we fabricated a mixed host emitting layer device F, mixing uniformly Bphen into CBP as emitting host, with no heterojuction structure of ITO/CBP:MoO3(15%)/CBP/[50%CBP:Bphen]:6%Ir(ppy)3/Bphen(40nm)/LiF/Al. This device achieved the peak current efficiency of39.8cd/A corresponding to peak power efficiency of41.6lm/W. And the lifetime of device F was eventually prolonged to3674h, which is about4times of the device C. The performance of the device were improved further by adopting double emitting layer and mixed host structure to optimize the emitting layer. |
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