The Preparation Of Organic Electroluminescent Devices And Their Stability Study

Organic light-emitting diodes (OLEDs), which have been a very active researcharea in organic semiconductor field, are expected to be a major breakthrough recently.Many famous company and college of the world have concentrated on the study ofOLEDs. In this thesis, the stability of OLEDs and the fabrication of passive matrixOLED has been focused on. Mainly include the several aspects bellows:1. A novel system for measuring the stability of several OLEDs has beenpresented. In the proposed system, high precision IC operational amplifiers areadopted to achieve driving power supply, silicon photocells are used to capturebrightness of OLED, and an acquisition board based on PCI bus is used to obtain dataconcerning the variation of brightness and voltage of OLED. The measuring systemcan provide four different power sources: constant voltage, alternating voltage,constant current and hybrid current-voltage source (a constant current source forforward bias and constant voltage source for reverse bias). Silicon photocells replacetraditional brightness photometers, which substantially reduces costs. An acquisitionboard based on PCI bus is selected, which decreases development difficult andshortens development time. The measuring results indicate that this system iscompetent to measure luminance delay and voltage increase, and it is stable andreliable.2. We investigated the influence of pulse current (PC) versus direct current (DC)driving mode on the stability of three organic light-emitting diodes based onN′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) and tris(8-hydroxyquinoline)aluminum (Alq3) with different and without hole-injection layers(HILs). Two different HIL materials were used: copper phthalocyanine (CuPc) and4,4′,4′′-tris(3-methylphenylphenylamino)triphenylamine (m-MDTATA). The influenceof PC versus DC driving mode on the stability depends on the type of devices.Devices without HIL showed expected improvement in the device lifetime under PCdriving mode compared to DC driving mode. On the other hand, devices with the HILshowed minor improvement or even deterioration in the device lifetime under PC driving mode compared to DC driving mode. These results are attributed to differentratios of the injected electrons/holes in devices. Adjusting the ratio of the injectedelectrons/holes by inserting different HTLs or other methods, the electron/holebalance is improved. In this situation, the positive role of PC driving mode on thestability does diminish gradually; the negative role will be dominating.3. The stability under higher electric field was investigated. In order to estimatethese values in reasonable amount of time, a scalable coulombic degradation waswidely used to estimate the lifetime of OLEDs. The scalable coulombic degradation isrelation between initial luminance and half-life, and increasing initial luminance isaccepted to accelerating test for lifetime estimation. A high electric field was observedwhen an OLED working with high initial luminance, and it might introduce new otherdegradation mechanism. In this paper, we measured the luminance degradation andoperating temperatures of the OLEDs based on tris (8-hydroxyquinoline) aluminum(Alq3) with two hole transport layer with different glass transition temperature (Tg) atdifferent initial luminances. It was found that high initial luminance leads to higherdevice temperature (exceed the Tgof the organic materials), and then results in theinstability of organic materials (crystallization or other morphological changes) indevices, which is expected to invalidate the use of lifetime estimation by the scalablecoulombic degradation.4. Aone-dimension thermal model has been implemented according to the theoryof heat transfer. A temperature distribution in typical OLEDs was obtained based onthis model. We found the maximal temperature inside OLED was appeared the pointin emitting layer close the cathode layer. The joule heating was mainly eliminatedthrough ITO anode, and its temperature was smaller. By way of simulations, we verifythe thermal conductivity and length of each function layer have no impact on thethermal performance. Furthermore, we find through simulations that a significanttemperature gradient appears inside the OLEDs only when the thermal conductivity oforganic layer is very little (about10-4level), such as the discrepancy between themaximal temperature and the substrate is17K when input power is0.731W. Moreover,we find that the temperature distribution in typical OLEDs may significant changewhen the surface character (convection heat-transfer coefficient and surfaceemissivity) change. So, the increasing surface emissivity with substrate and cathodesurface improvement, and the increasing convection heat-transfer coefficient withforced convection may reduce OLED device temperature. 5. Amonochrome passive matrix OLED with a cathode separator method has beendesigned and fabricated. Common available commercial polyimide and negativephotoresist are used to produce cathode separator with overhang and undercut onindium tin oxide （ITO） glass substrate. The organic layers and the cathode areevaporated on this structure, and are discontinuous at the edge of the cathodeseparator. The passive matrix OLED with the cathode separator method has theadvantages of high resolution, simple process, less crosstalking problem and less cost.In this paper, integrated fabricated process has been explored and improved. Manyproblems and the solutions in fabricated process were investigated.