The Study Of Top-emitting Organic Light-emitting Devices Based On Microcavity Structure And Silicon

Organic light-emitting devices (OLEDs) are considered to hold the most brilliant promise of next generation of flat-panel displays due to their high luminous efficiency, low driving voltage, a broad range of colors, thinness and portability. In comparison with the conventional bottom emitting OLED (BOLED), top-emitting OLED (TOLED) paves a feasible way for realizing high-resolution large-size full-color OLED-based displays on Si thin-film-transistor (TFT) substrates or active-matrix backplanes with complicated pixel circuits and thereby drawing great attention of researchers. In this dissertation, we predominantly focused on the fundamental researches such as OLED theory, performance improvement, and some applications to realizing OLED displays. The results are listed as follows:(1) For BOLED, we mainly investigated the enhancement of electron-injection ability and the calculation of electron-injection barrier-height, owing to the fact that the injection and transport ability of electrons is inferior to holes in most OLEDs. Firstly, we studied the improvement of OLED performance by using a composite electron-injection layer (c-EIL) of Liq/CsO_x. The efficiency of device using c-EIL was enhanced by ³0%. The enhancement of device efficiency was further verified by'Electron-only'devices and explained by dipole effect and step-barrier theory. Then, the chromaticity and electron-injection ability of blue OLEDs were significantly improved by using a dual electron-transport layer (d-ETL, e.g., Bpy-OXD/Alq₃ or Bpy-OXD/BPhen). This can be attributed to the hole-blocking function of Bpy-OXD which confines the carriers within the emitting layer (EML) and the step-barrier provided by the d-ETL which promotes carrier injection. Lastly, the electron-injection barrier-height of"metal/organic"interface (i.e., between the Al cathode and the most commonly used ETLs of Alq₃ and BPhen) was calculated by using"current-voltage (I-V) characteristics". The barrier height of 0.66 eV for Alq₃/Al and that of 0.83 eV for BPhen/Al were estimated. While the barrier height of 0.1 eV for Alq₃/LiF/Al and that of 0.098 eV for BPhen/CsO_x/Al were derived.(2) For TOLEDs with microcavity structure, we systematically investigated the microcavity effect and its effect on device performance. We first demonstrated efficient blue TOLEDs with single-mode resonant emission and low voltage by using [TBADN:DSA-Ph] as EML. The chromaticity can be adjusted from deep blue with CIE color coordinates of (0.15, 0.08) to green emission with CIE color coordinates of (0.17, 0.57) by altering the thickness of hole-transport layer. The device efficiency can be enhanced by 60% with the deposition of C₆₀ index-matching layer over the semitransparent cathode. The transmittance and reflectance of top contact through which the light is outcoupled was calculated by using a transfer matrix method, the results indicated that the optimal performance of blue TOLED was obtained in between the maximum and minimum transmittance of top contact as a result of the trade-off between wide-angle interference and multiple-beam interference within the cavity. The device efficiency was enhanced by 50% and the CIE color coordinates were negligibly affected by using a dual EML of [TBADN:DSA-Ph]/[Alq₃:DSA-Ph]. The improved performance of device with dual EML was attributed to the energy transfer from Alq₃ to DSA-Ph and DSA-Ph directly harvesting carriers in the EML of [Alq₃:DSA-Ph]. Then, highly efficient microcavity TOLED based on Ag anode and Ag semitransparent cathode was demonstrated. With Alq₃ as EML, the device showed a maximum luminous efficiency of 9.21 cd/A which is 2-3 times higher than those of the corresponding TOLED with Al/Ag semitransparent cathode and BOLED. This can be attributed to the significant microcavity effect and efficient carrier injection from Ag electrode. Lastly, fluorescent red OLEDs with narrow emission and negligible current-induced quenching by using PDT-doped emitting system of [Alq₃:PDT:rubrene] were demonstrated. With the incorporation of C₆₀ outcoupling layer, the TOLED exhibited excellent red emission with luminous efficiency of ³ cd/A and CIE color coordinates of (0.64, 0.36). The F?rster's radius in the EML of [Alq₃:PDT:rubrene] was calculated, and the results indicated that the energy transfer process is predominantly from the host of Alq₃ to the guest of PDT via the intermediation of rubrene.(3) For Si-based TOLEDs, we mainly focused on the performance improvement and related theory. Firstly, MoO_x is proven to be more efficient than SiO₂ in improving device performance, the efficiencies of p-Si/MoO_x device are almost double those of p-Si/SiO₂ device. Moreover, in comparison with the thermally-grown SiO₂ buffer layer, MoO_x can be deposited by conventional evaporation technology under vacuum conditions, which simplifies the fabrication process. Secondly, Si-based TOLED with EML of [TPBA:TPA] was demonstrated possessing superior performance, the luminous and power efficiencies were achieved 3.3 cd/A and 2.3 lm/W, respectively, and the maximum luminance was reached 1.3×104 cd/m2 @12 V. This can be attributed to the efficient energy transfer from the host of TPBA to the guest of TPA, highly fluorescent emission of TPA, and the optimization of device structure. In comparison with the TOLED using conventional Ag anode, the Si-based TOLED shows negligible microcavity effect and giant enhancement of pixel contrast ratio (PCR) as a result of low reflectance of Si anode. The experimetnal results were theoretically analyzed in detail. Lastly, efficient TOLEDs with low-cost Cu anode were investigated. The MoO_x modification can considerably enhance the work function of Cu anode, which accounts for the performance improvement. The TOLED with Cu anode is superior to the counterpart with Ag anode in several respects such as higher PCR, weak microcavity effect and lower leakage current.