Studying On The Microcavity Effect For Improving Performances Of Blue And White Top-emitting Organic Light-emitting Diodes

Top-emitting organic light-emitting diodes(TEOLEDs) have attracted much attention in the field of micro-display due to their advantages of high resolution, high aperture ratio, and compatibility to any substrates. While the microcavity effect in TEOLEDs weakens the device performances on luminance and chromaticity, especially in the blue and white TEOLEDs. So studying the microcavity effect and designing the microcavity structure is very important to obtain high-performance blue and white TEOLEDs.In this thesis, a theoretical model on microcavity effect is improved based on the dipole theory and the transfer matrix theory. And then the microcavity effect in a blue TEOLED is studied theoretically and experimentally. The emission intensity and the chromaticity of the blue TEOLED are improved effectively by optimizing the device structure. On the basis of above results, white TEOLED structures are designed to enhance the output of blue light and increase the proportion of blue component in the white emission. The corresponding efficiency and chromaticity of the white TEOLEDs are improved due to the optimization on the device structure.The main work includes:(1) A classic electromagnetic theory about dipole radiation is used to study the exciton radiation pattern. The orientation of dipole moment and the polarization of electromagnetic wave are considered to improve the precision of the theory. The electromagnetic wave propagation in a multilayer system(e.g. TEOLED) is calculated by the transfer matrix theory. Based on the theoretical model containing the dipole radiation theory and transfer matrix theory, the influence on the TEOLED performances from the microcavity effect is studied. The microcavity resonance spectrum and the luminescence spectrum can be calculated, and then the color coordinates, the color temperature, and the color rendering index can be acquired.(2) Conventional blue TEOLEDs have poor performances because the microcavity effect in TEOLED suppresses the output of blue light. In order to improve the device performances, a metal/Ag dual layer cathode is introduced into the blue TEOLED to increase the electron injection and the light output. Firstly, the influence of the complex refractive index of the metal on the cathode reflectance is calculated with the microcavity model, and we find Sm/Ag dual layer cathode has the small reflectivity. Then, the thicknesses of Sm and Ag are changed to adjust the chromaticity of the blue TEOLEDs, and we find the thinner Ag and thicker Sm is facilitate to acquire pure blue light in TEOLED. A series of blue TEOLEDs with different Sm/Ag cathodes are fabricated to verify the theoretical results and optimize the device chromaticity. The optimized blue TEOLED has the similar spectrum with the blue emitter, realizing the good chromaticity. Besides, the spectra during the viewing angles of 0-75°are almost unchanged, demonstrating the good stability is acquired in the blue TEOLED.(3) The chromaticity of white TEOLEDs is redshift from the standard white light because the output of blue light is suppressed by the microcavity effect. In order to improve the chromaticity of white TEOLEDs, the resonance of multi-beam interference in the microcavity is adjusted to enhance the output of blue light. Firstly, resonance wavelengths in the TEOLEDs with different thicknesses of hole transport layer(NPB) are calculated, and the corresponding devices are fabricated. The theoretical and experimental results indicate that the output of blue light is enhanced in the white TEOLED with 100 nm NPB, and the balanced white emission is acquired. Then, a carrier/exciton blocking layer is inserted between blue and red EMLs to control the exciton recombination zone in the bipolar host material CBP. The color shift of white emission is decreased and the color stability of the white TEOLED is consequently improved.(4) Wide-angle interference and multi-beam interference in microcavity are analyzed separately to improve chromaticity and efficiency of the white TEOLEDs. The resonance intensities of wide-angle interference and multi-beam interference in TEOLEDs are calculated considering influence factors of electrodes and exciton locations. We find increasing the anode reflectance and decreasing the cathode reflectance help to weaken the multi-beam interference and strengthen the wide-angle interference, which is facilitate to enhance the output of blue light. Then the white TEOLEDs with high reflection anode and low reflection cathode are fabricated. Furtherly, the role of wide-angle interference on the performances of TEOLEDs is revealed through using ?-doping technology and comparing the spectra of white TEOLEDs with different blue and red EML positions. The blue light intensity significantly increases and the chromaticity of white TEOLEDs is further improved with the use of enhanced wide-angle interference. The white TEOLED exhibits the higher efficiency than the bottom-emitting device with the same organic layers, and shows the excellent chromaticity approaching to the standard white light.(5) In white TEOLEDs, a dual blue EMLs based on a heterojunction is introduced to constrain most of the excitons in the blue EMLs and strengthen the intensity of the blue emission. In order to acquire the pure and stable white emission, some key factors are analyzed to design the EML structure. Firstly, the carrier transport in EMLs is studied with the single-carrier devices. We find the blue guests serve as the hole transport channel and electron trap, while the red guests act as the electron transport channel and hole trap. The trapping effect of red guests contributes to the red emission at the low voltage. Then, the exciton distribution in SPPO1 is determined by analyzing the emission intensity of ?-doped devices, which guides the design of blue and red EML structure. The energy transfer between the blue and red emitters is investigated by inserting an interlayer. We find there exists weak energy transfer which helps to improve the stability of white light. Finally, the excellent color stability with CIE coordinate drift of only(0.009, 0.001) in the luminance range of 10~104 cd/m2 is obtained in our optimized white TEOLED. The white TEOLED also exhibits the high performances with the CIE coordinates of(0.33, 0.41) and a current efficiency of 13.3 cd/A. The principle of stability is analyzed deeply.