Study On Modifying The Electrode Of Top-emitting Organic Light-emitting Devices

Since C.W. Tang has reported high brightness organic light-emitting devices (OLEDs) with low operating voltage for the first time in 1987, OLEDs have received more and more attentions. As a new flat panel display technology, OLEDs have many merits such as light weight, thin thickness, low cost, fast response speed, active emission, low energy consume, high brightness and efficiency, broad operating temperature, more choice of materials, availability for full color display and flexible display, etc. Up to now, various structure have been introduced to satisfy the commercial application. Top-emitting organic light-emitting devices (TOLEDs) now are being a focus in the field of OLED, since they are capable of resolving the competition problem between driving circuits and active emissive area, fabricating OLED displays with higher display quality yet without sacrificing aperture ratios of pixels. Besides the merits referred above, TOLEDs on silicon can achieve monolithic integration of OLED displays on a silicon chip and are also suitable for making microdisplay on silicon.In this paper, transition metal oxide was used to improve the hole-injection capacity of anode of top-emitting device. As an wide-gap semiconductor, MoO_x is a wide-gap semiconductor with a band gap of 3.1 eV, and an electron affinity of around -2.2 eV, which implied a valence band at around -5.3 eV. Compared with Ag (-4.3 ev), the valence band match the HOMO energy level of m-MTDATA better, and then the hole-injection capacity of anode wasimproved. Because of the low resistivity of 1.2×10^-2Ω·cm, the hole-injection capacity of MoO_x is independent with the change of thickness in a certain range which result in better controllability and repetitiveness. Because the organic layers of top-emitting device are sandwiched by two metal electrodes with certain reflectivity and the magnitude of total thickness is nanometer, there is strong microcavity effect in the top-emitting device. As we known, the optic microcavity is selectivity for the wavelength of light output. If the intrinsic EL spectrum of luminous material match the resonant wavelength of the cavity, the light output will be enhanced, otherwise, it will be weaken. To enhance the intrinsic EL spectrum of Alq3, the effective length of cavity composed of two electrodes should satisfy the phase matching condition.An optimized total thickness of 104.4 nm of organic thickness is acquired through calculation. In experiment, the device was optimized through adjusting the thickness of Alq3 simply, and the result of experiment is accord with that of calculation. At last, the turn-on voltage of device descends to 2.5 V, and a high efficiency of 8.9 cd/A is acquired at 4 V. The maximum efficiency of 11.7 cd/A and brightness of 99100 cd/m2 is acquired at 7 V and 12 V, respectively.The anode of conventional top-emitting device is deposited on the substrate, and the cathode is fabricated as transparent and semi-transparent electrode for the output of light. But for better integration on a-Si TFT, the structure of conventional top-emitting device should be inverted, i.e inverted top-emitting organic light-emitting device. But the reverse depositing sequence leads to a badly electron injection from bottom electrode, and difficulty for anode fabrication. Kim. et. al. reported the Mg:Al electrode acquired by co-deposition of Mg and Al has a low work function and melting temperature compared with pure Al electrode. We also adopt Mg:Al electrode in this paper, and a thin LiF insulating layer is inserted between electrode and the electron-transport layer to enhance the electron injection further attribute totunnelling. There are usually two kinds of anode in ITEOLEDs: One is ITO or IZO transparent anode fabricated by magnetron sputtering-deposition; Another is metal semitranspatent anode. In consideration of the radiate damage brought to the organic layer deposited on the substrate, the Ag semitransparent anode is adopted in our experiment. But the mismatch between Ag and m-MTDATA results in a bad injection. Through the comparison of two different single-carrier devices: Ag/MoO_x/NPB/MoO_x/Ag; Ag/Ag₂O/NPB/MoO_x/Ag, it shown that the MoO_x has better hole-injection capacity than that of Ag2O.The performance of ITEOLEDs is improved distinctly through using the MoO_x as hole-injection layer and protection layer at last. The turn-on voltage is as low as 5.5 V, and the Maximum efficiency and luminance is 3.7 cd/A (15 V) and 47000 cd/m² (18 V) respectively.