Studies On Performance Improvement In Top-emitting Organic Light-emitting Devices

Since top-emitting organic light-emitting devices(TEOLEDs) were introduced, it had attracted more and more attentions in virtue of its advantages on resolving the competition between pixel driving circuits and active emissive area in active matrix display and fabricating OLED displays with higher display quality without sacrificing aperture ratios of pixels. Besides the merits referred above, TEOLEDs on silicon can achieve monolithic integration of OLED displays on a silicon chip and are also suitable for making microdisplay on silicon. Top-emitting structures with double metallic electrodes is also an effective way to achieve deep blue light-emitting device with high efficiency and saturation because of microcavity effect which is capable of narrowing spectrum and amplifying intensity of light outcoupled. In order to be integrated on n-channel transistors which based on a-Si panel, inverted top-emitting organic light-emitting devices(ITEOLEDs) now are being a focus in the field of OLEDs.This paper is focus on how to achieve silicon-based blue top-emitting devices with high efficiency and color purity from the following aspects: the modification of metal anode to improve the hole-injection capacity, the optimization of effective cavity length, and the selection of methods to acquire devices with high saturation and efficiency. At last, we also discuss how to acquire a non-doped ITEOLEDs with high efficiencies.Because of mismatching between Femi energy level of Ag anode and the HOMO of organic hole-transporting layer, the TEOLEDs employing naked Ag as anode showed worse performance. In order to improve the hole-injection capacity of Ag, F4-TCNQ, which is a strong electron acceptor, was deposited on the surface of Ag anode, and a series of devices with sturctre Ag(45nm)/F4-TCNQ(Xnm)/m-MTDATA(45nm)/NPB(5nm)/Alq3(52nm)/LiF(1nm)/Al(1nm)/Ag(24nm) were fabricated to testify the effect of F4-TCNQ. By comparison, we found that the devices employing F4-TCNQ show better brightness at the same voltage, and the maximum current efficiency is enhanced to 7.0 cd/A. The improvement of the devices with MoO_x attributed to the dipole layer of which the positive dipole point to inside of Ag surface. The formation of such dipole layer is due to the different electronegatives of the contact materials which leads a redistribution of the electron density on the interface. To further testify our deduction, F4-TCNQ was replaced by MoO_x which also is an oxidant. The X-ray photoelectron spectroscopy (XPS) and Ultraviolet photoelectron spectroscopy (UPS) were also carried out for detailed analysis. By analyzing, we found the electron binding energy of Ag reduced when 3.8-nm-thick MoO_x was deposited on the top of Ag, and it is certain that the introduction of MoO_x change the surface energy level of Ag. Based on this, a series of devices Ag(45nm)/MoO_x(xnm)/m-MTDATA(45nm)/NPB(5nm)/Alq₃(52nm)/LiF(1nm)/Al(1nm)/Ag(24nm) were fabricated. The devices with MoO_x modification show excellent performance. The maximum brightness achieve 100000cd/m2 at 12 V, and the efficiency can achieve 8.8cd/A at 4V. By XPS analyzing, we found there is no reaction between Ag and MoO_x. According to this, we conclude that the cause of the formation of dipole layer when MoO_x was deposited on top of Ag is that the electrons tend to increase at MoO_x-side. The MoO_x shows similar effect for Al electrode. However, the essential reason is different. We found that there is a reaction between MoO_x and Al, and the dipole layer belongs to a kind of chemical dipoles.Because of the intrinsic characteristic, the emission of conventional blue device based on 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) shows worse saturation. In order to improve the saturation of blue device based on DPVBi, we employed microcavity effect presented in our TEOLED to narrow emissive spectra and amplify the intensity of the light outcoupled. In this paper, two different ways were adopted to modulate the effective cavity length. First, modulation of effective cavity length was acquired by adjusting the total thickness of organic layers sandwiched between electrode and anode. The change of thickness means change of conditions for choosing the mode of light emitted from the cavity. Devices with structures Ag(45nm)/Ag2O(uvo30s)/m-MTDATA(22.3nm)/NPB(5nm)/DPVBi(15nm)/ Alq3 (Xnm)/LiF(1nm)/Al(1nm)/Ag(24nm) were fabricated. When the thickness of Alq3, x, reduce from 37.5nm to 30nm, blue-shift of the emissive spectra occurs. The Commission Internationale de l'Eclairage coordinates (CIEx,y) of (0.14,0.08) is acquired when x is 35, moreover, the purity achieve 0.90. The maximum brightness of 11600cd/m² and efficienty of 0.96cd/A are acquired at 14V and 6V, respectively. The fabricating process of deep blue top-emitting device obtained by such way is very simply, but the decrease of total thickness of organic layers also leads to some other problems inevitably, such as: increase of leakage current, electrode quenching; In order to avoid the problem referred above for higher efficiencies, we propose a new way to adjust the effective cavity length. In such way, the phase shift adjustment layer (PSAL), which is capped on the top of Ag cathode, is employed firstly. The phase shift on reflection of top-electrode is relative to the thickness,refractive index of the capping layer. Accordingly, the effective cavity length can be adjusted by changing the thickness of PSAL. Based on the result simulated by computer, the devices with structures: Ag(45nm)/MoOx(1.7nm) /m-MTDATA(30nm)/NPB(5nm)/DPVBi(15nm)/Bphen(5nm)/Alq3(35nm)/LiF(1nm)/Al(1nm)/Ag(26nm)/Alq3(xnm) were fabricated. We found the peak of emissive spectra shift to 476nm when the thickness of Alq3 increase to 60nm, but the blue-shift doesn't occurs any more if we keep on increasing the thickness of Alq3. The CIEx,y of (0.13,0.15) belongs to deep blue region. The maximum current efficiency and brightness are 3.0cd/A and 20630cd/m², respectively.In comparison with the first way mentioned above, such way can resolve the problem deriving from reducing the thicknesses of the organic layers, and a deep blue top-emitting device with high efficiency and saturation can be acquired. Different depositing sequence lead to some new problems for carrier injection in inverted top-emitting organic light-emitting device, especially for electrodes. We found that it is ohmic contact between MoO_x and Ag by measuring the current-voltage characteristic of device: Ag/MoO_x(200nm)/Ag. In addition, we found that the hole injection capacity of MoO_x is not only better than that of Ag₂O but also independent of the deposition sequence according to the comparison of single-carrier devices: (1) Ag/MoO_x(5nm)/NPB(120nm)/MoO_x(5nm)/Ag;(2)Ag/Ag₂O(uvo30s)/NPB(120nm)/MoO_x(5nm)/Ag. MoO_x was used as a hole injection layer for enhance the hole-injection capacity of Ag top-anode in ITEOLED firstly. The device shows a low turn on voltage of 5.5 V without p-doping. The maximum brightness is enhanced to 7946cd/m² compared with the almost no emissions in the device without MoO_x. The insertion of MoO_x between Ag and organic layer also stop diffusion of Ag. To enhance the efficiency further, we substitute Mg:Al(9:1) for Al to reduce the injection barrier of electron. The current density of 100mA/cm2 was achieved at a lower voltage of 10V. Moreover, the reflectance of Mg:Al electrode was improved by employing Ag/Mg:Al compound electrode of which the reflectivity is comparative to that of Al. Depending on these improvement, the maximum brightness and efficiency achieve 47070 cd/m² at 18Vand 3.7cd/A at 15V, respectively.