Study On Performance Improvement In Top-emitting Organic Light-emitting Devices Based On Silicon

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. Top-emitting organic light-emitting devices (TEOLEDs) now are being a focus in the field of OLEDs, since they are capable of resolving the competition problem between pixel 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, TEOLEDs on silicon can achieve monolithic integration of OLED displays on a silicon chip and are also suitable for making microdisplay on silicon. This paper is focus on how to improve performances of silicon-based TEOLEDs from the following aspects: the optimization of the semitransparent cathode, the out-coupling layer, the suitable electron buffer layer, respectively, and the chioce of an emissive material with a large electroluminescence (EL) intensity and a high efficiency; design of the total thickness of organic layers and the out-coupling layer with microcavity theory and transfer matrix theory, respectively.Considering the difference of top cathode transparency in TEOLEDs, the research work mainly includes two kinds of TEOLEDs, in which one kind is based on transparent indium-tin oxide (ITO) or indium-zinc oxide (IZO) cathode and the other is based on semitransparent metal cathode. Though with a high transparence in the visible region, the normal ITO is acquired with a sputtering process, leading to the destruction to the bottom organic materials. The metal cathode with high transmission in the visible range, good electron injection, low resistivity, and good stability in environment is welcome since its easy deposition without damage. Multilayer AlAg cathode, instead of the normal Al/Ag bilayer, is designed and optimized with an improved transmission. With an additional 78 nm-thick tris-(8-hydroxyquinoline) aluminum (Alq₃) as the capping layer, the transmissivity of the cathode can be further enhanced to 55.3% at 550 nm and a transmissive spectra is obtained with a large full width at half maximum (FWHM) in the visible area, which is beneficial to the fabrication of white TEOLEDs. A TEOLED with the architechture of Si/SiO₂(1600nm)/Ag(50nm)/Ag₂O/m- MTDATA(40nm)/NPB(10nm)/DPVBi(30nm)/CBP:Ir(ppy)₃:Ir(piq)₂(acac)(30nm,1:5%:5%)/BCP(10nm)/Alq₃(20nm)/LiF(1nm)/[Al(2.5nm)/Ag(0.5nm)/Al(1.0nm)/Ag(0.5nm)/Al(0.5nm)/Ag(11nm)]/Alq₃(78nm) is fabricated with white emission. The maximum brightness of 39312 cd/m², arriving at 23 V, is enhanced with a factor of 1.9-3.1 from 7 to 21 V compared with that in the bilayer AlAg device without Alq₃-capping. The maximum luminous efficiency is 2.64 cd/A obtained at 13 V. CIE coordinates lie in the white emission region, changing from (0.457,0.368)(7 V) to (0.357,0.355)(23 V). The TEOLED with Alq₃ as the out-put layer is acquired with the relatively good white emission since white light from TEOLEDs with a capping layer is not easily obtained.In OLEDs, electrons are shown as the minority carrier since the electron injection barrier is much larger than that of holes. The enhancement of electron injection from the cathode can reduce the imbalance between holes and electrons and make more carriers recombination in emission area, which is another approach to improve OLED performances. In our paper, we acquire n-type ZnO as the cathode buffer layer with thermal deposition and fabricate TEOLEDs with structures of Si/SiO₂(1600nm)/Ag(100nm)/Ag₂O/m-MTDATA(45nm)/NPB(25 nm)/Alq₃(60nm)/ZnO/Al(4nm)/Ag(12nm), where the ZnO layer can be adjusted to an optimum thickness. At the same time, TEOLEDs without or with LiF buffer (1 nm or 0.5 nm) are compared to those with different ZnO buffer layers. As a result, the TEOLED with 3.5 nm ZnO shows the best EL performance in all devices with a low turn-on voltage of 3.1V, a maximum brightness of 41790 cd/m² (22V) and a high efficiency of 3.1 cd/A. We also analyze how this thin ZnO layer improves electron injection ability by x-ray photoelectron spectroscopy (XPS) analyse with the explanation as follows: The thermal deposition of ZnO onto Alq₃ layer leads to the splitting of N1s spectra in Alq₃ and a more than 1eV Zn3d spectra shift to the lower energy which indicate the formation of Zn-N chemical bond, resulting in the occurrence of Alq₃ cation. Some new energy levels, shown as the characteristics of the lowest unoccupied molecular orbit, are generated in the forbidden band of Alq₃, which are helpful to the electron injection. Besides, the highest occupied molecular orbit of ZnO is about 2eV higher than that of Alq₃ and it effectively block electron injection into the cathode, resulting in more carrier recombination in the emissive area and an enhanced EL performance.The light output is determined with the refractive index of the material onto the semitransparent cathode. A higher refractive index as the capping layer is, the better light output is. Here, zinc sulfide (ZnS) is utilized as the outcoupling layer in our TEOLEDs because of the advantages of a high refractive index about 2.4, easily thermal evaporation, and a low absorption in the visible region. With a high EL performance as the target, the total organic thickness, the cathode AlAg thickness, and the ZnS capping layer are optimized, respectively. As a result, the transmissivity of cathode reaches 80% at 530 nm with a 32 nm-thick ZnS capping. The TEOLED is fabricated with configuration of Si/SiO₂/Ag(100nm) /Ag₂O/m-MTDATA(45nm)/NPB(7nm)/Alq₃(50nm)/LiF(1nm)/Al(0.3nm)/Ag(18nm)/ZnS(32nm), in which the maximum luminance and current efficiency based on the Alq₃ emission reach 145475 cd/m²(13 V)and 12.2 cd/A (5 V), respectively. The leakage currents in TEOLEDs with ZnS output layer decrease to about 10-3 mA/cm², which can be explained as follows: Because the evaporation of ZnS crystal granules requires more heat energy and longer time than that of Al or Ag, strong heat radiation from Mo boat to those TEOLEDs during ZnS deposition process, which corresponds to an annealing process, results in an improved order of organic materials and, furthermore, in the reduction of leakage currents. In addition, the annealing process to the emissive Alq₃ leads to the occurrence of facial Alq₃ from isomer meridianal Alq₃, which shows 12–16 nm blue shift of EL spectra. Moreover, the introduction of ZnS output layer leads to a lower phase shift of Ag/ZnS, which causes blue shift of EL spectra.To get a large EL intensity and a high luminous efficiency in TEOLED which satisfy the requirement of silicon-based MOS driving circuits, a green fluorescent material with good emissive charcteristics is needed. Here, the 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]-pyrano[6,7,8-ij]quinolizin-11-one (C545T) is utilized in our TEOLED as the green emission with structure of Si/SiO₂/Ag(100nm)/Ag₂O/m-MTDATA(45nm)/ NPB(5 nm)/Alq₃:C545T(20 nm, 1:0.5%)/Alq₃(30 nm)/LiF(1 nm)/Al(0.5 nm)/ Ag (30 nm), where the total organic thickness is calculated with microcavity theory, leading to the resonant wavelength in the device consistent with the peak of C545T. As a result, a C545T -based TEOLED with a low turn-on voltage of only 2.5 V, a high EL intensity of 80215 cd/m² at 9 V, and a maximum current efficiency of 32.7 cd/A or an external quantum efficiency of 8.8% is fabricated with a magnified coefficient of 15.6. Also, at the low voltage of 4 V, this green emission has possession of a high luminance and a large efficiency, shown as 487 cd/m² and 27.8 cd/A, respectively, which satisfy the requirement of Si-based MOS driving circuits.