Study On Top-emitting Organic Light-emitting Devices Based On Silicon

Organic light-emitting devices (OLEDs) had been attracting much attentionfor their potential used in flat display panel. Since Tang first reported onOLEDS with high luminance and low turn on voltage, OLEDs had developedrapidly on materials and structures. And now, OLEDs are on the way toindustrialization. Top-emitting organic light-emitting devices (TOLEDs) basedon silicon now are being a focus in the filed of OLED, since they are capableof achieving monolithic integration of an OLED displays on a silicon chip,fabricating OLED displays for higher display quality yet without sacrificingaperture ratios of pixels and suitable for making microdisplay on silicon. Thispaper is focus on how to make a TOLED based on silicon with high brightnessand efficiency at low voltage through reasonable design of device structures.TOLED based on silicon with good performances should firstly satisfiesthe demand of low turn-on voltage. In the fabricating of conventional OLEDS,the deposition of materials which have lower Ionization Potential than that ofhole-transporting materials (NPB) between ITO anode and NPB ashole-injection layer is an effective way to enhance the brightness at lowvoltage. We used m-MTDATA as hole injection layer in our TOLED for itsexcellent uniformity, Shallower hole trap level, high stability in air. Secondly,a stable semi-transparent cathode is crucial for TOLED based on silicon.LiF/Al is usually used as an effective cathode for its high electron injectionability in OLED. LiF, Alq, and Al take part in the chemical reaction at theAlq/Lif/Al interface, as release Li atom. The released atoms react with Alqmolecules to form Alq anion, which faciliates the injection of electrons. Thethickness of Al is on the order of one nanometer or less, as is enough for thereaction between Lif, Alq, and Al. Under our experimental conditions, Al withthe thickness of two nanometers is enough to ensure the electron injection.Moreover, Ag is stable in air, which has highest conductivity and, minimalabsorption in the visible-light region. Therefore, we adopted LiF (1nm)/Al(2nm)/Ag (20nm) as a semi-transparent cathode for TOLED, which ensuredthe injection of electron and the conductivity and transmission of the cathode.Thirdly, the high reflectivity of bottom anode is essential for achieving highefficiency in TOLED based on silicon. Although Ag has highest reflectivityamong metals, it is not considered as an ideal anode for TOLED, because of itslow workfunction (-4.3eV). However, Ag2O, reported as P-type semiconductor,whose energy level is matches with the HOMO level of NPB has the band gapof 1.3eV, and the ionization potential of -5.3eV. UV-ozone treatment is asimple way to induce thin Ag2O film on Ag surface. Therefore, we usedsurface-modified Ag as anode and LiF/Al/Ag as cathode to prepare TOLEDbased on silicon using Alq as emitting layer. The luminance of the device at4V is 10cd/m2, and reaches 14090cd/m2 at 14V. The highest efficiency is1.7cd/A at 8V.The performance of the device is superior to that of thecontrolled device that used Ag as anode, as is owing to the good matchingbetween energy level of Ag2O and the HOMO level of m-MTDATA. The holeinjection is enhanced because of the existence of Ag2O film, as facilitated theinjection of electron. As a result, the luminance and efficiency are improved.However, organic materials were evaporated in full area when we preparedthe TOLED, which induced the high leakage current and resulted in lowefficiency of TOLED using rubrene sub-monolayer as emitting layer. Weresolved this trouble by adopting new shadow-mask technology whichpermitted the organic materials were deposited on the certain area and used thenew shadow-mask technology to prepare TOLED with double heterostructurewhich used QAD:Alq as emitting layer. The luminance and the efficiency ofthe device at 4V are 328cd/m2 and 4.5cd/A, respectively. The superiorperformance of the device is the result of high probability of excitonrecombination and the avoidance of high leakage current bring on byheterostructure and new shadow-mask technology.The microcavity exits in TOLED because the emitting layer of device islocated between two metal mirrors with certain reflectivity. We observed themore obvious microcavity by increasing the thickness of the bottom anode.The width at half maximum (FWHM) of electroluminescence spectra ofTOLED is only 44 nanometer, while that of electroluminescence spectra ofconventional OLED with the same structure as TOLED is 108 nanometer. Wecalculated the emission spectra of the TOLED. Compared with the testedspectra of the TOLED, the (FWHM) of the calculated spectra is 48 nanometer.The peak of the measured spectra and the calculated spectra are 512nanometerand 520 nanometer, respectively. The minor difference is the result ofsimplification in our calculation. In our calculation, we assumed the excitonsrecombined at the same place, which one diffuse length away from theNPB/Alq interface. In fact, the diffuse length is an average value and theexcitons recombined at different place within the emitting layer.The deposition of Alq on the semi-transparent cathode as anti-reflectivelayer is an effective way to improve the efficiency of the TOLED. Alq ischosen as anti-reflective layer for the merit of easier evaporation. Althoughthere are many methods to calculate the transmission of a series films, transfermatrix is usually used to calculate the transmission of a series of films, owingto it contains all the useful parametesr needed in calculating the opticalcharacteristics of films. Therefore, we analyzed the transmission of films ofAlq/Ag (20nm)/Alq (dnm)/air based on transfer matrix formalism. Thesimulating results revealed that when the thickness of Alq is 45nanometers thetransmission of films is 65% at 530 nanometer wavelength, compared with37% when there is no Alq deposited on Ag cathode. These calculated resultsare useful in improving the performance of TOLED by the way of enhancingthe transmission of semi-transparent cathode.Based on the TOLED, we also developed passive matrix TOLED displayon silicon. The row of cathode is realized by insulating columns fabricated byphotoresist. The pixel number is 32×24, and the resolution is6pixels/millimeter. We also designed the driving circuit.