Characteristics of indium zinc oxide films deposited using the facing targets sputtering method for OLEDs applications

The amorphous indium zinc oxide (IZO) thin films were deposited on polyethersulfone (PES) and glass substrates using the facing targets sputtering (FTS) system. The electrical, optical and structural properties of the IZO thin films deposited as functions of sputtering parameters on the glass and PES substrates. An optimal IZO deposition condition is fabricated for organic light-emitting device (OLED) based on glass and PES. The amorphous IZO anode-based OLEDs show superior current density and luminance characteristics.     

Anode material properties of Ga-doped ZnO thin films by pulsed DC magnetron sputtering method for organic light emitting diodes

This study examined the anode material properties of Ga-doped zinc oxide (GZO) thin films deposited by pulsed DC magnetron sputtering along with the device performance of organic light emitting diodes (OLEDs) using GZO as the anode. The structure and electrical properties of the deposited films were examined as a function of the substrate temperature. The electrical properties of the GZO film deposited at 200 °C showed the best properties, such as a low resistivity, high mobility and high work function of 5.3 × 10^− 4Ω cm, 9.9 cm²/Vs and 4.37 eV, respectively. The OLED characteristics with the GZO film deposited under the optimum conditions showed good brightness > 10,000 cd/m². These results suggest that GZO films can be used as the anode in OLEDs, and a lower deposition temperature of 200 °C is suitable for flexible devices.     

Solution-processed small molecule thin films and their light-emitting devices

Thin films of N,N′-bis-(3-Naphthyl)-N,N′-biphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), tris-(8-hydroxyquinoline)-aluminum (Alq₃) and their blends prepared by spin-coating process were investigated. Experimental results revealed that the NPB films prepared by spin-coating process have smoother surface than that of Alq₃, which was attributed to their different molecular structures. Organic light-emitting devices (OLEDs) with emitting layer prepared by spin-coating the blends of NPB and Alq₃ exhibited a maximum luminance and a current efficiency over 10,000 cd/m² and 3.8 cd/A respectively, and when 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-[l]benzopyrano[6,7,8-ij]quinolizin-11-one was doped in, a current efficiency of 8 cd/A can be obtained. Comparative device performance to the vapor-deposited OLEDs suggested that solution-process could be an alternative route for the fabrication of OLEDs based on Alq₃.     

Morphology and growth of e-beam deposited YSZ thin films

Yttria-stabilized zirconium (YSZ) thin films were grown from the tetragonal phase of ZrO₂ stabilized by 8 wt% of Y₂O₃ (8% of YSZ) ceramic powders using e-beam deposition technique (EB-PVD). The influence of the type of substrate on the microstructure of deposited YSZ thin films was analysed. YSZ thin films (2-3 μm of thickness) were deposited on three different types of substrates: optical quartz (SiO₂), porous Ni-YSZ substrates and Alloy 600 (Fe-Ni-Cr). The dependence of the substrate temperature (from 20 to 600 °C) on the thin film structure and the surface morphology were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that (i) the substrate temperature has an influence on the crystallite size, which varied between 12 and 50 nm, (ii) the substrate type has an influence on the growth mechanism of YSZ thin films, and (iii) a bias voltage applied to the substrate during the deposition of thin films has an influence on the densification of YSZ layers.     

Organic light emitting diodes based on dipyridamole drug

The dipyridamole drug [DIP: 2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido(5,4-d)pyrimidine] is widely used in treatment of coronary heart disease for its antiplatelet and vasodilating activities, and its high intensity photoluminescence (PL) has been widely reported. In this work, the fabrication and the characterization of a new OLED using the DIP molecule as an emitting layer is reported. The devices were assembled using a heterojunction between three organic molecular materials: the N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) or the 1-(3-methylphenyl)-1,2,3,4-tetrahydroquinoline-6-carboxyaldehyde-1,1′-diphenylhydrazone (MTCD) as hole-transporting layer, the DIP layer as an emitting layer and the tris(8-hydroxyquinoline aluminum) (Alq₃) as the electron transporting layer. All the organic layers were sequentially deposited in a high vacuum by thermal evaporation onto indium tin oxide substrates and without breaking vacuum. Continuous electroluminescence emission was obtained in all configurations upon varying the applied bias voltage from 4 to 30 V, the observed wide emission band was centered at 493 nm. The luminance of the devices was about 1500 (cd)/m² with 4.5 cd/A of efficiency for the best device. The charge transport behavior in the OLED is also discussed as a function of different carrier injection levels.     

Micropatterning of emitting layers by microcontact printing and application to organic light-emitting diodes

The microcontact printing (μCP) technique, which is a simple and low damage fabrication technique for thin films, was successfully applied to fabricate patterned emitting layers such as polyfluorene (PF). We fabricated micropatterns by transferring dried and uniform thin films, and observed strong electroluminescence (EL) from the fabricated organic light-emitting diodes (OLEDs) with the patterned emitting layers. The performance of the fabricated device was superior to that of a conventionally fabricated device. This demonstrates the well-controlled interfaces achieved by μCP. Furthermore, we succeeded in fabricating OLEDs with multiple emitting layers. These results show that this technique is promising for application to cost-effective, high luminance and multicolored OLED displays.     

Molecularly ordered aluminum tris-(8-hydroxyquinoline) thin films grown by hot-wall deposition

We report on the growth and microstructural analysis of molecularly ordered thin film layers of aluminum tris-(8-hydroxyquinoline) (Alq₃) by hot-wall deposition onto amorphous glass substrates. Using transmission electron microscopy (TEM), ordering on a scale of 100 nm was observed. Raman measurements of these films indicated that they corresponded to the α-polymorph of crystalline Alq₃, and photoluminescence measurements exhibited a single broad peak centered at 500 nm, which is also consistent with the α-form. As a comparison, we deposited films of Alq3 using organic molecular beam deposition (OMBD), which exhibited no molecular ordering from the TEM studies. For these films, strong point-to-point variations in the Raman spectrum, and the existence of a double peak in the photoluminescence at 500 and 522 nm were observed. These measurements indicate that the OMBD films possess a mixture of both α and amorphous phases.     

Experimental study of the organic light emitting diode with a p-type silicon anode

We have fabricated and studied an organic light emitting diode (OLED) with a p-type silicon anode and a SiO₂ buffer layer between the anode and the organic layers which emits light from a semitransparent top Yb/Au cathode. The luminance of the OLED is up to 5600 cd/m² at 17 V and 1800 mA/cm², the current efficiency is 0.31 cd/A. Both its luminance and current efficiency are much higher than those of the OLEDs with silicon as the anodes reported previously. The enhancement of the luminance and efficiency can be attributed to an improved balance between the hole- and electron-injection through two efficient ways: 1) restraining the hole-injection by inserting an ultra-thin SiO₂ buffer layer between the Si anode and the organic layers; and 2) enhancing the electron-injection by using a low work function, low optical reflectance and absorption semitransparent Yb/Au cathode.     

White OLED using β-diketones rare earth binuclear complex as emitting layer

In this work, the fabrication and the characterization of a white triple-layer OLED using a β-diketones binuclear complex [Eu(btfa)₃phenterpyTb(acac)₃] as the emitting layer is reported. The devices were assembled using a heterojunction between three organic molecular materials: the N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) as hole-transporting layer, the β-diketones binuclear complex and the tris(8-hydroxyquinoline aluminum) (Alq₃) as the electron transporting layer. All the organic layers were sequentially deposited under high vacuum environment by thermal evaporation onto ITO substrates and without breaking vacuum. Continuous electroluminescence emission was obtained varying the applied bias voltage from 10 to 22 V showing a wide emission band from 400 to 700 nm with about 100 cd/m² of luminance. The white emission results from a combined action between the binuclear complex, acting as hole blocking and emitting layer, blue from NPB and the typical Alq₃ green emission. The intensity ratio of the peaks is determined by the layer thickness and by the bias voltage applied to the OLED, allowing us to obtain a color tunable light source.     

YSZ thin films deposited by e-beam technique

YSZ thin films were grown evaporating cubic and tetragonal phase ZrO₂ stabilized by 8 wt.% of Y₂O₃ (8% of YSZ) ceramic powders by using e-beam deposition technique. Operating technical parameters that influence thin film properties were studied. The influence of substrate crystalline structure on growth of deposited YSZ thin film was analyzed there. The YSZ thin films (1.5-2 μm of thickness) were deposited on three different types of substrates: Al₂O₃, optical quartz (SiO₂), and Alloy 600 (Fe-Ni-Cr). The dependence of substrate temperature, electron gun power, and phase of ceramic powder on thin film structure and surface morphology was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The substrate temperature was changed in the range of 20-600° C (during the YSZ thin film deposition) and its influence on the crystallinity of deposited YSZ thin films was analyzed. It was found that electron gun power and substrate temperature has the influence on the crystallite size, and texture of YSZ thin films. Also, the substrate has no influence on the crystal orientation. The crystallite size varied between 20 and 40 nm and increased linearly changing the substrate temperature. The crystal phase of evaporated YSZ powder has the influence on the structure of the deposited YSZ thin films.     

OVPD und Anwendung optischer Spektroskopiemethoden zur Wachstumskontrolle. OVPD and Applications of Optical Spectroscopic Methods to Growth Control

Organic Vapor Phase Deposition (OVPD) is a new thin film growth technique which is very suitable for deposition of uniform thin films on larger substrate areas. The polarization sensitive methods, ellipsometry and Reflectance Anisotropy Spectroscopy (RAS), have huge potential for the control of the growth in the OVPD process. The capability of ellipsometry to determine the thickness and the optical constants of OVPD deposited films was demonstrated using as example an Alq₃ film. RAS showed high potential for the detection of very thin organic anisotropic films, as exemplified for an PTCDA film.     

Zinc phthalocyanine — Influence of substrate temperature, film thickness, and kind of substrate on the morphology

Zinc phthalocyanine (ZnPc), C₃₂H₁₆N₈Zn, is a planar organic molecule having numerous optical and electrical applications in organic electronics. This work investigates the influence of various deposition parameters on the morphology of vapour thermal evaporated ZnPc films. For this purpose, ZnPc is deposited at different substrate temperatures up to 90 °C and film thickness up to 50 nm onto various substrates. The morphology of this ZnPc layers is characterised by X-ray diffraction (XRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM) methods. XRD measurements show that all ZnPc films are crystalline in a triclinic (α-ZnPc) or monoclinic (γ-ZnPc) phase, independent from the kind of substrate, layer thickness, or substrate temperature. The ZnPc powder, the starting product for the thermally evaporated ZnPc films, is present in the stable monoclinic β-phase. Thus, the stacking of the ZnPc molecules changes completely during deposition. The crystallite size perpendicular to the substrate determined by XRD microstructure analysis is in the range of the layer thickness while the lateral size, obtained by AFM, is increasing with substrate temperature and film thickness. AFM and XRR show an increase of the layer roughness for thicker ZnPc layers and higher substrate temperatures during film deposition. The strain in the ZnPc films decreases for higher substrate temperatures due to enhanced thermal relaxation and for thicker ZnPc films due to lower surface tension.     

Enhance current injection of organic light emitting diodes by inserting an organic superlattice

Poor electron injection is a great concern for organic light emitting diodes (OLEDs). In order to improve the electron mobility, inserting organic superlattice structures in the electron transport layer was investigated in conventional OLEDs configuration. The superlattices are composed of alternating tris(8-hydroxyquinoline aluminium (Alq₃) and copper phthalocyanine (CuPc) thin films, which are used as electron and hole injection layers. Experimental results show superlattices with a 6-nm period have the largest injected current. Reduction of turn-on voltage and resistance of superlattice OLEDs were also observed. After thermal annealing, the current-voltage characteristic changes and shows the possibility of layer intermixing in organic superlattices.     

The structure and electrical characteristics of Si/Ge heterojunctions: I: Imperfections in the Si-Ge heteroepitaxial system obtained by deposition of germanium from a molecular beam

The type and distribution of imperfections in germanium and Ge_xSi_1?x layers deposited onto nn⁺-Si substrates were investigated using optical and electron microscopy and electron diffraction. The influence of deposition temperature and substrate treatment on the morphology of the growing films is discussed.     

Substrate temperature dependent morphology and resistivity of pulsed laser deposited iridium oxide thin films

Iridium oxide (IrO₂) thin films were deposited on Si (100) substrates by means of pulsed laser deposition technique at various substrate (deposition) temperatures ranging from 250 to 500 °C. Effects of substrate temperature on the crystalline nature, morphology and electrical properties of the deposited films were analyzed by using X-ray diffraction, Raman spectroscopy, Scanning electron microscopy and four-point probe method. It was found that the above properties were strongly dependent on the substrate temperature. The as-deposited films at all substrate temperatures were polycrystalline tetragonal IrO₂ and the preferential growth orientation changed with the substrate temperature. IrO₂ films exhibited fairly homogeneous thickness and good adhesion with the substrate, the average feature size increases with the substrate temperature. The room-temperature resistivity of IrO₂ films decreased with the increase of substrate temperature and the minimum resistivity of (42 ± 6) μΩ cm was obtained at 500 °C. The resistivity of IrO₂ films correlated well with the corresponding film morphology changes.     

ITO deposited by pyrosol for photovoltaic applications

The goal of this work is to investigate morphology, electrical and optical properties of indium-tin-oxide (ITO) deposited by pyrosol on glass and Si substrates at different temperatures and to implement such layers for the processing of Si-based solar cells. The influence of the methanol/H₂O ratio on general properties of ITO was investigated. Atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission spectra, ellipsometry and resistivity measurements were used for the analysis. It is shown that properties of ITO layers depend dramatically on the substrate used. It is shown that the resistivity of ITO layers deposited on a glass substrate is higher up to 2.5 times, compared to that of ITO layers deposited on a Si substrate at the same conditions, but in both cases decreases if the deposition temperature increases. Moreover, ITO layers deposited on a glass substrate are more flat and their refractive indexes are always lower for all deposition temperatures. An increase of the H₂O concentration in a film-forming solution leads to a decrease of the ITO film resistivity and to a slight increase of the roughness. An application of pyrosol deposited ITO films as the top transparent electrodes for the (p⁺nn⁺)Si and heterojunction ITO/n-Si solar cells is demonstrated.     

Thin films of boron nitride grown by CVD

For the first time, thin films of boron nitride were deposited by chemical vapour deposition on to polished silicon and other metal substrates using the inorganic compound H₃BNH₃ (aminodiborane) and ammonia as carrier gas. The substrate temperature was varied from 400 to 600°C. The films were chemically inert and adherent to the substrates. The FTIR spectrum of the film showed B-N-B absorption at 800 cm⁻¹, B-N stretching at 1056 cm⁻¹, and also a weak absorption at 1340cm⁻¹ corresponding to B-N-B bending vibration. Deposited films also exhibited X-ray diffraction pattern with interplanar spacing with (002) plane of hexagonal boron nitride.     

Enhancement of the lifetime in organic light-emitting devices fabricated utilizing wide-bandgap-impurity-doped emitting layers

The degradation behaviors of the electrical and the optical properties of organic light-emitting devices (OLEDs) fabricated with an emitting layer (EML) doped with or without a wide-bandgap-impurity were investigated. The OLEDs with a wide-bandgap-doped Alq₃ EML were more stable than those with an undoped Alq₃ EML. The existence of the doped wide-bandgap-impurity in the EML decreased the trap-charge density in the EML, resulting in an increase in the number of electrons in the Alq₃ EML. That increases in the number of electron in the Alq₃ EML for the OLEDs with a wide-bandgap-impurity decreased the staying time of the holes in the Alq₃ EML, resulting in an enhanced lifetime for the OLEDs. These results indicate that OLEDs with a wide-bandgap-impurity-doped EML hold promise for potential applications in long-lifetime OLED displays.     

Growth of pseudocubic perovskite-type SrRuO3 thin films on quartz substrate using pulsed laser deposition method

Strontium ruthenium oxide (SrRuO₃) thin films have been grown using pulsed laser deposition technique on silicon, Pt coated silicon and quartz substrates. The effect of substrate temperatures on the structural, microstructure, and electrical properties of the SrRuO₃ films on quartz substrate has been investigated using XRD, SEM, AFM and four-probe method, respectively. The lowest resistivity at room temperature for the SrRuO₃ thin film on quartz substrate has been achieved at substrate temperature of 700 °C. Furthermore, the comparisons of SrRuO₃ thin films deposited on various substrates have been done with respect to structural, microstructural and electrical properties. XRD patterns exhibit that all thin films are a single phase, pseudo-cubic perovskite structure. Study of surface morphology shows that grain size and roughness varies with respect to substrate. It is observed that SrRuO₃ thin films yield larger grain size and root mean square roughness on Pt/Si substrate. Investigation of electrical properties shows that SrRuO₃ thin films can serve the purpose of the bottom electrode in dielectric and ferroelectric devices.     

Structural and electrical properties of RuO2 thin films prepared by rf-magnetron sputtering and annealing at different temperatures

Conductive ruthenium oxide (RuO₂) thin films have been deposited at different substrate temperatures on various substrates by radio-frequency (rf) magnetron sputtering and were later annealed at different temperatures. The thickness of the films ranges from 50 to 700 nm. Films deposited at higher temperatures show larger grain size (about 140 nm) with (200) preferred orientation. Films deposited at lower substrate temperature have smaller grains (about 55 nm) with (110) preferred orientation. The electrical resistivity decreases slightly with increasing film thickness but is more influenced by the deposition and annealing temperature. Maximum resistivity is 861 μΩ cm, observed for films deposited at room temperature on glass substrates. Minimum resistivity is 40 μΩ cm observed for a thin film (50 nm) deposited at 540°C on a quartz substrate. Micro-Raman investigations indicate that strain-free well-crystallized thin films are deposited on oxidized Si substrates.