Low-cost and reliable thin film encapsulation for organic light emitting diodes using magnesium fluoride and zinc sulfide

We report highly efficient gas diffusion barriers for organic light emitting diodes (OLEDs) with an encapsulation structure composed of alternating magnesium fluoride (MgF₂) and zinc sulfide (ZnS) layers grown by vacuum thermal deposition. The half lifetime of yellow OLEDs under an initial luminance of 2000 cd/m² with rubrene as an emitter reached 245 h using three pairs of MgF₂/ZnS layers. The device lifetime was obviously improved using MgF₂ and ZnS as passivation layers before UV-cured epoxy seal without desiccant with the lifetime for the initial luminance dropping to 56% being over 500 h. This simple and inexpensive encapsulation method can potentially be applied to top-emitting OLEDs due to good light transmission characteristic of the passivation film.     

Electrical insulation and bending properties of SiOx barrier layers prepared on flexible stainless steel foils by different preparing methods

Stainless steel foils on which flexible display devices and integrated solar modules are prepared need to be coated by barrier layers for electrical insulation. In this study, SiO_x barrier layer was prepared on steel foils (SUS 304) by ion beam assisted deposition, Sol-gel deposition and plasma enhanced chemical vapor deposition, respectively. The electrical properties of the SiO_x films, such as resistance, reactance, leakage current density, breakdown field strength and performance index were investigated, and the bending properties were evaluated by bending tests. The best electrical insulation and bending properties of barrier could be achieved with 4 μm thick SiO_x layer prepared by plasma enhanced chemical vapor deposition.     

Thin film passivation of organic light emitting diodes by inductively coupled plasma chemical vapor deposition

The characteristics of an SiN_x passivation layer grown by a specially designed inductively coupled plasma chemical vapor deposition (ICP-CVD) system with straight antennas for the top-emitting organic light emitting diodes (TOLEDs) are investigated. Using a high-density plasma on the order of ∼ 10¹¹ electrons/cm³ formed by nine straight antennas connected in parallel, a high-density SiN_x passivation layer was deposited on a transparent Mg-Ag cathode at a substrate temperature of 40 °C. Even at a low substrate temperature, single SiN_x passivation layer prepared by ICP-CVD showed a low water vapor transmission rate of 5 × 10^− 2 g/m²/day and a transparency of ∼ 85% respectively. In addition, current-voltage-luminescence results of the TOLED passivated by the SiN_x layer indicated that the electrical and optical properties of the TOLED were not affected by the high-density plasma during the SiN_x deposition process.     

Water permeation through organic-inorganic multilayer thin films

We prepared organic (self-assembled monolayer (SAM))-inorganic (TiO₂) multilayer barrier films on polyethylene terephthalate substrate using atomic layer deposition and molecular layer deposition methods in the same deposition chamber. The water permeation was mainly blocked by the inorganic TiO₂ layer. While the lag time was proportional to the thickness of the TiO₂ layer, the steady-state permeation rate was relatively independent of the thickness. The multilayer approach was effective in extending the lag time due to both the tortuous path effect and the internal desiccant effect. Water permeation occurred sequentially in the organic-inorganic multilayer barriers by water accumulation in the organic SAM layers. The water vapor transmission rate was 7.0 × 10^− 4 g/m²·day during the lag time of 155 h at 60 °C and a relative humidity of 85% with 5-dyad barrier film.     

Investigation of brittle failure in transparent conductive oxide and permeation barrier oxide multilayers on flexible polymers

An oxide multilayer structure—consisting of an indium zinc oxide (IZO) conductive layer, a silicon oxide (SiO_x, x = 1.8) water vapor permeation barrier, and an aluminum oxide (Al₂O₃) interlayer—coated on polyethylene terephthalate (PET) is proposed as a transparent flexible substrate for display and photovoltaic applications. Vital properties of the multilayer, such as the low water vapor impermeability of the SiO_x barrier and the high conductance of the IZO film, degraded considerably because of the crack formation in bend geometries, attributed to the large difference between elastic properties of the oxide films and polymers. In order to suppress the crack formation, a 10-nm-thick Al₂O₃ interlayer was sputtered on Ar ion-beam treated PET surfaces prior to a SiO_x plasma-enhanced chemical vapor deposition (PECVD) process. Changes in the conductance and water vapor impermeability were investigated at different bending radii and bending cycles. It was found that the increases in resistance and water vapor transmission rate (WVTR) were significantly suppressed by the ion-beam PET pretreatment and by the sputtered Al₂O₃ interlayer. The resistance and WVTR of IZO/SiO_x/Al₂O₃/PET systems could be kept low and invariable even in severely bent states by choosing the SiO_x thickness properly. The IZO (135 nm)/SiO_x (90 nm)/Al₂O₃ (10 nm)/PET system maintained a resistance of 3.2 × 10^− 4 Ω cm and a WVTR of < 5 × 10^− 3 g m² d^− 1 after 1000 bending cycles at a bending radius of 35 mm.     

Amorphous silicon carbide thin films (a-SiC:H) deposited by plasma-enhanced chemical vapor deposition as protective coatings for harsh environment applications

We investigated amorphous silicon carbide (a-SiC:H) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) as protective coatings for harsh environment applications. The influence of the deposition parameters on the film properties was studied. Stoichiometric films with a low tensile stress after annealing (< 50 MPa) were obtained with optimized parameters. The stability of a protective coating consisting of a PECVD amorphous silicon oxide layer (a-SiO_x) and of an a-SiC:H layer was investigated through various aging experiments including annealing at high temperatures, autoclave testing and temperature cycling in air/water vapor environment. A platinum-based high-temperature metallization scheme deposited on oxidized Si substrates was used as a test vehicle. The a-SiO_x/a-SiC:H stack showed the best performance when compared to standard passivation materials as amorphous silicon oxide or silicon nitride coatings.     

Deposition and characterization of ultra-high barrier coatings for flexible electronic applications

A barrier structure consisting of SiO_x and SiN_x films was deposited on the polymer substrate at 80 °C via plasma-enhanced chemical vapor deposition (PECVD). However, the low radius of curvature (R_c) of the barrier-coated substrate may cause the inconvenience of the following fabrication processes. By depositing a 150 nm-SiN_x film, the R_c of the barrier-coated polycarbonate (PC) substrate can increase from 80 to 115 mm without inducing any cracks in the barrier structure. Furthermore, the thermal stress of the barrier structure can be adjusted via extending the PECVD process duration in the chamber and replacing PC by the polyethersulone (PES) substrate. The R_c can increase to ∼356 mm by depositing the 150 nm-SiN_x film on the other side of the PES substrate. Finally, the calcium test result of the barrier films/PES/SiN_x sample was calculated to be around 3.05 × 10⁻⁶ g/m²/day, representing that the barrier structure did not fail after modification.     

Light-emitting characteristics of organic light-emitting diodes with the MoOx-doped NPB and C60/LiF layer

The hole ohmic properties of the MoO_x-doped NPB layer have been investigated by analyzing the current density-voltage properties of hole-only devices and by assigning the energy levels of ultraviolet photoemission spectra. The result showed that the performance of organic light-emitting diodes (OLEDs) is markedly improved by optimizing both the thickness and the doping concentration of a hole-injecting layer (HIL) of N, N′-diphenyl-N, N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) doped with molybdenum oxide (MoO_x) which was inserted between indium tin oxide (ITO) and NPB. For the doping concentration of above 25%, the device composed of a glass/ITO/MoO_x-doped NPB (100 nm)/Al structure showed the excellent hole ohmic property. The investigation of the valence band structure revealed that the p-type doping effects in the HTL layer and the hole concentration increased at the anode interfaces cause the hole-injecting barrier lowering. With both MoO_x-doped NPB as a hole ohmic contact and C₆₀/LiF as an electron ohmic contact, the device, which is composed of glass/ITO/MoO_x-doped NPB (25%, 5 nm)/NPB (63 nm)/Alq₃ (37 nm)/C₆₀ (5 nm)/LiF (1 nm)/Al (100 nm), showed the luminance of about 58,300 cd/m² at the low bias voltage of 7.2 V.     

Effects of oxygen addition on electrochromic properties in low temperature plasma-enhanced chemical vapor deposition-synthesized MoOxCy thin films for flexible electrochromic devices

Electrochromic organomolybdenum oxide (MoO_xC_y) films are deposited onto 60 Ω/□ flexible polyethylene terephthalate/indium tin oxide substrates by low temperature plasma-enhanced chemical vapor deposition (PECVD) using a precursor of molybdenum carbonyl vapor, which is carried by argon gas, mixed with oxygen gas and synthesized by radio frequency power at room temperature (23 °C). The MoO_xC_y films with modified surface morphology and compositions of varying oxygen contents are proven to offer noteworthy electrochromic performance. Porous surface of the MoO_xC_y film (398 nm thick) provides Li⁺ ion diffusion coefficient value of 1.7 × 10^− 10 cm²/s for Li⁺ de-intercalation at a potential scan rate of 2 mV/s. High x/y value at high surface composition of oxygen to carbon in the MoO_xC_y film offers light modulation with transmittance variation of up to 63% and coloration efficiency of 36 cm²/C at a wavelength of 800 nm for 200 cycles of Li⁺ intercalation and de-intercalation. PECVD-synthesized MoO_xC_y thin films show promising electrochromic properties for applications in flexible electrochromic devices.     

Nanocomposite por-Si/SnOx layers formation for gas microsensors

Hetero-phase nanocomposite layers based on porous silicon and nonstoichiometric tin oxide (por-Si/SnO_x) were obtained by the chemical vapor deposition (CVD), magnetron sputtering, and molecular layer deposition methods. The structure, and the atomic and phase compositions of the nanocomposites were studied by means of transmission electron microscopy, energy-dispersive X-ray analysis (EDX), scanning electron microscopy, Raman spectroscopy, Auger spectroscopy, and X-ray photoelectron spectroscopy. The obtained data were indicative of the formation of por-Si/SnO_x nanocomposite layers up to 2 μm thick with x = 1.0-2.0. According to EDX data, in magnetron sputtering process the formation of por-Si/SnO_x nanocomposite layers proceeds on the externally exposed surface of polycrystalline por-Si skeleton elements with subsequent diffusion of tin atoms into the pores along the por-Si walls. The other two methods lead to formation of large SnO_x islands covering pores in the por-Si structure. Enhanced diffusion of tin atoms into porous matrix with D_eff ≈ 1 × 10⁻¹⁴ cm²/s was observed in samples annealed at 500 °C. Sensor heterostructures based on magnetron sputtered por-Si/SnO_x nanocomposite layers show high sensitivity to NO₂ environmental molecules and remarkable stability, thus offering promise in gas sensing applications.     

Nanostructured tantalum nitride films as buffer-layer for carbon nanotube growth

Tantalum nitride (TaN_x) films are usually used as barriers to the diffusion of copper in the substrate for electronic devices. In the present work, the TaN_x coating plays an extra role in the iron catalyzed chemical vapor deposition production of carbon nanotubes (CNT). The CNTs were grown at 850 °C on TaN_x films prepared by radio frequency magnetron sputtering. The correlation between the CNT morphology and growth rate, and the pristine TaN_x film nature, is investigated by comparing the evolution of the nano-composition, roughness and nano-crystallinity of the TaN_x films both after annealing and CVD at 850 °C.     

Protection of organic light-emitting diodes over 50 000 hours by Cat-CVD SiNx/SiOxNy stacked thin films

Silicon oxynitride (SiO_xN_y) films have been formed by adding proper amount of oxygen gas to usual forming condition of silicon nitride (SiN_x) films in catalytic chemical vapor deposition (Cat-CVD) method. The composition and refractive index of the film can be systematically controlled by changing oxygen flow rate. Organic light-emitting diodes (OLEDs) covered with SiN_x/SiO_xN_y stacked films have been completely protected from damage due to oxygen and moisture and their initial emission intensity is maintained over 1000 hours under 60 °C and 90% RH, which is equivalent to 50 000 hours in normal temperature and humidity conditions.     

Electrical characterization of high-k gate dielectrics on Ge with HfGeN and GeO2 interlayers

Two kinds of HfSiO_x/interlayers (ILs)/Ge gate stack structures with HfGeN- and GeO₂-ILs were fabricated using electron cyclotron resonance (ECR) plasma sputtering and the subsequent post deposition annealing (PDA). It was found that HfGe was formed by the deposition of Hf metal on Ge and changed to HfGeN by N₂ ECR-plasma irradiation, which was used as IL. Another IL was GeO₂, which was grown by thermal oxidation at 500 °C. For dielectrics with HfGeN-IL, PDA of 550 °C resulted in effective oxide thickness (EOT) of 2.2 nm, hysteresis of 0.1 V, and interface state density (D_it) = 7 × 10¹² cm^− 2 eV^− 1. For dielectrics with GeO₂-IL, PDA of 500 °C resulted in EOT of 2.8 nm, hysteresis of 0.1 V, and D_it = 1 × 10¹² cm^− 2 eV^− 1. The structural change of HfSiO_x/GeO₂/Ge during the PDA was clarified by using X-ray photoelectron spectroscopy, and the gate stack formation for obtaining the good IL was discussed.     

Plastic substrate with gas barrier layer and transparent conductive oxide thin film for flexible displays

A novel plastic substrate for flexible displays was developed. The substrate consisted of a polycarbonate (PC) base film coated with a gas barrier layer and a transparent conductive thin film. PC with ultra-low intrinsic birefringence and high temperature dimensional stability was developed for the base film. The retardation of the PC base film was less than 1 nm at a wavelength of 550 nm (film thickness, 120 µm). Even at 180 °C, the elastic modulus was 2 GPa, and thermal shrinkage was less than 0.01%. The surface roughness of the PC base film was less than 0.5 nm. A silicon oxide (SiO_x) gas barrier layer was deposited on the PC base film by a roll-to-roll DC magnetron reactive sputtering method. The water vapor transmission rate of the SiO_x film was less than 0.05 g/m²/day at 40 °C and 100% relative humidity (RH), and the permeation of oxygen was less than 0.5 cc/m² day atm at 40 °C and 90% RH. As the transparent conductive thin film, amorphous indium zinc oxide was deposited on the SiO_x by sputtering. The transmittance was 87% and the resistivity was 3.5 × 10^− 4 ohm cm.     

Silicon oxide diffusion barrier coatings on polypropylene

In this study the influence of process conditions for the plasma-enhanced chemical vapor deposition of SiO_x diffusion barrier coatings on polypropylene (PP) is investigated and compared to results obtained on polyethylene terephthalate (PET). It was observed that the thermal load during deposition is much more crucial in the case of PP. If the thermal load is not the limiting factor, the composite parameter (CP) energy input per mass of precursor showed to be valuable to describe plasma conditions at constant oxygen to monomer ratio. Low oxygen transmission rates (OTRs) of 5.1 ± 3.6 and 0.3 ± 0.1 cm³/m²day/atm were achieved on PP and PET foil, respectively, for an optimal CP of 4.1 × 10⁵ J/g. Fourier transform infrared (FTIR) spectroscopy revealed that low carbon and silanol content is necessary for good barrier performance. Low RF power, necessary to reduce thermal load on PP, can be compensated by increasing the oxygen to monomer ratio.For favorable plasma conditions, the dependence of the OTR on the coating thickness follows a similar trend for both substrate materials with a critical thickness of approximately 12 nm. The residual permeation can be correlated to the defect density at each stage of film growth by means of a simple correlation. Further support for permeation through defects is found by the activated rate theory, since the apparent activation energy of oxygen permeation is below typical values of amorphous glasses and remains unchanged due to the deposition of SiO_x on both substrates.     

Formation and structural characterization of nanocrystalline Si/SiC multilayers grown by hot filament assisted chemical vapor deposition using CH3SiH3 gas jets

We report on the formation and the structural characterization of nanocrystalline Si/SiC (nc-Si/SiC) multilayers on Si(100) by hot filament assisted chemical vapor deposition using CH₃SiH₃ gas pulse jets. Si rich amorphous SiC (a-Si_1 − xC_x, x ~ 0.33) was initially grown at the substrate temperature (T_s) of 600 °C with heating a hot filament at ~ 2000 °C. The following crystalline SiC layers were grown at T_s = 850 °C without utilizing a hot filament. When the a-Si_1 − xC_x layer was ultrathin (< 2 nm) on Si(100), this a-Si_1 − xC_x layer was transformed to a single epitaxial SiC layer during the subsequent SiC growth process. The Si{111} faceted pits were formed at the SiC/Si(100) interface due to Si diffusion processes from the substrate. When the thickness of the initial a-Si_1 − xC_x layer was increased to ~ 5 nm, a double layer structure was formed in which this amorphous layer was changed to nc-Si and nc-SiC was grown on the top resulting in the considerable reduction of the {111} faceted pits. It was found that nc-SiC was formed by consuming the Si atoms uniformly diffused from the a-Si_1 − xC_x layer below and that Si nanocrystals were generated in the a-Si_1 − xC_x layers due to the annealing effect during further multilayer growths.     

Multilayer amorphous hydrogenated carbon (a-C:H) and SiOx doped a-C:H films for optical applications

In this study SiO_x doped amorphous hydrogenated carbon (a-C:H) films were formed from hexamethyldisiloxane (with hydrogen transport gas) by closed drift ion beam deposition applying variable ion beam energy (300-800 eV). The band gap dependence on the deposition energy was determined and used in production of SiO_x doped a-C:H and a-C:H (formed from acetylene gas) multilayer (two and four layers) stack. Optical properties of the multilayer structures as well as individual layers were analysed in the UV-VIS-NIR range (200-1000 nm). It was shown that employing double or four layer systems, the reflectivity of the multilayer structure-crystalline silicon can be tuned to almost 0% at specific wavelength range (550-950 nm), important in solar cell applications.     

Deposition of WNxCy thin films for diffusion barrier application using the dimethylhydrazido (2) tungsten complex (CH3CN)Cl4W(NNMe2)

Tungsten nitride carbide (WN_xC_y) thin films were deposited by chemical vapor deposition using the dimethylhydrazido (2⁻) tungsten complex (CH₃CN)Cl₄W(NNMe₂) (1) in benzonitrile with H₂ as a co-reactant in the temperature range 300 to 700 °C. Films were characterized using X-ray diffraction (XRD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy and four-point probe to determine film crystallinity, composition, atomic bonding, and electrical resistivity, respectively. The lowest temperature at which growth was observed from 1 was 300 °C. For deposition between 300 and 650 °C, AES measurements indicated the presence of W, C, N, and O in the deposited film. The films deposited below 550 °C were amorphous, while those deposited at and above 550 °C were nano-crystalline (average grain size < 70 Å). The films exhibited their lowest resistivity of 840 µΩ-cm for deposition at 300 °C. WN_xC_y films were tested for diffusion barrier quality by sputter coating the film with Cu, annealing the Cu/WN_xC_y/Si stack in vacuum, and performing AES depth profile and XRD measurement to detect evidence of copper diffusion. Films deposited at 350 and 400 °C (50 and 60 nm thickness, respectively) were able to prevent bulk Cu transport after vacuum annealing at 500 °C for 30 min.     

Chemical vapor deposition growth of Fe3O4 thin films and Fe/Fe3O4 bi-layers for their integration in magnetic tunnel junctions

A possible route for the synthesis of Fe₃O₄, Fe, and Fe/Fe₃O₄ bi-layers with chemical vapor deposition by employing the same Fe₃(CO)₁₂ carbonyl precursor is presented. The comprehensive structural, chemical, and morphological investigation of the as-deposited thin single films and bi-layers is performed by X-ray diffraction, X-ray reflectivity, Raman spectroscopy, and time-of-flight secondary ion mass spectrometry depth profiling. We present the possibility of performing the deposition of pure metallic Fe and Fe₃O₄/γ-Fe₂O₃ by adjusting the deposition pressure from 10^- 3/^- 4 Pa to 1 Pa, respectively. The integration of Fe₃O₄ thin films in a magnetic tunnel junction stack fully synthesized by in situ atomic layer and chemical vapor deposition processes is also presented, showing good stack stability and marginal interdiffusion.     

Thermal effects and plasma damage upon encapsulation of polymer solar cells

A barrier structure consisting of silicon oxide and silicon nitride films was deposited via plasma-enhanced chemical vapor deposition (PECVD) for the encapsulation of polymer solar cells (PSCs). The total concentration of the solution and the ratio of P3HT and PCBM on the performance of polymer solar cells were studied by UV-Vis absorption spectroscopy, atomic force microscopy and photocurrent measurement. Base on these measurements, there is a compromise between light absorption and phase separation with increasing blend concentration. The PSCs were annealed at 80, 100, 120 and 140 °C for 10-60 min to investigate the thermal effects and to estimate the best deposition temperature of the barrier layers. Nevertheless, the devices with the encapsulation of barrier layers had relatively low power conversion efficiencies (PCE) of 0.98% comparing to the devices heated in the PECVD system (1.57%) at the same condition of 80 °C for 45 min due to the plasma damage during the film deposition process. After inserting a 5-nm TiO_x layer between Al/barrier structure and active layer against the plasma damage, the annealed devices presented an average PCE of 2.26% and demonstrated over 50% of their initial value after constant exposure to ambient atmosphere and sunlight for 1500 h.