Chapter 2 Literature Review
2.6. Motivations
Although conventional thermal method has been utilized for the synthesis of encapsulating adhesives, the reaction takes several hours for completion, which is time and energy-consuming. Therefore, fast and effective preparation for package materials has become a crucial topic. Furthermore, the common encapsulating materials exhibit poor gas barrier capability, resulting in insufficient lifetimes while they are applied for package of photoelectric devices. Short lifetimes are pivotal obstacles to their commercialization.
Chapter 3
Experimental Methods
3.1. Encapsulation of OLEDs
3.1.1. Experimental Flow for Encapsulation of OLEDs
The experimental flow for encapsulation of OLEDs was illustrated in Fig. 3-1.
Fig. 3-1. Experimental flow for encapsulation of OLEDs.
3.1.2. Materials for OLED Encapsulating Resins
Figure 3-2 illustrates the chemical structures of monomers for the preparation of UV-curable OLED encapsulating resins including PU-acrylate and silicone-acrylate.
All chemicals were obtained from UCB Co. and used without further purification.
Silica and alumina inorganic fillers with sizes = 30 to 100 nm were obtained from
Aldrich Co.
PU-acrylate Silicone-acrylate
Fig. 3-2. Chemical structures of monomers for the preparation of UV-curable OLED encapsulating resins.
The initiators (benzoyl peroxide; BPO) and photoinitiators (Irgacure 184/Irgacure 369/Irgacure ITX/Irgacure 651/Darocur 1173/Irgacure 819/triaryl sulfonium hexafluoroantimonate) for the preparation of UV-curable OLED encapsulating resins were purchased from Aldrich Co. and Ciba Co., respectively.
Their chemical structures are depicted in Fig. 3-3 and were utilized without further
C
Irgacure 819 sulfonium hexafluoroantimonate
Fig. 3-3. Initiators and photoinitiators for the preparation of UV-curable OLED encapsulating resins.
3.1.3. Preparation of OLED Encapsulation Resins 3.1.3.1. Synthesis of UV-curable PU-acrylate
PU-acrylate monomer (90 g), silica (10 g), Irgacure 184 (1 g), Irgacure 369 (0.5 g), and Irgacure ITX (0.5 g) were stirred for 30 min and irradiated by a UV lamp (100 W) for 10 min (Scheme 1). The weight-average molecular weight (Mw),
number-average molecular weight (Mn), Mw/Mn ratio and viscosity of PU-Acrylate were 153,000, 72,600, 2.11, and 16,300 cps, respectively.
Scheme 3-1
3.1.3.2. Synthesis of UV-curable Silicone-acrylate
Silicone-acrylate monomer (180 g), Irgacure 651 (2 g), Darocur 1173 (1 g), Irgacure 819 (0.5 g), and alumina (20 g) were mixed for 30 min and illuminated by a UV lamp (Entela UVP; wavelength = 365 nm) at the power of 100 W for 20 min
I-651, D-1173, I-819, alumina
The materials for OLED sample preparation include naphthyl phenyl benzidine (NPB; hole transport material), tris-[8-hydroxyquinoline] aluminum (Alq3; light emitting material), LiF (electron injection/protection material), 4,4’-bis(diphenyl vinylenyl)-biphenyl(ADS082BE; blue-color emitting material), 1,3-bis [2-(2,2’- bipyridine-6-yl)-1,3,4-oxadiazo-5-yl]benzene (Bpy-OXD; electron blocking material).
Their chemical structures are showed in Fig. 3-4. All chemicals were purchased from Aldrich Co. and used without further purification.
NPB ADS082BE
Fig. 3-4. Chemical structures of NPB, ADS082BE, Bpy-OXD, and Alq3. (continued on next page.)
N
Fig. 3-4. Chemical structures of NPB, ADS082BE, Bpy-OXD, and Alq3.
3.1.5. Preparation of OLED Samples layer; 3 nm) and Al (cathode; 80 nm) were sequentially deposited onto the ITO glass by using a vacuum evaporator (ULVAC VPC-060) schematically illustrated in Fig.
3-5. The deposition conditions are: background pressure = 4 10 6 torr; deposition rate of organic materials = 0.3-0.5 Å/sec; deposition rate of LiF and Al = 2-5 Å/sec. The structure of such a sample is illustrated in Fig. 3-6(a) and is denoted as Type 0 sample.
In order to investigate the gas barrier effect of the LiF protection layer, 80- and 100-nm thick LiF layers were further deposited on the Type 0 sample. It was termed as Type I sample and its structure is depicted in Fig. 3-6(b). The Type II sample shown in Fig. 3-6(c) is achieved by encapsulating the resin prepared previously via spin coating process (stage I: 1,500 rpm for 20 sec; stage II: 3,500 rpm for 30 sec).
Fig. 3-5. The vacuum evaporation system for OLED sample preparation.
(a) (b) (c)
Fig. 3-6. Structures of (a) Type 0; (b) Type I and (c) Type II OLED samples.
3.1.6. Preparation of Color/Luminance Tunable OLED Samples
Firstly, the ITO glass substrate (sheet resistance = 5 / ; substrate size = 5 cm 5 cm) was ultrasonically cleaned with the acetone, methanol as well as de-ionized water for 5 min and dried with a stream of nitrogen followed by baking in an 120 C-oven for 30 min. After treated by oxygen plasma for 90 sec, NPB (50 or 80
Rotary Pump
Turbo Pump VG
VM
VB
Material Source Sensor
Shutter Holder/Mask
nm), ADS082BE (35 or 50 nm), Alq3 (50 nm), LiF(3 nm), and Al (80 nm) were sequentially deposited on the ITO glass by a vacuum evaporator illustrated in Fig. 3-5.
Such a sample was designated as Device III and is illustrated in Fig. 3-7(a). The similar procedure was carried out to prepare the Device IV containing Bpy-OXD electron blocking layers with various thickness (10, 20 or 30 nm) as illustrated in Fig.
3-7(b)).
The silicone-acrylate was subsequently adopted for the encapsualtion of Device IV containing 20-nm thick Bpy-OXD via spin-coating technique (stage I: 1,000 rpm for 10 sec; stage II: 2,500 rpm for 20 sec) and cured in an UV oven (Entela UVP 2450 W) for 10 sec at room temperature. The thickness of sealing resin was approximately 50 m measured by a surface profiler (TENCOR P-10). Such a sample was termed as Device V as shown in Fig. 3-7(c).
(a) (b) (c)
Fig. 3-7. The structures of color/luminance tunable OLEDs. (a) Device III; (b) Device IV and (c) Device V.
3.1.7. Characterizations and Reliability Test of OLEDs
We measured the molecular weight, viscosity, and gas permeation rate with a GPC (Waters Alliance GPC V200), a viscosity meter (Viscolite 700), and a gas permeation meter (Illinois-8501), respectively. Moreover, the EL properties and lifetimes of OLEDs were recorded by a Keithley 2400 and Spectrascan PR650, respectively.
3.2. Encapsulation of OSCs
3.2.1. Experimental Flow for Encapsulation of OSCs
The experimental flow for encapsulation of OSCs was illustrated in Fig.3-8.
Fig. 3-8. Experimental flow for encapsulation of OSCs.
3.2.2. Materials for OSCs Encapsulating Resins
All of resin monomers, initiators, solvents and fillers (silica; 30 to 100 nm) used in the experiment were purchased from Aldrich Co. and used without further purification.
O C OCH2 1,6-hexanedol diacrylate and 3,4-epoxycyclohexane carboxylode (see Fig. 3-9 for chemcial structures) were the monomers for acrylics and epoxy resins, respectively.
BPO and triaryl sulfonium hexafluoroantimonate were utilized as thermal initiators and photoinitiators, respectively. Propylene glycol monomethyl ether acetate (PGMEA) was the solvent. The metal alloy (Invar) purchased from Goodfellow Co.
was 64 wt.% Fe: 36 wt.% Ni alloy with size about 100 nm.
1,6- Hexanedol diacrylate
3,4-Epoxycyclohexane carboxylode
Fig. 3-9. Chemical structures of materials for OSCs encapsulating resins.
3.2.3. Preparation of OSCs Encapsulation Resins
1,6-Hexanedol diacrylate (10 g), benzoyl peroxide (0.1 g), and PGMEA (100 mL) were heated at 100 C for 3 hrs. After blended with 3,4-epoxycyclohexane carboxylode 10 g and triaryl sulfonium hexafluoroantimonate (0.1 g), the acrylics/epoxy resins (polymer matrices) were obtained. Then, 50 wt.% of silica and various amounts of Invar (0-15 wt.%) were added into the epoxy-acrylate polymer matrices to form the organic/inorganic nanocomposite resins. The resin sample
Table 3-1. Designation of resin sample for the encapsulation of OSCs.
Sample designation Invar content (wt.%)
Nanocomposite I 0
Nanocomposite II 5
Nanocomposite III 10
Nanocomposite IV 15
3.2.4. Materials for OSCs Samples
Polyethylene dioxythiophene (PEDOT; hole transport material), Poly(3-hexylthiophene-2,5-diyl)(P3HT; p-type semiconductor), and [6,6]-phenyl C61-butyricacid methyl ester (PCBM; n-type semiconductor) were acquired from Aldrich Co. and utilized without further purification. Their chemical structures are illustrated in Fig. 3-10.
PEDOT P3HT PCBM
Fig. 3-10. Chemical structures of PEDOT, P3HT and PCBM.
3.2.5. Preparation of OSCs Samples
The ITO glass substrate (sheet resistance = 5 / ) was first ultrasonically cleaned with the acetone, methanol, and de-ionized water for 5 min. It was then dried by a stream of nitrogen, baked in a 120 C-oven for 30 min and treated by O2 plasma for 90 sec. 3 wt.% PEDOT was dissolved in water, filtered with a 0.2 µm filter and deposited onto the ITO glass by spin-coating (stage I: 1000 rpm for 20 sec; stage II:
3000 rpm for 30 sec). Afterwards, the P3HT powder (molecular weight = 87,000) was
O
ground together with PCBM (purity = 98%) to form P3HT/PCBM mixture (weight ratio = 1/1). Then 2 wt.% P3HT/PCBM mixture was dissolved in dichloromethane, filtered with a 0.2 µm filter, and deposited onto the PEDOT layer by spin-coating (stage I: 1000 rpm for 20 sec; stage II: 2000 rpm for 30 sec). Finally, the LiF and Al electrode were deposited onto the P3HT/PCBM by vacuum evaporator shown in Fig.
3-5. Figure 3-11(a) illustrates the device structure of solar cell prior to encapsulation.
Encapsulation of organic solar cells was achieved by spin-coating the nanocomposite IV on the top of Al electrode at the condition: stage I: 500 rpm for 10 sec; stage II: 1500 rpm for 20 sec followed by UV curing (Entela UVP 2450 W) for 10 seconds. The thickness of encapsulating layer was about 100 µm. Figure 3-11(b) illustrates the device structure of solar cell subjected to encapsulation.
(a) (b)
Fig. 3-11. Structures of organic solar cells (a) prior to and (b) after encapsulation. The layer thicknesses of devices are: ITO glass/PEDOT(30 nm)/P3HT:PCDM(50 nm)/LiF(10 nm)/Al(80nm).
3.2.6. Characterizations and Reliability Test of OSCs
probe (Everbeing SR-4), respectively. Thermomechanic analysis (TMA), adhesion strength, and film thickness were recorded on a Seiko SSC 5000 TMA, a micro-computer universal testing machine (Hung Ta Co.) with the standard test method (ASTM D1002) and a surface profiler (TENCOR P-10), respectively. The current-voltage (I-V) curves for OSCs were measured in the air by an electric meter (Keithley 238), whose accuracy can reach picoampere, under illumination of white light from a 300 W halogen lamp (Saturn Co.) whose intensity was recorded on a radiometer (IL-1700).
3.3. Encapsulation of LEDs
3.3.1. Experimental Flow for Encapsulation of LEDs
The experimental flow for encapsulation of LEDs was illustrated in Fig.3-12.
Fig. 3-12. Experimental flow for encapsulation of LEDs.
3.3.2. Materials for LED Encapsulating Nanocomposites
The chemical structure of silicone-acrylate monomer for the preparation of UV-curable LED encapsulating resins has been manifested in Fig. 3-2. All chemicals were obtained from UCB Co. and used without further purification. The inorganic fillers including silica and alumina particles with sizes = 30-100 nm were obtained from Aldrich Co.
3.3.3. Preparation of LED Encapsulating Nanocomposites
Silicone-acrylate monomer (180 g), Irgacure 651 (2 g), Darocur 1173 (1 g), Irgacure 819 (0.5 g), and alumina filler (20 g) were mixed for 30 min and then illuminated by a UV lamp (Entela UVP; wavelength = 365 nm) at the power of 100 W for 20 min (Scheme 2). The Mn, Mw, Mw/Mn ratio, and viscosity of silicone-acrylate nanocomposites were 37,300, 83,200, 2.23, and 7,200 cps, respectively.
3.3.4. Materials for LEDs Samples
The blue-light GaN chips and phosphors (yttrium aluminum garnet-Y3Al5O12; YAG) for LEDs were acquired from Tekcore Co. (Taiwan) and Nichia Co., respectively. All the materials were used directly without further purification.
3.3.5. Preparation of LED Samples
The phosphor was firstly dispersed in the silicone-acrylate encapsulating nanocomposites (phosphor:silicone-acrylate nanocomposites = 12:88 in weight ratio).
Then we dripped it into the bowel of lead frame which a commercial blue chip (460
illumination for 5 min to complete the preparation of LEDs samples with structure shown in Fig. 2-4.
3.3.6. Preparation of Compatibilizer for Polymer Alloys 3.3.6.1. Materials for Compatibilizers for LEDs
The LDPE pellet (NA 20766) with density = 0.923 g/cm3 and melt index = 8 g/10 min was supplied by USI Far East Co. of Taiwan. The GMA, and PE-g-acrylic acid (PB1009), whose chemical structures were shown in Fig. 3-14, were obtained from Japanese Oil and Fat Co. and BP Performance Polymer Inc., respectively. The initiator, 2,5-dimethyl-2,5-bis(t-butyl- peroxy)hexane (TX 101) as shown in Fig. 3-13, with a half-life approximately 6 min at 160 C was purchased from the Akzo Chemie Co.
Fig. 3-13. Chemical structures of GMA, PE-g-Acrylic acid, and TX 101.
3.3.6.2. Grafting Process of GMA and LDPE for Compatibilizers
The grafting reaction was manifested in Scheme 3 and carried out by using a
C
twin screw extruder (W&P, ZSK 25). The LDPE pellets were fed at 40 g/min into the hopper and the GMA/TX 101 solution and were injected into the twin screw extruder from the injection nozzle via a liquid chromatography pump (Fig. 3-14). The extrudate was pelletized and then dried prior to analysis. The crude polymer was dissolved in hot toluene, precipitated the product with methanol, and then dried in a reduced pressure at 50 C for 24 hrs. The element analyzer (Heraeus CHN-0-Rapid F002) was used to analyze the contents of oxygen of the dried polymer. Thin film samples for Fourier-transform infrared (FTIR) analysis were prepared by pressing the precipitated resin between two mylar sheets at 120 C and 5 ton/cm2 for 1 min. FTIR measurements were performed by using a Bio-Rad Digitlab Division UMA 300/FTS-40 spectrometer. The thermalgravimetric analysis (TGA) was carried out by using a Perkin-Elmer TGA7 and in temperatures ranging from room temperature to 470 C at a heating rate of 10 C/min in nitrogen ambient.
Scheme 3-3
Fig. 3-14. The procedure used for the grafting reaction.
3.3.7. Preparation of LED Reflector Cups with Polymer Alloys/Compatibilizer 3.3.7.1. Materials for Polymer Alloys
The PET, PPS (P-4), and nylon 6,6, whose chemical structures were shown in Fig. 3-15, were obtained from Shinkong Synthetic Fibers Co., Du Pont Co., and Phillips Petroleum Co., respectively.
Fig. 3-15. Chemical structures of PET, PPS, and nylon 6,6.
3.3.7.2. Manufacture of LED Reflector Cups
PPS powder, PET or nylon 6,6 pellets/glass fiber, TiO2 powder, and the PE-g-GMA compatibilizer were mixed in the twin screw extruder at 280 C at a screw speed of 300 rpm. After the extrudate was then cooled, palletized, and injection-molded, the polymeric reflector cups were manufactured as shown in Fig.
3-16.
Fig. 3-16. Polymeric reflector cups for LEDs.
3.3.8. Fabrication of LED Lamps
The LEDs samples fabricated previously (see Sec. 3.3.5) were placed into the polymeric reflector cups for form the LED lamps as shown in Fig. 3-17.
Fig. 3-17. Structures of LED lamps
3.3.9. Characterizations and Reliability Test of LEDs
We measured the viscosity and molecular weight with a viscosity meter (Viscolite 700) and a GPC (Waters Alliance GPC V200), respectively. The transparency, refractive index, and gas penetration were examined with a UV/Visible spectrometer (HITACHI U-3300), a ellipsometry (Filmetrics F20), and a gas
(Spectrascan PR650).
Impact test specimens were prepared by an injection molding machine (Toshiba IS55). Izod impact test was conducted on unnotched specimens at 25 C according to ASTM D256 test standard. Morphologies of fractured surfaces of PPS/PET alloys were examined by scanning electron microscopy (SEM, Cambridge model S360).
Chapter 4
Results and Discussion
4.1. UV-assisted Synthetic Procedure
Ultraviolet (UV) light is the high-energy electromagnetic radiation with wavelength ranging from 180 to 400 nm. The main principle of UV-assisted synthetic procedure is to induce the dissociation of photoinitiators, generating free radicals to result in initiation, propagation and termination for photo-polymerization [15].
Scheme 3-1 and 3-2 present the UV-assisted synthetic procedure of encapsulating adhesives. In this study, the reactions of PU-acrylate and silicone-acrylate with UV-assisted synthetic procedure takes only 10-20 min, while that with conventional thermal methods takes several hours for completion. Moreover, the viscosities of PU-acrylate and silicone-acrylate prepared by UV irradiation (16,300 cps for PU-acrylate; 7,200 cps for silicone-acrylate ) are larger than that prepared by conventional thermal methods (8,500 cps; reaction duration = 8 hrs for PU-acrylate and 5,300 cps; reaction duration = 20 hrs for silicone-acrylate ) and no solvent is necessary in the UV-assisted synthetic procedure. These advantages fit the requirements for the low-cost and clean production of encapsulating adhesives or other polymeric materials.
Moreover, the silica and alumina particles were well dispersed in polymer matrices (Figures. 4-1 and 4-2), thus the gas penetration can be improved.
475 nm
95 nm 475 nm
95 nm
Fig.4-1. TEM micrographs of PU-acrylate/silica nanocomposites.
95 nm
475 nm 95 nm
475 nm
Fig.4-2. TEM micrographs of silicone-acrylate/alumina nanocomposites.
4.2. Lifetime Enhancement of OLEDs
As shown in Fig. 4-3(a), the lifetimes of OLED samples are strongly affected by the thickness of the protection layer, LiF. Without LiF, the luminance sharply drops when the device is lighted up and the half-lifetime, defined as the duration while the luminance decays from the original amount to its half, is only 4 hrs, revealing the oxygen and moisture in the air deteriorate the metal electrodes and organic layers.
When LiF is deposited, however, the half-lifetime drastically rises to be 20 (LiF = 80 nm) and 31 hrs (LiF = 100 nm), respectively, representing the LiF has the resistance of the oxygen and moisture in the air and thus the lifetimes of lab-made OLEDs are prolonged. Experimental results manifest that the lifetimes of OLEDs can be modulated with the LiF thickness and the devices with thicker LiF films exhibit better barrier effects.
(a) (b)
Fig.4-3. Lifetimes of lab-made OLEDs (a) Type 0 and I (b) Type II.
Although the half-lifetimes of lab-made OLEDs have been promoted to reach 31 hrs, it remains unsatisfied. In order to further improve the lifetimes, lab-made encapsulating adhesives were spin-coated in the OLED devices and cured with the UV illumination as shown in Fig. 3-6(c). The LiF layer inserted between the cathode and the encapsulating adhesives not only exhibits the gas penetration as described above but also avoids the corrosion of encapsulating adhesives (i.e. PU-acrylate), which are slightly acidic, on the cathode (Al layer). As shown in Fig. 4-3(b), application of encapsulating adhesives dramatically enhances the half-lifetimes of OLED devices to 72 hrs, which are 2.3 folds to those of Type I (LiF = 100 nm) and 18 folds to those without encapsulation (Type 0). This result shows that the encapsulating adhesives can further block the entry of moisture and oxygen in the air into the OLEDs, suppressing the degradation of organic materials and metal electrodes. In comparison with commercial UV curable epoxy-based encapsulating adhesives (EPO-TEK OG112-4; Epoxy technology Inc.), the nanocomposite encapsulating adhesives exhibit longer lifetimes and shorter curing duration since the half-lifetimes and curing times of EPO-TEK OG112-4 are 35 hrs and 2 min, respectively. This indicates that PU-acrylate can be an alternative polymer matrix to epoxy resins for the encapsulation of OLEDs via the appropriate prescription of photoinitiators and dispersion of fillers. Compared with the literature data for the lifetimes of OLEDs [40], our OLED devices with multi-layer encapsulation exhibit better gas blocking capability since the half-lifetimes of devices with packages reported in the literature are only 3.1 folds to those without packages.
Experimental results have proved that LiF and lab-made encapsulating adhesives for OLEDs encapsulation have better lifetime enhancement than other known materials because PU-acrylate exhibits better water proof propensities than other
polymers [15] and high adhesion strength with LiF [10]. Moreover, LiF has tougher chemical resistance to encapsulating adhesives than other inorganic materials [41].
Consequently, the encapsulation combining the LiF and PU-acrylate can effectively reduce the gas permeability. In the near future, synthesis of UV-curable encapsulating adhesives with different polymer matrices and fillers will be carried out in order to further improve the lifetimes of OLEDs.
4.3 Electroluminescent Properties of Color/Luminance Tunable OLEDs and Their Lifetime Enhancement with Encapsulation
4.3.1. Color Modulation and EL Effect of OLEDs with Distinct NPB/ADS082BE Layer Thicknesses
As shown in Figs. 4-4(a)-(c) as well as Fig. 4-5, the EL hue and luminance of OLED samples strongly depend on the thickness of NPB (hole transport layer) and ADS082BE (blue-color emitting layer). With the bias voltage = 7 V and the NPB thickness = 80 nm, the OLED with thinner ADS082BE (35 nm) emits blue light (CIEx,y = (0.18,0.27)) and that with thicker ADS082BE (50 nm) emits deep-blue light (CIEx,y = (0.12,0.20)). Moreover, the luminance of former (350 cd/m2) is higher than that of the latter (10 cd/m2). This result indicates that the device with thicker ADS082BE can effectively confine all the holes in the ADS082BE layer, whose emitting efficiency is lower, so that the recombination almost takes places in the blue-color emitting layer; however, the device with thinner ADS082BE cannot completely restrict the holes in the ADS082BE layer and some holes can overcome
(Alq3), whose emitting efficiency is higher, inducing the red-shift of EL color and strong luminance.
When the layer thickness of NPB was reduced to be 50 nm, nevertheless, the OLED with thinner ADS082BE (35 nm), which is lighted up at 7 V, irradiates blue-green light (CIE x,y = (0.23,0.30)), revealing the decrease for electrical resistance of NPB can further increase the mobility of hole and raise the possibility of the recombination in the Alq3 layer. In addition, its luminance (450 cd/m2) at 7 V is highest among Device III due to its lowest electrical resistance. Since the EL efficiencies of NPB as well as ADS082BE are diverse and the location of recombination can be manipulated by the layer thickness of NPB/ADS082BE, the color/luminance tunable OLEDs can thus be fabricated.
Fig. 4-4. The EL spectra of Devices III: (a) NPB = 80 nm; ADS082BE = 35 nm; (b) NPB = 80 nm; ADS082BE = 50 nm; (c) NPB = 50 nm; ADS082BE = 35 nm.
(a) (b)
(c)
Fig. 4-5. Electroluminescent properties of Devices I (+:NPB = 50 nm; ADS082BE = 35 nm; :NPB = 80 nm; ADS082BE = 35 nm; :NPB = 80 nm;
ADS082BE = 50 nm)
(a) (b)
Fig. 4-6. The energy level diagrams for (a) Devices III and (b) Devices IV.
-6 -4 -2 0 2 4 6 8 10 12
0.1 1 10 100 1000 10000
Luminance(cd/m2 )
Voltage(V)
4.3.2. Introduction of Electron Blocking Layer
In order to promote the EL properties of blue-light OLEDs (i.e., Device III containing 80 nm NPB and 35 nm ADS082BE), Bpy-OXD (the electron blocking layer) was introduced into the device. Without Bpy-OXD, the electron can freely arrive at either ADS082BE or Alq3 layer and the recombination occurs when the hole and electron meet as shown in Fig. 4-6(a). With Bpy-OXD, however, the hole can be impeded in the boundary of Bpy-OXD/Alq3 since it cannot overcome the energy
In order to promote the EL properties of blue-light OLEDs (i.e., Device III containing 80 nm NPB and 35 nm ADS082BE), Bpy-OXD (the electron blocking layer) was introduced into the device. Without Bpy-OXD, the electron can freely arrive at either ADS082BE or Alq3 layer and the recombination occurs when the hole and electron meet as shown in Fig. 4-6(a). With Bpy-OXD, however, the hole can be impeded in the boundary of Bpy-OXD/Alq3 since it cannot overcome the energy