**2.1. Limited light extraction and improvement strategy**

OLEDs suffer from poor external efficiency that arises from Snell's law; i.e., light generated in a high-refractive-index layer tends to remain trapped in the layer due to total internal reflection [2,3]. In fact, whatever the internal quantum efficiency might be, the light

extraction efficiency of OLEDs with flat multi-layered structures including no additional surface modifications is typically only about 20% of the internal quantum efficiency [4,5]. In such OLEDs, the extracted emission cone to air is very small and only a small fraction of the light generated in the material can be outcoupled from the device but the rest is trapped within by the total internal reflection. The emitted light has to travel from the emissive layer (*n*organic = 1.6~1.7) through the ITO contact (*n*ITO = 1.8~2.0) and the glass substrate (*n*substrate = ~1.5) and finally into air (*n*air = 1). By using ray optics, simply the amount of extracted light out of incident light to the air can be estimated by the following relation [8].

$$
\eta\_{\parallel} = \frac{1}{2n^2} \tag{1}
$$

Effect of Photonic Structures in Organic Light-Emitting Diodes

To overcome this problem, an ordered monolayer of silica microspheres with a diameter 550 nm as a scattering medium has been used by Yamasaki *et al.* as shown in Fig. 1(b) [13]. They have used hexagonally closed packed silica microspheres to enhance the light extraction by a 2-D diffraction lattice. However, in this structure a spectral change is inevitable by the periodic configuration of microspheres, and the device does not show Lambertian emission

*Microlens array*: Figure 2 shows a microlens that has been used as an array for light extraction from the substrate modes [11,12]. The range of diameter of the semi-spherical lens is typically from a few m to hundreds of m Without such lenses, light emitted into substrate with large incidence angles is totally reflected internally by the flat interface between the substrate and air. However, by introducing semi-spherical lenses, the considerable degree of light is transmitted to the air with less total internal reflection because the incidence angle with respect to the surface normal of the lens is now lower than the critical angle. Recently this technology has mostly been used in OLEDs fabricated for lighting because low-cost fabrication of light extraction films is possible via imprint processes. However, the enhanced light extraction efficiency achieved so far is only 1.5-1.7 times of that of reference devices without lenses. To make the matter worse, the color variation with increase of angle is inevitable, preventing the Lambertian emission which is very essential for lighting. It is shown by Lim *et al*. that randomly fabricated microlenses can

solve this problem to a certain extent and improve the angle dependence [30].

**Figure 2.** Schematic structure of OLEDs with a microlens and light extraction.

efficiency gets doubled.

*Insertion of low-refractive-index materials*: Tsutsui *et al*. [5] have used ultrathin organic emissive layers as very poor waveguides with only a very few allowed modes. This allows a considerable amount of light to leak into the substrate, and eq. (1) is no longer valid. In addition, if the index of refraction of the substrate can also be lowered, the light output can be improved significantly as shown in Fig. 3. Tsutsui *et al*. have proposed the use of aerogels with a refractive index close to that of air (naerogel~1) and demonstrated that the out-coupling

*Distributed Bragg reflectors (Microcavity)*: Another promising light extraction technique is the use of microcavity structures [18-20,31,32]. In microcavity devices, the internal emission can

with the increase of output polar angle from 0° to 90°.

– Light Extraction and Polarization Characteristics 67

where *n* is the effective refractive index of the emissive layer with respect to the that of outcoupled space (air) By taking account of Fresnel reflections at the glass/air interface [9] the calculated extraction of light to the air is only about ~17%.

Resumption of at least part of the remaining trapped light (~80%) has been one of the most important issues in fabricating OLEDs for practical applications over the past years. The intense research efforts have been focused on, e.g., substrate surface roughening [10], microlenses [11,12], monolayer of silica spheres as a scattering medium [13], insertion of low-refractive-index materials [5], distributed Bragg reflectors (DBRs) [14-20], and onedimensional (1-D) or two-dimensional (2-D) photonic structures [21-28]. The research developments in these areas are described below:

*Light extraction by scattering*: One of the low-cost methods for enhanced light extraction efficiency is roughening the substrate surface as shown in Fig. 1(a). The roughness causes random light scattering at the interface between substrate and air, and thus a guided light can be extracted from OLEDs. Schnitzer *et al*. have demonstrated a 30% increase of external efficiency in GaAs light-emitting diodes (LEDs) using nanotextured surface [29]. In case of OLEDs, however, this surface roughening techniques have not been used widely because semiconducting organic materials have relatively low refractive index of 1.7 compared with inorganic materials (*n*=3.5). Although a randomly rusted surface can extract guided modes, it also interrupts the external air modes (within critical angle) by non-transparent surface. Therefore, inorganic LEDs with larger amount of guided modes have better extraction ratio than organic LEDs.

**Figure 1.** Schematic structure of OLEDs with modified substrate surface and light extraction.

To overcome this problem, an ordered monolayer of silica microspheres with a diameter 550 nm as a scattering medium has been used by Yamasaki *et al.* as shown in Fig. 1(b) [13]. They have used hexagonally closed packed silica microspheres to enhance the light extraction by a 2-D diffraction lattice. However, in this structure a spectral change is inevitable by the periodic configuration of microspheres, and the device does not show Lambertian emission with the increase of output polar angle from 0° to 90°.

66 Organic Light Emitting Devices

extraction efficiency of OLEDs with flat multi-layered structures including no additional surface modifications is typically only about 20% of the internal quantum efficiency [4,5]. In such OLEDs, the extracted emission cone to air is very small and only a small fraction of the light generated in the material can be outcoupled from the device but the rest is trapped within by the total internal reflection. The emitted light has to travel from the emissive layer (*n*organic = 1.6~1.7) through the ITO contact (*n*ITO = 1.8~2.0) and the glass substrate (*n*substrate = ~1.5) and finally into air (*n*air = 1). By using ray optics, simply the amount of extracted light

> 2 1 2*n*

where *n* is the effective refractive index of the emissive layer with respect to the that of outcoupled space (air) By taking account of Fresnel reflections at the glass/air interface [9]

Resumption of at least part of the remaining trapped light (~80%) has been one of the most important issues in fabricating OLEDs for practical applications over the past years. The intense research efforts have been focused on, e.g., substrate surface roughening [10], microlenses [11,12], monolayer of silica spheres as a scattering medium [13], insertion of low-refractive-index materials [5], distributed Bragg reflectors (DBRs) [14-20], and onedimensional (1-D) or two-dimensional (2-D) photonic structures [21-28]. The research

*Light extraction by scattering*: One of the low-cost methods for enhanced light extraction efficiency is roughening the substrate surface as shown in Fig. 1(a). The roughness causes random light scattering at the interface between substrate and air, and thus a guided light can be extracted from OLEDs. Schnitzer *et al*. have demonstrated a 30% increase of external efficiency in GaAs light-emitting diodes (LEDs) using nanotextured surface [29]. In case of OLEDs, however, this surface roughening techniques have not been used widely because semiconducting organic materials have relatively low refractive index of 1.7 compared with inorganic materials (*n*=3.5). Although a randomly rusted surface can extract guided modes, it also interrupts the external air modes (within critical angle) by non-transparent surface. Therefore, inorganic LEDs with larger amount of guided modes have better extraction ratio

**Figure 1.** Schematic structure of OLEDs with modified substrate surface and light extraction.

(1)

out of incident light to the air can be estimated by the following relation [8].

the calculated extraction of light to the air is only about ~17%.

developments in these areas are described below:

than organic LEDs.

*Microlens array*: Figure 2 shows a microlens that has been used as an array for light extraction from the substrate modes [11,12]. The range of diameter of the semi-spherical lens is typically from a few m to hundreds of m Without such lenses, light emitted into substrate with large incidence angles is totally reflected internally by the flat interface between the substrate and air. However, by introducing semi-spherical lenses, the considerable degree of light is transmitted to the air with less total internal reflection because the incidence angle with respect to the surface normal of the lens is now lower than the critical angle. Recently this technology has mostly been used in OLEDs fabricated for lighting because low-cost fabrication of light extraction films is possible via imprint processes. However, the enhanced light extraction efficiency achieved so far is only 1.5-1.7 times of that of reference devices without lenses. To make the matter worse, the color variation with increase of angle is inevitable, preventing the Lambertian emission which is very essential for lighting. It is shown by Lim *et al*. that randomly fabricated microlenses can solve this problem to a certain extent and improve the angle dependence [30].

**Figure 2.** Schematic structure of OLEDs with a microlens and light extraction.

*Insertion of low-refractive-index materials*: Tsutsui *et al*. [5] have used ultrathin organic emissive layers as very poor waveguides with only a very few allowed modes. This allows a considerable amount of light to leak into the substrate, and eq. (1) is no longer valid. In addition, if the index of refraction of the substrate can also be lowered, the light output can be improved significantly as shown in Fig. 3. Tsutsui *et al*. have proposed the use of aerogels with a refractive index close to that of air (naerogel~1) and demonstrated that the out-coupling efficiency gets doubled.

*Distributed Bragg reflectors (Microcavity)*: Another promising light extraction technique is the use of microcavity structures [18-20,31,32]. In microcavity devices, the internal emission can be effectively extracted via interference effects. In addition, microcavity provides us with spectral narrowing and spatial redistribution of the emission. Microcavity using inorganic distributed Bragg reflectors (DBRs) consisting of alternating inorganic layers with different refractive indices has been extensively studied over the past several decades. The advantage of DBRs is that they may have very high reflectivity and very low loss. These reflectors have a selective reflectivity in a specific wavelength range, which can form constructive interference, and effectively suppress other modes and induce a high reflectance over a certain range of wavelengths depending on the difference in refractive indices of constituting layers. This leads to spectral narrowing and intensity enhancement of spontaneous emission in microcavity OLEDs.

Effect of Photonic Structures in Organic Light-Emitting Diodes

(2)

The schematic structure of a microcavity is shown in Fig. 4. The total optical path difference between direct emission and emission after single-round reflection *L* (see Fig. 4) is given by:

> 2 4 *eff m i i i*

shift. The first, second, and third terms stand for effective penetration depth, summation of the optical thickness in each layer, and phase shift, respectively. Usually, the total thickness of the organic materials in OLED structure is about 100 nm and the ITO thickness is determined by considering good electrical conductance and high transparence. Therefore, the optical length of the cavity can be modified by varying the second term in eq. (2). If resonant condition for constructive interference is *2L=mλ*, the light with *λ* is selectively

*Diffractive resonators*: The application of Bragg grating to OLEDs has been reported by Matterson *et al*. [21] and Lupton *et al*. [22]. They have demonstrated an increase in the light extraction by Bragg-scattering of waveguided light using a corrugated photoresist layer. However, in this device structure the light must transmit through the absorptive gold and photoresist layers, which limit its absolute efficiency [24]. Later, Ziebarth *et al.* [23] have demonstrated a more conventional ITO-based electroluminescent (EL) device using a stamped Bragg grating into poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) layer. Even though the soft-lithography is beneficial in fabricating large-scale devices at low cost, waveguide absorption is strong in ITO and patterned PEDOT layers [24]. This results in reduction of grating effect particularly in the shorter wavelength region. Fujita *et al*. have also shown the improved electroluminescence from a corrugated ITO device using this concept [27,28]. They have used vacuum evaporated EL materials and square-shaped pattern substrates. Using vacuum evaporation, the organic materials deposited on patterned substrates retain their pattern shape due to the low adatom mobility of deposited organic molecules. This corrugated shape through all device structure enhances the light extraction efficiency by not only waveguided light diffraction but also surface plasmon. However, a square type pattern is not suitable for fabricating stable EL

The enhanced light extraction from OLEDs with diffractive resonators can be explained as follows. First, waveguided light propagation along the in-plane direction of the device is emitted to the surface direction by Bragg diffraction in the grating device. Figure 5 illustrates the mechanism of how the diffracted light can be extracted by Bragg diffraction in a 1-D grating sample for simple consideration. If the light incident on a material with a

1 1 2 21 2 sin sin 0, 0 *m dn n <sup>c</sup>*

 

 (3)

refractive index of *n*2 from that with *n*1, the diffraction condition is given by

 

 

*n L n L n*

where *neff* is the effective refractive index, *ni* is refractive index of *i*th layer,

enhanced and the color purity is also enhanced.

devices because of possible electrical short problems.

– Light Extraction and Polarization Characteristics 69

φ

*<sup>m</sup>* denotes phase

**Figure 3.** Schematic diagram of light extraction from OLED with (a) conventional structure and (b) structure with a thin active layer and a low-refractive index layer.5 Copyright 2001, Wiley-VCH.

**Figure 4.** Schematic structure of microcavity OLEDs.

The schematic structure of a microcavity is shown in Fig. 4. The total optical path difference between direct emission and emission after single-round reflection *L* (see Fig. 4) is given by:

68 Organic Light Emitting Devices

spontaneous emission in microcavity OLEDs.

n1

n2

n3

n1

(b)

n4

n2

n3

(a)

**Figure 4.** Schematic structure of microcavity OLEDs.

be effectively extracted via interference effects. In addition, microcavity provides us with spectral narrowing and spatial redistribution of the emission. Microcavity using inorganic distributed Bragg reflectors (DBRs) consisting of alternating inorganic layers with different refractive indices has been extensively studied over the past several decades. The advantage of DBRs is that they may have very high reflectivity and very low loss. These reflectors have a selective reflectivity in a specific wavelength range, which can form constructive interference, and effectively suppress other modes and induce a high reflectance over a certain range of wavelengths depending on the difference in refractive indices of constituting layers. This leads to spectral narrowing and intensity enhancement of

**Figure 3.** Schematic diagram of light extraction from OLED with (a) conventional structure and (b) structure with a thin active layer and a low-refractive index layer.5 Copyright 2001, Wiley-VCH.

$$L = \frac{\mathcal{\lambda}}{\mathcal{\mathbf{2}}} \left( \frac{n\_{eff}}{\Delta \mathbf{n}} \right) + \sum\_{i} n\_{i} L\_{i} + \left| \frac{\phi\_{m}}{4\pi} \mathcal{\lambda} \right| \tag{2}$$

where *neff* is the effective refractive index, *ni* is refractive index of *i*th layer, φ*<sup>m</sup>* denotes phase shift. The first, second, and third terms stand for effective penetration depth, summation of the optical thickness in each layer, and phase shift, respectively. Usually, the total thickness of the organic materials in OLED structure is about 100 nm and the ITO thickness is determined by considering good electrical conductance and high transparence. Therefore, the optical length of the cavity can be modified by varying the second term in eq. (2). If resonant condition for constructive interference is *2L=mλ*, the light with *λ* is selectively enhanced and the color purity is also enhanced.

*Diffractive resonators*: The application of Bragg grating to OLEDs has been reported by Matterson *et al*. [21] and Lupton *et al*. [22]. They have demonstrated an increase in the light extraction by Bragg-scattering of waveguided light using a corrugated photoresist layer. However, in this device structure the light must transmit through the absorptive gold and photoresist layers, which limit its absolute efficiency [24]. Later, Ziebarth *et al.* [23] have demonstrated a more conventional ITO-based electroluminescent (EL) device using a stamped Bragg grating into poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) layer. Even though the soft-lithography is beneficial in fabricating large-scale devices at low cost, waveguide absorption is strong in ITO and patterned PEDOT layers [24]. This results in reduction of grating effect particularly in the shorter wavelength region. Fujita *et al*. have also shown the improved electroluminescence from a corrugated ITO device using this concept [27,28]. They have used vacuum evaporated EL materials and square-shaped pattern substrates. Using vacuum evaporation, the organic materials deposited on patterned substrates retain their pattern shape due to the low adatom mobility of deposited organic molecules. This corrugated shape through all device structure enhances the light extraction efficiency by not only waveguided light diffraction but also surface plasmon. However, a square type pattern is not suitable for fabricating stable EL devices because of possible electrical short problems.

The enhanced light extraction from OLEDs with diffractive resonators can be explained as follows. First, waveguided light propagation along the in-plane direction of the device is emitted to the surface direction by Bragg diffraction in the grating device. Figure 5 illustrates the mechanism of how the diffracted light can be extracted by Bragg diffraction in a 1-D grating sample for simple consideration. If the light incident on a material with a refractive index of *n*2 from that with *n*1, the diffraction condition is given by

$$m\mathcal{X} = d\_c \left( n\_1 \sin \theta\_1 - n\_2 \sin \theta\_2 \right) \left( \theta\_1 > 0, \theta\_2 < 0 \right) \tag{3}$$

where λ is the wavelength, *n*i (*i*=1 and 2) is the effective refractive index, *d*c is the grating period, and *m* is the diffraction order. If we consider a waveguided light to have a high incident angle (θ1~90°) and diffracted light has the lowest diffraction angle (θ2~0°), then eq. (3) can be simplified as:

$$m\mathcal{X} = n\_{eff}d\_c\tag{4}$$

Effect of Photonic Structures in Organic Light-Emitting Diodes

(5)

2 *m nd eff c* 

For *m*=2, second-order diffracted light makes the counter propagating mode, which results in an optical feedback for lasing. Lasing does not occur along the guided direction because of the low quality of side surfaces but it is outcoupled to the surface normal direction by

Hence, the physical meaning of this relation is that both first- and second-order Bragg diffractions of waveguided light occur simultaneously in different directions. That means waveguided light satisfying the Bragg condition is perfectly extracted toward the surface normal direction by the first-order Bragg diffraction until the second-order Bragg diffracted

Nano-patterned structures are prepared to fabricate corrugated OLEDs. For periodic and quasi-periodic nano-patterned substrates, 1-D and 2-D grating and buckling structures,

(λ=488 nm)

θ ( )Azobenzene film

θ

Beam 2

Beam

Azobenzene

thinfilm

ror

Mirr

<sup>θ</sup> (a) Argon laser

To form a 1-D surface relief grating (SRG) structure, an azobenzene polymer thin film is irradiated using two Ar+ laser (488 nm) beams. This is achieved due to the mass transport from the region of constructive interference to that of destructive interference region in azobenzene polymer, leading to a volume decrease in the highly irradiated (constructive intereference) region with the increase in the irradiation time as shown in Fig. 6. This process is quite different from the other more conventional microscopic processes such as laser ablation and chemical etching. The major advantage of this photo-fabrication approach

Glass

first-order Bragg diffraction.

light decays along in-plane direction.

respectively, are prepared as described below:

**2.2. Device fabrication with nano-patterned structures** 

**Figure 6.** Schematic illustration of surface relief grating fabrication process.

Beam 1

(b)

– Light Extraction and Polarization Characteristics 71

where *n*eff is the effective refractive index. For *m*=1, first-order diffracted light can be extracted to surface normal direction resulting in an increased light extraction.

**Figure 5.** Typical structure of a diffractive resonator along one axis in the plane of the waveguide. 33 Copyright 2008, American Institute of Physics.

This diffraction relation can also be applied to the surface-emitting distributed feedback laser. Here we consider a situation where the waveguided and diffracted lights have high incident angles, i.e., θ1 = 90° and θ2 = - 90°. By substituting these angles into eq. (3), one obtains:

Effect of Photonic Structures in Organic Light-Emitting Diodes – Light Extraction and Polarization Characteristics 71

$$
\hbar \mathfrak{m} \mathcal{A} = \mathfrak{N}\_{\text{eff}} d\_c \tag{5}
$$

For *m*=2, second-order diffracted light makes the counter propagating mode, which results in an optical feedback for lasing. Lasing does not occur along the guided direction because of the low quality of side surfaces but it is outcoupled to the surface normal direction by first-order Bragg diffraction.

Hence, the physical meaning of this relation is that both first- and second-order Bragg diffractions of waveguided light occur simultaneously in different directions. That means waveguided light satisfying the Bragg condition is perfectly extracted toward the surface normal direction by the first-order Bragg diffraction until the second-order Bragg diffracted light decays along in-plane direction.

#### **2.2. Device fabrication with nano-patterned structures**

70 Organic Light Emitting Devices

(3) can be simplified as:

where λ is the wavelength, *n*i (*i*=1 and 2) is the effective refractive index, *d*c is the grating period, and *m* is the diffraction order. If we consider a waveguided light to have a high incident angle (θ1~90°) and diffracted light has the lowest diffraction angle (θ2~0°), then eq.

> *m nd eff c*

where *n*eff is the effective refractive index. For *m*=1, first-order diffracted light can be

Emitted light

2nd order diffracted light 1st order diffracted light

**Figure 5.** Typical structure of a diffractive resonator along one axis in the plane of the waveguide. 33

*wg m <sup>g</sup> k* sin

k0=free space wavevector

kwg=in-plane wavevector

kg=Bragg vector (= ) <sup>Λ</sup>

= ±*k* ± *k* <sup>0</sup>

θ

This diffraction relation can also be applied to the surface-emitting distributed feedback laser. Here we consider a situation where the waveguided and diffracted lights have high incident angles, i.e., θ1 = 90° and θ2 = - 90°. By substituting these angles into eq. (3), one

of a trapped waveguide mode 2π

Copyright 2008, American Institute of Physics.

obtains:

extracted to surface normal direction resulting in an increased light extraction.

n2

n1

(a)

(b)

(4)

Nano-patterned structures are prepared to fabricate corrugated OLEDs. For periodic and quasi-periodic nano-patterned substrates, 1-D and 2-D grating and buckling structures, respectively, are prepared as described below:

**Figure 6.** Schematic illustration of surface relief grating fabrication process.

To form a 1-D surface relief grating (SRG) structure, an azobenzene polymer thin film is irradiated using two Ar+ laser (488 nm) beams. This is achieved due to the mass transport from the region of constructive interference to that of destructive interference region in azobenzene polymer, leading to a volume decrease in the highly irradiated (constructive intereference) region with the increase in the irradiation time as shown in Fig. 6. This process is quite different from the other more conventional microscopic processes such as laser ablation and chemical etching. The major advantage of this photo-fabrication approach

is the possible precise control of grating depth by adjusting the light exposure energy and polarization states of writing beam.

The two recording lights are circularly polarized. Based on the following equation, the periodicity of the SRG can be controlled by changing the crossing angle between two recording laser beams.

$$
\Lambda = \frac{\mathcal{A}\_{\text{laser}}}{2\sin\theta} \tag{6}
$$

Effect of Photonic Structures in Organic Light-Emitting Diodes

However, the basic concept is almost the same in both cases except for deposition method

**Figure 7.** Schematic diagram of nano-imprint process. 34 Copyright 2008, American Institute of Physics.

We prepared ITO or Au stripes as an anode using sputter deposition or thermal evaporation. In case of organic EL devices, a two or three-layered structure consisting of (CuPc: used in 2-D grating device))/TPD/Alq3 is deposited successively by vacuum evaporation. For making the polymeric EL device, PEDOT doped with poly- (styrenesulfonate) PSS is spin coated as a hole transport layer (HTL) and heated at 60 °C for 20 min (*T*g of CYTOP : 108 °C) in an oven. Then MEH-CN-PPV is spin coated as an emissive

After coating organic or polymeric layers using a metal mask, lithium fluoride (LiF) as an electron injection layer (EIL) and Al as a cathode are deposited. The detailed experimental conditions including thickness of each layer were explained elsewhere [33,35,36]. The pixel

In this section, we examine light extraction characteristics from OLED devices with 1-D or 2- D DFB grating substrates. The waveguided light is extracted to normal direction by an imprinted low-refractive index layer (1-D DFB grating). Also, electrical characteristics in OLEDs with 2-D hexagonally nano-imprinted periodic structures are investigated to confirm the enhanced light extraction from this device (2-D DFB grating). We review previously reported results in view of light extraction characteristics and electrical

*Optical characterization of corrugated OLEDs with periodic structures (1-D grating)*: Figure 8(a) shows a schematic illustration of the Bragg diffraction process of waveguided light in periodic structures. When waveguided light is incident on the grating structure, the light is reflected by

with a size of 3 mm×3 mm was used for electroluminescence measurements.

**2.3. Effects of periodic DFB grating structures in OLEDs** 

characteristics from periodically corrugated OLEDs.

and materials used.

layer (EML) on the HTL successively.

– Light Extraction and Polarization Characteristics 73

where *λlaser* is the wavelength of the laser and is an incidence angle. The formed SRG pattern is transferred onto a UV curable epoxy or a CYTOP (perfluoropolymer, Asahi Glass Co. Ltd.,), so that a patterned substrate with an SRG is obtained.

For a buckling fabrication, standard commercial poly(dimethylsiloxane) (PDMS) materials (Wacker ELASTOSIL RT 601) are mixed with a curing agent in a weight ratio of 9:1 and then spin-coated on a pre-cleaned glass substrate. The coated PDMS is cured at 100 °C for 1 h and then an aluminum layer is deposited on it. A structure thus prepared is heated to 100 °C with an external radiation source by thermal evaporation at a pressure below 1 × 10-3 Pa. It is then cooled to the ambient temperature by keeping it in a chamber for more than 30 min and venting to atmosphere. The difference in the thermal expansion coefficients between the aluminum film and PDMS generates a buckled structure on the PDMS film. The PDMS replica is formed by pouring PDMS over the buckled PDMS master and by curing it at 100 °C for 1 h. The PDMS replica can be easily peeled off from the PDMS master. For the second deposition of a 10-nm-thick aluminum layer, the buckled PDMS replica is used as the substrate. After deposition, the buckling pattern of the replica is transferred to a glass substrate after UV curing in curable resin (Norland Optical Adhesive 81) for 10 min. For the third deposition of a 10-nm-thick aluminum layer, the buckled PDMS replica thus fabricated is used as the substrate, and the above process is repeated. After the deposition, the buckled resin master is again produced, from which, finally, the buckled PDMS replica mould for making devices is fabricated.

The method of replicating nano-patterned structures (azobenzene film and buckling) onto substrates is shown in Fig. 7 [34]. First, the patterned azobenzene polymer film or buckling pattern is converted to a master mould of PDMS. After the heat treatment at 50 °C for 1 h the silicon rubber becomes firm and it can easily be separated from azobenzene polymer substrate as a free-standing film. The CYTOP solution (CTL-109A, Asahi Glass Co. Ltd.) is drop-cast on a glass substrate and is heated under the patterned silicon rubber mould pressed on it at 60 °C for 1 h. For the UV curable epoxy (Norland Optical Adhesive 81), UV light is irradiated to harden the epoxy layer under silicon rubber mould. The patterned structure is thus replicated onto a substrate.

We fabricated OLEDs on patterned nano-structured substrates. The device structures introduced in this chapter are classified in two parts, organic and polymeric devices. The organic layers in the organic device are coated by thermal evaporation (2-D grating & buckling devices), whereas spin coating (1-D grating device) is used in the polymeric device. However, the basic concept is almost the same in both cases except for deposition method and materials used.

72 Organic Light Emitting Devices

recording laser beams.

making devices is fabricated.

structure is thus replicated onto a substrate.

polarization states of writing beam.

is the possible precise control of grating depth by adjusting the light exposure energy and

The two recording lights are circularly polarized. Based on the following equation, the periodicity of the SRG can be controlled by changing the crossing angle between two

> 2sin *laser*

where *λlaser* is the wavelength of the laser and is an incidence angle. The formed SRG pattern is transferred onto a UV curable epoxy or a CYTOP (perfluoropolymer, Asahi Glass

For a buckling fabrication, standard commercial poly(dimethylsiloxane) (PDMS) materials (Wacker ELASTOSIL RT 601) are mixed with a curing agent in a weight ratio of 9:1 and then spin-coated on a pre-cleaned glass substrate. The coated PDMS is cured at 100 °C for 1 h and then an aluminum layer is deposited on it. A structure thus prepared is heated to 100 °C with an external radiation source by thermal evaporation at a pressure below 1 × 10-3 Pa. It is then cooled to the ambient temperature by keeping it in a chamber for more than 30 min and venting to atmosphere. The difference in the thermal expansion coefficients between the aluminum film and PDMS generates a buckled structure on the PDMS film. The PDMS replica is formed by pouring PDMS over the buckled PDMS master and by curing it at 100 °C for 1 h. The PDMS replica can be easily peeled off from the PDMS master. For the second deposition of a 10-nm-thick aluminum layer, the buckled PDMS replica is used as the substrate. After deposition, the buckling pattern of the replica is transferred to a glass substrate after UV curing in curable resin (Norland Optical Adhesive 81) for 10 min. For the third deposition of a 10-nm-thick aluminum layer, the buckled PDMS replica thus fabricated is used as the substrate, and the above process is repeated. After the deposition, the buckled resin master is again produced, from which, finally, the buckled PDMS replica mould for

The method of replicating nano-patterned structures (azobenzene film and buckling) onto substrates is shown in Fig. 7 [34]. First, the patterned azobenzene polymer film or buckling pattern is converted to a master mould of PDMS. After the heat treatment at 50 °C for 1 h the silicon rubber becomes firm and it can easily be separated from azobenzene polymer substrate as a free-standing film. The CYTOP solution (CTL-109A, Asahi Glass Co. Ltd.) is drop-cast on a glass substrate and is heated under the patterned silicon rubber mould pressed on it at 60 °C for 1 h. For the UV curable epoxy (Norland Optical Adhesive 81), UV light is irradiated to harden the epoxy layer under silicon rubber mould. The patterned

We fabricated OLEDs on patterned nano-structured substrates. The device structures introduced in this chapter are classified in two parts, organic and polymeric devices. The organic layers in the organic device are coated by thermal evaporation (2-D grating & buckling devices), whereas spin coating (1-D grating device) is used in the polymeric device.

Co. Ltd.,), so that a patterned substrate with an SRG is obtained.

(6)

**Figure 7.** Schematic diagram of nano-imprint process. 34 Copyright 2008, American Institute of Physics.

We prepared ITO or Au stripes as an anode using sputter deposition or thermal evaporation. In case of organic EL devices, a two or three-layered structure consisting of (CuPc: used in 2-D grating device))/TPD/Alq3 is deposited successively by vacuum evaporation. For making the polymeric EL device, PEDOT doped with poly- (styrenesulfonate) PSS is spin coated as a hole transport layer (HTL) and heated at 60 °C for 20 min (*T*g of CYTOP : 108 °C) in an oven. Then MEH-CN-PPV is spin coated as an emissive layer (EML) on the HTL successively.

After coating organic or polymeric layers using a metal mask, lithium fluoride (LiF) as an electron injection layer (EIL) and Al as a cathode are deposited. The detailed experimental conditions including thickness of each layer were explained elsewhere [33,35,36]. The pixel with a size of 3 mm×3 mm was used for electroluminescence measurements.
