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

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 characteristics from periodically corrugated OLEDs.

*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 a photonic band gap and simultaneously diffracted in the direction perpendicular to the photonic crystal surface because the Bragg condition is satisfied in this direction. The angle *θ* of the emission direction with respect to the surface normal is governed by the conservation of momentum in the plane of the waveguide [21-24,37,38] given by

$$k\_0 \sin \theta \ = \pm k\_{\text{avg}} \pm k\_g = \pm \frac{2\pi n\_{eff}}{\lambda} \pm m \frac{2\pi}{\Lambda} \tag{7}$$

Effect of Photonic Structures in Organic Light-Emitting Diodes

(8)

Figure 9 shows the angle dependence of EL spectra for the two EL devices with and without the grating structure. For measuring the EL spectra, a detector with a diameter of 5 mm is located 10 cm apart from the device surface. A sharp peak (632 nm) has been observed in the normal direction (*θ* =0°), and a peak splitting has been found to occur by increasing the detection angle because the grating diffracted waveguided light travels in the opposite direction. The wavelengths of the separated two peaks have been measured as a function of the detection angle as shown in Fig. 10(a). The measured wavelength positions agree well

> 2 21 3 2 1 3 32 arcsin sin arcsin sin arcsin sin *n nn n nn*

considering all refractions in PPFVB/glass/air, as shown in Fig. 10(b). This means that extraction angle of Bragg diffracted light is closely related to refractive indices of stacked

Calculated ( )

θ

Calculated ( Measured θ1

**0 5 10 15 20**

**Extraction angle (degrees)**

Calculated ( ) θ3

**Figure 10.** (a) Measured extraction angles of Bragg diffracted light. *θ*1 and *θ*3 denote calculated extraction angles in CYTOP and air, respectively. (b) Schematic illustration for refraction of Bragg diffracted light in each stacked material. 33 Copyright 2008, American Institute of Physics.

To compare the effects of refractive indices of imprinted materials, we have calculated light extraction angles from materials with various refractive indices, as shown in Fig. 11. The wavelength of vertically emitted light is assumed to be 632 nm. According to eq. (8), as the refractive index becomes high, the light extraction angle becomes wider. For *n*=1.80, the extraction angle of light with wavelength of 800 nm is 65°, whereas the total light extraction angle is only 23° for *n*=1.00. In the case of CYTOP (*n*=1.34), the total light extraction angle is

with the lines given by eq. (8) with *θ3*, which is given by

 

**750 800**

**550**

**(nm)**

(a)

(b)

materials.

– Light Extraction and Polarization Characteristics 75

where *λ* is the wavelength, *k*wg and *k*g are respectively the wavenumbers of the waveguided light and the grating with a period *Λ*, *neff* is the effective refractive index of the waveguide mode, *k0* is the free-space wavenumber of the diffracted light, and *m* is the diffraction order. If *Λ* and *neff* are known, eq. (7) gives the emission angle of extracted light as a function of wavelength for the first-order Bragg diffraction of waveguided light.

**Figure 8.** (a) Schematic illustration of Bragg diffraction of waveguided light. (b) AFM image of patterned azobenzene polymer. 33 Copyright 2008, American Institute of Physics.

**Figure 9.** Angular dependence of electroluminescence spectra from OLED devices with flat (green curve at *θ* =0°) and patterned CYTOP layer. 33 Copyright 2008, American Institute of Physics.

Figure 9 shows the angle dependence of EL spectra for the two EL devices with and without the grating structure. For measuring the EL spectra, a detector with a diameter of 5 mm is located 10 cm apart from the device surface. A sharp peak (632 nm) has been observed in the normal direction (*θ* =0°), and a peak splitting has been found to occur by increasing the detection angle because the grating diffracted waveguided light travels in the opposite direction. The wavelengths of the separated two peaks have been measured as a function of the detection angle as shown in Fig. 10(a). The measured wavelength positions agree well with the lines given by eq. (8) with *θ3*, which is given by

74 Organic Light Emitting Devices

a photonic band gap and simultaneously diffracted in the direction perpendicular to the photonic crystal surface because the Bragg condition is satisfied in this direction. The angle *θ* of the emission direction with respect to the surface normal is governed by the conservation of

> <sup>2</sup> <sup>2</sup> sin *eff wg g*

where *λ* is the wavelength, *k*wg and *k*g are respectively the wavenumbers of the waveguided light and the grating with a period *Λ*, *neff* is the effective refractive index of the waveguide mode, *k0* is the free-space wavenumber of the diffracted light, and *m* is the diffraction order. If *Λ* and *neff* are known, eq. (7) gives the emission angle of extracted light as a function of

*k kk m*

**Figure 8.** (a) Schematic illustration of Bragg diffraction of waveguided light. (b) AFM image of

θ **= 0**°

θ **= 3**°

θ **= 5**°

θ **= 1**°

**Figure 9.** Angular dependence of electroluminescence spectra from OLED devices with flat (green curve at *θ* =0°) and patterned CYTOP layer. 33 Copyright 2008, American Institute of Physics.

**500 600 700 800**

**Wavelength (nm)**

patterned azobenzene polymer. 33 Copyright 2008, American Institute of Physics.

**(a.u.)**

**Intensity** 

*n*

 (7)

momentum in the plane of the waveguide [21-24,37,38] given by

wavelength for the first-order Bragg diffraction of waveguided light.

0

$$\theta\_3 = \arcsin\left(\frac{n\_2}{n\_3}\sin\theta\_2\right) = \arcsin\left(\frac{n\_2}{n\_3}\sin\left(\arcsin\left(\frac{n\_1}{n\_2}\sin\theta\_1\right)\right)\right) \tag{8}$$

considering all refractions in PPFVB/glass/air, as shown in Fig. 10(b). This means that extraction angle of Bragg diffracted light is closely related to refractive indices of stacked materials.

**Figure 10.** (a) Measured extraction angles of Bragg diffracted light. *θ*1 and *θ*3 denote calculated extraction angles in CYTOP and air, respectively. (b) Schematic illustration for refraction of Bragg diffracted light in each stacked material. 33 Copyright 2008, American Institute of Physics.

To compare the effects of refractive indices of imprinted materials, we have calculated light extraction angles from materials with various refractive indices, as shown in Fig. 11. The wavelength of vertically emitted light is assumed to be 632 nm. According to eq. (8), as the refractive index becomes high, the light extraction angle becomes wider. For *n*=1.80, the extraction angle of light with wavelength of 800 nm is 65°, whereas the total light extraction angle is only 23° for *n*=1.00. In the case of CYTOP (*n*=1.34), the total light extraction angle is

35°. Thus nano-imprinted CYTOP layer can extract waveguided light with high directionality. Such characteristics provide an advantage for small- or medium-size OLEDs, which are mainly viewed from the forward direction [39].

Effect of Photonic Structures in Organic Light-Emitting Diodes

*Electrical characterization of corrugated OLEDs with periodic structures (2-D grating)*: The current–voltage (I-V) characteristics of an EL device with a 2-D grating (2-D grating device) has been measured and compared with those of an EL device without grating (non-grating device). The 2-D grating device shows a higher current level compared to the non-grating device, as shown in Fig. 13(a). Both EL devices show a power-law dependence of *I~V6-7* over a large current and voltage range. Because of large trap concentration and low mobility in organic semiconductors, the carrier transport in OLEDs is trap-charge-limited current

**With 2-D grating**

**1 10**

**Voltage (V)**

**012345**

**Voltage (V) Voltage (V)**

**Without 2-D grating**

**Without 2-D grating**

**Vtr**

**Figure 13.** (a) Current-voltage plot measured from a 2-D grating and non-grating devices. (b) Magnified current-voltage plots in low voltage region.35 Copyright 2008, The Japan Society of Applied

current must satisfy TCLC conduction more quickly.

observed. In this case, the current density *J* is described by

One may intuitively think that higher current effect in 2-D grating devices is simply due to the increase of interface contact area by corrugation between electrode and organic semiconductors. However, this cannot explain the increase of transition voltage (*Vtr*) at which the conduction model changes from ohmic to TCLC, as indicated by two arrows in Fig. 13(a). If the increase in the interface contact area is a major effect, *Vtr* in the grating device must be shifted to a voltage lower than that of the non-grating device because higher

**With 2-D grating**

At low voltages, low-mobility ohmic conduction via thermally generated free charge is

Physics.

(TCLC) [40], which is known to show power law dependence.

**m2**

(a)

**)**

**density (A/**

**d**

**A/cm2**

**A**

**nt density (**

**Curre**

(b)

**)**

**10-3 c**

**10**

**10-5**

**10-7**

**(x 10-5)**

**10-6 Current**

**10-4**

– Light Extraction and Polarization Characteristics 77

**Figure 11.** Calculated extraction angles in imprinted materials with various refractive indices as a function of wavelength. 33 Copyright 2008, American Institute of Physics.

The effect of a grating on normally-directed EL has been observed by collecting EL spectra between ±18.4°, as shown in Fig. 12. The enhancement of EL spectra in the device with patterned CYTOP layer has been observed over the wavelength range from 540 nm to 728 nm. However, it should be noted that the highest EL intensity is observed only around 650 nm, whereas vertically directed emission peak position is 632 nm. This results from different transmittance of ITO at various wavelengths as shown in Fig. 12. Because the wavelength of the highest EL intensity is closely related to both natural fluorescence and waveguide absorption in ITO layer, the light can be extracted more efficiently due to the high transmittance and fluorescence at 650 nm. Hence, the grating effect is higher in longer wavelength region which has higher transmittance. If ITO with high transparency is possible to be deposited at room temperature, the grating effect of CYTOP with high transmittance will be increased.

**Figure 12.** Overall EL spectra within ±18.4° from OLEDs with flat and patterned CYTOP layers. Two dotted lines show transmittance of CYTOP/Glass and ITO/CYTOP/Glass, respectively.33 Copyright 2008, American Institute of Physics.

*Electrical characterization of corrugated OLEDs with periodic structures (2-D grating)*: The current–voltage (I-V) characteristics of an EL device with a 2-D grating (2-D grating device) has been measured and compared with those of an EL device without grating (non-grating device). The 2-D grating device shows a higher current level compared to the non-grating device, as shown in Fig. 13(a). Both EL devices show a power-law dependence of *I~V6-7* over a large current and voltage range. Because of large trap concentration and low mobility in organic semiconductors, the carrier transport in OLEDs is trap-charge-limited current (TCLC) [40], which is known to show power law dependence.

76 Organic Light Emitting Devices

transmittance will be increased.

American Institute of Physics.

35°. Thus nano-imprinted CYTOP layer can extract waveguided light with high directionality. Such characteristics provide an advantage for small- or medium-size OLEDs,

**0 10 20 30 40 50 60 70**

**n=1.00, d=423nm n=1.50, d=363nm**

**n=1.80, d=334nm n=1.34, d=380nm**

> **0.8 1.0**

> **0.2 0.4 0.6**

> **0.0**

**ITO/PPFVP/Glass**

ITO/CYTOP/Glass

**PPFVB/Glass**

CYTOP/Glass

**n**

**Transmittan**

**ce**

**Figure 11.** Calculated extraction angles in imprinted materials with various refractive indices as a

The effect of a grating on normally-directed EL has been observed by collecting EL spectra between ±18.4°, as shown in Fig. 12. The enhancement of EL spectra in the device with patterned CYTOP layer has been observed over the wavelength range from 540 nm to 728 nm. However, it should be noted that the highest EL intensity is observed only around 650 nm, whereas vertically directed emission peak position is 632 nm. This results from different transmittance of ITO at various wavelengths as shown in Fig. 12. Because the wavelength of the highest EL intensity is closely related to both natural fluorescence and waveguide absorption in ITO layer, the light can be extracted more efficiently due to the high transmittance and fluorescence at 650 nm. Hence, the grating effect is higher in longer wavelength region which has higher transmittance. If ITO with high transparency is possible to be deposited at room temperature, the grating effect of CYTOP with high

**Collecting**

**angle (18.4**°)

**0 10 20 30 40 50 60 70**

**Extraction angle (degree)**

**Figure 12.** Overall EL spectra within ±18.4° from OLEDs with flat and patterned CYTOP layers. Two dotted lines show transmittance of CYTOP/Glass and ITO/CYTOP/Glass, respectively.33 Copyright 2008,

**Grating**

**No grating**

**500 600 700 800**

**gratingWavelength (nm)**

function of wavelength. 33 Copyright 2008, American Institute of Physics.

**800 1000**

> **200 400 600**

> > **0**

**u.)**

**Intensity (a.**

which are mainly viewed from the forward direction [39].

**750**

**700**

**600**

**550 Wa**

**650**

**m)**

**m**

**velength (n**

**v**

**800**

**Figure 13.** (a) Current-voltage plot measured from a 2-D grating and non-grating devices. (b) Magnified current-voltage plots in low voltage region.35 Copyright 2008, The Japan Society of Applied Physics.

One may intuitively think that higher current effect in 2-D grating devices is simply due to the increase of interface contact area by corrugation between electrode and organic semiconductors. However, this cannot explain the increase of transition voltage (*Vtr*) at which the conduction model changes from ohmic to TCLC, as indicated by two arrows in Fig. 13(a). If the increase in the interface contact area is a major effect, *Vtr* in the grating device must be shifted to a voltage lower than that of the non-grating device because higher current must satisfy TCLC conduction more quickly.

At low voltages, low-mobility ohmic conduction via thermally generated free charge is observed. In this case, the current density *J* is described by

$$J = q\mu\_n n\_0 V / d\_{t'} \tag{9}$$

Effect of Photonic Structures in Organic Light-Emitting Diodes

Even though the depth of patterned shape is only 50 nm, the 2-D grating device does not show any breakdown during applying voltage. Generally thin EL devices can easily suffer from breakdowns because the internal field distribution is very sensitive to interface roughness and dust particles. It is therefore very important that the patterned electrode shape must be optimized for the stability of EL devices. Otherwise, the patterned electrode may result in worse device condition without realizing any high light extraction efficiency. Because of this reason, one should use those patterned electrode structures which give minimal 'partial reduction thickness of organic layers'. This means that if the field distribution between cathode and anode is uniform, the possibility of breakdown may be reduced even when the depth of patterned shape is high. For studying the effect of the shape of patterned electrodes, we have calculated the static field distribution in EL devices for patterned electrodes of square and sinusoidal shapes. Although the light is diffracted by 500-nm-pitched lines, which has the same width as the interfered periodicity of the two Ar+ laser beams (Fig. 6(b)), the electric field distribution is related to the distance between closest protrusions. Hence the distance used for the calculation is 580 nm. (See Fig. 14(c)). As shown in Fig. 14(a), a high electric field gets localized at the edges of square-shaped cathode and anode electrodes. However, if the patterned electrodes are of sinusoidal shapes the field distribution becomes almost uniform. Figure 14(b) shows static field distribution in sinusoidal-shaped electrode. Note that we use different ranges of relative field intensity in Fig. 14(a) and (b) to clearly visualize the field distribution as color variations. Although the field is concentrated in the intermediate regions between the top and bottom of the patterned electrodes (see Fig. 14(b)), the field distribution becomes much more uniform compared with the case of Fig. 14(a). Figure 14(d) represents the depth profile of a patterned azobenzene film obtained along a red line in the AFM image shown in Fig. 14(c). The shape at the upper region is approximated as sinusoidal. This shape results in no breakdown of 2-

Next, we describe the relationship among the reduction of thicknesses, current efficiency, and diffraction effects. Figure 15(a) displays the external current efficiency versus current density. In the 2-D grating device, a higher efficiency is obtained in a high current density region. However, below a current density of 3×10-5A/cm2, the efficiency of the non-grating device is found to be slightly higher than that of the 2-D grating device, as shown in Fig. 15(b). How can we explain this? As mentioned above, the major difference between a 2-D grating device and a non-grating device is in *RB* or effective thickness of the bulk layer; i.e., *RB* is lower and the layer is thinner in the 2-D grating device than that in the non-grating device. Hence, we should discuss the dependence of current efficiency on the emitter thickness [41]. For this purpose, the recombination probability (*Prec*), which is directly proportional to the EL yield, is considered. *Prec* is defined by the ratio of the recombination

/ 1/ 1 / *rec t t rec rec t P*

This gives *P*rec = 1 when τrec/τt=0 and *P*rec decreases with increasing τrec/τt. The thickness

  (11)

D grating devices even though leakage current is high.

time *τrec* and the transit time *τt* of the charge carriers as:

dependence in t comes only from:

 – Light Extraction and Polarization Characteristics 79

where *q* is the electronic charge, *μn* is electron mobility, *n0* is a thermally generated background free charge density, *V* is the applied voltage, and *dt* is the organic layer thickness. In order to find what induces the low voltage ohmic current, we have examined the *I-V* plot in the low voltage range. According to Fig. 13(b), the 2-D grating device shows a higher ohmic current than that of the non-grating device. This means that the 2-D grating device has a lower total resistance *Rtotal* which is a sum of junction resistance (*RJ*), bulk resistance (*RB*) of organic layers and electrode resistance (*REL*) and is given by:

$$R\_{total} = R\_f + R\_B + R\_{EL} \tag{10}$$

Here the ohmic resistance induced by Al and Au (*REL*) and the junction resistance (*RJ*) induced by interfacial barrier between electrode and organic layer are the same in both samples. Hence *RB* in the 2-D grating device must be smaller than that in the non-grating device. This result may be understood from the concept of 'partial reduction thickness of organic layers' proposed by Fujita *et al*. [27,28]. They have observed improved electroluminescence from a corrugated ITO where the reduction of thickness of organic layers is effectively induced by each edge of Al and ITO square-shape patterned electrodes shown as black areas in Fig. 14(a). At the edge of each patterned electrode, a higher electric field develops (See Fig. 14 (a)) and this results in reduction of operating voltage. Thus, the increased low voltage current in the 2-D grating device may be explained due to the lower bulk resistance (*RB*) and hence the lower total serial resistance.

**Figure 14.** Calculated static field distribution between (a) square- and (b) sinusoidal-shape patterned electrodes. (c) AFM image of a patterned UV epoxy layer. (d) Depth profile along a red line in (c).35 Copyright 2008, The Japan Society of Applied Physics.

Even though the depth of patterned shape is only 50 nm, the 2-D grating device does not show any breakdown during applying voltage. Generally thin EL devices can easily suffer from breakdowns because the internal field distribution is very sensitive to interface roughness and dust particles. It is therefore very important that the patterned electrode shape must be optimized for the stability of EL devices. Otherwise, the patterned electrode may result in worse device condition without realizing any high light extraction efficiency. Because of this reason, one should use those patterned electrode structures which give minimal 'partial reduction thickness of organic layers'. This means that if the field distribution between cathode and anode is uniform, the possibility of breakdown may be reduced even when the depth of patterned shape is high. For studying the effect of the shape of patterned electrodes, we have calculated the static field distribution in EL devices for patterned electrodes of square and sinusoidal shapes. Although the light is diffracted by 500-nm-pitched lines, which has the same width as the interfered periodicity of the two Ar+ laser beams (Fig. 6(b)), the electric field distribution is related to the distance between closest protrusions. Hence the distance used for the calculation is 580 nm. (See Fig. 14(c)). As shown in Fig. 14(a), a high electric field gets localized at the edges of square-shaped cathode and anode electrodes. However, if the patterned electrodes are of sinusoidal shapes the field distribution becomes almost uniform. Figure 14(b) shows static field distribution in sinusoidal-shaped electrode. Note that we use different ranges of relative field intensity in Fig. 14(a) and (b) to clearly visualize the field distribution as color variations. Although the field is concentrated in the intermediate regions between the top and bottom of the patterned electrodes (see Fig. 14(b)), the field distribution becomes much more uniform compared with the case of Fig. 14(a). Figure 14(d) represents the depth profile of a patterned azobenzene film obtained along a red line in the AFM image shown in Fig. 14(c). The shape at the upper region is approximated as sinusoidal. This shape results in no breakdown of 2- D grating devices even though leakage current is high.

78 Organic Light Emitting Devices

<sup>0</sup> / , *n t J q nV d* 

where *q* is the electronic charge, *μn* is electron mobility, *n0* is a thermally generated background free charge density, *V* is the applied voltage, and *dt* is the organic layer thickness. In order to find what induces the low voltage ohmic current, we have examined the *I-V* plot in the low voltage range. According to Fig. 13(b), the 2-D grating device shows a higher ohmic current than that of the non-grating device. This means that the 2-D grating device has a lower total resistance *Rtotal* which is a sum of junction resistance (*RJ*), bulk

Here the ohmic resistance induced by Al and Au (*REL*) and the junction resistance (*RJ*) induced by interfacial barrier between electrode and organic layer are the same in both samples. Hence *RB* in the 2-D grating device must be smaller than that in the non-grating device. This result may be understood from the concept of 'partial reduction thickness of organic layers' proposed by Fujita *et al*. [27,28]. They have observed improved electroluminescence from a corrugated ITO where the reduction of thickness of organic layers is effectively induced by each edge of Al and ITO square-shape patterned electrodes shown as black areas in Fig. 14(a). At the edge of each patterned electrode, a higher electric field develops (See Fig. 14 (a)) and this results in reduction of operating voltage. Thus, the increased low voltage current in the 2-D grating device may be explained due to the lower

**Figure 14.** Calculated static field distribution between (a) square- and (b) sinusoidal-shape patterned electrodes. (c) AFM image of a patterned UV epoxy layer. (d) Depth profile along a red line in (c).35

resistance (*RB*) of organic layers and electrode resistance (*REL*) and is given by:

bulk resistance (*RB*) and hence the lower total serial resistance.

Copyright 2008, The Japan Society of Applied Physics.

(9)

*R RRR total J B EL* (10)

Next, we describe the relationship among the reduction of thicknesses, current efficiency, and diffraction effects. Figure 15(a) displays the external current efficiency versus current density. In the 2-D grating device, a higher efficiency is obtained in a high current density region. However, below a current density of 3×10-5A/cm2, the efficiency of the non-grating device is found to be slightly higher than that of the 2-D grating device, as shown in Fig. 15(b). How can we explain this? As mentioned above, the major difference between a 2-D grating device and a non-grating device is in *RB* or effective thickness of the bulk layer; i.e., *RB* is lower and the layer is thinner in the 2-D grating device than that in the non-grating device. Hence, we should discuss the dependence of current efficiency on the emitter thickness [41]. For this purpose, the recombination probability (*Prec*), which is directly proportional to the EL yield, is considered. *Prec* is defined by the ratio of the recombination time *τrec* and the transit time *τt* of the charge carriers as:

$$P\_{rec} = \ \tau\_t \ / \ \left(\tau\_t + \tau\_{rec}\right) = 1 / \left(1 + \tau\_{rec} \ / \ \tau\_t\right) \tag{11}$$

This gives *P*rec = 1 when τrec/τt=0 and *P*rec decreases with increasing τrec/τt. The thickness dependence in t comes only from:

$$
\pi\_t = \left. d\_t \right| \left. \mu \right| \mathbf{F}\_t \tag{12}
$$

Effect of Photonic Structures in Organic Light-Emitting Diodes

**Figure 16.** (a) Schematic illustration of diffraction of the guided light. (b) Schematic illustration of the angle range for total reflection and diffraction at 500 nm wavelength. (c) The diffraction limit of incidence angle θ1 for the first diffraction as a function of wavelength. 35 Copyright 2008, The Japan

light to an angle below the critical angle (61°) can be emitted due to the total internal reflection. However, in grating devices, the incident light within the angle range between *θ<sup>1</sup>* and 90° is diffracted and then emitted at an angle between *θ2* and -90°. For example, in the case of a light with 500 nm wavelength, *θ1* and *θ2* are 0° and 7.6°, respectively. This means that the incident light within the angle range between 0° and 90° can be emitted at an angle

Society of Applied Physics.

– Light Extraction and Polarization Characteristics 81

where *dt* is the emitter layer thickness, the carrier mobility, and *F* the applied electric field operating on the sample. According to eqs. (11) and (12), at a given electric field, increasing emitting layer thickness will increase *τt* and hence *Prec*. Employing this theory, 2-D grating device must show lower current efficiency due to the short transit time by reduction of thickness.

**Figure 15.** (a) Extracted current efficiency against current density measured from a 2-D grating and non-grating devices. (b) Magnified extracted efficiency vs current density in low current density region. 35 Copyright 2008, The Japan Society of Applied Physics.

It should be noted, however, that a higher current efficiency in the 2-D grating device increases even though the non recombined current is higher. The enhanced current efficiency in the 2-D grating device can be explained as follows. First, the waveguided light propagating along the in-plane direction of the device is emitted to the surface direction by Bragg diffraction in the 2-D grating device. Figure 16(a) shows 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 the refractive index of *n2* from that of *n1*, the diffraction condition is given by

$$
\hbar \, m \mathcal{X} = d\_c (n\_1 \sin \theta\_1 - n\_2 \sin \theta\_2) \tag{13}
$$

where *d*c is a periodic distance and *m* is a diffraction order. Because the refractive indices are *n1*=1.7 and *n2*=1.5 in EL layer and epoxy/glass, respectively, the 1st-order diffracted light (*m* = 1) can be emitted, as shown in Fig. 16(b) and (c). In flat devices without grating, only EL Effect of Photonic Structures in Organic Light-Emitting Diodes – Light Extraction and Polarization Characteristics 81

80 Organic Light Emitting Devices

thickness.

/ , *t t*

where *dt* is the emitter layer thickness, the carrier mobility, and *F* the applied electric field operating on the sample. According to eqs. (11) and (12), at a given electric field, increasing emitting layer thickness will increase *τt* and hence *Prec*. Employing this theory, 2-D grating device must show lower current efficiency due to the short transit time by reduction of

**1.0 Without 2-D grating**

**Figure 15.** (a) Extracted current efficiency against current density measured from a 2-D grating and non-grating devices. (b) Magnified extracted efficiency vs current density in low current density region.

It should be noted, however, that a higher current efficiency in the 2-D grating device increases even though the non recombined current is higher. The enhanced current efficiency in the 2-D grating device can be explained as follows. First, the waveguided light propagating along the in-plane direction of the device is emitted to the surface direction by Bragg diffraction in the 2-D grating device. Figure 16(a) shows 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 the refractive index of *n2* from that of *n1*, the diffraction

1122 ( sin sin ) *m dn n <sup>c</sup>*

where *d*c is a periodic distance and *m* is a diffraction order. Because the refractive indices are *n1*=1.7 and *n2*=1.5 in EL layer and epoxy/glass, respectively, the 1st-order diffracted light (*m* = 1) can be emitted, as shown in Fig. 16(b) and (c). In flat devices without grating, only EL

  (13)

35 Copyright 2008, The Japan Society of Applied Physics.

condition is given by

 

**With 2-D grating**

**0.0 5.0x10-4 1.0x10-3 1.5x10-3 0.0**

**With 2-D grating**

**Current density (A/cm2**

**0.0 4.0x10-5 8.0x10-5**

**Current density (A/cm2**

**)**

**Without 2-D grating**

**)**

*d F* (12)

**1.5**

**0 5**

**1.0**

**0.5 ent efficienc**

**0.0**

**cy (cd/A)**

**Curr**

**e**

(b)

**0.5Curren**

**nt efficiency**

**2.0 y (cd/A)**

(a)

**Figure 16.** (a) Schematic illustration of diffraction of the guided light. (b) Schematic illustration of the angle range for total reflection and diffraction at 500 nm wavelength. (c) The diffraction limit of incidence angle θ1 for the first diffraction as a function of wavelength. 35 Copyright 2008, The Japan Society of Applied Physics.

light to an angle below the critical angle (61°) can be emitted due to the total internal reflection. However, in grating devices, the incident light within the angle range between *θ<sup>1</sup>* and 90° is diffracted and then emitted at an angle between *θ2* and -90°. For example, in the case of a light with 500 nm wavelength, *θ1* and *θ2* are 0° and 7.6°, respectively. This means that the incident light within the angle range between 0° and 90° can be emitted at an angle

in the range between 7.6° and -90°. It should be noted that an incident light within 61°~90° cannot be emitted in flat devices without a grating. In other words, the light diffracted from the incidence angle in the range between 61° and 90° contributes to additional light extraction in 2-D grating devices obtained, resulting in an increase in the of light output.

Effect of Photonic Structures in Organic Light-Emitting Diodes

**Figure 17.** AFM analysis of buckling pattern. (a) Buckled structure formed by a 10-nm-thick Al layer. (b),(c) Buckled structures formed by deposition of a 10-nm-thick Al layer twice and three times, respectively. Resin layers imprinted with a buckled PDMS replica were used for measurement. Inset: FFT patterns of each image. (d) Power spectra from FFTs as a function of wavelength for buckled patterns obtained with deposition of a 10-nm-thick Al layer once (black), twice (red) and three times

The observation that the FFT ring patterns are of similar size indicates that the characteristic wavelength does not change after redeposition. Moreover, the FFT ring patterns after multiple deposition processes display more diffuse patterns, indicating a broader distribution. The power spectra in Fig. 17(d) represent the unchanged peak wavelengths at ~410 nm and the broader distributions in the long wavelength side for the multiple depositions. In addition, the surface area ratio after deposition twice and three times significantly increases from ~1.4% to ~9.0% and 11.3% corresponding to depths of 40–70 nm

The devices with buckling show higher current density (*J*) and luminance (*L*) than those without buckling and a device with triple buckling shows higher *J* and *L* than that with only double buckling (Fig. 18(a)). It has been reported that the larger *J* in the corrugated device mainly results from a stronger electric field because of the partially reduced organic layer thickness in the intermediate region between the peak and valley of the sinusoidal patterned gratings [27,35]. Measurements have also been made on devices without buckling but with the organic layer thickness decreased by 20% and 40%. As mentioned in 2.3, current density (*J*) for these devices is shown by dotted and dashed curves in Fig. 18. The current density in the device with triple buckling lies between that in the reference devices and in devices with thinner organic layers. This suggests that the thickness of the organic layers on buckling is partially reduced by ~20–40%. In the devices with double, triple and without buckling, the

(blue).36 Copyright 2010, Nature Publishing Group.

and 50–70 nm, respectively.

– Light Extraction and Polarization Characteristics 83

Another aspect of enhancing the light extraction in a 2-D grating device is by recovering the quenched light coupled with surface plasmon mode. This effect can be observed in an Alq3 based system because the excitons have no preferred orientation in an Alq3 layer, whereas conjugated polymer systems show a lower effect because the dipole moments lie in the plane of the film due to spin casting. Hobson *et al*. [42] have found that a further recovery of the trapped light can be obtained by the surface plasmon with the help of a periodic grating formed on substrate particularly in Alq3-based EL devices. This effect can also explain the increased current efficiency in Alq3-based EL devices because the corrugation remains intact on the Al electrode layer.
