**3.4. Polarization conversion in surface-plasmon-coupled emission from corrugated OLEDs with buckling structures**

94 Organic Light Emitting Devices

wavelength [62,67].

**L-CPEL**

**R-CPEL**

**Linear+45°**

**Linear 45**

 **Linear-45°**

**(a.u.)**

**Intensity (**

**u**

**Intensity (a.**

**.)**

(a) (b) **V = 0 V(=3**λ**/2)**

(c) (d)

**V = 4.5 V(=5**λ**/4)**

**500 600 700 800 Wavelength (nm)**

**500 600 700 800 Wavelength (nm)**

In order to apply this concept to OLEDs, we have attached a phase retarder to an EL device. This situation is different from the transmittance measurement system because here the EL device has a metallic mirror as a cathode. The output of EL light is R-CP-EL, as explained in Fig. 26. Hence different polarization states are also possible by controlling the birefringence of the NLC layer. To evaluate the degree of polarization quantitatively, R-, L-circular or linear polarizer with the direction of +45° and -45° is inserted in the emissive EL devices between the phase retarder and detector. The output of EL light transmitted from the L-PCLC is R-CP-EL within the wavelength range corresponding to the stopband. The emitted R-CP-EL can be changed into a different polarization by the phase retardation. Figure 28 shows the polarized EL spectra with different polarizations as applied voltage increases from 0 (Fig. 28(a)) to 4.5 (Fig. 28(c)), 6 (Fig. 28(b)), and 7.5 V (Fig. 28(d)). Thus EL light with different polarizations can be selectively emitted by varying the voltage. Outside of the stopband of PCLC, the intensity of opposite polarized light becomes higher because the stopband of PCLC cannot cover a wide wavelength range. It should be noted, however, if a multilayered-PCLC with different pitches is used, the polarization rate can be high over all

**(a.u.)**

**Intensity (**

**u.)**

**u**

**Intensity (a.**

**L-CPEL**

**LPEL-45°**

**LPEL-45**

**LPEL+45°**

**R-CPEL**

**V = 6 V(=**λ**)**

**500 600 700 800 Wavelength (nm)**

**500 600 700 800 Wavelength (nm)**

**V = 7.5 V(=3**λ**/4)**

**Figure 28.** Measured polarized electroluminescence from OLED. Selectively emitted light of (a) L-CP-EL, (b) R-CP-EL, (c) LP-EL(+45°) and (d) LP-EL(-45°) under fields of 0, 6, 4.5, and 7.5 V, respectively. 63

Copyright 2008, American Institute of Physics.

The fabrication process of buckling and OLED devices is almost the same as described in section 2.2. The only difference is the use of a thinner ITO (40 nm) than previous one (120 nm) to extract TM mode preferentially by a surface plasmon coupled emission [68].

To characterize the outcoupled SP mode by buckles, we have calculated its in-plane propagation vectors and plotted the grating period for the emission angles of 0°, 20°, 40°, and 60° as a function of the wavelength of the outcoupled light in Fig. 29(a). Considering the distribution maximum of the buckling periodicity at ~410 nm, it is reasonable that the main diffraction of the SP mode for the normal direction occurs at the emission wavelength of ~690 nm. In addition the FWHM of the periodicity distribution from 300–600 nm allows outcoupling of the SP mode over the entire emission wavelengths by the first- and secondorder diffractions. As the emission angle increases, the main diffraction wavelength shifts from ~690 nm for 0° to ~580 nm, ~490 nm, and ~440 nm for 20° , 40° , and 60° , respectively.

We have measured the linearly polarized electroluminescence spectra of the devices with and without buckles at the emission angles of 0°, 20°, 40°, and 60°, and then calculated the lightenhancement ratio (the intensity ratio of the two spectra in the devices with and without buckles) as a function of emission wavelength. Figure 29(b) presents the enhancement ratio of the TM-polarized light. The broad peak intensities for each emission angle are consistent with the main diffraction wavelengths calculated in Fig. 29(a), as indicated by arrows. It is very interesting to note that the TE-polarized light also gets enhanced by buckles as shown in Fig. 29(c). This enhancement is even greater than that for TM-polarized light, particularly at larger emission angles, although generally the SP mode is considered to be excited only by TMpolarized light and the diffraction gratings do not convert the polarization state of an incident light upon diffraction. However, it is also known that the polarization conversion can occur if the grating wavevector is not parallel to the plane of incidence [69-73]. So-called conical diffraction occurs at 0°–90° azimuthal angles by the grating with different wavevectors with respect to the incidence plane, where even TE-polarized light may excite the SP mode because of the existence of the electric field component parallel to the grating vector. In other words, the SP mode excited by a TM-polarized light can be outcoupled to the TE- as well as TMmodes radiation. As the azimuthal angle increases from 0° to 90°, the outcoupled TM mode decreases and the outcoupled TE mode increases by the conical diffractions [69,70]. As far as we know, this was the first report on the polarization state of the extracted SP mode, although a qualitative description on the polarization state can be found for the outcoupled SP mode from a silver cathode with a 2-D corrugated structure [74].

Because the grating vectors in a buckling structure are random over all azimuthal angles, the SP mode in the device with buckles also experiences conical diffractions at all azimuthal angles and then the polarization conversion of the outcoupled light occurs. For example, *k*0sinθ, *k*SP, and *k*G for the emission wavelength at 600 nm are graphically presented in Fig. 30. Here only one grating wavevector from a 1-D grating with a periodicity of 410 nm is assumed. The radius of the solid circle (blue) corresponds to *k*SP, the momentum space

Effect of Photonic Structures in Organic Light-Emitting Diodes

within the solid circle (black) represents the escape zone to air mode, and that between the dotted and solid circle (black) indicates the glass mode. As the azimuthal angle of the SP vector increases, the polar and azimuthal angles of the outcoupled light increase and simultaneously the polarization conversion to the TE mode becomes strong. At an angle of 35°, below the azimuthal angle, the SP mode is outcoupled to the air mode by the grating, between 35°–55° it is trapped to the glass substrate, and above 55° it propagates into the ITO/organic layer with the highly TE-converted polarization. In such a restricted condition of a one-directional grating with a definite periodicity, this ITO/organic mode does not outcouple. However, a conical diffraction to air is expected to occur in our buckling structure over all the possible azimuthal angles, 0–360° because of the grating wavevector distributed over all azimuthal directions. Therefore, the enhancement of the TE-polarized light is observed as shown in Fig. 29(c). However, the greater enhancement of the TEpolarized light than that of the TM-polarized light for all polar angles indicates that more TE-polarized light must be outcoupled to the air mode through the diffraction by buckles, because of the polarization conversion to the TE mode being weak at low azimuthal angles below 35°. Considering the dimension of the emitting area (3 mm × 3 mm) and glass thickness (1.0 mm), most light propagating to the glass substrate cannot undergo reflection or scattering at the corrugated Al layer. Hence the scattering of the glass mode by buckles can be ignored. We believe that the TE-converted light propagating to the ITO/organic layer by the diffraction at an azimuthal angle above 55° can be coupled to the TE0 leaky guided mode [75], which can then be outcoupled again by the diffraction through the grating vectors with different directions. The broad periodicity and random orientation of buckles contribute to the additional extraction of the TE-polarized light for all polar angles, thereby

producing a higher enhancement of the TE-polarized light over all polar angles.

**Figure 30.** Momentum representations of SP mode (blue circle, *k*SP), glass light-line (black dotted circle), air light-line (black solid line), grating wave vector (red arrow, *k*G), and the outcoupled light to air mode (black arrow, *k*0sinθ) for the emission wavelength of 600 nm. θ and φ represent the polar and azimuthal angle, respectively. Only one-directional grating vector from one-dimensional grating with a periodicity

of 410 nm is assumed. 68 Copyright 2011, Wiley-VCH.

– Light Extraction and Polarization Characteristics 97

**Figure 29.** (a) Relation between the outcoupled emission wavelength and the grating period for the emission angles of 0° (black), 20° (red), 40° (green), and 60° (blue), satisfying the first- and second-order (only for 0°) diffractions condition. The dashed horizontal line represents the peak wavelength of 410 nm in the periodicity of the buckles used as the grating. (b) Enhancement ratios of TM-polarized light by buckles at the same angles as (a), 0° (black), 20° (red), 40° (green), and 60° (blue) from top to bottom, obtained by dividing the spectrum (measured through a polarizer) of the device with buckles by that without buckles. (c) Enhancement ratios of TE-polarized light by buckles with the same information as (b). 68 Copyright 2011, Wiley-VCH.

within the solid circle (black) represents the escape zone to air mode, and that between the dotted and solid circle (black) indicates the glass mode. As the azimuthal angle of the SP vector increases, the polar and azimuthal angles of the outcoupled light increase and simultaneously the polarization conversion to the TE mode becomes strong. At an angle of 35°, below the azimuthal angle, the SP mode is outcoupled to the air mode by the grating, between 35°–55° it is trapped to the glass substrate, and above 55° it propagates into the ITO/organic layer with the highly TE-converted polarization. In such a restricted condition of a one-directional grating with a definite periodicity, this ITO/organic mode does not outcouple. However, a conical diffraction to air is expected to occur in our buckling structure over all the possible azimuthal angles, 0–360° because of the grating wavevector distributed over all azimuthal directions. Therefore, the enhancement of the TE-polarized light is observed as shown in Fig. 29(c). However, the greater enhancement of the TEpolarized light than that of the TM-polarized light for all polar angles indicates that more TE-polarized light must be outcoupled to the air mode through the diffraction by buckles, because of the polarization conversion to the TE mode being weak at low azimuthal angles below 35°. Considering the dimension of the emitting area (3 mm × 3 mm) and glass thickness (1.0 mm), most light propagating to the glass substrate cannot undergo reflection or scattering at the corrugated Al layer. Hence the scattering of the glass mode by buckles can be ignored. We believe that the TE-converted light propagating to the ITO/organic layer by the diffraction at an azimuthal angle above 55° can be coupled to the TE0 leaky guided mode [75], which can then be outcoupled again by the diffraction through the grating vectors with different directions. The broad periodicity and random orientation of buckles contribute to the additional extraction of the TE-polarized light for all polar angles, thereby producing a higher enhancement of the TE-polarized light over all polar angles.

96 Organic Light Emitting Devices

68 Copyright 2011, Wiley-VCH.

**Figure 29.** (a) Relation between the outcoupled emission wavelength and the grating period for the emission angles of 0° (black), 20° (red), 40° (green), and 60° (blue), satisfying the first- and second-order (only for 0°) diffractions condition. The dashed horizontal line represents the peak wavelength of 410 nm in the periodicity of the buckles used as the grating. (b) Enhancement ratios of TM-polarized light by buckles at the same angles as (a), 0° (black), 20° (red), 40° (green), and 60° (blue) from top to bottom, obtained by dividing the spectrum (measured through a polarizer) of the device with buckles by that without buckles. (c) Enhancement ratios of TE-polarized light by buckles with the same information as (b).

**Figure 30.** Momentum representations of SP mode (blue circle, *k*SP), glass light-line (black dotted circle), air light-line (black solid line), grating wave vector (red arrow, *k*G), and the outcoupled light to air mode (black arrow, *k*0sinθ) for the emission wavelength of 600 nm. θ and φ represent the polar and azimuthal angle, respectively. Only one-directional grating vector from one-dimensional grating with a periodicity of 410 nm is assumed. 68 Copyright 2011, Wiley-VCH.

To confirm the polarization conversion by buckles on diffraction, a buckled resin layer on a glass substrate, coated with a 100-nm-thick Al layer, is irradiated using a linearly polarized He–Ne laser (632.8 nm) at an incident angle of 60° and the scattered light from the surface normal observed through a linear polarizer. We have found that the incident TM-polarized light is largely converted into the TE mode upon diffraction. The ratio of TE- to TMpolarized light intensities was around 0.7, irrespective of the incident azimuthal angle. This result is consistent with the enhancement of the TE-polarized light by buckles in the device structure shown in Fig. 29(c).

Effect of Photonic Structures in Organic Light-Emitting Diodes

*Nano and Bio Research Division, Daegu Gyeongbuk Institute of Science and Technology, Sang-Ri,* 

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*Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama,* 

*Hyeonpung-Myeon, Dalseong-Gun, Daegu, Republic of Korea* 

**Author details** 

Soon Moon Jeong

Hideo Takezoe\*

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*Meguro-ku, Tokyo, Japan* 

– Light Extraction and Polarization Characteristics 99
