**2.4. Effects of quasi-periodic buckling structure in OLEDs**

Buckling patterns are produced spontaneously by thermal evaporation of Al films on poly(dimethylsiloxane) (PDMS) substrates preheated to 100 °C using an external heat source. Al layers with a thickness of 10 nm are deposited on thermally expanded PDMS. After cooling to ambient temperature, the buckling process spontaneously occurs, releasing the compressive stress induced by the difference between the thermal expansion coefficients of PDMS and Al films [43-45]. Figure 17(a), (b) and (c) shows atomic force microscopy (AFM) images of buckles formed by a 10-nm-thick Al layer applied once, twice and three times, respectively. The vague symmetric ring in the fast Fourier transform (FFT) pattern shows that the buckling structure has a characteristic wavelength with a wide distribution and without preferred orientation of the periodic structure. The characteristic wavelength can be obtained by the power spectrum of FFT as a function of wavenumber *k*=2π/λ. Figure 17(d) presents the power spectra of various buckles plotted against the wavelength instead of the wavenumber for direct comparison with outcoupled spectra of OLEDs. The buckling structure of the 10-nm-thick Al layer shows a peak periodicity at wavelengths of ~400 nm (Fig. 17d), resulting in a ~1.4% increase in the surface area ratio of the buckled to flat PDMS with a buckle depth as low as 25–30 nm as shown in Fig. 17(a).

In general, the depth of buckling structure *D* depends on the buckling periodicity λ, which is proportional to the thickness *d* of thin films and the imposed compressive strain (stress) Δ as *D*~λ Δ1/2 [46,47]. The buckles need to have a large depth for efficient diffraction and require a shorter buckling periodicity than that shown in Fig. 17(a) to be effective for an emission peak at a wavelength of ~525 nm. However, there is a trade-off between these factors, because *D* is proportional to λ. We have therefore adopted an alternative method assuming that the larger the compressive stress, the deeper are the buckles at a constant wavelength [48]. We have introduced additional compressive stresses by further deposition of a 10-nm-thick Al layer, once or twice more, on a buckled PDMS replica fabricated from a buckled PDMS mould after the first deposition of an Al layer (Figs. 17(b) and (c)).

on the Al electrode layer.

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.

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

Buckling patterns are produced spontaneously by thermal evaporation of Al films on poly(dimethylsiloxane) (PDMS) substrates preheated to 100 °C using an external heat source. Al layers with a thickness of 10 nm are deposited on thermally expanded PDMS. After cooling to ambient temperature, the buckling process spontaneously occurs, releasing the compressive stress induced by the difference between the thermal expansion coefficients of PDMS and Al films [43-45]. Figure 17(a), (b) and (c) shows atomic force microscopy (AFM) images of buckles formed by a 10-nm-thick Al layer applied once, twice and three times, respectively. The vague symmetric ring in the fast Fourier transform (FFT) pattern shows that the buckling structure has a characteristic wavelength with a wide distribution and without preferred orientation of the periodic structure. The characteristic wavelength can be obtained by the power spectrum of FFT as a function of wavenumber *k*=2π/λ. Figure 17(d) presents the power spectra of various buckles plotted against the wavelength instead of the wavenumber for direct comparison with outcoupled spectra of OLEDs. The buckling structure of the 10-nm-thick Al layer shows a peak periodicity at wavelengths of ~400 nm (Fig. 17d), resulting in a ~1.4% increase in the surface area ratio of the buckled to flat PDMS

In general, the depth of buckling structure *D* depends on the buckling periodicity λ, which is proportional to the thickness *d* of thin films and the imposed compressive strain (stress) Δ as *D*~λ Δ1/2 [46,47]. The buckles need to have a large depth for efficient diffraction and require a shorter buckling periodicity than that shown in Fig. 17(a) to be effective for an emission peak at a wavelength of ~525 nm. However, there is a trade-off between these factors, because *D* is proportional to λ. We have therefore adopted an alternative method assuming that the larger the compressive stress, the deeper are the buckles at a constant wavelength [48]. We have introduced additional compressive stresses by further deposition of a 10-nm-thick Al layer, once or twice more, on a buckled PDMS replica fabricated from a

buckled PDMS mould after the first deposition of an Al layer (Figs. 17(b) and (c)).

**2.4. Effects of quasi-periodic buckling structure in OLEDs** 

with a buckle depth as low as 25–30 nm as shown in Fig. 17(a).

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

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 and 50–70 nm, respectively.

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

Effect of Photonic Structures in Organic Light-Emitting Diodes

consistent with the reported studies in which, as the Alq3 layer thickness decreases below 30 nm, the carrier recombination probability decreases and the exciton-quenching effects at the Al cathode increase, thereby decreasing the internal quantum efficiency of the devices [25,40,49,50]. Therefore, the great enhancement of current and power efficiency in the devices with buckling is obviously caused not by a change of internal quantum efficiency, but by an increase in the outcoupling efficiency, that is, enhanced extraction of the waveguide light.

To investigate the outcoupling of the TE0 and TM0 modes, we have measured the electroluminescence spectra of these devices. Contrary to the enhancement emerging as new sharp peaks in conventional corrugated OLEDs [21-24,27,28,33], our buckled devices exhibit enhancement over the entire electroluminescence spectrum (Fig. 19(a)). We have evaluated

**Figure 19.** Electroluminescence spectral characteristics. (a) Electroluminescence spectra of devices without buckling (black) and double (red) and triple (blue) buckling, measured from the surface normal at a current density of 5 mA/cm2. (b) Enhancement ratio of intensity by buckling as a function of emission wavelength, obtained by dividing the spectrum of the device with double (red) and triple (blue) buckling by that without buckling. The wavelengths of the TE0 and TM0 modes are indicated by arrows at 655 and 720 nm, respectively. (c) Angular dependence of light intensity for devices without buckling (black) and with double (red) and triple (blue) buckling. All data were normalized with the intensity of the devices without buckling in the normal direction. Each dashed line represents a guide to the ideal Lambertian emission pattern. All devices with and without buckling show the Lambertian emission pattern with a

maximum intensity in the normal direction. 36 Copyright 2010, Nature Publishing Group.

– Light Extraction and Polarization Characteristics 85

**Figure 18.** Device performance. (a) Current density–luminance–voltage characteristics of typical OLEDs without buckling (black) and with double (red) and triple (blue) buckling. The dotted and dashed lines represent the current density of devices without buckling but with the organic layer thickness decreased by 20% and 40%, respectively. (b) Current efficiency (cd/A) and power efficiency (lm/W) as a function of luminance (cd/m2) for OLEDs without buckling (black) and with double (red) and triple (blue) buckling. 36 Copyright 2010, Nature Publishing Group.

current efficiencies are found to be 3.05 cd/A (double buckling), 3.65 cd/A (triple buckling) and 1.67 cd/A (without buckling), and the power efficiencies 1.64, 2.1 and 0.73 lm/W, respectively, at a luminance of 2,000 cd/m2. These efficiency increases correspond to enhancements of ~83% with double buckling, and 120% with triple buckling in the current efficiency and 120% with the double buckling and 190% with triple buckling in the power efficiency (Fig. 18(b)). We attribute the greater enhancement of efficiencies in the devices with triple buckling than those in double buckling to an increase in the optical confinement factor due to the greater buckling depth [28]. The observed enhancement in the power efficiency higher than in the current efficiency may be attributed to the reduction in operating voltage due to the partial decrease in the organic layer thickness in the corrugated structure (see Fig. 18(a)). One may expect that the decreased thickness of the N,N'-bis(3 methylphenyl)-N,N'-diphenylbenzidine (TPD) and Alq3 layers may lead to a better charge balance with a better internal quantum efficiency because of the stronger electric field dependence of electron mobility in the Alq3 layer than that of hole mobility in the TPD layer. However, the devices without buckling but with decreased thickness of the organic layer show no improvement in the current efficiency. The device with a decrease in thickness of 40% shows a significantly decreased current efficiency of 0.86 cd/A at 2,000 cd/m2. This is consistent with the reported studies in which, as the Alq3 layer thickness decreases below 30 nm, the carrier recombination probability decreases and the exciton-quenching effects at the Al cathode increase, thereby decreasing the internal quantum efficiency of the devices [25,40,49,50]. Therefore, the great enhancement of current and power efficiency in the devices with buckling is obviously caused not by a change of internal quantum efficiency, but by an increase in the outcoupling efficiency, that is, enhanced extraction of the waveguide light.

84 Organic Light Emitting Devices

**Figure 18.** Device performance. (a) Current density–luminance–voltage characteristics of typical OLEDs without buckling (black) and with double (red) and triple (blue) buckling. The dotted and dashed lines represent the current density of devices without buckling but with the organic layer thickness decreased by 20% and 40%, respectively. (b) Current efficiency (cd/A) and power efficiency (lm/W) as a function of luminance (cd/m2) for OLEDs without buckling (black) and with double (red)

current efficiencies are found to be 3.05 cd/A (double buckling), 3.65 cd/A (triple buckling) and 1.67 cd/A (without buckling), and the power efficiencies 1.64, 2.1 and 0.73 lm/W, respectively, at a luminance of 2,000 cd/m2. These efficiency increases correspond to enhancements of ~83% with double buckling, and 120% with triple buckling in the current efficiency and 120% with the double buckling and 190% with triple buckling in the power efficiency (Fig. 18(b)). We attribute the greater enhancement of efficiencies in the devices with triple buckling than those in double buckling to an increase in the optical confinement factor due to the greater buckling depth [28]. The observed enhancement in the power efficiency higher than in the current efficiency may be attributed to the reduction in operating voltage due to the partial decrease in the organic layer thickness in the corrugated structure (see Fig. 18(a)). One may expect that the decreased thickness of the N,N'-bis(3 methylphenyl)-N,N'-diphenylbenzidine (TPD) and Alq3 layers may lead to a better charge balance with a better internal quantum efficiency because of the stronger electric field dependence of electron mobility in the Alq3 layer than that of hole mobility in the TPD layer. However, the devices without buckling but with decreased thickness of the organic layer show no improvement in the current efficiency. The device with a decrease in thickness of 40% shows a significantly decreased current efficiency of 0.86 cd/A at 2,000 cd/m2. This is

and triple (blue) buckling. 36 Copyright 2010, Nature Publishing Group.

To investigate the outcoupling of the TE0 and TM0 modes, we have measured the electroluminescence spectra of these devices. Contrary to the enhancement emerging as new sharp peaks in conventional corrugated OLEDs [21-24,27,28,33], our buckled devices exhibit enhancement over the entire electroluminescence spectrum (Fig. 19(a)). We have evaluated

**Figure 19.** Electroluminescence spectral characteristics. (a) Electroluminescence spectra of devices without buckling (black) and double (red) and triple (blue) buckling, measured from the surface normal at a current density of 5 mA/cm2. (b) Enhancement ratio of intensity by buckling as a function of emission wavelength, obtained by dividing the spectrum of the device with double (red) and triple (blue) buckling by that without buckling. The wavelengths of the TE0 and TM0 modes are indicated by arrows at 655 and 720 nm, respectively. (c) Angular dependence of light intensity for devices without buckling (black) and with double (red) and triple (blue) buckling. All data were normalized with the intensity of the devices without buckling in the normal direction. Each dashed line represents a guide to the ideal Lambertian emission pattern. All devices with and without buckling show the Lambertian emission pattern with a maximum intensity in the normal direction. 36 Copyright 2010, Nature Publishing Group.

the wavelength dependence of the enhanced emission by considering the intensity ratio of the two spectra in the devices with and without buckling (Fig. 19(b)). The calculated peak wavelengths of the TE0 and TM0 modes for the first-order diffraction are consistent with the broad peak intensities in Fig. 19(b), although the enhancement due to the TM0 mode is not distinct because of the weak emission intensity above 700 nm. The relatively flat enhancement by a factor of ~2.2 around λ0=525 nm in the devices with triple buckling is partially due to the relatively weak first- and second-order diffraction TE0 and TM0 modes, whereas the remarkable enhancement (factor of 4.0) around 655 nm is mainly due to the strong first-order diffraction in TE0 and TM0 modes (see Fig. 19(b)). These results indicate that a further enhancement of more than a factor of at least 2.2 can be expected if the peak wavelength of the buckles is optimized for the TE0 and TM0 modes to be diffracted at around 525 nm in the normal direction. Moreover, the broad distribution of periodicity in the buckling structure suggests that the entire emission wavelength range over blue, green and red in white OLEDs can be simultaneously outcoupled by only one grating structure. The angular dependence of the light intensity for the devices is shown in Fig. 19(c). It is interesting to note that all devices with and without buckling show a Lambertian emission pattern with a maximum intensity in the normal direction. According to the Bragg equation, the first-order diffraction angles of the TE0 and TM0 modes around the main emission wavelength of 525 nm by the grating period of 410 nm are expected to be between 20° and 40°. However, because *k*G has random orientation and broad periodicity due to the buckling, it is distributed over all azimuthal directions in contrast to one- or two-directional *k*G in conventional corrugated OLEDs [21-24,27,28,33]. Thus, the outcoupled emission concentrates into the normal direction, resulting in the Lambertian emission pattern.

Effect of Photonic Structures in Organic Light-Emitting Diodes

outside of the stop band rapidly decreased, because the emissive material had wider

For evaluating the degree of circular polarization at a certain wavelength λ, a *g*-factor is

() () () 1 () 2 <sup>2</sup>

where *I*L/R is the intensity of left/right-handed CP (L-CP, R-CP) light, and *r* is the left/righthanded intensity ratio, *I*L(λ)/*I*R(λ). It is evident that |*g*(λ)| is zero for nonpolarized light (*r*()=1) and is equal to -2 for pure, single-handed circularly polarized light (*r*()= ∞ or 0). The *g*(λ) values found were 0.001 [56], 0.25 [57], and 1.6 [58], but only in a narrow wavelength range. Woon *et al.* and Geng *et al.* respectively reported circularly polarized PL [60] and EL [61] with a constant *g*(λ) value over a wide spectral range covering most of the emission band. However, the bandwidth [60] and *g*(λ) value [61] were still insufficient for

To achieve a tunable polarization of electroluminescence, we have used combination of voltage dependent nematic liquid crystal (NLC) phase retarders and photon recycling concept [62,63]. The phase retardation arises between two optical eigenmodes during light propagation in an anisotropic medium as a phase retarder. Upon emerging from the phase retarder, the relative phase of the two eigenmodes is found to be different from that at the incidence, and thus their polarization state becomes different as well [64-66]. Now suppose we apply a voltage (*V*) across the cell filled with NLC, by which the liquid crystal molecules change their orientation toward the field direction, if the NLC has positive dielectric anisotropy. With increasing the voltage, the birefringence e 0 *nE n E n* – decreases, where *ne* and *no* are refractive indices for extraordinary-(*e*-) and ordinary-(*o*-) light waves, respectively, and the retardation () decreases as well. Hence, as the *e*- and *o*-waves propagate through the NLC cell, their relative phase difference changes, and the state of

We have introduced another polarization characteristics, namely polarization conversion in surface plasmon (SP) coupled emission by buckling structures. In section 2.4, we have demonstrated that the quasi-periodic buckling structures with broad distribution and directional randomness can effectively enhance the light-extraction efficiency by outcoupling the waveguide modes without introducing spectral changes and directionality [36]. In this study, however, we could not differentiate the outcoupling of transverse electric (TE) mode from that of the surface-plasmon (SP) mode (transverse magnetic (TM) mode) by buckles because of the broad periodicity of the buckling structure and the similar propagation vectors of the TE and SP modes. The explanation of polarization conversion in the surface-plasmon-coupled emission presented here is based on a trial method for distinguishing TE and TM modes in light enhancement in OLEDs with buckling pattern. However in this trial approach, an interesting phenomenon of polarization conversion in SP

*I I r <sup>g</sup> II r* 

 

*L R L R*

application to commonly used emissive materials with wide emission band.

() () () 1

(14)

 

emission band than the stop band width formed.

used which is defined as:

polarization of the wave also changes.

coupling has been observed.

– Light Extraction and Polarization Characteristics 87
