1. Introduction

Organic light-emitting diodes (OLEDs) are currently being exploited for applications in flat panel display technology and solid-state lighting owing to their superior features, such as flexibility, transparency, lightweight, fast response, wide viewing angle, and wide color gamut [1–5]. An OLED comprises several organic thin films sandwiched between a transparent anode and metal cathode. The organic stacks include a hole injection layer and hole transport layer (HTL) for carrying holes, an electron injection layer and electron transport layer (ETL) for carrying electrons, and an emissive layer (EML), wherein electrons and holes recombine to form excitons for light emission. However, numerous optical losses occur when light propagates from the internal organic layers of an OLED into the air, limiting the light out-coupling efficiency of OLEDs. Since the apparent difference in refractive indices (n) between the organic layers in an OLED (n ${\approx} $ 1.5−2.0) and air (n = 1.0), total internal reflection occurs when light propagates from a high refractive index media to a low one. Thus, only partial light escapes out of the device into the air, which is termed air mode. In addition, some light is trapped in the glass substrate, which is termed substrate mode. The light trapped in the indium tin oxide (ITO) and organic layers is called waveguide (WG) mode. Moreover, the emissive dipole in OLEDs is typically close to the metal cathode, thereby potentially exciting the surface plasmon in the organic layers/metal interface. This is termed the surface plasmon polariton (SPP) mode [6–10]. Due to these aforementioned optical losses, only about 20% of the light in the EML of OLEDs can escape into the air.

To enhance the light extraction efficiency (LEE) of OLEDs, over the past decade, several researchers have invested a lot of efforts into developing light extraction methods to extract the trapped light in the interior of OLEDs, wherein external extraction techniques, including microlens array films and/or hemispherical macrolens are incorporated on the surface of the glass substrate for extracting the substrate mode [11–14]. The nano-scaled textured structures, such as the scattering layer, photonic crystal grating, and diffraction grating, used as internal extractors are employed inside OLEDs for extracting the trapped light from the SPP and WG modes [15–26]. For instance, Sun et al. demonstrated a light extraction enhancement of 30% from the SPP mode in a corrugated OLED by employing a nano-scaled periodic grating structure [18]. Similarly, Fung et al. exploited the nano-modified ITO substrate as an internal light extractor to harvest the trapped light for enhancing the efficiency of a green OLED, achieving an EQE enhancement of 36.9% compared to the planar one. To the best of our knowledge, the LEE indeed exhibits sinusoidal behavior under various ETL thicknesses in planar OLEDs owing to several reasons, such as the microcavity effect and optical loss modes. SPP mode decays over the distance between the dipole and the metal and the WG mode increases with increasing the ETL thickness. Consequently, with the interplay of these optical effects, the light out-coupling efficiency (air mode) displays sinusoidal behavior [27]. The ETL thickness of OLEDs significantly affects device performance. Although light out-coupling efficiency regarding ETL thickness dependence has been investigated in planar OLEDs, no studies have reported such findings in corrugated OLEDs. Corrugated OLEDs have recently been identified as a straightforward and efficient approach utilized to improve the LEE of OLEDs [28–30]. Although light extraction enhancement by using corrugated OLEDs has been widely studied, most of the studies have explored the phenomenon for certain device structures (i.e., first antinode or second antinode conditions) and have rarely examined the effects on corrugated OLEDs with various layer thicknesses (i.e., ETL, HTL). The link between ETL thickness and planar OLEDs has certainly been studied widely. It is now well known that the ETL thickness significantly impacts device performance due to the optical interference effect. Therefore, we believed that it was noteworthy to further investigate the effect of ETL thickness on corrugated OLEDs. Additionally, this study has also paid attention to realizing the physical mechanism of light extraction. We found that the physical mechanism for light extraction enhancement in corrugated OLEDs reported in previous studies merely involved indicating how the enhancement occurred qualitatively rather than quantitively [31–33], meaning that the researchers deduced that the origin was from the SPP and/or WG modes. They did not precisely identify the proportion of efficiency improvement from each optical mode. In this study, however, we have invested efforts to find this out.

Here, we investigated the effects of ETL thickness on LEE based on our proposed corrugated OLED with a promising light extraction [28]. Each layer of the corrugated OLED exhibited a periodically corrugated structure, determined by the first nanoimprinted poly(methyl methacrylate) (PMMA) layer on the glass substrate. The detailed nanoimprinting process was performed through optical blaze grating. To realize the impact of LEE on various ETL thicknesses, different ETL thicknesses were adopted to change the distribution of carrier recombination, exciton formation, and optical interference and obtain different ratios of optical loss modes in OLEDs, thereby potentially leading to an increase or decrease in light extraction and device efficiency. Therefore, a series of corrugated OLEDs with various ETL thicknesses (15 to 215 nm) were manufactured. In addition, their referenced planar OLEDs were also fabricated for comparison. Upon comparison with the planar OLED, a significant LEE enhancement of 83.3% was obtained in the corrugated OLED at the ETL of 115 nm. Moreover, light emission was divided into transverse electric (TE) and transverse magnetic (TM) emissions, and the attribution from the SPP and WG modes to EQE enhancement was distinctly verified. The experimental results and theoretical calculations indicate that the origin of efficient LEE enhancement of 83.3% was due to the effective extraction efficiency from the SPP and WG modes.

2. Experiment

2.1 Corrugated OLED fabrication

Figure 1(a) shows the process flow of corrugated OLEDs with an internal nano-sized grating structure. First, liquid polydimethylsiloxane (PDMS) was mixed with a hardener and poured on a commercial blazed grating (Model #43–776, Edmund Optics; 2400 grooves/mm, corresponding to a pitch of 417 nm) and then placed in an oven for curing. Second, the PDMS mold with the grating coated on the mold surface was demolded through peeling. A PMMA layer was laminated between the PDMS mold and glass substrate followed by UV-curing at 1 joule/cm². After separating the glass from the PDMS mold, a PMMA glass substrate with a nanoimprinted grating structure was formed. 125 nm of indium zinc oxide (IZO) was deposited onto the PMMA glass substrate as the anode, having a sheet resistance of 53 Ω/sq. Organic layers and a metal cathode were deposited through thermal evaporation under vacuum pressure of approximately 8 × 10⁻⁶ and 8 × 10⁻⁵ torr, respectively. Figure 1(a) shows the device structure. A N,N’-Di(naphthalen-1-yl)-N,N’-diphenyl-benzidine (NPB), with a thickness of 60 nm, was used as the HTL. Tris-(8-hydroxyquinoline)aluminum(III) (Alq₃), with a thickness of 10 nm, served as an EML material, and 4,7-diphenyl-1,10-phenanthroline (BPhen) acted as the ETL, whose thicknesses of 15, 40, 115, and 215 nm were applied for investigating the dependence of thickness. The electron injection layer and cathode comprised lithium fluoride (LiF) and Al, with thicknesses of 0.9 and 120 nm, respectively.

 

Fig. 1. (a) Process flow of corrugated OLEDs; (b) Cross-sectional SEM image of the corrugated OLED at a tiled angle of 52°.

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2.2 Measurements

Scanning electronic microscopy (SEM; JEOL JSM-6500F) and atomic force microscopy (AFM; Bruker) were employed to identify the dimensions of the PDMS mother mold, as shown in Fig. S1. Figure S1(a) depicts the top view of the SEM image and the cross-sectional image at a tilted angle of 52°. The grating pitch and height of the PDMS mother mold were 422 nm and 86 nm, respectively. For further confirmation, Figs. S1(c)–(d) show the AFM image and cross-sectional profile, wherein the AFM results reveal that the pitch and height of the PDMS mother mold were 420 nm and 84 nm, respectively. These values were close to the SEM results. On the other hand, Fig. 1(b) shows the cross-sectional SEM image at a tilted angle of 52° and displays every layer of the corrugated OLED device, including the dimensions of the nanoimprinted grating structure—the height of 79 nm and side lengths of 128 nm and 289 nm. Therefore, the pitch of the nanoimprinted grating structure in the OLED was approximately 417 nm, and the dimension of the nanoimprinted grating structure in the OLED was almost equivalent to that of the PDMS mother mold, suggesting that a perfect imprinting process was implemented, and no damage occurred during the fabricating processes. Device characterization, which includes luminance-current density-voltage (L-J-V) characteristics, current efficiency versus luminance (CE-L) curves, and electroluminescence spectrum, was carried out by employing a source meter (Keithley, 2400) and a spectrometer (Konica Minolta, CS-1000). The LEE of the planar and corrugated OLEDs was evaluated by using an integral sphere to obtain their EQE values.

3. Results and discussions

3.1 Planar OLEDs with various ETL thicknesses

Most of the previous studies have investigated ITO-based planar OLEDs; however, only a few studies have explored IZO-based planar OLEDs. To obtain convincing results, we started with the ITO case. Planar OLEDs with ITO as the anode were fabricated, exhibiting various ETL thicknesses, for confirming the electroluminescence (EL) characteristics mentioned in the literature. The ETL thickness varied from 15 to 240 nm. Figure 2(a) shows the current density versus voltage (J-V) curves of these planar OLEDs, whose driving voltage increased from 2.8 to 8.6 V (at J = 1 mA/cm²) when increasing the ETL thickness due to an increase in device resistance. Figure 2(b) shows the current efficiency (CE) versus luminance curves, where CE varied with the ETL thickness. The maximum values for each thickness have been plotted in Fig. 2(c). The CE profile demonstrated sinusoidal behavior. The low efficiency of 17 cd/A obtained at the thin ETL of 15 nm was due to carrier unbalance induced by efficient electron transporting. As the thickness was increased to 40 nm, electron transporting slowed down, leading to a carrier balance in OLED and a maximum CE of 3.4 cd/A. As the ETL thickness increased beyond 40 nm (65, 115 nm), the CE gradually dropped, and the lowest CE of 0.2 cd/A was obtained at 115 nm. This was due to the lack of carrier unbalance once again, which was induced by insufficient electron transporting in OLED. The CE increased and attained the second maximum value when the ETL was increased from 115 nm to 215 nm. Such enhancement in efficiency must only have originated from optical effect rather than electrical effect. This optical effect was assigned to the weak microcavity effect [32–36]. The maximum EQE, as a function of ETL thickness, is shown in Fig. 2(d), where the profile displays sinusoidal behavior [27]. Furthermore, a homemade numerical model was exploited to simulate power dissipation coupled to the optical loss modes for planar OLEDs. The detailed theory, simulation process, and relevant parameters have been explained in previous literature [37]. As a result, the simulation of power dissipation on different optical loss modes with different ETL thicknesses has been presented in Fig. 2(d), where the variation of air mode seemingly exhibits sinusoidal variation, corresponding well with previous studies. In addition, such simulation results were also in good agreement with the experimental results, affirming that our ITO-based planar OLED was reliable.

 

Fig. 2. Performance of the ITO-based planar OLED under various ETL thicknesses: (a) J-V characteristics; (b) CE-L curves; (c) CE as a function of ETL thickness; (d) The maximum EQE and simulation results for power dissipation under various optical loss modes for the ITO-based planar OLED.

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Furthermore, planar OLEDs, using IZO (with a low-temperature fabrication process) as the anode, were fabricated. The transparent conductive anode was fabricated sequentially, after finishing the corrugated substrate, with a thermal sensitive PMMA as material for the nanostructure. However, the high-temperature fabrication of ITO would have destroyed the nanostructure on the substrate; thus, a low-temperature IZO electrode was adopted. The ETL thickness of IZO-based planar OLEDs varied from 15 to 215 nm. Figures 3(a) and (b) show the J-V characteristics and CE–L curves, respectively. Figure 3(c) shows CE as a function of ETL thickness. The efficiency profile of the corrugated OLEDs was similar to that of ITO, as shown in Fig. 2(c). The efficiency performance of IZO-based planar OLEDs exhibited sinusoidal variation, also shown in Fig. 3(d). Based on this information, corrugated OLEDs were manufactured based on the IZO-based planar OLED to investigate the LEE effect.

 

Fig. 3. Performance of the IZO-based planar OLED under various ETL thicknesses: (a) J-V characteristics; (b) CE-L curves; (c) CE as a function of ETL thickness; (d) The maximum EQE and simulation results for power dissipation under various optical loss modes for the IZO-based planar OLED.

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3.2 Light extraction efficiency for corrugated OLEDs with various ETL thicknesses

Corrugated OLEDs with various ETL thicknesses (15 to 215 nm) were fabricated. Their J-V curves are shown in Fig. S2. The J-V characteristics of the corrugated OLEDs were similar to that of the planar ones. For device efficiency, since the angular light extraction pattern, which meant that the trapped light for corrugated OLEDs was not probably extracted in the normal direction of substrate. Thus, an integral sphere was employed to measure the EQE and evaluate the LEE rather than perform a CE comparison. Figure 4(a) shows the EL spectra of the planar and corrugated OLEDs with various ETL thicknesses, measured at a constant J = 5 mA/cm². The EL spectra showed different emissive profiles. With an increase in ETL thickness, the central peaks of the spectra for both the cases (planar and corrugated) would experience red-shift first follow by blue-shift. This was caused due to the microcavity effect in OLEDs [21]. Moreover, the intensity of light emission for the corrugated OLEDs increased because the corrugated structure benefited to outcouple the trapped light from the WG and/or SPP modes. A maximum EQE enhancement was obtained at the ETL thickness of 115 nm, and the spectral peak intensity increased by approximately two-folds from 0.036 to 0.074 µw. The EQE is represented in Fig. 4(b). The EQE summary for the planar and corrugated OLEDs is outlined in Table 1. The EQE profile exhibits sinusoidal variation, caused by the interplay between the carrier balance and optical interference effect in OLEDs, as shown in Fig. 4(b) [32]. The EQE of corrugated OLEDs was apparently enhanced, and a peak EQE of 1.79% was achieved at the ETL thickness of 40 nm. Additionally, the proportion of EQE enhancement must have originated from the power loss in the optical modes within the planar OLEDs, also shown in Fig. 4(b), especially from the WG and SPP modes. Thus, EQE enhancement, defined as the EQE of the corrugated OLED minus the EQE of the planar OLED, including the power dissipation of the WG and SPP modes extracted from Fig. 4(b), are both illustrated in Fig. 4(c), wherein the EQE enhancement decreases with an increase in ETL thickness due to the power dissipation of the WG and SPP modes, which also decreases with an increase in ETL thickness. EQE enhancement as a function of ETL thickness for the planar and corrugated OLEDs is shown in Fig. 4(d), wherein EQE enhancement can be defined as the EQE of the corrugated OLED divided by that of the planar OLED. EQE enhancement varied with the ETL thickness, and the maximum enhancement by 83.3% was obtained at the ETL thickness of 115 nm. Such variation in EQE was obtained owing to the varying power dissipation coupled to the optical mode, as shown in Fig. 4(b). To realize EQE enhancement from each optical mode, information, such as the angular spectra of TE and TM, was needed to determine the mechanism underlying light extraction. Unfortunately, this information could not be obtained through integral sphere measurement but could potentially be accessed through angular spectrum measurement, provided a polarization lens was placed in front of the spectrometer.

 

Fig. 4. Planar and corrugated OLEDs at various ETL thicknesses: (a) EL spectra measured at a constant J =5 mA/cm²; (b) EQE and simulation results for power dissipation under various optical loss modes; (c) EQE enhancement (defined as the EQE of corrugated OLED minus that of planar OLED) and power dissipation of the SPP and WG modes extracted from Fig. 3(b); (d) EQE enhancement (defined as the EQE of corrugated OLED over the EQE of planar OLED).

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Table 1. EQE measured by integral sphere

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3.2 EQE of TE and TM emissions in OLEDs

A schematic diagram for angular spectrum measurement is illustrated in Fig. 5(a) [38], where the corrugated OLEDs were mounted on a rotational stage, and an angle of 0° was set as the normal direction of the device. The angle between the normal direction of the device and the direction of the spectrometer could be precisely controlled by using a rotary motor. A spectrometer (Minolta, CS-1000) was employed to measure the angular spectrum of the OLED at a fixed position. All the angular spectra were then integrated to determine the EQE of the device. In this study, theta dependence measurement was performed since the major LEE was in the theta direction based on the results of our previous study [29]. Figures 5(b)−(e) display the angular EL spectra of the planar and corrugated OLEDs with different ETL thicknesses. Enhanced emission in the corrugated OLEDs was observed either in the positive or negative viewing angle. A distinct emission enhancement and a large wavelength shift were also observed in the case of ETL thickness of 115 nm (Fig. 5(d)), which originated due to the effective light extraction from the WG and SPP modes. Furthermore, the enhanced difference between the planar and corrugated OLEDs (at viewing angles from −80° to 80°) was calculated by subtracting the planar OLED spectra from the corrugated OLED spectra, as shown in Figs. 6(a)−(d). One or two enhanced emission peaks were observed, and the peaks shifted as the viewing angle changed. For example, two enhanced peaks (approximately at 500 and 600 nm) were observed at the viewing angle of 20 degrees in Fig. 6(b). Both the peaks shifted toward blue with an increase in viewing angle. This was probably caused due to the extraction from WG and/or SPP modes. Moreover, the observable difference in the outcoupling wavelength at different viewing angles was caused due to dispersion relation.

 

Fig. 5. (a) A schematic diagram of angular spectrum measurement. The EL spectra of planar and corrugated OLEDs at various viewing angles; The EL spectra of the planar (black lines) and corrugated OLEDs (red lines) at ETL thicknesses of (b) 15 nm, (c) 40 nm, (d) 115 nm, (e) 215 nm, and various viewing angles (−80° to 80°).

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Fig. 6. Enhancement of planar and corrugated OLEDs with various ETL thicknesses of 15 nm (a) 40 nm (b) 115 nm (c) 215 nm, and (d) at various viewing angles (−80° to 80°).

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Both the SPP and WG modes can contribute to TM emissions; in contrast, only the WG mode can contribute to TE emissions. A polarized lens was placed in front of the device to obtain angular EL spectra of TE and TM for the planar and corrugated OLEDs, as shown in Figs. S3−S6. Figures 7(a) and 7(b) show the EQE (total, TE and TM) as a function of ETL thickness for the planar and corrugated OLEDs, respectively. Table 2 summarizes the EQE performance measured through the angular spectrum. The total EQE can be equally divided into the EQEs of TE and TM, whose sum was almost equivalent to the total EQE (Figs. 7(a) and (b)). In addition, the EQE of TE and TM was almost equal to each other in the planar OLED. However, the EQE of TM in the corrugated OLED was significantly higher than that of TE at the ETL thickness of 15 nm (Fig. 7(b)), largely owing to the contribution of the SPP mode. To delve even further and investigate the origin of EQE enhancement, EQE enhancements for both the planar and corrugated OLEDs were performed, as illustrated in Fig. 7(c). As mentioned above, since EQE enhancement must have originated due to the trapped light extraction from the SPP and WG modes, the simulated power dissipation of the SPP and WG modes for the planar OLED was also performed individually for verification purposes, as shown in Fig. 7(c). According to the trend of EQE with respect to the ETL thickness, EQE enhancement in TE certainly originated from the WG mode; however, not all of it would have originated from the WG mode since the partial WG modes were coupled into TM. Therefore, the EQE enhancement in TM was dominated by the SPP mode at a thin ETL of 15 and 40 nm, whereas EQE enhancement was dominated by the WG mode at a thick ETL of 115 and 215 nm, resulting in a small hump.

 

Fig. 7. EQE as a function of ETL thickness for (a) planar and (b) corrugated OLEDs measured by using the angular spectrum; (c) EQE enhancement of TE and TM emissions of the corrugated and planar OLEDs and the power dissipation in the SPP and WG modes for the planar OLED, where EQE enhancement is defined as the EQE of the corrugated OLED minus that of the planar OLED.

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Table 2. EQE measured by the angular spectrum method.

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3.3 EQE enhancement from the SPP and WG modes

To realize the proportion of EQE enhancement from the SPP and WG modes, it was necessary to extract the coupled-enhanced EQEs of the SPP and WG modes from TM. Upon extracting one, the other could also be obtained. The dispersion relation (ω-k_x plot) of the EQE enhancement in TE and TM at various ETL thicknesses was plotted, as shown in Fig. 8, according to the analysis presented in our previous study [28]. Moreover, the simulated dispersion lines based on the theoretical calculation for the WG and SPP modes were also plotted to realize the origin of EQE enhancement (Fig. 8). For a thin ETL (15 nm), no enhancement was observed in TE, and EQE enhancement was mainly attributable to TM, as shown in Fig. 8(a), corresponding to the SPP modes (m= ${\pm} $ 1). With increasing thickness (40, 115, 215 nm), enhancement from the SPP mode underwent significant decay due to the emission dipole gradually being away from the metal–dielectric interface (Figs. 8(a)–(c)). On the other hand, the WG mode of TE and TM began to get involved in the enhancement with increasing thickness, as shown in Figs. 8(b) –(d). Its notable contribution occurred at the thickness of 115 nm, with multiple attributions of the WG modes (m=${\pm} $ 1, ${\pm} $ 2). The little SPP and WG modes contributed to EQE enhancement at the super-thick ETL thickness of 215 nm, as shown in Fig. 8(d). This was caused because the SPP was a type of evanescent wave existing close to the metal–organic dielectric interface. The corrugated grating merely enhanced the nearby propagating light because the height of the corrugated structure was approximately 79 nm.

 

Fig. 8. Dispersion relation of EQE enhancement in TM and TE emissions with various ETL thicknesses of (a) 15, (b) 40, (c) 115, and (d) 215 nm, where the solid yellow lines represent the theoretical enhancement from the WG mode, and the red lines represent those from the SPP mode [28].

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The simulation results reveal that the EQE enhancement in TM was almost entirely from the SPP modes (ETL = 15 nm). Based on this factor, if the same extraction efficiency for the SPP mode is assumed at various thicknesses, EQE extraction would be proportional to the power dissipation of the SPP mode in the planar OLED (Fig. 7(c)). Thus, EQE enhancement from the SPP mode as a function of ETL thickness can be derived as shown in Fig. 9. In addition, once EQE enhancement from the SPP mode is obtained, it helps in determining the overall EQE enhancement from the WG mode. Eventually, the total EQE enhancement, EQE (SPP), and EQE (WG) based on varying ETL thicknesses, is demonstrated in Fig. 9. Accordingly, the significant EQE enhancement of 83.3% at 115 nm was assigned to the joint contribution of the SPP and WG modes. The total EQE enhancement was 0.5%, from which 0.39% was contributed from WG and 0.11% from SPP.

 

Fig. 9. EQE enhancement for the total, SPP and WG modes at various ETL thicknesses.

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4. Conclusion

In summary, we have demonstrated a corrugated OLED with a significant LEE of 83.3% through ETL thickness modulation, where the enhancement originated from the SPP and WG modes. Light extraction to EQE from the SPP and WG modes at various ETL thickness was verified by combining the findings with the experimental results and theoretical calculations. The SPP mode was found to mainly dominate light extraction at thin ETL (15 nm), whereas the SPP and WG modes were both found to dominate light extraction in different ratios at moderate ETL thicknesses (40 to 115 nm). As a result, an optimal LEE of 83.3% was obtained at the ETL of 115 nm with the interplay of the SPP and WG modes, corresponding to an enhancement of 0.5% in EQE, out of which 0.39% originated from the WG mode and the remainder (0.11%) from the SPP mode.