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Abstract
Although there have been tremendous achievements ever since the first work on an organic electroluminescent (EL) device that emitted polarized light, the development of flexible polarized emission organic light-emitting devices (OLEDs) is not without hurdles, and the challenge towards real-world applications still requires tremendous effort. In this paper, we proposed highly linearly polarized light-emission from flexible green OLEDs capitalized on integrated ultrathin metal-dielectric nanograting. The acquired polarized device with meticulously optimized geometric parameters yields an angle-invariant average extinction ratio beyond 20.0 dB within a viewing angle range of ± 60°. The detailed analysis illustrates that surface plasmons and cavity modes are simultaneously contributed to the TM-polarized light selection. We hope that the presented approach will open new opportunities for designing flexible polarized light sources.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Organic light-emitting diodes (OLEDs) have been extensively investigated and commercialized as a promising technology for applications such as full-color flat-panel displays and interior lighting [1–3]. Until now, however, extensive scientific efforts devoted to OLEDs using both polymers and small molecules have been focused merely on unpolarized light [4]. A polarized light, especially linearly polarized light, as an important and appealing functional expansion, has been increasingly demanded in various applications, such as power-saving backlight sources [5,6], optical storage [7], optical communications [8], and three-dimensional (3D) display system [9]. Ever since the first work on an organic electroluminescent (EL) device based on aligned conjugated polymers that emitted polarized light [10], several approaches and mechanisms have been explored to control the polarization of EL light emitted from OLEDs. For example, it has been widely reported that intrinsic linearly polarized light emission can be attained from incorporating uniaxially oriented materials (e.g., oligomers or liquid crystalline) into emissive layers by uniaxial alignment methods including mechanical stretching/shearing/rubbing alignment [10–12], alignment on specific substrates [4,13], Langmuir-Blodgett deposition [14] and LC self-organization [15]. Although there have been yielding tantalizing achievements through aforementioned methods, the inadequate polarization ratio still constitutes the major hurdle for real application. Moreover, the damage and contamination of the emissive layer inevitably deteriorate the polarization efficiency. Other approaches illustrated that the polarized luminescence can be acquired through the integration of nanostructures, such as photonic crystals [16], and metal gratings [17], which has gained abundant attention as a prominent polarized light source due to their conspicuous features, including high extinction ratio (ER), compact structure, and desired performance. Nonetheless, despite the fact that this class of methods can achieve high degree of polarization, the luminous efficiency still remains deficient for practical applications. Furthermore, most of these strategies featuring integrated nanostructures were designed for use on rigid glass, which is stymied by one overriding challenge for being compatible with flexible substrates. Thereby, the development of flexible polarized emission OLEDs is not without hurdles, and the challenge towards real-world applications still require tremendous efforts in the next few years.
Here, we propose and experimentally investigate a highly linearly polarized light emission from flexible organic light-emitting devices capitalized on integrated ultrathin metal-dielectric nanograting. The suggested nanograting structure, which is integrated with a metallic Aluminum (Al) nanograting at the top and a modified dielectric UV-curable polyurethane acrylate nanograting at the bottom, is constructed on the flexible green OLED substrate using our developed soft-nanoimprinting technology. The geometric structure parameters related to the capability of selecting linearly polarized light are meticulously optimized and the resulting OLED device with the optimal parameters yields extinction ratio measured as high as 20.0 dB with respect to the normal direction of the nanograting surface. Detailed analysis provides valuable insights into the polarization selection mechanisms and reveals that surface plasmons and cavity modes simultaneously contributed to the TM-polarized light selection.
2. Results and discussion
2.1 Device structure design illustration
Figure 1 illustrates the schematic configuration of the proposed flexible green OLED capitalized on integrated ultrathin metal-dielectric nanograting, which directly produces highly linearly polarized light (TM-polarized light). The ultrathin metal-dielectric nanograting is composed of a metallic Al nanograting at the top and a dielectric UV-curable polyurethane acrylate (PUA, PhiChem) nanograting at the bottom, which is integrated on the external substrate surface of the flexible green OLED substrate. The heights of the top metallic Al grating and the bottom dielectric PUA grating are depicted by T1 and T2, respectively, and both metallic and dielectric gratings have the same period of P and the fill factor of F. To probe the effects of various geometric structure parameters on the ability of selecting linearly polarized light, the developed device architecture provided in Fig. 1 should be theoretically designed. Broadly, the theoretical methods can be classified into two categories in OLED optical simulation according to the implicit assumption property of all layers: isotropic [18] and anisotropic [19]. Although the layer anisotropy has some effect on the out-coupling efficiency and far-field distribution of the device, the diffraction effect and polarization selectivity of the grating play the key role for final device efficiency and polarization efficiency under the condition that the OLED electrical structure is determined. Hence, the convenient and simple finite difference time domain (FDTD) method (RSoft 8.1 Design Group, Inc) was employed to find the optimal geometric structure parameters based on the assumption that all layers of the device are isotropic [20]. In our simulation, the thickness of bilayer cathode of Liq/Al, emission material layer (EML), ITO, and polyethylene terephthalate (PET) substrate were set as 100 nm, 115 nm, 120 nm and 200 nm, respectively, and a polarized point source (520 nm) was arranged in the middle of the EML. The complex optical functions of metallic Al were fitted using the Drude-Lorentz model (RSoft Fullwave), and the values of frequency-dependent refractive index (n) and extinction coefficient (k) of the organic layers were set as 1.75 and zero, respectively, in the wavelength range of 300-800 nm. The perfectly matched layer (PML) boundary conditions in all dimensions were adopted and the calculated mesh accuracy was set as λ/dx = 20 in the overall two-dimensional structure.
Fig. 1. (a) Configuration of the proposed linearly polarized flexible OLED incorporating an integrated metal-dielectric nanograting. (b) Geometric architecture parameters of a metal-dielectric nanograting. Here, T1 and T2 denote the depth of the PUA grating and the thickness of Al, respectively, while P and F refer to the period and fill factor of the nanograting, respectively.
To find optimal geometric parameters for the metal-dielectric nanograting, the initial grating parameters of P, F, T1 and T2 are fixed at 100 nm, 0.5, 100 nm, and 20 nm, respectively, simultaneously considering a delicate trade-off among these parameters and the corresponding fabrication capability. The extinction ratio, which is usually employed to characterize the performance of the linearly polarized OLED, can be attained from two parameters based on the following formula:
Fig. 2. Calculations of the wavelength-dependent transmittance for TM polarized light and the corresponding ER based on nanograting parameters: (a, b) period (F = 0.5, T1 = 100 nm, T2 = 20 nm), and (c, d) fill factor (P = 100 nm, T1 = 100 nm, T2 = 20 nm).
To obtain the optimal height of the bottom dielectric PUA grating, the wavelength-dependent transmittance for TM polarized light and the corresponding extinction ratios based on various of T1 were also simulated and provided in Figs. 3(a) and 3(b), respectively. While the ${\textrm{T}_{\textrm{TM}}}$ value noticeably increases as the height of the PUA grating monotonically beyond the long wavelength range between 600 nm to 800 nm, its counterpart gradually decreases within the short wavelength domain especially below 500 nm. This pronounced fact directly implies that a proper trade-off between the parameter of T1 and the electroluminescence (EL) spectra of OLED device is straightforward required. Correspondingly, the average optimal fER approaches 24.5 dB when the structure parameter of T1 was set to 80 nm. To explore the dependence of the polarized light selection properties on Al height, the similar calculations are also performed and are mapped in Figs. 3(c) and 3(d), respectively. Apparently, the transmittance of the TM polarized light drastically descends with the increment of the height of metallic Al particularly within the long wavelength range beyond 500 nm [Fig. 3(c)]. Additionally, although the device with 10 nm-thick Al displays an optimal average transmittance of 77.8%, the 20 nm-thick of Al with a relatively lower value of 72.2% was chosen as our optimized result. This is due to the easy oxidation of the 10 nm thick Al nanograting, which then causes inevitable degradation of the polarization selection capability. Accordingly, the average extinction ratio ${\textrm{f}_{\textrm{ER}}}$ over the visible wavelength range can still achieve as high as 25.6 dB when T2 equals to 20 nm. Obviously, this extinction value still far surpasses the requirement of the practical commercialization [Fig. 3(d)]. In a bid to investigate the angular behavior of the proposed polarized device, angle-resolved TTM and fER as a function of wavelength were explored and presented in Figs. 3(e) and 3(f), respectively. As can be seen from these figures, the transmittance of TM polarized light and the corresponding fER sustain almost constant over a wide angular range up to 60°. This favorable angle-invariant feature can be ascribed to the suppressed phase shift accumulated during the propagation through the ultrathin grating cavity medium [21]. Interestingly, it is worth noting that both the calculated values gradually becomes even higher with larger incident angles beyond 60°. This meaningful feature can be attributed to the fact that the impinging TE polarized light on the grating at the vertical direction becomes much smaller for larger oblique incidence, and thereby leads to lager reflective efficiency for TE polarization [22].
Fig. 3. Calculations of the wavelength-dependent transmittance for TM polarized light and the corresponding fER based on various geometric parameters: (a, b) thickness of the dielectric (T1) (P = 100 nm, F = 0.5, and T2 = 20 nm), and (c, d) thickness of the metallic Al (T2) (P = 100 nm, F = 0.5, and T1 = 100 nm), and (e, f) incident angle.
2.2 Polarized flexible green OLED
To appraise the feasibility of our developed approach, the metal-dielectric nanograting was fabricated based on the optimized structural parameters and subsequently incorporated onto substrate of flexible green phosphorescent OLEDs using following procedures. The nanograting of Si master template with optimized geometric parameters (P = 100 nm, F = 0.6, and T = 80 nm) was fabricated using e-beam lithography [23,24]. Thereafter, the obtained Si template was fluorosilane-treated to acquire essential hydrophobic properties for an easier demolding process in upcoming nanoimprinting lithography procedure [25,26]. Next, the predesigned nanograting structure was directly transferred to perfluoropolyether (PFPE) transferring stamp by the Si master mold using UV soft-nanoimprinting lithography technology as we investigated previously [27–29]. Subsequently, the prestructured ITO-coated PET substrates (sheet resistance = 20 Ω sq-1) were first cleaned in an ultrasonic bath with detergent, acetone, ethanol and deionized water and then by UV-zone treatment for 15 min. Then, the prepared substrates were transferred into a thermal evaporation chamber for green OLEDs fabrication (base pressure = 10−6 Torr). The phosphorescent emitter unit (EML) consists of a 10 nm-thick 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) for hole injection, a 45 nm-thick di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane(TAPC) for hole transport, a 40 nm-thick 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) for electron transport and hole blocking, and a 20 nm-thick 10 wt% Bis (2-phenylpyridine) iridium(III) acetylacetonate (Ir(ppy)2(acac)) doped into 4,4f-N,Nf-dicarbazole- biphenyl (CBP) for green emission. Liq (2 nm)/Al (100 nm) bilayer was employed as the cathode. To fabricate metal-dielectric nanograting on flexible OLED device, the modified PUA resin, which was obtained through mixing the Six-functional urethane acrylate mixed with siloxane epoxy (1:1.06), was drop-dispensed on the PET substrate after necessary encapsulation procedure. Subsequently, the PFPE stamper was put onto the prepared PUA resin and imprinted at 0.3 MPa pressure under a UV illumination at the intensity of 800 mW cm-2 for 30 seconds. As a consequence, the PUA resin nanograting was successfully acquired after lifting off the PFPE stamper. Finally, the polarized flexible OLED was fabricated after 20 nm-think Al film was thermally evaporated on the PUA resin nanograting. Figure 4(a) and the inset provide the scanning electron microscope (SEM) images (FEI Quanta 200 FEG) of the patterned PUA dielectric grating and cross-sectional Al/PUA grating, respectively. From these images an Al/PUA grating has a period of ∼ 100 nm and a fill factor of ∼ 0.6 without surface bump. Moreover, the height of the patterned PUA grating and Al grating are nearly consistent with the theoretical optimized results of ∼ 80 nm and ∼20 nm, respectively, concretely validating the feasibility of our developed approach. The optical transmittances of PET substrates without and with metal-dielectric nanograting were measured using a UV/vis/near-IR spectrometer (Shimadzu, UV3600) by integrating a polarizer and are depicted in Fig. 4(b). Compared to the bare PET substrate with an average transmittance of 85.1% for TM polarized light, the substrate with the nanograting displays a relatively lower average TM polarized light transmittance of 68.7% over the whole visible spectrum. However, this experimental result is still satisfied taking into account the absorptivity of metallic Al film. To further investigate the polarization behavior of flexible OLEDs with integrated grating, the electroluminescence (EL) spectra intensity of TM polarized light (θ = 0°) and TE polarized light (θ = 90°) was measured with the experimental setup illustrated in Fig. 4(c) and are displayed in Fig. 4(d). In the experimental setup, a PhotoReasearch PR-655 spectrometer connected with a programmable Keithley 2400 power source was employed to simultaneously collect current, voltage and luminance. As mapped in Fig. 4(d), the EL spectra of TM polarized light dropped dramatically as the polarizer was rotated 90°, indicating that the proposed device exhibits desirable polarization characteristics and this feature is in accordance with the visualized photos of the polarized device after rotating the polarizer at three different orientations of θ = 0°, 45° and 90°, respectively [Insets in Fig. 4(d)]. Moreover, the current efficiency (CE) of flat control device and the polarized device were carried out without polarizer [Fig. 4(e)]. It is seen that the flat device yields a CE of 48.6 cd A-1 (@ 20 mA cm-2), while the polarized device shows a CE of 14.6 cd A-1 (@ 20 mA cm-2). Considering that the blocked TE polarized emission light and imperfect TM polarized light transmittance property of the integrated grating, the obtained smaller CE of the polarized device is reasonable and it is also higher than that of the previously reported results [10,14]. To analytically describe the polarization ratio results, both the simulated and tested angle-resolved (φ) extinction ratio of fER were characterized and are denoted in Fig. 4(f). It is observed that the developed device yields angle-invariant measured fER as high as 20.0 dB with respect to the normal direction of the nanograting surface within a viewing angle range (φ) of ± 60°. Furthermore, the obtained fER increases with the angle of φ and displays fulfilling angle robustness capability, which corresponds very well to the simulated results. Specifically, it is obvious that there exists a discrepancy between the calculation and experiment, which may stem from the imperfect Al coverage on the sidewall due to imperfect evaporation process as well as the natural oxidation of Al film. Furthermore, the performance stability of a polarized device was tested by repeatedly bending the substrates to a radius of curvature of about 5 mm at a constant current of 20 mA cm-2 (Fig. 5). The extinction ratio of fER exhibits stable trend under repeated bends with only a small decrease, less than ∼ 10% after 500 bending cycles. In contrast, the CE of the device shows a quick degradation mainly due to the brittle ITO layer, which illustrates superior bending stability of the proposed integrated grating structure. Obviously, better flexible performance would be realized if the suggested grating integrated on other flexible OLEDs using foldable transparent conductive electrode instead of an ITO electrode.
Fig. 4. (a) SEM images of dielectric (UV-resist) nanogratings. Inset: the cross sectional SEM image of the metal-dielectric nanogationg. (b) The measured TM polarized light transmittance spectra of bare PET substrate and the metal-dielectric nanograting constructed on PET substrate, respectively. (c) Schematic illustration of the measurement setup. (d) The EL spectra intensity of a polarized OLED as the polarizer rotates at an angle of θ. Insets: photos of polarized emission of the polarized OLED after rotating linear polarizer at different orientations. (e) The current efficiency as a function of current density. (f) Extinction ratio versus incident angle of φ. The error bars indicate the statistics obtained from 5 OLEDs per configuration.
2.3 Theoretical analysis
In an attempt to expound the physical working mechanism underlying the polarization-selective, the electric-field profiles and the corresponding time-average energy flux (Poynting vector) for the device without and with integrated gratings were simulated (RSoft 8.1 and corresponding codes generated in-house) and provided in Fig. 6 and Fig. 7, respectively. For the flat device, the initially both TM and TE polarized light sources behave as an identical spherical wave, which is confined within the flat structure and rapidly decays during the escaping propagation from the device (Fig. 6). Actually, the near-field radiation patterns are strongly dependent on the dipole orientation as the boundary condition of the electric field and the magnetic field are different. These observed identical spherical wave morphologies of different polarized waves of the flat device can be directly attributed to the large size setting in the simulation. For the polarized device, as mapped in Fig. 7(a), the impinging TM polarized light excites surface plamons (SPs), which induces surface charge oscillations, resulting in associated electromagnetic fields located at the Al/PUA interface [30]. Meanwhile, cavity modes (CMs) are also excited within the groove of the grating [31]. In other words, SPs and CMs are all responsible for enhanced extraordinary optical transmission (EOT) for TM-polarized incident light. This was further confirmed by the Poynting vector distribution [Fig. 7(b)], which clearly displays that the nanograting grooves function as light channels due to SPs and CMs resonances, collecting and channeling the TM impinging light through the grooves. In contrast, the free electron oscillation is excited and located at the Al/PUA interface in the grating direction where the incoming light energy flow cannot squeeze into grating groove gap and most of them are reflected and confined within the polarized device [Figs. 7(c) and 7(d)] [32].
Fig. 6. Simulation of the reference flat device with TM/TE polarized light source. (a) cross-section electric field intensity. (b) The corresponding time-average energy flux (Poynting vector) distribution.
Fig. 7. Calculations of the cross-section electric field intensity for TM polarized light and the corresponding time-average energy flux based on different polarized light sources: (a, b) TM polarized light, (c, d) TE polarized light.
3. Conclusions
In summary, highly linearly polarized light-emission flexible OLED with integrated ultrathin metal-dielectric nanograting has been proposed and experimentally demonstrated. The attained polarized flexible green OLEDs exhibits distinctive polarization behavior and yields a current efficiency of 14.6 cd A-1 at a current density of 20 mA cm-2 with a characterized average extinction ratio of ∼ 20 dB in the entire visible range. Moreover, the resultant device retains a favorable angle robustness of extinction ratio capability. Detailed theoretical analysis reveals that surface plasmons and cavity modes are simultaneously responsible for enhanced optical transmission for generating TM polarized light of the device. In particular, the developed grating architecture can be tightly constructed on the flexible device substrate by our developed soft-nanoimprinting lithography, which could lead to a new breed of more compact and efficient high performance flexible polarized light source.
Funding
National Natural Science Foundation of China (51875231, 61775076); Natural Science Foundation of Jiangsu Province (BK20161303); Six Talent Peaks Project in Jiangsu Province (DZXX-011); Qinglan Project of Jiangsu Province of China; 333 High-level Talents Training Program of Jiangsu Province.
Disclosures
The authors declare no conflicts of interest.
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