Dual-color emissive OLED with orthogonal polarization modes

doi:10.21203/rs.3.rs-3202940/v1

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Dual-color emissive OLED with orthogonal polarization modes

https://doi.org/10.21203/rs.3.rs-3202940/v1

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published 13 Feb, 2024

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Linearly polarized organic light-emitting diodes (LP-OLEDs) have become an appealing functional expansion in polarization optics and optoelectronic applications. However, this current LP-OLED generally presents comprehensively lower polarization performance, cost-prohibitive process and monochromatic modulation limit. Herein, a switchable dual-color OLED with orthogonal linearly polarization modes based on a dielectric/metal (D/M) nanograting-waveguide hybrid-microcavity geometry was first proposed, via the cost-efficient laser interference lithography and vacuum thermal evaporation methods. This acquired LP-OLED presented a reported transverse-electric/transverse-magnetic (TE/TM) polarization extinction ratio of 15.8 dB with a divergence angle of ± 30°, an excellent external quantum efficiency of 2.25%, and an orthogonal polarized colors from green to sky-blue. Detailed theoretical model illustrated that the rasterization of D/M-cathode further satisfied the momentum matching between waveguide and air mode. This diffracts both the targeted TE waveguide of sky-blue light that was localized furthest by modulating electron transport layer thickness and the off-confined green TM polarization light. Thereby, a polarization-encrypted colorful optical image was theoretically proposed. This work represents a significant step toward the low-cost LP-OLED with excellent polarization performance and toward the electrically-inspired polarization encryption for color images.

Physical sciences/Optics and photonics/Optical materials and structures

Physical sciences/Physics/Optical physics/Nanophotonics and plasmonics

Organic light-emitting diodes (OLEDs) have many distinct merits, such as high contrast ratio, wide viewing angle, self-luminescence, low driving voltage, flexibility, lightweight, and so on, resulting in their commercialization for full-color display panels, eco-friendly lighting sources as well as the most promising technology in solid-state light source, optical data communications, and photonics.^1–10 As an important freedom of light waves, the linearly polarized characteristic endows OLEDs with controllable spatial pattern of the emission spectra, which is completely distinguished from the conventional OLED that a Lambertian-like air mode emission with a non-directional emission.^11,12 This characteristic affords LP-OLED with great potentials for full resolution autostereoscopic naked-eye three-dimensional displays, visible light communication and information anticounterfeiting and encryption, inspiring various efforts been explored for the linearly polarized OLED (LP-OLEDs).^13–17

The most sticking point and a major obstacle for LP-OLED is to acquire a high linear polarization extinction ratio (ER).¹⁸ As for LP-OLED, organic single crystal-based OLED presents an inherent in-plane anisotropic nature for direct LP light emission. Nevertheless, compared to the thin-film LP-OLED, these organic single crystal-based ones are becoming incompetent for the construction of large-area LP electroluminescence devices.^19–20 Since for those planar OLEDs, some photons are confined with the substrate and waveguide layer, as well as propagating along the organic/metal interface.²¹ As a result, the leak into air acts as background light, leading to complex emission profiles with scarcely polarization extinction ratio and low light extraction efficiency (LEE).²² This ratio is defined as ER_TM/TE=10lgI_TM/I_TE, where I_TM and I_TE are intensities of transverse-magnetic (TM) and transverse-electric (TE) polarized emission at the same wavelength. The first LP-OLED can be tracked into 1995, in which, Dyreklev et al. introduced aligned conjugated polymers as the emitting layer, endowing the resulting device with a polarization anisotropy of 3.1, but with an EQE as low as 0.1%.²³ Consequently, several orientation methods, like friction-transfer processing, tensile alignment, Langmuir-Blodgett deposition, and pre-aligned substrates^24–26, have been developed rapidly for the emission of linearly polarized light; whereas, these methods usually give rise to a certain damage to the active layer and a wide emission peak with low color purity, as well as a lower ER. An alternative way to directly capitalize a planar OLED on integrated nanograting could precisely regulate the internal light field distribution and achieved high ER^27–33. In spite of this, these externally integrated optical structures, like birefringent photonic polarizer, generally result in at least 50% light loss and large emission angle^34–35, and also result in passively modulating optical properties for a narrowed bandwidth and a resulting pure linear polarized color, due to the weak light-matter interactions.^36–37

Therefore, an effective strategy should be adopted to improve the comprehensive performance of LP-OLED, of which, the method of embedding a nanograting structure into stacked OLED device has recently been identified as a straightforward and efficient approach.^38–39 In 2021, So et. al demonstrated a LP-OLED with a small divergence angle less than 3°, a moderate ER_TE/TM of 11.1 dB at ~ 520 nm and a high EQE of 7%, providing one of the few successful cases of polarization modulation by embedding an optical cavities using a combination of interference lithography and reactive ion etching.³⁹ It should be noted that the great challenge for internally integrated LP-OLED devices is how to realize a comprehensive enhancement in EQE, ER and small divergence angle, as well as to develop a cost-efficient method to replace the high-cost and time-consuming fabrication process that is involved with electron beam etching, ion etching, and nano-imprinting lithography technology^40–41. All the above-mentioned issues inevitably hinder the commercialization process, especially for large-area LP-OLED. Furthermore, considering the dynamic holographic display and polarized colorful image encryption, multi-wavelength modulation with different polarization mode in one single OLED device, is highly demanded, but no reported. Therefore, it is highly demanded to develop a reasonable integration strategy for LP-OLEDs with multi-color orthogonal polarization control, orienting toward virtual reality, power-saving backlight sources, optical storage and encryption.^42–44

Herein, a one-dimensional dielectric/metal (D/M) nanograting as large as 3×3 cm² was obtained through a combined low-cost methods of laser interference lithography (VIL) and vacuum thermal evaporation (VTE) for further being embedded into OLED device. Via optimizing the electron transport layer (ETL) thickness, the TE waveguide mode at desired wavelength was significantly confined within metal reflection waveguide, and as a result, high-intensity TE waveguide mode was diffracted into the air free space, due to the forming of corrugated OLED. Finally, a dual-color orthogonal polarization mode emissive OLED, with a main TE mode vertical emission presenting an ER of 15.8 dB, a suppressed full width at half maximum (FWHM) of 28 nm and a divergence angle of ± 30° for sky-blue light, along with a green TM mode light emission, was realized. The devices show great theoretical strategy for the polarization-encryption of colorful images. This presented design concept can be extended into the full-color gamut linearly polarized modulation with high ER and excellent EQE, providing a powerful platform to manufacture low-cost, large-area, multi-color emissive LP-OLED with multiple polarization for three-dimensional display, augmented/virtual reality, high-density data storage, and optical encryption.

Design and construction of large-area corrugated dielectric/metal nanograting.

As displayed in Fig. 1a, in order to form a high polarized emission with an extremely narrow emission cone, corrugated OLED structures embedded with a dielectric/metal (D/M) nanograting geometry, were reasonably designed and constructed, through finite-difference time domain (FDTD) simulation and the combined methods of laser interference lithography and vacuum thermal evaporation. Herein, a structure of glass substrate/cathode/electron injection layer (EIL)/electron transport layer (ETL)/emitting layer (EML)/hole transport layer (HTL)/hole injection layer (HIL)/anode (120-nm-Al) was utilized to construct an inverted bottom-emitting OLED. Of these, the cathode for corrugated OLED was composed by a D/M nanograting with a structure of photoresist/100-nm-MgF₂/25-nm-Ag; and a planar 25-nm-Ag film was also fabricated for planar OLED, as a control group. The EML layer was deliberately composed by the commercialized host materials, N,N’-dicarbazolyl-3,5-benzene (mCP) and 4,4’4”-tris(N-carbazolyl) triphenyl-amine (TCTA) and blue emissive guest materials Ir-complex iridium (III)bis-(4,6-difluorophenyl-pyridinato-N,C²)-picolinate (FIrPiC) with dual layers of TCTA:FIrPic (8% wt) and mCP:FIrPiC (8% wt).⁴⁵ The chemical structures of emitting materials are shown in Fig. 1b and corresponding EL spectrum for conventional OLEDs based on ITO cathode ranges from 440 to 650 nm with a FWHM of 67 nm and a dual-peak at 470 and 500 nm, respectively⁴⁵.

In order to produce a regular pattern, a maskless laser lithographic technique⁴⁶ was employed, by using an interference pattern from two obliquely incident beams at 343 nm wavelength, to illuminate the spin-coated photoresist film, as shown in Fig. 1c. With the illumination and development of the photoresist, an interference pattern of coherent light was recorded into the photoresist. After the subsequent vacuum thermal evaporation process of MgF₂ with a thickness of 100 nm and a thinner 25-nm-Ag film, a D/M nanograting was successfully transferred to the cathode. The period of the interference pattern and thus that of the grating recorded on the substrate, were given by the grating diffraction equation (Supplementary Note 1). Herein, the thickness of the bottom Ag layer was reasonably set as 25 nm with a refractive index (n) of 0.13 and extinction coefficient (k) of 2.66 (Supplementary Fig. S1a), to ensure the conductivity and transparency as the transparent electrodes for OLEDs. To optimize the type of dielectric material, the simulated far-field distribution of corrugated OLED in normal direction was calculated (Supplementary Fig. S1b) and the resulting output intensity reaches its maximum when the effective refractive index (n_eff) is located at 1.60. Considering the n for photoresist is ~ 1.70, along with the equivalent refractive index theory, the MgF₂ with a lower n (1.38 at λ = 470 nm) and lower optical loss for blue light region, was deliberately incorporated (Supplementary Fig. S1b). As a result, the 100-nm-thick MgF₂ layer was deposited on the corrugated photoresist film to ensure the deep enough groove depth. On the other hand, the relatively suppressed n of substrate, is reasonable to suppress the substrate mode and enhance the light extraction efficiency of linear polarized light.

Via precisely controlling the recording angle β, laser power, exposure time and developing time of photoresist film, along with the thickness of MgF₂ layer, a perfect MgF₂/photoresist grating with a certain groove depth of 80 nm, a period of 300 nm, and a ridge width of 150 nm, was successfully realized for the intracavity resonance of 470 nm wavelength, As presented in Fig. 1c. Excitingly, via modulating the β value, a large-area Ag/MgF₂/photoresist grating was realized with a size of 3×3 cm², when β was set as 34.9°. This combined method of LIL and VTE offers an excellent, relatively easy and cost-efficient technique for large-area corrugated OLED.

Simulation of planar OLEDs with highest TE waveguide mode confinement

Considering rare reports on sky-blue LP-OLED with high ER and EQE, one of the main tasks of this work was focused on the efficient coupling output of 470 nm light for high polarization with narrow linewidth. In order to achieve pure TE waveguide emission at 470 nm wavelength, desired geometric structure of the stacked OLED should be rationally designed to completely suppress the emission of TM waveguide, SPP, substrate and air mode, along with diffracting the localized TE waveguide mode efficiently by using corrugated structure. According to the measured reflection and transmission spectra in Supplementary Fig. S2a, a reflectivity of 70% and as high as 97% are provided from the bottom 25-nm-Ag cathode and the 120-nm-Al anode, respectively. Along with the sandwiched organic layer, a planar Fabry-Pérot (F-P) nanocavity is successfully constructed, and thereby, forming a metal reflection waveguide (MRW) and improving mode confinement within the gain-active medium.⁴⁷

In order to achieve a maximum confinement of waveguide mode, reasonably enlarging the effect refractive index of waveguide layer was necessary. Herein, due to the relatively higher n of ETL (TmPyPb) than that of TAPC (HTL) (Supplementary Fig. S1b), the thickness of ETL was deliberately regulated. First, the intensity and corresponding proportion changes of different modes at λ₀ of 470 and 500 nm, including air mode, substrate (Sub) mode, absorption mode, surface plasmons (SPP) mode and waveguide (WG) mode, as the increase of ETL thickness, were simulated for planar OLED (Fig. 2a and 2d). It was notable to find that the air mode emission at 470-nm-wavelength was suppressed almost completely when the ETL thickness ranges from 30 to 115 nm, combined with an increasing SPP mode and decreased substrate and waveguide modes. Similarly, the air mode at λ₀ = 500 nm, presented an analogous tendency, but with a slightly suppressed valley and right-shift, which was mainly ascribed to the difference of wavelength response to the F-P cavity.⁴⁸ To clearly understand the influence of MRW geometry on the mode distribution for TE and TM polarization at the wavelength of 470 nm, the far-field angular emission pattern of both polarization modes for the planar OLED with different ETL thicknesses were simulated, and the corresponding emission intensities were represented by a blue line in the illustration of Fig. 2b and 2c. Apparently, the TE mode at various ETL thickness, was emitted at a specific angle with a complete confinement into the waveguide at the ETL thickness range of 34 ~ 113 nm. These observed emission pattern and the changed intensity could be illustrated by the followingformula^49–50:

$$\frac{4\varvec{\pi }{\varvec{n}}_{\varvec{e}\varvec{f}\varvec{f}}\varvec{h}\varvec{c}\varvec{o}\varvec{s}}{{}_{0}}-{}_{1}-{}_{2}={\varvec{m}}_{1}2\varvec{\pi }$$

where is the incident angle of photons to the metal film, $ℎ$ the cavity length or organic WG thickness, ${n}_{eff}$ the effective refractive index, ${m}_{1}$ the order and  the reflection phase shift. Detailed analysis could be found in Supplementary Note 2. Herein, the n_eff is determined as 1.51 when 107-nm-thick ETL with n of 1.81 and EML with n of 1.75 (Supplementary Fig. S2b). As seen in Supplementary Fig. S3 and Fig. 2h, for any frequency photons that propagate with a certain angle α between two electrodes, only when the optical path difference between two adjacent outgoing beams satisfied the coherent phase length condition, the modes finished the emitting behavior with a uniquely determined view angle α₁ that was equal to arcsin(n_eff/sinα). For TE mode at λ = 470 nm, its view angle was enlarged as the increased ETL thickness, presenting a nearly horizontal emission by breaking phase matching at the case of a matched thickness of 107 nm; whereas, in this case, the TM emission started to present a profile. This was ascribed to fact that the TE mode was completely confined into the waveguide layer (Fig. 2e), but large section of TM waveguide mode excited surface plasmons (SPP), resulting in associated electromagnetic fields located at Ag/organic interface (Fig. 2f).

In order to separately determine the optimal ETL thickness for the maximum confinement of TE polarized mode into waveguide layer, all mode intensities at λ = 470 and 500 nm as the function of ETL thickness were simulated. As shown in Fig. 2g, multimode photon localization was successfully controlled by modulating the waveguide (ETL) thickness, for instance, when the ETL was controlled at 107 nm, these unwanted extra modes (Sub and air mode) were suppressed to the maximum extent, and therefore, confining the target single TE mode at desired 470 nm wavelength.

Simulation of corrugated OLEDs with highest TE light extraction efficiency

Based on above simulation and analysis, a reasonable Bragg grating geometry was further embedded to achieve TE polarized light extraction. Thereby, another task in this work is to fine-tune the geometric parameters of D/M nanograting-waveguide structure to realize a switchable dual-color OLED with high orthogonal polarization modes. Of these, the corrugated OLED structure was constructed with a ETL thickness of 107 nm, a groove depth of 80 nm, a period of 300 nm, and a ridge width of 150 nm for photoresist/MgF₂ nanograting.

The resulting simulated electric-field intensity contours for plane wave incidence in TE and TM modes for corrugated LP-OLEDs, were displayed in Fig. 2i and 2j. Of these, the TE mode light was transmitted into air free space, while TM polarized mode was transformed by the grating, forming a certain SPP mode, as schematic in Fig. 2k. This phenomenon was consistent with the simulated position distribution of TE and TM modes (Supplementary Fig. S4a) that most of the TM mode was located at the interface of metal and dielectric, generating SPP mode and that the TE mode formed waveguide mode. More than that, due to the strong quenching by the metal layer, TM mode experienced severely attenuation⁵¹ and only TE mode intensity changed periodically with the ETL thickness. Herein, we defined a light extraction efficiency ratio (LEER) between that of planar and corrugated OLED, as $\text{L}\text{E}\text{E}\text{R}=\frac{{\text{L}\text{E}\text{E}}_{\text{c}\text{o}\text{r}\text{r}\text{u}\text{g}\text{a}\text{t}\text{e}\text{d}}}{{\text{L}\text{E}\text{E}}_{\text{p}\text{l}\text{a}\text{n}\text{a}\text{r}}}$. For TE polarized mode, the LEER was approached to ~ 15 time when the ETL thickness was determined as 107 nm, in which case, whereas, it was close to zero for TM polarized mode (Supplementary Fig. S4b). Essentially, this phenomenon was resulted from the momentum-match between TE waveguide light and air photon. Bragg gratings efficiently promoted out-coupling by providing the TE waveguide light with additional momentum to couple to air (Fig. 2k, Fig. 4d and 4e). The momentum matching condition is given by the Bragg Eq. 5²:

$$\overrightarrow{{\varvec{k}}_{0}}=\overrightarrow{{\varvec{k}}_{\varvec{w}\varvec{g}}}\pm {\varvec{m}}_{2}\overrightarrow{{\varvec{k}}_{\varvec{G}}}$$

here $\overrightarrow{{\varvec{k}}_{0}}=\frac{2\pi }{\overrightarrow{{}_{0}}}$,$\overrightarrow{{\varvec{k}}_{\varvec{w}\varvec{g}}}=\frac{2\pi {n}_{eff}}{\overrightarrow{{}_{0}}}$, $\overrightarrow{{\varvec{k}}_{\varvec{G}}}=\frac{2\pi }{\overrightarrow{}}$is the momentum of light in air, waveguide, and grating, respectively; ${m}_{2}$ is the diffraction order. For a given wavelength ${}_{0}$, TE waveguide mode coupled to air could be tuned by the grating period, the effective refractive index and the waveguide layer thickness (Fig. 2l). Herein, the intensity and wavelength of emitted TE waveguide mode into air were governed by the ETL thickness and grating period Ʌ, that overcomes the phase matching of F-P cavity and meanwhile satisfies the momentum matching of grating structure. Therefore, a 300-nm-period grating was adopted for 107-nm-thick ETL to extract strongest waveguide mode of 470 nm wavelength. This negligible TM diffraction mode was ascribed to the polarization selectivity of grating structure for waveguide mode, along with the weak coupling between the momentum of $\overrightarrow{{k}_{SPP}}$ and $\overrightarrow{{k}_{G}}$.

Furthermore, the mode dispersion for TE and TM polarization mode presented a distinct diffraction profile with a maximum intensity and negligible emission intensity at λ = 470 nm, respectively (Fig. 2m and 2n). The simulated ER_TE/TM revealed the excellent linear polarization characteristic of the optimized LP-OLED with a high ER, a small divergence angle and a narrow bandwidth at λ = 470 nm. This was ascribed to the fact that the corresponding waveguide mode at desired wavelength exactly broke the phase matching of F-P cavity and therefore, matching momentum condition of the grating geometry at the case of optimized ETL thickness and grating period.

LP-OLED device performance characterization and analysis.

To validate our design concept, we fabricated corrugated OLEDs with a grating period of 300 nm and ETL thickness of 107 nm to investigate the effect of D/M nanograting-waveguide geometry on the device performance, including the EQE, ER, and the electroluminescence angular dispersion characteristic. Herein, the corrugated OLED with an inverted structure of glass/photoresist-MgF₂-Ag nanograting/LiF/TmPyPb/mCP:FIrPiC/TCTA:FIrPiC/TCTA/TAPC/ HATCN/Al (Fig. 3a), was fabricated as depicted in Fig. 1c. In addition, planar OLEDs with an ETL thickness of 107 nm and 60 nm, respectively, were also fabricated for comparison, which were named as planar and reference OLED, respectively. According to the cross-sectional scanning electron microscope (SEM) images of these multilayer (Fig. 3b), it was clearly observed the significant corrugated geometry until the top thick-Al anode for the stacked OLED. The elemental map revealed its notably structural characteristic.

The EL spectra, current density-voltage-luminance curves, angle-resolved EL intensity in normal direction at emission peak of 470 nm wavelength, and the corresponding TE and TM polarized mode emission, were measured with the same setup, as shown in Fig. 3c-3l. Contrast to the reference OLED with 60-nm-ETL, both planar and corrugated OLED with 107-nm-ETL, presented a significantly narrowed emission spectra with a distinguishable EL peak at λ ~ 470 nm and almost a disappeared peak at λ ~ 500 nm. Of these, the optimized corrugated OLED displayed a suppressed FWHM of 28 nm, from 205 nm for reference OLED, with a CIE from (0.31,0.48) to (0.13,0.35). This clearly demonstrated that the successful introduction of D/M nanograting-waveguide geometry significantly shifted the EL spectra to desired blue-sky region. From Fig. 3e, the turn-on voltages (V_on) of the optimized LP-OLED were also suppressed to 4.7 V and the corresponding luminance exceeded 48 cd m^− 2 at 8 V, significantly higher than that of 7 cd m^− 2 for planar OLED. The EQE values of both planar and corrugated OLED devices were measured to be 1.23% and 2.25% at 8 V, respectively. Detailed parameters of all devices are summarized in Table 1. Furthermore, The corrugated LP-OLED presented a relatively higher storage stability than that of the planar and conventional OLED, as summarized in Supplementary Fig. S5. This may be mainly ascribed to the hybrid microcavity effect on the exciton lifetime and the more uniform photons distribution.

We experimentally assessed the performances of the emitted TE and TM polarized mode dispersion by employing the angle-resolved emission spectra measurement via rotating the polarizer and θ(Supplementary Fig. S6). For the planar OLED, the air mode was significantly suppressed with a slight TE mode emission at the wavelength from 460 to 500 nm (Fig. 3h, i). The EL intensities at both specific wavelengths were deliberately extracted, as displayed in Fig. 3g, revealing the weak polarized emission at both wavelengths. Excitingly, the optimized corrugated OLED presented a high ER_TE/TM of 15.8 dB at λ = 470 nm, small divergence angle of ± 30°, and a narrowed FWHM of 28 nm(Fig. 3f, 3j and 3k). This was highly consistent with the simulation results as Fig. 2m. In contrast, there is almost no waveguide mode or SPP mode diffraction features in TM mode at emission peak of 470 nm wavelength (Fig. 3l). It should be noted that the polarization ratio of the linearly polarized light source required for commercialization is 14.7 ~ 16.0 dB, demonstrating the great commercial application potential of our devices.

Table 1

Summary performance parameters of planar and corrugated OLED devices with ETL thickness of 107 nm
Device	V_turn−on (V)	EL intensity (a.u.)	FWHM (nm)	η_C,max (cd/A)	η_p,max (lm/W)	ER_max (dB)	EQE MAX/at 8 V (%)
Planar OLED	5.2	13	38	4.15	2.33	4.9 (@500 nm)	1.93/1.23
Corrugated OLED	4.7	103	28	3.81	1.60	15.8 (@470 nm)	2.25/2.23

Acquired TE mode emission and corresponding development status.

Accordingly, for the optimized LP-OLEDs, the TE polarized mode presented a sky-blue color, with a θ of 90°, as seen in Fig. 4a and Supporting Video; whereas, the intensity of sky-blue TE polarized light dropped dramatically as the polarizer was rotated to 0°, ultimately displaying a green TM polarized light (Fig. 4b). This created a dual-color emissive OLED with orthogonal polarization mode. Furthermore, the resulting emission pattern took on a dual-intersecting bright arcs, corresponding to the two back propagation waveguide modes of grating diffraction, as displayed in Fig. 4c, which was completely different from the Lambertian profile of planar OLED, as shown in the inserted map. Moreover, the inner side of the arc displayed a high luminance of sky-blue, whereas, a low green luminance for the outer side. This could be explained using reciprocal space. Compared with the simulated mode dispersion of dielectric materials for planar OLED device (Fig. 4d), the total internal reflection and dispersion formed waveguide (WG), substrate (Sub) and air mode, were significantly modulated by the insertion of grating momentum (Fig. 4e). Owing to the additional momentum $\overrightarrow{{k}_{G}}$ by the nanograting to couple the light within the waveguide into the air, two dual-colorful arcs were observed with a short-wavelength in the inner side for the air dispersion with a stronger resonance and higher emission intensity when approaching to the arc center. This was highly consistent with experimental result in Fig. 4e.

To further assess this statement quantitatively, detailed comparison with state-of-the-art LP-OLED, was summarized in Table S1, of which, the negative data represented 10lg(TM/TE). Figure 4f depicted a comparison of LP-OLED emitting TE polarization mode, via the strategies of molecular alignment, external and embedded optical nanostructures. It was clearly demonstrated that all of these parameters were among the best level for LP-OLED devices. Also, to the best of our knowledge, this was the first attempt to employ organic semiconductor for a dual-color emissive OLED with orthogonal linearly polarized modes, and this design strategy could be further utilized to design LP-OLEDs at whole visible color gamut for high TE polarization (Fig. 4g). As the ETL thickness was increased from 100 to 200 nm, combined with the corresponding period being located at 300 ~ 400 nm, desired colorful emission ranging from 480 to 640 nm could be achieved. Thereby, another narrow bandgap OLED with a red TE mode and a green TM mode light, could also be achieved, in which, its grating period and ETL thickness were controlled at 385 nm and 192 nm, respectively. Accordingly, a dynamically modulation of dual-color polarized light emission at any wavelength with high EQE, narrow bandwidth, strong polarization, and small divergence angle could be effectively realized via simply matching the emission wavelength, grating period, and ETL thickness.

Electroluminescent linear polarization-based colorful image encryption.

Now that we have established these high performance LP-OLEDs with dual-color orthogonal polarization, this distinct emission behavior inspires us to further explore their application for novel color-image encryption. The connection between electroluminescence and keys in color image encryption applications is presented in Fig. 5. As seen in Fig. 5a, a full-color display could be achieved by combining two types of corrugated OLEDs. As the optical axis of the polarizer rotates from 0° to 90°, two dual-color orthogonal polarization transition were observed, with one from green TM polarized light to a blue TE one for the wide-bandgap OLED and with another from green TM polarized light to a red TE in the narrow-bandgap device. Taking Van Gogh's Iris in Fig. 5c(i), as an example, in which, this complete color array is decomposed into several series of color slices. These cover all the colors of the encrypted target and serve as a guide for single-color device array, as shown in Fig. 5b. Different from the original Iris picture, the product obtained from a polarization direction of 90° was deliberately adopted as the target encryption. Based on the presented coupling theory model, each color slice was determined by the device grating period and ETL thickness, as revealed in Fig. 5b(i-v) for blue, cyan, green, yellow, and red colors, respectively. Based on this, with the introduction of a polarizer, the picture color changes significantly as the rotation of optical axis, and only when it is rotated to 90°, the encrypted target can be reproduced,. This is completely distinguished from the original picture observed with the naked eye (Fig. 5c). The fore-polarization process, along with multicolor polarization encryption from micro-LP-OLED, facilitate a double encryption for colorful images. To the extent of our knowledge, this is the first attempt for polarization-controlled chromo-encryption based on electroluminescent devices.

In summary, we have demonstrated, for the first time, the controllable fabrication of a dual-color emissive LP-OLED with orthogonal polarization mode, which is achieved by a dielectric/metal nanograting-waveguide geometry through the combined cost-efficient methods of laser interference lithography and vacuum thermal evaporation. The fine modulation of ETL waveguide thickness, combined with the subtly embedding of Ag/MgF₂ nanograting geometry into OLED devices, coincidentally satisfied the momentum matching of grating structure and air space, and thus, selectively diffracting the desired sky-blue TE mode emission that confined within waveguide layer. Additionally, a certain green TM mode that beyond the waveguide confinement frequency, was also emitted into the air free space, forming a dual-color spatial pattern with orthogonal polarization. To the extent of our knowledge, it was the first theoretical proposal to employ an electroluminescent linear polarization emission with orthogonal polarization for dual encryption of colorful images. The diversity of freedom degrees, like the grating geometry and waveguide thickness and material types, can be enable a wealth of application in solid-state colors, high density optical data storage and encryption.

Ag/MgF ₂ nanograting fabrication on the glass substrate.

Glass substrates were cleaned with deionized water, ethanol and acetone in sequence. Prior to spin-coating of the photoresist layers, those substrates were dried and then exposed to ultraviolet-ozone for 20 min. After the glass was cooled to room temperature, an ultrathin layer of photoresist (AP-R3170) was spin-coated onto the glass substrate at 3,000 r.p.m. for 60 s and then baked at 110 ℃ for 70 s. A laser source with a wavelength of 343 nm was split into two pathways and eventually merged and interfered on the photoresist and thereby generated the grating pattern with a period of 300nm after development (AR 300 − 47). The template was cleaned using O₂ plasma (120 W, 90 s) and loaded into a thermal evaporation chamber. MgF₂ grating was fabricated on cleaned template that was pre-deposited with 100-nm-thick MgF₂, which was formed using thermal evaporation at a pressure of 3×10^− 4 Torr. Finally, a 25-nm-thick Ag layer was deposited on the MgF₂ nanograting by thermal evaporation to obtain the Ag/MgF₂ nanograting.

Fabrication of OLEDs

The Bottom-emitting OLEDs were fabricated on the Ag/MgF₂ grating substrate by thermal evaporation of the following materials in sequence: Al as anode; 1,4,5,8,9,11-hexaazatriphenylene-hexanitrile (HATCN) as hole injection layer; 1,1’-bis((di-4-tolylamino) phenyl) cyclohexane (TAPC) as hole transporting layer; 4,4’4’’-tris(N-carbazolyl) triphenyl-amine (TCTA) as electron blocking layer; iridium (III)bis-(4,6-difluorophenyl-pyridinato-N,C²)-picolinate (FIrpic) doped N, N’-dicarbazolyl-3,5- benzene (mCP) or TCTA as emitting layer; 1,3,5-tri(m-pyridin-3-ylphenl) benzene (TmPyPb) as electron transporting layer; and LiF/Ag as cathode. The device structure is Ag (25nm)/LiF (0.8nm)/ TmPyPb (107 nm)/mCP:FIrpic (8%, 15nm)/TCTA:FIrpic (8%, 15nm)/TCTA (5nm)/TAPC (40nm)/ HAT-CN (10nm)/Al (120nm). The evaporation rate of organic materials is 0.1 ~ 1 Å/s, and that of metal materials is 0.1 ~ 2 Å/s, during the evaporation process. All materials are evaporated at a pressure of 3×10^− 4 pa。

Characterization of OLEDs

The EQE–luminance–voltage curves and EL spectra for the planar and granting OLEDs were measured with a programmable source meter (Keithley 2400) and a spectrophotometer (Spectrascan PR650), which measures power per steradian per unit wavelength per unit area (in units of W/(nm·sr·m² )). The angular distribution of the EL was measured using a source meter (Keithley 2400), a linear polarizer (GCL-0510), a goniometer, and a fiber optic spectrometer (Ocean Optics S2000). SEM measurement was carried out on a JEOL JSM-7500F scanning electron microscope. Atom force microscopy (AFM) imaging was performed on a Dimension Icon AFM (Bruker Corporation). The film thickness was measured using XP-2 stylus profilometer.

Theoretical Modeling

The optical reflectivity of the electrode, the near-field intensity distribution, the far-field modal dispersion and OLED optical mode distribution were calculated using the FDTD method together with in-house generated codes, where the structure parameters of the grating were set as the 300 nm according to the AFM measurements. In the simulation, the wavelength-dependent refractive index (n, k) of Ag is calculated using Drude model, and the refractive index (n, k) of the other materials were measured by an ellipsometer within the wavelength region of 400 ~ 850 nm, from which the material models were automatically generated and adopted in the FDTD. The periodic boundary conditions in X direction and perfectly matched layer boundary conditions in Y direction were employed to optimize the grating structure due to the periodic of the nanograting The mesh accuracy was set as 𝜆∕dx = 18 in the overall architecture and a smaller mesh (dx, dy = 1 nm) was added in regions of the OLED. For modal dispersion in reciprocal space, MATLAB software was used.

Competing interests

The authors declare no competing interests.

Author contributions

Ruixiang Chen: Experimental investigations, Device fabrication and characterization, Optical simulation, Experimental images, Writing-original draft. Ningning Liang: Conceptualization, Investigation, Methodology, Supervision, Writing-review & editing. Tianrui Zhai: Conceptualization, Investigation, Methodology, Supervision. All authors discussed the experiments and results.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22275005) and the Beijing Natural Science Foundation (Z180015). The authors gratefully acknowledge the following persons: Prof. Quanfu An, and Prof. Yongzhe Zhang of Beijing University of Technology for providing the AFM and film thickness measurements, respectively.

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Dual-color emissive OLED with orthogonal polarization modes

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Abstract

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Introduction

Results

Simulation of planar OLEDs with highest TE waveguide mode confinement

Simulation of corrugated OLEDs with highest TE light extraction efficiency

Conclusion

Methods

Fabrication of OLEDs

Characterization of OLEDs

Theoretical Modeling

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Competing interests

Author contributions

Acknowledgements

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