Young Yun Kima,
Jung Jin Parka,
Seong Ji Yea,
Woo Jin Hyunb,
Hyeon-Gyun Imc,
Byeong-Soo Baec and
O Ok Park*a
aDepartment of Chemical and Biomolecular Engineering (BK21+ Graduate Program), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea. E-mail: oopark@kaist.ac.kr
bDepartment of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave. SE, Minneapolis, MN 55455, USA
cDepartment of Materials Science and Engineering, Korea Advanced Insititute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
First published on 5th July 2016
The use of polymer light-emitting diodes (PLEDs) as future displays and lightings has been of interest because of their advantages such as lightness, thinness, high contrast ratio, fast response time and high flexibility. Light-extraction structures such as microlens arrays have been designed to improve the efficiency of PLEDs. In this work, we have fabricated novel microlens arrays based on an organic–inorganic hybrid sol on a flexible substrate. Al2O3 nanoparticles dispersed in microlens arrays are expected not only to extract more light but also to block gas molecules. Further light-extraction from Al2O3 nanoparticles in the microlens arrays was confirmed by an increase in diffusive reflectance and photoluminescence intensity. As a result, the flexible PLEDs with the arrays showed enhanced maximum current efficiencies by 47% and 38% for SPB-02T (blue) and PDY-132 (yellow) emissive materials. They also showed improved stability, and high flexibility at a bending radius of 4 mm. Furthermore, Al2O3 nanoparticles were incorporated into irregular microlens arrays consisting of two different sizes of hemispherical lenses, to minimize changes in emission spectra and radiation pattern as well as to improve efficiency.
Internal quantum efficiency of OLEDs has approached almost unity by the recent development in highly efficient emitters and by precise engineering of interlayers for adjusting energy levels, which minimize energy barriers and improve charge transport characteristics.4,13,14 In contrast, extraction of photons generated inside the device still needs improvement because the extraction efficiency would reach only about 20%, due to differences in refractive indexes among layers.15,16 The rest of the generated photons are lost by three main pathways namely, ITO/organic mode, surface plasmon mode, and substrate/air mode.16 Photon loss from the first two modes can be recovered by using high-index substrates,17,18 by substituting the cathode metal with other materials,19 and by introducing a low-index grid,20,21 a corrugation structure22–25 or a photonic crystal.26,27 In case of the substrate/air mode, which comprises a major portion of the trapped or wave-guided modes, adaptation of the optical structure to the substrate is required to extract more light.
Several studies have been reported including insertion of scattering media28,29 and a corrugation pattern30 as well as introduction of microlens arrays for the out-coupling of the substrate/air mode.31–36 Introduction of microlens arrays, which reduces incident angles of light from the substrate, is an effective approach to extract light that would be trapped or wave-guided without them. However, previous studies suggest that a major drawback of microlens arrays is the changes in emission characteristics such as emission spectrum and radiation patterns at different viewing angles.31–33 In addition, the fabrication processes for microlens arrays sometimes include photolithography, e-beam lithography or interference lithography, which demand high cost and pose additional difficulties in manufacturing.31,33,35,36 Therefore, simple and cost-effective fabrication of microlens arrays that introduces minimal changes in optical properties is very much necessary. Additionally, maximizing luminance improvements and introducing additional functionality for microlens arrays are important in consideration of cost and time required in their fabrication. The flexibility of such arrays should also be improved for their application to flexible devices.
Several recent studies have been conducted to meet the demands concerning microlens arrays, such as reduction of fabrication cost and difficulty, introduction of new functionality, and reduction of optical side effects. Galeotti et al. proposed a simple technique, the breath-figure method, to reduce the cost and difficulty of their fabrication.34 Koh et al. reported the mixing of green and red phosphors for microlens arrays to impart an additional functionality as a color filter.35 Hwang et al. described the reduction of optical haze in microlens arrays by selective etching techniques.36 However, there have been no attempts to solve the aforementioned issues at the same time by one simple process. In addition, flexibility of microlens arrays have not been reported.
In this report, we would like to describe novel microlens arrays containing nano-sized Al2O3 particles used in enhancing efficiency and stability of the PLEDs. The arrays, which can be fabricated through simple and cost-effective methods under mild conditions, exhibit high flexibility. Because of an organic–inorganic hybrid network of silica and an epoxy compound can be readily formed with brief thermal curing, the fabrication of microlens arrays is simple and inexpensive, and the resulting arrays exhibit excellent flexibility. Since Al2O3 nanoparticles are known to exhibit good light-scattering ability due to their high refractive index and inherent barrier properties against oxygen and water vapour, inclusion of Al2O3 nanoparticles in the microlens arrays will enable further light extraction and retarded degradation of the PLEDs. Undesirable changes in emission characteristics including emission spectra and radiation patterns on viewing angle were minimized by adapting Al2O3 nanoparticles in an irregular microlens array.
To replicate the colloidal self-assembled patterns, a polydimethylsiloxane (PDMS) replica mould was made by mixing of pre-polymer and curing agent (Sylgard 184, Dow Corning) as a ratio of 10:
1. The colloidal pattern on slide glasses was treated by trichlorosilane (Aldrich) vapor for 10 minutes to prevent the direct attachment of PS particles to the PDMS replica mould. The mixed solution of PDMS was poured on the pre-treated colloidal pattern and thermally cured at 70 °C for 4 hours.39 The PDMS replica mould was then fabricated by detaching cured PDMS carefully from the colloidal pattern.
As for a transfer of the microlens array pattern on the desired substrate, the silica hybrid sol solution was firstly spin-coated on the substrate which was treated with oxygen plasma for 5 minutes at 18 mW (PDC-32G, Harrick Plasma) before the spin-coating. Then the PDMS replica mould was applied on the silica sol film and cured at 70 °C for 30 minutes. By the careful detachment of PDMS from the silica sol film, the microlens array was formed at the substrate. For the microlens array with Al2O3 nanoparticles, Al2O3 nanoparticles (Nanodur, Nanophase Technology, diameter = 40–50 nm) were dispersed in ethanol and mixed with the hybrid silica sol solution.
For the device with a microlens array, the array was formed at the backside of the substrate by the procedure described above. As an anode, PEDOT:PSS (Clevios PH1000) mixed with 5 vol% with ethylene glycol (J. T. Baker) and 0.1 wt% Zonyl FS-300 fluorosurfactant (Aldrich) was spin-coated on the substrate at 1000 rpm for 40 seconds followed by thermal annealing at 130 °C for 20 minutes.40 This procedure was repeated twice to form a thicker anode film to achieve a low level of surface resistance and ensure operation stability of a polymer light-emitting diodes. A thickness of resulting film was about 150 nm. Then PEDOT:PSS (Clevios P VP AI 4083) as a hole-transporting layer was spin-coated at 2500 rpm for 40 seconds and annealed at 115 °C for 20 minutes. The substrate was subsequently transferred to a glove box followed by spin-casting of a blue light-emitting polymer, SPB-02T (Merck) from 10 mg mL−1 solution in chlorobenzene. The resulting film was thermally annealed at 110 °C for 20 minutes to remove solvent. Finally, the sample was transferred and equipped to a thermal evaporator. Lithium fluoride (LiF) and aluminum (Al) were deposited sequentially. Thicknesses and deposition rates for both materials were 1 nm and 100 nm, 0.2 Å s−1 and 2 Å s−1, respectively. The emissive area of the device was defined by overlapped area of the anode and the cathode, which was 3 × 3 mm2.
Fig. 1b represents the fabrication procedure of the microlens array. Three-dimensional colloidal structures as master moulds were prepared by a simple dip-coating method using an oven to promote the self-assembly of polystyrene (PS) particles aided by the binder polymer, polyvinylpyrrolidone (PVP).39 Polydimethylsiloxane (PDMS) pre-polymer and curing agent were prepared and poured onto the colloidal crystal. After they were thermally cured, they were carefully detached from the colloidal structure to form a replica mould. The silica hybrid sol mixed with Al2O3 nanoparticles was spin-coated onto a poly(ethylene naphthalate) (PEN) substrate, and the PDMS replica was applied on top of the resulting silica hybrid sol film. A PEN film was selected because of its good thermal property (processing temperature = 160 °C), optical transparency, and chemical and mechanical stability.44 The silica hybrid sol film with PDMS was cured thermally in an oven. Finally, the microlens array with embedded Al2O3 nanoparticles was fabricated by detaching the PDMS replica mould from the film.
A colloidal master pattern composed of PS particles with 1.5 μm diameter was observed by scanning electron microscope (SEM) (Fig. 2a). PS particles assembled into a close-packed hexagonal pattern in the dip-coating process. The diameter of PS was chosen to be 1.5 μm, the largest diameter that could be synthesized with high regularity by dispersion polymerization in the present study. In addition, a previous study suggested that the out-coupling efficiency tends to converge at about 30 percent when the diameter of the microlens is larger than 1 μm.45 The microlens array was then formed on the PEN substrate by pattern transfer from the PDMS replica mould. Fig. 2b clearly shows that hemispherical microlenses were formed without any differences in size and shape from the original master mould. The microlens array was composed of silica hybrid sol, which was prepared by mixing silica particles with tetraorthosilicate and organic (3-glycidoxypropyl)trimethoxysilane (GPTMS) molecules under mildly acidic conditions. Thermal curing of the sol results in the formation of an organic–inorganic hybrid network, which is a blend of the inorganic network of silica formed by hydrolysis and poly-condensation, and the organic network formed by ring-opening polymerization of the GPTMS.
Therefore, the microlens array pattern could be replicated well by simple thermal curing at short time. The fabricated pattern could retain its shape and size even after the bending test. SEM observation (inset in Fig. 2b), revealed that cracks or defects did not form after 1000 cycles of the bending test at a bending radius of 4 mm (data not shown). Therefore, the microlens array consisting of the hybrid silica sol is highly suitable for applications in flexible PLEDs. The Al2O3 nanoparticles (Nanophase Technology Corp) are shown in the transmission electron microscope (TEM) result in Fig. 2c.
Fig. 3b presents diffuse reflectance spectra of the PEN film, the film with a microlens array, and the film with Al2O3-embedded microlens arrays. Diffuse reflectance is the effectiveness of reflection of light such that incident and reflected angle of light are not identical. The spectra were recorded by passing the light to the opposite side of the microlens array. A higher value of diffuse reflectance means greater scattering of light coming from the interior of the device.
The spectrum of bare PEN film shows a low defined level of diffuse reflectance (3.0%), which might be due to inherent scattering by processing fillers in the film.47 Although the PEN film used in this study is optical grade, a small amount of inherent scattering spot can cause this phenomenon. Upon attachment of the microlens array to the PEN film, the diffuse reflectance increased to 8.0% at 450 nm because the array could affect path of the light entering the film, or the small inclusions in the silica hybrid film could also scatter light. Diffuse reflectances of the array mixed with 0.8 and 2.6 wt% of Al2O3 nanoparticles respectively increased to 8.9% and 11.7% at 450 nm. Thus it is concluded that the Al2O3 nanoparticles in the microlens array scattered more light from the array and from the substrate. The improved light scattering by Al2O3 nanoparticles in the array originated from a higher refractive index of the nanoparticles than that of silica, which is 1.77 and 1.5, respectively.48
Because the light scattering by Al2O3 nanoparticles is more efficient at shorter wavelengths, we recorded photoluminescence (PL) spectra of PEN films with and without microlens array using a blue light-emitting polymer, SPB-02T. Samples for PL measurement had the same structure as that of the flexible blue PLEDs except for a cathode (Fig. S1 in ESI†). Upon introduction of the array, the PL intensity increased by 142% relative to that of a reference sample. Furthermore, incorporation of up to 2.6 wt% of Al2O3 nanoparticles in the array caused the PL intensity to increase by 31% relative to that of the microlens array. This result implies that the nanoparticles can act as efficient light-scattering centres that extract light from the sample interior. The PL intensity tended decreased as more Al2O3 nanoparticles than 2.6 wt% were inserted in the array (Fig. S2 in ESI†). Therefore, 2.6% was set as an optimum concentration of Al2O3 nanoparticles.
Samples | La (cd m−2) | Max. PEa (lm W−1) | Max. CEa (cd A−1) | Rmax.CEa |
---|---|---|---|---|
a L: luminance at 20 mA cm−2, max. PE: maximum power efficiency, max. CE: maximum current efficiency, Rmax.CE: relative maximum current efficiency. | ||||
Reference | 535 | 1.68 | 2.75 | 1 |
Regular microlens array | 708 | 2.13 | 3.56 | 1.29 |
Regular microlens array + Al2O3 2.6% | 785 | 2.38 | 4.03 | 1.47 |
Irregular microlens array | 682 | 2.10 | 3.52 | 1.28 |
Irregular microlens array + Al2O3 2.6% | 791 | 2.38 | 3.97 | 1.44 |
The microlens array was fabricated opposite to the side where the electrodes, emissive layer, and transporting layers were deposited. Consequently, the current density of the flexible PLEDs remained unchanged regardless of whether the microlens arrays were attached to the substrate (Fig. S3 in ESI†). Because the bare microlens array and that with embedded Al2O3 improved the luminance of flexible PLEDs without changing electrical characteristics, the PLEDs showed higher power and current efficiencies (Fig. 4b). Upon attachment of the array to the PEN substrate, the maximum current efficiency of the PLED increased by 29% relative to that of a bare PEN. Upon mixing of the Al2O3 nanoparticles with the array at 2.6 wt%, the maximum current efficiency of the flexible PLED further increased by 13% relative to that of flexible PLED with the array, and by 47% compared to that of flexible PLED on bare PEN. Therefore, incorporation of Al2O3 nanoparticles in the microlens array is a simple, cost-effective way of increasing the efficiency of the PLEDs.
The normalized luminances over time at 20 mA cm−2 for the flexible blue PLEDs were measured to determine the improvement in stability of the PLED due to gas blocking by Al2O3 nanoparticles in the microlens array. All PLEDs without any encapsulation tended to lose their luminance over a short period under ambient conditions (Fig. 4c). Specifically, the flexible PLEDs degraded rapidly because they were formed on a flexible substrate with poor barrier properties against gas compared with those of glass. In addition, some regions of the emissive layer were directly exposed to ambient air and light because they were not completely covered by a cathode.
Degradation of the exposed emissive polymers could promote the migration of reactive radicals, water, or oxygen gas molecules into nearby unexposed emissive polymers. Hence, the absolute stability of the flexible PLED in this work may be greatly improved simply by changing the processing method such as inkjet printing or selective patterning of emissive layer in order to cover it fully by cathode. Therefore, we simply focused on the relative stability of the flexible PLEDs. Application of the microlens array to the substrate efficiently delayed the degradation of luminance. As a result, relative luminances after they were exposed in ambient air for 18 hours were increased from 0.23 for reference to 0.46 for microlens array, and 0.57 for microlens array with 2.6% Al2O3 nanoparticles. To confirm the enhancement of stability in the flexible PLEDs, water-vapor transmission rates (WVTR) of PEN films were measured (Fig. S4 and S5 in ESI†). The WVTR was reduced from 2.978 g per cm3 per day of bare PEN, to 2.418 and 1.448 g per cm3 per day of PEN coated with silica, and silica with 2.6% of Al2O3 nanoparticles, respectively. Therefore, the improved stability in flexible PLEDs was due to the impeded passage of gas molecules through the silica hybrid film, which consists of organic and inorganic hybrid cross-linked networks.55,56 Degradation of the PLEDs was further slowed by the introduction of Al2O3 nanoparticles in the microlens array. The delayed penetration of the gas molecules may be driven from the blocking of the molecules, not the absorbing them, because the WVTR remained same after the exposure of the samples to the water vapour over 18 h (Fig. S5†). This result indicates that the nanoparticles block the penetration of oxygen and water vapour molecules into the emissive and transporting layers, thereby enhancing stability of the PLED.
The bending test was conducted to confirm the flexibility of the PLEDs. The microlens array showed high flexibility because it consisted of an organic–inorganic hybrid network, therefore the PLED with the array containing Al2O3 also showed high flexibility. When we conducted the bending test at the bending radius of 4 and 6 mm, the luminance of the device stayed at almost same level even after 1000 cycles (Fig. 4d). On the whole, both the efficiency and the stability of the PLEDs were improved using a single layer of the microlens array, and they were fabricated by a simple and cost-effective way. Additionally, the device with the microlens array could be operated in the bent state because of organic–inorganic hybrid network consisting microlens array.
The microlens arrays with Al2O3 nanoparticles were used in a flexible, yellow polymer light-emitting device to determine whether their advantageous effects were not restricted to the blue devices. The device structures are shown in the inset in Fig. S4a.† All of the layers except an emissive layer were retained and the emissive layer was changed to PDY-132, a yellow light-emitting polymer. Fig. S6a in ESI† shows that the luminance of the yellow flexible PLEDs with the microlens array increased from 1265 to 1599 cd m−2 at 20 mA cm−2 (Table S1†); PLEDs with Al2O3 nanoparticles incorporated in the microlens array showed a luminance further increase to 1885 cd m−2. Subsequently, the array improved maximum current efficiency by 24%, and introduction of Al2O3 nanoparticles into the array enhances it more by 12% (Fig. S6b†). A relatively stronger light scattering by Al2O3 nanoparticles at shorter wavelength cause slightly higher enhancement in current efficiency in the blue PLED than yellow PLED. Still, this result suggests that the array PLEDs irrespective of their emission wavelength.
We fabricated an irregular microlens array by a slightly modified method from previous study to reduce optical side effects caused by the microlens array.37 PS particles having diameters of 1 and 1.5 μm were mixed to form a master pattern, and the pattern was transferred to the PEN substrates through the same procedure used to prepare the regular microlens array.
The inset in Fig. 5a displays that PS particles having different diameters were arranged to form somewhat irregular patterns via a simple dip-coating process. The irregular patterns then were transferred to the PEN substrate to form an irregular microlens array (Fig. 5a). Despite of their smaller diameter, 1 μm-sized hemispheres shows light out-coupling efficiency comparable to that of the hemispherical microlenses with 1.5 μm size.45 In addition, inclusion of the smaller hemispheres in the microlens array resulted in a high packing density. Therefore, we expected that the efficiency of the PLED with an irregular microlens array is similar to that of a PLED with a regular microlens array. The structure of device fabricated in this experiment are depicted in an inset of Fig. 5b. We fabricated a SPB-02T-based PLED with an irregular array on a PEN substrate. Irregular microlens arrays were as effective as regular microlens array in enhancing the current efficiency. The maximum current efficiencies achieved with the PLEDs with irregular and regular microlens array increased by 28 and 29%, respectively, compared to a PLED without the array. Incorporation of Al2O3 nanoparticles into the irregular array increased the maximum current efficiency by 44% relative to that of the reference device.
Emission spectra of SPB-02T-based PLEDs with and without microlens arrays are presented in Fig. 5c. Introduction of regular microlens array into a PLED changed the emission spectrum of the PLED relative to that of a reference device. The CIE 1931 colour coordinates for reference device were initially (0.157, 0.205), changing to (0.161, 0.223) upon attachment of a regular microlens array. Even when we incorporated Al2O3 nanoparticles into the array, the emission spectrum of the PLED varied from that of reference device (colour coordinates of (0.160, 0.223)). But when we used an irregular microlens array with the nanoparticles in a PLED, the colour coordinates of the emission spectrum were (0.157, 0.207), which are very close to that of the reference device. As a result, the irregular array minimized the unwanted changes in the emission spectrum of the SPB-02T-based PLED. The refractive angle is directly related to the wavelength of light. When we introduce the irregular microlens array with embedded Al2O3 nanoparticles, the array spreads the light from the PLED in random direction and prevents the light from propagating collectively with specific refractive angles. Therefore, the array can minimize the amplification or offset of the light with specific wavelength. This result coincides with the previous report.37
The viewing-angle dependence of the emissions among PLEDs with and without the microlens arrays are shown in Fig. 5d. Because regular microlens array extracted light entering at a high incident angle with respect to the substrate, the emission intensity at a larger viewing angle for the PLED with a regular array was strong. The radiation pattern became similar to that of the reference device except at 75° upon incorporation of the Al2O3 nanoparticles into the regular microlens array. When we introduce an irregular microlens array with Al2O3 nanoparticles into the PLED, however, the emission patterns at different angles of the PLED became nearly the same as that of reference device. The irregular array with Al2O3 nanoparticles spread light in every direction, thus the radiation patterns became similar to that without the array. These findings suggested that the incorporation of the nanoparticles into the irregular array improved efficiency and stability, as well as minimized the changes in emission spectrum and radiation patterns at different viewing angles.
Footnote |
† Electronic supplementary information (ESI) available: Photoluminescence spectra of SPB-02T films with and without microlens arrays, voltage–current density curves of blue polymer light-emitting diodes, water vapor transmission rates of the samples, and device characteristic curves for yellow polymer light-emitting diodes with and without microlens arrays. See DOI: 10.1039/c6ra12718e |
This journal is © The Royal Society of Chemistry 2016 |