Highly Efficient Color Conversion of 3D Photonic Crystal Phosphor Films for Micro‐Light‐Emitting Diode Applications

The color conversion efficiency of phosphor films can be significantly improved by producing them in the shape of photonic crystals (PhCs). When light with photonic band‐edge wavelengths is incident on a PhCs phosphor film, the excitation photons are resonantly absorbed, enhancing the emission intensity. This study develops three‐dimensional PhC phosphor films based on self‐assembled inverse opal structures. PhC phosphors are constructed using a silk fibroin polymer and red organic fluorescent dyes (λpeak ≈ 610 nm) doped into it. The photonic structures are designed using electromagnetic wave simulations and photonic band structure analyses. The fabricated PhC phosphor films are integrated with blue micro‐light‐emitting diode (micro‐LED) chips (λpeak ≈ 452 nm), resulting in a color conversion efficiency increase of more than 1.5 times that of bulk phosphor films. Additionally, experiments are performed using various micro‐LED chips with a size of less than 100 μm. Overall, the results of this study demonstrate the potential of PhC phosphor films in future micro‐LED‐based devices and applications, including displays and communication systems.


Introduction
[3][4][5][6] Despite the development of highly efficient blue micro-LEDs, the "green gap" causes a significant decrease in the efficiency of green LEDs at longer wavelengths, and red LEDs experience low device efficiency owing to the low crystallinity of the active layer material in the red wavelength region. [7]To overcome these problems, color conversion materials such as phosphors and quantum dots have been suggested as alternatives. [8,9]Many studies have been conducted on highefficiency color conversion materials for converting blue light into green or red light.However, to achieve practical levels of high luminance, a sufficiently thick color conversion material layer is required; this poses significant challenges in terms of subsequent fabrication processing and price competitiveness. [10][13] Thus far, PhC structures for LED applications have mainly been applied to surfaces of semiconductor structures [14][15][16][17][18] or structured yttrium aluminum garnet (YAG) phosphors [19,20] to improve light extraction efficiency.In addition, the interaction between excitation photons and optically active materials can be increased by structuring phosphorescent or fluorescent materials into PhCs, resulting in the efficient color conversion of the excitation light without changing the shape of the emission spectrum.The method of improving color conversion efficiency using structurally engineered phosphors is not limited to a specific type of luminescent material.PhC phosphors are of different types, including those based on color conversion materials with a high internal quantum efficiency (IQE), such as quantum dots, [21][22][23][24][25] and those based on materials with low IQE, such as up-conversion nanoparticles. [26]lthough methods for efficiently exciting phosphors using PhC phosphor have been proposed, their practical application remains challenging.Specifically, it is required to improve the total amount of light that the phosphor structures can emit.When processing the PhC phosphors into one-or two-dimensional shapes based on sophisticated lithography methods, precise tuning is possible to match the PBE wavelength to the excitation light wavelength.However, in the one-or two-dimensional PhC phosphor structures reported thus far, the thickness of the color conversion material DOI: 10.1002/adpr.202300297 The color conversion efficiency of phosphor films can be significantly improved by producing them in the shape of photonic crystals (PhCs).When light with photonic band-edge wavelengths is incident on a PhCs phosphor film, the excitation photons are resonantly absorbed, enhancing the emission intensity.This study develops three-dimensional PhC phosphor films based on selfassembled inverse opal structures.PhC phosphors are constructed using a silk fibroin polymer and red organic fluorescent dyes (λ peak % 610 nm) doped into it.The photonic structures are designed using electromagnetic wave simulations and photonic band structure analyses.The fabricated PhC phosphor films are integrated with blue micro-light-emitting diode (micro-LED) chips (λ peak % 452 nm), resulting in a color conversion efficiency increase of more than 1.5 times that of bulk phosphor films.Additionally, experiments are performed using various micro-LED chips with a size of less than 100 μm.Overall, the results of this study demonstrate the potential of PhC phosphor films in future micro-LED-based devices and applications, including displays and communication systems.
layer is limited to approximately half the wavelength of light (hundreds of nanometers) to localize the guided mode in the slab waveguide, [27] resulting in a limited maximum color conversion efficiency of the excitation light (Table S1, Supporting Information).Therefore, it is necessary to develop PhC phosphors with improved light conversion efficiency or introduce new structures in which the shape of the phosphor structure is not limited to the slab waveguide geometry.
This study develops three-dimensional (3D) PhC phosphor films based on inverse opal structures comprising a silk fibroin polymer and rhodamine 6 G (R6G) organic fluorescent dye.Using the fabrication method of inverse opals based on opal templates constructed by the self-assembly of nanospheres, 3D PhC phosphor films can be economically and rapidly produced without requiring sophisticated fabrication equipment.The 3D PhC structure within the fabricated phosphor film is formed with a thickness of 10 μm or more.Further, it exhibits a characteristic photonic bandgap (PBG) wavelength proportional to the diameter of the nanospheres used for the opal template.When the PhC phosphor film is excited by photons with a wavelength corresponding to the photonic band-edge (PBE), the color conversion efficiency is expected to improve compared to the case when the bulk phosphor film is excited.In particular, the practical applicability of the PhC phosphor film can be verified using a blue light-emitting micro-LED (λ peak % 452 nm) as an excitation light source for the phosphor.

Results and Discussion
Polymethyl methacrylate (PMMA) nanospheres were used to form face-centered cubic (FCC)-structured opal templates via self-assembly to create inverse opal structures, as illustrated in Figure 1a.The lattice constant of the inverse opal structure was dependent on the diameter of the PMMA nanospheres.For the experiment, three types of nanospheres with diameters of 220, 250, and 300 nm were prepared (Figure S1, Supporting Information).The empty voids in the opal structure were infiltrated with the silk fibroin polymer, and the selective removal of PMMA produced a 3D-patterned polymeric film, as depicted in Figure 1b.[30] To create luminescent films that emit red light when excited by blue light, a silk fibroin polymer was doped with the R6G organic dye.
Figure 1c shows the simulation results for the reflectance of the silk inverse opal structure as a function of its period.It is obvious that the photonic bandgap and the corresponding PBE wavelengths are proportional to the period of the PhC.The optimized PhC phosphor structure used in this study was designed for excitation light with a wavelength of 450 nm.Notably, the longer PBE wavelength of the PhC structure with a period of 221 nm (green line) and the shorter PBE wavelength of the PhC structure with a period of 256 nm (blue line) closely matched the wavelength of the excitation light source.The measured reflectance spectra, as shown in Figure 1d, demonstrate that the photonic bandgap occurs at wavelengths λ PBG = 382, 417, and 483 nm for the inverse opal structures fabricated using spheres with diameters of 220, 250, and 300 nm, respectively.The real period values of the fabricated inverse opal structure were determined by comparing the measured photonic bandgap wavelength with the calculated results (Figure 1e).The organic-based inverse opal structure has a crystallinity than the inorganic-based inverse opal structure such as titania, [31] so the reflectance value is relatively low and Fabry-Perot oscillation is not observed in the spectrum. [29,30]Nevertheless, the flexibility of organic-based films makes them highly applicable to biomedical applications and flexible display devices.We conducted electromagnetic wave simulations based on the finite-difference time-domain (FDTD) method to analyze the photonic band structure of the PhC (Figure S2, Supporting Information) and the behavior of light when it was incident on a PhC phosphor film.As shown in Figure 2a, the reflectance peak corresponds to the first photonic bandgap at the L point in the photonic band structure. [29,30,32]The interaction between the excitation light and luminescent material is expected to be enhanced at PBE wavelengths (PBE1 and PBE2) because of the slow group velocity of light (dω/dk).Figure 2b shows the simulated reflectance and absorption spectra of the light that is incident on the PhC phosphor film.The number of photons absorbed by the phosphor increased when the wavelength of the incident light was tuned to the PBE.The most enhanced light absorption was predicted to occur at the PBE1 wavelength of the dielectric band, where the electromagnetic field was highly localized in the phosphor material (Figure 2c). [33]e determined the appropriate lattice constant for the PhC phosphor that could be excited by monochromatic light with a wavelength of 450 nm via numerical calculations.As shown in the simulated absorption spectra in Figure 2d, the wavelength of the PBE (black-dotted line), where the absorption peak appears, is proportional to the period of the structure.The absorption spectrum of the PhC phosphor with a period of 221 nm (green line) shows that the PBE1 wavelength is close to 450 nm.In contrast, PBE1 of the PhC with periods of 203 nm (red line) or 256 nm (blue line) appears at wavelengths sufficiently shorter or longer than 450 nm so that no PBE resonance occurs due to the incident light from the excitation source.The plot in Figure 2e shows the absorption enhancement factor of the PhC phosphor films as a function of the excitation wavelength.The enhancement factor was calculated by comparing the absorption of the PhC phosphors with that of reference phosphors at a given excitation wavelength.We set the reference as a flat film with the same amount of phosphor as PhC phosphor to calculate the absorption enhancement.In other words, the flat film was set to have a thickness of 26% of the PhC phosphor film since the volume ratio of air and phosphor in the inverse opal structure is approximately 74:26.Owing to the PBE resonance effect, more than three times the absorption of the excitation light is expected in the PhC phosphor films compared with the reference bulk phosphor film.
Furthermore, an investigation of phosphor excitation using a light source with a finite emission bandwidth is necessary to demonstrate the practical merits of the proposed PhC phosphor. [34]We assumed an excitation light with a Gaussian profile and a full-width at half-maximum of 30 nm, which is the typical bandwidth of commercial LED sources.Figure 2f shows the absorption enhancement factor of the PhC phosphor films as a function of the peak wavelength of the excitation source with a Gaussian spectral profile.It should be noted that the maximum absorption significantly decreases, leading to a decrease in the enhancement factor from 3 to 1.6 at a peak wavelength of 452 nm.The reason for the decrease in absorption is straightforward-a wide range of light sources includes photons with wavelengths different from the optimized one (λ = λ PBE1 ).The photons released from the wide-spectrum light source with wavelengths less than λ PBE1 fall into the PBG.Consequently, their propagation through the PhC is restricted, and only the luminescent agents near the surface can be excited, leading to a low excitation efficiency.However, the absorption level of the PhC phosphor is still significantly higher than that of the bulk phosphor.In particular, the absorption of the PhC phosphor at the peak wavelength is approximately 1.6 times higher than that of the bulk phosphor.When an excitation light source with a peak wavelength of 452 nm was used, absorption enhancement factors of 1.31, 1.62, and 0.87 were predicted for the PhC phosphors with periods of 203, 221, and 256 nm, respectively, implying that color conversion using the PhC phosphor film with a period of 221 nm is the most suitable method for excitation with blue LED sources.Moreover, the increase in the absorption of pump photons remains effective across a wide range of pump wavelengths, indicating significant flexibility in tuning the wavelength between the peak wavelength of the wide-spectrum pump source and the PBE of the PhC phosphor, which is crucial when considering the use of PhC phosphors in LED applications.
Figure 3a shows a schematic of our PhC-phosphor-capped micro-LED device, using which we experimentally investigated the photoluminescence emitted from the PhC phosphor films.The phosphor film was stacked on a blue (λ peak = 452 nm) LED chip with a size of 60 μm Â 60 μm. [35]The PhC phosphor films with various periods and a reference phosphor film were excited with blue light, and their photoluminescence spectra were measured.The energy corresponding to the absorbed excitation light was converted into emission light, with an energy proportional to the IQE of the phosphor material.The blue excitation light emitted from our micro-LEDs was converted to red light by the phosphor.Therefore, the greater the number of excitation photons absorbed, the stronger the intensity of the red emission from the phosphor (Figure S3, Supporting Information).Figure 3b shows the photoluminescence emission spectra of the phosphor films.Fluorescence enhancement factors of 1.25, 1.54, and 1.06 were observed for the PhC phosphors with periods of 203, 221, and 256 nm, respectively.Regardless of the low IQE of R6G dye at the 450 nm excitation wavelength, [36] we found that the color conversion efficiency (or external quantum efficiency) was improved (Table S2, Supporting Information).The measured fluorescence enhancement showed a trend similar to the absorption enhancement that was predicted via our simulations (Figure 2f ).It is noteworthy that the fluorescence of the PhC phosphor with a period of 256 nm showed a weak level of enhancement that was not predicted by calculations, which is due to unexpected scattering of light.Nevertheless, the most efficient color conversion occurred with the PhC phosphors with a period of 221 nm (green solid line), where the PBE resonance wavelength was significantly similar to the peak wavelength of the micro-LED used as the excitation light source (Figure 3c,d).Figure 3e,f shows photographs of the bulk and PhC phosphor films (Λ = 221 nm) excited by the blue LED source, respectively (Video S1, Supporting Information).The R6G emission was significantly enhanced when the PhC structure was applied to the phosphor film.The quantum-confined Stark effect (QCSE) is observed in InGaN/GaN quantum wells (QWs) because of the strong piezoelectric fields generated within the well layers.As shown in Figure 4a, in a LED constructed with an InGaN/GaN QW structure, the injection of an electric current into the QWs leads to carrier screening of the QCSE, eliminating the potential across the QW and causing a shift toward shorter wavelengths in the LED output spectrum. [37]When we injected 1-1000 μA of current into our blue micro-LED chip, the peak wavelength of the LED blueshifted from 454 to 450 nm.As expected from the results of the absorption enhancement factor calculation, shown in Figure 2f, a large tolerance exists in tuning between the PBE wavelength and the peak wavelength of the LED excitation source.Figure 4b shows the fluorescence enhancement factor of the PhC phosphor films as a function of the current injected into the micro-LED excitation source.Although the peak wavelength of the excitation light source was blueshifted owing to the increase in the amount of current injected, the color conversion efficiency of the PhC phosphor did not deteriorate (Figure S4, Supporting Information); this result is crucial for considering the use of PhC phosphor in LED applications.
Depending on their size, LED chips can be used in various applications, including signage (>200 μm), TV or monitor displays (50-100 μm), smartwatch displays, and virtual reality/ mixed reality (10 μm).The improvement in the color conversion efficiency is because of the PBE effect in the L-point direction perpendicular to the surface of the PhC phosphor film; this is valid regardless of the size of the lateral area of the excitation light source.The R6G fluorescence spectra emitted from the PhC phosphor films with periods of 203, 221, and 256 nm were measured using micro-LED excitation sources with various chip sizes (6-100 μm, Figure S5, Supporting Information).As shown in Figure 4c, the fluorescence enhancement factor of the PhC phosphor with a period of 221 nm uniformly remained greater than 1.5 regardless of the LED chip size, indicating that our PhC phosphors are suitable for use in a wide range of micro-and mini-LED applications.

Conclusion
This study proposed a 3D PhC phosphor film suitable for micro-LED applications.Numerical simulations based on FDTD were used to design a PhC phosphor structure suitable for blue micro-LED excitation light sources.The experimental results indicate that the fluorescence intensity of the 3D PhC phosphor film, whose PBE wavelength was well-tuned to match the peak wavelength of the excitation source, was improved by approximately 1.5 times compared with that of the reference bulk phosphor film.The results of this study demonstrate that our PhC phosphor platform is not only a novel method for creating highly efficient phosphors in the future but also a practical and impactful technology.Moreover, as the concept of PhC phosphors is not limited to phosphors such as organic dyes, quantum dots, and nanoparticles, PhC phosphors are expected to be applied to various high-efficiency light-emitting elements and devices.We also expect that the practicality of our work will be further improved if phosphor films are prepared using phosphors with high IQE at the excitation wavelength.

Experimental Section
Fabrication of 3D PhC Phosphor Films: The 3D PhCs were prepared using colloidal PMMA nanospheres (1% concentration dispersed in water, Phosphorex).A 5 μL-PMMA nanosphere solution was dropped onto the silicon substrate and subsequently heated on a hot plate to 80 °C for 5 min.As the solvent evaporated, the PMMA nanospheres self-assembled into an opal template with an FCC structure.The silk fibroin aqueous solution, which was prepared in the same manner as that used in our previous study, was doped with an R6G organic dye (Sigma-Aldrich) with a concentration of 1 mg mL À1 .An aqueous solution of R6G-doped silk fibroin was poured onto the fabricated opal template and dried for 24 h.Finally, the dried film was immersed in acetone to selectively remove the PMMA nanospheres.The PhC phosphor film we fabricated consists of an inverse opal layer about 10 μm thick and a handling layer about 50 μm thick.Therefore, a phosphor with a thickness of approximately 7 μm thinner than PhC phosphor films was designated as the reference (Figure S6, Supporting Information).
Numerical Calculations: All numerical calculations were performed using the FDTD method with a commercial software package (Lumerical FDTD Solutions).The results were analyzed using scripts provided by the vendor to extract the photonic band structure and reflectance/absorption spectra.Especially, the absorption per unit volume is calculated from the divergence of the Poynting vector, [38] P abs ¼ 0.5 realð∇ ⋅ SÞ ¼ 0.5 realðiωE ⋅ DÞ ¼ 0.5 ωjEj 2 imagðεÞ (1)   where ω is the angular frequency of light and ω is the permittivity of the material.That is, the total absorption in phosphor region is obtained by integrating P abs in space as follows: To design a 3D periodic structure, periodic boundary conditions were applied in the x-and y-directions, and a structure with a thickness of 10 μm in the z-direction was assumed.The refractive indices of the materials were measured using ellipsometry.The absorption spectra of the excitation source with a Gaussian spectral profile were calculated by integrating the product of the absorption spectra of the monochromatic source and the Gaussian function as follows: Abs mono ðλÞ ⋅ where λ peak is the peak wavelength of the broadband excitation source, and σ is the standard deviation of the Gaussian function.
Optical Measurement: The optical properties of the PhC films were examined using a visible/near-infrared fiber-optic spectrometer (USB-2000, Ocean Optics).To measure the reflectance spectra, we used a 1 Â 2 fiber coupler, which allowed white light to be reflected by the sample, simultaneously collecting the reflected optical signal.The reflected optical signal was directly connected to the spectrometer through a fiber.The fluorescence spectra emitted from the phosphor films were recorded using the same spectrometer.The micro-LED chips used as the excitation light source were fabricated using 7 μm-thick GaN epi-grown Si wafer.The three types of micro-LED chips were centered in a 1 Â 1 cm test elements group (TEG) pattern by sharing a common n-type electrode and each p-type electrode, respectively (Figure S6, Supporting Information). [35]The current injected into the LED was controlled using a source meter (2450 Source Measure Unit, Keithley).

Figure 1 .
Figure 1.Design and fabrication of 3D PhC phosphors.a) Fabrication steps for 3D PhC phosphor films.b) SEM image of 3D PhC phosphors.Inset shows the structural color of the 3D PhC structure, without phosphor (left) and with phosphor (right).c) Calculated reflectance spectra of the 3D PhC by period.d) Measured and e) simulated reflectance spectra of the 3D PhC with periods of 203, 221, and 256 nm.

Figure 2 .
Figure 2. Numerical calculations for the 3D PhC structure.a) Photonic band structure of the 3D PhC near the L point of the FCC structure.b) Reflectance and absorption spectra of the 3D PhC phosphor.c) Electric field intensity profile when the light of PBE1 wavelength is incident.d) Calculated absorption spectra of the 3D PhC phosphor by period.Absorption enhancement factor when the phosphor films are excited by e) monochromatic light sources and f ) broadband LED sources.

Figure 3 .
Figure 3. a) Schematic of the 3D PhC phosphor film integrated on the micro-LED chip.b) Fluorescence spectra from the phosphor films.Photographed images of the c) power-off and d) powered micro-LED chip, e) reference phosphor and f ) 3D PhC (Λ = 221 nm) phosphor films integrated on the micro-LED chip.

Figure 4 .
Figure 4. Characterization of 3D PhC phosphors integrated on micro-LED chips.a) Peak wavelength of blue micro-LED as a function of injection current.b) Fluorescence enhancement factor of the 3D PhC phosphors as a function of injection current into the LEDs.c) Fluorescence enhancement factor of the 3D PhC phosphors as a function of micro-LED chip size.