Advanced Performance of Photoluminescent Organic Light-Emitting Diodes Enabled by Natural Dye Emitters Considering a Circular Economy Strategy

Abstract

Organic optoelectronic devices receive appreciable attention due to their low cost, ecology, mechanical flexibility, band-gap engineering, brightness, and solution process ability over a broad area. In this study, we designed and studied organic light-emitting diodes (OLEDs) consisting of an assembly of natural dyes, extracted from noble fir leaves (evergreen) and blue hydrangea flowers mixed with poly-methyl methacrylate (PMMA) as light emitters. We experimentally demonstrate the effective conversion of blue light emitted by an inorganic laser/photodiode into longer-wavelength red and green tunable photoluminescence due to the excitation of natural dye–PMMA nanostructures. UV-visible absorption and photoluminescence spectroscopy, ellipsometry, and Fourier transform infrared methods, together with optical microscopy, were performed for confirming and characterizing the properties of light-emitting diodes based on natural dyes. We highlighted the optical and physical properties of two different natural dyes and demonstrated how such characteristics can be exploited to make efficient LED devices. A strong pure red emission with a narrow full-width at half maximum (FWHM) of 23 nm in the noble fir dye–PMMA layer and a green emission with a FWHM of 45 nm in blue hydrangea dye–PMMA layer were observed. It was revealed that adding monolayer MoS₂ to the nanostructures can significantly enhance the photoluminescence of the natural dye due to a strong correlation between the emission bands of the inorganic–organic emitters and back mirror reflection of the excitation blue light from the monolayer. Based on the investigation of two natural dyes, we demonstrated viable pathways for scalable manufacturing of efficient hybrid OLEDs consisting of assembly of natural-dye polymers through low-cost, purely ecological, and convenient processes.

1. Introduction

Organic light-emitting diodes (OLEDs) are at the forefront of modern display and lighting technologies, powering nearly everything from smartphone screens to large monitors. In 1987, Tang et al. invented organic light-emitting diodes [1] which, compared to inorganic LED technology, offer several key benefits, including improved energy efficiency, the ability to produce brighter colors, and greater design flexibility for both displays and lighting solutions. Furthermore, organic photoluminescent (PL) materials offer outstanding flexibility of chemical modification, high contrast, wide color gamut, fast response, light weight, and precise control over their fluorescence behaviors for the development of customized materials for specific applications [2,3,4,5]. Despite their recent achievement in consumer electronics, there are still some deficiencies in OLED technology that limit further improvement of emission performance. Note that their rapid growth in consumer electronics and extensive application are exacerbating the problem of end-of-life sustainability. Traditional electronic devices often have complex, multi-layered constructions where components are bonded together using irreversible methods, such as strong adhesives, making it extremely difficult to extract valuable materials. Moreover, adding functional groups to change PL colors still suffers from intricate molecular designs and requires multi-step synthesis processes [3,6]. These challenges require new strategies that enable efficient and scalable methods for altering their PL properties. One of the most viable solutions for realizing the development of new organic light-emitting diodes is the incorporation of natural materials to fabricate highly effective devices [7]. In recent years, there has been a renewed interest in natural dyes due to their eco-friendliness, ready availability, affordability, non-toxicity, and sustainability. The advantages of natural dyes over synthetic dyes also include their large abundance, improved safety for humans, and their extraction being from renewable resources. Natural pigments, such as chlorophylls, can be explored as a photoluminescent layer in OLEDs because of their relatively less complicated extraction, easy preparation, and large absorption coefficients [8]. Recently, natural-derived materials for sustainable organic electronics such as light-emitting diodes, organic photovoltaics OPVs, etc., have been investigated by novel start-ups and large electronics companies [9,10]. Natural dyes are abundant in the earth; they can be extracted from flower petals, leaves, roots, and barks in the form of anthocyanin, carotenoid, flavonoid, and chlorophyll pigments. In natural dyes, chlorophylls are photoactive molecular building blocks existent in most photosynthetic systems. Usually, dye molecules capture UV-Vis-IR light to perform photosynthesis and can convert it into photoluminescent emissions of different colors [11]. Natural dye molecules turn over billions of times and remain stable for years. Chlorophylls in natural dyes are biomolecules that are the primary component of plant species which use light energy for photosynthesis and perform better in near-ultraviolet environments, making them a preferred photoluminescent material for OLEDs [12,13,14]. It is important to borrow the same physico-chemical principles from nature to fabricate new OLEDs. Natural dyes are luminescent material with PL spectra covering a wide range of the visible band, which allows for the rendering of all colors visible to the human eye. It is well known that the most significant challenge in developing organic luminophores is achieving strong, narrowband emission. Nowadays, solving this problem is crucial for the further advancement of OLEDs. For example, in OLED displays, each subpixel should emit a distinct spectrum for red, green, and blue, with each color appearing as a relatively narrow peak in a spectrum range to ensure high color purity and clear separation of primary colors. Meeting the demands of eco-friendly technologies to produce high quality OLEDs on a large scale also remains a big challenge.

Here, we suggest using natural dyes as an environmentally and economically superior alternative to traditional synthetic dyes for developing hybrid OLEDs. We have shown that the process for the synthesis of photoluminescent cells based on natural dyes is one which is low-cost and can be considered as a viable option for hybrid OLEDs in future research. To facilitate the extraction of natural dye compounds, a solvent-assisted extraction method utilizing methanol was employed. Methanol is an effective and eco-friendly solvent for natural dye extraction due to its high polarity, ability to dissolve a broad range of organic compounds, and preservation of dye stability. In this work, the broad spectra of optical properties of fabricated natural dye–PMMA nanostructures were studied in the UV-Vis-IR spectral range using different experimental methods. We used the photoluminescent features of chlorophyll dyes derived from noble fir leaves (evergreen) and blue hydrangea flowers and mixed ones with poly-methyl methacrylate (PMMA) to create hybrid OLEDs with two distinct fluorescence colors, green and red, with a narrow full-width at half maxima (FWHM). It was shown that such natural dye–polymer composites can be conveniently manufactured as color conversion layers for application in light-emitting devices to meet high performance demands. The obtained experimental results show that fluorescent natural dye–PMMA layers can be combined with a standard inorganic GaN blue light photodiode (PD) to create electroluminescent hybrid OLEDs with three distinct colors, such as red, green, and blue (the last color is chosen due to PD emission). It was revealed that transferring monolayer MoS₂ on the substrate can significantly enhance the photoluminescent performance of the natural dye–PMMA layers. Such combination of the ultrathin two-dimensional semiconductors and natural dyes can be presented as an efficient means to improve the emission behaviors of organic light OLEDs due to overlapping of the PL bands of the MoS₂ monolayer and dye layer, as well as the extraordinary refractive index of MoS₂.

In the present investigation, only two natural dyes extracted from noble fir leaves (evergreen) and blue hydrangea flowers were deployed as photoluminescent layers for developing hybrid OLEDs for novel organic devices with great performance and high efficiency. This study demonstrates viable pathways for manufacturing efficient hybrid OLEDs through low-cost and ecologically promising processes.

2. Experimental Methods

2.1. Preparation of Samples

Chlorophyll dye was extracted using the procedures reported by Schiller et al. for spinach leaf [15]. Fresh and healthy leaves from the noble fir (typically used as Christmas trees) and blue hydrangea flowers were collected locally and thoroughly rinsed with tap water followed by distilled water to remove all the dust and unwanted visible particles, then they were cut into small pieces and dried at room temperature. An amount of 10 g of noble fir or blue hydrangea was crushed in a mortar into fine powder. The powders were were put in different glass beakers, and 100 mL of methanol was added to each beaker. The resulting mixture was placed into an ultrasonic bath for 30 min with a frequency of 30 kHz using degas mode and a sonication power of ~100 W (Hilsonic HS2900S0, Croft Business Park, Bromborough Wirral, UK) for preparing homogeneous chlorophyll dye-based liquid. The optimum temperature for solving purposes was about 45–50 °C. Chlorophyll-based samples were filtered twice through Millex HV (0.2 μm) filter membrane (Merck Life UK Limited, Glasgov, UK), and the filtrates were stored in a cupboard for analysis and future use. To avoid auto-oxidation, the filtrates were placed in dark bottles. Finally prepared chlorophyll dye dispersion was introduced to the Poly(methyl methacrylate) (PMMA 950, 3% anisole) solution under continuous mixing. Hybrid chlorophyll dye–PMMA solutions were prepared by mixing the dye–methanol dispersion with 33, 50, and 67 vol. % of PMMA 950, 3% anisole liquid. The resulting mixture was placed into an ultrasonic bath for 30 min with a frequency of 37 kHz using degas mode to prepare a homogeneous chlorophyll dye-based liquid. The maximal photoluminescence efficiency and stability of the final solution were achieved for compounds with approximately equal volumes of chlorophyll dye and PMMA. Thus, we focus on the properties of 50 vol. %. dye–PMMA solutions. We used standard Si/SiO₂(90 nm) wafers as substrate. Chlorophyll dye–PMMA solution was spin-coated on a Si/SiO₂ substrate with a slow rotation speed of ~500 rpm to obtain a film thickness of ~1 μm. Finally, the samples were baked at 150 °C on a hot plate for 15 min to evaporate residual methanol–anisole liquid and obtain a polymerized stable layer. Note that the applied method of extraction of natural dyes from leaves and flowers for optical applications coincides with that for creating natural compounds such as potent antimicrobial agents for nanomedicine [16].

To combine the natural dye–PMMA with the MoS₂ monolayer (grown using the chemical vapor deposition (CVD) method on the Si/SiO₂ substrate and acquired from 2D Semiconductors Company, Scottsdale, AZ, USA), the Si/SiO₂/MoS₂ nanostructure was spin-coated with a dye–PMMA solution. Then, after drying overnight, the samples were baked on a hot plate at 120 °C for 20 min in order to improve MoS₂ adhesion to the Si/SiO₂ layer and boost homogeneity of the nanostructure.

2.2. Optical Microscopy Images

By using different types of natural dyes, it is possible to create thin layers that shine in various colors, including blue, green, and red. Figure 1 shows representative optical microscopy images showing noble fir and blue hydrangea chlorophyll cells entrapped within a PMMA matrix. Note that the natural dye–PMMA composites for glowing layers with different colors can remain bright even under illumination by a microscopy lamp of broad visible spectrum. The Ultra Plus Carl ZEISS scanning electron microscope (SEM) (Carl Zeiss, Jena, Germany) was used for high-resolution imaging of the noble fir dye–PMMA nanostructure. SEM provides information about the samples’ surface morphology (Figure 1c). The PMMA matrix demonstrates a non-porous amorphous structure. In creating natural dye–PMMA nanocomposites, PMMA serves as a polymer support, where continuous nets of natural dye clusters are embedded.

2.3. Photoluminescence Spectroscopy

The photoluminescence (PL) spectra in the range of 400 to 900 nm were analyzed using a diffraction spectrometer, Horiba Ltd., Kyoto, Japan. Room-temperature PL emission was measured with excitation by a GaN-based light-emitting diode laser (Kyocera, SLD Laser Inc., Goleta, CA, USA) with λ = 405 nm and a half-width of Δλ ~ 2 nm [17,18]. In the case of the proposed OLED-like structure, we excited the natural dye–PMMA layers at a wavelength of λ = 440 nm and a half-width of Δλ ~ 25 nm, which are typical characteristics of GaN-based LEDs on which an organic luminophore coating could be applied for white light rendering.

2.4. Reflection/Absorption Spectroscopy

Absorbance spectra of natural dyes in PMMA matrix were measured at room temperature using two different methods. First, the absorbance was extracted from experimental data of the diffuse reflectance determined by employing a fiber-coupled spectrometer (USB4000, Ocean Optics) (Ocean Optics, Inc., Dunedin, FL, USA) and applying the Kubelka–Munk algorithm [19]. The Kubelka–Munk equation determines the relationship between the diffuse reflectance (R_dif) of a film and its absorption (k) and scattering (s) coefficients, typically expressed in the common form, k/s = (1 − R_dif)²/2R_dif. This method is widely applied in diffuse reflectance spectroscopy to determine optical properties, like the band gap of materials, by converting measured reflectance into absorption spectra in approximation of s = 1 [20].

The chlorophyll concentration (Chlorophyll a and Chlorophyll b) of the resulting methanol–dye solution was estimated using the classical spectrophotometric method. According to the Lambert–Beer law, the relationship between the concentration and optical density is as follows [21]:

C_a (mg/L) = 12.74 A₆₆₃ − 2.69 A₆₄₅

(1)

C_b (mg/L) = 22.9 A₆₄₅ − 4.68 A₆₆₃

(2)

C_total (mg/L) = C_a + C_b

(3)

where A₆₆₃ and A₆₄₅ are the absorbance of chlorophyll at wavelengths of 663 and 645 nm, respectively. In Equations (1)–(3) C_a, C_b, and C_total are the concentrations of chlorophyll (a), chlorophyll (b), and total chlorophyll, respectively. The estimated concentration was equal to approximately 150 and 180 ppm for noble fir and blue hydrangea dye, respectively.

2.5. Ellipsometric Measurements

To determine the optical constants n* = n + ik of the deposited natural dye–PMMA layers on the Si/SiO₂ substrate, we used the ellipsometry method. To this aim, a variable-angle focused-beam spectroscopic ellipsometer Woollam M 2000F (J. A. Woollam Co., Inc., Lincoln, NE, USA) was employed. The Woollam ellipsometer is based on the rotating polarizer–compensator–analyzer setup and utilizes a diode array spectrophotometer to extract spectral ellipsometric parameters. For measurements of the two ellipsometric parameters, Ψ and ∆, a broad wavelength range of 240 to 1200 nm and a 75 W Xe arc lamp (Hamamatsu Photonics Com. Iwata City, Japan) was used. Focusing optics allowed us to achieve a small spot size on the surface of the sample, approximately 50 μm × 70 μm for ~60–70° angles of incidence. Measured ellipsometric parameters Ψ (ellipsometric reflection) and Δ (ellipsometric phase) directly correspond to the sample reflections, r_p and r_s, which are the amplitude reflection coefficients for p- and s-polarized light. The ellipsometer also allowed us to separately measure the intensity reflection coefficients for p- and s-polarized light, R_p = |r_p|² and R_s = |r_s|². The main ellipsometric equation can be written as [22,23].

$$\rho ={\displaystyle \frac{r_p}{r_s}}=\tan \psi e^{i\Delta }$$

(4)

The complex optical constants n* = n + ik of the dye–PMMA nanostructures were determined using the standard Woollam’s ellipsometric software WVASE32 for the case of bulk material [22,23,24]:

$$n^{*}=n_2+{ik}_2={\{{sin}^2{\theta }_i+{sin}^2{\theta }_i{tan}^2{\theta }_i{\displaystyle \frac{{\left(1-\rho \right)}^2}{{\left(1+\rho \right)}^2}}\}}^{1/2}$$

(5)

Measurements of the ellipsometric parameters Ψ and ∆ were performed at an angle of incidence θ_i = 63°, which is approximately equal to the pseudo-Brewster angle in the visible region. Note that the ellipsometric measurement of Ψ and ∆ is the most sensitive and precise at the pseudo-Brewster angle [23]. Based on the optical constants n and k, it is possible to estimate the absorption coefficient, α = 4πk/λ (where λ is the wavelength of the incident light). The absorption coefficient, α, is the main characteristic of Lambert’s law: I = I₀exp(αt) (where I₀, I are the intensity of the incident and transmitted light, respectively, and t is the thickness of the film) [25].

2.6. Fourier Transform Infrared Spectroscopy

Fourier transform infrared (FTIR) spectroscopy was performed with the help of a Bruker Vertex 80 system (Bruker UK Limited, Coventry, UK) equipped with a Hyperion 3000 microscope (Bruker UK Limited, Coventry, UK) for focusing measurements. A variety of sources and detectors combined with aluminum-coated parabolic reflectance lens enable this system to be used from visible to mid-IR wavelengths [26].

3. Results and Discussion

3.1. Photoluminescence Spectra

The room-temperature photoluminescence spectra of chlorophyll dye of noble fir and blue hydrangea show non-symmetric dependence with peaks at λ ~ 675 nm (red light) and λ ~ 560 nm (green light), respectively (Figure 2a,b). Using 405 nm laser excitation, the 675 nm emission emerges as the main component for the noble fir dye–PMMA layer, and the occurrence of a small peak at 722 nm indicates the existence of two luminescence species. In contrast to the noble fir–PMMA layer, the PL spectra of the blue hydrangea dye–PMMA layer contain an additional shoulder near λ ~ 525 nm, which is a shorter wavelength than the strong main peak at λ ~ 560 nm. The spectral positions of the PL peaks for noble fir dye–PMMA and blue hydrangea dye–PMMA do not change when measured at different areas of the samples. Color purity is another key criterion of OLEDs, which depends on the width of the emission spectrum and is typically measured by the full-width at half-maximum (FWHM). It should also be noted that the PL peak in the red region at λ ~ 675 nm demonstrates a smaller FWHM of Δλ ~ 23 nm than those in the case of the short wavelength (λ ~ 560 nm) of green PL emission (Δλ ~ 45 nm) (Figure 2a,b). We measured the efficient yield of the PL noble fir dye, which usually refers to the intensity of emitted photons over the intensity of the excitation laser light. To determine the efficiency, the ratio between the PL intensity at λ ~ 675 nm and the corresponding value of the peak of the exciting laser (λ ~ 405 nm) was evaluated as follows: η = I_PL(max)/I_laser(max) ≈ 0.17 (Figure 2c). Measurements of the stability of the PL signals for the methanol–dye solutions were performed at room temperature in ambient conditions from a constant working area of a 3 mm diameter with excitation using a 405 nm blue laser at constant power. The PL intensity emanating from the samples was monitored over time. It was revealed that the slow decay of PL intensity (~5%) was observed only on the first day of testing, and then the emission signal reached a stable level.

The intensity of PL of the chlorophyll dye in the red region can be significantly enhanced by adding into the device the MoS₂ monolayer between the Si/SiO₂ substrate and natural dye–PMMA nanostructure. Figure 2d demonstrates the enhancement of PL emission intensity by more than five times due to the presence of a MoS₂ monolayer in the fabricated device. Note that MoS₂ is an effective luminescent material with a peak of emission near 675 nm, in the same region as that for noble fir dye (compare Figure 2a,c,e). Moreover, a small FWHM of PL for the MoS₂ monolayer can significantly enhance the efficiency of top-emitting natural dye–PMMA nanostructures due to a significant overlap between both PL spectra. Note that the MoS₂ monolayer also possesses high reflectance in the visible range of spectrum due to the large values of the real part of the refractive index (Figure 2f). Optical constants of the MoS₂ monolayer (with a thickness of about 0.7 nm) were calculated using the three-phase ellipsometric model, consisting of the Si/SiO₂ (90nm) substrate, MoS₂ monolayer, and ambient air based on measured ellipsometric parameters Ψ and Δ for different incidence angles (50–70°). Two dominant peaks are observed at the wavelengths of ~660 nm and ~607 nm in the k and R plots, which are related to A and B excitonic peaks. In the shorter wavelength range of 370–470 nm, C, D excitonic peaks occurred [26,27]. Note that MoS₂ monolayers display extraordinarily large values for the real part of the refractive index, about n ~ 5–6 at λ ~ 400–500 nm, as shown in Figure 2f. Such a highly reflective MoS₂ monolayer works as a back reflection mirror and promotes the collection of a greater amount of the excited light from the blue laser (or photo-diode) in the chlorophyll dye–PMMA nanostructure. The PL efficiency of the Si/SiO₂/MoS₂/noble fir dye–PMMA nanostructure is close to the PL quantum yield (PLQY) of commonly used reference materials, such as rhodamine R6G in an ethanol solution (PLQY of 94%) [28]. Note that PLQY is a measure of a PL’s efficiency, defined as the ratio of the number of photons emitted by fluorescence to the number of photons absorbed.

We can assume that, due to the high refractive index of the MoS₂ monolayer, the density of trapped light in the dye–PMMA cavity becomes higher. Thus, the MoS₂/PMMA–dye structure can act as a waveguide, which confirms the appearance of some interference dips in the reflectance spectra. The large enhancement of absorption and corresponding photoluminescence can be explained in terms of the optical interference produced by multi-reflection and multi-refraction at the interfaces. The interference may enhance the local electrical fields around the MoS₂ monolayer and inside the cavity. Using MoS₂ nanostructures for emission enhancement in LEDs seems to be an effective method in light of a recent study [27,29]. It was shown that doping of the MoS₂ photonic crystal by transition metals can significantly change the transmission spectrum of MoS₂, which resulted in strong waveguide mode propagation involving plasmonic Fano-resonances. Furthermore, appropriate doping has been shown to change the peak transmission shape and maximum wavelength of the MoS₂ layer. We can suggest using a doped MoS₂ layer for the enhancement of photoluminescent emission in hybrid OLEDs based on different natural dyes.

3.2. Absorption Spectra

The optical properties of noble fir dye–PMMA and blue hydrangea dye–PMMA nanostructures were evaluated from diffuse reflection by applying the Kubelka–Munk approach and ellipsometric measurements. The absorption spectra in a broad spectral range of 350–900 nm of chlorophyll dyes in the PMMA matrix are shown in Figure 3a. The two types of chlorophyll dyes exhibit prominent absorption peaks at wavelengths of ~400 and ~670 nm. Moreover, the absorption peak of the blue hydrangea dye–PMMA nanostructure is severely blue-shifted by ~40 nm from 400 nm compared to the spectral position for the noble fir dye–PMMA nanostructure. It is well known that these major types of absorption bands originate from chlorophyll ‘a’ and chlorophyll ‘b’ [30,31]. Note that the blue hydrangea dye–PMMA nanostructure displays an additional band at λ ~ 442 nm (Figure 3a). Chlorophyll b exhibits a main absorption peak at a short wavelength in the spectral range (400–500 nm), while chlorophyll a characterizes an in vivo long-wavelength absorption maximum at ~680 nm, which shifts to ~670 nm in organic solvents. For total characterization, we should emphasize that chlorophyll c demonstrates maximum absorption at λ ~ 400–450 nm, which is what we observed for the blue hydrangea dye–PMMA nanostructure. Finally, regarding chlorophyll absorption, we can underline that chlorophyll d absorbs in vivo at 720 nm or in the far-red region [31].

The microscopic origin of absorption in natural dyes can be associated with the corresponding π–π* transition which connects with the aromatic molecular structure of the dyes, namely the >C=O group. This group mainly contributes to dye absorption at wavelengths near 400 nm. On the other hand, the blue hydrangea dye–PMMA nanostructure manifests the band beyond 500 nm, which is a result of electron transfers between the donor and acceptor levels in the dye molecules, through the π-bridge upon light absorption (through non-bonding orbitals, n). The absorption moves to a longer wavelength at ~700–730 nm as the amount of delocalization in the dye molecules increases [32]. Thus, both dye–PMMA nanostructures show three intense absorption peaks in the spectral range of 300–700 nm: (i) both dyes have a sharp peak which is associated with the presence of an aromatic structure (300–420 nm), (ii) the second absorption peak occurs due to n–π* electron transition in the >C=O group (430–500 nm), and (iii) the third peak is the result of the dye compound conjunctions (670–720 nm) or, in other words, delocalization of electronic states in the dye molecules [31,32].

The resulting complex refractive index n* = n + ik is extracted from ellipsometric measurements and plotted for noble fir dye–PMMA and blue hydrangea dye–PMMA nanostructures in Figure 3b, showing a correlation between the dependences for absorbance A and the imaginary part k presented in Figure 3a and Figure 3b, respectively. The spectrum of the imaginary part k exhibits two main bands at λ ~ 400 and 670 nm. We observed an increase in the amplitude of k for blue hydrangea dye–PMMA nanostructures together with a shift to high photon energy of the absorption peak near 400 nm. The most obvious tendency for the real part of the refractive index, n, is its general convergence to values typical for the pure PMMA matrix and their modulation in the blue range of the spectra with the addition of the natural dye. Note that features in the n and k spectra (Figure 3b) are not as pronounced as the dependences presented in Figure 3a for the absorption. This is because the ellipsometric measurements contain a significant contribution from diffuse scattering, which is very difficult to account for in the evaluation of the real values of n and k; it is more correct to regard the measurements as the effective values of n and k. The methanol-extracted dye maintains a refractive index at an average level of 1.52, and the shape of an absorption coefficient resembles the dependences displayed in Figure 3b.

It is well known that an ideal luminescent organic material should demonstrate a large Stokes shift between the maximum of absorption and PL peak. Such scenario will minimize the spectral overlapping between the tails of the absorption and emission spectra and prevent reabsorption of the reemitted photons by the neighboring dyes [33]. Our measurement of PL emission by chlorophyll dyes demonstrated a large Stokes shift between excitation laser light of 405 nm and PL peaks, which are located at 560 nm for blue hydrangea dye–PMMA and 675 nm for noble fir dye–PMMA nanostructures, respectively. The dyes extracted from plant leaves and flowers primarily contain two types of chlorophylls, namely chlorophyll a and chlorophyll b. Note that the contents of chlorophyll a are usually three times higher than chlorophyll b in the leaf tissue. The molecular formula for chlorophyll a is C₅₅H₇₂N₄O₅Mg, and that for chlorophyll b is C₅₅H₇₀N₄O₆Mg [34]. The absorption spectra of the blue hydrangea dye–PMMA nanostructures exhibit a stronger response in the blue region of the electromagnetic spectrum in comparison to the noble fir dye–PMMA nanostructures. We can assume that chlorophyll c (C₃₅H₃₀O₅N₄Mg) is a blue-green accessory photosynthetic pigment found in blue hydrangea dye, which is essential for stronger harvesting of blue light. In dye form, different chlorophylls become excited by light and quickly release energy in the form of fluorescence. It is possible that a and b preferentially convert the absorbed energy into red PL light while chlorophyll c converts it into green PL light.

3.3. FTIR Spectroscopy

The FTIR spectra collect significant information about the dyes’ chemical structures. For pure noble fir dye deposited on Si-SiO₂ substrates (we excluded the contribution in FTIR spectra from PMMA molecules, and reflectance was measured with respect to Ag mirror), a peak detected at 3560 cm⁻¹ was associated with the O–H stretching mode. The same peak moved to 3580 cm⁻¹ in the case of blue hydrangea dye (Figure 3c). Hydroxyl (O–H) group stretching vibrations are normally linked with the presence of phenols in chlorophyll dye b [32]. Furthermore, the features in the region between 1650 and 1750 cm⁻¹ correspond to C=O bond-specific vibrations. Similarly, the peak at 1470 cm⁻¹ is associated with C–C stretching modes, reflecting a characteristic feature of aromatic compounds. Both FTIR spectra show clear peaks at about 1100 and 1300 cm⁻¹, but the region between those peaks varies strongly for different dyes. Note that C–O stretching vibrations that occur in chlorophyll b are usually located in the spectral region of 1000–1300 cm⁻¹. The broad peak around 3570 cm⁻¹ corresponds to the O–H stretching mode, while the C–O vibration occurs at 1690 cm⁻¹. Similarly, C=C stretching modes occur at 1675 cm⁻¹, and C–O vibrations are located at 1143 cm⁻¹. Furthermore, the appearance of the bandwidth at 600 cm⁻¹ is the result of the C–H bond bending in aromatic rings (Figure 3c) [32]. The FTIR spectra of the pure methanol–dye solution show the following peaks: (1) bands from 2843 to 3627 cm⁻¹ correspond to C–H and O–H stretching; (2) small shoulder bumps around 1640 and 1087 cm⁻¹ rise due to C=C stretching; and (3) bands ranging from 1333 to 1425 cm⁻¹ occur because of the vibration modes of alcohols. Hence, FTIR spectroscopy yields partial confirmation of the chemical components present in the methanol–dye extracts [35], and various vibrational modes were identified in the chlorophyll dyes.

3.4. PL Performance Based on Natural Dyes Excited by Blue GaN Photodiode

Developing high-performance blue, green, and red hybrid OLEDs is a major challenge, and the choice of light emitters is critical for overcoming this issue. To evaluate the PL performance of natural dye nanostructures, we covered blue gallium nitride (GaN) light-emitting photodiodes (PDs) using a corresponding dye–PMMA nanostructure, as shown in the inset of Figure 4. Such PDs are essential for next-generation display technologies owing to their high resolution, low power consumption, and long lifespan. The spectral stability of the PD emission was evaluated across a driving voltage range of 2.5–4.0 V and a current of ~1 mA. It was found that the GaN PD exhibits a small redshift (439–443 nm) as the voltage increases, whilst maintaining a narrow emission bandwidth (Δλ ~ 20 nm) and a uniform intensity distribution throughout the same voltage range. Thus, the bottom-emitting blue PD emits at λ ~ 440 nm, which excites the PL in the top noble fir dye–PMMA or blue hydrangea dye–PMMA nanostructures (Figure 4a,b).

Top-emitting chlorophyll dye devices achieve enhanced red and green color purity (FWHM of 25–45 nm) under a low running voltage applied to GaN PD (2.5–3.5 V and currents of 0.5 mA), which provides brightness of ~900 cd m⁻². This study provides an important strategy to develop high-performance green and red hybrid OLEDs based on natural dyes. Furthermore, it should be stressed that our experimental results demonstrate that photoluminescent natural dye–PMMA nanostructures can be combined with standard inorganic GaN blue light PDs to create electroluminescent hybrid OLEDs with three distinct colors, such as red, green, and blue (where the last color is directly associated with PD emission).

There are two major components in a hybrid LED: a blue light source and a natural dye light-converting layer. In operation, a portion of the blue light emitted from the light source (blue PD) is converted to longer wavelengths by the light converter-dyed chlorophyll. The overall efficiency of a hybrid LED is directly related to the efficiency of the inorganic PD that converts electricity to the blue lights and the efficiency of the dye light converter that converts blue light into other wavelengths. The next step of our research is to optimize the performance of hybrid OLED devices by adding photoluminescent layers and improving the effective energy transfer to excited states of dye molecules through multiple resonance effects within the fabricated complex nanostructures.

3.5. A Circular Economy Strategy for Photoluminescent Hybrid LEDs Based on Natural Dyes

Bio-based nanostructures have drawn much interest as they are promising materials for real-world applications. Henari et al. [36] demonstrate the advantages of green chemistry for the production of different noble metal nanostructures, which can be new avenues for the production of cost-effective, eco-friendly nanosystems for high-performance optical devices. In this study, we show that with a carefully designed system combining natural dye thin films with monolayer TMDs, it is possible to turn a single blue emitter into a tunable multicolor OLED using low-cost material already familiar to manufacturers. This suggested design of hybrid OLEDs could significantly reduce the ecological footprint and cost of OLED manufacturing as it requires fewer scarce inputs and fewer processing steps. These hybrid OLEDs can work well on flexible surfaces, opening options for sleek luminaires, thin backlights, and future smart-building panels. The next step for researchers should be to explore brightness, efficiency, and long-term stability in order to transform lab prototypes into real-world lighting products. It is important to analyze the creation of hybrid OLEDs based on natural dyes to ensure the full circularity of this environmentally friendly prototype. Natural dyes, particularly chlorophylls, promise an environmental advantage for the proposed technology. They are bioavailable, non-toxic, and low-cost and have a large Stokes shift (e.g., excitation at 405 nm and emission at 675 nm), which prevents reabsorption and increases device efficiency. The integration of the PMMA polymer matrix and the functional MoS₂ monolayer create a “circular paradox”, which is when an increase in technical efficiency leads to a decrease in recycling efficiency. Note that PMMA is a thermoplastic that is ideal for chemical recycling (depolymerisation) back to pure monomer methyl methacrylate (MMA). This recycling significantly reduces the carbon footprint compared to the production of fossil-fuel-based virgin plastics. On the other hand, monolayer MoS₂ is an extremely valuable two-dimensional semiconductor, essential for enhancing photoluminescence in devices. Usually, its integration is performed via polymer-assisted transfer, where PMMA is often used as a carrier. Even after “cleaning”, PMMA residues in the form of nanoparticles of about 100 nm in size remain on the surface of the monolayer MoS₂ [37]. These residues critically complicate the clean recovery of MoS₂ for reuse, as polymer contamination reduces the functional value of the recovered 2D material. To document and improve the overall environmental profile of optoelectronic devices, a Life Cycle Assessment (LCA) needs to be conducted, in accordance with ISO 14040 and ISO 14044 [38]. Performing an LCA will allow us to compare the carbon footprint of this bio-OLED with that of traditional OLEDs, which is critical for perspective positioning in the market, especially given that bioplastics are typically more expensive than conventional plastics. An LCA is necessary because of Design for Disassembly (DfD), which is a fundamental principle that involves designing new devices so that they can be easily disassembled for material recovery, reuse, or recycling [39]. The most critical challenge to the circular economy arises at the micro level–interlayer interfaces, especially between the PMMA polymer and the 2D material MoS₂. Therefore, DfD perspectives should focus on creating selective dissociation mechanisms. The ideal solution is to introduce an intermediate, temporary, or thermolabile interface layer that can be dissolved by an environmentally safe, moderately polar solvent (e.g., water or alcohol) or degraded by mild heating, allowing clean separation of the PMMA–dye layer from MoS₂ without damaging it. An effective circular pathway for this hybrid system requires the development of an integrated three-step system that takes into account the unique chemical properties of each material. PMMA is one of the most suitable polymers for chemical recycling because it can be efficiently depolymerized into the monomer MMA, which can then be reused in primary production.

To ensure the high purity of the monomer required for optical devices, it is critical to avoid contamination with organic dyes. After mechanical dismantling of the PMMA–dye layer, pre-treatment is required. This may involve mild solvolysis or oxidative degradation of the dispersed natural dyes to minimize their amount before pyrolysis. This step should be optimized to ensure biodegradation of the dyes. The procedure of thermal depolymerization of the pre-purified PMMA allows for optical-grade MMA monomers to be obtained. This will allow the use of the recovered monomer to create new high-tech components. It is worth noting that natural dyes (chlorophylls) are bio-based components with circularity achieved not through chemical recycling but, importantly, through returning to the biological cycle. A DfD process will be needed for effectively separating natural dyes for biodegradation, ensuring the purity of PMMA before its chemical recycling. To achieve this, PMMA-free MoS₂ transfer technologies need to be developed and validated.

4. Conclusions

In this study, we demonstrated that natural dyes with high-intensity PL can be used as color conversion layers, applied in organic light-emitting devices to achieve high performance. We proposed fabricating hybrid OLEDs using two natural dyes derived from noble fir leaves (evergreen) and blue hydrangea flowers as active PL dyes and underlined the impact of their optical properties on emission performance. It was shown that polymer composites based on natural dye–PMMA with high-purity fluorescence can be used as color conversion layers, applied in organic light-emitting devices. The optical properties of noble fir dye–PMMA and blue hydrangea dye–PMMA nanostructures were evaluated from measurements of the diffuse reflectance, ellipsometric characteristics, and FTIR spectroscopy. Importantly, the addition of an ultrathin MoS₂ monolayer can significantly enhance the resulting intensity of the PL emitted by natural dye–PMMA nanostructures due to the overlapping of the spectra of PL emission from constituent layers and a strong back reflectance from MoS₂ nanostructures, which allows for the strong trapping of light in the dye–PMMA-Si/SiO₂/MoS₂ cavity. It was shown that noble fir dye–PMMA and blue hydrangea dye–PMMA nanostructures can emit light in the red and green spectral regions with a small FWHM. Such color purity is an essential key criterion of hybrid OLEDs for future commercial displays.