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Abstract
Color balance is a critical concept in the application of functional transparent polymers from a customer’s standpoint. In this study, multiple polar and non-polar fluorescent dyes are embedded simultaneously for the first time in a polydimethylsiloxane (PDMS) polymer matrix. Five dyes successfully coexist with the optimum blending ratio. Furthermore, simultaneous dispersing of polar and non-polar dyes in the polymer is achieved. Absorption and photoluminescence characteristics of multiple fluorescent dyes in PDMS medium are systemically deconvoluted and discussed. The competitive average visible transmittance and color balance of synthesized multi-fluorescent dye embedded PDMS is demonstrated by high color rendering index and CIE color space coordinates close to the white point. Additionally, the luminescent solar concentrator device demonstrates improved power conversion efficiency and light utilization efficiency than the pure PDMS waveguide-based device. Moreover, the long-term storage stability is demonstrated successfully. The findings, therefore, demonstrate the applicability of multi-fluorescent dye embedded PDMS to advanced transparent devices.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
Corrections
7 December 2022: A typographical correction was made to the author affiliations.
1. Introduction
As a result of the development of numerous dyeing techniques involving organic/inorganic dyes and pigments, the coloration of polymers has been a major concern for a century. In modern times, the range of applications for polymers with transparent or flexible properties is expanding. Accordingly, it remains essential to develop a dyeing technique that does not impair these properties. Polydimethylsiloxane (PDMS), one of the siloxane-based elastomers, is transparent, stretchable, inexpensive, biocompatible, non-flammable, corrosion-resistant, and easy to handle. Thanks to these properties, PDMS can be utilized in numerous fields, including flexible and stretchable sensors [1–4], luminescent solar concentrators (LSCs) [5–9], substrates of the transfer process [10,11], optical filters [12,13], microfluidic chips [14–17], and biomedical research [18,19].
PDMS polymers are generally synthesized through a curing process after mixing a varnish-type precursor and a curing agent in appropriate proportions. When an organic/inorganic pigment or dye is applied to PDMS, its dyeing is achievable. In this instance, the optical characteristics of the dyed PDMS are dependent on the properties of the applied colorant, and it is crucial to disperse the dye particles uniformly without aggregation. In general, inorganic pigments are materials with a high refractive index that result in high opacity, whereas organic dyes are materials with a low refractive index that, when applied, provide transparency [20].
Even at thicknesses of 10 mm or more, conventional PDMS exhibits near 90% transmittance in the visible region (Supplement 1, Fig. S1). In addition, it possesses a refractive index of 1.42 [21], and the critical angle of total internal reflectance at the interface with the air is approximately 44.76 degrees, making it suitable for use as a host material functioning as a waveguide of the LSC device. Moreover, by applying a fluorophore, it is possible to impart fluorescence properties while maintaining transmittance. To date, poly(methyl methacrylate) (PMMA) [22–24] and polycarbonate (PC) polymers [25–27] have been predominantly used as host materials in thin-film or bulk LSCs; however, research on PDMS [5–9] is limited. Because PDMS can easily be cast as a thin film to a thick block, it can simultaneously serve as the dye host and the whole substrate of the device.
As part of transparent photovoltaics, fluorescent dyes [28–33], quantum dots [23,34–39], and scattering nanoparticles [33,40] can be incorporated into the host materials for efficient transmission of incident light to the edge side of LSCs. LSCs are ideally suited for applications requiring large-area transparency, such as conventional windows for buildings and automobiles, but researchers have primarily focused on device efficiency and visible light transmittance. Conversely, focus on the color balance of transmitted light has not been emphasized enough [41–43]. Not only is the transparency but also the color balance of the transmitted light crucial for windows [44]. The color of an object is relative and varies significantly based on the wavelength constituting the light source. In other words, if a particular wavelength range of the natural light that enters through the window is filtered, the color of indoor objects (such as wallpaper, furniture, etc.) may appear different, which may significantly affect customer satisfaction [45].
Utilizing a single phosphor, the majority of LSC research has focused on high quantum yield (QY), optical efficiency, and large Stokes shift [46,47]. Recently, structures employing two or more types of fluorophores have been reported as a means of enhancing the photostability of organic materials and utilizing a broader spectrum of light [39,48,49] or maximising the shift in wavelength [32,50–53]. Typical multi-fluorophore devices have two design types: a tandem structure in which substrates containing each fluorophore are stacked [33,39,48,49,53–56]; and a mixture structure in which fluorophores are dispersed together in a film or bulk substrate [29,32,50,51,57]. Since most of these multi-fluorophore devices use only two types of fluorophores, they cannot provide perfectly balanced color transmission. Some studies have emphasized the significance of the color balance of photovoltaic devices considering building integrated photovoltaic (BIPV) applications [44,58,59]; however, a balanced multiple fluorescent strategy is unprecedented.
In the current study, a multi-fluorescent dye dispersed PDMS block with a balanced color of transmitted light was fabricated. Five dyes successfully coexisted with the optimum blending ratio. Furthermore, we achieved simultaneous dispersing of polar and non-polar dyes in the polymer; two dyes were dissolved in non-polar p-xylene with a dipole moment of zero, while three dyes were dissolved in ethanol with a dipole moment of 1.66 (at 20 °C). The optimized multi-fluorescent dye dispersed PDMS generated a high average visible transmittance (AVT) of 70.7%. Its International Commission on Illumination (CIE) 1931 color space coordinates (x, y, z) of (0.31543, 0.32968, 0.35489), CIE 1976 color space coordinates (a*, b*) of (0.77, 0.52), and color rendering index (CRI) of 94.6 attest to its remarkable color balance. The Si solar cell integrated luminescent solar concentrator device was then assembled, exhibiting 6.6 times improved power conversion efficiency (PCE) along with 5.0 times higher light utilization efficiency (LUE). Ultraviolet-visible (UV-vis) absorbance and photoluminescence properties, and long-term (2 years) storage stability of six fluorescent dyes in PDMS were investigated. These findings demonstrate the feasibility of applying the simultaneously mixed fluorophore concept to a variety of transparent windows.
2. Materials and methods
2.1 Materials
7-Diethylamino-4-methylcoumarin (Coumarin 1, D87759, 99%), 3-(2-benzothiazolyl)-N,N-diethylumbelliferylamine (Coumarin 6, 442631, 98%), Acridine Orange hemi(zinc chloride) salt (Basic Orange 14, 158550, dye content 85%), sulforhodamine B sodium salt (Kiton Red 620, S9012, technical grade, dye content 75%), Nile Blue chloride (Nile Blue, 222550, dye content 85%), 3,3’-diethylthiadicarbocyanine iodide (DTDCI, D87759, 99%), p-xylene (134449, ReagentPlus, 99%), and glycerol (G9012, >=99.5%) were purchased from Sigma Aldrich (Seoul, South Korea). Ethyl alcohol (E1261, anhydrous, 99.9%) was provided by Samchun Chemical (Pyeongtaek, South Korea). Polydimethylsiloxane (PDMS) elastomer (SYLGARD 184 silicone elastomer kit) was purchased from Dow Corning (Midland, MI, USA). All materials were utilized without any additional purification.
2.2 Preparation of fluorescent dye solutions
Stock solutions of all fluorescent dyes were prepared. Specifically, the concentrations of C1 and C6 stock solutions in p-xylene were 2.0 mg ml−1, and the concentrations of BO14, KR620, NB, and DTDCI stock solutions in ethanol were 0.2, 1.0, 0.1, and 0.2 mg ml−1, respectively. For ultraviolet-visible absorbance and photoluminescence (PL) analysis, C1, C6, BO14, KR620, NB, and DTDCI sample solutions were prepared at concentrations of 0.1–10, 0.5–25, 0.1–10, 0.5–25, 0.1–10, and 0.1–10 µg ml−1, respectively.
2.3 Synthesis of fluorescent dye dispersed PDMS
To disperse fluorescent dye molecules in PDMS, the prepared stock solutions were added dropwise to the oligomer varnish to the desired concentration while stirring by hand. After thoroughly combining the solutions with the oligomer, the solvent was removed from the stock solution under vacuum (ca. 1.5 Torr). Then, 10 wt.% of a curing agent was added, followed by stirring, and bubbles were thoroughly removed under vacuum. The degassed mixture was poured into a 20 mm × 20 mm × 12 mm or 50 mm × 50 mm × 12 mm U-shaped mold with angled sides. Specifically, a 3D printer (Style Plus – A15CR, Cubicon, Seongnam, South Korea) was utilized to print the polylactic acid (PLA) mold. To remove the jagged printing pattern, the interior was coated with polyimide (PI) tape. For the purpose of smoothing the front and back surfaces of the PDMS block, both open sides of the mold were sealed with flat cover glasses (1000412, Paul Marienfeld, Lauda-Königshofen, Germany). Consequently, the mixture was thermally cured in the oven at 50 °C for 24 hours. For ultraviolet-visible absorbance and photoluminescence analysis, the fluorescent dyes C1, C6, BO14, KR620, NB, and DTDCI were added at concentrations of 3.0, 8.0, 3.0, 3.0, 2.0, and 2.0 µg gPDMS−1, respectively. Owing to the limitation of the instrument's sample holder, PDMS samples for PL analysis were cured in a 15 mm × 15 mm × 40 mm PMMA cuvette. The multi-fluorescent dye dispersed PDMS was fabricated by repeatedly adding stock solution and vacuum process for each dye.
2.4 Fabrication of solar cell integrated light luminescent concentrator devices
The solar cell integrated LSC device was fabricated by affixing a single-sided passivated emitter and rear cell (PERC) structured silicon solar cell to a 50 × 50 mm2 by 12 mm thick PDMS block. The Si solar cell was sliced to 50 × 12 mm2 using an ablation laser. The 2 mm wide string ribbons were then soldered to the positive and negative electrodes. The cell was fixed to the edge of the PDMS block with a rubber band. In order to minimize flux loss at the interface between the edge of the PDMS block and the Si solar cell, glycerol was introduced as an index matching gel. Additionally, the remaining empty edges were painted black to block direct illumination and light scattering or reflection [42,60,61].
2.5 Characterization
The UV-vis spectroscopy was conducted using a UV-vis spectrometer (V-670, JASCO, Tokyo, Japan) in a wavelength range of 300–1,000 nm. 10 × 10 mm2 cuvettes were utilized for liquid samples. AVT was evaluated in the wavelength range of 380–780 nm according to the following equation:
2.6 Colorimetric calculation
The procedure depicted in Supplement 1, Scheme S1, was used to perform the colorimetric calculation for evaluating the color balance of the samples, and their coordination values were marked in the most widely used CIE 1931 color space (CIE XYZ). The transmitted spectral power distribution (SPD) of samples was calculated as the product of the incident SPD and transmittance spectra of samples. Standard illuminant D65, designated as a standard daylight source by the CIE, was used as incident SPD [63]. The tristimulus values of samples were determined by integrating the product of the SPD of samples and the color matching function (CMF, CIE 1931 2° standard observer) [64] over the range of 380–780 nm, which is the customary wavelength integration limit. By normalizing these values, three color space parameters were finally calculated, and the results are depicted in the CIE XYZ. To amplify the resulting subtle color difference, the CIE 1976 L*, a*, b* color space (CIELAB) was utilized. The previously calculated tristimulus values were transformed into CIELAB coordinates. The calculation of the CRI utilized the same SPD as the CIE XYZ calculation. Standard illuminant D65 was used as the reference light source, and the Euclidean distance (chrominance) between the reference light source and the sample was calculated in CIE 1976 L*, u*, v* color space (CIELUV).
3. Results and discussion
3.1 Selection of fluorescent dye
In order to achieve color balance in the visible region, the absorption wavelength range of the following five commonly used dyes (Supplement 1Fig. S3(b–f)) was considered: 3-(2-Benzothiazolyl)-N,N-diethylumbelliferylamine, commonly called Coumarin 6 (C6), is derivative of coumarin family with absorption and emission wavelengths of approximately 445 and 505 nm, respectively, in solution. Acridine Orange, also known as Basic Orange 14 (BO14), absorbs approximately 490 nm and emits 525 nm wavelength lights in solution. Sulforhodamine B is also known as Kiton Red 620 (KR620), and its solution exhibits absorption and fluorescent properties near 565 and 585 nm, respectively. Additionally, Nile Blue (NB) solution exhibits absorption and emission in wavelengths of 640 and 660 nm, respectively. To solve the color balance mismatch that occurred by NB, 3,3’-Diethylthiadicarbocyanine iodide (DTDCI) was introduced to replace NB. Its absorption and emission wavelengths in solution are 655 and 685 nm, respectively. Since the light below the wavelength of 400 nm is out of the active range of a typical solar cell exploiting light in the visible-infrared region, we attempted to convert it into low-energy light that could be utilized. 7-Diethylamino-4-methylcoumarin, also known as Coumarin 1 (C1), was selected (Supplement 1, Fig. S3(a)). Since C1 solution absorbs light near 370 nm, it was anticipated that introducing C1 into a transparent polymer would not affect its visible range transmittance and color balance, even at high concentrations. In addition, C1 emits fluorescence over the 400 nm wavelength region. The fluorescent dye combination proposed in this study is only one example of a feasible color balance; countless other combinations of colorants are possible.
3.2 Fluorescent dye solution
It is well known that the absorption and emission properties of fluorescent dye depend on the interaction between the molecules of the dye and the molecules of the solvent [65]. Our operating conditions and resulting fluorescent dye properties may differ from those provided by references or chemical suppliers. Therefore, we first compiled a database by investigating the optical properties of each selected dye according to its concentration in solution.
Figure Supplement 1, S4(a),(b)) display the transmitting properties of C1 and C6 fluorescent dyes dissolved in p-xylene. The C1 and C6 solutions exhibit distinct absorption properties in the wavelength regions of 300–400 and 360–480 nm, respectively. As shown in Fig. 1(a), the C1 solution was transparent while the C6 solution was yellowish-green. Additionally, their respective peak absorption wavelengths were 370 and 440 nm. The absorption saturation was witnessed near 2.5 µg ml−1 concentration for C1 solution. As the absorption wavelength range is outside of the visible spectrum, the determination of its concentration is independent of visible transmittance. Absorption of C6 solution was saturated between 10–25 µg ml−1.
Fig. 1. Digital images of florescent dye solutions under (a) indoor light and (b) UV lamp. C1, C6, KR620, BO14, NB, and DTDCI solutions are left to right.
BO14, KR620, NB, and DTDCI fluorescent dyes were dissolved in ethanol, and their transmission characteristics are shown in Supplement 1, Fig. S4(c-d). The fluorescent dye solutions displayed yellow, pink, navy blue, and sky blue, in that order (Fig. 1(a)). The absorption wavelength regions for BO14, KR620, NB, and DTDCI were 400–510, 460–600, 500–690, and 550–700 nm, respectively. Their peak absorption wavelengths were, in order, 490, 555, 625, and 655 nm. BO14 and NB solutions exhibited saturation of absorption near 5 µg ml−1 concentration, and new secondary peaks appeared in the direction of shorter wavelengths. Meanwhile, the absorption of KR620 and DTDCI solutions were saturated near concentrations of 10 and 5 µg ml−1, respectively. Not only primary absorption but also wavelengths below 300 nm were enhanced above saturation concentration. In Fig. 2(a), the absorbance spectra of all fluorescent dye solutions have been normalized. The peak absorption trends according to corresponding fluorescent dye concentration were plotted in Supplement 1, Fig. S5. For all dyes, increasing linear trends to their saturation concentration correspond well with the Beer-Lambert law.
Fig. 2. Normalized (a) UV-vis absorbance and (b) photoluminescence spectra of fluorescent dye solutions. The representative concentrations for each dye solution are 0.5, 2.5, 5.0, 5.0, 5.0, and 2.5 µg ml−1 for C1, C6, BO14, KR620, NB, and DTDCI, respectively. The baseline of UV-vis absorbance spectra for C1 and C6 solutions was p-xylene, and for another was ethanol.
Supplement 1, Fig. S6 illustrates the measured PL emission results. The excitation wavelengths for each fluorescent dye were matched to their peak absorption wavelengths. Intriguingly, ethanol-based BO14, KR620, NB, and DTDCI solutions exhibited the maximum PL intensity at the same concentration showing the maximum absorption. In contrast, PL emissions of p-xylene-based C1 and C6 solutions were reduced over a particular concentration. The solutions of C1, C6, BO14, KR620, NB, and DTDCI emitted the maximum PL at concentrations of 0.5, 5.0, 10.0, 10.0, 10.0, and 2.5 µg ml−1, respectively. As presented in Table 1 and Fig. 2(b), their peak positions were 411, 485, 523, 583, 669, and 648 nm, and corresponding Stokes shifts were 51, 45, 33, 28, 44, and 29 nm, respectively.
Table 1. Absorption (λabs) and emission wavelengths (λem) of fluorescent dyes in solution and PDMS matrix.
3.3 Fluorescent dye dispersed transparent PDMS
As stated previously, the absorption and emission properties of fluorescent dye are highly medium-dependent [65]. As the dispersing medium had been changed from solvent to polymer, we performed the optical analysis of fluorescent dye dispersed PDMS to investigate expected optical property change. Six color PDMS samples were fabricated successfully. In Fig. 3, all colored PDMS blocks exhibited uniform dye dispersion, devoid of aggregation and staining.
Fig. 3. Digital image of fabricated (a) pure and (b) C1, (c) C6, (d) BO14, (e) KR620, (f) NB, and (g) DTDCI dispersed color PDMS blocks. Top and bottom columns show color under indoor light and UV lamp, respectively. Bottom column shows the concentration of C1, C6, BO14, KR620, NB, and DTDCI were 3.0, 8.0, 3.0, 2.0, 2.0, and 2.0 µg gPDMS−1, respectively. The block size is 20 × 20 × 15 mm3.
Supplement 1, Fig. S7 depicts the transmittance characteristics of a single fluorescent dye dispersed color PDMS. In the PDMS medium, a striking distinction between non-polar dyes (C1 and C6) and polar dyes (BO14, KR620, NB, and DTDCI) was revealed. Even though C1 and C6 were dispersed at a quite high concentration relative to solutions, their absorption was greatly diminished and broadened. Based on the phenomenon observed during the fabrication process, the deep green color of the C6 mixed oligomer faded after curing. This implies a change in the chemical state of the C6 dye molecule, which may result in the destruction of its structure or a reaction with other components. A change in C1 was challenging to observe because it was colorless (i.e., transparent). Considering that these two fluorescent dyes belong to the coumarin family, their reaction mechanisms may be comparable. Peak absorption wavelengths (λabs) were also shifted by 370 → 305 nm (Δλabs = 65 nm) for C1, and 475 → 405 nm (Δλabs = 60 nm) for C6 PDMS.
In PDMS, the absorption properties of BO14, KR620, NB, and DTDCI were also slightly diminished and broadened compared to the ethanol medium. Enhanced absorption at 400 nm wavelength was also observed. The increase in transmission was greater than anticipated, taking into account the dimension of the sample and the difference in concentration. Nevertheless, the resulting transmittance values were still close to the expected range, and their characteristic peaks were preserved. Also, there was no discernible color difference between the PDMS before and after the curing process. The absorption peak shifts were 490 → 475 (Δλabs = 15 nm) for BO14, 555 → 540 (Δλabs = 15 nm) for KR620, 625 → 575 (Δλabs = 50 nm) for NB, and 655 → 610 (Δλabs = 45 nm) for DTDCI in PDMS. Table 1 and Fig. 4(a) display the peak absorption wavelength values and the normalized absorbance plots, respectively.
Fig. 4. Normalized (a) UV-vis absorbance and (b) photoluminescence spectra of fluorescent dye-dispersed color PDMS blocks. The baseline of absorbance spectra for C1 and C6 solutions was p-xylene, and for another was ethanol. PL spectra were extracted from convoluted original peaks.
When certain solvents are used as dispersing agents in PDMS, their bubbles possibly form and become trapped during the curing process. Consequently, the haziness of PDMS could be significantly increased. In addition, the case involving solvents whose refractive index is mismatched with PDMS (1.42), could cause a distortion. Ethylene glycol, with a well-matched refractive index (1.43), and a high boiling point, was introduced as a pre-dissolving agent for fluorescent dye in PDMS [66]. However, adding a large volume of solution could deteriorate the physical property of the resulting PDMS, especially if the solvent is not completely removed. We chose ethanol as the solvent for polar fluorescent dyes, which is easy to remove due to its low boiling point of 78.4 °C. Even when the dye solution was mixed with oligomer varnish, precipitation and aggregation of dye molecules did not occur, and uniform dispersion persisted after the removal of ethanol and curing of PDMS. On the basis of these facts, we propose the possibility that these dyes could be surrounded by trace amounts of ethanol molecules remaining after the mass removal of ethanol. Consequently, it is assumed that the luminescent properties of fluorescent dyes dissolved in ethanol are still maintained in the PDMS medium. Additionally, it suggests that other polar dyes can be dispersed in PDMS similarly.
The interaction between incident light and internal polymer chains may impart inherent light-scattering properties to transparent polymers [67–70]. Therefore, the investigation of the light-scattering effect of pure transparent PDMS preceded the PL analysis of colored PDMS (Supplement 1, Fig. S8). In contrast to the typical solution sample, strong incident light peaks were detected. In addition, we infer that the previously mentioned transmittance loss in color PDMS could be a result of this light scattering. On the other hand, all scattered incident light peaks were detected as red-shifted by approximately 3 nm. The result implies that the inelastic scattering of the incident light occurred within the PDMS medium [71].
Owing to the strong light-scattering properties of PDMS, the PL analysis of color PDMSs revealed peaks of the scattered incident light (Supplement 1, Fig. S9) that were significantly stronger than emissions from the fluorescent dyes. It is also known as the instrument profile function (IPF) and closely resembles the Gaussian distribution [62]. Consequently, we attempted to separate the peaks from the fluorescent dyes through the peak deconvolution of the original spectrum. The peak deconvolution was performed solely for the purpose of separating peaks from the IPF, and it should be noted that the results were derived without a thorough consideration of the transition states of fluorophores. Table 1 and Fig. 4(b) display the emission peak wavelengths and normalized PL for fluorescent dyes in PDMS, respectively. The excitation wavelengths for each color PDMS were determined based on the peak absorption wavelength determined by absorption measurement results. It was demonstrated that, with the exception of KR620, all fluorescent dyes exhibited distinct PL properties despite being introduced into PDMS at low concentrations with transmittances of at least 70%. As depicted in Fig. 3, these emissions were visible to the naked eye. The KR620 PL peak has the same shape as the peaks of other fluorescent dyes, but the relative intensity was very low. The IPFs observed in each colored PDMS sample correspond to the transparent PDMS sample's IPFs. The shifts of the peak emission wavelength according to the change of the medium were 411 → 370 (Δλabs = 41 nm) for C1, 485 → 470 (Δλabs = 15 nm) for C6, 523 → 523 for BO14 (Δλabs = 0), 583 → 572 for KR620 (Δλabs = 11 nm), 669 → 637 for NB (Δλabs = 32 nm), and 684 → 678 for DTDCI (Δλabs = 6 nm). The Stokes shifts of C1, C6, BO14, KR620, NB, and DTDCI in PDMS were 65, 65, 48, 32, 62, and 68 nm, respectively, and increased for all dyes compared to those in solution.
3.4 Multi-fluorescent dye dispersed transparent PDMS
Dispersing materials with different polarities coexist homogeneously in a medium is generally challenging. Nevertheless, based on the absorbance region and intensity data for each concentration of the single-color PDMS obtained above, we supposed that polar and non-polar fluorescent dyes could coexist in PDMS without additives and color balance could be achieved by balancing their concentrations. From this insight, multi-fluorescent dye dispersed PDMS was fabricated; each fluorescent dye C1, C6, BO14, KR620, and NB was dispersed onto PDMS concentrations of 0.9, 4.0, 1.5, 1.5, 1.0 µg gPDMS−1 (designated as combination 1), which were expected to exhibit a transmittance of approximately 70% in each characteristic absorption region. As expected, polar and non-polar dyes could coexist homogeneously in PDMS. However, the resultant PDMS appeared light purple in color (left inset of Fig. 5(a)). As shown in Fig. 5(a) (blue line), the absorbance analysis confirmed that the absorption of the 450–650 nm and 650–700 nm regions was unbalanced, resulting in a distinct hue. This phenomenon resulted from the absorption property of NB, which absorbs red light poorly in PDMS (Supplement 1, Fig. S7(e)). To solve the problem, we replaced NB with DTDCI (combination 2), which can absorb wavelengths up to 700 nm in PDMS. In addition, the concentrations of C6, BO14, and KR620 were adjusted to 10.0, 1.35, and 1.35 µg gPDMS−1, respectively. For DTDCI, a balanced concentration of 1.98 µg gPDMS−1 was used. As shown in Fig. 5(a), PDMS of dye combination 2 delivered a balanced visible light transmittance in the range of 60–70% and a dull grey hue, indicating that it was not weighted toward a specific color (right inset of Fig. 5(a)).
Fig. 5. UV-vis transmittance spectra of multi-fluorescent dye dispersed PDMS blocks. Three fluorescent dye combination results are divided into two graphs to eschew complexity. Inset (a) shows digital images of dye combination 1 (left) and 2 (right) PDMS. Top and bottom columns show color under indoor light and UV lamp, respectively. Inset (b) shows a digital image of dye combination 3 PDMS.
Deconvolution of the combined absorption peak of dye combinations 1 and 2 PDMSs was performed in order to confirm whether the fluorescent dye persists its unique absorption characteristics even in a mixed and dispersed state in the PDMS medium (Supplement 1, Fig. S10). Fitting was weighted based on the absorbance data in PDMS of each dye (i.e., the absorbance of colored PDMS with transparent PDMS as the baseline) (Supplement 1, Table S1); the cumulative fit was calculated by adding the reference absorbances of all dyes and transparent PDMS (fixed weight of 1). The final result was plotted by converting the y-axis to a transmittance value so that transparency could be understood intuitively. Consequently, the absorption characteristics of combinations 1 and 2 were surprisingly consistent with the sum of the intrinsic absorption characteristics of the constituent dyes (i.e., Beer-Lambert Law), indicating that there was no significant change in the intrinsic properties of the dyes; there was indistinct physicochemical interaction between adjacent dyes. The result supports that multiple dyes with different dissolution characteristics (i.e., polarity) can be dispersed in PDMS while retaining their individual absorbance properties.
To determine if the emission properties of each fluorescent dye are present in the mixed state in PDMS, PL analysis of the dye combination 2 was conducted. Since five dyes with different absorption characteristics were combined, the sample was analyzed five times at different excitation wavelengths to observe the fluorescence of each dye. As shown in Supplement 1, Fig. S11(a), the IPFs were detected in the same manner; thus, the fluorescent dye peaks were deconvoluted from the original spectrum (Supplement 1, Fig. S11(b-f)). It should be noted, however, that peak deconvolution was technically performed to preserve the shape of the dye emission peak as much as possible, and the deconvoluted peaks were chosen without a substantial physical background. At the excitation wavelengths of 305 and 540 nm, unlike the single-dye PDMS, a broad peak appeared to be overlapping multiple dyes. In this instance, the peak closest to the excitation wavelength was chosen. As depicted in Fig. 6, the PL result of the dye combination 2 PDMS exhibited the PL nature of the constituent fluorescent dyes, as in absorption. Thus the finding implies that even if multiple fluorescent dyes with different dissolution properties are mixed in PDMS, each dye can keep the individual absorption and emission characteristics. However, the possibility of Förster resonance energy transfer (FRET) is not discussed here owing to the low fluorescent dye concentrations.
Fig. 6. Normalized photoluminescence spectra of multi-fluorescent dye dispersed PDMS. (Combination 2) Excitation wavelengths were 305, 405, 475, 540, and 655 nm for C1, C6, BO14, KR620, and DTDCI, respectively. PL spectra were extracted from the original spectra shown in Fig. S11.
It is evident that the concentration of C1 has does not affect the visible transmittance, as it absorbs only the UV region (Supplement 1, Fig. S7). Consequently, we fabricated multi-fluorescent dye PDMS with increased C1 concentration to 63 µg gPDMS−1, which is 70 times, while keeping the concentrations of others (designated as combination 3). The absorbance analysis result of the dye combinations 2 and 3 PDMS (Fig. 5(b)) demonstrated, as anticipated, selectively increased UV region absorption with visible region balance. AVT values were calculated as 93.7 and 70.7% for pure PDMS and dye combination 3 PDMS blocks.
3.5 Color balance assessment of multi-florescent dye dispersed PDMS
The color balance of the multiple fluorescent dye PDMS was evaluated by CIE XYZ and CIELAB color coordinates, and CRI. The calculated CIE XYZ color coordinates for the fluorescent dye solutions and PDMSs used in this study are depicted in Fig. 7(a), and their values are provided in Supplement 1, Tables S2-S4. The x, y, and z values of dye combinations 2 and 3 PDMS were extremely close to 0.31271, 0.32902, and 0.35827, which were the values of the Standard Illuminant D65 (i.e., the white point). By comparing the distance between the coordinates, CIELAB makes it possible to judge the difference in chromaticity between samples intuitively, and the white point is (a*, b*) = (0, 0). The a* and b* axes range from −128 to 128, and a positive value of a* indicates the degree of red and negative values of green color, and positive values of b* indicate yellow and negative values of blue. The L*, a*, and b* values of the samples are indicated in Fig. 7(b) and listed in Supplement 1, Tables S2–S4. The CIELAB coordinates for the dye combination 3 PDMS were (a*, b*) = (0.77, 0.52), indicating that the sample has no distinct color. Therefore, the calculated CRI for the dye combination 3 PDMS was high as 94.6. Therefore, the fabricated multi-fluorescent dye PDMS was evaluated for its excellent color balance; accordingly, the light passing through it exhibited almost no color distortion (Supplement 1, Fig. S12).
Fig. 7. Mapping of calculated color coordinates for prepared samples on (a) CIE XYZ and (b) CIELAB color spaces. Dye combination 3 is marked as the representative of multi-fluorescent dye dispersed PDMS.
3.6 Photoelectric characteristics of Si solar cell integrated LSC device
In order to evaluate the LSC performance of the fabricated multiple fluorescent dye PDMS, the photoelectric characteristics of the device to which a diced Si solar cell is attached were analyzed. The average short-circuit current density (JSC), VOC, fill factor, and PCE of the solar cell were 44.71 ± 0.41 mA cm−2, 0.661 ± 0.002 V, 68.8 ± 0.9%, and 20.3 ± 0.3% (Supplement 1, Figs. S13 and S14). According to the Experimental and Characterisation section, the flat area of the PDMS block was 50 × 50 mm2, and its thickness was 12 mm. To prevent the LSC device from being overestimated by direct illumination and scattered or reflected light, a 40 × 40 mm2 anti-reflection mask and a bottom anti-reflection sheet were implemented. Consequently, the results were adjusted based on the assumption that solar cells were attached to all four edges [42].
When multiple fluorescent dye PDMS was applied as a waveguide, it exhibited significantly improved J–V characteristics than transparent PDMS (Fig. 8(a) and Supplement 1, Fig. S15). The relatively high VOC and JSC values could be attributed to the fact that multiple fluorescent PDMS guides more light to the edge than transparent PDMS. As a result, the PCE of the device increased 6.6 times, from 0.035 ± 0.003% to 0.234 ± 0.010%. The result is relatively lower than other studies (Table S5); however, it is hard to compare directly because the presented parameters and measuring conditions are not standardized [42,60,61]. The primary efficiency loss would come from the decrease of PL intensity, as shown in Supplement 1, Figs. S9 and S11. The detailed parameter values are shown in Table S6. As a result of EQE spectra (Fig. 8(b)), high quantum efficiency was detected near 360 nm, which corresponds to the fluorescence region of the C1 dye (Fig. 6), which occupies the most weight in multiple fluorescent PDMS. In a similar context, the fluorescence from C1 dye was also indirectly observed from reflectance analysis, shown in Supplement 1, Fig. S16. In addition, it seems that the fluorescence and scattering effects of the other fluorescent dyes generated the current in the 450–700 nm region. Integrated JSC from EQE profile was 1.197 mA/cm2, matching well with JSC value from J–V measurement. LUE, defined as the product of AVT and PCE, improved 5.0 times, from 0.033 ± 0.002 to 0.165 ± 0.007. Supplement 1, Fig. S17 demonstrates that pure PDMS and multi-fluorescent dye dispersed PDMS LSCs satisfied the photon balance at every wavelength [42,61]. According to all fluorescent dyes down-convert incident photons, the m value was assigned to 1.
Fig. 8. (a) Representative J–V curve and (b) EQE spectra (solid line) with integrated current density of LSC devices (dotted line) using pure PDMS and multi-fluorescent dye dispersed PDMS as a waveguide.
Optical efficiency (ηopt) for geometric gain factor (G) can be defined as follows [72]:
Afront is illuminated LSC window area, Aedge is area of edge Si solar cells, ILSC is short-circuit current of LSC device, and ISC is short-circuit current of Si solar cell. Consequently, ηopt of 4.03 ± 0.15% and 12.25 ± 0.34% at G of 0.77 were calculated for pure PDMS and multiple fluorescent dye embedded PDMS, respectively. Utilizing high QY dyes and high-efficiency solar cells would result in superior performance.3.7 Stability of single- and multi-fluorescent dye dispersed PDMS
PDMS is utilized in a variety of applications due to its renowned structural, mechanical, chemical, and thermal durability [73–75]. In terms of stability, the organic material is typically inferior to the inorganic material. Based on the absorbance analysis, we evaluated the 2-year storage stability of the fluorescent dyes used in this research. Fig. S18 and S19 depict the transmittance spectra of fresh and 2-year aged pure PDMS, and color PDMS blocks dispersed with a single dye, respectively. Except for C1, the remaining dyes persisted in their inherent absorption properties. The absorbance of C6, BO14, KR620, and NB decreased slightly, whereas the absorbance of DTDCI decreased by more than half. The C1 peak was obscured by a significantly increased ultraviolet absorption. In Fig. 9(a), the transmittance characteristics of 2-year aged multi-fluorescent dye dispersed PDMS deviated from the hypothesis that the overall transmittance would decrease slightly. To determine the cause, peak deconvolution was performed using the transmittance results of each 2-year aged single dye dispersed PDMS material (Table S7 and Fig S20). As a result, the weight of BO14 and KR620 increased substantially, while the absorbance of DTDCI barely decreased. In accordance with the CIELAB color space (Fig. 9(b)), the resultant PDMS was yellow (Fig. 9(a), inset). Supplement 1, Table S8 lists the colorimetric parameters in detail. Meanwhile, the CRI value dropped to 80.8. It is worth mentioning that five fluorescent dyes were still dispersed uniformly after 2-year storage, as confirmed by repeated UV-vis transmittance analyses for nine spots (Supplement 1, Fig. S21). These results imply that, except for C1, the remaining fluorescent dyes could be long-term stable in PDMS, suggesting the potential for indoor applications of multi-fluorescent dye dispersed PDMS.
Fig. 9. (a) UV-vis transmittance spectra and (b) mapping on CIELAB color space of fresh ((a) dot line and (b) hollow star) and 2-year aged ((a) solid line and (b) filled star) multi-fluorescent dye dispersed PDMS blocks.
4. Conclusions
In this study, the concept of multi-fluorescent dye dispersed PDMS was successfully demonstrated. Simultaneous dispersion of polar and non-polar fluorescent dyes in the PDMS matrix was accomplished. Moreover, the UV-vis absorbance and PL analyses showed that mixed fluorescent dyes exhibit individual optical properties. By blending at the optimal concentration ratio, color-balanced and fluorescing transparent LSC was fabricated successfully along with competitive AVT. CIE XYZ and CILAB color spaces, as well as CRI evaluations, indicated that multi-fluorescent dye dispersed PDMS has an excellent color balance. Furthermore, the photoelectric measurements of the LSC device presented its potential for BIPV application. In addition, it demonstrated proper two-year storage stability. If this concept were combined with strategies of utilizing UV and NIR region, LSC exploiting a wide range (i.e., UV-visible-NIR) light with neutral color would be developed. Further research into suppressing PL loss, lightfastness and thermal stability, FRET, and the introduction of high QY and Stokes shift dyes would facilitate the widespread implementation of this concept.
Funding
Korea Institute of Planning and Evaluation for Technology in Food, Agriculture and Forestry (421036-03); Korea Institute of Energy Technology Evaluation and Planning (20213091010020); National Research Foundation of Korea (NRF-2020R1A2C1101085, NRF-2022M3J7A1066428).
Acknowledgments
Authors acknowledges the financial support by the National Research Foundation (NRF) of Korea, which is funded by the Ministry of Science, ICT & Future Planning (NRF-2020R1A2C1101085 and NRF-2022M3J7A1066428), Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20213091010020), and Korea Institute of Planning and Evaluation for Technology in Food, Agriculture and Forestry (IPET) and Korea Smart Farm R&D Foundation (KosFarm) through Smart Farm Innovation Technology Development Program, funded by Ministry of Agriculture, Food and Rural Affairs (MAFRA) and Ministry of Science and ICT (MSIT), Rural Development Administration (RDA) (421036-03).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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