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Abstract
Bead-on-string fibers have been electrospun from terbium acetylacetonate hydrate (TAH) doped PMMA. The beads with a size of about 5μm and fibers with diameters about 300-600 nm are confirmed to form cross-links by SEM, which is helpful to the stability of silk-particle coexistence network. Under ultraviolet (UV) radiation, luminescence comparison between TAH/PMMA bulks and bead-on-string fibers with fluorescence enhancement in the latter verifies the effectiveness of electrospun. Further, when the beads and fibers reach the proper range, the greatest improvement of intensity is obtained. The greatest emission powers and the emission photon numbers of TAH/PMMA bead-on-string fibers in visible region are identified to be 32.07μW and 8.8 × 1013cps, respectively, which are nearly four times higher than those of the TAH/PMMA bulks under the 308nm UVB-LED pumping, and the highest luminous efficacy is up to 13.94%. Improved fluorescence behavior and conclusive photon quantification demonstrate the potential of TAH/PMMA bead-on-string fibers as UV-visible conversion layer of flexible solar cells applying in wearable electronic devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photovoltaics is the most advanced way of providing electricity far from any mains supply, and energy harvesting has grown from long-established concepts into devices for charging mobile electronics [1–4]. At present, the solar cell based on semiconductor photovoltaic effect is one of the effective means to utilize solar energy, which converts light energy into electrical power, however, the characteristics of fragility and inflexibility limit their application in portable fields [5–8]. Flexible solar cells need pliable substrates such as amorphous silicon and polyester film, etc., which possess some advantages of flexibility, portability and light weight compared with the hard substrate cell. As a future application, solar cells integrated into self-tempering apparels and self-charging textiles offer great potential business value [9,10]. Nevertheless, the low utilization of solar radiation, particularly poor response in ultraviolet radiation, is still a limitation remained to be solved. Hence an elastic light conversion material that absorbs UV radiation effectively into solar cells is desirable to investigate and develop further.
Among RE ions, Tb3+ is a promising optical activator and is widely used in phosphors for fluorescent lamps and displays because it allows facile photon management and converts UV radiation to visible emission on a large scale [11–15]. The rare earth mcro inorganic luminescent powders have high brightness, while the poor plasticity curbs the application. In recent years, in order to ameliorate the disadvantages, researchers produced fluorescent micro-nano beads by incorporating inorganic phosphor powders into polymer, which promotes the further application of the powders [16–19]. From another point of view, it is possible that the organic material itself shapes silk-particle structure, under that condition, not only fluorescent properties are comparable to those of inorganic and organic composites materials, but also flexibility can be free utmostly. With this focus of research, herein the bead-on-string fibers with dual properties of both microbeads and fibers are formed in one step by electrospinning, and the silk-particle structure holding high surface-to-volume ratio is conducive to adhesion of fiber thus obtaining stable network and knitting property. The soft bead-on-string fibers can provide favorable conditions for flexible material used as UV-visible conversion layer [20–25]. The schematic diagram for flexible solar cells which adopts bead-on-string fibers is depicted in Fig. 1. Compared with the bulks, the ultrafine structure of bead-on-string fibers can realize a high absorption and a strong fluorescence emission [26].
In this work, the bead-on-string fibers of terbium acetylacetonate hydrate (TAH) doped PMMA were formed by electrospinning technique, meanwhile, whose bulks is also obtained as a comparison. In an attempt to understand the underlying potential of the bead-on-string fibers, the responses to UV radiation in the bulks and the bead-on-string fibers were compared, and the brighter green luminescence recorded in TAH/PMMA bead-on-string fibers indicates that electrospun plays a positive role in the enhancement of characteristic luminescence intensity. The comparison of absolute spectral parameters further confirms that the TAH/PMMA bead-on-string fibers are promising candidates as UV-visible conversion layer to improve the photoelectric conversion efficiency of flexible solar cell.
2. Experiments
Poly methyl methacrylate (PMMA) were dissolved in mixed solvent consisting of tetrahydrofuran (THF) and N, N-Dimethylformamide (DMF) with PMMA concentration at 18wt%, 21wt%, 24wt% and 26wt% which were labeled as A, B, C and D, respectively. Subsequently, the terbium acetylacetonate hydrate accounting for 0.8% of PMMA weight was introduced. Then, the mixed solution was stirred until uniformity and transparency for the following electrospinning. The electrospinning equipment is composed of three major components, a high voltage power supply, a spinneret and a receiving device, as shown in Fig. 2. During the process of electrospinning, the solution injected in a 5ml plastic syringe was fed to the tip by using a stepper at a rate of 7.03mm/h. Along with the running of the high voltage power of 13.10kV, TAH/PMMA composite bead-on-string fibers were generated and collected on the conducting aluminum foil with a collection distance of 12cm, respectively. As an integral part of comparison, the TAH/PMMA bulks were obtained by evaporating the solvent for spinning solution slowly at room temperature.
The microstructures of bead-on-string fibers were observed by a JEOL JSM-7800F scanning electron microscope. A Hitachi F-7000 spectrometer equipped with a photomultiplier (PMT) tube as detector and a commercial CW Xe-lamp as pump source was used to record emission and excitation spectra. The differential scanning calorimetry (DSC) and the thermogravimetric analysis (TGA) were determined by American TA company SDT 600 at the rate of 20°C/min from room temperature to 600°C under N2 atmosphere. The absolute spectral parameters for the bulks and the bead-on-string fibers of TAH/PMMA were measured in an integrating sphere of 25cm inner diameter (Labsphere) which was connected to a QE65000 standard CCD detector (Ocean Optics) with a 600μm-core optical fiber. The current of the exciting 308 nm UVB light emitting diode (UVB-LED) was fixed at 20mA with reference voltage of 5.78V. A standard halogen lamp (Labsphere, SCL-050) was used to calibrate this measurement system, and the spectral power distribution was obtained through fitting the factory data based on the black body radiation law.
3. Results and discussion
3.1 Morphology of TAH/PMMA bead-on-string fibers
The TAH/PMMA bulks with a good transmittance is shown in Fig. 3(a), which is hard and easily broken. Figure 3(b) presents macro morphology of electrospun bead-on-string fibers, which is soft filamentous state as like a cloud of cotton in natural light. The ultrathin structure contributes to the full absorption for ultraviolet radiation compared with the transparent TAH/PMMA bulks [27–30].
Fig. 3 Photographs of TAH/PMMA bulks (a) and macro morphology of electrospun TAH/PMMA bead-on-string fibers of C (b).
The microcosmic appearance of electrospun fibers is presented in Fig. 4, in which, the micro morphology of A is non-fibrous, and with the increment of concentration, the bead-on-string fibers are emerged at B with more beads, and at C, the bead starts reducing, subsequently, getting into smooth fiber state at D. The bead-on-string fibers have the more stable network by creating knots for fiber to improve adhesion of fiber structures which are promising candidates as photoelectric conversion layer in flexible solar cell for wearable electronic device [31–33].The shift demonstrates that the concentration plays a key role at obtaining glazed fiber structure.
Fig. 4 SEM micrographs of electrospun TAH/PMMA of A, B, C and D corresponding to (a), (b), (c) and (d) under 1000 magnification, respectively.
In order to further exhibit the transformation process from bead-on-fibers to smooth fibers, the microstructure of the TAH/PMMA fibers at a higher magnification are provided at Fig. 5. In the lower polymer solution concentration, because the molecular chains are not entangled enough, the solution jet cannot effectively resist the external force, and break down. In the process of stretching, the solution jet gets inhomogeneous force, thereby the inconsistent molecular chain alignment come into being, finally, the bead-on-string fibers are formed. In Fig. 5(b), the bead-on-string fibers are arranged at random and are overlapped with each other, which well demonstrate the silk-particle structure with a complicated combination between beads and fibers, appearing flexible and supple condition, and the bead is conspicuous with size about 5μm and the non-uniform fiber diameter is about 300−600nm. With the increment of polymer concentration, the diameters become coarsen about 800-1300nm accompanying that the bead is further stretched, as shown as Fig. 5(c). Figure 5(d) exhibits smooth morphology with non-uniform diameters 1500-2000nm.
Fig. 5 SEM micrographs of electrospun TAH/PMMA fibers of A, B, C and D corresponding to (a), (b), (c) and (d) under 2000 magnification.
3.2 Thermal properties of TAH/PMMA
Thermal property is of great significance to PMMA for practical applications as optical materials, which is strongly influenced by the chain length of the polymer [34–36]. The thermal stability of TAH doped PMMAs is illustrated in Fig. 6. The curves show that most of the organic ingredients were removed under 600°C. The endothermic peaked at a temperature about 340°C indicates the degradation of PMMA, and the exothermic peak at 390°C is due to the carbon and carbon oxide release of PMMA, where the weight-loss events occur. The higher decomposition temperature implies that the terbium acetylacetonate hydrate doped PMMA have good thermal stability.
3.3 Fluorescence characteristics of TAH/PMMA bead-on-string fibers
Figure 7(a) and 7(b) give the typical emission spectra of TAH/PMMA bulks. Under different light excitation, all the fluorescence spectra exhibit a broad band at 300−400nm because of π–π* transition and narrow emission band between 450 and 650 nm with four sharp peaks at approximately 490, 548, 583 and 621nm due to the 5D4→7FJ (J = 6, 5, 4, 3) transitions of Tb3+ [37–42], and differential luminescence is exhibited as the inserted photos, respectively. The dominance of ligand radiation is more obviously under the excitation of 276nm than that of 288nm owing to the formation of different energy transfer mechanisms. Under 276nm UV radiation, the energy transfer is initiated by the excitation of the ground state (S0) of the acetylacetonate (acac) ligand to its excited singlet state (S2), and the excited S2 partly non-radiatively relax to S1 followed by intersystem crossing to the excited triplet state (T1). However, at 288nm excitation, there are a different circumstance, acac gets directly excited to S1, then relaxes non-radiatively to T1. Afterwards the excitation energy of the T1 is transferred to the level 5D4 of Tb3+ by non-radiative relaxation process [43], and finally, radiative transitions of Tb3+ takes place with visible emissions as presented as Fig. 8(a). It is worth investigating this difference of fluorescence intensity for A, B, C and D, and the Fig. 7(c) and 7(d) give the typical emission spectra of TAH/PMMA bead-on-string or smooth fibers under the excitation of 276 and 288nm. Comparing with the bulks, the emission intensities of TAH/PMMA fibers change considerably, one reason behind fluorescence enhancement is light scattering, which contributes to the absorbance of ultraviolet radiation. Moreover, the fluorescence emission, on the one hand, escapes; on the other hand, it enhances the scattering. Another reason is the accumulation of rare-earth ions on the periphery of the nanofibers due to the larger surface to volume ratio. With the increment of polymer concentration, the emission intensity of Tb3+ are strengthened obviously in the beginning, and the tendency becomes stagnant when the polymer concentration exceeds 24wt% (C), which illustrates fluorescence intensity is related to the number/size of bead /fiber diameter. The fiber scatters the light to the bead, thereby further enhancing the efftiveness of emission enhancement. When the beads and fibers reach the proper range, the greatest improvement in strength is obtained. Figure 8(b) and 8.(c) portray the different interactions between ligand and Tb3+, and in the bulks, the reciprocities of ligands occupy a larger proportion, nevertheless in the bead-on-string fibers, the excited ligand transfers energy to Tb3+, which give full play to the antenna effect. The dominance of ligand emission is greatly weakened, while the emission of Tb3+ is evidently strengthened possessing a master status, which can also be reflected by the photos of bead-on-string fibers [44,45], implying that electrospining procedure not only contributes to the absorption of sample in ultraviolet region, but also strengthens the energy transfer process from ligand to Tb3+.
Fig. 7 Emission spectra of TAH/PMMA bulks (a, b) and TAH/PMMA fibers of A, B, C and D (c, d). Inserted photos: fluorescence of TAH/PMMA bulks (a, b) and TAH/PMMA bead-on-string fibers of C (c, d).
Fig. 8 Energy transfer mechanism of TAH (a), and the interactions between ligand and Tb3+ in bulks (b) and bead-on-string fibers (b).
Figure 9(a) is the excitation spectrum of TAH/PMMA bulks monitoring the ligand emission at 325nm. The excitation spectrum consists of an intense broad band peaking at 280nm and effective excitable range are 200−310nm. The excitation spectrum of TAH/PMMA bulks monitoring the green emission of Tb3+ at 548nm is dominated by a broad band at 260−320nm, as shown in Fig. 9(b), and the excitability is relatively low, implying the radiation intensity of Tb3+ is subject to limitations [46–49]. After fibrosis, the excitable intensities of ligand and Tb3+ are revealed in Fig. 9(c) and 9(d), and the numerical variation relationship between ligand and Tb3+ is consistent with that of Fig. 7, that is, ligand absorption enhances and Tb3+ emission behavior free. The excitability of ligand has a slightly change in intensity monitored at 325nm, and the regularity is not obvious with the increment of concentration, while the Tb3+ increases significantly, suggesting energy transfer process is strengthened thereby raising the emission of Tb3+ as a main emitting center, which further illustrates the potential of TAH bead-on-string fibers as UV-visible conversion layer for flexible solar cells.
Fig. 9 Excitation spectra of TAH/PMMA bulks (a, b) and TAH/PMMA fibers of A, B, C and D (c, d) monitored at 325 and 548nm, respectively.
3.4 Absolute spectral parameters of TAH/PMMA bead-on-string fibers
Integrating sphere coupled with CCD detector measurement was applied for absolute spectral parameter characterization, which is helpful for assessing luminescence materials [50,51]. Lightful green luminescence was observed in Tb3+-doped PMMA samples under the excitation of 308nm UVB-LED in an integrating sphere, and the spectral power distribution P(λ) of TAH/PMMA bulks is shown in Fig. 10(a), which consists of a weak broad band and five emission bands located at 490, 548, 583, 621 and 650nm assigned to 5D4→7FJ (J = 6, 5, 4, 3, 2) transitions, respectively. With the implementation of fibrosis, the emissions of TAH/PMMA bulks are obvious strengthened as presented in Fig. 10(b). The photon distribution offers fundamental information in the optical field and relevant application. Based on the spectral power distributions, the photon distributions can be derived by
where λ is the wavelength, ν is the wavenumber, h is the Planck constant, c is the vacuum light velocity, and P(λ) is spectral power distribution. The emission photon distribution curves of TAH/PMMA bulks and fibers are derived from Eq. (1) with photo distributions as presented in Fig. 11(a) and 11(b). Moreover, the spectral power distribution intensity and photon number distribution reach to maximum at C compared with others. And the trend is in accordance with fluorescence intensity for B and C, while which has been reversed for A and D, which illustrate that the surface luminescence of non-fibrous state of A is relatively high due to the strong scattering, while the optical properties of smooth state of D is relatively stable. Furthermore, the maximum spectral power and photon number distributions of Tb3+ in bead-on-string fibers at C are up to 1.64μW/nm and 1.35 × 1011 cps/cm−1 at 548nm, which are six times than those of 0.25μW/nm and 0.20 × 1011cps/cm−1 in bulks, respectively. Meanwhile, in the visible region, the emission power and emission photon number of thin bead-on-string fiber layers are calculated to be 32.07μW and 8.8 × 1013cps that are merely four times than 8.36μW and 2.14 × 1013cps in bulks, which are attributed to the heightening of energy transfer process and the boosting of Tb3+ as the main emission center. The enhanced green fluorescence of C is shown in the inserted photo, suggesting the potential of bead-on-string fibers as photoelectric conversion material.Fig. 10 Spectral power distributions of TAH/PMMA bulks (a) and TAH/PMMA fibers of A, B, C and D (b) under the excitation of 308 nm UVB-LED. Inserted photos: fluorescence in integrating sphere of TAH/PMMA bulks (a,)and TAH/PMMA bead-on-string fibers of C (b).
Fig. 11 Photon distributions of TAH/PMMA bulks (a) and TAH/PMMA fibers of A, B, C and D (b) under the excitation of 308 nm UVB-LED.
The luminous efficacy is essential to assess luminescence prospect of terbium acetylacetonate hydrate doped poly methyl methacrylate. Under the excitation of 308nm UVB-LED, the total luminous flux Φv of the bulks and fibers corresponding to A, B, C and D can be calculated by
where V(λ) is the relative eye sensitivity and Km is the maximum luminous efficacy at 555nm (683lm/W). The luminous flux distributions are presented in the Fig. 12, and the relevant total luminous fluxes Φv in the whole visible spectral region are listed in Table 1.Fig. 12 Luminous flux distribution of fluescence of (a) bulks and (b) fibers corresponding to A, B, C and D under the excitation of 308nm UVB-LED with excitation power 115.6mW.
Table 1. Luminous efficacy of bulks and fibers corresponding to A, B, C and D.
Under the excitation power of 115.6mW for 308nm UVB-LED, with the increment of electrospining polymer concentration, the total luminous flux values show continued upward tendency, and reach to maximum ~16117μlm at C and the luminous efficacy gets 13.94%, which much higher than the bulks whose are 2625μlm and 2.27%. Based on above, electrospun fibrosis can be considered as a means to strengthen the photoelectric conversion efficiency of flexible solar cells for wearable electronic device, which is convenient for us traveling.
4. Conclusions
The fibers derived from TAH/PMMA are prepared through electrospinning process, which is verified as bead-on-string fibers. And the beads with size about 5μm and fibers with diameters about 300-600nm form cross-links that contribute to the stability of silk-particle coexistence network. Under ultraviolet (UV) radiation, luminescence comparison between TAH/PMMA bulks and fibers with enhanced green fluorescence illustrates the effectiveness of electrospun in increasing emission behavior. Further, the greatest improvement in strength is obtained when the beads and fibers reach the proper range. Under 308nm UVB-LED pump source, the highest emission powers, emission photon numbers and luminous efficacy in the visible region are derived to be 32.07μW, 8.8 × 1013cps and 13.94% for bead-on-string fibers which is far higher than the bulks with 8.36μW, 2.14 × 1013cps and 2.27%, respectively. Improved emission behavior and higher photon emission indicate that TAH/PMMA bead fibers can play the UV-visible conversion layer of flexible solar cells.
Funding
CityU Strategic Grant (7004788); National Natural Science Foundation of China (61275057).
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