Vol. 25, No. 17 | 21 Aug 2017 | OPTICS EXPRESS 20598 · Furthermore, when deposited on the reflector surface of a remote phosphor-converted light-emitting diode lamp, nanofiber films

Highly reflective nanofiber films based on electrospinning and their application on color uniformity and luminous efficacy improvement of white light-emitting diodes

Yong Tang, Guanwei Liang, Junchi Chen, Shudong Yu, Zongtao

Li,* Longshi Rao, AND Binhai Yu Key Laboratory of Surface Functional Structure Manufacturing of Guangdong High Education Institutes, School of Mechanical and Automotive Engineering, South China University of Technology, Guangdong, China *[email protected]

Abstract: Based on electrospinning technology, in this study, we fabricated poly(lactic-co-glycolic acid) (PLGA) nanofiber films with high reflectivity and scattering properties. Various films with different thicknesses and fiber diameters were fabricated by changing the electrospinning time and solution concentration, respectively. Detailed optical measurements demonstrate that the film reflectance and scattering ability increase with the thickness, whereas fiber diameter contributes little to both properties. With optimized film thickness and fiber diameter, nanofiber films feature whiteness with a reflectance of 98.8% compared to the BaSO4 white plate. Furthermore, when deposited on the reflector surface of a remote phosphor-converted light-emitting diode lamp, nanofiber films witness a correlated color temperature deviation decrease from 8880 K to 1407 K and a luminous efficiency improvement of 11.66% at 350 mA. Therefore, the nanofiber films can be applied in lighting systems as a highly reflective coating to improve their light efficacy and quality. © 2017 Optical Society of America

OCIS codes: (230.2090) Electro-optical devices; (230.3670) Light-emitting diodes; (290.0290) Scattering.

References and links

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Vol. 25, No. 17 | 21 Aug 2017 | OPTICS EXPRESS 20598

#300435 https://doi.org/10.1364/OE.25.020598 Journal © 2017 Received 20 Jun 2017; revised 1 Aug 2017; accepted 1 Aug 2017; published 15 Aug 2017

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1. Introduction

One-dimensional polymer nanofibers have attracted much attention due to advantages such as their large specific surface areas, and high aspect ratio. Nowadays, electrospinning is recognized as a low-cost, simple, and efficient way to fabricate nanofibers (NFs) with high aspect ratios [1–3]. Many functional polymers, such as polystyrene (PS) [4], poly(vinylidene fluoride)(PVDF) [5], and poly(lactide-co-glycolide) (PLGA) [6], have been intensively electrospun with excellent properties such as superhydrophobicity, piezoelectricity and so on. The variety of electrospun nanofibers is of great potentials in many applications, including but not limited to energy storage [7], biomedical engineering [8], environmental engineering [9], sensing and detection [10, 11], and optics [12]. Recently, research on the optical application of electrospun nanofibers has attracted much interest. Electrospun NF films exhibit high scattering since nano-sized fibers can scatter visible light greatly, based on the Mie theory [13, 14]. Chang et al. [15] fabricated PolymethylMethacrylate fibers with different diameters to study their scattering properties and found the light scattering band of those fibers had a strong linear relationship with the fiber diameter. Tsuboi et al. [16] indicated that the size of the pores would affect the scattering properties of NF films, and applied Polyvinylpyrrolidone(PVP) nanofibers to the gold surface to enhance the surface plasmon resonance. Inspired by the sunlight scattering ability of hollow hairs on the lower surfaces of poplar leaves, Ye et al. [17] prepared hollow fibrous films with high reflectance by coaxial electrospinning and pointed out that these white films could be used as backlight coatings to improve lighting efficiency. Liao et al. [18] encapsulated cellulose nanofibers with epoxy resin to achieve light transmittance greater than 85%. However, the utilization of the excellent optical scattering performance of electrospun films in angular color uniformity (ACU) improvement of LED lighting systems has not been reported yet.

At present, the most common method to produce white light is to mix the blue light emitted from blue LED chips with yellow light produced by exciting phosphor atoms. The remote phosphor (RP) structure, which separates the phosphor from the chip, is widely used in lighting systems to improve luminous efficiency by virtue of reducing the absorption of backscattered light by the LED chip [19, 20]. However, the remote phosphor structure exhibits a serious yellow ring effect due to the mismatching of the Lambertian blue radiation and isotropic yellow emission; therefore, the pursuit of ACU is still a popular topic of research [21]. There are many ways to enhance the ACU; Wang et al. [22] designed a free-form lens to decrease the yellow ring effect, Ding et al. [23] developed a micropatterned array optical film which was utilized to control light distribution, thus decreasing the correlated color temperature (CCT) deviation. In addition, we improved the color uniformity of chip-on-board (COB) white light-emitting diodes (WLEDs) by using remote phosphor films with two freeform surfaces [24]. However, all the above methods are at the expense of luminous flux


loss. A low-cost and facile phosphor coating method based on electrohydrodynamics was proposed by Sun et al. [25], which can realize high ACU by changing the phosphor layer morphology. Other methods such as the use of the distributed Bragg reflector structure [26],phosphor-in-glass converter [27],sapphire substrate structure [28], a patterned phosphor-in-class with a crater-type lens [29], were also employed in improving the ACU. However, these devices are difficult and costly to prepare. It is well noted that reflective coatings are widely used to achieve light control, which play a large role in improving the light quality. Xi et al. [30] prepared a one-dimensional SiOxNy coating over the reflective cup surface, but the reflectivity of the coating was only about 80%. A porous scattering coating with a 99% reflectivity was obtained using micro-foam processes by Kuo et al. [31] Kim et al. [32] used bead blasting to roughen the surface reflector, thus reducing the re-absorption of downconverted light by the LED chip. However, the above studies only focus on the improvement of light efficiency, and the ACU is not considered.

Therefore, in this study, electrospun nanofiber films are used for the first time to enhance the ACU of RP-LEDs. We study the influence of the film thickness and fiber diameter on the optical properties of the reflective layer in order to bestow the electrospun PLGA film with strong diffusion properties. More specifically, we employ a bi-directional reflectance distribution function (BRDF) to analyze the diffusion properties of the film and its diffuse reflectance is measured. The optimal scattering characteristics of the corresponding electrospinning parameters are utilized to prepare a diffusion coating on the reflector surface. Finally, we experimentally determine the role of the electrospun PLGA film for RP-LED lamps.

2. Experiment and measurement

2.1 Sample preparation

(1) Preparation of electrospinning materials

The electrospinning solution was prepared by dissolving different weight ratios of the PLGA 75/25 polymer (molecular weight of 70000 g/mol, Jinan Daihang Biological Engineering Co., Ltd) in a mixture solution of dimethylformamide (DMF, Richjoint®) and acetone (Guangzhou Chemical Reagent Factory) (volume ratio equals 2:1). After 5 hours of magnetic stirring at 50 °C and 350 rpm, the polymer solution was allowed to stand for 24 h to remove the air bubbles. Polished aluminum plates (20 mm × 20 mm × 1.5 mm) with greater than 85% reflectivity were used as fiber collecting substrates.

(2) Phosphor film preparation

The phosphor film was fabricated by the spin-coating method. An optical quartz glass was used as the phosphor-coated substrate. First, a mixture of polydimethylsiloxane (PDMS, Dow Corning Sylgard-184) and the cross-linking agent was mixed to attain a mass ratio of 10:1. Then, a YAG phosphor (YAG04 from Intematix) and PDMS mixture with a mass ratio of 1.6:2 was prepared. Finally, 1 mL of the mixture was dispensed on the glass and the solidification was followed in an oven at 150 °C for 1 h after constant spinning at 600 rpm for 1 min. The white LED lamp consists of a blue LED (3535, Foshan National Star) and a parabolic reflector cup, with the remote phosphor film placed on the upper surface.

(3) Electrospinning of PLGA nanofibers

In the experiment, the PLGA spinning solution was electrospun using a home-made far field electrospinning device, as shown in Fig. 1. The polished aluminum plates and parabolic reflector were used as the nanofiber deposition substrates. The positive output of the high-voltage power supply (model DW-P501-1ACDF from Tianjin Dongwen High Voltage Power supply Co., Ltd.) was connected to a 23 g stainless steel hypodermic needle with an inner diameter of 0.317 mm, which was used as a spinneret. The polished aluminum plates or


parabolic reflector, served as the collector, were placed on a piece of copper foil (20 mm × 20 mm), which was grounded to form a fixed rectangular electrode. The distance between the needle tip and the collector (working distance) was 20 cm. An appropriate amount of spinning fluid was inserted in a 1 mL plastic syringe connected with the needle serving as a spinneret. The syringe was fastened to the Lange syringe pump (model LSP01-1A, produced by Baoding Lange Constant Flow Pump Co., Ltd.), which quantitatively controlled the flow rate for the electrospinning process. The entire electrospinning device was enclosed within a transparent plexiglass box and vertically electrospun at room temperature.

Fig. 1. Schematic of the far field electrospinning system.

In this study, the film thickness and fiber diameter are demonstrated as two key parameters that influence the scattering properties. Films with various thicknesses can be obtained by simply varying the electrospinning time, whereas fiber diameter is determined by multiple spinning parameters, such as the solution concentration, applies voltage, the needle tip to the collector, and the flow rate. Studies have shown that the polymer concentration of the solution has an important effect on the fiber morphology and diameter [33]. When studying the effect of the solvent on the morphology and diameter of PLA fibers, Casasola et al. [34] pointed out that as the solution concentration increased, the fiber diameter increased significantly. Therefore, by adjusting the solution concentration between 0.2 g/ml to 0.5 g/ml, also referred to as 20 wt% to 50 wt%, we can obtain electrospun films with various fiber diameters. The other spinning parameters were set as follows: applied voltage was 13 kV, working distance was 20 cm, and flow rate was 0.5 mL/h.

2.2 Measurement and characterization

The morphologies of electrospun PLGA nanofibers nanostructures were characterized by a field emission scanning electron microscope (FEG-SEM, Zeiss® Merlin) under an accelerated voltage of 5.0 kV. The diameter of the PLGA nanofibers were measured by imaging software (Image J) and 50 fibers were measured for each sample to determine the average diameter of the nanofibers. Besides, we used a surface profilometer (Vecco Dektak 150, America) to measure the thickness of the electrospun films.

Since the asymmetric bidirectional reflectance distribution function (BRDF) [23] has been widely employed to characterize scattering properties, we utilized it to study the diffuse reflectance characteristics of the electrospun films. The BRDF is a function of both the incident and scattering directions, and is defined as follows:

ii

i

( , , , )( , , , , ) ,

( , )r i r r

i r ri i

dLBRDF

dE

θ ϕ θ ϕθ ϕ θ ϕ λθ ϕ

= (1)


Here, iθ and iϕ represent the zenith and azimuth angles of the incident light, respectively, and

rθ and rϕ the zenith and azimuth angles of the scattering light, respectively. λ is the

wavelength of the incident light, rL denotes the scattering radiance, and iE is the incident

irradiance. The BRDF measurement of the PLGA films, CCT distribution, luminous intensity, and

luminous flux were also measured in our home-made system. As shown in Fig. 2, the system mainly includes a rotating axis arm surrounding the testing bench, which is combined with a spectrometer (Ocean USB2000 + ) and an optical fiber probe to detect emitted light. When measuring the BRDF, a blue laser is the emission light source, and an adjustable DC power supply (KESSLY ®) is used to drive the LED light at a constant current of 350 mA as the source for CCT measurement. During the testing process, the sample was placed 316 mm away from the spectral probe, and the rocker revolved around the sample center between −90° to 90° to test the relevant parameters. We used an ultraviolet to visible light spectrophotometer equipped with a 60 mm-diameter integrating sphere (dual-beam UV-Vis spectrophotometer TU-1901) to conduct the diffuse reflectance measurement of the samples. The luminous flux and the spectrum of the lamps are tested within an integrating sphere system with a diameter of 0.5 m, which includes the spectrometer (Otsuka LE5400) and a DC power supply (Keithley 2450) driving the lamp between 50 mA to 1 A.

Fig. 2. System for the measurement of the BRDF for PLGA films.

3. Results and discussion

3.1 Fiber morphology

Figure 3 shows the SEM images of electrospun PLGA with a solution concentration of 25 wt% to 50 wt%. The experimental results show that when the solution concentration is less than 25 wt%, more beads appear on the fibers. Actually, the low solution concentration has a low viscosity, and the surface tension plays a great role on the fiber morphology, causing the formation of beads [35]. Although lower solution concentration can produce smaller fibers, it may lead to incomplete stretching and produce beads on the fibers. When the solution concentration is increased to 30 wt%, the beads disappear completely and smooth fibers with an average diameter of 475 nm are obtained. Above this concentration, the average fiber diameter increased as the solution concentration increased. Table 1 lists the average diameter distribution of the fibers under five different solution concentrations. The studies show that as the solution concentration increases, the fiber diameter also gradually increases, which is consistent with the result of Deitzel et al. [35] When the solution concentration is less than 25 wt%, the diameter of the fiber and the growth rate is relatively small and gentle, respectively.


The average fiber diameter is 864 nm at 50 wt%. It can be seen that the solution concentration has a great influence on the fiber morphology, which guides the samples preparation.

Fig. 3. SEM images of PLGA nanofibers fabricated with different PLGA solution concentrations of (a, e) 25 wt%, (b, f) 30 wt%, (c, g) 40 wt%, and (d, h) 50 wt%.

Table 1. Diameter of PLGA NFs with the PLGA concentration

PLGA Concentration (wt%) 20 25 30 40 50

Diameter (nm) 259 ± 46 283 ± 50 475 ± 104 705 ± 102 864 ± 114

3.2 Optical performance of the thin film

Electrospun fiber films are formed by the randomly staggered micro/nanofibers with high aspect ratios which produce high light scattering as result of a strong whiteness index. Pan et al. [36] point out that the fiber thickness is linearly related to the deposition time under appropriate spinning parameters, and Munir et al. also provide the same conclusion [37]. Therefore, we select a 35% solution concentration and obtain PLGA films with different thicknesses by controlling deposition time under the optimum spinning parameters. These original fiber films exhibit different optical properties as the deposition time increases, such as high diffuse reflectance which can cause visual shock from the intensity of the whiteness. The sample, the diffusion experiment and the relationship between the thickness and deposition time are shown in Fig. 4. It can be observed that the thickness increases with the deposition time, vary from 1.6 μm to 142.8 μm.

Fig. 4. (a) Image of the PLGA NF films, (b) patterns of green laser light scattering, and (c)the relationship between the thickness and deposition time. The peel-off-test image of the PLGA coating is inset.


The fibrous layers are deposited on highly reflective polished aluminum plates, which reflect the transmitted light again to ensure the extraction light. The fiber diameter and the unique structure of the films can cause significant light scattering, resulting in a high diffuse reflectance. The optical properties of the PLGA fiber films were systematically tested, including the BRDF and the diffuse reflectance. Figure 5 shows the variation of the BRDF, the mirror energy, and the diffuse reflectance of the PLGA films with different thickness. As shown in Fig. 5(a), the distribution of the BRDF of the films changes gradually from concentrated to uniform in the mirror reflection angle, and the curve flattens and slightly rises with the continuous deposition of PLGA fibers on the polished aluminum plates. The specular reflection energy decreases and transforms into the diffuse reflectance energy until the specular reflection disappears completely. However, when the fibers continue to deposit, the specular reflection of the fiber also slightly improves. It can be seen that the diffuse reflectance of the films reaches a balance when the film thickness is more than 6.7 μm, and the BRDF changes slightly. During the collection process, the fibers are stacked to form a dense fiber film with porosity which contributes to the scattering of light. For an isotropic single fiber, the light scattering on its surface and the shape of the porosities may change the direction of light transmission on the same layer of the fibers, thus enhancing the light mixture. However, the amount of fibers per unit area increases as the film thickness increases, and thus the pore sizes between the fibers become smaller, resulting in the enhancement of scatter properties; a similar explanation is given by Tsuboi et al. [16] When the film thickness is increased to 12.4 μm, the BRDF of the films does not change any more, showing that the fiber scatter properties have already reached their limit. In particular, we extracted the specular energy of the films from the BRDF by removing data in the range of about 4° on either side of the peak and fitting them with the average between two consecutive data points, as shown in Fig. 5(b). The results show that the specular energy decreases rapidly in 1 min and is even removed, which further proves the strong scattering ability of electrospun fiber films. It demonstrates that electrospinning is a facile and efficient method to produce strong scattering films. We can obtain films with strong scattering properties by simply controlling the fiber thickness.

The sample reflection includes the diffuse reflection of the films and the specular reflection of the aluminum plate. As shown in Fig. 5(c), the diffuse reflectance of the PLGA films gradually increases in balance with increasing thickness. When the film is thin, the polished aluminum mirror reflection occupies the main position, and thus the mirror reflected light could not reach the spectrophotometer of the radiation detector. With the increase of the fiber density per unit area, the stacking fiber layer reduces the specular reflection component and increases to saturation with the increase of fiber thickness, which produces a weak mirror reflection. The inset in Fig. 5(c) shows the average reflectivity measurement of the visible band. It indicates the average reflectivity gradually increases as the thickness of the films increases and ends up to a balance. When the thickness is changed from 1.6 μm to 142.8 μm, the average reflectivity changes from 67.62% to 94.42%, which is an increase of 39.62% related to the initial reflectivity. Compared to the BaSO4 white plate, the diffuse reflectance of visible light is as high as 98.8% at 380 nm and the average reflectance is 94.42% when the film thickness is 142.8 μm. Therefore, high reflectivity of the films can be obtained in visible light. The BRDF of film with 127.2 μm indicates that the specular reflectance stays constant, and the change in diffuse reflectance of the films eventually reaches stability. The BRDF and diffuse reflectance test results show that the thickness of the films contributes greatly to the light scattering.


Fig. 5. Optical properties of PLGA films with different thickness:(a) BRDF, (b) mirror energy change, and (c) diffuse reflectance. The inset is the average diffuse reflectivity of films with different thickness.

Fig. 6. Optical properties of PLGA films with different diameters: (a) the BRDF, and (b) the diffuse reflectance. The inset is the average diffuse reflectivity of films with different diameter.

On the other hand, samples with different diameter were prepared and measured to further study the diameter influence on the optical properties. Figure 6 shows the BRDF and the diffuse reflectance test results for different diameter samples. The results indicate that the BRDF of the different samples are saturated and have almost identical diffusion properties, but the samples with a smaller average diameter have higher reflectivity. These may be due to the unevenness of the fibers with large overlap of the diameter distribution, because electrospinning is a complex process of multiple factors. Our results are similar to those of Lee et al. [38] in their study of the effects of Polyacrylonitrile nanofiber diameters on light scattering. Here, films with small fiber diameters contribute little to achieving high diffuse reflectance. However, the large diameter is not suitable for preparation of a light scattering


film when the film thickness is uniform. Therefore, in order to control the optical properties of the films and apply them to lighting systems, it is necessary to consider the diameter of the fibers and the thickness of the films.

3.3 Application of NF films on LEDs for ACU improvement

Based on the preparation and test results of the above diffuse films, this paper proposes a PLGA diffusion film with an average fiber diameter of 475 nm and a film thickness with 194.7 μm. And it was prepared on the parabolic reflector of LED lamps to obtain strong BRDF characteristics and high diffuse reflectance, which contribute to the enhancement in ACU and improvement in light efficiency of the LED lighting system. Figure 7 shows the schematic diagram of the RP structure lamp. High ACU is a requirement of LED lamps, but traditional designs exist the yellow ring effect over a larger angle and the presence of more blue light in the center, which causes visual discomfort [39]; this is especially true in the poor ACU of RP structure lamps [21]. The high diffuse reflectance of the PLGA films not only enhances the ACU, but also improves the luminous efficiency of the lamp.

Fig. 7. (a) Schematic diagram of the light transmission. (b) Photograph of the lamp with/without the PLGA film. (c) Correlating blue illumination patterns.

Herein, the PLGA films prepared on the LED reflector shows significant forward light scattering properties, which effectively achieves even light distribution. After the application of the NF film on the parabolic reflector, the CCT decreases from 5442 K to 5234 K (350 mA). Figure 8(a) shows the angular CCT distributions. The results show that the color temperature is very high in the center of the traditional reflector and there is a clear yellow ring at a large viewing angle, which has been proved by Liu et al. either [40]; however, the lamp with PLGA films effectively modifies the small angle color temperature and enhances the ACU. The central color temperature is much higher for conventional lamps, caused by blue light escaping without exciting the phosphor in the central section of the lamp. The brightness of the lamp in the central section is significantly stronger than at the edge due to the mismatching of Lambertian blue radiation and the parallel light reflecting from the parabolic reflector. The schematic of the light scattering is showed in Fig. 7(a). Due to the strong diffusion properties of the films, the large angle blue light is changed from parallel exiting to Lambertian emission, and the blue light emitted by the Lambert radiation excites the phosphor more than the parallel exit of the blue light by increasing the path of the blue light. Thereby, the utilization efficiency of phosphor is increased and the center of the color temperature is reduced. It is also possible to effectively collect the back-scattered light from the fluorescent layer due to the strong scattering ability of the PLGA film [32], which further reduces the center color temperature and improves the ACU. The light intensity in the small angle also increased, as shown in Fig. 7(c). As shown in Fig. 8(d), the chromaticity coordinates of the LED lamps with PLGA films is closer toward yellow region. Figure 8(b) confirms the increase of the light intensity by different angle measurements. At the intensities of 0°, −30°, and −75°, the emission of the yellow light increased, but the blue light emission


at 0° is kept constant. It is further illustrated that the PLGA films improved the light exit and controlled their distribution effectively.

Fig. 8. (a) CCT distribution. (b) Emission spectra at the zeniths of 0°, −30°, −75°. (c) Current-dependent CCT. (d) Chromaticity coordinates of the LED lamps with/without PLGA films in CIE 1931.

To study the CCT stability of two lamps, the current was changed from 50 mA to 1 A, and the results are shown in Fig. 8(c). When the current changed from 50 mA to 1 A, the CCT of the lamp with the electrospun film increases from 5152 K to 5342 K with an increase of 190 K, while the CCT of the traditional lamp increases by 246 K, specifically. Among them, CCT deviation decreases from 8880 K to 1407 K (350 mA), which is mainly due to the PLGA films enhancing the utilization efficiency of the phosphor. Through directly changing the scatter ability of the reflector cup, the device shows a great range of ACU improvement compared to the previous works, which mainly aimed at the design of phosphor layer [41–43]. When the current increases, the blue LED chip produces more blue photons to excite the phosphor and become yellow photons, but traditional remote phosphor exhibits poor light color mixing. However, improving the utilization of the phosphor is a conducive way to improve CCT stability [44]. The PLGA films contribute to distributing the blue light and increasing the utilization efficiency of the phosphor as well as improving the light mixture, so the lamp with PLGA films show better CCT stability with increasing current.

In order to study the optical effects of PLGA films for the lamps in detail, we tested the luminous flux under different currents ranging from 50 mA to 1000 mA and calculated their luminous efficiencies, as shown in Fig. 9(a). The luminous flux gradually increased but the luminous efficiency reduced as the current increased. Specifically, compared to traditional lamps, the luminous flux of the lamp with PLGA film increased by 11.66% while the luminous efficiency increased by 10.89% at 350 mA. The improvement in luminous efficiency is mainly due to the increase utilized efficiency of the phosphor excitation, and the spectrum of Fig. 9(b) supports the conclusion. It should be noted that the difference between the two spectrums exists only in the yellow section, because the blue sections are coincident.


Therefore, we extracted the total energy of the blue light and the excited yellow light and the residual blue light energy, as shown in Fig. 9(d). The total blue light energy of the lamp with the PLGA film is 357.6 mW, which is 9.4 mW higher than that of the traditional. Both of the residual blue light energies are the same, but the excitation of the yellow light of the lamp with PLGA film increases by 21.5 mW while that of the traditional lamp increases by 182.7 mW. Thanks to the high diffuse reflectance of the PLGA films, more blue light is emitted in the forward direction and the total amount of blue light increased by 10.46 mW, which causes increased yellow light excitation. At the same time, the strong scattering characteristic of the films increase the emission of the backscattered yellow light, decreasing the light absorbed by the substrate [45]. CCE is defined as the power of the yellow emission light divided by the power of the blue excitation light. According to the CCE calculation method reported by Zhang et al. [46], we can calculate the CCE of different lamps by the Eq. (2) throughout the spectrum:

4

3

2

1

' '

( ),

( )

Y emY em

B ex B ex

I dPCCE

P I d

λ

λλ

λ

λ λ

λ λ

−−

− −

= =

(2)

where _Y emI and _B exI denote the electroluminescence (EL) intensity of the yellow emission

light and the blue excitation light (which is the intensity difference between the blue LED reference emission without the RP film and the blue component of the WLED), respectively. [ 1λ , 2 λ ] and [ 3λ , 4 λ ] are the range of blue light (380 nm to 490 nm) and the emission range

of yellow light (490 nm to 780 nm), respectively.

Fig. 9. (a) Luminous flux and luminous efficiency. (b) Emission spectrum of white LEDs with and without PLGA films. (c) Current-dependent color conversion efficiency and light efficacy of radiation for LED lamps with/without PLGA films. (d) Radiant flux of blue and yellow light (350 mA).

Light efficacy of radiation (LER) is the indicator used to determine the efficiency of light based on the human eye. The LER was calculated by the following formula:


( ) ( )d

683(lm / ) ,( )d

R SLER W

S

λ λ λ

λ λ=

(3)

where 683 lm/W is the biggest LER at 555 nm based on the human eye, R( λ ) is the human eye sensitivity function, and S( λ ) is the EL intensity of the light. Figure 9(c) shows the CCE and LER test results with different currents. The CCE shows that the lamps with PLGA film have a higher light conversion efficiency of 8.12% compared to that of the traditional lamp, and is more stable under larger currents, which may be due to the improvement of the light extraction efficiency. The LER of the lamps with PLGA film is greater than 350 lm/W under large currents and is more suitable for lighting than traditional lamps.

To adequately study the effect of the electrospun PLGA films on the optical performances of WLEDs, we can easily prepare the phosphor films with different mass ratios to achieve different lighting effect. Table 2 illustrates the optical performances of WLEDs without and with electrospun PLGA film under different mass ratio of phosphor films. At the low mass ratio, lamp with electrospun film behaves a total CCT of 7298 K, compared to 8336 K to the lamp without electrospun film. As can be seen, as the total CCT decreases, the total CCT decrement and CCT deviation decreases gradually. Besides, the luminous flux efficiency improves slightly. This is because the backscattering of the phosphor film increases, and the light absorption on the reflector increases consequently. From the results, it is demonstrated that the electrospun films extract more backscattering light and achieve light distribution, which proves that the electrospun films could maintain the stable optical properties.

Table 2. Change in total CCT and CCT deviation and luminous flux improvement of the system without and with electrospun film configuration for different mass ratio of YAG

and PDMS

Mass Ratio of YAG and

PDMS

Total CCT (K) Total CCT

Decrement (K) CCT Deviation Decrement (K)

Luminous Flux efficiency (%) Lamp without

electrospun film Lamp with

electrospun film

0.4 8336 7298 1038 \ 11.18

0.65 5897 5528 369 \ 11.29

0.8 5442 5234 208 7393 11.66

0.9 4735 4605 130 5992 11.86

1 4566 4450 116 2834 11.99

1.2 4257 4184 73 580 12.11

2.8 3772 3754 18 3 12.3

In this way, optical PLGA diffusion film with a unique porous structure can be easily

prepared by a simple process of electrospinning, which does not involve chemical or high temperature treatment in contrast to conventional methods. Therefore, these results demonstrate we cannot only achieve the enhancement of the ACU, but also improve the luminous efficiency of the lamp by using PLGA films. The strong diffusion properties of the PLGA films indicate that the electrospinning process is suitable for preparing simple and low-cost diffusion films for LED systems.

4. Conclusion

In this study, we demonstrated that the electrospun PLGA fiber films had good diffusion properties and could be used in LED light system. Various diffusion films with different thicknesses and fiber diameters were fabricated by changing the electrospinning time and solution concentration, respectively. In particular, we achieved samples with strong diffusion properties by controlling the thickness of the coating and the fiber diameter. And they were


characterized by the BRDF and diffuse reflectance which was as high as 98.8% in the visible light. When applied to LED lamps, the PLGA nanofiber films with unique structures could effectively control the light distribution and mix the rays evenly, which leads to the enhancement of the ACU by decreasing the CCT deviation from 8880 K to 1407 K (350 mA). Furthermore, the effects of the electrospun PLGA films on luminous efficiency, the CCE, and the LER were also measured. Our results indicate that the use of the films can indeed improve luminous efficiency by 11.66% and achieve more stable CCE and larger LER, obviously improving the lighting quality. Besides, the electrospun films could maintain the stable optical properties when the phosphor films changed with different mass ratios. As the backscattering light increases, lamps with electrosoun films own a lower CCT deviation and higher luminous efficiency. We believe that the application of the PLGA films with micro/nanostructure prepared by electrospinning have potential value in LEDs, especially in the preparation of high diffuse reflectance optical films used as backlight coatings.

Funding

National Natural Science Foundation of China (NSCF) (Grant Nos. 51405161, U1401249 and 51375177); Science and Technology Program of Guangdong Province (Grant Nos. 2014B010121002 and 2015B010132005).


Documents

Vol. 25, No. 17 | 21 Aug 2017 | OPTICS EXPRESS 20598 · Furthermore, when deposited on the reflector surface of a remote phosphor-converted light-emitting diode lamp, nanofiber films