Highly Elastic and Flexible Phosphor Film for Flexible LED Lighting and Display Applications

, -,*, -,*, -, -, U -,

(1. College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China; 2. Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan 030024, China; 3. College of Textile Engineering, Taiyuan University of Technology, Taiyuan 030024, China; 4. College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China)*Corresponding Authors, E-mail: jiahusheng_tyut@163.com; zaq6014567@126.com

HighlyElasticandFlexiblePhosphorFilmforFlexibleLEDLightingandDisplayApplications

JIAJing1,JIAHu-sheng1,2*,ZHANGAi-qin2,3*,SHENQian-qian1,LIDong-xin1,LIUXu-guang2,4

(1.CollegeofMaterialsScienceandEngineering,TaiyuanUniversityofTechnology,Taiyuan030024,China;2.KeyLaboratoryofInterfaceScienceandEngineeringinAdvancedMaterials,TaiyuanUniversityofTechnology,Taiyuan030024,China;3.CollegeofTextileEngineering,TaiyuanUniversityofTechnology,Taiyuan030024,China;4.CollegeofChemistryandChemicalEngineering,TaiyuanUniversityofTechnology,Taiyuan030024,China)
*CorrespondingAuthors,E-mail:jiahusheng_tyut@163.com;zaq6014567@126.com

Flexible LEDs have attracted significant interest in recent years for lighting and display applications. We present a polydimethylsiloxane based phosphor film that is capable of high elasticity and flexibility while actively emitting light. It not only exhibits good thermal stability in a wide range of -50-230℃, but also retains the optical properties as raw phosphors. The prepared transparent PDMS thin film and the corresponding phosphor film enable complete flexibility and elasticity, the largest elongation is up to400% and275%, respectively. Besides, white LEDs were fabricated using prepared YAG-doped phosphor film, showing averageTcof6925K, CRI of71and mean luminous efficiency of115.7lm/W. Furthermore, the proposed photoluminescent films in two colors and a flexible3×3LED array glowing with three colors were fabricated using thin elastic and transparent rubber and subjected to stretching, rolling and folding to demonstrate their promising use in flexible lighting and display applications.

light-emitting diodes; flexible design; phosphor film; optical properties

1 Introduction

The future is a world for screen. Not only televisions, computers, mobile phones and open-air large screens, but also the surface of tables or walls, and the displays inside vehicles will become various screens showing information. Though thin-film transistor liquid crystal display (TFT-LCD) technology applications have been commonplace, active matrix organic light-emitting diode displays (OLEDs) using glass or plastic substrates have been produced. As the optimal display technology at this stage[1-2], OLEDs have already been seen in smart phones, televisions, tablets and a new generation of wearable application. Apparently, TFT-LCD is being replaced with OLEDs step by step. As such, the luminescence mechanism of LEDs is the same as that of OLEDs,i.e., RGB color signals are emitted directly from diodes when a current passes through. Nevertheless, to date there are few real LEDs based self-luminous displays with small screen size and very high price on the market. Thus LEDs are applied more in lighting field, correspondingly, different colors and sorts of phosphor powders excited by blue or near ultraviolet (NUV) LED chips have been constantly exploited.

On the other hand, flexible displays, which can be bent, rolled, or even folded, are developing in fast speed because of the mega market demand for personal mobile devices. The most common approach to obtain flexible LEDs is to employ stretchable transparent electrodes[3]or flexible substrates[4-6], for example, electrodes based on indium tin oxide (ITO) films[7], graphene[8], single- or multi-walled carbon nanotubes (SWNTs or MWNTs)[9-10], polyethylene-dioxythiophene∶polystyrene-sulfonate(PEDOT∶PSS)[11], or nanowires[12]. In addition, colloidal quantum dot (QD) flexible sheets of CdSe/ZnS have recently appeared in flexible LEDs for lighting and display applications[13-15].

To our knowledge, the literature regarding stretchable transparent phosphor-based film hybridized with light emitting diodes to achieve flexible high performance lighting and displays is rare. Consequently, a highly stretchable and transparent hydroxyl-terminated polydimethylsiloxane(PDMS) film is prepared through bulk polymerization and heat curing without filler or catalyst, and then a flexible multicolor phosphor membrane based on alphabet templates is presented, which enables both flexibility and high elasticity. Array of tiny flexible fluorescent film panels, each emitting different color (such as red, green and blue) through selective blending of photoluminescence phosphor, could be a new class of display technique for LED display devices with NUV excitation light. Besides that, the proposed flexible phosphor film can be utilized as remote phosphor agents for the packaging structure of chip-on-board (COB) LEDs and flip chip LEDs. Most notably a major change in conventional lamps design concept may occur since super elastic, flexible and self-adhesive phosphor film can wrap around LED excitation sources in any shape or form.

2 Experiments

2.1 Materials

Octamethylcyclotetrasiloxane (D4, 98%), potassium hydroxide (KOH, 82%), ethyl orthosilicate (TEOS,40% SiO2), tetrahydrofuran (THF, 99%). All chemicals were used without further purification. Ce-doped yttrium aluminum garnet (YAG∶Ce3+) and oxy-nitride red phosphors (ZYP630H, Beijing Nakamura-Yuji), teflon (PTFE) coagulating molds with alphabet template slot, which have three sizes of 28 mm×28 mm×1 mm, 60 mm×60 mm×0.5 mm and 40 mm×40 mm×4 mm, and nine-block moulds with 40 mm×40 mm made by a 3D printer using polypropylene (PP), 1 W blue LED chips withλpof 445 nm (made in China) were used in this work.

2.2 Synthesis of Transparent PDMS Rubber and Preparation of Multicolor Phosphor Films

The silicone oil and TEOS were blended at the ratio of 5∶1 in weight in the presence of a little THF acting as diluent. Afterwards, inorganic phosphor powder accounting for 10% of mass of silicone oil was mixed into it, or directly injected into character template slots. Subsequently, the uniform mixture was filled into PTFE molds and small air bubbles were removed under vacuum. Finally, phosphor film was baked and cured at 110 ℃ for 5 h and then dried in oven at 47 ℃ for 7 d. The fully dried films were peeled off from the mold surface and used as flexible free-standing sheets of phosphors ready for further experiments. The synthesis route of proposed transparent PDMS elastomer is shown in Fig.2.

Fig.1 GPC chromatogram of synthetic hydroxyl silicone oil

Fig.2 Synthesis route of the transparent PDMS

2.3 Characterization

The molecular weight of synthetic hydroxyl silicone oil was determined using gel permeation chromatography (GPC, TDAmax, Malvern). Qualitative analysis of the structures of prepared silicone oil and crosslinked PDMS was taken using Fourier transform infrared spectrometer (FTIR, Tensor 27, Bruker). The kinetics of degradation for prepared PDMS elastic membrane was measured using thermogravimetric/differential thermal analysis instrument(TG/DTA, STA409C, Netzsch). Differential scanning calorimetry(DSC) curves on transparent PDMS films were detected using Q2000 apparatus (TA Instruments). The tensile testing was conducted at an electronic universal testing machine with rectangular specimen sheets that had a gauge length of 20 mm, a width of 10 mm and a thickness of 2 mm. Light transmittance and absorption spectrum were collected using Hitachi-3900 UV-Vis spectrometer (the slit width was 2 nm). Fluorescence spectra were measured on an Edinburgh LFS-920 spectrometer and the absolute quantum yield of different samples was determined through an absolute method by employing an integrating sphere. Electroluminescence(EL) spectra were analyzed using a computer controlled PMS-80 UV-Vis-near IR spectrometer with an integrating sphere. All measurements were made at room temperature unless otherwise stated.

3 Results and Discussion

3.1 FTIR Spectroscopy

The IR spectra of synthetic silicone oil and cross linked PDMS are given in Fig.3. It is quite clear that the band shape and wavenumber of silicone oil are basically in accordance with those of cross linked PDMS, although the absorbance for the latter is much higher as a result of more sample quantities. The wide absorption band containing two O—H stretching vibrations at 3 449 and 3 132 cm-1can be observed, which may be due to the formation of intermolecular hydrogen bond between end OH groups of PDMS, and then the O—H stretching vibration frequencies decrease to become a wide band[16]. As for crosslinked PDMS, the same two absorption bands are not obvious rather than disappeared because of different tick label. Furthermore, the band intensities remain unchanged for the reason that the number of terminal hydroxyl groups drops by 50% after the crosslinking reaction in spite of more sample quantities.

The characteristic asymmetrical stretching vibration of —CH3occurs at 2 965 cm-1. In addition to this vibration, another weaker absorption band at 2 905 cm-1is caused by the —CH3rotamer in crosslinked network structure, as shown in Fig.3(b). And the symmetrical bending vibration of —CH3is located at 1 402 and 1 400 cm-1for silicone oil and crosslinked PDMS, respectively. Moreover, the backbone of PDMS is made up of inorganic siloxane (Si—O—Si), whose asymmetrical stretching vibration band is split into two bands at 1 096 and 1 022 cm-1or so, and the corresponding symmetrical stretching vibration is at 800 cm-1. Absorption bands at 866 and 700 cm-1are assigned to the stretching vibration of Si—C. Besides, absorption bands at 1 261 and 482 cm-1may be caused by impurities in testing samples. The above results indicate that the synthesized polymer has the chemical structure depicted in Fig.2.

Fig.3 IR spectra of synthetic silicone oil (a) and crosslinked PDMS in the solid state(b)

3.2 Thermal Analysis

The thermal properties of prepared PDMS film were investigated by thermogravimetric (TG) analysis and differential thermal gravimetric (DTG) analysis, differential scanning calorimetry (DSC) under nitrogen atmosphere. As shown in Fig. 4, the onset decomposition temperature (Tonset) of the polymer is 231.8 ℃, as determined from the intersection of extended baseline and line composed of point of 5% weight loss and point of 50% weight loss[17]. No weight loss occurs between 100 ℃ andTonset, showing that there is no coordination water in polymer. The maximum decomposition rate occurs at 472 ℃ with a weight loss of 31.5%, and eventually complete decomposition of sample is reached at 527 ℃ with residue of 1.88%.

The glass transition temperature (Tg) of the polymer is not apparent on the second heating curve in the range of -50-400 ℃, as seen from Fig.4(b). The exothermic peak on cooling curve is attributed to new cohesional entanglement formed from the shifts of local molecular chain segments to lower state and the corresponding energy release. The above results confirm that prepared PDMS film has good thermal stability in a wide range of -50-230 ℃, and is thermally stable enough for fabrication of LEDs, given that the junction temperature of LED is generally less than 150 ℃[18-19].

Fig.4 Thermal properties analysis of prepared PDMS elastic membrane. (a) TG (square) and DTG (circle) curves at 10 ℃/min in N2. (b) DSC results at 10 ℃/min in N2.

3.3 Optical Properties

The light transmittance of prepared PDMS film in visible wavelength is more than 70% and even 80% especially in blue and green regions between 464-547 nm (refer to Fig. 5), completely transparent glass slides are as the blank control sample, indicating that synthesized PDMS elastomer has high transparency. Hence, YAG particles inside proposed phosphor film can obtain enough exciting light that passes through transparent PDMS matrix. Moreover, there is one characteristic absorption peak at 455 nm in the absorption spectrum of YAG phosphor film, demonstrating it can be excited by blue light to emit yellow light as original YAG phosphors.

The solid-state excitation and emission spectra of prepared YAG phosphor film and raw YAG phosphor recorded at room temperature are presented in Fig.6(a) and (b). Their peak shapes are very similar for either photoluminescence excitation (PLE) spectra or photoluminescence (PL) spectra. The excitation spectra for both show a double-peak structure. Along with the sub-highest peaks at around 344 nm due to the transitions from2F5/2→2D5/2of Ce3+, the highest peaks appear at 467 and 451 nm in blue light region are related to the2F7/2→2D5/2transitions of Ce3+and ensures that the prepared YAG phosphor film can match blue chips withλpof 451 nm or so.

Fig.5 Light transmittance of prepared PDMS film (square) and absorption spectrum of YAG phosphor film (circle) in visible wavelength

On the other hand, the red curve in Fig.6(b) displays typical emission band centered at 541 nm when excited at 450 nm, which is compounded of emission bandsvia2D3/2→2F7/2and2D3/2→2F5/2transitions of Ce3+[20], the corresponding yellow-green fluorescence of prepared phosphor film can mix with unconverted blue light to produce white light, and the correlated 1931 Commission Internationale de L’Eclairage(CIE) coordinates for the YAG phosphor film (0.408 8, 0.566 4) are closer to the green light region than that for raw YAG phosphor powders (0.430 4, 0.546 6), as marked in Fig.6(c). In addition, the absolute quantum yield for YAG phosphor film (0.60) is slightly smaller than that for raw YAG phosphors (0.72) excited at 450 nm.

Fig.6 PLE and PL spectra of raw YAG phosphor (a), YAG phosphor film (b) and the CIE chromaticity coordinates (c) for raw YAG phosphor (1) and YAG phosphor film (2).

3.4 Appearance

PDMS is commonly used for packaging materials with high transparency, good thermal stability and gas-permeability. In particular, in this study it is introduced for the elasticity and flexibility. Fig.7(a) and (b) show the appearance of prepared transparent PDMS film and YAG-doped phosphor film under natural light. Obviously, both have good flexibility. The proposed flexible phosphor film with silicone and YAG phosphor at mass ratio of 10∶1 can allow sufficient blue light pass through to mix with converted yellow light for white light, and also can scatter the incident blue light efficiently[21]. The real glowing effect of proposed multicolor phosphor film under NUV illumination or over high-power blue COB LEDs is displayed in Fig.7(c) and (d), corresponding scalability can be seen clearly from Fig.7(e) and (f).

Fig.7 High elasticity and flexibility multicolor photoluminescence film. (a) Photograph of transparent PDMS film (under natural light). (b) Visual appearance of YAG-doped phosphor film (under natural light). (c) Multicolor fluorescent film under NUV illumination of 365 nm. (d) Multicolor fluorescent film over high-power blue COB LEDs of 452 nm. (e) and (f) Scalability of transparent multicolor film under NUV illumination of 365 nm.

3.5 Mechanical Properties

Fig.8 Tensile fracture behavior of synthetic PDMS and corresponding YAG phosphor film under uniaxial stretching at 5 mm/min

The most convenient techniques to solve the problem on low strength of rubber are to add reinforce fillers, for instance, white carbon black, clay and calcium carbonate[22-24], or to prepare interpenetrating polymer networks(IPNs) structure generally composed of immiscible polymers, which leads to phase separation so that the resulting material is normally opaque[25-27]. Therefore, the future challenge is to improve the weak mechanical strength of proposed phosphor film without sacrificing its trans-parency.

3.6 White LED Devices

The fabricated LED lamp beads made of standard blue chips and slices from prepared YAG phosphor film are driven at 2.9 V. One drop of commercial silicone (Dow Corning) is used to fasten phosphor film while not influencing transmission of blue light within the packaging. The median is determined by means of 5 sets of data. When blue emission with peak wavelength of 445 nm is radiated from LED chip to excite YAG phosphor film, some of incident blue light is converted into yellow light, and the rest is diffused, finally these rays are blended to generate white light. The measured electroluminescence (EL) spectra and relevant CIE coordinates are shown in Fig.9. The CIE chromaticity coordinates corresponding to assembled white-LED with YAG phosphor film (0.308 5, 0.311 8) are close to standard white light (0.33, 0.33) according to the 1931 CIE coordinate diagram[28-30].

Fig.9 (a) EL spectra of assembled LEDs with or without YAG phosphor film. The mosaic photo shows visible emissions from different LED lamp beads. (b) Corresponding CIE chromaticity coordinates.

As given in Tab.1, the mean color temperature (Tc) of fabricated WLED is 6 925 K, the mean color rendering index (CRI) is 71.1, and the mean luminous efficiency is 115.7 lm/W (for naked blue LED chip it is only 13.96 lm/W), meaning that prepared YAG-doped phosphor film can absolutely be applied in white lighting applications as a result of good optical properties. The three main reasons why the luminous efficiency of white LEDs can be improved largely compared to that of naked blue LED are as follow. First, white light is a mixture of blue light radiated from blue chip and yellow-green light emitted by YAG phosphor, and the calculation equation of total luminous flux of a white LED[31]is:

(1)

whereKm(lm/W) is the maximum spectral luminous efficacy of eyes on light,Φeλ(W) is the radiant flux of a light source, andVλis the relative luminous

efficiency function of visible light. In other words, total luminous flux of a white LED is the integral of each wavelength of the spectrum multiplied by its relative luminous efficiency function. Wide yellow-green band area means relatively more luminous flux for fabricated white LED in Fig.9(a), therefore total luminous flux of a white LED and corresponding luminous efficiency are much higher than those of naked blue LED in the same drive current condition (120 mA). Second, the refractive index of GaN-based blue LED chip (2.4) is bigger than that of the air (1.0), causing most of the photons of blue light would be totally reflected at the interface between chip and air, and then lost in the chip. By contrast, phosphor film on the surface of chip can significantly increase the external quantum efficiency of blue LED chip due to higher refractive index of transparent silicone (about 1.5) and scattering by phosphor particles. At last the phosphor in the conformal film structure has good thermal conduction for blue chip[32], which leads to less heat accumulation, lower running temperature, and higher luminous efficiency of white LEDs.

Tab.1 Optical data for different fabricated LED lamp beads driven at 2.9 V

3.7 Flexible Phosphor-based Lighting and Displays

As shown in Fig.10, the proposed phosphor-based photoluminescence film makes flexible lighting, which can be wrapped directly in various shapes of LED excitation sources, for example, cubes and cylinders. The background color of photograph images is basically blue due to the fluorescer inside white papers as the luminous body’s backdrop. Also, nine independent pixels of 2 mm spacing glowing with three colors under NUV excitations were fabricated using thin elastic and transparent rubber and were subjected to stretching, rolling and folding to demonstrate the potential applications of suggested phosphor film in flexible displays. Each individual pixel shows good optical emission with a size of 10 mm×10 mm.

Fig.10 Flexible lighting using different phosphors on white papers under NUV excitations of 365 nm: (a) original toy bricks, (b) oxy-nitride red phosphors, (c) silicates gamboge phosphors. Nine pixels photoluminescent display of 5 mm thick employing proposed rubber under NUV illumination of 365 nm: (d) undeformed, (e) rolled, (f) folded.

4 Conclusion

A highly elastic and transparent PDMS film was prepared by a simple and pollution-free method, and then a flexible multicolor phosphor-based photoluminescence film and yellow YAG phosphor film were presented. As far as prepared YAG phosphor film be concerned, it not only retains the optical properties of raw YAG phosphors, but also exhibits good thermal stability in a wide range of -50- 230 ℃. The combination of blue chip and yellow YAG phosphor film can satisfy the requirements of white lighting applications, which shows averageTcof 6 925 K, CRI of 71 and mean luminance efficiency of

115.7 lm/W (for naked blue chip it is only 13.9 lm/W). Moreover, the prepared transparent PDMS thin film and the corresponding phosphor film enable complete flexibility and elasticity， the largest elongation is up to 400% and 275%, respectively. Ultimately, we succeeded in utilizing the suggested highly elastic, flexible and transparent phosphor-based photoluminescence film onto flexible lighting and LED arrays display. This method provides a promising solution to realize flexible/stretchable multicolor devices having high performance and low cost, compared with existing flexible LEDs employing stretchable transparent electrodes or flexible substrates.

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贾静(1987-)，女，山西原平人，博士研究生，2013年于太原理工大学获得硕士学位，主要从事柔性荧光薄膜的研究。

E-mail: jiajing.chn@icloud.com张爱琴(1974-),女，山西临猗人，副教授，硕士生导师，2012年于太原理工大学获得博士学位,主要从事有机光电材料的研究。

E-mail: zaq6014567@126.com贾虎生(1964-)，男，山西太原人，教授，博士生导师，1996年于西北工业大学获得博士学位，主要从事白光LED荧光粉与器件、半导体异质结设计及光电化学行为等方面的研究。

E-mail: jiahusheng_tyut@163.com

2017-04-13；

2017-06-13

山西省科技攻关计划项目(201603D121017,201601D102020)； 山西省自然科学基金(2015021075); 山西省高校科技创新项目(2016124); 山西省科技创新重点团队(201513002-10); 山西省研究生教育创新项目(2016BY055)资助

LED柔性照明及显示用超弹性柔性荧光膜

贾 静1， 贾虎生1,2*， 张爱琴2,3*， 申倩倩1， 李栋信1， 刘旭光2,4
(1. 太原理工大学 材料科学与工程学院， 山西 太原 030024；2. 太原理工大学 新材料界面科学与工程教育部重点实验室， 山西 太原 030024；3. 太原理工大学 轻纺工程学院， 山西 太原 030024； 4. 太原理工大学 化学化工学院， 山西 太原 030024)

柔性LED是近年来照明及显示领域研究的热点之一。本文提出了一种新的基于有机硅胶(PDMS)制备的兼具超弹性和柔性的荧光薄膜，它不仅在-50～230 ℃这一较宽的温度范围内展现了良好的热稳定性，还保持了原料荧光粉的光学性能。所制备的透明PDMS基质膜和相应的荧光膜具有完全的柔性和超弹性，其最大伸长率分别高达400%与275%。此外，采用所制掺YAG荧光膜和普通商用1 W蓝光芯片简单封装的白光LED灯珠满足日常白光照明的应用要求，呈现出约6 925 K的平均色温，约71的平均显色指数，115.7 lm/W左右的平均发光效率。最后，基于所提出荧光膜成膜工艺而制备的三色3×3柔性阵列显示，可以轻易被拉伸、卷曲和折叠,显示了它在柔性照明及显示器件方面具有应用价值和潜力。

LED； 柔性设计； 荧光膜； 光学性能

Supported by Program for Science and Technology Development of Shanxi(201603D121017,201601D102020); Natural Science Foundation of Shanxi Province(2015021075); Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi(2016124); Shanxi Provincial Key Innovative Research Team in Science and Technology(201513002-10); Graduate Innovation Program of Shanxi Province(2016BY055)

O482.31DocumentcodeA

10.3788/fgxb20173811.1493