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Fabrication of Flexible White Light Emitting Diodes from Photoluminescent Polymer Materials with Excellent Color Quality Huang Yu Lin, Chin-Wei Sher, Chih-Hao Lin, Hsien-Hao Tu, Xin Yin Chen, YiChun Lai, Chien-Chung Lin, Huang-Ming Philip Chen, Peichen Yu, HsinFei Meng, Gou-Chung Chi, Keiji Honjo, Teng-Ming Chen, and Hao-Chung Kuo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b03386 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017

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ACS Applied Materials & Interfaces

Fabrication of Flexible White Light Emitting Diodes from Photoluminescent Polymer Materials with Excellent Color Quality Huang-Yu Lin,¶ Chin-Wei Sher, ¶, # Chih-Hao Lin, ¶ Hsien-Hao Tu, ¶ Xin Yin Chen, ¶ Yi-Chun Lai, ¶ Chien-Chung Lin,∥,* Huang-Ming Chen, ¶ Peichen Yu, ¶ Hsin-Fei Meng, † Gou-Chung Chi, ¶ Keiji Honjo, ‡

¶

Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan # §

Hong Kong University of Science and Technology, Hong Kong

Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan ∥

Institute of Photonic System, National Chiao Tung University, Tainan, Taiwan †

‡

Teng-Ming Chen, § and Hao-Chung Kuo¶,*

Institute of Physics, National Chiao Tung University, Hsinchu, Taiwan

ICE Cube Center WOW 3DI Laboratory, Rm.307 Bldg. J3, 4259, Nagatsuta-cho, Midori-ku Yokohama, Kanagawa, Japan

*

To whom correspondence should be addressed: (H.C. Kuo) [email protected]; (C.C. Lin)

[email protected].

KEYWORDS: Light emitting diode; Polyfluorene; Quantum dots; Flexible device, Color rendering index BRIEFS: A simple approach for demonstrating flexible LEDs with excellent color quality and reliability.

ABSTRACT This study developed flexible light-emitting diodes (LEDs) with warm white and neutral white light. A simple ultraviolet flip-chip sticking process was adopted for the pumping source and combined with ACS Paragon Plus Environment

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polymer and quantum dot (QD) films technology to yield white light. The polymer-blended flexible LEDs exhibited higher luminous efficiency than the QD-blended flexible LEDs. Moreover, the polymer-blended LEDs achieved excellent color rendering index values (Ra = 96 and R9 = 96), with high reliability, demonstrating high suitability for special applications like accent, down or retrofit lights in the future. In the places such as museum, kitchen or surgery room, its high R9 and high CRI characteristics can provide high quality of services.

INTRODUCTION Recently, white light emitting diodes (LEDs) have received considerable attention because of their high luminousity, high light density, stable reliability, and energy efficiency.1,2 For daily life requirements related to lighting or displays, conventional high-efficiency white LEDs with gallium nitride blue chip and yttrium aluminium garnet phosphor have been developed.3,4 To improve LEDs and extend their application range, developing high-quality white LEDs with special features is crucial. To further extend the application of white LEDs, high color quality and bending characteristics are critical criteria. Organic LEDs (OLEDs) offer the highest potential for fabricating flexible lighting devices. They have received considerable attention because of their excellent color quality and thinness and are thus used in artistic lighting and displays. Because of the increasing prevalence of wearable technologies, numerous researchers have developed high-quality flexible OLEDs and inorganic LEDs. Many research groups have focused on developing electrode designs for flexible substrates, such as nitride nanowires, reduced graphene oxide, and cellulose/epoxy substrates,5–7 as well as improving the mechanical strength of InGaN/GaN epitaxial LEDs subjected to external bending strain.8 Rogers et al. used flexible LED technology integrated with a specialized epitaxial semiconductor layer to create or transfer-print microLED chips and systematically analyzed interconnected components in related nanoscale systems.9–13 The main factors related to flexible LEDs that require consideration include luminous efficiency, bending curvature, and reliability,14,15 as well as color quality parameters such as the color rendering ACS Paragon Plus Environment

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index (CRI) and CRI R9. Recently developed OLEDs can be used as flexible devices and provide highCRI lighting. However, according to the results of a benchmark image test (Supporting Information, Fig. S1), achieving a high CRI results in a degraded luminous efficiency.16–29 When they performed the same benchmark test, Zhou et al. achieved a high CRI (96) but a low efficiency (5.5 lm/W).22 Su et al. developed an OLED with high luminous efficiency (44 lm/W) but low CRI (68).16 Therefore, achieving a balance between CRI and luminous efficiency in OLEDs is challenging. In this study, we demonstrate flexible white LEDs that were developed using two types of flexible film; one was fabricated using quantum dots (QDs), and the other was fabricated using polymers. QDs are well-known lighting materials because of their high quantum yield, size-dependent tunable bandgap, and narrow emission linewidth. Different reported designs have placed QDs in standard LED packages.30-32 Currently, polymer LEDs (PLEDs) are attracting great interest because of their high emission efficiency and easy solution process, which make them suitable for numerous specific lighting applications such as interior design, cars, and displays. Much research has investigated polymer films for many applications, Peter J. Skabara’s group demonstrated the excellent hybrid inorganic/organic white light LEDs CHDV as an encapsulating, UV-curable matrix, a dilute solution (1% w/v) of organic converter material combined with a blue chip.

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In this study, we selected polyfluorene (PFO) to

fabricate films for our research.34-37 The polyfluorene (PFO) polymer exhibits not only the optimal luminous efficiency but also the excellent color render property, and was the popular materials as the active layer for OLED. PFO group polymers were with the broadband emission wavelength and can be control the band-gap by the ligand doping to modulate emission wavelength.38-39 As the PFO with the excellent optical property, several researches have reported the use of the hybrid polymer/QD LEDs to obtain the excellent optical characteristic.40-42 In the past, our group have been investigated the hybrid QD/polymer structure to optimized white LEDs, by discussion, PFO was the good candidate for w-LED and would be used in this study.43-44 Other recognized photoluminescent polymers, such as F8BT, have similar emission wavelengths to those required in this study. However, the quantum yield of F8BT is approximately 50%, which is lower than that of PFO,45 and F8BT is poorly excited at the wavelength of ACS Paragon Plus Environment

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400 nm,46 thus making it unsuitable for the present study. When the spectrum, color quality, and quantum yield were all taken into consideration, PFO was selected for use in the fabrication of a flexible LED that would be compared with QD samples. This study used a QD film consisting of red, green, and blue CdSe/ZnS QDs and a polymer film comprising PFO, PFO-GreenB, and PFO-mono dibenzothiophene (DBT) to develop warm white and neutral white flexible LEDs, respectively. The results demonstrate that using polymers in flexible LEDs yields LEDs with excellent color quality and efficiency.

EXPERIMENTAL METHODS Flexible substrate’s electrode design: Polyimide (PI) is a favorable candidate for the development of reliable flexible substrates. We defined the area of the electrode through wet etching and photolithography, and then coated it with 30 µm of bonding adhesive to facilitate the adhesion of the metal electrode to the substrate. Next, a 14.2 g copper stripe with a thickness of approximately 30 µm was coated onto the substrate using an e-gun. Following the anisotropic conductive adhesive bonding approach, the gap between the bonded anode and cathode pads was set to approximately 100 × 1500 µm2 (L × W).47 The flexible design and top view of the chip bonding is displayed in Supporting Information Figure S2. UV flip-chip bonding on the flexible substrate: First, we prepared UV flip-chips (Lextar Electronics Corporation, Taiwan) with a wavelength of 400 nm and chip size of approximately 1143 × 1143 µm. Next, we bonded the UV flip-chips onto the electrode layout by using a silicone-based anisotropic conductive adhesive (Dexerials Corporation) at a temperature of 230 °C and pressure of 22.5 kg for 180 s. Finally, the flexible LED was molded using 1-mm-thick polydimethylsiloxane (PDMS). After thermal treatment and pressurizing, conduction was induced by the metal particles between the bonding and electrode pads. The anisotropic conductive adhesive filled the gap between the two pads but did not enable conduction because of the pressure in this location. The procedure used to fabricate the UV LED-based flexible substrate is illustrated in Figure 1a–c. ACS Paragon Plus Environment

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Emission layer fabrication: Polymer powder (0.008 g) was blended into toluene (15 mL) and stirred for approximately 10 min. The polymer mixture was mixed with various ratios of the polymers (PFO:PFOGreenB:PFO-DBT = 60:50:1 for 3800 K; PFO:PFO-GreenB:PFO-DBT = 100:100:1 for 5900 K), blended in PDMS at a ratio of 2:1, and then stirred for approximately 30 min. To produce the QD mixture, red, green, and blue CdSe/ZnS QD solutions (5 mg/mL) were prepared at different ratios (red:green:blue = 15:3:2 for 3700 K; red:green:blue = 15:10:1 for 5600 K), blended in the PDMS at a ratio of 2:1), and then stirred for approximately 30 min. The component ratio of each QD mixture was adjusted to have a similar correlated color temperature (CCT) to its PFO counterpart. Finally, we poured the polymer and QD mixtures into different glass molds and baked them at 40 °C for 8 h to produce polymer films and QD films. Because CCT determines how the human eye perceives a light source, we attempted to keep the CCT of each device similar when different emission layers were employed in the LEDs. The component ratio of each QD mixture was adjusted to have a similar CCT to its PFO counterpart. The procedures employed to combine the flexible emission layer and UV LED arrays are displayed in Figure 1d–h. RESULTS AND DISCUSSIONS Figure 2 presents the absorption and photoluminescence (PL) emission spectra of the PFO; GreenB; PFO-DBT; and red, green, and blue QDs. The PFO exhibited a strong emission band, peaking at approximately 420, 440, and 465 nm, and it emitted blue light, as shown in Figure 2a. Chartreuse and red rays were generated by the PFO-GreenB and PFO-DBT samples, respectively, which exhibited strong emission bands peaking at approximately 525 and 675 nm, as displayed in Figure 2b and c. Figure 2d–f presents the PL spectra of the blue, green, and red QDs, exhibiting strong emission peaks at approximately 450, 535, and 630 nm, respectively. The UV flip-chip was employed as a flexible shortwave pumping source because these materials had strong emission peaks in the UV region. When a current of 1.6 mA/cm2 was applied, the emission spectrum of one type of white flexible LEDs with QD and polymer mixture films yielded neutral white light, as indicated in Figure 3a and c, whereas the other type yielded warm white light, as shown in Figure 3e and g. As a function of injection ACS Paragon Plus Environment

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current density ranging from 0.4 to 10 mA/cm2, the luminous efficiency and lumen flux of the neutral white and warm white flexible LEDs are presented in Figure 3b and d and Figure 3f and h, respectively. The inset photographs in Figure 3 show the neutral white and warm white PDMS layers fabricated from QDs or polymers. Both the polymer-fabricated neutral white and warm white flexible LEDs exhibited higher luminous efficiency than the QD-blended flexible samples (99.8% enhancement for the neutral white and 95.5% enhancement for the warm white light; obtained at a current of 3.8 mA/cm2). The luminous efficiency saturation as the current injection was increased was caused by a decrease in the efficiency of the UV chip. Figure S3 shows that the efficiency depends on the current density variation of a UV-based flexible LED. The 8.4% efficiency decrease at high current injection was a result of heating of the active region, carrier leakage, and Auger recombination.48-49 Carrier losses occurred either inside or outside the quantum well region due to nonradiative recombination. The phenomenon had little effect at low current but became dominant at high current injections.50 The quantum yield of the solidand liquid-type QDs and polymer fabricated films of these photoluminescence materials is presented in Table S1. The major concern with a quantum-dot-embedded device is that the quantum efficiency drops when the solvent dries up because of self-aggregation.51-52 In addition to the self-aggregation effect, serious degradation of QD performance is observed when the device is exposed to air because of oxygen and moisture erosion.

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In our previous study, QDs in the liquid-type package for LEDs exhibited

only a 5% decrease in performance after 1000 h of storage; however, once the solvent had dried up, the QDs tended to become the solid type and this led to a more than 70% reduction in performance.55 Thus, QDs in liquid form can be highly efficient, but they lose a large percentage of this efficiency when they solidify. Table S1 lists the measured quantum yields of the solid-type QDs used in this paper. We discovered that the polymer materials had superior quantum yields to the QDs; thus, it is conceivable that the efficiency of a polymer-based device is much higher than that of a QD-based device (Supporting Information, Figure S4). The polymer-fabricated flexible LEDs exhibited higher external quantum efficiency (EQE) than the QD-blended samples at both warm and neutral color temperatures (Supporting information, Figure ACS Paragon Plus Environment

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S5). EQE was determined through an integrated sphere system in which all emitted photons were collected and their characteristics analyzed. The formula for calculating the EQE is as follows.56

(1) where ∫λ/hcP(λ)dλ denotes total number of output photons and I/e indicates the number of electrons injected into the LED per second. In this study, the polymer films yielded higher brightness and quantum efficiency than did the QD films. Figure 4a–d presents the CRI and various Munsell codes (R1–R9) of the warm and neutral white flexible LEDs at a current density of 1.6 mA/cm2. The polymer-film-fabricated flexible LEDs achieved markedly higher CRIs (Ra) and CRI Nos. 1 to 9 (R1–R9) than the QD film samples. The polymer white flexible LEDs exhibited a high CRI of 90 and 96 at neutral and warm color temperatures, respectively. In addition to CRI, CRI R9 is a crucial criterion for lighting and is indicated by ability of strong red (scarlet) color rendering.57 The polymer-fabricated warm white flexible LED exhibited a high R9 (96) at 3800 K, which we believe indicates that it can render the shape and hue of organic tissue. Although the color quality performance of the QD-film-fabricated LEDs was not as expected in this study, the QDbased devices are not necessarily poor. For example, QDs are good for display and yield a large color gamut.58-59 Regarding the CRI, studies have employed 675 QDs (CdS/ZnS:Cu) to optimize a white LED and improve its CRI Ra and R9.60-61 We believe that PL polymers and QDs are promising materials for use in flexible LEDs in the future. The CIE 1931 coordinates of the flexible LED devices in this study are plotted in Figure 4e. All of the flexible samples’ coordinates were within the proximity of the standard Planckian locus and were denoted as white light, which is suitable for many lighting applications. The CRI Ra and R9 variations of the QD-film- and polymer-film-blended flexible LEDs when the current was 0.04–10 mA/cm2 are illustrated in Figure 5. In this study, all CRI and R9 values obtained for the QD-blended white flexible samples were average, irrespective of whether they were driven by a low or high current. By contrast, the polymer-fabricated samples achieved very high CRI (96.6) and R9 ACS Paragon Plus Environment

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(99) when the current density was 5.2 mA/cm2. Moreover, the color quality of the polymer-blended samples not only yielded excellent rendering, but also remained stable when the current was varied. To verify the reliability of our flexible devices, we tested their performance under bending. Bending reliability was obtained when the QD-based and polymer-based flexible LEDs were subjected to different bending curvatures, as displayed in Figure 6a. In the bending test, the flexible samples were bonded onto the surfaces of cylinders with different diameters (D = 80, 60, 50, and 30 mm) (Supporting Information, Figure S6). According to the results, both of the flexible LED samples exhibited excellent bending reliability, as indicated by the luminous efficiency as a function of current, which varied nonsignificantly when the bending diameter was 30 mm. However, we found that once the bending diameter was smaller than 30 mm, the LED devices tended to pop up and detach from the flexible substrate. In addition to the bending test, the samples’ reliability was also tested through a thermal characteristic and lifetime test. The polymer-fabricated LEDs exhibited a lower surface temperature than the QD samples did, probably because the degraded conversion efficiency of QD devices generates more output heat 62-63 (Supporting Information, Fig. S7). CONCLUSIONS This study demonstrated flexible LEDs that generated warm white and neutral white light. White light was generated by the films blended with QDs and with polymer by using a UV flip-chip as the pumping source. The polymer film was blended with PFO, PFO-GreenB, PFO-DBT, and silicon glue and demonstrated higher luminous efficiency than the QD film samples did. Moreover, the polymerfabricated warm white flexible sample exhibited a high CRI (Ra = 96 and CRI R9 = 96). In addition, the flexible LEDs were shown to be highly reliable when subjected to bending, thermal characteristic, and lifetime tests. We believe that these flexible white LEDs, which demonstrate excellent color quality and reliability, are suitable for use as surgery lighting and in special applications such as museum lighting ACKNOWLEDGMENTS: The authors express their gratitude to Epistar Corporation and Dexerials Corporation for their technical support. This research was funded by the Ministry of Science and Technology of Taiwan through grant ACS Paragon Plus Environment

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numbers MOST 101-2221-E-009-046-MY3, MOST104-3113-E-009-002-CC2, and MOST102-2221-E009-131-MY3. Supporting Information Available: The Supporting Information is available free of charge on the http://pubs.acs.org.

For the experiments on the flexible substrate and details of the electrode layout design, the standard and datasheet has been marked; the reliability of the flexible devices, such as their quantum efficiency, depends on the current and storage lifetime characteristic; bending tests using different bending diameters and thermal distribution images have been investigated.

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Figure 1. Procedure used to fabricate (a)–(c) a flexible UV LED-based substrate and (d)–(g) polymer and QD films; (h) the flexible UV substrate was covered to obtain warm white and neutral white flexible LEDs.


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Figure 2. PL emission and absorption spectra of (a) PFO; (b) PFO-GreenB; (c) PFO-DBT; and (d)–(f) blue, green, and red QDs, respectively.


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Figure 3. (a)(c) Emission spectra of the neutral white flexible LEDs fabricated with QD and polymer films. (b)(d) Luminous efficiency obtained using the QD film and polymer film with the current density set from 0.4 to 10 mA/cm2. (e)(g) Emission spectra of the warm white flexible LEDs fabricated with the QD and polymer films. (f)(h) Luminous efficiency obtained using the QD and polymer films with the current density set from 0.4 to 10 mA/cm2 .


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Figure 4. Correlation of CRI values and various Munsell codes for QD film-fabricated (a) neutral white and (c) warm white flexible LEDs, and polymer film-fabricated (b) neutral white and (d) warm white flexible LEDs. (e) The CIE 1931 coordinates for these flexible white LED devices.


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Figure 5. (a)–(d) Variation in the CRI Ra and CRI R9 values for the QD film- and polymer filmfabricated neutral and warm white flexible LEDs with the current density set from 0.04 to 10 mA/cm2 .


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Figure 6. (a) Bending test of the flexible LEDs under various bending curvatures from 30 to 80 mm. The luminous flux and voltage of (b) QD film- and (c) polymer film-fabricated white flexible LEDs.

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Fabrication of Flexible White Light-Emitting Diodes ... - ACS Publications

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