White polymer light-emitting devices for solid-state lighting: materials, devices, and recent progress.

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White Polymer Light-Emitting Devices for Solid-State Lighting: Materials, Devices, and Recent Progress Lei Ying, Cheuk-Lam Ho, Hongbin Wu,* Yong Cao, and Wai-Yeung Wong* mass production of large-area flexible devices.[1–13] According to the standard colorimetric system set up by Commission Internationale de L’Eclairage (CIE) in 1931, all colors are correlated to two coordinates in the system, and ideal white light emission is situated at the equal energy point with CIE coordinates of (0.333, 0.333). Generally, to realize the white light emission, one can mix the three primary colors (red, green and blue, RGB) in a certain proportion, or two complementary lights (orange or yellow and blue) as long as the connection line of their coordinates lies across the white light region. To address this point, various systems including inorganic phosphors,[14] nanocrystals,[15] organic small molecules,[12] polymers[13] and inorganic/organic hybrids[16] are utilized to generate white light emission. In comparison to the white light-emitting device systems which relied on inorganic phosphors or nanocrystals, white polymer light-emitting devices (WPLEDs) can offer more versatile and advanced properties in terms of broadened electroluminescent (EL) spectra, unlimited choices of chemical modifications as well as the employment of low-cost solution processing techniques. In addition, the efficiencies of WPLEDs can potentially be enhanced by tailoring spectral radiation with a high luminous efficacy, which is associated with the spectral power distribution of a white-light source. It should also be noted that for solid-state lighting, all the photons should be taken into account for illumination since they can be redirected to the forward viewing direction by engineering the lighting fixtures.[17] The champion power efficiency (PE), which is defined as the output light power from a device per electrical power input, has been reported up to 100 lm W−1 by using organometallic phosphorescent materials,[18] which is much higher than that achieved with typical incandescent light bulb of ca. 15 lm W−1, indicating a bright future of organic semiconductors for solid-state lighting. Additionally, color quality comprising appropriate color temperature (CT) and high color rendering index (CRI) is also needed to be considered for solid-state lighting based on WPLEDs. The CT is preferred to range within 2500–6000 K for solid-state lighting, which can be determined by comparing the chromaticity of the light source with that of the ideal black body radiator. By considering that organic materials normally present relatively broad EL spectra, the light of WPLEDs generally

White polymer light-emitting devices (WPLEDs) have become a field of immense interest in both scientific and industrial communities. They have unique advantages such as low cost, light weight, ease of device fabrication, and large area manufacturing. Applications of WPLEDs for solid-state lighting are of special interest because about 20% of the generated electricity on the earth is consumed by lighting. To date, incandescent light bulbs (with a typical power efficiency of 12–17 lm W−1) and fluorescent lamps (about 40–70 lm W−1) are the most widely used lighting sources. However, incandescent light bulbs convert 90% of their consumed power into heat while fluorescent lamps contain a small but significant amount of toxic mercury in the tube, which complicates an environmentally friendly disposal. Remarkably, the device performances of WPLEDs have recently been demonstrated to be as efficient as those of fluorescent lamps. Here, we summarize the recent advances in WPLEDs with special attention paid to the design of novel luminescent dopants and device structures. Such advancements minimize the gap (for both efficiency and stability) from other lighting sources such as fluorescent lamps, light-emitting diodes based on inorganic semiconductors, and vacuum-deposited small-molecular devices, thus rendering WPLEDs equally competitive as these counterparts currently in use for illumination purposes.

1. Introduction White organic light-emitting devices (WOLEDs) are receiving much research attention because of their promise for applications in solid-state lighting sources, back-lighting for liquid-crystal displays as well as full-color flat-panel displays due to their unique advantages of light-weight, high solidstate quantum efficiency as well as great potential for the Dr. L. Ying, Prof. H. B. Wu, Prof. Y. Cao, Prof. W.-Y. Wong Institute of Polymer Optoelectronic Materials and Devices State Key Laboratory of Luminescent Materials and Devices South China University of Technology Guangzhou 510640, P. R. China E-mail: [email protected] Dr. C.-L. Ho, Prof. W.-Y. Wong Institute of Molecular Functional Materials Department of Chemistry and Institute of Advanced Materials Hong Kong Baptist University Waterloo Road, Kowloon Tong, Hong Kong, P. R. China E-mail: [email protected]

DOI: 10.1002/adma.201304784

Adv. Mater. 2014, DOI: 10.1002/adma.201304784

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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exhibits high CRI of over 80 and can be perceived as natural white light by the human eye. One of the key issues of applying WPLEDs for solid-state lighting is on the stability, which embraces both color stability and long-term operation stability. The color stability in terms of the bias dependent color shift for WPLEDs is essentially more challenging than their monochromatic counterparts, as the individual color emitters are spatially distributed within the emissive layer. The proposed explanations include the shift of the recombination zones,[19,20] saturation of the long-wavelength emitters,[21] the competition between charge trapping and unperturbed charge transport,[22,23] or the different voltage dependencies of trap-assisted and bimolecular recombination.[24] On the other hand, the long-term operation stability is always associated with the organic semiconductors. However, this can be circumvented through developing more efficient polymeric emitters, reducing device resistance by rationally matching the energy levels of emitters with the host materials and electrodes, controlling the film morphology of each individual layer, or incorporating advanced encapsulating techniques to avoid ambient corrosion by moisture and oxygen.

Lei Ying received his PhD degree from South China University of Technology in 2009. Then he worked at University of California, Santa Barbara as a postdoctoral research fellow. In 2013, he joined South China University of Technology and was promoted to Associate Professor. His current interests include developing new organic semiconductors for optoelectronic devices.

Cheuk-Lam Ho received her B.Sc.(Hons.) (2004) and Ph.D. (2007) degrees both from Hong Kong Baptist University. Her Ph.D. and postdoctoral research in the laboratory of Prof. Wai-Yeung Wong mainly deals with the synthesis of new functional metallophosphors and metallopolymers for light-emitting and photovoltaic applications. She is currently a Research Assistant Professor at Hong Kong Baptist University.

Hongbin Wu is a Professor in South China University of Technology, Guangzhou, China. He received a Ph.D. in materials physics from the South China University of Technology under the supervision of Prof. Yong Cao in 2006 and joined the University as a research fellow in the same year and later was promoted to Professor in 2010. His current research interests focus on polymer optoelectronic devices including light-emitting devices and solar cells.

2. Device Engineering for WPLEDs 2.1. Single Layer WPLEDs The elegant approach that can lead to efficient white light emission involves blending the copolymers in a single layer, which can emit each of the individual colors and give rise to white emission by adjusting the proportion of each component.[25–27] Functionalized polyfluorenes represent one of the most attractive color-tunable polymers for PLEDs applications, because of their high photoluminescence efficiency, relatively good thermal and electrochemical stability. Zou et al. reported efficient and color-stable WPLEDs based on a newly synthesized efficient blue-emitting polymer poly[(9,9-bis(4-(2-ethylhexyloxy) phenyl)fluorene)-co-(3,7-dibenzothiophene-S,S-dioxide)] (PPF3,7-SO10), which can possess dual function as both the host material and blue emitter.[28] Based on an appropriate blending ratio with two typical EL polymers, green-emitting poly[2-(4(3′,7′-dimethyloctyloxy)-phenyl)-p-phenylene-vinylene] (P-PPV) and orange-red emitting poly[2-methoxy-5-(2-ethylhexyloxy)1,4-phenylenevinylene] (MEH-PPV), efficient white emission can be achieved with a relatively high luminous efficiency (LE) of 8.7 cd A−1. The devices showed appropriate CT values of 2500–6500 K and high CRI of 72–79, and quite stable upon a change of current density, stress and annealing at high temperature, which may hold promise for application in solid-state lighting.[28] Given that the improvement of efficiency of individual emission color can effectively enhance the overall efficiency, Yang et al. developed three novel blue, green and red light-emitting conjugated poly(9,9-bis(4-(2-ethylhexyloxy)phenyl)fluorene based copolymers containing dibenzothiophene-S,S-dioxide (FSO) moiety. Figure 1 demonstrates the molecular structures of the copolymers. By physically blending the copolymers PPF-SO (blue), PPF-SO-BT (green) and PPF-SO-DHTBT (red) in a single emissive layer with an optimized composition, a well-balanced

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Yong Cao, a physical chemist, is academician of the Chinese Academy of Sciences, fellow of the World Academy of Sciences for the Advancement of Science in Developing Countries, and Professor of South China University of Technology, China. He obtained his B.S. from Department of Chemistry, Leningrad (now San Petersburg) University and Ph.D. degree from Tokyo University, Japan, respectively. He was visiting senior researcher at University of California, Santa Barbara in 1988–1990 and senior scientist of UNIAX Corporation in 1990–1998. His current research interests focus on polymer optoelectronic materials and the corresponding devices.

Wai-Yeung Wong was born in Hong Kong, and graduated with both B.Sc.(Hons.) (1992) and Ph.D. (1995) degrees from The University of Hong Kong. After his postdoctoral stays at Texas A&M University in 1996 and The University of Cambridge in 1997, he joined Hong Kong Baptist University as Assistant Professor in 1998, rising through the academic ranks to Chair Professor in early 2011. His research focuses on synthetic inorganic/organometallic chemistry, with special emphasis on developing metallopolymers and metallophosphors with energy functions and photofunctional properties.




PROGRESS REPORT Figure 1. Molecular structures of the copolymers, EL spectrum and PE – L characteristics of the device based on PFN as the electron injection layer. Reproduced with permission.[29] Copyright 2013, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

white emission was achieved with CIE coordinates of (0.33, 0.36) and the maximum LE = 6.7 cd A−1 and PE = 5.5 lm W−1.[29] It was also realized that after incorporating a 20 nm thick poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt2,7-(9,9-dioctylfluorene)] (PFN) electron injection layer between the light-emitting polymer blend and the cathode, the WPLED performance can be greatly improved through more efficient electron injection and more balanced charge carrier transport. The maximum LE = 9.8 cd A−1, PE = 8.9 lm W−1 and CIE coordinates of (0.36, 0.37) were realized with a slight roll-off of PE as a function of luminance as illustrated in Figure 1. Moreover, the optimized devices can have an attractive CT close to 4700 K and an excellent CRI > 90. It was also noted that the white emission was almost unchanged as the current density increased from 20 to 260 mA cm−2 with the shift of CIE coordinates < 0.02, which can be ascribed to the formation of amorphous films that can suppress the tendency to phase separation. The excellent stability enabled the resulting WPLEDs to have superior device efficiency, CRI and color stability for solid-state lighting.[29] The key approach to realize high efficiency WOLEDs is to utilize both singlet and triplet excitons for light emission, which can allow for a conversion of up to 100% of injected charges into emitted photons and lead to a theoretical internal quantum efficiency of unity.[30] To address this point, much effort has been focused on physically blending phosphorescent dopants with fluorescent host materials to attain hybrid WPLEDs by utilizing both high-energy singlet excitons and low-energy triplet excitons,[31–39] or blending phosphorescent dyes into nonfluorescent host materials to achieve white emission based on all-phosphorescent emitters.[17,40,41] The former case is desirable for obtaining efficient WPLEDs with high color quality and long-term stability, while the latter one is beneficial for achieving high efficiency. Efficient hybrid WPLEDs based on fluorescent blue and phosphorescent green and red-emitters were achieved via a simple solution-processed procedure by using a deep-blue emitting poly(fluorene-co-dibenzothiophene-S,S-dioxide) (PF-


FSO) as the blue-emitter and host, which was doped with two low-energy phosphorescent iridium complexes. It is interesting to note that despite the low-lying triplet energy level of the host, phosphorescent quenching by the polyfluorene copolymer host can be significantly suppressed with poly(N-vinylcarbazole) (PVK) as the hole-injecting anode buffer layer. White emission with a well-balanced RGB emission from the host and the triplet emitters resulted in white emission color with CIE coordinates of (0.278, 0.312), with a peak LE of 15.1 cd A−1 and a CRI of 79–86.[42] While PF-FSO can have the dual function as both the fluorescent blue emitter and host material, white emission with EL spectra covering the entire visible light region from 400 to 780 nm can be attained by simultaneously incorporating electrophosphorescent sky-blue, yellow and saturated-red emitters with appropriate proportion, affording the “near” pure white CIE coordinates of (0.356, 0.334) and a high CRI of 90. Further device optimization by incorporating a bilayer electrode consisting of a water/alcohol-soluble conjugated polymer and Al electron-injection cathode can lead to an enhancement of 50% in device efficiency, resulting in the peak LE of 21.4 cd A−1 and maximum PE of 15.2 lm W−1.[43] The all-phosphorescent WPLEDs consist of both high and low energy phosphorescent emitters, which can lead to an improved quantum efficiency by utilizing the blue phosphorescent emission so that theoretically 100% internal quantum efficiency can be achieved. Considering that the incorporated high triplet energy phosphorescent emitters may cause back energy transfer to the polymer host materials, thereby high triplet energy level polymer hosts should be employed. The most widely used polymer is the non-conjugated poly(N-vinylcarbazole) (PVK) due to its comparatively high triplet energy level of ca. 2.75 eV which can be used as efficient host for various phosphorescent emitters. All-phosphorescent WPLEDs were achieved by blending sky-blue emitting iridium(III) bis(2-(4,6difluorophenyl)pyridinato-N,C2)picolinate (FIrpic) with the newly synthesized phosphorescent dendritic orange complexes (Ir-D1 and Ir-D2) into the PVK matrix. By incorporating a type



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of low conductivity poly(3,4-ethylenedioxythiophene):poly(styren esulfonate) (PEDOT:PSS, P8000, conductivity of 1 × 10−6 S cm−1) as anode buffer layer, the hole injection can be restricted so that more balanced charge fluxes and broadened recombination regions in the emissive layer can be achieved, which can in turn lead to high efficiency devices with low efficiency roll-off at high luminance. The white emission comprising two complementary colors exhibited a maximum LE and PE of 37.0 cd A−1 and 19.4 lm W−1, respectively with CIE coordinates at (0.359, 0.452).[44] N

N

N N

N

N

Ir

Ir

N

N N

3

N

Ir-D1

3

Ir-D2 Based on PVK as a polymer host, Wu and Wong et al. fabricated devices by doping phosphorescent sky-blue emitter FIrpic and yellow-emitting iridium complexes (Ir(DPA-Flpy)3 and Ir(Flpy)2(acac)) together in an appropriate proportion, and the electron-transport material 1,3,4-bis[(4-tert-butylphenyl)-1,3,4oxadiazolyl]phenylene (OXD-7) was added in the emissive layer to enhance the electron-transport properties. White emission with LE and PE of 40 cd A−1 and 20 lm W−1 and CIE coordinates of (0.395, 0.452) were achieved for Ir(DPA-Flpy)3, respectively, and the resulting efficiency data was among the best value for WPLEDs up to 2009.[45] The higher performance of Ir(DPAFlpy)3 than Ir(Flpy)2(acac) can be attributed to the hole-trapping properties of Ir(DPA-Flpy)3 with diphenylamino groups. Specific advantages of these WPLEDs include the single-emissive-layer device architecture based on solution-processing technology, and the achieved white emission is located in the white-yellow area, which is suitable for lighting instead of a perfect white emission for display applications. Here, no additional holeblocking or electron-transport layer is necessary through extra vacuum-deposited strategy, which allows full exploitation of the low-cost fabrication of polymer-based optoelectronic devices. O

Further investigation of WPLEDs relied on additive color mixing from a combination of the efficient electrophosphorescent blue (B) emitter (FIrpic), the green (G) emitter (iridium(III) tris(2-(4-tolyl)pyridinato-N,C2), Ir(mppy)3), and a new yellowemitting (Y) iridium complex functionalized with sterically hindered diarylfluorene chromophore (Ir(DPF)3) (R = H, Me) and a red (R) iridium(III)-based dendrimer (Ir-G2).[46] The CIE coordinates of WPLEDs at different blend ratios of the emitters and the molecular structures of Ir(DPF)3 and Ir-G2 are shown in Figure 2. Even though the initial efforts by blending two complementary blue and yellow emitters in PVK host attained high luminous efficacy, the resulting moderate color quality with CRI between 52 and 56 is not impressive. To avoid the trade-off problems between high LE and color quality, the BGYR fourcolor systems were prepared and the optimized device showed maximum LE of 45.4 cd A−1 and PE of 20.2 lm W−1 with the color coordinates of (0.304, 0.506). By further applying low conductivity PEDOT:PSS (P8000) as the anode buffer layer, the peak forward-viewing LE was increased to 60.1 cd A−1 at 5.7 V and the peak forward-viewing efficiency was increased to 37.4 lm W−1 at 4.8 V.[47] However, it was found that the obtained WPLEDs degraded significantly during operation, which can be attributed to the poor stability of PVK host and the blue emitter FIrpic. Therefore, in order to find practical applications for WPLEDs in the next stage of development, rational design and synthesis of more stable polymer host materials and blue emitters is indispensable. On the basis of the structural framework of sky-blue FIrpic, two new blue-emitting iridium complexes of POFIrpic and SOFIrpic were designed and synthesized by introducing the polarized P=O or S=O moieties into the 3′-position of the 2-(2′,4′-difluorophenyl)pyridine (dfppy) to enlarge the energy gap, which resulted in a blue shift of the peak emission to 460 nm for single component device (with CIE coordinates of (0.168, 0.294) and (0.176, 0.294) for POFIrpic and SOFIrpic, respectively). By blending POFIrpic with the phosphorescent green emitter (fac-tris[2-phenylpyridinato-C2,N]iridium(III), Ir(ppy)3) and red emitter (bis(1-phenyl-isoquinoline)(acetylacetonate)iridium(III), Ir(piq)2(acac)) into the PVK matrix, whiteemitting device comprising three emitting components of POFIrpic, Ir(ppy)3 and Ir(piq)2(acac) showed a maximum LE of 25 cd A−1 and an improved CRI value of 82.[48] In comparison to the previously reported device based on FIrpic as the blue emitter,[17] the POFIrpic-based device showed slightly improved device performance and CRI value.

O

N

N

Ir Ir

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4


F

O

2

N

Ir(DPA-Flpy)3

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F O P

Ir(Flpy)2(acac)

Ir

2 POFIrpic

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F O O S

Ir

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Even though there are various disadvantages of using nonconjugated polymers including low charge carrier mobility and substantial degradation during long-term operation,




conjugated polymer hosts with high triplet energy levels and appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are comparatively rare. Pei et al. reported a new type of conjugated polymer host PmPTPA by attaching a triphenylamine unit to poly(m-phenylene) backbone.[49] PmPTPA exhibited relatively high triplet energy of 2.65 eV since the phenyl rings adjacent to the other biphenyl structure are essentially twisted.[50] Meanwhile, tethering of the triphenylamine unit leads to the HOMO energy level of –5.35 eV that can facilitate hole injection. Efficient blue light-emitting device based on PmPTPA as host and FIrpic as dopant was achieved with maximum LE of 17.9 cd A−1 and the triplet energy back transfer can be prevented due to the high triplet energy level of the host. Subsequently, WPLEDs with phosphorescent blue, green and red dopants dispersed in PmPTPA showed maximum LE of 22.1 cd A−1, which is comparable to those achieved by non-conjugated PVK host. The LE – J characteristics of the devices based on PmPTPA as the host are shown in Figure 3. Recently, it was recognized that despite the presence of the low-lying triplet states, conjugated polyfluorenes and their derivatives can be utilized as the host for green


PROGRESS REPORT

Figure 2. The CIE coordinates of the devices shown in the CIE 1931 chromaticity diagram based on the yellow-emitting iridium complex Ir(DPF)3 (R = Me) and red-emitting Ir-G2. Reproduced with permission.[47] Copyright 2011, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

phosphorescent complex fac-tris(2-(4-tertbutyl)-phenylpyridine)iridium (Ir(t-BuPPy)3), as long as there is a thin layer of PVK incorporated as the anode buffer layer.[51] The reduced phosphorescent quenching was also found to be associated with the exciton formation and charge carrier recombination within the PVK layer and the PVK/poly(9,9dioctylfluorene) (PFO) interface due to the accumulation of holes.[52] The attained LE of 26.4 cd A−1 for sky-blue emission was among the best result reported to date based on conjugated polymer as the host, and the lower turn-on voltage and higher PE are attributed to the higher mobility of PFO. Through double doping with a yellow-emitting metallophosphor Ir(DPF)3 (R = H), WPLEDs with maximum LE of 40.9 cd A−1 and CIE coordinates of (0.32, 0.48) can be achieved.[52] It was also reported that by incorporating a yellow phosphorescent iridium complex of higher triplet energy (triplet energy level, ET = 2.2 eV) into the amyloid fibrils with a low triplet energy polyfluorene derivative (ET = 2.1 eV) as the matrix, the undesired Dexter back transfer process from phosphorescent emitters to the polyfluorene matrix can be effectively suppressed, leading to a white light emission with high CRI of 92.[53] These findings broadened the selection of polymer hosts available for highly efficient phosphorescent blue-emitting devices and can find potential applications in full color displays and solidstate lighting applications in the future.

Figure 3. LE – J characteristics of blue- and white-emitting devices based on PmPTPA as the host and molecular structure of PmPTPA Reproduced with permission.[49] Copyright 2011, American Chemical Society.



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Figure 4. Schematic view of the multi-layered WPLEDs together with the chemical structures of the active species used as the emitters. The bottom graphs are the EL spectra recorded at increasing driving voltage and the corresponding CIE coordinates. Reproduced with permission.[57] Copyright 2013, American Chemical Society.

2.2. Multi-layered WPLEDs Other than white emission from the single emission layer, the fabrication of multi-layered WPLEDs by solution processing remains a great challenge, since the successive deposition of each active layer is detrimental to the surface morphology of the preformed emissive layer. Also, it renders mixing of the two layers due to the similar solubility of the conjugated polymers. One effective way to circumvent this problem is to use materials with different solubility so that they can be deposited from orthogonal solvents.[54,55] Xu et al. reported the bilayer WPLEDs by spin-coating PFO and P-PPV mixture from p-xylene solution on top of the preformed PVK and poly[(9,9-dioctyl-2,7-fluorene)co -5,5-(4′,7′-di(3-hexyl-thien-2-yl)-2′,1′,3′-benzothiadiazole)] (PFO-DHTBT) layer, since the PVK and PFO-DHTBT mixture can be barely dissolved in p-xylene during the spin-coating procedure. The relative intensity of the green and blue bands depend on the blend ratio. By adjusting the weight ratio of the components in both layers, the EL spectrum of the device can be controlled and white emission with LE of 4.4 cd A−1 was achieved with CIE coordinates of (0.33, 0.32).[56] Bolink et al. demonstrated solution-processed bilayer WPLEDs by depositing blue-emitting polymer CB02 (obtained from Merck OLED Materials GmbH) using mesitylene as the solvent on the top of a highly efficient ionic transition-metal complex (iTMC) of 6-phenyl-2,2′-bipyridine(bis[2-(phenyl)pyridinato] iridium(III) hexafluorophosphate ([Ir-(ppy)2(Hpbpy)](PF6)) layer, and the corresponding schematic view of the multi-layered WOLEDs devices and their EL behavior are illustrated in Figure 4.[57] Since the hydrophilic iTMC has a poor solubility in organic

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solvents, the extremely low polarity of mesitylene ensures no etching of the underlying hydrophilic [Ir(ppy)2(Hpbpy)](PF6) layer during the processing of CB02, and white emission with CIE coordinates of (0.324, 0.337) can be realized by fine tuning of the thickness of both light-emitting layers as well as the driving voltage.[57] It is also worth mentioning that the multi-layered WPLEDs can be attained based on a cross-linking strategy, rendering the preformed light-emitting polymer layers insoluble through post-deposition treatment. It has been demonstrated that by integrating a cross-linkable azide functionalized polyfluorene derivative onto the inorganic near-ultraviolet InGaN/GaN lightemitting diode platform, high quality white light emission with a high CRI of 91 can be achieved after a post-thermal induced cross-linking of the azide functionalized polyfluorene layer.[58] Meerholz et al. reported that the cross-linking occurs by irradiating the film with UV-light in the presence of a photoacid to generate protons which can lead to cationic ring-opening polymerization (CROP) of the oxetane.[59,60] Further investigation demonstrated that the presence of protons in commercially available Baytron P poly(3,4-ethylene dioxythiophene)/ poly(styrene sulfonate) (PEDOT:PSS) can act as a photoacid, which can subsequently cause a surface-induced cross-linking reaction of oxetane-functionalized materials with the reactive chain ends remaining exposed on the surface, and allow for the cross-linking of another layer of oxetane-functionalized material on the top. Of particular interest is that this kind of layer by layer cross-linking approach can result in more stable devices in terms of increased lifetime than the photo-initiated crosslinking method, since no radical cations are involved and the




2.3. Improvement of Device Performance with the Use of Interfacial Layer It has been realized that water/alcohol-soluble PFN and its derivatives can be used as an efficient cathode buffer layer to improve the performance of WPLEDs.[29,42] The optimization of WPLEDs with PFN interlayer by incorporating a low conducting PEDOT:PSS achieved a more balanced hole/ electron transport in the active layer, leading to distinctly improved device performance.[64] Gong et al. reported efficient multi-layered WPLEDs in which an emissive layer consists of a luminescent polyfluorene host of PFO-ETM (PFO endcapped with 5-biphenyl-1,3,4-oxadiazole), poly[(9,9-dioctyl-2,7fluorene)-co-(2,2-fluoren-9-one)] (PFO-F, containing 1 mol% of 2,2-fluoren-9-one) and an iridium complex of tris(2,5-bis2’-(9’,9’-dihexylfluorene)pyridine)iridium(III) (Ir(HFP)3), which was sandwiched between a water-soluble PVK derivative (PVKSO3Li) as a hole-injection/hole-transport layer (HIL/HTL) and a water-soluble PBD derivative of 4-(5-(4-tert-butylphenyl)-1,3,4oxadiazole-2-yl)biphenyl-4’-yl sulfonic acid (t-Bu-PBD-SO3H) as an electron-injection/ electron-transport layer (EIL/ETL).[32] Each layer was spin-cast sequentially from orthogonal solvent due to the discrepant solubility C8H 17 C8H 17 of each species. It was noted that the selected HIL/HTL and EIL/ETL materials can effecN tively render the band alignment that would favor charge injection from the electrodes to PFN result in good hole and electron transport to the emissive layer, as well as confine the charge carriers inside the emissive layer for recombination.[32] Additionally, the incorporated PVK-SO3Li and t-Bu-PBD-SO3H layers can block the electrode metals from diffusion into the emissive layer, and concomitantly protect the emissive layer against ambient contamination. Therefore, these multi-layered PLEDs emit illumination-quality white light with high brightness of 19 500 cd m−2 and LE of 9.5 cd A−1 at the current density of 200 mA cm−2, together with high CRI of 92 and CT value of ∼6400 K which are insensitive to the applied voltage, current density and luminance.[31] An et al. demonstrated that for WPLEDs with a water/ alcohol-soluble polymer PFN as the EIL, the solvent used for casting PFN layer plays a significant role in the device performance. Dramatically improved device performance with a peak


PROGRESS REPORT

decomposition products of the photo-initiator can be avoided in the active OLED layers, and the positive effects are more pronounced for the emissive layer than the hole-transport layer. Consequently, multi-layered WPLEDs consisting of separate fluorescent red-, green-, and blue-emitting layers were fabricated, which showed a LE of 4.5 cd A−1 at a brightness level of 100 cd m−2.[61] An additional strategy addressing to the processing challenge is based on the utilization of light-emitting polymers with highly fluorinated side chain,[62] which provides a great potential for the fabrication of multi-layered WPLEDs due to their unique solubility in fluorinated solvents as well as hydrophobic and lipophobic characteristics.[63]

LE of 18.5 cd A−1 for forward-viewing was realized based on a solvent mixture of water:methanol (1:3, v:v), while devices based on a pure methanol solution showed degraded performance. The variation in device performance can be attributed to the washing out of OXD-7 component in the emissive layer due to the rinse effect, which resulted in imbalanced charge carrier transport rather than electron injection, as can be further supported by the built-in potential measurement.[65] The analogous processing solvent effect was realized when conjugated oligoelectrolyte of fluorene trimer bearing (N,N,N-trimethylammonium)hexyl substituents and tetrakis(1-imidazolyl)borate counter ions (FFF-BIm4) was used as the EIL for WPLEDs.[66] Jen et al. also reported that by inserting a water/alcohol soluble neutral conjugated surfactant of poly(9,9-bis(6’(diethanolamino)hexyl)-hexyl)fluorene] (PFN-OH) between the emissive layer and Al cathode, highly efficient white phosphorescent PLEDs can be achieved.[67] It was also realized that the morphology of PFN-OH thin film could be significantly altered by varying the solvent for spin-coating, and the maximum LE can be dramatically increased from 4.83 cd A−1 for device using smooth and uniform PFN-OH film to 20.7 cd A−1 for device using rough and aggregate PFN-OH film, which can be attributed to the improved hole-blocking ability due to mixed solvent-induced aggregation of the PFN-OH chain.[68] They also demonstrated that the electron-transporting ability of a water/ alcohol-soluble neutral conjugated polymer poly[9,9-bis(2-(2-(2diethanolaminoethoxy)ethoxy)ethyl)fluorene] (PF-OH) can be dramatically improved upon doping with Li2CO3 salt, while the current density of hole carrier was significantly decreased in the doped PF-OH layer, as confirmed by the measurements of electron-dominate and hole-dominate devices, respectively. Very high efficiency WPLEDs with a maximum forward viewing LE of 36.1 cd A−1 (61.4 cd A−1 for total viewing) and PE of 23.4 lm W−1 (39.8 lm W−1 for total viewing) were achieved.[69]

n

n

n N

HO

OH

OH

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N PFN-OH

OH

O P O O

O O PO PF-EP

Another efficient ethanol-soluble conjugated polymer that can be used as efficient EIL/ETL is the phosphonate-functionalized polyfluorene poly[9,9-bis(6’-diethoxylphosphorylhexyl) fluorene] (PF-EP). WPLEDs based on a three-color white–emitting polymer[70] as the single emissive layer and PF-EP/LiF/Al instead of Ca/Al as the cathode, achieved more efficient electron injection and the coordinated protecting effect of PF-EP from diffusion of Al atoms into the emissive layer and excitonquenching near cathode interfaces, and gave forward viewing LE of 15.4 cd A−1 and CIE coordinates of (0.37, 0.42) with CRI of 85, which represents the best results to date among the nondoped WPLEDs.[71] By simply doping Li2CO3 in an alcohol-soluble neutral conjugated PF-EP, it was found that the phosphate groups can make Li ions movable in the PF-EP film, resulting



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Figure 5. Molecular structures of triarylamine-based hole-transporting materials, and the energy diagram and the exciplex mechanism of the device. Reproduced with permission.[79], Copyright 2011, American Chemical Society.

8

in a great improvement in the electron injection and transport ability of PF-EP.[72] However, while most of the interfacial layers are mainly focused on the cathode, effort devoted to the anode buffer layer including hole injection or transport layer is limited,[73–75] thus providing great opportunities for the further improvement of device performance by anode decoration.

tate ligand O^N^C^N (O^N^C^N = 5,5-dibutyl-2-(2,4-difluoro-5(pyridin-2-yl)phenyl)-5H-indeno[1,2-b]pyridin-9-olate, which provided excimer emission located at ca. 620 nm. Coupled with the high-energy vibronic structured emission band at 480–520 nm, high-efficiency WPLEDs with the maximum LE of 17.0 cd A−1, CIE coordinates of (0.43, 0.45) and CRI of 78 were achieved.[80]

2.4. Excimer- or Exciplex-based WPLEDs

3. Single White Light-emitting Polymers

For multi-layered PLEDs, the transient bimolecular excited state (exciplex) may be formed at the heterojunction interface of the hole transport layer and emissive layer, if electron is confined in the emissive layer while hole is still located in the hole transport layer.[76] Excimer is an emissive excited state whose wavefunction overlaps two adjacent molecules of like composition,[2] and its emission is typically located in the yellow part of the visible spectrum.[77] Even though the exciplex emission is fundamentally undesirable for the achievement of good color purity of monochromatic light-emitting devices, it can be judiciously utilized as the low energy band emission to compensate for the high energy band blue emission so that the entire visible spectrum can be covered to present white emission.[78] To address this point, Cheng et al. utilized fluorenone defects in PFO to induce exciplex emission in the multi-layered device containing triarylamine-based cross-linkable hole-transporting materials (HTMs, with the molecular structures shown in Figure 5).[79] When DV-Me-TPD was used as HTM, the peak emission wavelength due to the exciplex emission was located at 609 nm, which is an appropriate red-colored component for white light emission. By doping a small amount of green emitter 4,7-bis(9,9dihexylfluoren-2-yl)-2,1,3-benzothiadiazole (BFBT) into PFO, white emission comprising three primary colors was achieved based on device with a configuration of ITO/PEDOT:PSS/DVMe-TPD/PFO:BFBT/Cs/Al (Figure 5), which showed a maximum LE of 5.28 cd A−1 with CIE coordinates of (0.32, 0.42).[79] In addition to the fluorescent exciplex-based WPLEDs, phosphorescent excimer or exciplex can also be utilized to reach white emission in conjunction with efficient phosphorescent blue-light emitters. Che et al. reported a new cyclometalated platinum(II) complex supported by a fluorinated rigid tetraden-

Single white light-emitting polymer systems are of particular interest due to their exclusive advantages of relatively stable EL spectra and slow roll-off of device efficiency, since the intrinsic phase separation for the single-layer polymer-blend system and the interfacial mixing of different layers for the multi-layered device architectures can be effectively suppressed. However, the realization of single white light-emitting polymers is synthetically challenging. The principal strategy for the molecular design of such single white-emitting polymers involves the covalent tethering of chromophores with either two complementary colors (blue and yellow) or three primary colors (blue, green and red) to the main or side chain,[81–94] or employing hyperbranched[95–97] or cross-linked[98] molecular framework. By finetuning the chromophore content through controlling the feed ratio of monomers during copolymerization, simultaneous emission of all incorporated chromophores can be attained by managing energy transfer or charge trapping, which subsequently results in continuous broad EL spectra for the white light. It is also worth noting that a variety of strategies have been employed to improve the efficiency of devices based on single whiteemitting polymers, i.e., by utilizing chromophores with high emission quantum yields,[86] covalently attaching low-energy chromophores to the side chain with an alkyl spacer,[85,88,93] thermal annealing of the polymer films before deposition of the cathode,[86,91,99] controlling film morphology[100] or incorporating an additional cathode interfacial layer to facilitate electron injection from cathode,[101] and so forth. Additionally, single white-emitting polymer emission can also be realized from poly[(9,9-dioctylfluorene)-co(dibenzothiophene-S,S-dioxide)] random copolymers of PFOFSO. The incorporated electron-deficient FSO unit can form





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intramolecular charge transfer state with fluorene segments that results in a broad and red-shifted emission band.[102–104] It was recognized that with increasing content of the incorporated FSO unit from 2 to 30 mol%, the EL color changed from PFOlike blue to greenish-white with the CIE coordinates of (0.24, 0.41).[105] By covalently incorporating a small amount of red fluorophore 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole into the main chain of PFO-FSO, the EL spectra exhibited broadened profiles with the CIE point at (0.35, 0.29) shifting toward the

pure white point.[106] A further investigation was to incorporate carbazole moiety into PFO-FSO main chain to generate a bipolar host material, upon which single white-emitting polymers were attained by covalently incorporating a bisphenylamine functionalized 2,1,3-benzothiadiazole derivative as the red emitter.[107] Another set of single white-emitting polymers was proposed relying on the mechanism of electron trapping on host or host pendant instead of the conventional charge trapping on the low-energy dopant moieties, as shown in Figure 6.[108]

Figure 6. The difference between the two mechanisms of charge trapping on dopant and electron trapping on host, and the molecular structure of copolymers with electron-trapping side groups. Reproduced with permission.[108] Copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.




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Phosphonate-functionalized polyfluorenes comprising lowenergy dopants in the main chain were chosen as a platform. By considering that the phosphonate groups on the side chains have stronger electron affinity than those of the low-energy dopants, the electrons are supposed to be confined mostly by the host pendants in the EL process. Therefore, the dopant content has to be improved with respect to the copolymers without electron-trapping effect, since the charge trapping in dopant is efficiently suppressed. The high concentration of the chromophore is at a centesimal level of 1 mol%, which is one order of magnitude higher than that of other intramolecular charge transfer based copolymers (0.01–0.1 mol%). This indicated that the concentration of chromophores of this polymer set did not affect the spectra much, which is beneficial for decreasing the batch-to-batch variation in the synthesis of single white-emitting polymers and enhancing the reliability and reproducibility for mass fabrication. The best white emission was achieved with CIE coordinates of (0.34, 0.35) at 8 V.[108]

1-x

O O P

O

N

O P O O

S

N

x n

0.99

O O PO

O P O O

N

N Se

0.01 n

O O PO

Various processing techniques have been demonstrated to be able to improve the performance of WPLEDs. Analogous to the physically blending systems, the most effective approach to enhance the efficiency of single white light-emitting polymers is to incorporate phosphorescent emitters, so that both singlet and triplet excitons can be used to lead to 100% internal quantum efficiency.[109,110] Cao et al. reported hybrid single white-emitting polyfluorenes by simultaneously utilizing fluorescent blue and green species in the main chain, and phosphorescent red emitter covalently tethered to the side chain. White emission with CIE coordinates of (0.31, 0.34) can be achieved through adjusting the proportion of both green (2,1,3-benzothiadiazole, BT) and phosphorescent red emitters to 0.03 mol%, the resulting copolymer PFBT3-Phq3 exhibited LE of 4.6 cd A−1 with relatively stable EL spectra at various applied voltages as portrayed in Figure 7.[111] There are also a number of single white-emitting polymers reported on the basis of grafted triplet chromophores in conjunction with different backbone to attain a white emission with two complementary colors.[112–114] An alternative approach involves the covalent incorporation of low-energy red emitter into the main chain of the host material, which offers copolymers showing maximum LE of 5.3 cd A−1 and CIE coordinates of (0.32, 0.34) together with high CRI between 84 and 89, indicating their great potential for solidstate lighting.[115] An additional method is to chemically coordinate the phosphorescent iridium complex core into the main chain of polyfluorene via an ancillary β-diketonate ligand.[116,117] This approach provides single white-emitting polymers with a moderate LE of 3.25 cd A−1 and CIE coordinates of (0.28, 0.32).[117] The best-performing single white-emitting polymer

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via simultaneous exploitation of singlet and triplet excitons was achieved by using polyfluorene derivatives containing osmium complex in the backbone, which exhibited LE of 10.7 cd A−1 with CIE coordinates of (0.36, 0.29).[118] Although the utilization of low-energy red or yellow phosphorescent emitters can lead to superior efficiency, single white-emitting polymers containing high-energy blue and green emitters in the conjugated polymers remain challenging, as the host materials of high triplet energy level are scarce. The coil-to-coil type triarylamine (TPA) oxadiazole (OXA) diblock copolymer system that contains blue and red styryl heteroleptic iridium complexes randomly in each different block was developed, and the non-conjugated backbone with TPA and OXA side chains has a relatively high triplet energy level that can act as an efficient host for phosphorescent sky-blue emitter.[119] The site isolation of the two emitters was realized due to the incompatibility of these two blocks, which can be targeted to suppress energy transfer from higher to lower energy emitters, hence eventually providing a self-assembled solution-processed single-layer WPLED with improved color balance.[120] 0.99 0.01 n A fluorinated poly(arylene ether phosN N S phine oxide) backbone with high triplet energy of 2.96 eV and appropriate HOMO/ O LUMO energy levels of –5.7/–2.3 eV was P O O developed by Wang et al. which was successfully used to construct blue electrophosphorescent polymers by covalently tethering FIrpic into the side chain. The blue electrophosphorescent copolymer exhibited LE as high as 19.4 cd A−1, indicating that the fluorinated poly(arylene ether phosphine oxide) can be potentially used as the platform for the development of highperformance all-phosphorescent single white-emitting polymers.[121,122] On the basis of such fluorinated poly(arylene ether

Figure 7. EL spectra of the devices based on copolymer PFBT3-Phq3 at various applied voltages. Reproduced with permission.[111] Copyright 2006, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.




F

O P

O

0.5

F

O P

F

F

N

F O

0.5-x-y

F

O P

F O

F

x

F O

N

(CH2)8

O O

N

O

2

N

the absorption profile of the orange-emitting core, illustrating an improved Förster energy transfer from polyfluorene arm to the core that can lead to comparatively higher LE of 18.0 cd A−1 and CIE coordinates of (0.33, 0.35). The high efficiency of these hyperbranched architectures could be attributed to the suppressed concentration quenching of the dopant units, more efficient energy transfer from polymer host to orange dopant and thermal annealing-induced crystalline α-phase polyfluorene (PF) in an amorphous F O host.[97] P O Despite the various advantages of hypery F branched polymeric platform available to date, the phosphorescent conjugated copolymers O with hyperbranched structures are mainly (CH2)8 monochromatic green-emitting[126,127] and red-emitting[128] polymers, while the single O white-emitting polymer has not been reported O Ir yet. We speculated that as long as hyperN branched structure was involved in all-phosN phorescent single white-emitting polymers, much higher efficiency can be anticipated. To achieve a color-stable white EL spectrum, a cross-linked network that fixes the luminescent chromophores is an ideal structure. However, the poor solubility of cross-linked polymer network inhibits its processability. Ma et al. reported the first white emissive network films formed by in situ electrochemical copolymerization (ECP). Figure 8 illustrates the schematic representation of ECP technique. The films obtained by this method exhibited excellent structural and phase stability because the cross-linked structures in ECP films can fix the position of the molecules and avoid phase separation.[98] Precisely controlled dopant content of the cross-linked ECP film can be achieved through varying the precursor content in the electrolyte solution, which afforded white-emitting devices with well-balanced RGB distribution and full coverage of the whole visible range and an excellent CRI value of 88. The best device performance with LE of 6.7 cd A−1 was attained with CIE coordinates of (0.33, 0.35). It was also realized that the EL spectra were extremely stable over a wide range of driving voltage of 8–22 V, which can be 2

Ir

F

F

F

In addition to the linear molecular structure, hyperbranched π-conjugated polymers also emerged as a new class of materials that have attracted increasing attention for light-emitting applications,[124] since the concentration quenching originated from the strong interchain interaction can be effectively suppressed by the branched structure, while the twisted molecular conformation can lead to superior LE.[125] Wang et al. developed hyperbranched single white EL polymers by combining blueemitting polyfluorene multi-arms with star-shaped orange cores.[95,96] By a fine-tuning of the core composition in conjunction with a careful management of charge carrier trapping and energy transfer from core to arm, single hyperbranched whiteemitting polymer system was developed. Devices based on three-armed polymer exhibited a pure white emission with LE of 16.6 cd A−1 and CIE coordinates of (0.33, 0.36).[95] An additional study by extending the arm number to six resulted in a better overlap of the emission band of polyfluorene arm with

C8H17 C 8H17 p

C 8H 17

C8H17

PROGRESS REPORT

phosphine oxide) backbone, highly efficient all-phosphorescent single white-emitting polymers were designed and synthesized by successively copolymerizing the blue and yellow monomers. Simultaneous blue and yellow triplet emissions were observed to generate white EL with a champion LE of 18.4 cd A−1 (8.5 lm/W, 7.1%) and CIE coordinates of (0.31, 0.43), demonstrating the great potential of all-phosphorescent single white-emitting polymers.[123]

C 8H17

p

C 8H17

C8 H17

C8H 17 q

q

N N

S

C8 H17

S

N

C 8H 17

N S N

C 8 H 17

m NS

N

N

N

N

C8H17

N

i

C8H 17

C 8H17

N S N

C 8H 17

N N

N N S

N

N

C8H17

C 8H17

C 8H 17

C8H 17

C 8H17

C8H 17

C8H 17


i

m

C 8H17 n

C8H 17

h

n


h


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Figure 8. Schematic representation of the ECP technique. (Reproduced with permission.[98] Copyright 2010, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

ascribed to the uniform dispersion of dopant units in the ECP films, and absence of phase separation due to the fixed position of molecules. This kind of ECP film fabrication strategy provides versatile opportunities to develop WPLEDs with efficient emission together with excellent spectral stability.[98]

4. Future Opportunities and Perspectives Efforts devoted to develop novel highly efficient polymer systems and their device engineering have led to enormous progress in white polymer light-emitting devices, which have significantly narrowed the gap with their inorganic and vacuum-processed small-molecule counterparts. From materials point of view, developing novel polymers with appropriate energy levels that can match with the electrodes and maintain charge balance at high current density while simultaneously avoid the negative concentration quenching effects, are highly desirable. In contrast, the device engineering aspects including tandem and inverted structures that can lead to more rational device geometry so that all electrically generated excitons can be well-controlled and utilized are also of pivotal importance. Specific attention should also be paid on the collection of all generated photons since they can be redirected to the forward

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viewing direction by engineering the lighting fixtures. In addition, by considering that the majority of the light generated in the organic materials is confined due to the factors including the total internal reflection, surface plasmon coupling, as well as metal absorption, out-coupling efficiency (ηout) of only approximately 20% was obtained according to the Snell’s law. Consequently, future attention should be focused on developing advanced out-coupling technologies toward highly efficient WPLEDs by reducing wastage of the generated light. Of particular challenge for the commercialization of WPLEDs in the solid-state lighting market is the useful lifetime, which is typically defined as the working time of the device efficiency to be decayed to 95%, 90% or 50% with respect to its initial value under a continuous operation period. However, the lifetime tests at normal operation conditions is really time consuming, while accelerating testing conditions such as enhanced driving voltages or elevated operational temperatures may suffer from spectral stability issue which may lead to less reliable results. Principally, developing highly efficient and stable monochromatic polymeric blue-, green- and red-emitters with high glass transition temperatures is highly favored, since the device degradation starts from the components with shortest lifetime and any changes of monochromatic color may have great influence on the white emission. In addition, optimizing




Acknowledgements L. Ying and C.-L. Ho contributed equally to this work. W.-Y. Wong acknowledges the financial support from the National Natural Science Foundation of China (No. 91222201), National Basic Research Program of China (973 Program) (2013CB834702), Hong Kong Baptist University (FRG2/11–12/156), Hong Kong Research Grants Council (HKBU202410 and HKUST2/CRF/10) and Areas of Excellence Scheme, University Grants Committee of HKSAR, China (Project No. [AoE/P-03/08]). L. Ying, H.-B. Wu, W.-Y. Wong and Y. Cao also thank the National Natural Science Foundation of China (Nos. 51303056, 51373145, 91333206, 51010003, 51225301 and 61177022) and the National Basic Research Program of China (No. 2009CB623602) for the financial support. Received: September 24, 2013 Revised: December 20, 2013 Published online:

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Application of Nanostructures in Electrochromic Materials and Devices: Recent Progress.

Flexible energy-storage devices: design consideration and recent progress.

Neurosurgical materials and devices.

Certification and classification programs for dental materials and devices. List of certified dental materials and devices revised to Jan 1, 1977. Council on Dental Materials and Devices.

Organic Materials for Time-Temperature Integrator Devices.

Shunt Devices for the Treatment of Adult Hydrocephalus: Recent Progress and Characteristics.

New nanobiocomposite materials for bioelectronic devices.

Expansion of the acceptance program for dental materials and devices: alloys for cast dental restorative and prosthetic devices. Council on Dental Materials and Devices.

Hydrogen-bonded supramolecular conjugated polymer nanoparticles for white light-emitting devices.

Ionic current devices-Recent progress in the merging of electronic, microfluidic, and biomimetic structures.

Materials and structures for stretchable energy storage and conversion devices.

Recent microfluidic devices for studying gamete and embryo biomechanics.

Perovskite solar cells: from materials to devices.

Recent developments in paper-based microfluidic devices.

Recent studies in intrauterine devices: a reappraisal.

Free-Standing Conducting Polymer Films for High-Performance Energy Devices.

Recent Progress on PEDOT-Based Thermoelectric Materials.

Fullerene-Grafted Graphene for Efficient Bulk Heterojunction Polymer Photovoltaic Devices.

Amine-Based Interfacial Molecules for Inverted Polymer-Based Optoelectronic Devices.

Stretchable Materials for Robust Soft Actuators towards Assistive Wearable Devices.

Nature-Inspired Structural Materials for Flexible Electronic Devices.

Recent Advances in Electrospun Nanofiber Interfaces for Biosensing Devices.

Recent Progress in Advanced Materials for Lithium Ion Batteries.

Progress and Design Concerns of Nanostructured Solar Energy Harvesting Devices.