A Systematic Approach to Achieving High Performance Hybrid Lighting Phosphors with Excellent Thermal‐ and Photostability

The authors have designed and synthesized a family of high‐performance inorganic–organic hybrid phosphor materials composed of extended and robust networks of one, two, and three dimensions. Following a bottom‐up solution‐based synthetic approach, these structures are constructed by connecting highly emissive Cu₄I₄ cubic clusters via carefully selected ligands that form strong Cu? N bonds. They emit intensive yellow‐orange light with high luminescence quantum efficiency, coupled with large Stokes shift, which greatly reduces self‐absorption. They also demonstrate exceptionally high framework‐ and photostability, comparable to those of commercial phosphors. The high stabilities are the result of significantly enhanced Cu? N bonds, as confirmed by the density functional theory (DFT) binding energy and electron density calculations. Possible emission mechanisms are analyzed based on the results of theoretical calculations and optical experiments. Two‐component white phosphors obtained by blending blue and yellow emitters reach an internal quantum yield as high as 82% and correlated color temperature as low as 2534 K. The performance level of this subfamily exceeds all other types of Cu–I based hybrid systems. The combined advantages make them excellent candidates as alternative rare‐earth element‐free phosphors for possible use in energy‐efficient lighting devices.     

Extremely Efficient White Organic Light‐Emitting Diodes for General Lighting

Highly power‐efficient white organic light‐emitting diodes (OLEDs) are still challenging to make for applications in high‐quality displays and general lighting due to optical confinement and energy loss during electron‐photon conversion. Here, an efficient white OLED structure is shown that combines deterministic aperiodic nanostructures for broadband quasi‐omnidirectional light extraction and a multilayer energy cascade structure for energy‐efficient photon generation. The external quantum efficiency and power efficiency are raised to 54.6% and 123.4 lm W^?1 at 1000 cd m^?2. An extremely small roll‐off in efficiency at high luminance is also obtained, yielding a striking value of 106.5 lm W^?1 at 5000 cd m^?2. In addition to a substantial increase in efficiency, this device structure simultaneously offers the superiority of angular color stability over the visible wavelength range compared to conventional OLEDs. It is anticipated that these findings could open up new opportunities to promote white OLEDs for commercial applications.     

Merging Biology and Solid‐State Lighting: Recent Advances in Light‐Emitting Diodes Based on Biological Materials

Solid‐state lighting (SSL) is one of the biggest achievements of the 20^th century. It has completely changed our modern life with respect to general illumination (light‐emitting diodes), flat devices and displays (organic light‐emitting diodes), and small labeling systems (light‐emitting electrochemical cells). Nowadays, it is however mandatory to make a transition toward green, sustainable, and equally performing lighting systems. In this regard, several groups have realized that the actual SSL technologies can easily and efficiently be improved by getting inspiration from how natural systems that manipulate light have been optimized over millennia. In addition, various natural and biocompatible materials with suitable properties for lighting applications have been used to replace expensive and unsustainable components of current lighting devices. Finally, SSL has also started to revolutionize the biomedical field with the achievement of efficient implantable lighting systems. Herein, the‐state‐of‐art of (i) biological materials for lighting devices, (ii) bioinspired concepts for device designs, and (iii) implantable SSL technologies is summarized, highlighting the perspectives of these emerging fields.     

Tetraphenylimidazole‐Based Excited‐State Intramolecular Proton‐Transfer Molecules for Highly Efficient Blue Electroluminescence

Aiming for highly efficient blue electroluminescence, we have designed and synthesized a novel class of tetraphenylimidazole‐ based excited‐state intramolecular proton‐transfer (ESIPT) molecules with covalently linked charge‐transporting functional groups (carbazole‐ and oxadiazole‐functionalized hydroxyl‐substituted tetraphenylimidazole (HPI), i.e., HPI‐Cbz and HPI‐Oxd, respectively). High T_g (ca. 130 °C) amorphous films of HPI‐Cbz and HPI‐Oxd showed intense and ideal blue‐light emission (λ_max = 462 and 468 nm, Φ_PL = 0.44 and 0.38) with a large Stokes shift of over 160 nm and a narrow full width at half‐maximum of less than 65 nm. Organic light‐emitting devices using HPI‐Cbz and HPI‐Oxd as the emitting layer generated an efficient blue electroluminescence (EL) emission peaking at around 460 nm with excellent CIE coordinates of (x, y) = (0.15, 0.11). A maximum external quantum efficiency of 2.94%, and a maximum brightness of 1 229 cd m⁻² at 100 mA cm⁻², as well as a low turn‐on voltage of 4.8 V were achieved in this work.     

Red Phosphorescent Iridium Complex Containing Carbazole‐Functionalized β‐Diketonate for Highly Efficient Nondoped Organic Light‐Emitting Diodes

A novel red phosphorescent iridium complex containing a carbazole‐functionalized β‐diketonate, Ir(DBQ)₂(CBDK) (bis(dibenzo[f,h]quinoxalinato‐N,C²) iridium (1‐(carbazol‐9‐yl)‐5,5‐dimethylhexane‐2,4‐diketonate)) is designed, synthesized, and characterized. The electrophosphorescence properties of a nondoped device using the title complex as an emitter with a device configuration of indium tin oxide (ITO)/N,N′‐diphenyl‐N,N′‐bis(1‐naphthyl)‐1,1′‐diphenyl‐4,4′‐diamine (NPB; 20 nm)/iridium complex (20 nm)/2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (BCP; 5 nm)/tris(8‐hydroxyquinoline) (AlQ; 30 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) are examined. The results show that the nondoped device achieves a maximum lumen efficiency as high as 3.49 lm W^–1. To understand this excellent result observed, two reference complexes Ir(DBQ)₂(acac), where acac is the acetyl acetonate anion, and Ir(DBQ)₂(FBDK), [bis(dibenzo[f,h]quinoxalinato‐N,C²) iridium (1‐(9‐methyl‐fluoren‐9‐yl)‐6,6‐dimethylheptane‐3,5‐diketonate)], have also been synthesized, and as emitters they were examined under the same device configuration. The maximum lumen efficiency of the former compound is found to be 0.26 lm W^–1 while that for the latter is 0.37 lm W^–1, suggesting that the excellent performance of Ir(DBQ)₂(CBDK) can be attributed mainly to an improved hole‐transporting property that benefits the exciton transport. In addition, a bulky diketonate group separates the emitter centers from each other, which is also important for organic light‐emitting diodes.     

半导体照明关键技术研究

以GaN基功率型发光二极管为核心的半导体照明光源,已经展示了广阔的应用前景,但其流明效率和公认的200 lm/W的路线图目标相比还有较大的发展空间.从材料外延、管芯制作、器件封装和系统应用等方面介绍了半导体照明关键技术的最新进展,其中包括清华大学集成光电子学国家重点实验室的研究成果,并展望了未来的研究方向.     

Molecular Engineering of Blue Fluorescent Molecules Based on Silicon End‐Capped Diphenylaminofluorene Derivatives for Efficient Organic Light‐Emitting Materials

Blue fluorescent materials based on silicone end‐capped 2‐diphenylaminofluorene derivatives are synthesized and characterized. These materials are doped into a 2‐methyl‐9,10‐di‐[2‐naphthyl]anthracene host as blue dopant materials in the emitting layer of organic light‐emitting diode devices bearing a structure of ITO/DNTPD (60 nm)/NPB (30 nm)/emitting layer (30 nm)/Alq₃ (20 nm)/LiF (1.0 nm)/Al (200 nm). All devices exhibit highly efficient blue electroluminescence with high external quantum efficiencies (3.47%–7.34% at 20 mA cm^?2). The best luminous efficiency of 11.2 cd A^?1 and highest quantum efficiency of 7.34% at 20 mA cm^?2 are obtained in a device with CIE coordinates (0.15, 0.25). A deep‐blue OLED with CIE coordinates (0.15, 0.14) exhibits a luminous efficiency of 3.70 cd A^?1 and quantum efficiency of 3.47% at 20 mA cm^?2.     

Built‐In Haze Glass‐Fabric Reinforced Siloxane Hybrid Film for Efficient Organic Light‐Emitting Diodes (OLEDs)

Substrates with high transmittance and high haze are desired for increasing the light outcoupling efficiency of organic light‐emitting diodes (OLEDs). However, most of the polymer films used as substrate have high transmittance and low haze. Herein, a facile route to fabricate a built‐in haze glass‐fabric reinforced siloxane hybrid (GFRH) film having high total transmittance (≈89%) and high haze (≈89%) is reported using the scattering effect induced by refractive index contrast between the glass fabric and the siloxane hybrid (hybrimer). The hybrimer exhibiting large refractive index contrast with the glass fabric is synthesized by removing the phenyl substituents. Besides its optical properties, the hazy GFRH films exhibit smooth surface (R_sq = 0.2 nm), low thermal expansion (13 ppm °C⁻¹), high chemical stability, and dimensional stability. Owing to the outstanding properties of the GFRH film, OLED is successfully fabricated onto the film exhibiting 74% external quantum efficiency enhancement. The hazy GFRH's unique optical properties, excellent thermal stability, outstanding dimensional stability, and the ability to perform as a transparent electrode enable them as a wide ranging substrate for the flexible optoelectronic devices.     

Study of Configuration Differentia and Highly Efficient,Deep‐Blue,Organic Light‐Emitting Diodes Based on Novel Naphtho[1,2‐d]imidazole Derivatives

Two novel naphtho[1,2‐d]imidazole derivatives are developed as deep‐blue, light‐emitting materials for organic light‐emitting diodes (OLEDs). The 1H‐naphtho[1,2‐d]imidazole based compounds exhibit a significantly superior performance than the 3H‐naphtho[1,2‐d]imidazole analogues in the single‐layer devices. This is because they have a much higher capacity for direct electron‐injection from the cathode compared to their isomeric counterparts resulting in a ground‐breaking EQE (external quantum efficiency) of 4.37% and a low turn‐on voltage of 2.7 V, and this is hitherto the best performance for a non‐doped single‐layer fluorescent OLED. Multi‐layer devices consisting of both hole‐ and electron‐transporting layers, result in identically excellent performances with EQE values of 4.12–6.08% and deep‐blue light emission (Commission Internationale de l'Eclairage (CIE) y values of 0.077–0.115) is obtained for both isomers due to the improved carrier injection and confinement within the emissive layer. In addition, they showed a significantly better blue‐color purity than analogous molecules based on benzimidazole or phenanthro[9,10‐d]imidazole segments.     

A Novel Fluorene‐Triphenylamine Hybrid That is a Highly Efficient Host Material for Blue‐, Green‐, and Red‐Light‐Emitting Electrophosphorescent Devices

TFTPA (tris[4‐(9‐phenylfluoren‐9‐yl)phenyl]amine), a novel host material that contains a triphenylamine core and three 9‐phenyl‐9‐fluorenyl peripheries, was effectively synthesized through a Friedel‐Crafts‐type substitution reaction. Owing to the presence of its sterically bulky 9‐phenyl‐9‐fluorenyl groups, TFTPA exhibits a high glass transition temperature (186 °C) and is morphologically and electrochemically stable. In addition, as demonstrated from atomic force microscopy measurements, the aggregation of the triplet iridium dopant is significantly diminished in the TFTPA host, resulting in a highly efficient full‐color phosphorescence. The performance of TFTPA ‐based devices is far superior to those of the corresponding mCP‐ or CBP‐based devices, particularly in blue‐ and red‐emitting electrophosphorescent device systems. The efficiency of the FIrpic‐based blue‐emitting device reached 12 % (26 cd A^–1) and 18 lm W^–1 at a practical brightness of 100 cd m^–2; the Ir(piq)₂acac‐based red‐emitting device exhibited an extremely low turn‐on voltage (2.6 V) and a threefold enhancement in device efficiency (9.0 lm W^–1) relative to those of reference devices based on the CBP host material.     

Modulation of Solid‐State Aggregation of Square‐Planar Pt(II) Based Emitters: Enabling Highly Efficient Deep‐Red/Near Infrared Electroluminescence

The design of square‐planar Pt(II) complexes with highly efficient solid‐state near infrared (NIR) luminescence for electroluminescence is attractive but challenging. This study presents the fine‐turning of excited‐state properties and application of a series of isoquinolinyl pyrazolate Pt(II) complexes that are modulated by steric demanding substituents. It reveals that the bulky substituents do not always disfavor metallophilic Pt···Pt interactions. Instead, π–π stacking among chelates, which are fine‐tuned by the associated substituents, also exerts strong influence to the metal‐metal‐to‐ligand charge transfer (MMLCT) transition character. Theoretical calculations indicate that Pt···Pt contacts become more relevant in the trimers rather than the dimers, especially in their T₁ states, associated with a change from mixed ³LC/³MLCT transition in the monomer/dimer to mixed ³LC/³MMLCT transition character in the trimer. Electroluminescence devices affording intense deep‐red/NIR emission (near 670 nm) with unprecedentedly high external quantum efficiency over 30% are demonstrated. This work provides deep insights into formation MMLCT transition of square‐planar Pt(II) complexes and efficient molecular design for deep‐red/NIR electroluminescence.     

Scalable All‐Evaporation Fabrication of Efficient Light‐Emitting Diodes with Hybrid 2D–3D Perovskite Nanostructures

Quasi‐2D (Q2D) lead halide perovskites have emerged as promising materials for light‐emitting diodes (LEDs) due to their tunable emission, slowed‐down carrier diffusion, and improved stability. However, they are primarily fabricated through solution methods, which hinders its large‐scale manufacture and practical applications. Physical‐vapor‐deposition (PVD) methods have well demonstrated the capability for reproducible, scalable, and layer‐by‐layer fabrication of high quality organic/inorganic thin films. Herein, for the first time, the full‐evaporation fabrication of organic–inorganic hybrid ((BA)₂Cs_n?1Pb_nBr_3n+1) Q2D–3D PeLEDs is demonstrated. The morphology and crystal phase of the perovskite are controlled from 3D to 2D by modulating material composition, annealing temperature, and film thicknesses. The confinement of carriers in 3D layers and the energy funnel effect are discovered and discussed. Importantly, a record high external quantum efficiency (EQE) of 5.3% based on evaporation method is achieved. Moreover, a centimeter‐scale PeLED (1.5 cm × 2 cm) is obtained. Furthermore, the T₅₀ lifetime of the device with an initial brightness of 100 cd m^?2 is found to be 90 min with a thin layer PMMA passivation, which is among the longest for all PVD processed PeLEDs. Overall, this work casts a solid stepping stone towards the fabrication of high‐performance PeLEDs on a large‐scale.     

Barium Hydroxide as an Interlayer Between Zinc Oxide and a Luminescent Conjugated Polymer for Light‐Emitting Diodes

A study of hybrid light‐emitting diodes (HyLEDs) fabricated with and without solution‐processible Cs₂CO₃ and Ba(OH)₂ inorganic interlayers is presented. The interlayers are deposited between a zinc oxide electron‐injection layer and a fluorescent emissive polymer poly(9‐dioctyl fluorine–alt‐benzothiadiazole) (F8BT) layer, with a thermally evaporated MoO₃/Au layer used as top anode contact. In comparison to Cs₂CO₃, the Ba(OH)₂ interlayer shows improved charge carrier balance in bipolar devices and reduced exciton quenching in photoluminance studies at the ZnO/Ba(OH)₂/F8BT interface compared to the Cs₂CO₃ interlayer. A luminance efficiency of ≈28 cd A^?1 (external quantum efficiency (EQE) ≈ 9%) is achieved for ≈1.2 μm thick single F8BT layer based HyLEDs. Enhanced out‐coupling with the aid of a hemispherical lens allows further efficiency improvement by a factor of 1.7, increasing the luminance efficiency to ≈47cd A^?1, corresponding to an EQE of 15%. The photovoltaic response of these structures is also studied to gain an insight into the effects of interfacial properties on the photoinduced charge generation and back‐recombination, which reveal that Ba(OH)₂ acts as better hole blocking layer than the Cs₂CO₃ interlayer.     

Monothiatruxene‐Based,Solution‐Processed Green,Sky‐Blue,and Deep‐Blue Organic Light‐Emitting Diodes with Efficiencies Beyond 5% Limit

The development of blue materials with good efficiency, even at high brightness, with excellent color purity, simple processing, and high thermal stability assuring adequate device lifetime is an important remaining challenge for organic light‐emitting didoes (OLEDs) in displays and lightning applications. Furthermore, these various features are typically mutually exclusive in practice. Herein, four novel green and blue light‐emitting materials based on a monothiatruxene core are reported together with their photophysical and thermal properties, and performance in solution‐processed OLEDs. The materials show excellent thermal properties with high glass transition temperatures ranging from 171 to 336 °C and decomposition temperatures from 352 to 442 °C. High external quantum efficiencies of 3.7% for a deep‐blue emitter with CIE color co‐ordinates (0.16, 0.09) and 7% for green emitter with color co‐ordinates (0.22, 0.40) are achieved at 100 cd m^?2. The efficiencies observed are exceptionally high for fluorescent materials with photoluminescence quantum yields of 24% and 62%, respectively. The performance at higher brightness is very good with only 38% and 17% efficiency roll‐offs at 1000 cd m^?2. The results indicate that utilization of this unique molecular design is promising for efficient deep‐blue highly stable and soluble light‐emitting materials.     

Synthesis of Semiconducting Functional Materials in Solution: From II‐VI Semiconductor to Inorganic–Organic Hybrid Semiconductor Nanomaterials

This Feature Article provides a brief overview of the latest development and emerging new synthesis solution strategies for II–VI semiconducting nanomaterials and inorganic‐organic semiconductor hybrid materials. Research on the synthesis of II–VI semiconductor nanomaterials and inorganic–organic hybrid semiconducting materials via solution strategies has made great progress in the past few years. A variety of II–VI semiconductor and a new family of [MQ(L)_0.5] (M = Mn, Zn, Cd; Q = S, Se, Te; L = diamine, deta) hybrid nanostructures can be generated using solution synthetic routes. Recent advances have demonstrated that the solution strategies in pure solvent and a mixed solvent can not only determine the crystal size, shape, composition, structure and assembly properties, but also the crystallization pathway, and act as a matrix for the formation of a variety of different II–VI semiconductor and hybrid nanocomposites with diverse morphologies. These II–VI semiconductor nanostructures and their hybrid nanocomposites display obvious quantum size effects, unique and tunable optical properties.     

Tetradentate Platinum Complexes for Efficient and Stable Excimer‐Based White OLEDs

A series of tetradentate platinum complexes that exhibit both efficient monomer and excimer emission are synthesized. Via small modifications to the cyclometalating ligands, both the monomer and excimer emission energy can be separately tuned. Devices employing all of the developed emitters demonstrate impressively high external quantum efficiencies (EQEs) within the range of 22% to 27% for concentrations between 2% and 16%. The halogen‐free design of the complexes also enables the fabrication of single, doped, white organic light‐emitting diodes (OLEDs) with long operational lifetimes. A balanced white device employing the complex Pt2O2, achieves a device operational lifetime to 80% of the initial luminance estimated at over 200 h at 1000 cd m^–2, while also achieving 12.5% peak EQE for a warm white light with a color rendering index of 80. Furthermore, a highly doped device exhibiting nearly exclusive excimer emission showed an impressive operational lifetime, which is estimated at more than 400 h for 1000 cd m^‐2.     

Highly Efficient Greenish‐Yellow Phosphorescent Organic Light‐Emitting Diodes Based on Interzone Exciton Transfer

Phosphorescent organic light emitting diodes (PHOLEDs) have undergone tremendous growth over the past two decades. Indeed, they are already prevalent in the form of mobile displays, and are expected to be used in large‐area flat panels recently. To become a viable technology for next generation solid‐state light source however, PHOLEDs face the challenge of achieving concurrently a high color rendering index (CRI) and a high efficiency at high luminance. To improve the CRI of a standard three color white PHOLED, one can use a greenish‐yellow emitter to replace the green emitter such that the gap in emission wavelength between standard green and red emitters is eliminated. However, there are relatively few studies on greenish‐yellow emitters for PHOLEDs, and as a result, the performance of greenish‐yellow PHOLEDs is significantly inferior to those emitting in the three primary colors, which are driven strongly by the display industry. Herein, a newly synthesized greenish‐yellow emitter is synthesized and a novel device concept is introduced featuring interzone exciton transfer to considerably enhance the device efficiency. In particular, high external quantum efficiencies (current efficiencies) of 21.5% (77.4 cd/A) and 20.2% (72.8 cd/A) at a luminance of 1000 cd/m² and 5000 cd/m², respectively, have been achieved. These efficiencies are the highest reported to date for greenish‐yellow emitting PHOLEDs. A model for this unique design is also proposed. This design could potentially be applied to enhance the efficiency of even longer wavelength yellow and red emitters, thereby paving the way for a new avenue of tandem white PHOLEDs for solid‐state lighting.     

Efficient Charge Carrier Injection and Balance Achieved by Low Electrochemical Doping in Solution‐Processed Polymer Light‐Emitting Diodes

Charge carrier injection and transport in polymer light‐emitting diodes (PLEDs) is strongly limited by the energy level offset at organic/(in)organic interfaces and the mismatch in electron and hole mobilities. Herein, these limitations are overcome via electrochemical doping of a light‐emitting polymer. Less than 1 wt% of doping agent is enough to effectively tune charge injection and balance and hence significantly improve PLED performance. For thick single‐layer (1.2 µm) PLEDs, dramatic reductions in current and luminance turn‐on voltages (V_J = 11.6 V from 20.0 V and V_L = 12.7 V from 19.8 V with/without doping) accompanied by reduced efficiency roll‐off are observed. For thinner (<100 nm) PLEDs, electrochemical doping removes a thickness dependence on V_J and V_L, enabling homogeneous electroluminescence emission in large‐area doped devices. Such efficient charge injection and balance properties achieved in doped PLEDs are attributed to a strong electrochemical interaction between the polymer and the doping agents, which is probed by in situ electric‐field‐dependent Raman spectroscopy combined with further electrical and energetic analysis. This approach to control charge injection and balance in solution‐processed PLEDs by low electrochemical doping provides a simple yet feasible strategy for developing high‐quality and efficient lighting applications that are fully compatible with printing technologies.     

Os(II) Based Green to Red Phosphors: A Great Prospect for Solution‐Processed,Highly Efficient Organic Light‐Emitting Diodes

Preparation and characterization of a new class of emissive Os(II) complexes ( 1 – 5 ) have been achieved using 3‐(thiazol‐2‐yl), 3‐(benzothiazol‐2‐yl), 3‐(imidazol‐2‐yl) and 3‐(benzimidazol‐2‐yl) azole chelates, together with the incorporation of one diphosphine ancillary for satisfying the octahedral coordination arrangement. The resulting Os(II) complexes, except for 5 , all show bright emission spanning visible region from green to saturated red, and their structural versus spectroscopic properties have been comprehended by absorption/emission together with computational approaches. Subsequently, a series of solution‐processed OLEDs using 1 – 4 as the dopant were successfully fabricated, demonstrating excellent device performances with external quantum efficiency (EQE) of 15%, current efficiency exceeding 48 cd/A, and power conversion efficiency up to 50 lm/W. Complex 4 is the only Os(II) based phosphor so far suitable for the fabrication of highly efficient green‐emitting OLEDs.