Dopant Emission Research Articles

Pure blue phosphorescent platinum(ii) emitters supported by NHC-based pincer type ligands with unitary emission quantum yields

Pincer [Pt(CNHC^CAr^CNHC)L] complexes are excellent emissive dopants and sensitizers for high-efficiency pure blue phosphorescent OLEDs and sky blue phosphor-sensitized fluorescent OLEDs delivering outstanding blue indexes of 229 and 122, respectively.

Single and Dual Doping of Blue‐Emissive ZnSeTe Quantum Dots with Transition Metal Ions

AbstractDoping particularly with transition metal ions is an effectual means to modulate photoluminescence (PL) of quantum dots (QDs). The precedent doped QDs rely primarily on single doping with either Cu+ or Mn2+ into the most common host compositions of (Zn)CdS, ZnSe, and InP, with little success on co‐doping with both impurities enabling their simultaneous emissions. Herein, single and dual doping are explored with Cu+ and/or Mn2+ into blue‐emissive ZnSeTe QDs. PL of the singly doped ZnSeTe QDs synthesized via a co‐nucleation process, comprising host and dopant emissions, is spectrally tuned by varying the concentration of each dopant. For the effective dual doping toward co‐emergence of both dopant‐related emissions, a two‐step doping strategy, where co‐nucleation doping of Mn2+ is temporally decoupled from diffusion doping of Cu+, is devised. In the resulting co‐doped ZnSeTe QDs, successfully showing three distinct emission components, the relative spectral distribution of triple emissions is readily modulated by individually adjusting Cu and Mn concentrations, producing color‐quality tunable white emissions. Upon elaborate multishelling, the co‐doped QDs display outstanding PL quantum yields of 72–75%. Singly and dually doped QDs are further applied for the fabrication of QD light‐emitting diodes, and their electroluminescence is examined compared to PL.

Bis(Triphenylamine)Benzodifuran Chromophores: Synthesis, Electronic Properties and Application in Organic Light-Emitting Diodes.

A series of bis(triphenylamine)benzodifuran chromophores have been synthesized and fully characterised. Starting from suitably functionalized benzodifuran (BDF) precursors, two triphenylamine (TPA) moieties are symmetrically coupled to a central BDF unit either at 4,8-positions through double bonds (1) and single bonds (2) respectively, or at 2,6-positions through double bonds (3). Their electronic absorption and photoluminescence properties as well as redox behaviour have been investigated in detail, indicating that the π-extended conjugation via vinyl linkers in 1 and 3 leads to comparatively strong electronic interactions between the relevant redox moieties TPA and BDF. Due to intriguing electronic properties and structural planarity, 3a has been applied as a dopant emitter in organic light-emitting diodes. A yellowish-green OLED exhibits a high external quantum efficiency (EQE) of 6.2%, thus exceeding the theoretical upper limit most likely due to energy transfer from an interface exciplex to an emissive layer and/or favorable horizontal orientation.

Lead-Free Halide Light-Emitting Diodes with External Quantum Efficiency Exceeding 7% Using Host–Dopant Strategy

Lead-free halide light-emitting diodes (LEDs) are fabricated using nontoxic and earth-abundant CsCu2I3 with a strong yellow emission at a peak wavelength of 568 nm. CsCu2I3-based host–dopant emitters are formed by vacuum thermal evaporation (VTE) film codeposition process instead of the commonly used solution-based film deposition process. Using the VTE process, extremely thin (30 nm) host–dopant emitters have successfully been formed with the CsCu2I3 dopant and various organic host molecules. A bright yellow emission with a photoluminescence quantum yield value of 84.8% is achieved in the 0.5% CsCu2I3-doped halide emitter film due to the successful spatial localization of charge carriers and excitons using an organic host with appropriate energy levels to CsCu2I3. With the further enhancement in charge balance using the cohost system, a record-breaking lead-free halide LED has been fabricated with an EQE of 7.4%. The lead-free halide LEDs are also highly stable in the device operation with LT70 of 20 h at 100 cd/m2.

Luminescent Cu doped CdTe nanocrystals via cation exchange of Cu7Te5 nanocubes: From undoped to doped emission

In II-VI group, the doping in CdTe nanocrystals (NCs) is more difficult than other chalcogenides. In this communication, CdTe nanocrystals containing Cu impurities were carefully synthesized based on controlled reverse cation exchange process between as-prepared Cu7Te5 nanocubes and Cd2+ ions. By well-defined Cu7Te5 nanocubes, the obtained CdTe NCs kept the original morphology. The concentration of Cu impurities in CdTe NCs was controlled by the regulation such reverse cation exchange. Additionally, the regulation from band gap (BG) photoluminescence (PL) to the coexistence of the bandgap emission, dopant emission, and surface-state emission was realized. This tailoring from undoped to doped emission in the case of Cu impurities is helpful to study the Cu related doping in telluride NCs.

High‐Performance Nondoped Organic Light‐Emitting Diode Based on a Thermally Activated Delayed Fluorescence Emitter with 1D Intermolecular Hydrogen Bonding Interactions

AbstractNondoped organic light‐emitting diodes (OLEDs) have drawn enormous attention for their merits of process simplicity, reduced fabrication cost, and phase stability. Herein, a novel approach of utilizing intermolecular hydrogen bonding for enhancing performance of nondoped OLEDs with a new thermally activated delayed fluorescence (TADF) emitter 10‐(4‐(2,6‐di(pyrimidin‐5‐yl)pyridin‐4‐yl)phenyl)‐10H‐phenoxazine (DPmP‐PXZ) is investigated. Endowing with suitable intermolecular hydrogen bonds linking the ends of neighboring DPmP‐PXZ molecules, the molecules tend to form extended 1D molecular chain in solid. This 1D structure effectively keeps the electron‐rich PXZ cores in neighboring molecules apart from each other such that triplet‐related exciton quenching can be well suppressed. In addition, it also improves the balance of carrier mobilities and the optical out‐coupling from the emitting layer. With these merits, OLEDs using DPmP‐PXZ as a dopant emitter, from 10 to 100 wt%, can maintain high external quantum efficiencies (EQEs) of over 20%. More importantly, its nondoped OLED shows a yellow emission and an excellent maximum EQE of 21.8% with little efficiency roll‐off which is even comparable with state‐of‐the‐art yellow OLEDs. These results provide new insight on the role of hydrogen bonding in molecular packing and its subsequent influences on the performance of OLEDs.

Tuning the Photophysical and Electrochemical Properties of Aza‐Boron‐Dipyridylmethenes for Fluorescent Blue OLEDs

AbstractA series of substituted aza‐boron‐dipyridylmethene (aD) compounds are demonstrated as fluorescent dopant emitters in blue organic light emitting diodes (OLEDs). Replacing the meso‐carbon of a dipyridylmethene dye with nitrogen to form the aD chromophore leads to a destabilization of the highest occupied molecular orbital in aD, as evidenced both from their experimentally determined photophysical and electrochemical properties. These properties are consistent with theoretical calculations of the molecular energetics. These aD derivatives emit violet to blue light, peaking between 400 and 460 nm with photoluminescent quantum yields over 85%. The aD compounds have small energy differences (<400 meV) between their singlet and triplet excited states. OLEDs fabricated with an aza‐boron‐dipyridylmethene emitting fluorophore give an external quantum efficiency of 4.5% on glass substrates, close to the theoretical maximum for fluorescent OLEDs.

Investigating HOMO Energy Levels of Terminal Emitters for Realizing High‐Brightness and Stable TADF‐Assisted Fluorescence Organic Light‐Emitting Diodes

AbstractBy simple modification of the functional groups on the boron–nitrogen‐containing skeleton, the energy level of the highest occupied molecular orbital (EHOMO) of emitters can be easily adjusted. Blue‐emission derivatives are developed, which are capable of showing small full width at half maximums and high photoluminescence quantum yields. Blue thermally activated delayed fluorescence (TADF)‐assisted fluorescence organic light‐emitting diodes (TAF‐OLEDs) based on two new emitters as the terminal emitter are fabricated, resulting in high external quantum efficiency (EQE) of up to 21.9%, high color purity, and high brightness (Lmax = 63 777 cd m−2). By analyzing the transient electroluminescence spectra of the TAF‐OLEDs, it is found that a smaller EHOMO difference between TADF‐assistant dopant (TADF‐AD) and terminal emitter efficiently helps to decrease hole trapping inside the emitting layer, hence resulting in a lower efficiency rolloff and a longer operational device lifetime. TAF‐OLEDs based on CzBNCz as a terminal emitter having the closest EHOMO to that of TADF‐AD show a maximum EQE of 21.9% together with a reduced efficiency rolloff (EQEs of 21.2% and 19.8% at 100 and 1000 cd m−2, respectively). This research provides a designing principle for a terminal emitter in TAF‐OLEDs with well‐matched energy levels towards reaching the requirements of commercial displays.

Photoluminescence of ZnO:Eu3+ and ZnO:Tb3+ coatings formed by plasma electrolytic oxidation of pure zinc substrate

Eu3+ and Tb3+ doped ZnO optical coatings were formed by the Plasma Electrolytic Oxidation (PEO) method from the pure Zn sheet, with various dopant concentrations. The morphologies show no significant change with the introduced dopant or their concentrations, with typical appearance of PEO formed coatings. All elements are uniformly distributed, and concentration of incorporated dopants is related to the concentrations of the Eu2O3 or Tb4O7 particles added to the basic electrolyte. Wurtzite structure of ZnO was identified in all the samples, with no change induced by the dopants. Under 260 nm excitation ZnO emits broad-band in the visible region attributed to oxygen interstitial defects and oxygen vacancies. Photoluminescence (PL) of ZnO:X3+ (X = Eu, Tb) is a sum of PL from ZnO and 4f–4f dopant emissions. Eu3+ and Tb3+ are most effectively excited in charge-transfer band and 4f5d levels, respectively. By increasing concentrations of the dopants the broad PL decreases in favor of the 4f–4f dopant transitions, an indication of the energy transfer. ZnO:Eu3+ spectra are dominated by the hypersensitive 5D0→7F2 transition. Eu3+ ions are incorporated in a highly asymmetric environment. PL emission at 541 nm is the most intense in ZnO:Tb3+. Judd-Ofelt intensity parameters rise with increasing Eu3+ concentration, up to Ω2 = 7.8·10−42 cm2 and Ω4 = 4.4·10−42 cm2. Radiative lifetime of 5D0 level drops to 1.12 ms. Chromaticity of PL varies with concentration only when the samples are irradiated by UV light, from white to orange-red or green for Eu3+ or Tb3+ doped samples, respectively. Eu3+ under 394 nm and 464 nm irradiation gives 99.5% color-purity.

Blue Emissive fac/mer‐Iridium (III) NHC Carbene Complexes and their Application in OLEDs

AbstractThe photophysical and electrochemical properties of N‐heterocyclic carbene complexes of Iridium (III) (Ir(C^C:)3, where C^C: = N‐phenyl,N‐methyl‐pyrazinoimidazol‐2‐yl (pmpz), N,N‐di‐p‐tolyl‐pyrazinoimidazol‐2‐yl (tpz)) are reported. Facial and meridional isomers of Ir(pmpz)3 are prepared, but only the facial isomer can be isolated for Ir(tpz)3. The fac‐Ir(pmpz)3 and fac‐Ir(tpz)3 complexes have emission maxima at 465 nm in polystyrene, whereas the emission maximum of mer‐Ir(pmpz)3 is redshifted to 495 nm. The emission energies and photoluminescent quantum yield (ΦPL) in solution decrease on going from nonpolar to polar solvents; however, all the complexes are efficient emitters in polystyrene at room temperature (ΦPL = 88–96%) and 77 K. Blue phosphorescent organic light emitting diodes employing fac‐Ir(tpz)3 as an emissive dopant achieves a high external electroluminescence efficiency (≈18 ± 1%) and brightness (29 000 cd m−2) at low current density.

Controllable molecular doping in organic single crystals toward high-efficiency light-emitting devices

Organic single-crystalline semiconductors have drawn significant attention in the area of organic electronic and optoelectronic devices due to their superiorities of highly ordered structure, high carrier mobility and low impurity content. Molecular doping technique has made great progress in improving device performance via optimizing the optical and electrical properties of organic semiconductors. In particular, this technique has been attempted by taking fluorescent dye-molecules as the emissive dopants to tune emission color and improve device performance of organic single crystals. Up to now, there are few reports about the use of molecular doping in organic single crystals to optimize their intrinsic electrical properties. Here, we have introduced the controllable molecular doping as a feasible approach toward manipulating charge carrier transport properties of organic single crystals. Upon optimization of doping concentration, balanced carrier transport can be realized in 5,5′-bis(4-trifluoromethyl phenyl) [2,2’] bithiophene (P2TCF3)-doped 1,4-bis(4-methylstyryl) benzene (BSB–Me) crystals. Organic light-emitting devices (OLEDs) based on these doped crystals achieve a maximum luminance of 423 cd/m2 and current efficiency of 0.48 cd/A. It demonstrates that high-efficiency crystal-based OLEDs are of great significance for the development of organic electronics, especially for display and lighting applications.

Aqueous synthesis of highly luminescent ternary alloyed Mn-doped ZnSeS quantum dots capped with 2-mercaptopropionic acid

Highly photoluminescent and water dispersible ternary alloyed Mn-doped ZnSeS and core/shell Mn:ZnSeS/ZnS quantum dots (QDs) with pure dopant emission were synthesized through a simple aqueous route using thiolactic acid (2-MPA) as a capping ligand. Transmission electron microscopy and X-ray diffraction show that Mn:ZnSeS nanocrystals are of spherical shape, with a diameter of 2.4 nm and a cubic zinc blende structure. With the overcoating of the ZnS shell, the particle size increases to 3.7 nm, which confirms the epitaxial growth of the shell on Mn:ZnSeS cores. The photoluminescence (PL) quantum yield depends on the Mn loading and reaches 22% for Mn:ZnSeS cores doped with 10% Mn and 41% after the growth of ZnS at the surface of the cores due to the effective elimination of surface-trap states. Mn:ZnSeS QDs exhibit also long PL lifetimes (up to 681 μs) indicating that the emission originates from the spin forbidden Mn2+ 4T1 → 6A1 transition. Electron paramagnetic resonance and X-ray photoelectron spectroscopy results suggest that Mn2+ ions are located at the interface of core/shell Mn:ZnSeS/ZnS QDs. Further, the stability of Mn:ZnSeS/ZnS QDs was also investigated along with their transfer in organic phase using octanethiol.

High Efficiency Sky-Blue Gold(III)-TADF Emitters*.

Highly efficient sky-blue luminescent gold(III) complexes with emission quantum yields up to 82 %, lifetimes down to 0.67 μs and emission peak maxima at 470-484 nm were prepared through a consideration of pincer gold(III) donor-acceptor complexes. Photophysical studies and time-dependent density functional theory (TDDFT) calculations revealed that the emission nature of these gold(III) complexes is most consistent with TADF. Solution-processed OLEDs with these gold(III) complexes as dopants afforded electroluminescence maxima at 465-473 nm with FWHM of 64-67 nm and maximum external quantum efficiencies (EQEs) of up to 15.25 %. This research demonstrates the first example of gold(III)-OLEDs showing electroluminescence maxima at smaller than 470 nm, and highlights the potential of using gold(III)-TADF emitters in the development of high efficiency blue OLEDs and blue emissive dopant in WOLEDs.

Highly efficient Eu3+-activated Ca2Gd8Si6O26 red-emitting phosphors: A bifunctional platform towards white light-emitting diode and ratiometric optical thermometer applications

To fulfill bifunctional applications of the white light-emitting diode (white-LED) and optical thermometry, Eu3+-activated Ca2Gd8Si6O26 phosphors were prepared. Upon 394 nm excitation, dazzling red emission with admirable color purity of 93.8% was seen and the optimal doping content of the Eu3+ ions in the Ca2Gd8Si6O26 host was 30 mol%. The Ca2Gd8Si6O26:2.4Eu3+ phosphors showed a high quantum efficiency of 53.9%. Via investigation of the relation between the dopant content and emission intensity, one knows that the concentration quenching mechanism was decided by electric dipole-dipole interaction. Though estimating the values of Ω2 and Ω4 based on the Judd-Ofelt theory, it was proved that the Eu3+ ions took the low symmetry sites in the Ca2Gd8Si6O26 host lattices. Furthermore, the temperature-dependent emission spectra manifested that the resultant compounds had splendid thermal stability with activation energy of 0.25 eV. Utilizing the prepared compounds as the red-emitting components, a white-LED was packaged and it can emit high quality white light with high color rendering index (84.8) and low correlated color temperature (4981 K). Additionally, through analyzing the temperature-dependent excitation spectrum, the optical thermometric behaviors of the designed phosphors were studied. The maximum absolute and relative sensitivities of the studied compounds were 0.0014 K-1 and 0.276% K−1, respectively. These results provided a new insight into searching for novel bifunctional luminescent materials towards white-LED and ratiometric optical thermometer applications.

Using Fourier-Plane Imaging Microscopy for Determining Transition-Dipole-Moment Orientations in Organic Light-Emitting Devices

We use Fourier-plane imaging microscopy (FIM) to determine the transition-dipole-moment orientation in doped organic emissive thin films. The use of FIM enables precise, sensitive, and rapid measurement of dipole orientation in the emission layer of an organic light-emitting device (OLED). An optical model of a stratified birefringent multilayer is introduced for interpreting results obtained by FIM. Using the model, we determine the average orientation of transition-dipole-moment vectors of three phosphorescent dopant emitters. The dipole alignment measured by FIM quantitatively explains the difference in OLED efficiencies using these archetype dopant molecules. FIM provides a nondestructive tool to measure and ultimately improve the outcoupling efficiency of OLEDs and other light-emitting devices.

Tuning photophysical and electroluminescent properties of phenanthroimidazole decorated carbazoles with donor and acceptor units: Beneficial role of cyano substitution

Four new multi-functionalized carbazole derivatives featuring phenanthroimidazole, N-phenylcarbazole and cyano-substituents are designed and synthesized. The dyes are characterized by photophysical, electrochemical and thermal properties and the dependency of auxiliary chromophores on the functional properties critically analyzed. The dyes showed tunable emission from violet to blue region depending on the chromophore attached to the carbazole core. The cyano-substituted dyes exhibited enhanced intramolecular charge transfer in optical properties as compared to N-phenylcarbazole substituted dyes. Consequently, cyano-substituted dyes displayed solvatochromism in emission and possess large Stokes shift. All the dyes showed remarkable thermal stability attributable to robust phenanthroimidazole and carbazole chromophores. The fluorescent sensory properties of all the dyes toward acid are also explored and found that the emission of cyano derivatives displayed blue shift due to protonation of phenanthroimidazole unit. Finally, the dyes are employed as dopant emitter in multilayer solution-processed organic light-emitting diode which displayed blue electroluminescence.

Fine‐Tuning the Physicochemical and Electroluminescence Properties of Multiply‐Substituted Bipolar Carbazoles by Functional Group Juggling

AbstractThe nature of chromophores often plays a decisive role in imparting properties to functional materials. In this work, we demonstrate that the position of the chromophoric unit on the carbazole core is crucial to tune the photophysical and electroluminescence properties. A series of bipolar compounds featuring carbazole donor and cyano acceptor groups are used as model compounds to evaluate the proposed hypothesis. It is revealed that the conjugation connectivity and chromophore density significantly affects the functional properties of the dyes. A compound featuring carbazole on the C2 and C7 positions of a carbazole core displayed a more bathochromic absorption band than its structural isomer with carbazole on C1 and C8 positions. A major change in emission properties is achieved on changing the position of the carbazole and cyano units on carbazole nucleus. The emission spectra of the dyes are sensitive to solvent polarity and displayed positive solvatochromism attributable to the photoinduced charge transfer from the peripheral carbazole donor to the cyano acceptor. They are demonstrated as dopant emitters in solution‐processed multilayer organic light emitting devices and exhibited deep‐blue or sky‐blue emission with CIE coordinates varying in the range: 0.14≤x≤0.20, 0.07≤y≤0.16. A compound containing a carbazole donor on C3 and C6 and a cyano acceptor on C2 and C7 positions of the carbazole core exhibited the best performance in an OLED with a maximum external quantum efficiency of 3.4 %.

High-Efficiency Narrow-Band Electro-Fluorescent Devices with Thermally Activated Delayed Fluorescence Sensitizers Combined Through-Bond and Through-Space Charge Transfers

Organic light-emitting diodes utilizing thermally activated delayed fluorescence sensitizers and multiple-resonance (MR) dopants may simultaneously offer high efficiencies and narrow-band emissions...

A Multifunctional Blue‐Emitting Material Designed via Tuning Distribution of Hybridized Excited‐State for High‐Performance Blue and Host‐Sensitized OLEDs

AbstractActualizing full singlet exciton yield via a reverse intersystem crossing from the high‐lying triplet state to singlet state, namely, “hot exciton” mechanism, holds great potential for high‐performance fluorescent organic light‐emitting diodes (OLEDs). However, incorporating comprehensive insights into the mechanism and effective molecular design strategies still remains challenging. Herein, three blue emitters (CNNPI, 2TriPE‐CNNPI, and 2CzPh‐CNNPI) with a distinct local excited (LE) state and charge‐transfer (CT) state distributions in excited states are designed and synthesized. They show prominent hybridized local and charge‐transfer (HLCT) states and aggregation‐induced emission enhancement properties. The “hot exciton” mechanism based on these emitters reveals that a balanced LE/CT distribution can simultaneously boost photoluminescence efficiency and exciton utilization. In particular, a nearly 100% exciton utilization is achieved in the electroluminescence (EL) process of 2CzPh‐CNNPI. Moreover, employing 2CzPh‐CNNPI as the emitter, emissive dopant, and sensitizing host, respectively, the EL performances of the corresponding nondoped pure‐blue, doped deep‐blue, and HLCT‐sensitized fluorescent OLEDs are among the most efficient OLEDs with a “hot exciton” mechanism to date. These results could shed light on the design principles for “hot exciton” materials and inspire the development of next‐generation high‐performance OLEDs.

Structural, optical properties, energy transfer mechanism and quantum cutting of Tb3+ doped ZnS quantum dots

Tb-doped ZnS (ZnS:Tb3+) QDs with various Tb concentration have been successfully fabricated by the wet chemical method. The X-ray diffraction (XRD) and transmission electron microscopy have revealed that the synthesized QDs had a zinc blende structure and the particle diameter was about 5 nm. The optical properties were studied through photoluminescence (PL) spectra and time decay curves. XRD, emission spectra and PL decay kinetics have confirmed the successful incorporation of Tb3+ ions into ZnS host. The PL spectra also revealed that the intensity of dopant emission was significantly enhanced due to the energy transfer from the emission of host material. The interaction mechanism and the energy transfer between Tb3+ ions were studied by Inokuti-Hirayama model. The efficiency of the energy transfer process from ZnS host to Tb3+ ions and the nature of this interaction mechanism were determined. For the first time, the quantum cutting process based on energy transfer between Tb3+ ions has also been studied for ZnS:Tb3+ QDs.