Wide Range Of Doping Concentrations Research Articles

Sterically Wrapped Oxygen‐Bridged Boron Derivatives for High‐Performance Blue Organic Light‐Emitting Diodes

AbstractNear ultraviolet (NUV) organic light‐emitting diodes (OLEDs) have great advantages in the field of light excitation sources, chemical and biological sensors, etc. However, the molecular design of NUV emitters still faces the challenges of spectral broadening and red‐shift. In this study, attaching weak oxygen‐bridged triarylboron acceptor to weak carbazolyl donor is able to address intractable problems by weakening donor–acceptor (D–A) charge transfer. Simultaneously, sterically wrapped and modified oxygen‐bridged triarylboron acceptor by bulk substituents (multiple carbazolyl) can suppress intermolecular π–π stacking through steric hindrance effect. These design strategies are able to obtain short‐wavelength, narrow‐band emission, and great color purity. As a result, carbazolyl derivatives xCz‐BO‐based (x = 2, 3, 4, 5) emitters show EL emission peaks from 406 to 422 nm in the NUV light region and small full‐width at half‐maximums (FWHMs) of 32–41 nm over a wide range of dopant concentrations (10–40 wt.%). The most violet Commission International de I'Eclairage (CIE) color coordinate is (0.16, 0.03). Moreover, 5Cz‐BO can be employed as host for vacuum‐/solution‐processed blue OLEDs and these devices exhibit high performance with EQEs of over 20%.

Growth of heavily doped LiNbO3:Ce crystals, their physicochemical and optical characteristics and defect structure

A series of LiNbO3:Ce crystals was grown using the Czochralski method in a wide range of dopant concentrations ([Ce] = 0.55–2.41 wt% in the melt). The physicochemical characteristics of the crystal-melt Li2O-Nb2O5-CeO2 system during the growth of the LiNbO3:Ce crystals have been studied. Optical characteristics and defect structure of the crystals were studied using the methods of photoinduced light scattering (PILS), laser conoscopy, optical spectroscopy and XRD. In the range of dopant concentrations in the crystal [Ce] ≈ 0.8–0.88 wt%, pronounced threshold effects have been detected in the LiNbO3:Ce crystals. It has been shown that LiNbO3:Ce crystals with a high dopant concentration can be used as “black” lithium niobate – a temperature-independent acoustoelectronic material free from pyroelectric effects.

Stereo effects for efficient synthesis of orange-red multiple resonance emitters centered on a pyridine ring.

Despite theoretical difficulties, we herein demonstrate an effective strategy for the inaugural synthesis of an orange-red multiple resonance (MR) emitter centered on a pyridine ring via stereo effects. Compared to conventional benzene-centered materials, the pyridine moiety in the novel MR material acts as a co-acceptor. This results in a significant spectral redshift and a narrower spectrum, as well as an improved photoluminescence quantum yield (PLQY) due to the formation of intramolecular hydrogen bonds. As envisioned, the proof-of-concept emitter Py-Cz-BN exhibits bright orange-red emission peaking at 586 nm with a small full width at half maximum (FWHM) of 0.14 eV (40 nm), exceeding both the wavelength and FWHM achieved with benzene-centered BBCz-Y. Benefiting from high PLQYs (>92%) and suppressed chromophore interactions, the optimized organic light-emitting diodes achieved high maximum external quantum efficiencies of 25.3-29.6%, identical small FWHMs of 0.18 eV (54 nm), and long lifetimes over a wide range of dopant concentrations (1-15 wt%). The performance described above demonstrates the effectiveness of this molecular design and synthesis strategy in constructing high performance long-wavelength MR emitters.

Cycloalkyl Fused Dibenzofuran Derived Green Ir(III) Complexes Possessing High Horizontal Emitting Dipole Orientation Ratios and Color Stability at High Doping Concentrations

AbstractIn this study, the photophysical characteristics and electroluminescence of three green Ir(III) complexes with newly created cycloalkyl fused dibenzofuran ligands are examined. The results demonstrate that the cycloalkyl fused dibenzofuran ligand can modify the intermolecular interactions and alter the horizontal emitting dipole orientation ratios of dopants in phosphorescent organic light‐emitting diodes (PhOLEDs). One of the three phosphors, Ir(TBF)2(mppy), shows high photoluminescence quantum yield of 0.96 and a high horizontal emitting dipole orientation ratio of 0.80. Therefore, an optimized PhOLED using Ir(TBF)2(mppy) exhibits a peak external quantum efficiency (EQE) of 27.1% and a very small efficiency roll‐off with a high EQE of 25.1% at 10 000 cd m−2. It is significant that the high EQE device maintains the CIE chromaticity coordinates over a wide doping concentration range (3–10 wt%), thus creating new possibilities for the dopant design strategy for the practical use of PhOLEDs.

Role of Metallic Adlayer in Limiting Ge Incorporation into GaN.

Atomically thin metal adlayers are used as surfactants in semiconductor crystal growth. The role of the adlayer in the incorporation of dopants in GaN is completely unexplored, probably because n-type doping of GaN with Si is relatively straightforward and can be scaled up with available Si atomic flux in a wide range of dopant concentrations. However, a surprisingly different behavior of the Ge dopant is observed, and the presence of atomically thin gallium or an indium layer dramatically affects Ge incorporation, hindering the fabrication of GaN:Ge structures with abrupt doping profiles. Here, we show an experimental study presenting a striking improvement in sharpness of the Ge doping profile obtained for indium as compared to the gallium surfactant layer during GaN-plasma-assisted molecular beam epitaxy. We show that the atomically thin indium surfactant layer promotes the incorporation of Ge in contrast to the gallium surfactant layer, which promotes segregation of Ge to the surface and Ge crystallite formation. Understanding the role of the surfactant is essential to control GaN doping and to obtain extremely high n-type doped III-nitride layers using Ge, because doping levels >1020 cm−3 are not easily available with Si.

Developing a two-parabolic band model for thermoelectric transport modelling using Mg2Sn as an example

Thermoelectrics is a field driven by material research aimed at increasing the thermal to electrical conversion efficiency of thermoelectric (TE) materials. Material optimisation is necessary to achieve a high figure of merit (zT) and in turn a high conversion efficiency. Experimental efforts are guided by the theoretical predictions of the optimum carrier concentration for which generally the single parabolic band (SPB) model is used which considers the contribution to electronic transport only from the majority carriers’ band. However, most TE materials reach peak performance (maximum zT) close to their maximum application temperature and when minority carrier effects become relevant. Therefore, single band modelling is insufficient to model the behaviour of TE materials in their most practically relevant temperature range. Inclusion of minority effects requires addition of the minority carrier band and necessitates the use of a two-band model—the simplest and, for most cases, sufficient improvement. In this study, we present a systematic methodology for developing a two-band model using one valence and one conduction band for any given TE material. The method utilises in part the SPB model and in part a simple cost function based analysis to extract material parameters like density of states masses, band gap, deformation potential constant etc., based on easily available experimental data. This simple and powerful method is exemplified using Mg2Sn, chosen due to its low band gap, the availability of experimental data in a wide range of dopant concentrations and its practical importance, being an end member of the highly popular Mg2(Si,Sn) solid solutions. Using the experimental data for p- and n-type Mg2Sn from literature, a two-band model was obtained. Optimum carrier concentration and maximum zT were predicted from both SPB and two-band models and at 650 K pronounced differences between the two models, which could prevent realisation of maximum zT, were observed, demonstrating the practical necessity to model the effect of minority carriers.

Robust Luminescent Molecules with High-Level Reverse Intersystem Crossing for Efficient Near Ultraviolet Organic Light-Emitting Diodes.

Organic light-emitting diodes (OLEDs) radiating near ultraviolet (NUV) light are of high importance but rarely reported due to the lack of robust organic short-wavelength emitters. Here, we report a short π-conjugated molecule (POPCN-2CP) with high thermal and morphological stabilities and strong NUV photoluminescence. Its neat film exhibits an electroluminescence (EL) peak at 404 nm with a maximum external quantum efficiency (ηext,max ) of 7.5 % and small efficiency roll-off. The doped films of POPCN-2CP in both non-polar and polar hosts at a wide doping concentration range (10-80 wt%) achieve high-purity NUV light (388-404 nm) and excellent ηext,max s of up to 8.2 %. The high-level reverse intersystem crossing improves exciton utilization and accounts for the superb ηext,max s. POPCN-2CP can also serve as an efficient host for blue fluorescence, thermally activated delayed fluorescence and phosphorescence emitters, providing excellent EL performance via Förster energy transfer.

Robust Luminescent Molecules with High‐Level Reverse Intersystem Crossing for Efficient Near Ultraviolet Organic Light‐Emitting Diodes

AbstractOrganic light‐emitting diodes (OLEDs) radiating near ultraviolet (NUV) light are of high importance but rarely reported due to the lack of robust organic short‐wavelength emitters. Here, we report a short π‐conjugated molecule (POPCN‐2CP) with high thermal and morphological stabilities and strong NUV photoluminescence. Its neat film exhibits an electroluminescence (EL) peak at 404 nm with a maximum external quantum efficiency (ηext,max) of 7.5 % and small efficiency roll‐off. The doped films of POPCN‐2CP in both non‐polar and polar hosts at a wide doping concentration range (10–80 wt%) achieve high‐purity NUV light (388–404 nm) and excellentηext,maxs of up to 8.2 %. The high‐level reverse intersystem crossing improves exciton utilization and accounts for the superbηext,maxs. POPCN‐2CP can also serve as an efficient host for blue fluorescence, thermally activated delayed fluorescence and phosphorescence emitters, providing excellent EL performance via Förster energy transfer.

Sterically Wrapped Multiple Resonance Fluorophors for Suppression of Concentration Quenching and Spectrum Broadening

AbstractMultiple resonance (MR) emitters are promising for highly efficient organic light‐emitting diodes (OLEDs) with narrowband emission; however, they still face intractable challenges with concentration‐caused emission quenching, exciton annihilation, and spectral broadening. In this study, sterically wrapped MR dopants with a fluorescent MR core sandwiched by bulk substituents were developed to address the intractable challenges by reducing intermolecular interactions. Consequently, high photo‐luminance quantum yields of ≥90 % and small full width at half maximums (FWHMs) of ≤25 nm over a wide range of dopant concentrations (1–20 wt %) were recorded. In addition, we demonstrated that the sandwiched MR emitter can effectively suppress Dexter interaction when doped in a thermally activated delayed fluorescence sensitizer, eliminating exciton loss through dopant triplet. Within the above dopant concentration range, the optimal emitter realizes remarkably high maximum external quantum efficiencies of 36.3–37.2 %, identical small FWHMs of 24 nm, and alleviated efficiency roll‐offs in OLEDs.

Sterically Wrapped Multiple Resonance Fluorophors for Suppression of Concentration Quenching and Spectrum Broadening.

Multiple resonance (MR) emitters are promising for highly efficient organic light-emitting diodes (OLEDs) with narrowband emission; however, they still face intractable challenges with concentration-caused emission quenching, exciton annihilation, and spectral broadening. In this study, sterically wrapped MR dopants with a fluorescent MR core sandwiched by bulk substituents were developed to address the intractable challenges by reducing intermolecular interactions. Consequently, high photo-luminance quantum yields of ≥90 % and small full width at half maximums (FWHMs) of ≤25 nm over a wide range of dopant concentrations (1-20 wt %) were recorded. In addition, we demonstrated that the sandwiched MR emitter can effectively suppress Dexter interaction when doped in a thermally activated delayed fluorescence sensitizer, eliminating exciton loss through dopant triplet. Within the above dopant concentration range, the optimal emitter realizes remarkably high maximum external quantum efficiencies of 36.3-37.2 %, identical small FWHMs of 24 nm, and alleviated efficiency roll-offs in OLEDs.

Investigation on photoluminescence and Judd–Ofelt parameters in Eu ion-doped CaZrO3 in relation to the local structural property

In this study, we investigated the photoluminescence (PL) of Eu ion-doped CaZrO3 in a wide doping concentration range (x = 0.005–0.1). The change in the local structure with Eu3+ doping was analyzed by employing Rietveld refinement on the X-ray diffraction patterns. X-ray photoemission spectroscopy revealed that the valence states of Eu ions are mainly trivalent in the CaZrO3 matrix for all doping concentrations. The emission spectra of our samples showed series peaks originating from the f-f transitions of Eu3+ ions in the red–orange region under ultraviolet photoexcitation. The time-resolved PL measurement demonstrated that the rising time decreased at higher x and that the average decay times were not affected by the doping concentration. The Judd–Ofelt parameters calculated from the PL spectra indicated that the asymmetry ratios changed non-monotonically with the increasing doping concentration of Eu3+, which is in good agreement with the local lattice properties of our samples.

Investigation of NV centers charge states in CVD diamond layers doped by nitrogen and phosphorous

Nitrogen-vacancy color centers in CVD diamond are promising candidates for many quantum applications. In particular, they can be used as sensors for electric and magnetic fields with high sensitivity and nanometer scale spatial resolution. When state readout is performed optically, only the NV− charge state can be used, therefore, it is important to have the maximum concentration of NV centers in the NV− state. In this work, the effect of combined nitrogen-phosphorus doping of diamond on the balance of charge states of NV centers is studied in a wide range of dopant concentrations and at different power of the laser used for photoluminescence excitation. It is shown that the use of a combination of doping with nitrogen and phosphorus makes it possible not only to ensure that NV centers in diamond predominantly exist in the NV− state, but also to stabilize the NV− state under conditions of high power of excitation laser. Cooling with liquid nitrogen allows to further increase the fraction of the NV− state. The obtained results may be applicable for a number of practical tasks, such as optimization of the sensitivity of magnetometers.

Low‐Threshold Lasing from Copper‐Doped CdSe Colloidal Quantum Wells

AbstractTransition metal doped colloidal nanomaterials (TMDCNMs) have recently attracted attention as promising nano‐emitters due to dopant‐induced properties. However, despite ample investigations on the steady‐state and dynamic spectroscopy of TMDCNMs, experimental understandings of their performance in stimulated emission regimes are still elusive. Here, the optical gain properties of copper‐doped CdSe colloidal quantum wells (CQWs) are systemically studied with a wide range of dopant concentration for the first time. This work demonstrates that the amplified spontaneous emission (ASE) threshold in copper‐doped CQWs is a competing result between the biexciton formation, which is preferred to achieve population inversion, and the hole trapping which stymies the population inversion. An optimum amount of copper dopants enables the lowest ASE threshold of ≈7 µJ cm−2, about 8‐fold reduction from that in undoped CQWs (≈58 µJ cm−2) under sub‐nanosecond pulse excitation. Finally, a copper‐doped CQW film embedded in a vertical cavity surface‐emitting laser (VCSEL) structure yields an ultralow lasing threshold of 4.1 µJ cm−2. Exploiting optical gain from TMDCNMs may help to further boost the performance of colloidal‐based lasers.

Defect Structure of Zinc-Doped LiNbO3 Crystals in a Wide Range of Dopant Concentrations

IR absorption spectra of zinc-doped LiNbO3 crystals in the stretching region of OH groups, Raman spectra of the crystals, and their optical absorption spectra provide evidence of drastic changes in the mechanism responsible for the incorporation of Zn into the structure of the crystals and the nature of hydrogen bonds and the corresponding proton-containing charged defect complexes ( $$V_{{{\text{Li}}}}^{ - }$$ –OH–, NbLi–OH−, and $${\text{Zn}}_{{{\text{Nb}}}}^{{{\text{3}} - }}$$ –OH−) near a concentration threshold at ≈6.76 mol % ZnO in the melt.

Structural evolution and phase diagram of the superconducting iron selenides Lix(C2H8N2)yFe2Se2(x=0∼0.8)

Here we report on the structural and electronic phase diagram of lithium and ethylenediamine intercalated FeSe in a wide range of dopant concentration $(x=0\ensuremath{\sim}0.8)$. Undoped ${({\mathrm{C}}_{2}{\mathrm{H}}_{8}{\mathrm{N}}_{2})}_{y}\mathrm{F}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{e}}_{2}$ crystallizes in an orthorhombic phase. With increasing lithium doping, an orthorhombic to tetragonal phase transition occurs at $x=0.35$, and the superconducting tetragonal phase persists until $x=0.5$. Meanwhile, the ${T}_{c}$ is found dependent strongly on dopant concentration, raising rapidly from 30 K at $x=0.35$ to 45 K at $x=0.5$. The crystal structures of $\mathrm{L}{\mathrm{i}}_{0.31(3)}{({\mathrm{C}}_{2}{\mathrm{H}}_{8}{\mathrm{N}}_{2})}_{0.52(7)}\mathrm{F}{\mathrm{e}}_{2.03(2)}\mathrm{S}{\mathrm{e}}_{2}$ are determined by using high-resolution neutron diffraction data at 5, 60, 150, and 295 K, respectively. The distortion of the FeSe tetrahedron is enhanced significantly from 150 to 295 K, meanwhile, the normal-state Hall resistivity changes sign from negative to positive in the same temperature range. The dominant hole carrier in electron-doped $\mathrm{L}{\mathrm{i}}_{0.5}{({\mathrm{C}}_{2}{\mathrm{H}}_{8}{\mathrm{N}}_{2})}_{y}\mathrm{F}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{e}}_{2}$ above 230 K suggests that the temperature-induced structure distortion may lead to a reconstruction of the Fermi surface topology and the appearance of hole pockets.

Molecular beam epitaxy growth and temperature-dependent electrical characterization of carbon-doped GaAs on GaAs(1 1 1)B

Here we present a study of the solid source molecular beam epitaxy growth of carbon doped GaAs(1 1 1)B oriented layers in a wide doping concentration range from 1016 to 1020cm-3. The layers have been fabricated on GaAs(1 1 1)B substrates with a 1° miscut towards (2 1 1) and carbon was supplied by a heated graphite filament source. Atomic force microscopy revealed smooth surfaces (root mean square roughness 0.3 nm) up to the highest carbon concentration. The carbon concentration was determined by secondary ion mass spectroscopy. All samples exhibit p-type conductivity and temperature-dependent Hall- and resistivity measurements show carrier freeze out for doping concentration in the 1016 cm−3 and 1017 cm−3 range, whereas we observe degenerated behavior for carbon concentrations above ca. 1×1019 cm−3. The hole mobility decreases with increasing carbon concentration. The acceptor ionization energy for carbon was determined via low-temperature photoluminescence measurements to 26.3 meV. From the free hole density determined from Hall-measurements at 300 K and the overall carbon concentration, we estimate that approximately 100% of the carbon atoms are electrically active acceptors for carbon concentrations below 1×1018 cm−3. For concentrations above 1×1019 cm−3 compensation sets in and the doping efficiency is well below 100%. For comparison, samples in the standard (1 0 0)-orientation have been produced and carbon incorporation behavior during growth is very similar.

A t-butyl modification approach of acceptor moiety for stable deep blue emission in thermally activated delayed fluorescent devices

A t-butyl modification method of an acceptor moiety was developed as a molecular design approach of blue emitters to shift the emission color to deep blue region and to stabilize the emission color according to doping concentration. A phenyl unit of triphenyltriazine acceptor moiety of blue thermally activated delayed fluorescent emitter was modified with either one or two t-butyl units to study the effect of the t-butyl unit on the electroluminescence emission of the blue thermally activated delayed fluorescent emitters. It was found that two t-butyl units introduced in the phenyl unit of the triphenyltriazine shifted the emission color to deep blue emission while keeping the deep blue color over wide doping concentration range.

Dielectric and piezoelectric properties and electrical conductivity of LiNbO3:ZnO crystals in a wide range of dopant concentrations

The dielectric and piezoelectric properties and electrical conductivity of initially multidomain LiNbO3:ZnO crystals have been studied in a wide range of dopant concentrations with the aim of determining the composition range of the anomalous increase in unipolarity. The results demonstrate that the development of spontaneous unipolarity during high-temperature annealing takes place only in LiNbO3:ZnO crystals grown from melts in a “near-threshold” composition range (~5.4 < Cm ≤ 6.76 mol % ZnO). The effect is accompanied by a considerable and reproducible increase in the static piezoelectric modulus d333. The increase in the piezoelectric modulus, Δd333, rises linearly with increasing jump in electrical conductivity, Δσ, near a temperature T* ≈ 800 K.

Surface characterization of sputtered N:TiO2 thin films within a wide range of dopant concentration

Nitrogen-doped titanium dioxide thin films were deposited using a new approach, which involves the sputtering of a sintered TiN target in a reactive DC magnetron discharge running in O2/Ar mixtures. Based on the higher chemical affinity of titanium for oxygen compared to nitrogen, samples with N concentrations ranging between 1.1 and 9.3 at% were conveniently prepared by adjusting the gas composition in the deposition chamber. Surface characterization of the prepared thin films was done by using X-ray photoelectron spectroscopy, atomic force microscopy and contact angle measurements, while film crystallinity was checked with an X-ray diffraction. The current results show how the dopant content can be controlled by adjusting the oxygen intake in the films during reactive discharge deposition. The XPS spectra clearly show that nitrogen–titanium bonds are formed. Nitrogen doping induces broadening of the valence band, and, consequently, narrowing of TiO2 band gap. Additionally, doping affects the surface morphology, as demonstrated by atomic force microscopy and contact angle data. The effect of carbon contaminant removal under UV irradiation on the hydrophilic conversion process is discussed.

Growth of large LiNbO3〈Mg〉 crystals

We have analyzed conditions for the growth of large (≥80 mm in diameter) LiNbO3〈Mg〉 crystals in a wide range of dopant concentrations in the melt (2.9–5.8 mol % MgO). We have established conditions for the growth of large LiNbO3〈Mg〉 crystals uniform in doping level from melts containing magnesium concentrations above a certain threshold (Cmelt 5.0–5.8 mol % MgO) and optimized the high-temperature electrodiffusion annealing process, which allowed us to obtain single-domain, microstructurally homogeneous, large LiNbO3〈Mg〉 crystals containing 4.9–5.15 mol % magnesium.