Enhanced efficiency of organic photovoltaic cells with Sr2SiO4:Eu2+ and SrGa2S4:Eu2+ phosphors

We report the effect of yellow Sr₂SiO₄:Eu²⁺ and green SrGa₂S₄:Eu²⁺ phosphors on the efficiency of organic photovoltaic (OPV) cells. Each phosphor was coated on the back side of indium tin oxide (ITO)/glass substrates by spin coating with poly(methyl methacrylate) (PMMA). The maximum absorption wavelength of the active layer in the OPV cells was ～512 nm. The emission peaks of Sr₂SiO₄:Eu²⁺ and SrGa₂S₄:Eu²⁺ were maximized at 552 nm and 534 nm, respectively. The short circuit current density (J_sc) and power conversion efficiency (PCE) of the OPV cells with Sr₂SiO₄:Eu²⁺ (8.55 mA/cm² and 3.25%) and with SrGa₂S₄:Eu²⁺ (9.29 mA/cm² and 3.3%) were higher than those of the control device without phosphor (7.605 mA/cm² and 3.04%). We concluded that phosphor tuned the wavelength of the incident light to the absorption wavelength of the active layer, thus increasing the J_sc and PCE of the OPV cells.     

Photoluminescence properties of novel white phosphor of Dy3+-doped LaBSiO5 glass

A single-phased white emitting phosphor LaBSiO₅:Dy³⁺ was successfully synthesized via a solid state reaction. The X-ray diffraction results confirmed that the doped Dy³⁺ ions did not change the lattice structure. Several strong excitation peaks of LaBSiO₅:Dy³⁺ were found around 300–450 nm. Under excitation of 350 nm, the LaBSiO₅:Dy³⁺ exhibited white emission by combining the two emission peaks at 478 and 574 nm corresponding to the typical ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions. The optimal substitution proportions of Dy³⁺ for La³⁺ was determined to be 1 mol% and the critical distance of Dy³⁺ was 25.9628 Å. Moreover, the CIE chromaticity coordinates of LaBSiO₅:Dy³⁺ phosphor was (0.3116, 0.3474) which is close to the ideal white light coordinates (0.333, 0.333), indicating that the phosphor has a potential application as a single component ultraviolet-convertible white light emitting phosphor.     

Giant Red-Shifted Emission in (Sr,Ba)Y2O4:Eu2+ Phosphor Toward Broadband Near-Infrared Luminescence

Near-infrared (NIR) light-emitting diodes (LEDs) light sources are desirable in photonic, optoelectronic, and biological applications. However, developing broadband red and NIR-emitting phosphors with good thermal stability is always a challenge. Herein, the synthesis of Eu²⁺-activated SrY₂O₄ red phosphor with high photoluminescence quantum efficiency and broad emission band ranging from 540 to 770 nm and peaking at 620 nm under 450 nm excitation is designed. Sr/Ba substitution in SrY₂O₄:Eu²⁺ has been further utilized to achieve tunable emission by modifying the local environment, which facilitates the giant red-shifted emission from 620 to 773 nm while maintaining the outstanding thermal stability of SrY₂O₄:Eu²⁺. The NIR emission is attributed to the enhanced Stokes shift and crystal field strength originated from the local structural distortions of [Y1/Eu1O₆] and [Y2/Eu2O₆]. The investigation in charge distribution around Y/Eu provides additional insight into increasing covalency to tune the emission toward the NIR region. As-fabricated NIR phosphor-converted LEDs demonstration shows its potential in night-vision technologies. This study reveals the NIR luminescence mechanism of Eu²⁺ in oxide-based hosts and provides a design principle for exploiting Eu²⁺-doped NIR phosphors with good thermal stability.     

Application research of CdS: Eu3+ quantum dots-sensitized TiO2 nanotube solar cells

Trivalent Eu³⁺-doped CdS quantum dot (CdS: Eu³⁺ QD)-sensitized TiO₂ nanotube arrays (TNTAs) solar cells are prepared by using the direct adsorption method. The influences of sensitization time, sensitization temperature, and Eu³⁺ ion concentrations are investigated systematically. The photo-current of the CdS: Eu³⁺ QDs/TiO₂ nanotubes appear at the main absorption region of 320–480 nm, and the maximum incident photon to the current conversion efficiency (IPCE) value is 21% at 430 nm when the sensitization condition is 4% doping Eu³⁺ concentration, 60 °C sensitization temperature, 8 h sensitization time. Compared with the un-doped CdS QD-sensitized TNTAs, the conversion efficiency and IPCE of CdS: Eu³⁺ QDs/TNTAs are two times and three times than that of un-doped CdS QDs sensitized TNTAs. This scenario exhibits the potential applications of rare earth elements in QD-sensitized solar cells.     

The effects of Ce3+ doping on the structural and optical properties of SrS nanoparticles synthesized by solid state diffusion method

A solid state diffusion method (SSDM) is employed to synthesize the nanocrystalline SrS phosphors doped with varying concentrations of Ce³⁺ ions. The surface morphology and structural properties of as prepared phosphors are characterized by various techniques including X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction pattern (SAED) and energy dispersive X-ray spectroscopy (EDX). The optical properties are investigated and discussed in terms of absorption spectra, photoluminescence (PL) and thermoluminescence (TL) measurements. The cubical crystal structure and average crystallite size estimated by X-ray diffractograms are found to lie in nano-range is also supported by HRTEM analysis. FESEM is used to describe the surface morphology and confirm the crystallite size of as-prepared nanophosphors. The absorption spectra and corresponding Tauc׳s plots confirm the wide band-gap nature of prepared samples. When excited with 375 nm wavelength of ultraviolet (UV) light at room temperature, the SrS:Ce³⁺ nanophosphors exhibit two emission bands at around 459 (blue region) and 551 nm (green region) originating from the ²T_2g(5d)→²F_5/2(4f) and ²T_2g(5d)→²F_7/2(4f) transitions respectively. The results of thermoluminescence (TL) studies in terms of TL glow curves, corresponding kinetic parameters and dopant concentration dependence of intensity for nanocrystalline SrS:Ce³⁺ phosphor are presented and discussed.     

Preparation and luminescence of europium-doped lanthanum fluoride–benzoic acid hybrid nanostructures

Europium-doped lanthanum fluoride (LaF₃:Eu³⁺) nanoparticles were synthesized using a solvothermal method, and they were then capped with benzoic acid (BA) ligands to form LaF₃:Eu³⁺–BA hybrid nanostructures. The LaF₃:Eu³⁺–BA hybrid nanostructures showed strong luminescence as a result of energy transfer from BA to the Eu³⁺ ions of the LaF₃:Eu³⁺ nanoparticles. The dominant excitation band for the LaF₃:Eu³⁺–BA hybrid nanostructures ranged from 200 nm to 300 nm. It has been shown that the luminescence of LaF₃:Eu³⁺–BA hybrid nanostructures strongly depends on the pH value and content of benzoic acid used in the preparation of the hybrid nanostructures. An X-ray diffraction technique, transmission electron microscopy, luminescence spectroscopy, Fourier transform infrared spectroscopy and a UV–vis spectrophotometer were used to characterize the products.     

Thermoluminescence studies of ultraviolet and gamma irradiated erbium(III)- and ytterbium(III)-doped gadolinium oxide phosphors

Present paper deals with the thermoluminescence (TL) properties of Gd₂O₃:Er³⁺, Yb³⁺ phosphor synthesized by a conventional solid state reaction method. The TL measurements were performed under UV and gamma irradiations. The structural and morphological analysis of the resulting phosphor was carried out by X-ray diffraction (XRD) study and field emission gun scanning electron microscopic (FEGSEM) technique. The functional group determination of prepared phosphor was carried out by Fourier transform infrared (FTIR) analysis. Elemental analysis was carried out by energy dispersive X-ray analysis. The particle size was determined by transmission electron microscopic (TEM) technique. For UV irradiation UV source providing 254 nm wavelength was used. Whereas Co⁶⁰ gamma source was used for gamma irradiation. The TL response of Gd₂O₃:Er³⁺, Yb³⁺ phosphor for two different radiations was compared and studied in detail. The process and possible mechanism for TL were investigated and discussed with the help of energy level models. The kinetic parameters such as order of kinetics, activation energy and frequency factors were evaluated by peak shape method and curve fitting technique. Effects of varying concentrations of Er³⁺ and Yb³⁺ was also investigated. The TL studies were further investigated by applying computerized glow curve deconvolution (CGCD).     

Preparation and luminescent properties of novel red phosphors for white-light emitting diodes (W-LEDs) application

A series of Eu³⁺–Gd³⁺ co-doped solid solution of Ca_0.54Sr_{0.46–1.5x–1.5z}Eu_zGd_x (MoO₄)_y (WO₄)_1−y (x=0.01–0.20, y=0–1.0, z=0.01–0.30) have been prepared by solid-state reactions. It is found that appropriate amount of Mo⁶⁺ or W⁶⁺, Eu³⁺ and Gd³⁺concentrations can enhance the luminescent intensity and improve crystal structure. These phosphors can be effectively excited by ultraviolet light at 394 nm and blue light at 465 nm (f–f transition) and emits red light (616 nm) with line spectrum. The wavelengths at 394 and 465 nm are nicely fitted in with the widely applied output wavelengths of ultraviolet or blue LED chips.     

Shape controlled synthesis and formation mechanism of Y2(C2O4)3 and Eu3+–Y2O3 phases via hydrothermal microemulsion

The flat or truncated flat cube Y₂O₃ precursor powders were synthesized in a reverse microemulsion and hydrothermal. Hexadecyl Trimethyl Ammonium Bromide (CTAB) and Organophosphate (OP) were used as the mixed surfactants, n-pentanol was used as assistant, and cyclohexane was used as oil phase. Y(NO₃)₃ and Na₂C₂O₄ solution were used as the water-phrase and starting materials. The resulting products were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), d thermogravimetric – differential scanning calorimetry (TG–DSC), and photoluminescence spectroscopy (PL). The results show that the monoclinic rod-like Y₂(C₂O₄)₃ have been obtained via hydrothermal microemulsion and the sizes of the particles were diameters of 100 nm and lengths up to about 1–2 μm. The precursor calcined products were doped with Eu³⁺, and the prepared phosphors showed well-defined red luminescence due to radiative transitions from ⁵D₀ to ⁷F_J (J=1, 2) levels of Eu³⁺ ions. Furthermore, We reported Eu³⁺–Y₂O₃ phases represented a new class of optically active materials.     

Photoluminescent and photocatalytic properties of bismuth doped strontium aluminates blended with titanium dioxide

Bismuth co-doped long persistent phosphor (LPP) powders were obtained by a combustion synthesis technique followed by a post-annealing under carbon atmosphere. Bismuth content was varied from 1.0 to 15.0 mol%. X-ray diffraction analysis revealed that the powders show mainly a mixture of three phases: the SrAl₂O_4, the SrAl₁₄O₂₅ and the Sr₂Al₆O₁₁ crystalline phases. Photocatalyst composites were obtained by wet mixing of TiO₂ anatase and LPP powders followed by annealing in air at 450 °C. Photoluminescence measured spectra under 380 nm excitation show a tunable emission from green (510 nm) to greenish-blue (463 nm) in which peak wavelength localization is related to the Bi content. Photoluminescence intensity decreases as Bi content increases. Degradation of methylene blue solutions, irradiated by UV light (254 nm), was monitored by the decrease of its 650 nm absorption peak in regular periods. The best photocatalytic activity is observed when in the composite blend a 2.0 mol% of Bi content was used, and complete methylene degradation is reached after 210 min. These photocatalyst composite powders are potential candidates to clean-up wastewater applications, and might be potential candidates for photocatalytic hydrogen generation in aqueous solutions.     

Fabrication and study of properties of the PLA/Sr2MgSi2O7:Eu2+, Dy3+ long-persistent luminescence composite thin films

In this paper, Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ (SMS) particles were first synthesized by sol–gel method and then modified with 3-aminopropyltriethoxysilane (APS) to improve their dispersibility and compatibility in the polylactic acid (PLA) matrix. The structure of pure SMS particles was analyzed by XRD and XPS. The properties of SMS particles before and after modification were characterized by FT-IR and SEM. PLA/SMS composite films containing 15 wt% of SMS particles were prepared by spin coating on silicon wafer. Their morphology and luminescence properties were examined. It was found that the composite films can be excited by a broad band from 330 nm to 425 nm with the highest excitation intensity at 360 nm. The fluorescent and phosphorescent emission bands of the composite films and SMS particles all have a major emission peak at 468 nm. Decay curves of the composite films have a similar tendency with that of the pure SMS particles, except for the lower intensity.     

Photoluminescence properties of YVO4:Eu3+,Ba2+ nanoparticles prepared by an ion exchange method

YVO₄:Ba²⁺ nanoparticles with a Ba²⁺ doping concentration x=0%, 1%, 3%, 5%, 7% and 9% were synthesized by a solvothermal method and then they were codoped with Eu³⁺ ions by an ion exchange method to form the YVO₄:Eu³⁺,Ba²⁺ nanoparticles. It was found that the photoluminescence intensity of the as-prepared YVO₄:Eu³⁺,Ba²⁺ nanoparticles steadily increased with x until x=7%, and then decreased for higher x. Thermal annealing resulted in considerable enhancement in their photoluminescence, and higher annealing temperature led to stronger photoluminescence enhancement. The emission intensity of the YVO₄:Eu³⁺,Ba²⁺ (x=7%) nanoparticles annealed at 500 °C was about 205% stronger than the sample without Ba²⁺ doping. Thermal annealing of the ion-exchanged YVO₄:Eu³⁺,Ba²⁺ nanoparticles at 500 °C and 700 °C resulted in photoluminescence enhancement of about 14 times and 27 times, respectively. The asymmetric ratio of Eu³⁺ in the ion-exchanged YVO₄:Eu³⁺,Ba²⁺ nanoparticles was found to increase after annealing.     

Photoluminescence of phosphorous doped Ge on Si (100)

Photoluminescence (PL) of selectively grown phosphorus (P) doped germanium (Ge) is investigated. 350–600 nm thick P-doped Ge is grown on 100 nm thick P-doped Ge buffer layer, which is annealed at 800 °C before the main part of Ge deposition. In the case of Ge deposited at 325 °C, approximately two times higher PL intensity is observed by P doping of ~3.2×10¹⁹ cm⁻³. Further increase of PL intensity by a factor of 1.5 is observed by increasing the growth temperature from 325 °C to 400 °C due to improved crystal quality. Varying PH₃ partial pressure at 400 °C, red shift of the PL occurred with increasing P concentration due to higher bandgap narrowing. With increasing P concentration up to ~1.4×10¹⁹ cm⁻³ at 400 °C the PL peak intensity increases by filling electrons into the L valley and decreases due to enhanced point defect concentration and degraded crystallinity. By post-annealing at 500–800 °C, the PL intensity is further increased by a factor of 2.5 because of increased active P concentration and improved crystal quality. Reduced direct bandgap energy by introducing tensile strain is also observed.     

White OLED based on a temperature sensitive Eu3+/Tb3+ β-diketonate complex

A mixed lanthanide β-diketonate complex of molecular formula [Eu_0.45Tb_0.55(btfa)₃(4,4′-bpy)(EtOH)] (btfa^– = 4,4,4–trifluoro–1–phenyl–1,3–butanedionate; 4,4′-bpy = 4,4′-dipyridyl; EtOH = ethanol) was synthesized and its structure was elucidated by single crystal X-ray diffraction. The temperature dependence of the complex emission intensity between 11 and 298 K is illustrated by the Commission Internacionale l’Éclairage (CIE) (x, y) color coordinates change within the orange-red region, from (0.521, 0.443) to (0.658, 0.335). The existence of Tb³⁺-to-Eu³⁺ energy transfer was observed at room temperature and as the complex presents a relatively high emission quantum yield (0.34 ± 0.03) it was doped in a 4,4′-bis(carbazol-9-yl)biphenyl (CBP) organic matrix to be used as emitting layer to fabricate a white organic light-emitting diode (WOLED). Continuous electroluminescence emission was obtained varying the applied bias voltage showing a wide emission band from 400 to 700 nm. The white emission results from a combined action between the Eu³⁺ and Tb³⁺ peaks from the mixed Eu³⁺/Tb³⁺ complex and the other organic layers forming the device. The intensity ratio of the peaks is determined by the layer thickness and by the bias voltage applied to the OLED, allowing us to obtain a color tunable light source.     

Memory characteristics of Al2O3/La2O3/Al2O3 multi-layer films with various blocking and tunnel oxide thicknesses

In order to investigate charge trap characteristics with various thicknesses of blocking and tunnel oxide for application to non-volatile memory devices, we fabricated 5 and 15 nm Al₂O₃/5 nm La₂O₃/5 nm Al₂O₃ and 15 nm Al₂O₃/5 nm La₂O₃/5, 7.5, and 10 nm Al₂O₃ multi-stack films, respectively. The optimized structure was 15 nm Al₂O₃ blocking oxide/5 nm La₂O₃ trap layer/5 nm Al₂O₃ tunnel oxide film. The maximum memory window of this film of about 1.12 V was observed at 11 V for 10 ms in program mode and at ?13 V for 100 ms in erase mode. At these program/erase conditions, the threshold voltage of the 15 nm Al₂O₃/5 nm La₂O₃/5 nm Al₂O₃ film did not change for up to about 10⁴ cycles. Although the value of the memory window in this structure was not large, it is thought that a memory window of 1.12 V is acceptable in the flash memory devices due to a recently improved sense amplifier.     

Cobalt-doped cerium oxide nanoparticles: Enhanced photocatalytic activity under UV and visible light irradiation

CeO₂ nanoparticles (NPs) were synthesized by coprecipitation using cerium(III) nitrate hexahydrate as the precursor and ethanol as the solvent. Different concentration of cobalt-doped cerium oxide NPs (3mol % and 6 mol %) were prepared by adding various concentrations of cobalt chloride to cerium nitrate. The as-synthesized NPs were characterized through X-ray diffraction (XRD) measurements, ultraviolet (UV)–visible spectroscopy, Photoluminescence (PL) spectroscopy, and transmission electron microscopy (TEM). XRD results reveal that the as-prepared CeO₂ NPs had a face-centered cubic structure with crystallite size in the range of 5–8 nm. TEM analyses showed that the CeO₂ NPs and Co-doped CeO₂ NPs had a homogenous size distribution (sizes were within 5–12 nm). Band-edge absorption of CeO₂ NPs redshifted upon increasing the Co concentration as compared to undoped CeO₂ NPs. PL spectra reveal a peak shift of CeO₂ emission upon cobalt doping, which were due to an increase in oxygen defects localized between the Ce4f and O2p energy levels (i.e., via formation of Ce³⁺ states). Photocatalytic degradation of methylene blue in aqueous solution under UV and visible (sunlight) irradiation in the presence of pure CeO₂ NPs and of Co-doped CeO₂ NPs was investigated. The efficiency of photocatalytic degradation of CeO₂ NPs increased with the Co concentration both under UV irradiation and under visible light. Co-doped CeO₂ NPs (6 mol%) showed degradation efficiencies of 98% and 89% at 420 min of exposure to UV irradiation and to visible light, respectively.     

Synthesis and luminescent properties of ternary complexes EuXLa1−X(TTA)3Dipy

Rare-earth ternary complexes Eu_XLa_1?X(TTA)₃Dipy (X = 0, 0.1, 0.25, 0.5, 0.75, 0.9, 1.0) were synthesized. Characterization with DTA-TG, IR, elemental analysis and fluorescent spectra had also been carried out. It is found that the enhanced luminescence of Eu³⁺ ions by La³⁺ ions occurs in ternary complexes, and when X = 0.25, Eu_0.25La_0.75(TTA)₃Dipy has the highest luminescence efficiency and lifetime. It is proved by TG curve that the complexes are stable, and we monitored the spectra of Eu_XLa_1?X(TTA)₃Dipy[PVK:Eu_XLa_1?X(TTA)₃Dipy/BCP/AlQ/Al] at the different rate r min^?1. The results showed that the La³⁺ ion acts as an energy transfer bridge that helps energy transfer from PVK to Eu³⁺.     

Properties of fluorine-doped SnO2 thin films by a green sol–gel method

Fluorine doped tin oxide (FTO) films were fabricated on a glass substrate by a green sol–gel dip-coating process. Non-toxic SnF₂ was used as fluorine source to replace toxic HF or NH₄F. Effect of SnF₂ content, 0–10 mol%, on structure, electrical resistivity, and optical transmittance of the films were investigated using X-ray diffraction, Hall effect measurements, and UV–vis spectra. Structural analysis revealed that the films are polycrystalline with a tetragonal crystal structure. Grain size varies from 43 to 21 nm with increasing fluorine concentration, which in fact critically impacts resultant electrical and optical properties. The 500 °C-annealed FTO film containing 6 mol% SnF₂ shows the lowest electrical resistivity 7.0×10⁻⁴ Ω cm, carrier concentration 1.1×10²¹ cm⁻³, Hall mobility 8.1 cm²V⁻¹ s⁻¹, optical transmittance 90.1% and optical band-gap 3.91 eV. The 6 mol% SnF₂ added film has the highest figure of merit 2.43×10⁻² Ω⁻¹ which is four times higher than that of un-doped FTO films. Because of the promising electrical and optical properties, F-doped thin films prepared by this green process are well-suited for use in all aspects of transparent conducting oxide.     

Properties of In2O3 films obtained by thermal oxidation of sprayed In2S3

In₂S₃ thin films were grown by the chemical spray pyrolysis (CSP) method using indium chloride and thiourea as precursors at a molar ratio of S:In=2.5. The deposition was carried out at 350 °C on quartz substrates. The film thickness is about 1 µm. The films were then annealed for 2 h at 550, 600, 650 and 700 °C in oxygen flow. This process allows the transformation of nanocrystal In₂O₃ from In₂S₃ and the reaction is complete at 600 °C. X-ray diffraction spectra show that In₂O₃ films are polycrystalline with a cubic phase and preferentially oriented towards (222). The film grain size increases from 19 to 25 nm and RMS values increase from 9 to 30 nm. In₂O₃ films exhibit transparency over 70–85% in the visible and infrared regions due to the thickness and crystalline properties of the films. The optical band gap is found to vary in the range 3.87–3.95 eV for direct transitions. Hall effect measurements at room temperature show that resistivity is decreased from 117 to 27 Ω cm. A carrier concentration of 1×10¹⁶ cm^?3 and mobility of about 117 cm² V^?1 s^?1 are obtained at 700 °C.