PL Decay Lifetime Research Articles

Synthesis, characterization and the photoinduced electron-transfer energetics of CdTe/CdSe type-II core/shell quantum dots

The CdTe/CdSe type-II core/shell quantum dots (QDs) were chemically synthesized in a noncoordinating solvent. The optical properties and structures of the synthesized QDs were characterized by absorption spectroscopy (Abs), photoluminescence (PL) spectroscopy, PL-decay lifetime, X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). The PL peak of the CdTe/CdSe core/shell QDs could be tuned from 698 nm (1.77 eV) to 780 nm (1.59 eV) by changing the thickness of CdSe shell and fixing the size of CdTe core. The PL quantum yields (QYs) of these QDs were from 14.1% to 31.7% and their full width at half maximum (FWHM) of PL peaks was kept below 0.12 eV during the growth process of CdSe shell on CdTe core. Compared to CdTe cores, the CdTe/CdSe type-II core/shell QDs have much longer PL decay times, up to ~100 ns? Based on Marcus theory, the charge transfer (CT) of CdTe/CdSe 1–5 monolayers (MLs) type-II core/shell QDs has been studied and the effects of quantum confinement on the structure parameters have been revealed. The reorganization energies (RE) of the CdTe/CdSe (1–5 ML) type-II core/shell QDs for the electron transfer (ET) were in the range from 60 meV to 106 meV and was found that ET takes place in the Marcus inverted region (-ΔG0ET > λCT) which is typical for these QDs.

Synthesis and optical properties of tunable dual emission copper doped CdTe1-xSex alloy nanocrystals

Tunable and dual emission Cu2+-doped CdTe1-xSex alloy nanocrystals (NCs) were successfully synthesized by the wet chemical method. The morphology and chemical compositions of the synthesized NCs were assessed by transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). The structural and optical properties were characterized by X-ray diffraction (XRD), absorption spectroscopy (Abs), photoluminescence (PL) spectroscopy, PL-decay lifetime. With the Cu2+ dopant concentration in the range of 0.5–5% and x of 0.5 in the precursors, the fabricated Cu2+-doped CdTe1-xSex NCs exhibited the dual emissions in the visible window at about 665 nm and near infrared window ranging from 830 to 840 nm which correspond to the excitonic emission of CdTe1-xSex NCs and Cu2+ dopant ions, respectively. With the increase in dopant concentration, the fluorescence lifetime of Cu-doped CdTe0.5Se0.5 NCs significantly improved (up to μs) and the lattice constant of NCs decreased. By varying x from 0 to 1.0, both excitonic and dopant emissions of Cu-doped CdTe1-xSex NCs could be tuned in a wild range from 758 to 563 nm and from 918 to 748 nm, respectively. Interestingly, the crystal structure of these NCs changed gradually from the zinc blende (ZB) to wurtzite (WZ) structure when x increased. The results revealed that the dual emission, fluorescence lifetime and crystal structure of the Cu-doped NCs could be controlled by varying dopant concentration and chemical composition of the host.

Enhanced the stability of Cesium lead bromide perovskite nanocrystals

Despite colloidal CsPbBr3 nanocrystals (NCs) exhibit an excellent optoelectronic characteristics, the stability greatly restricts their potential applications due to the ionic nature. The CsPbBr3 NCs were synthesized in a Br-rich strategy by adding 1,9-DN. The PL intensity of CsPbBr3 NCs was increased and the average PL decay lifetime was reduced due to the decrease of defects. The chemical, illumination and thermal stability of CsPbBr3 NCs were obviously improved, which may be caused by the increase of ligand and the decrease of VBr on the surface of CsPbBr3 NCs as adding 1,9-DN. The results provide a novel strategy to prepare emissive materials with environmental stability, implying great promise in optoelectronic applications.

Highly luminescent CsPbX3 (X=Cl, Br, I) perovskite nanocrystals with tunable photoluminescence properties

All-inorganic cesium lead halide perovskite nanostructures have become an outstanding luminous optoelectronic material for illumination and display. In this paper, we synthesized monodispersed colloidal single- and mixed-halide CsPbX3 (X = Cl, Br, I) nanocrystals with various morphologies and brightly luminescent properties. By varying the amount of PbX2 (X = Cl, Br, I), the prepared CsPbX3 nanocrystals possess high photoluminescence quantum yield (PLQY, about 70%), narrow full width at half-maximum (FWHM) (4–17 nm), tunable band gap (407–680 nm), and highly strong photostability. In addition, it is found that the PL decay lifetime of CsPbX3 nanocrystals ranged from 1 to 6 ns, and their PL mechanism was studied thoroughly. These corresponding results demonstrate that all-inorganic colloidal CsPbX3 perovskite nanocrystals are expected to have a promising application in the field of optoelectronic material.

Investigation of energy transfer mechanisms in rare-earth doped amorphous silica films embedded with tin oxide nanocrystals.

Three different types of rare earth (RE3+) ions-doped silica thin films are fabricated by a soft chemistry-based method. By introducing tin oxide (SnO2) nanocrystals with larger cross-sections as sensitizers, the characteristic emission intensity of RE3+ ions in amorphous silica thin films can be enhanced by more than two orders of magnitude via the energy transfer process. The possible energy transfer processes under different local environment are revealed by using Eu3+ ions as an optical probe. Quantitative studies of PL decay lifetime and temperature-dependence PL spectra suggest that the partial incorporation of RE3+ ions into SnO2 sites gives rises to the change of crystal-field symmetry and the significant enhancement of energy transfer efficiency. Further, typical analytical energy dispersive X-ray spectroscopy (EDS) mapping results prove that part of Eu3+ ions doped into the SnO2 sites after annealing at 1000 °C. We anticipate that our results would shed light on the future research on the energy transfer mechanisms under different local structures of RE3+ ions.

Tunable photoluminescence of CsPbBr3 perovskite quantum dots for their physical research

All-inorganic cesium lead halide perovskite nanostructure CsPbBr3 quantum dots have brought extensive attention in photoelectric applications. In this paper, CsPbBr3 quantum dots were synthesized with the diameter ranges from 8.7 to 11.8 nm by adjusting reaction temperature from 140 to 180 °C. In addition, for the monodisperse CsPbBr3 nanocrystals, it was found that radiative lifetime exists between 5 and 20 ns and PL decay lifetimes ranges from 4 to 11 ns, and their PL mechanism was studied thoroughly.

Shape-Controlled Synthesis of All-Inorganic CsPbBr3 Perovskite Nanocrystals with Bright Blue Emission.

We developed a colloidal synthesis of CsPbBr3 perovskite nanocrystals (NCs) at a relative low temperature (90 °C) for the bright blue emission which differs from the original green emission (∼510 nm) of CsPbBr3 nanocubes as reported previously. Shapes of the obtained CsPbBr3 NCs can be systematically engineered into single and lamellar-structured 0D quantum dots, as well as face-to-face stacking 2D nanoplatelets and flat-lying 2D nanosheets via tuning the amounts of oleic acid (OA) and oleylamine (OM). They exhibit sharp excitonic PL emissions at 453, 472, 449, and 452 nm, respectively. The large blue shift relative to the emission of CsPbBr3 bulk crystal can be ascribed to the strong quantum confinement effects of these various nanoshapes. PL decay lifetimes are measured, ranging from several to tens of nanoseconds, which infers the higher ratio of exciton radiative recombination to the nonradiative trappers in the obtained CsPbBr3 NCs. These shape-controlled CsPbBr3 perovskite NCs with the bright blue emission will be widely used in optoelectronic applications, especially in blue LEDs which still lag behind compared to the better developed red and green LEDs.

Performance and Stability of Polymer Solar Cells Based on the Blends of Poly(3-Hexylthiophene) and Indene-C60 Bis-Adduct**Supported by the Tianjin Natural Science Foundation under Grant Nos 13JCYBJC18900 and 13JCZDJC26700, and the National High Technology Research and Development Program of China under Grant No 2013AA014201.

The performance and morphology stability of polymer bulk heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) as the donor and indene-C60 bisadduct (ICBA) or methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the acceptor are compared. Effect of the different donor and acceptor weight ratios on photovoltaic performance of the P3HT:ICBA device is studied. The optimal device achieved power conversion efficiency of 5.51% with Jsc of 10.86mA/cm2, Voc of 0.83 V, and fill factor (FF) of 61.1 % under AM 1.5G (100 mW/cm2) simulated solar illumination. However, the stability measurement shows that cells based on P3HT:ICBA are less stable than those of the device based on P3HT:PCBM. Atomic force microscope results reveal that the morphology of the P3HT:ICBA film changed considerably during the storage periods due to unstable interpenetrating D-A network. This observation can be explained by the fact that there is lack of intermolecular hydrogen bonds in the P3HT:ICBA system. However, in the P3HT:PCBM system the molecules in the blend film are firmly held together in the solid state by means of intermolecular hydrogen bonds originating from C-H⃛Os bonds (where Os comes from the singly-bonded O atom of PCBM), forming a stable three-dimensional network. The measured PL decay lifetimes for P3HT:PCBM and P3HT:ICBA systems are 33.66 ns and 35.34 ns, respectively, indicating that the P3HT:ICBA system has a less efficient exciton separation efficiency than that of P3HT:PCBM, which may result in the interfacial photogenerated charges accumulated on the D: A interface. Such progressive phase segregation between P3HT and ICBA eventually leads to the degradation in performance and deteriorates the stability of the device. We also present an approach to enhance the stability of P3HT:ICBA systems by adding PCBM as the second acceptor. Our results show that by carefully tuning the contents of PCBM as the second acceptor, more stable polymer solar cells can be obtained.

Controlling CH3NH3PbI(3-x)Cl(x) Film Morphology with Two-Step Annealing Method for Efficient Hybrid Perovskite Solar Cells.

The methylammonium lead halide perovskite solar cells have become very attractive because they can be prepared with low-cost solution-processable technology and their power conversion efficiency have been increasing from 3.9% to 20% in recent years. However, the high performance of perovskite photovoltaic devices are dependent on the complicated process to prepare compact perovskite films with large grain size. Herein, a new method is developed to achieve excellent CH3NH3PbI3-xClx film with fine morphology and crystallization based on one step deposition and two-step annealing process. This method include the spin coating deposition of the perovskite films with the precursor solution of PbI2, PbCl2, and CH3NH3I at the molar ratio 1:1:4 in dimethylformamide (DMF) and the post two-step annealing (TSA). The first annealing is achieved by solvent-induced process in DMF to promote migration and interdiffusion of the solvent-assisted precursor ions and molecules and realize large size grain growth. The second annealing is conducted by thermal-induced process to further improve morphology and crystallization of films. The compact perovskite films are successfully prepared with grain size up to 1.1 μm according to SEM observation. The PL decay lifetime, and the optic energy gap for the film with two-step annealing are 460 ns and 1.575 eV, respectively, while they are 307 and 327 ns and 1.577 and 1.582 eV for the films annealed in one-step thermal and one-step solvent process. On the basis of the TSA process, the photovoltaic devices exhibit the best efficiency of 14% under AM 1.5G irradiation (100 mW·cm(-2)).

Highly efficient near-infrared emission in Er^3+ doped silica films containing size-tunable SnO_2 nanocrystals

Co-doping size-tunable SnO2 nanocrystals into Er(3+) ions embedded silica thin films produces an enhancement of Er-related near-infrared emission by three orders of magnitude. Selective PL and PLE measurements show that energy transfer process occurs between SnO2 nanocrystals and Er(3+) ions. Quantitative studies of PL decay lifetime and photoluminescence temperature-dependence demonstrate that both high energy transfer efficiency from SnO2 nanocrystals to Er(3+) ions and the partial incorporation of Er(3+) ions into SnO2 nanocrystals contribute to the near-infrared emission enhancement. All these results indicated that SnO2 nanocrystals with suitable size have great potentials in fabricating high-efficiency near-infrared luminous materials as sensitizers of Er(3+) ions.

Nonradiative and Radiative Recombination in CdS Polycrystalline Structures

Properties of polycrystalline CdS layers, employed in formation of the CdS-Cu2S heterostructures, have been studied by combining contactless techniques of the time and spectrally resolved photoluminescence (TR-PL) spectroscopy and microwave-probed photoconductivity (MW-PC) transients. The confocal microscopy has been employed to correlate the homogeneity of photoluminescence and grain size in CdS layers. Three types of samples with crystallite grain size of <1 μm (the I-type) and of 2–10 μm of homogeneous (II-type) and inhomogeneous (III-type) grain distribution have been separated. The simultaneous record of MW-PC and TR-PL responses ensures the same sampling area on the layer under investigation, as both (MW-PC and TR-PL) signals are generated by the same UV laser excitation beam. Two PL bands peaked at 500 and 700 nm were revealed. It has been demonstrated that photoluminescence intensity strongly depends on the properties of the polycrystalline 15–26 μm thick CdS layers with equilibrium carrier density of about1.5×1013 cm−3, which serve as the substrates to form CdS-Cu2S junctions. The different carrier decay components were ascribed to different microareas with characteristic MW-PC and PL decay lifetimes of 2–10 ns, ascribed to microcrystallites with PL instantaneous decay lifetimes of 40–200 ns, and MW-PC decay lifetimes in the range of 100–1000 μs attributed to the inter-crystallite areas of CdS polycrystalline material.

Time-Resolved Microphotoluminescence Study of Cu(In,Ga)Se2

The carrier recombination processes in Cu(In1-x,Gax)Se2 (CIGS) thin films were investigated by time-resolved microscopic-photoluminescence (µ-PL) measurement at room temperature. For films with x = 0.45, the spatial distribution of the donor–acceptor pair luminescence is much larger than the grain size. The PL decay lifetime is directly correlated with the µ-PL intensity, but the spectral shape is identical regardless of the sample position. These results suggest that the nonuniform distribution of nonradiative recombination centers mainly affects the carrier recombination in CIGS thin films. At relatively high Ga concentrations (x ≥0.7), the spatial inhomogeneity in µ-PL intensity is enhanced and decay accelerated, suggesting an increase in the density of nonradiative recombination centers. Thus, suppression of nonradiative recombination is critical to enhancing the performance of CIGS-based solar cells.

Time-Resolved Photoluminescence Studies of Indium-Rich InGaNAlloys

Time-resolved photoluminescence (PL) spectroscopy has be used toinvestigate indium-rich InGaN alloys grown on sapphire substrates bymetal organic chemical vapor deposition. Photoluminescence measurementindicates two dominant emission lines originating from phase-separatedhigh- and low-indium-content regions. Temperature and excitationintensity dependence of the two main emission lines in these InGaNalloys have been measured. Temperature and energy dependence of PLdecay lifetime show clearly different decay behaviour for the two mainlines. Our results show that photo-excited carriers are deeplylocalized in the high indium regions while photo-excited carriers canbe transferred within the low-indium-content regions as well as tohigh-content regions.

Exciplex emission and energy transfer in white light-emitting organic electroluminescent device

We report a white light organic electroluminescent (EL) device achieved through exciplex formation between organic materials as a green color light source. Exciplex formation at 500 nm originated from poly( N-vinylcarbazole) (PVK) and 2,5-bis(5- tert-bytyl-2-benzoxazolyl)thiophene (BBOT), exciplex formation at 550 nm between from N, N′-diphenyl- N, N′-bis(3-metylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) and BBOT was assumed to correspond to the complex formation covering the green color region. In the mixed emitting materials, the PL decay lifetimes of BBOT and poly(3-hexylthiophene) (P3HT) were increased as much as 35–68 and 1.5–19 ns, respectively. This indicates that the energy transfer occurred from PVK to BBOT and P3HT, enhancing the quantum efficiency of these materials. We achieved white light emission with brightness as great as 12.3 μW/cm 2 at 20 V, corresponding to CIE coordinates of x=0.29 and y=0.353. The EL spectrum of the device was changed with a function of the applied voltage.

A Novel Approach to the Assessment of Semiconductor Hetero-Interfaces in Multilayer Structures

ABSTRACTWe demonstrate the novel use of CV simulation to determine the level of charge at each interface in multilayer GaAs/AlGaAs heterostructures. The gettering of charged centres at the earliest grown interfaces is quantitatively appraised and the parallel application of PL decay lifetime measurements determines their effectiveness as recombination centres. A direct relationship is found between the magnitude of the charged states and the recombination velocity. Estimates of a 1eV activation energy and a hole capture cross section of 10−15 cm2 were also obtained. This suggests that the dominant recombination centre could be observed as a hole trap.