Time-Resolved Photoluminescence

Time-resolved microscopy is a widely used approach for imaging and quantifying charge and energy transport in functional materials. While it is generally recognized that resolving small diffusion lengths is limited by measurement noise, the impacts of noise have not been systematically assessed or quantified. This article reports modeling efforts to elucidate the impact of noise on optical probes of transport. Excited state population distributions, modeled as Gaussians with additive white noise typical of experimental conditions, are subject to decay and diffusive evolution. Using a conventional composite least-squares fitting algorithm, the resulting diffusion constant estimates are compared with the model input parameter. The results show that heteroscedasticity (i.e., time-varying noise levels), insufficient spatial and/or temporal resolution, and small diffusion lengths relative to the magnitude of noise lead to a surprising degree of imprecision under moderate experimental parameters. Moreover, the compounding influence of low initial contrast and small diffusion length leads to systematic overestimation of diffusion coefficients. Each of these issues is quantitatively analyzed herein, and experimental approaches to mitigate them are proposed. General guidelines for experimentalists to rapidly assess measurement precision are provided, as is an open-source tool for customizable evaluation of noise effects on time-resolved microscopy transport measurements.

The authors demonstrate a new route to the fabrication of individual aluminosilicate microtubes that can act as micron-scale optical cylindrical resonators. The microtubes were prepared using a simple vacuum assisted wetting and filtration through a microchannel glass matrix. Microphotoluminescence spectra of the microtube cavity show sharp resonant modes with quality factors up to 3200. A strong reduction of the emission decay time at high excitation power confirms the occurrence of amplified spontaneous emission from a single microtube.

In,Ga)As/GaP(001) quantum dots (QDs) are grown by molecular beam epitaxy and studied both theoretically and experimentally. The electronic band structure is simulated using a combination of k•p and tight-binding models. These calculations predict an indirect to direct crossover with the In content and the size of the QDs. The optical properties are then studied in a low-In-content range through photoluminescence and time-resolved photoluminescence experiments. It suggests the proximity of two optical transitions of indirect and direct types.

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Formation of zinc oxide nanostructured thin films at different temperatures on Al-coated silicon and lithium niobate substrates by hydrothermal method was presented. The comparison of morphology of nanostructured thin films formed by hydrothermal and electrodeposition method was carried out. The opportunity to use the hydrothermal method instead electrodeposition to obtain nanostructured films on a conductive layer was shown. The dependence of morphology and crystallinity from growth temperature was established using scanning electron microscopy and X-ray diffractometry. The application of synthesized films as sensing layer of acoustic wave based and electrochemical sensors could enhance its sensitivity to pollutants in gas or liquid phases by an active area increasing.

This paper reports a facile fabrication of vertical ZnO nanowalls on ITO coated glass substrates by using an aqueous solution growth method at low temperature. The formation of nanowalls is ascribed to selective dissolution of (001) planes of the chemical bath deposited dense ZnO rods. The morphology of the etched ZnO nanowalls is determined by the structure of the as-grown ZnO rod arrays, which can be readily controlled by tuning aqueous solution parameters such as: initial pH value of chemical bath solution and the growth temperature. With verticaly aligned ZnO nanowalls as electrode in inverted polymer solar cells, the average performance of devices with open circuit voltage, short circuit current density, fill factor, and power conversion efficiency are measured as 0.56 V, 7.56 mW cm −2 , 0.49 and 2.14%, respectively. The results indicate that the two-dimensional structure of ZnO nanowalls can effectively serve as an electrode for inverted polymer solar cells.

Nonpolar (11$ \bar 2 $0) a ‐plane GaN thin films were grown on r ‐plane (1$ \bar 1 $02) sapphire substrates by low‐pressure metal organic chemical vapor deposition (MOCVD). The stress characteristics of the a ‐plane GaN films were investigated by means of polarized Raman scattering spectra in backscattering configurations. The experimental results show that there are strong anisotropic in‐plane stresses within the epitaxial a ‐plane GaN films by calculating the corresponding stress tensors. The temperature dependence of Raman scattering spectra was studied in the range from 100 K to 550 K. The measurements reveal that the Raman phonon frequencies decrease with increasing temperature. The temperature at which nonpolar a ‐plane GaN films are strain free is discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We develop a contactless method based on photoluminescence measurements in the modulated mode: the high-frequency modulated photoluminescence. The high frequency domain allows accessing to carrier dynamics in the nanosecond time scale which is typical for thin films materials. To illustrate the experimental method, we analyze Cu(In,Ga)Se 2 photovoltaic absorbers where recombination mechanisms in the bulk, surface and grain boundaries are not completely understood. We correlate the data with classical time resolved photoluminescence. We show that the combination of the two methods allows, with the help of one dimensional simulations, an estimation of carrier traps and recombination centers parameters in thin films samples.

In,Ga)As/GaP(001) quantum dots (QDs) are grown by molecular beam epitaxy and studied both theoretically and experimentally. The electronic band structure is simulated using a combination of k•p and tight-binding models. These calculations predict an indirect to direct crossover with the In content and the size of the QDs. The optical properties are then studied in a low-In-content range through photoluminescence and time-resolved photoluminescence experiments. It suggests the proximity of two optical transitions of indirect and direct types.

Hazardous substances produced by anthropic activities threaten human health and the green environment. Gas sensors, especially those based on metal oxides, are widely used to monitor toxic gases with low cost and efficient performance. In this study, electron beam lithography with two-step exposure was used to minimize the geometries of the gas sensor hotplate to a submicron size in order to reduce the power consumption, reaching 100 °C with 0.09 W. The sensing capabilities of the ZnO nanofilm against NO2 were optimized by introducing an enrichment of oxygen vacancies through N2 calcination at 650 °C. The presence of oxygen vacancies was proven using EDX and XPS. It was found that oxygen vacancies did not significantly change the crystallographic structure of ZnO, but they significantly improved the electrical conductivity and sensing behaviors of ZnO film toward 5 ppm of dry air.

Photoinduced electronic and structural changes of a hydrogen-generating supramolecular RuPt photocatalyst are studied by a combination of time-resolved photoluminescence, optical transient absorption, and X-ray absorption spectroscopy. This work uses the element specificity of X-ray techniques to focus on the interplay between the photophysical and -chemical processes and the associated time scales at the catalytic Pt moiety. We observe very fast (<30 ps) photoreduction of the Pt catalytic site, followed by an ∼600 ps step into a strongly oxidized Pt center. The latter process is likely induced by oxidative addition of reactive iodine species. The oxidized Pt species is long-lived and fully recovers to the original ground state complex on a >10 μs time scale. However, the photosensitizing Ru moiety is fully restored on a much shorter ∼300 ns time scale. This reaction scheme implies that we may withdraw two electrons from a catalyst that is activated by a single photon.

A new approach is demonstrated for investigating charge and spin diffusion as well as surface and bulk recombination in unpassivated doped semiconductors. This approach consists in using two complementary, conceptually related, techniques, which are time-resolved photoluminescence (TRPL) and spatially resolved microluminescence (μPL) and is applied here to p+ GaAs. Analysis of the sole TRPL signal is limited by the finite risetime. On the other hand, it is shown that joint TRPL and μPL can be used to determine the diffusion constant, the bulk recombination time, and the spin relaxation time. As an illustration, the temperature variation of these quantities is investigated for p+ GaAs.

Time-resolved photoluminescence (PL) has been employed to study the optical transitions and their dynamic processes and to evaluate materials quality of InGaN epilayers grown by metalorganic chemical vapor deposition. Our results suggest that the PL emissions in InGaN epilayers result primarily from localized exciton recombination. The localization energies of these localized excitons have been obtained. In relatively lower quality epilayers, the localized exciton recombination lifetime τ, decreases monotonically with an increase of temperature. In high quality epilayers, τ increases with temperature at low temperatures, which is a well-known indication of radiative exciton recombination. Our results demonstrate that time-resolved PL measurements uniquely provide opportunities for the understanding of basic optical processes as well as for identifying high quality materials.

Semiconductor quantum dot/rod/emitter upconverter nanostructures are promising for photovoltaic applications due to their broadband absorption and tunability. However, the best reported upconversion quantum yield is only 2%, limited by competing carrier relaxation mechanisms. In this work, both steady-state and excitation wavelength-dependent time-resolved photoluminescence are used to measure carrier separation and transfer dynamics as a function of nanostructure morphology. We synthesize and measure core-only, core/rod intermediates, and full core/rod/emitter structures and perform three-dimensional (3D) modeling of the electron and hole wavefunctions for each of these structures. We observe that addition of the rod to the core quantum dot improves passivation and carrier separation. An increase in core homogeneity and rod alloying are both found to result in higher radiative recombination rates relative to nonradiative recombination, in agreement with an increase in upconversion efficiency, as previously reported by the authors. Collectively, these results illustrate how the nanostructure morphology and composition influence carrier separation and recombination, suggesting a path toward improved upconversion performance.

The emission dynamics properties of self-assembled InAs quantum dots with different cap layers are investigated by steady-state and time-resolved photoluminescence (PL). The uniformity of the dots can be improved by the proper cap layer structure. It is found that the InGaAs and InAlAs combination cap layer structure could more effectively release the strain in QDs, suppress the indium segregation, enhance the confinement effect on carriers and reduce the thermally induced carrier escape from QDs. This has led to QDs with narrower emission, above 1.3 µm, and to improved temperature stability of PL intensity and has resulted in an increase of the PL decay time to 10 ns at 160 K.

The microluminescence surface scan technique ͑MSST͒ is used to investigate the lateral transport of photocarriers in a thin InGaAs-InP quantum well, under high optical excitation intensity ͑0.3 to 30 KW/cm 2) and in the temperature range from 7 to 200 K. The size of the in-plane photocarrier distribution depends on both optical excitation intensity and temperature. A phonon-wind-driven mechanism is used to explain the behavior of the distribution at temperatures below 15 K. Further, the attenuation of the phonon flux during the photocarrier cloud expansion plays a key role in the phonon-wind mechanism. Finally, it is found that the phonon wind becomes quenched at high optical excitation intensity, which is probably correlated to an increasing carrier velocity greater than the sound velocity.

We report charge recombination rates in lead iodide perovskites with alloys of formamidinium (FA) and methylammonium (MA) cations, FA x MA 1−x PbI 3 , 0 ≤ x ≤ 0.9, prepared through ion exchange with minimal differences in morphology and charge trap density. Our results show that the trap-mediated recombination rate increases over 2 orders of magnitude with decreasing MA content. These results are consistent with a proposed mechanism that MA molecules are more effective in screening charged defects. In contrast, band-toband charge recombination rates are minimized at an intermediate alloy composition. These findings reveal that the monovalent cations impact recombination processes through different mechanisms.

In this work, WO3 nanostructures were synthesized with different complexing agents (0.05 M H2O2 and 0.1 M citric acid) and annealing conditions (400 °C, 500 °C and 600 °C) to obtain optimal WO3 nanostructures to use them as a photoanode in the photoelectrochemical (PEC) degradation of an endocrine disruptor chemical. These nanostructures were studied morphologically by a field emission scanning electron microscope. X-ray photoelectron spectroscopy was performed to provide information of the electronic states of the nanostructures. The crystallinity of the samples was observed by a confocal Raman laser microscope and X-ray diffraction. Furthermore, photoelectrochemical measurements (photostability, photoelectrochemical impedance spectroscopy, Mott–Schottky and water-splitting test) were also performed using a solar simulator with AM 1.5 conditions at 100 mW·cm−2. Once the optimal nanostructure was obtained (citric acid 0.01 M at an annealing temperature of 600 °C), the PEC degradatio...

Nowadays, most of the Research and Development (R&D) efforts devoted to perovskite solar cells have been focused on achieving higher power conversion efficiencies, by using small sized substrates with even smaller active areas. The present paper proposes a method for optimizing the deposition of thin layers of organicinorganic hybrid perovskite methylammonium lead iodide (CH3NH3PbI3) on large surfaces (up to 75 mm x 75 mm) via spin coating procedures, performed in a clean room environment at room temperature and by considering their optical properties to assess their quality. Thus, in order to map the deposited area and realize studies about their optical properties, the absorption coefficient, the refractive index and layers thicknesses were determined by using a high accuracy spectroscopic ellipsometer system based on rotating compensator ellipsometer (RCE) technology from the J.A. Woolam Co. as functions of the wavelength, within a spectral range from 250 to 800 nm. Also, the sam...

The selenium-doped ZnO nanomaterial has successfully grown the surface of FTO (Fluorine Tin Oxide) using a seed-mediated hydrothermal method at a temperature of 90°C for 5 hours. In this research, the doping selenium by variation the volume of selenium solution at 0 mL, 0.025 mL dan 0.2 mL. This is an impact on the optical properties and morphology of ZnO nanorods. The samples were characterized using UV-Vis spectroscopy, X-Ray Diffraction and Field Emission Scanning Electron Microscopy (FESEM). The UV-Vis spectra showed that strong absorption occurs in the wavelength range of 300-380 nm. The 0.025 mL Se doped ZnO was the highest absorption compared to other samples. The XRD pattern exhibited five peaks at an angle of 2θ: 31.70 °; 34.4 °; 36.2 °; and 47.5 °. representing the orientation of the crystal planes (100), (002), (101), and (102) of hexagonal lattice. The FESEM images showed that Se doped ZnO with hexagonal face shape. The 0.2 mL Se doped ZnO was the most uniform compared t...

The Mg concentration dependence of the performance of inverted type organic solar cells based on Mgdoped ZnO nanorod arrays and poly(3-hexylthiophene) (P3HT) has been investigated. The Mg dopants with various concentrations (0, 1, 3 and 5 at.%) were introduced during the hydrothermal growth of the ZnO nanorod arrays on fluorine-doped tin oxide (FTO) glass substrate. The P3HT was deposited onto Mg-doped ZnO nanorod arrays by spin coating technique, followed by deposition of Ag as anode using magnetron sputtering technique. The length and density of Mg-doped ZnO nanorods increased, whereas the diameter decreased with the Mg concentration. The short circuit current density (J sc) and open circuit voltage (V oc) improved with increasing of Mg concentration up to 3 at.%, which could be attributed to increased interfacial area for more efficient exciton dissociation and reduced charge recombination as a result of lower number of oxygen interstitials which act as electron traps in ZnO. However, the J sc and V oc started to decrease at Mg concentration of 5 at.%, mainly due to poor infiltration of P3HT into the high-density 5 at.% Mg-doped ZnO nanorod arrays and increase of Mg dopant-related trapping centers. The highest power conversion efficiency of 0.36 ± 0.02% was achieved at Mg doping concentration of 3 at.%, an enhancement of 225% as compared to that based on undoped ZnO nanorod arrays.

The current work focuses on the incorporation of excess amount of gallium (Ga) in zinc oxide (ZnO) nanowires to generate oxygen interstitials for developing p-type conductivity. Ga has been used as an n-type dopant in ZnO lattice by reducing its inherent vacancies up to 3% molar ratio of Ga and Zn employing chemical bath deposition technique. However, the addition of more than 4% of gallium nitrate in the bath solution creates oxygen interstitials which are confirmed from the presence of X-ray photoelectron spectroscopy (XPS) peak at 532.6 eV and the relevant cathodoluminescence (CL) peak at 626 nm of the grown Ga-doped ZnO nanowires. The p-type conductivity of such ZnO nanowires has been confirmed from current-voltage, capacitance-voltage and Hall voltage measurements. The reproducibility of the results indicates the incorporation of excess Ga to be a promising technique for growing p-type ZnO nanowires.

We have studied optical properties of single In 0.1 Ga 0.9 N quantum wells with GaN barriers in close proximity to the wafer surface (<10 nm). We have found that at room temperature a balance of radiative, non-radiative recombination and complex surface states effects results in an optimum cap thickness of 3nm for achieving highest brightness emitters. At low temperature, we observe a behaviour that suggests that some surface states act as trapping centres for carriers rather than as a non-radiative recombination channel. Temperature dependence of the photoluminescence decay curves shows that carrier lifetimes in all the wafers increase at lower temperatures and reach similar maximum value. Main features of the evolution of lifetimes with temperature can be explained satisfactory by a combination of radiative, non-radiative recombination and above mentioned twofold surface effects. Detailed picture of the carrier dynamics is however complex and needs to include the modification of the electrostatic potential in the quantum wells positioned in the surface depletion regions.

for useful discussions regarding TRMC measurements. This work was supported by the Photochemistry and Radiation Research program in the DOE Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, under contract No. DE-AC36-99GO10337 to NREL. Additional support is also provided by the NREL Director's Discretionary Research and Development (DDRD) fund.

Gallium nitride ͑GaN͒ and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet ͑UV͒ emitters and detectors ͑in photon ranges inaccessible by other semiconductors͒ and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The point defects include native isolated defects ͑vacancies, interstitial, and antisites͒, intentional or unintentional impurities, as well as complexes involving different combinations of the isolated defects. Further improvements in device performance and longevity hinge on an in-depth understanding of point defects and their reduction. In this review a comprehensive and critical analysis of point defects in GaN, particularly their manifestation in luminescence, is presented. In addition to a comprehensive analysis of native point defects, the signatures of intentionally and unintentionally introduced impurities are addressed. The review discusses in detail the characteristics and the origin of the major luminescence bands including the ultraviolet, blue, green, yellow, and red bands in undoped GaN. The effects of important group-II impurities, such as Zn and Mg on the photoluminescence of GaN, are treated in detail. Similarly, but to a lesser extent, the effects of other impurities, such as C, Si, H, O, Be, Mn, Cd, etc., on the luminescence properties of GaN are also reviewed. Further, atypical luminescence lines which are tentatively attributed to the surface and structural defects are discussed. The effect of surfaces and surface preparation, particularly wet and dry etching, exposure to UV light in vacuum or controlled gas ambient, annealing, and ion implantation on the characteristics of the defect-related emissions is described.

AlGaN/GaN heterostructures with different thicknesses of AlN interlayer (AlN-IL) were grown by metal organic chemical vapour deposition (MOCVD) system. The results obtained from atomic force microscopy revealed that, surface roughness decreases with increase in the interlayer thickness. Scanning electron microscopy and transmission electron microscopy images portrayed good interfaces between AlGaN/GaN heterostructures containing AlN as the IL. From the high-resolution X-ray diffraction data, the aluminium composition was estimated as 21-24%, and the AlGaN layer thicknesses were found to be 22-26 nm in the AlGaN layers using the epitaxy smooth fit software. Using reciprocal space mapping, the strain between AlGaN and GaN layers scanned along (1 0 5) plane were analyzed for the AlGaN/GaN heterostructures. Photoluminescence (PL) spectroscopy revealed a significant shift in the AlGaN peaks for samples both in the presence and absence of AlN-IL. Time resolved photoluminescence results exhibit dominate decay time in AlGaN/GaN heterostructures samples containing AlN-IL of around 3 nm thick. It is to be noted that, mobility increased from 1246 to 2000 cm 2 /volt-sec due to the presence of AlN-IL at 300 K.

Carrier tunnelling through GaAs barriers of different thicknesses is investigated in vertically InGaAs/GaAs quantum rings (QR's). Shorter PL decay time of the ground state emission of high-energy component in the sample with thicker spacer (1.5 nm) is ascribed to both tunnelling effect between the two QR families and vertical coupling between layers in the stacks. We found that tunnelling time between QR's followed the Wentzel-Kramers-Brillouin (WKB) approximation. The non resonant tunnelling rate between QR's is found to be different by one order of magnitude from the rate in quantum dots (QD's).

The instability exhibited by perovskite solar cells when exposed to the environment under illumination is one major obstacle for the entry of perovskite technology on the photovoltaic market. In this work, we use the external quantum efficiency (EQE) technique to study the photoinduced degradation of two types of solar cells having CH3NH3PbI3 as absorber layer, one deposited by spin coating with an n-i-p architecture, and the other deposited by evaporation with an inverted p-in structure. We also study the effect of different encapsulants to protect the cells against atmospheric agents. We find that EQE provides information regarding the areas of the cell most susceptible to degradation, in addition to providing an estimate of the optical gap and the Urbach energy of the absorbent material. We confirm that the combined action of illumination and the environment markedly accelerate the degradation, which is reflected in the deterioration of all the parameters of the cell. The rear part of the cell is the first region to suffer the lightinduced degradation. On the other hand, the cells deposited by evaporation and with a good encapsulation process are highly stable, since after 30 hours of exposure just small spectral change is noticed in the red/infrared region of the EQE spectrum.

The Mg concentration dependence of the performance of inverted type organic solar cells based on Mgdoped ZnO nanorod arrays and poly(3-hexylthiophene) (P3HT) has been investigated. The Mg dopants with various concentrations (0, 1, 3 and 5 at.%) were introduced during the hydrothermal growth of the ZnO nanorod arrays on fluorine-doped tin oxide (FTO) glass substrate. The P3HT was deposited onto Mg-doped ZnO nanorod arrays by spin coating technique, followed by deposition of Ag as anode using magnetron sputtering technique. The length and density of Mg-doped ZnO nanorods increased, whereas the diameter decreased with the Mg concentration. The short circuit current density (J sc) and open circuit voltage (V oc) improved with increasing of Mg concentration up to 3 at.%, which could be attributed to increased interfacial area for more efficient exciton dissociation and reduced charge recombination as a result of lower number of oxygen interstitials which act as electron traps in ZnO. However, the J sc and V oc started to decrease at Mg concentration of 5 at.%, mainly due to poor infiltration of P3HT into the high-density 5 at.% Mg-doped ZnO nanorod arrays and increase of Mg dopant-related trapping centers. The highest power conversion efficiency of 0.36 ± 0.02% was achieved at Mg doping concentration of 3 at.%, an enhancement of 225% as compared to that based on undoped ZnO nanorod arrays.

One of the challenges in improving the efficiency of hybrid solar cells is to remove crystal defects from the semiconducting nanoparticles. Here we report the synthesis, characterization and applications of transition metal doped ZnO nanorods in hybrid bulk heterojunction solar cells. Mn x Zn 1Àx O and Ni x Zn 1Àx O with dopant concentrations ranging from x = 0.01-0.10 were successfully synthesized by a novel facile solution-processed wet chemical method, which gave better yield at low cost and temperature. The morphological and optical properties of the material were investigated by Transmission electron microscopy (TEM), UV-Visible and Fluorescence spectroscopies. TEM measurements confirmed the reduction in size of nanorods upon doping. UV-Visible spectra of doped nanorods showed blue shift with respect to the reference undoped ZnO. The defect emissions were completely disappeared from the fluorescence spectra upon doping at high concentrations. These doped nanorods in combination with P3HT were employed in the hybrid solar cells which gave better current densities than their corresponding undoped counterparts.

A detailed study of the photonic modes in microtube cavity of ~ 7-8 µm outer diameter is presented. We demonstrate a new route to the fabrication of individual microtubes with the maximum length of 200 µm, using a vacuum assisted wetting and filtration through a microchannel glass matrix. The microtubes were studied using micro-photoluminescence spectroscopy and luminescence lifetime imaging confocal microscopy. In the emission spectra of the microresonators we find periodic very narrow peaks corresponding to the whispering gallery modes of two orthogonal polarizations with quality factors upto 3200 at room temperature. In order to identify the peaks in the observed mode structure, we have adopted the boundary-value solution to the problem of scattering of electromagnetic waves by a dielectric microcylinder. A strong enhancement in photoluminescence decay rates at high excitation power suggest the occurrence of amplified spontaneous emission from a single microtube. The evanescent field in these photonic structures extends a couple of micrometers into the surroundings providing the possibility for efficient coupling to an external photonic device.

Truxene derivatives, due to their molecular structure and properties, are good candidates for the passivation of defects when deposited onto hybrid lead halide perovskite thin films. Moreover, their semiconductor characteristics can be tailored through the modification of their chemical structure, which allows-upon light irradiation-the interfacial charge transfer between the perovskite film and the truxene molecules. In this work, we analysed the use of the molecules as surface passivation agents and their use in complete functional solar cells. We observed that these molecules reduce the non-radiative carrier recombination dynamics in the perovskite thin film through the supramolecular complex formation between the Truxene molecule and the Pb 2+ defects at the perovskite surface. Interestingly, this supramolecular complexation neither affect the carrier recombination kinetics nor the carriers collection but induced noticeable hysteresis on the photocurrent vs voltage curves of the solar cells under 1 sun illumination.

Strong temperature dependence of optical properties has been studied in visible InAlAs/AlGaAs quantum dots, by employing photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. The fast redshift of the exciton emission peak was observed at much lower temperature range compared to that observed in the InAs/GaAs QDs. In TRPL we did not observe the constant decay time even at low temperature. Instead, the observed decay time increases quickly with increasing temperature, showing 2D properties in the transient dynamic process. We attributed our results to the strong lateral coupling e!ect, which results in the formation of the local minibands or extended states from the discrete energy levels.

The current work focuses on the incorporation of excess amount of gallium (Ga) in zinc oxide (ZnO) nanowires to generate oxygen interstitials for developing p-type conductivity. Ga has been used as an n-type dopant in ZnO lattice by reducing its inherent vacancies up to 3% molar ratio of Ga and Zn employing chemical bath deposition technique. However, the addition of more than 4% of gallium nitrate in the bath solution creates oxygen interstitials which are confirmed from the presence of X-ray photoelectron spectroscopy (XPS) peak at 532.6 eV and the relevant cathodoluminescence (CL) peak at 626 nm of the grown Ga-doped ZnO nanowires. The p-type conductivity of such ZnO nanowires has been confirmed from current-voltage, capacitance-voltage and Hall voltage measurements. The reproducibility of the results indicates the incorporation of excess Ga to be a promising technique for growing p-type ZnO nanowires.

In the present work flame spray pyrolysis synthesis and characterization of the nano-scale phosphors; M′-YTaO 4 and M′-Y(Ta 0.85 Nb 0.15)O 4 have been studied for the first time. Phase and elemental analysis of the produced nanophosphors were carried out by X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) measurements, respectively. The surface morphology and particle size of the nanophosphors were identified using scanning electron microscopy (SEM). The XRD results reveal that the nanophosphors have monoclinic M′-YTaO 4 and M′-Y(Ta 0.85 Nb 0.15)O 4 phases belonging to the presence of M´-form of fergusonite structure. The particle sizes of the nanophosphors were found to be in the range of 50-100 nm. The spectroscopic characterization was performed by both radioluminescence (RL) and thermally stimulated luminescence (TSL) measurements after exposure to X-ray irradiation. Also, photoluminescence and decay times were investigated under UV excitation. The nanophosphors can be concluded as appropriate emissive materials for imaging, display and scintillator applications due to the efficient photoluminescence, moderate radioluminescence (RL) and thermally stimulated luminescence characteristics.

One of the most fascinating characteristics of perovskite solar cells (PSCs) is the retrieved obtainment of outstanding photovoltaic (PV) performances withstanding important device configuration variations. Here we have analyzed CH 3 NH 3 PbI 3-x Cl x in planar or in mesostructured (MS) configurations, employing both titania and alumina scaffolds, fully infiltrated with perovskite material or presenting an overstanding layer.

One of the challenges in improving the efficiency of hybrid solar cells is to remove crystal defects from the semiconducting nanoparticles. Here we report the synthesis, characterization and applications of transition metal doped ZnO nanorods in hybrid bulk heterojunction solar cells. Mn x Zn 1Àx O and Ni x Zn 1Àx O with dopant concentrations ranging from x = 0.01-0.10 were successfully synthesized by a novel facile solution-processed wet chemical method, which gave better yield at low cost and temperature. The morphological and optical properties of the material were investigated by Transmission electron microscopy (TEM), UV-Visible and Fluorescence spectroscopies. TEM measurements confirmed the reduction in size of nanorods upon doping. UV-Visible spectra of doped nanorods showed blue shift with respect to the reference undoped ZnO. The defect emissions were completely disappeared from the fluorescence spectra upon doping at high concentrations. These doped nanorods in combination with P3HT were employed in the hybrid solar cells which gave better current densities than their corresponding undoped counterparts.

A pyrene-hydroxyquinoline tethered ingeniously designed and developed azine based Schiff base luminophoric system, 2-((Z)-((E)-(pyren-1-ylmethylene)hydrazono)methyl)quinolin-8-ol (PHQ) has been explored as a selective 'TURN ON' chromofluorogenic sensor for hazardous Hg 2 + ions in ethanol/H 2 O (9 : 1,v/v) medium with detection limit of 0.22 μM and binding constant of 1.09 x 10 3 M À 1. The detail scrutiny of mechanism of interaction between PHQ and Hg 2 + is rationalized through various techniques like 1 H NMR titration, FT-IR, mass spectral analysis, theoretical computations, Job's plot, dynamic light scattering (DLS), transmission electron microscopy (TEM), time resolved excited state decay dynamics and fluorescence reversibility studies, which concretely declare a synergistic effect of supramolecular aggregation induced enhanced emission and deactivation of photo-induced electron transfer (PET) processes. On the other hand, a 'TURN OFF' luminescence feature of highly fluorescent PHQ hydrosol is utilized for the selective screening of powerful explosive picric acid (PA) in aqueous medium and mechanism is deciphered by means of steady state, time-resolved emission as well as computational studies which altogether reflect a combined effect of PET and ground state complexation between probe and PA. The commendable detection limit of 55 nM and Stern-Volmer quenching constant of 1.48 x 10 4 M À 1 with high quenching efficiency of 84% increases the pertinency of the nano-probe.

In the present work we report the effect of quantum confinement, biaxial and uniaxial strains on electronic properties of two dimensional (2D), one dimensional (1D) and layered system of Methylammonium lead iodide (MAPI) in cubic phase using first principles calculations based on density functional theory for its implications to solar cell. Our studies show that the bandgap of MAPI is dimension dependent and is maximum for 1D. We also found that the band gap of 2D MAPI can be modulated through application of biaxial strain, which shows linear relation with strain; compressive strain decreases the band gap whereas tensile strain increases the band gap. 1D MAPI shows near parabolic response towards strain which increases with compressive and tensile strain. Our studies show that the 2D MAPI is better for a solar cell due to lower effective mass of electron and hole arising from the strong s-p anti-bonding coupling. The calculated solar cell parameters suggest that the 2D MAPI is best suited for solar cell applications. The calculated open circuit voltage, fill factor and efficiency are highest for 2D MAPI. The highest theoretical efficiency of 2D MAPI is 23.6% with mesoporous (mp)-TiO 2 electrode.

Dot-in-a-well (DWELL) heterostructures have been extensively researched in quantum dot based infrared photo-detectors as it offers tuning of detection peak wavelength, low dark current and a higher operating temperature with optimized quantum well thickness. In this paper, we have correlated experimentally observed opto-electronic properties of three different InAs quantum dots in DWELL configuration with varying ternary capping (In 0.15 Ga 0.85 As-Strain reducing ternary alloy) thickness from 4 to 8 nm to the proposed simulation model for achieving high efficiency in device performance. Low temperature (8 K) photoluminescence (PL) spectra exhibits a blue shift of around 24 nm along with a decrease in intensity as the capping thickness increases above 6 nm. Decrease in full width at half maximum (FWHM) value for PL peak was observed as the capping thickness is increased from 6 to 8 nm. These are attributed to formation of InGaAs quantum wells via dots sublimation process. Presence of InGaAs wells for 8 nm capped sample was confirmed using low temperature photoluminescence measurement at 2.54 W/cm 2 and photoluminescence excitation (PLE). Time resolved photoluminescence spectroscopy gave further insight into the carrier dynamics of the grown structures and confirms undesirable quantum well formation in 8 nm capped sample. Improved optical characteristics with formation of defect-free structure having larger quantum dot was achieved in 6 nm capped sample affirmed using transmission electron microscopy images. Low dark current density with high confinement energy was achieved from 6 nm capped DWELL structure. Dominant spectral response peak was obtained at 7.56 mm from all the samples. A concentration-dependent theoretical strain model using the Schr€ odinger equation was developed to calculate the potential, ground-state and inter-sub band energy-levels. It was validated by comparing experimentally achieved PL and PLE peaks along with spectral response peaks. The theoretical model was used to calculate the energy levels corresponding to conduction and valence bands for InGaAs well and indium concentration in the well was obtained to be around 30%. Similarly, concentration dependent 2D to 3D transition for In x Ga 1-x As (0 x 1) quantum dots formation was modelled using People-Bean relation which matches with the proposed simulation.

Photoluminescence intensity enhancement has been observed for self-assembled InAs quantum dots (QDs) coated with gold nanoparticles and associated to surface Plasmon (SP) coupling effects. Photoluminescence intensity enhancement was investigated for different temperatures, excitation energy, laser power, GaAs cap layer thickness, and QD morphology. It was found that the SP-QD coupling efficiency increases with the increase of the temperature. It was also observed that this effect is enhanced for QDs grown on (3 1 1)B GaAs.

Laser-based patterning of CIGSe solar cell absorber layers for monolithic series connection by nanosecond (ns) and picosecond (ps) laser pulses introduces material modifications in the vicinity of the patterned lines, which affect the CIGSe composition, the electrical properties, the extent of the dead area and finally the efficiency of the complete solar cell. Here, spectral and time-resolved photoluminescence is used to show that Cu vacancies and anti-sites are formed next to the edges of the scribe lines, which lead to the formation of additional energy states in the band gap near the valence band edge. A significant reduction in charge carrier lifetime is noted as the scribed lines are approached from unaffected areas of the material. Since this effect is absent in the mechanical needle patterning, it can clearly be attributed to the influence of the laser pulses. Despite the laserinduced material modification, laser-based CIGSe patterning outperforms needle-based patterning. Advantages regarding the width of the affected edge areas were observed when using ns pulses instead of ps pulses, which could be due to the lower peak intensities.

Gallium nitride ͑GaN͒ and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet ͑UV͒ emitters and detectors ͑in photon ranges inaccessible by other semiconductors͒ and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The point defects include native isolated defects ͑vacancies, interstitial, and antisites͒, intentional or unintentional impurities, as well as complexes involving different combinations of the isolated defects. Further improvements in device performance and longevity hinge on an in-depth understanding of point defects and their reduction. In this review a comprehensive and critical analysis of point defects in GaN, particularly their manifestation in luminescence, is presented. In addition to a comprehensive analysis of native point defects, the signatures of intentionally and unintentionally introduced impurities are addressed. The review discusses in detail the characteristics and the origin of the major luminescence bands including the ultraviolet, blue, green, yellow, and red bands in undoped GaN. The effects of important group-II impurities, such as Zn and Mg on the photoluminescence of GaN, are treated in detail. Similarly, but to a lesser extent, the effects of other impurities, such as C, Si, H, O, Be, Mn, Cd, etc., on the luminescence properties of GaN are also reviewed. Further, atypical luminescence lines which are tentatively attributed to the surface and structural defects are discussed. The effect of surfaces and surface preparation, particularly wet and dry etching, exposure to UV light in vacuum or controlled gas ambient, annealing, and ion implantation on the characteristics of the defect-related emissions is described.

The Mg concentration dependence of the performance of inverted type organic solar cells based on Mgdoped ZnO nanorod arrays and poly(3-hexylthiophene) (P3HT) has been investigated. The Mg dopants with various concentrations (0, 1, 3 and 5 at.%) were introduced during the hydrothermal growth of the ZnO nanorod arrays on fluorine-doped tin oxide (FTO) glass substrate. The P3HT was deposited onto Mg-doped ZnO nanorod arrays by spin coating technique, followed by deposition of Ag as anode using magnetron sputtering technique. The length and density of Mg-doped ZnO nanorods increased, whereas the diameter decreased with the Mg concentration. The short circuit current density (J sc) and open circuit voltage (V oc) improved with increasing of Mg concentration up to 3 at.%, which could be attributed to increased interfacial area for more efficient exciton dissociation and reduced charge recombination as a result of lower number of oxygen interstitials which act as electron traps in ZnO. However, the J sc and V oc started to decrease at Mg concentration of 5 at.%, mainly due to poor infiltration of P3HT into the high-density 5 at.% Mg-doped ZnO nanorod arrays and increase of Mg dopant-related trapping centers. The highest power conversion efficiency of 0.36 ± 0.02% was achieved at Mg doping concentration of 3 at.%, an enhancement of 225% as compared to that based on undoped ZnO nanorod arrays.

Colloidal semiconductor nanoplatelets (NPLs) are quasi 2D-nanostructures that are grown and processed inexpensively using a solution based method and thus have recently attracted considerable attention. We observe two features in the photoluminescence spectrum, suggesting two possible recombination channels. Their intensity ratio varies with temperature and two distinct temperature regions are identified; a low temperature region (10K < T < 90K) and a high temperature region (90K < T < 200K). This ratio increases with increasing temperature, suggesting that one recombination channel involves holes that are weakly localized with a localization energy of 0.043meV. A possible origin of these localized states are energy-variations in the xy-plane of the nanoplatelet. The presence of positive photoluminescence circular polarization in the magnetically-doped core/multi-shell NPLs indicates a hole-dopant exchange interaction and therefore the incorporated magnetic Manganese ions act as a marker that determines the location of the localized hole states. 1 Time-resolved measurements show two distinct timescales (τ fast and τ slow) that can be modeled using a rate equation model. We identify these timescales as closely related to the corresponding recombination times for the channels. The stronger hole localization of one of these channels leads to a decreased electron-hole wave function overlap and thus a decreased oscillator strength and an increased lifetime. We show that we can model and understand the magnetic interaction of doped 2D-colloidal nanoplatelets which opens a pathway to solution processable spin controllable light sources.

The minority-carrier lifetime is a crucial parameter for the improvement of electronic or optoelectronic materials and particularly solar cells. We have investigated the potential of the time-correlated single photon counting (TCSPC) for the measurement of time-resolved photoluminescence (TRPL) and the direct determination of the effective minority-carrier lifetime in silicon. The TRPL technique was found to be in very good agreement with quasi-steady state and transient photoconductance results obtained from Sinton Instruments and microwave photoconductance from Semilab. We have demonstrated the ability to evaluate the effective carrier lifetime of silicon substrates over a broad range of values (between a few μs and less than 500 μs).

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