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Through luminescence downshifting (LDS) effect, high energy solar radiation is converted to a wavelength region where the response of the photovoltaic devices is better. Additionally, LDS layers deposited on organic materials-based devices could provide a protective feature for the atmospheric agents, like ultraviolet photons, oxygen and moisture, that cause material degradation. This work reports an optimization procedure for the optical properties of a LDS layer composed by the organic dye commercially called Kremer Blue dispersed on poly-methyl methacrylate (PMMA) inert matrix. With optimized values for photoluminescence quantum yield (PLQY) and absorbance parameters, this LDS layer became an efficient, low cost UV filter for solar cells based on poly (3-hexylthiophene):[6,6]‐phenyl C61‐butyric acid methyl ester (P3HT:PC61BM) blend, at the same time that provoked a significant increasing (around 6%) in the device's photocurrent production. Such optimization procedure can be applied to other materials that could be candidates as luminescence down-shifters for the various photovoltaic technologies currently available.

This study analyzed the interfaces between poly(3-hexylthiophene) (P3HT) on an Au substrate (Au/P3HT) and on Au coated with poly(3,4-ethyldioxythiophene) doped with poly(4-styrenesulfonate) (Au/PEDOT:PSS/P3HT). The different morphologies on the nanostructured Au surface were obtained by applying cyclic potentials using cyclic voltammetry to produce different interfaces in the two systems studied. Under atomic force microscopy (AFM), the number of potential activation cycles was found to produce different roughnesses on the surfaces of the Au electrodes. The interfaces formed were examined by electrochemical impedance spectroscopy (EIS) to identify charge transfer processes for the different segments of the P3HT. The systems were characterized optically by measuring photoluminescence (PL), emission decay time and photoluminescence quantum yield (PLQY). It was observed that the differences in the surface morphologies of the substrates significantly influenced the electrical and optical properties of the P3HT at the interfaces.

The polymer Poly[tris(2,5-bis(hexyloxy)-1,4-phenylenevinylene)-alt-(1,3-phenylenevinylene)] (PTDPV) has a broad range of visible emission extending from green to red, and thus may be useful for obtaining a white light emitting blend. Nevertheless, the amount of works found in the literature dealing with the optical emission properties of PTDPV is very small. In this work, we performed a study of the optical properties of this material using photoluminescence and of the vibrational properties using Raman and Fourier Transform Infrared Spectroscopy (FT-IR) techniques. At the same time, we use the Density Functional Theory (DFT) method to calculate the optical, vibrational and molecular properties of PTDPV. We have obtained the best DFT results using a hybrid functional and a simple basis set (DFT / B3LYP and 6–31 g *), without addition of correction or polarization factors (+). To reach these conclusions, we compared the HOMO and LUMO values obtained via DFT with those found in the literature. Furthermore, the Raman and IR simulations obtained using this basis set were compared with the experimental results of the PTDPV, showing great agreement. From the vibrational modes obtained, it was possible, using the Lin model [ [1] , [2] , [3] ], based on the Franck Condon approximations, to reconstruct the photoluminescence spectrum of the PTDPV. Based on these reconstructed spectra, it was possible to establish the contribution of the different vibrational modes to the vibronic emissions of the photoluminescence spectrum. The quality of the reconstruction obtained with the DFT results and that obtained with the experimental ones are very close, showing the possibility of using in this reconstruction the calculated vibrational modes when the experimental ones are not available. To demonstrate the application of this material, PFO:PTDPV blends were prepared, with which white photoluminescence was obtained. OLEDs prepared with these blends, in suitable proportions, emitted white light using low excitation power, demonstrating the feasibility of using PTDPV in OLEDs for lighting, which could be prepared by solution on large areas.

Mixtures of luminescence downshifting (LDS) materials has been used to increase the efficiency of poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) bulk heterojunction solar cell. This layers convert more energetic photons to lower energies that are better matched with wavelength peak of the external quantum efficiency (EQE) of a P3HT:PC61BM solar cell. Experimental studies were used to optimise the optical properties of LDS layers including the maximum of absorbance and the photoluminescence quantum yield (PLQY). To provide the significant improvements, combinations of LDS mixtures were prepared to provide the greatest absorption and PLQY. The approach is shown to simultaneously improve the photocurrent and increase the lifetime of the device by absorbing UV light. By optimising the optical properties of the LDS mixture, a relative increase of about 20% in the photocurrent density produced by the P3HT:PCBM cell could be achieved, which to our knowledge is one of the most significant reported for OPVs.

Segregation is an effect observed when CH3NH3Pb(I1-xBrx)3 mixed perovskites are illuminated with photon energy greater than their band gap. It is concerned to the formation of low band-gap I-rich domains that constitute attractive centers for the recombination of photogenerated electron-hole pairs, causing a significant red-shift of photoluminescence of the material with consequences for the operation of optoelectronic devices, such as light emitting diodes and photovoltaic cells. This effect is commonly observed as reversible, with a spontaneous remixing of the halogens occurring when the material is left in the dark. While the segregation process is already relatively well documented in the literature, much less attention has been paid to the causes and understanding of the remixing process. In this work are presented results that demonstrate the strong dependence of the remixing with the pressure to which the material is submitted. When the perovskite film is subjected to low pressure, it is noticed that the maximum displacement of PL during the dark time is significantly lower than in the case of the film at ambient pressure, indicating that under low pressure the remixing process is hampered. The dependence of the remixing process with the pressure can be used in new hybrid perovskite applications, such as in photoinscription to memory storage.

White light emitting diodes (WOLEDs) were prepared from easily processable materials, well known in the literature (PFO and P3OT), with the intention of exploring the blueshift of the P3OT emission, when in low concentration, in blends with PFO in beta phase. The photoluminescence (PL) study, through donor excitation, of PFO:P3OT blends with different relative concentrations, served as basis for comparing and predicting the behavior of these blends when excited via carrier injection. Despite the good PL results, the white emission of the PFO:P3OT blends with low P3OT concentration was not replicable via electroluminescence (EL), due to the lack of a more intense green component. To circumvent this problem, new PFO:P3OT blends were prepared with the insertion of CdSe(ZnS) quantum dots (QDs), which have green emission. With this new composition, the devices presented white emission (0.33:0.33) at low voltages and current (8 V, 0.250 mA). For the PFO:CdSe(ZnS):P3OT devices, the change in the applied voltage also showed the possibility of tuning the emission of these structures. Therefore, this blend is very attractive to make solution processed, low cost, large area, and flexible lighting panels.

The density functional theory (DFT) simulation method has been highlighted in last years, due to its ability to predict optical, electronic and molecular properties of different materials, presenting good agreement with experimental results. In this work, we conducted a study of the structural and optical properties of the copolymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-( N -(4-sec-butylphenyl)diphenylamine)] (TFB) which appears with great emphasis in the literature being applied mainly as hole transporting and electron blocking layer in devices. Raman and FT-IR measurements were taken, and the obtained results were compared with those calculated via DFT method (DFT/B3LYP and 6–31g*). The comparison between experimental and DFT results were performed, and a good agreement between both results was observed, showing discrepancy lower than 7% for the vibrational peak positions, in energy. Also, photoluminescence (PL) measurements were performed at room and liquid nitrogen temperature. The PL spectra were reconstructed via Franck Condon, through the Huang-Rhys parameters, by using the vibrational modes obtained both experimentally and via DFT. This procedure allowed to determine the contribution of the different vibrational modes to the photoluminescence vibronic bands.

Organic photovoltaic devices (OPVs) have a shorter working lifespan and lower power conversion efficiency if compared to traditional silicon-based solar cells. Intending to increase the lifespan and improve the performance of OPVs, in this work we have applied a layer of Cadmium Sulfide (CdS) Quantum Dots (QDs) encapsulated in Zinc Sulfide (CdS/ZnS) to act as a luminescence down-shifting (LDS) layer in poly(3-hexylthiophene-2,5-diyl) (P3HT):phenyl-C61-butyric-acid-methyl ester (PCBM) organic solar cell. The QDs were synthesized by the hot injection route and deposited by casting on a device with ITO/PEDOT:PSS/P3HT:PCBM/Al structure. OPVs with and without the LDS layer of QDs were tested over time of exposure to the illumination of an AM1.5 solar simulator. The results showed that in the cells with the QDs layer, there was an increase in Open Circuit Voltage (Voc), Short Circuit Current Density (Jsc), Fill Factor (FF), and Power Conversion Efficiency (η), after 70 min of exposure, reaching a peak of these parameter values around 120 min of light exposure, while in the cells without the LDS layer, these parameters have decreased over the entire exposure time. This behavior was explained taking into account the improvement of the photoluminescence (PL) yield by the QDs film and the illumination process.

During the last few years, great interest has been given to research in organic solar cells. Apart from the low cost techniques needed for the production of the active layers, the plasticity of these types of materials offers an additional benefit compared to traditional silicon-based solar cells. However, the commercialization of organic photovoltaic technologies is limited by rapid degradation and low efficiency. Recently, the use of luminescent down-shifting (LDS) films has been explored as a tool for increasing the external quantum efficiency of organic-based devices. A previously observed superposition in the emission–absorption spectra of Kremer blue (Kb) and Coumarin 7 (C7) dyes suggests that an energy transfer process can take place among these molecules. Decay time photoluminescence measurements in K b 100 − x C 7 x samples were carried out to confirm a Foster resonance energy transfer process. An optimization process following the design of experiments methodology was carried out to adjust the optical properties of K b 100 − x C 7 x films for applications in the P 3 H T: P C 61 B M organic photovoltaic device. An increment of 18.6% in the short circuit current density ( J s c) of P 3 H T: P C 61 B M devices coated with a K b 92.5 C 7 7.5 LDS film was observed. A degradation test shows a rapid drop in the electric current density of devices without the LDS film, which we attribute to the so-called burn-in, widely reported in the literature. For devices coated with the K b 92.5 C 7 7.5 LDS film, no rapid drop in the electric current density was observed, suggesting that the use of the luminescent layer mitigates the photoinduced burn-in loss.

The optical properties of electrochemically synthesized P3HT films were studied. After the synthesis, the samples were divided into two groups: as prepared (P3HT-E) and the chemically reduced (dedoped) in a basic medium (P3HT-D). These films were characterized by photoluminescence (PL), Raman and photoluminescence time decay techniques. Comparing the PL spectra of the films, it is noted that P3HT-D has a higher PL intensity. The films were exposed to a continuously incident 405 laser and the PL spectra were monitored every 10 min. For both samples, the photoluminescence intensity along the photo-irradiation showed an increase, although the P3HT-E sample presented a greater increase. Regarding the Raman spectra before and after the photo-irradiation, it was noticed that for P3HT-D there were no changes, whereas P3HT-E showed significant changes. The Raman spectrum of P3HT-E sample at the photo-irradiated point was very similar to the Raman spectrum of P3HT-D. Thus, it was proposed that laser irradiation induces a reduction process similar to chemical reduction. Although both processes lead to an increase in the PL intensity, there is a difference in the position of the emission bands. Unlike chemical doping, the photoreduction does not remove the charge transfer complexes of the polymer matrix, but only breaks their bond with the backbone chain. From the data of PL decay, it was observed that in the film P3HT-E, there was a predominance of longer times, evidencing the presence of barriers for the energy transfer, while, for the P3HT-D, the contribution of shorter decay times predominates, evidencing a greater energy transfer between the chains.

Self-assembled cadmium sulfide (CdS) thin solid films were synthesized by chemical bath deposition (CBD) technique for different deposition times (20 min.–24 h) on transparent and flexible polyester substrates using sodium acetate complex agent. CBD-CdS films were also deposited on glass (BK7) and quartz substrates, as reference. CBD-CdS films on polyester substrate showed a homogeneous deposition, reduction of chemical or structural defects (green emission), and large photoluminescence efficiency in comparison of CBD-CdS films deposited on Si-based substrates. Simulation of PL temperature dependence for polyester CdS films using the rate equation for the carrier population showed the presence of two decay pathways due to cadmium and sulphide defects at crystallite.

Optical properties of self-assembled quantum dots (SAQDs) samples were investigated through photoluminescence (PL) with variation of temperature and excitation density. Two types of InAs SAQDs were studied: dots grown directly on GaAs, and dots grown inside an InGaAs quantum well, heterostructure known as dot-in-a-well (DWELL). The QDs structures were produced through conventional growth (with several deposition rates) as well as through pulsed growth mode, which consists in deposition cycles with growth interruption intervals. In the samples of QDs grown on GaAs, two PL peaks were identified and their behavior as a function of temperature can be explained by taking into account a bimodal distribution of dot sizes. For the DWELL sample only one PL peak was identified, which was assigned to the emission of the ground state of QDs belonging to a larger Gaussian distribution of dot sizes. SAQDs samples showed an anomalous behavior of the peak energy, integrated intensity and linewidth (FWHM) of the PL signal, as a function of temperature. To explain the observed results, a model was proposed that takes into account the size distribution of dots and the excited state emission of the family of larger dots. The QDs can be distributed in a bimodal size distribution, and the emission from the excited state can be observed, or not, depending on the growth conditions and on the potential profile of the dot. Such aspects can be combined to explain several peculiarities of the PL spectra observed in this and other published papers on SAQDs optical properties.

In this work we used different methods and parameters of deposition in order to obtain thin films of poly(p-phenylene vinylene) (PPV) with different structural properties. These properties were characterized by measurements of absorbance, Raman scattering, and photoluminescence (PL) as a function of temperature. The analysis of experimental data allowed us to observe the dependence of optical spectra with the effective conjugation length and the interaction between the polymer chains. In particular, we were able to correlate the dependence of the PL line shape with temperature and the values of emission anisotropy factor of the films with the intra and interchains energy migration processes. These results led us to conclude that such processes are essential ingredients for a proper analysis of the optical response of thin films of conjugated polymers.

In the present paper the layer-by-layer self-assembly (LbL) technique was used to obtain poly( p -phenylenevinylene) (PPV) films exchanging the counter-ion Cl of the poly(xylylidene tetrahydrothiophenium) (PTHT ) precursor polymer by Congo Red (CR) azodye. The objective of the present paper is to study the azodye photoisomerization trans → cis → trans process as a function of the PPV thermal annealing procedure. The LbL PTHT/CR films were thermally converted into LbL PPV/CR films at low (110 °C) and conventional (230 °C) temperatures, and characterized through both absorption and emission techniques. We found that the unexpected emission band above 650 nm, attributed to the azodye, for LbL PPV/CR film thermally converted at lower temperature (110 °C) is an evidence of the considerable decrease of CR degradation in the polymeric matrix. Emission anisotropy and selective photoluminescence were carried out to characterize qualitatively the exciton energy transfer.

In this work, MEH-PPV and BDMO-PPV films were deposited by spin-coating on “dirty” textile substrates of canvas, nylon, canvas with resin, jeans and on glass and the temperature dependence of the optical properties of them was studied by photoluminescence and Raman (300 K) techniques. The temperature dependence of the energy, of the half line width at half height of the purely electronic peak, of the integrated PL intensity and of the Huang-Rhys factor, S = I (01) / I (00) , were obtained directly from the PL spectrum. For an analysis of the vibrational modes involved, Raman measurements were performed on substrates with and without polymers deposited and the results compared with those found in the literature. The films of MEH-PPV and BDMO-PPV showed optical properties similar to those films deposited on other substrates such as glass, metals, etc. It was observed an inversion of the first vibrational band in relation to the purely electronic peak with increasing temperature in the films deposited on nylon and canvas. The vibrational modes obtained by Raman were used to compose the simulation of the PL line shape of BDMO-PPV films on canvas and nylon, using a model proposed by Lin [29] .

In this work, poly (3-methylthiophene) (P3MT) and poly (3-octylthiophene) (P3OT) films were synthesized electrochemically in non-aqueous media through the oxidation of the monomers, (3-methylthiophene and 3-octylthiophene), using a standard three-electrode cell in acetonitrile with 0.100 mol L-1 LiClO4. The polymeric thin films were deposited on platinum plates for optimal quality control of the process. It was observed that the material as-prepared for the anodic electropolymerization undergoes a natural process of de-protonation as a function of time. Moreover, the partial dedoped form, obtained in NH4OH solution, presents a good chemically stabilite form but, when radiated with blue light again becomes unstable. Films obtained by these methods have been characterized by cyclic voltammetry, Raman and photoluminescence spectroscopy. Both Raman and photoluminescence (PL) spectra results led to the characterization of two structures (pristine and non-pristine forms of thiophene rings) which formed the P3MT and P3OT polymer chain. These results were associated with the stabilization of pristine chains and mixed chains (non-pristine structures), radical cation and dication forms, in the polymeric film. Their bands in the Raman and PL spectra are wide and asymmetric and their adjustments by Gaussian functions were necessary, indicating that there are three distinct contributions to the vibration and two to the emission spectra in the formed polymeric material.

In this work, poly (3-methylthiophene) (P3MT) or poly (3-octylthiophene) (P3OT) films were synthesized electrochemically with polydiphenylamine (PDFA), in layers and blended copolymers, in non-aqueous media through the oxidation of the monomers, using a standard three-electrode cell in acetonitrile with 0.100 mol L −1 LiClO 4 . The polymeric thin films were deposited on platinum plates for optimal quality control of the process. The electrochemical behaviors of as-prepared and partial dedoped films obtained by these methods was verified by cyclic voltammetry (CV) and have been characterized by UV–vis reflectance, resonance Raman, electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. Based on data obtained by UV–vis reflectance spectroscopy, it was possible to characterize the chemical species in the polymer matrix using the resonant Raman method. Both Raman and PL spectra results led to the characterization of three structures (aromatic, semi-quinone and quinone forms of thiophene rings) which formed the homopolymers and copolymers chain. The EPR measurements were taken to quantify the semi-quinone species stabilization at 298 and 77 K temperatures. The ionization potential ( I P ), electron affinity ( E.A. ) and gap energy ( E g ) parameters were determined based on the technical analyses carried out.

This work reports on the results of magnetophotoluminescence (MPL) measurements carried out in a sample containing two Al 0.35 Ga 0.65 As/GaAs, coupled double quantum wells (CDQWs), with inter-well barriers of different thicknesses, which have the heterointerfaces characterized by a distribution of bimodal roughness. The MPL measurements were performed at 4 K, with magnetic fields applied parallel to the growth direction, and varying from 0 to 12 T. The diamagnetic shift of the photoluminescence (PL) peaks is more sensitive to changes in the confinement potential, due to monolayer variations in the mini-well thickness, rather than to the exciton localization at the local potential fluctuations. As the magnetic field increases, the relative intensities of the two peaks in each PL band inverts, what is attributed to the reduction in the radiative lifetime of the delocalized excitons, which results in the radiative recombination, before the excitonic migration between the higher and lower energy regions in each CDQW occurs. The dependence of the full width at half maximum (FWHM) on magnetic field shows different behaviors for each PL peak, which are attributed to the different levels and correlation lengths of the potential fluctuations present in the regions associated with each recombination channel.

In this work we simulate the photoluminescence (PL) spectra of BDMO-PPV thin films prepared by spin-coating technique on glass and on copper, as a function of temperature (12–300 K). Simulations were done using two theoretical models based on (i) the SSH theory where the line shape of the purely electronic transition is partly generated by localized states and partly by delocalized states and (ii) the semi-empirical model containing the coupling between localized molecular excitons and vibrational modes in Franck–Condon approach. Four active vibrational modes have been considered: C–C stretching coupled to a C–H bending of the phenyl ring at 1111.5 cm −1 , inter-rings C–C stretching at 1282.2 cm −1 , CC stretching coupled to a C–H bending of the vinyl group at 1309.3 cm −1 , C–C stretching of the phenyl ring at 1580.2 cm −1 . Additional vibrational mode of 403 cm −1 associated with C–C–C out-of-plane bending allowed leastwise for this material to adjust well with the characteristic asymmetry of the purely electronic transition. Finally, application of theoretical models are strongly dependent on the well-resolved PL spectra, i.e., electronic transition peak presented a relatively thinner HWHM and an asymmetric line shape.

In this work, poly(3-octylthiophene) (P3OT) films were synthesized electrochemically in non-aqueous media through the oxidation of the monomer, (3-octylthiophene), using a standard three-electrode cell in acetonitrile with 0.05 mol L S1 LiClO4 or 0.05 mol L S1 Et4NBF4. The polymeric films were deposited on fluorine tin oxide (FTO). The partial dedoping was obtained in NH4OH solution, providing a good chemical stability of the formed material. The films obtained through this method have been characterized by Fourier-transform infrared spectroscopy (FT-IR), electron paramagnetic resonance (EPR), UV–Vis absorption, and photoluminescence (PL) spectroscopy. The FT-IR and EPR spectra together gave the results that led to characterization of two structures (pristine and non-pristine forms of thiophene rings) while forming the P3OT polymer chain. These results were associated with the stabilization of pristine chains and mixed chains (non-pristine structures) in the polymeric film. Their bands in the PL spectra are wide and asymmetric and their adjustments by Gaussian functions was necessary; this was the main indication that there are two distinct contributions to the emission spectra. These two contributions are attributed to the emission by mixed chains (Gaussian centered at higher energy) and by pristine chains (Gaussian of lower energy) present in the formed polymeric material. Copyright 2010 John Wiley & Sons, Ltd.

In this work, we present a detailed study on the optical properties of two GaAs/Al 0.35 Ga 0.65 As coupled double quantum wells (CDQWs) with inter-well barriers of different thicknesses, by using photoluminescence (PL) spectroscopy. The two CDQWs were grown in a single sample, assuring very similar experimental conditions for measurements of both. The PL spectrum of each CDQW exhibits two recombination channels which can be accurately identified as the excitonic e 1 -hh 1 transitions originated from CDQWs of different effective dimensions. The PL spectra characteristics and the behavior of the emissions as a function of temperature and excitation power are interpreted in the scenario of the bimodal interface roughness model, taking into account the exciton migration between the two regions considered in this model and the difference in the potential fluctuation levels between those two regions. The details of the PL spectra behavior as a function of excitation power are explained in terms of the competition between the band gap renormalization (BGR) and the potential fluctuation effects. The results obtained for the two CDQWs, which have different degrees of potential fluctuation, are also compared and discussed.

Self-sustained cast films formed from heterogeneous blends of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) conjugated polymer and thermoplastic polyurethane (TPU) were investigated by photoluminescence (PL) and scanning electron microscopy (SEM). A blue shift is observed for the pure electronic transition PL peak (E00) with decreasing MEH-PPV concentration. The two clear shoulders in the PL spectra at higher energy than the E00 peak appear due to the formation of small conjugation segments of the MEH-PPV molecules at the interface of the spherical MEH-PPV domains. This assumption and the origin of the blue shift were confirmed by correlating the average size of the MEH-PPV domains, observed from SEM images, and the analysis of the PL spectra at low temperatures. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

We present a comparative study carried out on the optical and electrical characteristics of undoped and Te doped AlGaAsSb/AlAsSb Bragg mirrors with 6.5 pairs of layers and bulk undoped and Te doped AlGaAsSb epilayers alloys lattice matched on InP, grown by molecular beam epitaxy, using SIMS, photoluminescence, reflectivity and IxV techniques. The temperature dependence of PL transitions observed in the Bragg mirrors are similar to that observed in bulk samples and associated with the donor and acceptor recombinations in alloys with electrostatic potential fluctuations described by quasi-donor-acceptor-pair (QDAP) models. We verified by SIMS the presence of a macro fluctuation in the Te profile concentrations in the growth direction of the doped Bragg mirror. The influence of doping, alloy and macrofluctuation of incorporated Te on the optical and electrical properties of doped Bragg mirror is analyzed. The AlGaAs/AlAsSb seems a very attractive option for VCSEL technology.

A detailed study of the photoluminescence properties in undoped and Te-doped AlGaAsSb alloys lattice matched to InP is presented. Photoluminescent temperature and excitation intensity dependences are employed to discuss the origin of the dominant transition and the influence of the presence of Al and Te on the fluctuation of the electrostatic potential in the epitaxial layers. The behavior of PL dominant transitions is associated with the quasi-donor-acceptor-pair (QDAP) model at a low temperature interval. The temperature dependence of photoluminescence intensities showed characteristics similar to those observed for amorphous semiconductors and disordered superlattices. The presence of two V elements, or the presence of Sb in the ternary and quaternary (III)-III-V-V alloys, causes a reduction in photoluminescence intensity temperature dependence, when compared to (III)-III-III-V alloys.

Photoluminescence is one of the most used spectroscopy techniques for the study of the optical properties of semiconducting materials and heterostructures. In this work the potentiality of this technique is explored through the investigation and characterization of structural imperfections originated from fluctuations in the chemical composition of ternary and quaternary alloys, from interface roughnesses, and from unintentional compounds formed by the chemical elements intermixing at the interfaces. Samples of GaAs/AlGaAs, GaAsSb/GaAs, GaAsSbN/GaAs and GaAs/GaInP quantum well structures are analyzed to verify the influence of the structural imperfections on the PL spectra

In this work we present a study on the carbon incorporation in InGaP layers grown by Chemical Beam Epitaxy as a function of growth temperature. Hall measurements show that the electron concentration increased from 2.3×1014 to 8.9×1016 cm−3 as the growth temperature increased from 500 °C to 560 °C. Lower growth temperature InGaP layers are more resistive. Using photoluminescence and a carbon-implanted sample, we identify an acceptor level at nearly 45 meV from the top of the valence band. Part of incorporated carbon acts as an acceptor and part as a donor. The electrical and photoluminescence measurements show that the ratio between acceptors and donors increase as the growth temperature decreases. Due to this compensation, samples grown at lower temperatures (500 °C) present a resistively 104 times higher than those grown at high temperature (560 °C).

Com o objetivo de estudar as propriedades ópticas do polímero TFB, e de sua blenda com o polímero P3HT, foram realizadas medidas de absorção e fotoluminescência (PL) para ambos polímeros, e de uma blenda TFB:P3HT (95:05), depositados sob lâminas de vidro, pela técnica de deposição por centrifugação. Foram utilizados diferentes solventes na preparação das amostras, a fim de comparar os possíveis efeitos dos mesmos na emissão e transferência de energia entre os dois polímeros. Medidas de tempo de decaimento foram realizadas para verificar se esse parâmetro se altera para o TFB preparado a partir de diferentes solventes, e quando na presença de P3HT. Os resultados das medidas de fotoluminescência dos polímeros isolados mostram que não há mudança na forma da emissão desses polímeros conforme se alteram os solventes utilizados no preparo das soluções. Entretanto, no caso da blenda, ocorre mudança na transferência de energia entre os polímeros, dependendo do solvente, deslocando a cor de emissão da amostra para o azul ou vermelho. No espectro de fotoluminescência da blenda é observada uma emissão em 575 nm, a qual não aparece na fotoluminescência das amostras do TFB e P3HT isolados. A partir das medidas de tempo de decaimento, vimos que, nas amostras de TFB, não há alteração considerável com a mudança do solvente. Já para a blenda, nota-se que a presença do P3HT faz com que o tempo de decaimento do TFB diminua, especialmente quando o solvente utilizado foi o clorofórmio.

AlGaAs structures with different aluminum concentration (x=0.0, 0.17, 0.30, and 0.40) were characterized by photoluminescence and photoreflectance techniques. The temperature dependence of optical transitions in the temperature ranging from 2 to 300 K were investigated. Y. P. Varshni [Physica (Utrecht) 34, 194 (1967)], L. Vina et al. [Phys. Rev. B 30, 1979 (1984)], and R. Passler [Phys. Status Solidi B 200, 155 (1997)] models were used to fit the experimental points. The Passler model gave the best adjustment to the experimental points. The tree models showed that the empirical parameters obtained through the adjustment of the experimental data in the three different models are aluminum composition dependent in the ternary alloy.

AlxGa1-xAs/GaAs double quantum well structures with different well thickness and different barrier aluminum concentration (x = 0.17, 0.30, 0.40) were characterized by the photoluminescence technique. The temperature dependence of excitonic transitions in the temperature range of 2 K to 300 K were investigated. The photoluminescence data obtained give clear evidence of the influence of the aluminum concentration on the temperature dependence of excitonic transitions in the quantum wells. Varshni [Physica (Utrecht) 34, 194 (1967)], Vi na et al. [Phys. Rev. B 30, 1979 (1984)] and Passler [Phys. Stat. Sol. (b) 200, 155 (1997)] models were used to fit the experimental points.

The temperature dependence of excitonic transitions in double quantum well heterostructures in the temperature range of 2–300 K were investigated. A crossing between excitonic transition experimental curves as a function of temperature in quantum wells of the same thickness and different barrier height is observed. The influence of the barrier height on the temperature dependence of excitonic states in the quantum wells is analyzed.

We report electrical and optical characteristics of a Si-doped (Al)GaInAs digital alloy/AlInAs Bragg mirror lattice matched to InP grown by molecular beam epitaxy. A 98.2% reflectivity with a 107 nm stop band width centred at 1.54 μ m is obtained. An average voltage drop of 16 mV per period at a current density of 1 KA cm − 2 is observed for a mean electron concentration of about 5.5 × 10 18 cm − 3 . The influence of structural and intrinsic properties of the heterostructure on the electrical resistivity and optical reflectivity is analysed.

International audience; This work examines the photodegradation of poly(3-octylthiophene) (P3OT) by UV-irradiation at 405 nm under vacuum and as a function of temperature (300 and 10 K). Two different P3OT films were studied. The first one was electrochemically synthesized on platinum plate and chemically reduced in a basic medium (P3OT-E) whereas the second one, chemically synthesized was spread out on glass by casting process from polymer previously dissolved in chloroform (P3OT-Q). After 200 s under UV illumination, the photoluminescence (PL) spectra observed on the P3OT-E samples showed an unexpected increase of some eight times in the emission intensity. In contrast, the PL spectra of P3OT-Q, subjected to the same temperature, pressure and photoirradiation time, showed a drop of to around half the initial signal intensity. Similarly experiments were performed at low temperature 10 K. In this temperature condition, PL spectra of P3OT-E and P3OT-Q obtained under photoirradiation, presented similar decreases of the intensity of their fluorescence. In order to explain these different results, Raman spectroscopy studies had been performed, and it was been possible to follow the formation of C=O group on the side alkyl chain after 200 min of irradiation. From the results obtained, we proposed a mechanism to explain the unexpected intensification of the PL emission signal for P3OT-E at 300 K. This mechanism was based on the destabilization of the charge transfer complex between the radical cations in P3OT-E and the residual dioxygen.

We have performed a systematic study of photoluminescence (PL) line shape in n-type σ-doped In 0.15 Ga 0.85 As/GaAs quantum wells (6 QWs). Samples grown by molecular beam epitaxy with well widths of 60, 100, 300 A, with sheet doping concentration ranging from 10 11 to 10 13 cm -2 , were analysed. Two positions were considered for the doping planes, at the centre (symmetric) and at the edge (asymmetric) of the QW. The luminescence spectrum from the symmetric 60 A δ QW exhibits a dominating broad structure that at low excitation condition and low temperature has an almost rectangular form. The square shape of this spectrum is explained by the collective recombination of electrons from the conduction band with thermalized photogenerated holes. For the symmetric L W = 300 A samples, the PL spectra display an almost symmetric peak followed by a small shoulder on the higher or lower energy side depending on the carrier concentrations. For the asymmetric 300 A δ QW sample two distinct peaks were observed. They were correlated to transitions involving the fundamental and first excited electronic subbands.

Photoluminescence measurements at different temperatures have been performed to investigate the effects of confinement on the electron-phonon interaction in GaAs/AlGaAs quantum wells (QWs). A series of samples with different well widths in the range from 150 up to 750 Å was analyzed. Using a fitting procedure based on the Pässler-p model to describe the temperature dependence of the exciton recombination energy, we determined a fit parameter which is related to the strength of the electron-phonon interaction. On the basis of the behavior of this fit parameter as a function of the well width thickness of the samples investigated, we verified that effects of confinement on the exciton recombination energy are still present in QWs with well widths as large as 450 Å. Our findings also show that the electron-phonon interaction is three times stronger in GaAs bulk material than in Al(0.18)Ga(0.82)As/GaAs QWs.

We present the results of our studies on the emission properties of ${\text{In}}_{0.53}{\text{Ga}}_{0.47}\text{As}/{\text{In}}_{0.525}{\text{Ga}}_{0.235}{\text{Al}}_{0.25}\text{As}$ single and coupled double quantum wells (CDQWs) with different degrees of potential fluctuation. We have verified that the curve of the temperature $(T)$ dependence of the emission peak energy $({E}_{\text{PL}})$ is significantly influenced by the potential fluctuations (which are magnified by the presence of the internal barrier in the CDQW) as well as by the excitation density used in the photoluminescence (PL) measurements. As the excitation power increases, two effects occur simultaneously: the filling of the band-tail states related to potential fluctuations and the band-gap renormalization (BGR) caused by the increase in the density of photogenerated carriers. As the optical density increases, the ${E}_{\text{PL}}$ can shift to either higher (blueshift) or lower (redshift) energies, depending on the temperature at which the measurements are carried out. The temperature at which the displacement changes from a blueshift to a redshift is governed by the magnitude of the potential fluctuations and by the variation of BGR with excitation density.

The temperature dependence of the photoluminescence properties of a thin film of poly[2-methoxy-5-(2(')-ethylhexyloxy)-p-phenylene-vinylene], MEH-PPV, fabricated by spin coating, is analyzed. The evolution with temperature of the peak energy of the purely electronic transition, of the first vibronic band, of the effective conjugation length, and of the Huang-Rhys factors are discussed. The asymmetric character of the pure electronic transition peak and the contribution of the individual vibrational modes to the first vibronic band line shape are considered by a model developed by Cury et al. [J. Chem. Phys. 121, 3836 (2004)]. The temperature dependence of the Huang-Rhys factors of the main vibrational modes pertaining to the first vibronic band allows us to identify two competing vibrational modes. These results show that the electron coupling to different vibrational modes depends on temperature via reduction of thermal disorder.

The mechanism for low-temperature photoluminescence (PL) emissions in GaAsSb/AlGaAs and GaAsSbN/GaAs strained-layer single quantum wells (SQWs), grown by molecular-beam epitaxy, is studied in detail, using PL spectroscopy as a function of temperature and excitation intensity. In all samples, the PL peak energy as well as the full width at half maximum (FWHM), as a function of temperature, present anomalous behaviors, i:e., the PL peak energy shows a successive red/blue/redshift (S-shaped behavior) and the FWHM shows a successive blue/red/blueshift (“inverted S-shaped curve”) with increasing temperature. At sufficiently low excitation intensity and in a narrow temperature interval (50 ‐ 80 K), the nitrogen-containing samples present two clear competitive PL peaks. The low-energy PL mechanism (8 ‐ 80 K) is dominated by localized PL transitions, while the high-energy PL mechanism is dominated by the ground state (e1-hh1) PL transition. Additionally, these PL peaks show different temperature dependence with the low-energy PL peak, showing a stronger redshift than the high-energy PL peak. A competition process between localized and delocalized excitons is used to discuss these PL properties.

We investigate the effects of the interface morphology on the electronic properties of InAs/In systems using in-air atomic force microscopy and low temperature photoluminescence. Atomic force microscopy results show that the distribution of InAs strained film into three-dimensional islands and the two-dimensional wetting layer—typical of the Stranski–Krastanov growth mode—is strongly affected by the characteristics of the substrate and by the morphology of the InP buffer layer. The differences in the optical data are correlated to the different interface characteristics observed by atomic force microscopy. We discuss the origin of emission peaks taking into account the diffusion process of adsorbed atoms on the different types of surface.

Submitted by Guilherme Lemeszenski (guilherme@nead.unesp.br) on 2013-08-22T19:01:22Z No. of bitstreams: 1 S0103-97332004000400022.pdf: 163059 bytes, checksum: cd009132f334dd2586e6a14554357af3 (MD5) Made available in DSpace on 2013-08-22T19:01:22Z (GMT). No. of bitstreams: 1 S0103-97332004000400022.pdf: 163059 bytes, checksum: cd009132f334dd2586e6a14554357af3 (MD5) Previous issue date: 2004-06-01 Made available in DSpace on 2013-09-30T20:01:20Z (GMT). No. of bitstreams: 2 S0103-97332004000400022.pdf: 163059 bytes, checksum: cd009132f334dd2586e6a14554357af3 (MD5) S0103-97332004000400022.pdf.txt: 13097 bytes, checksum: 41e6751a54fdae567496162132c6b3bb (MD5) Previous issue date: 2004-06-01 Submitted by Vitor Silverio Rodrigues (vitorsrodrigues@reitoria.unesp.br) on 2014-05-20T15:15:41Z No. of bitstreams: 2 S0103-97332004000400022.pdf: 163059 bytes, checksum: cd009132f334dd2586e6a14554357af3 (MD5) S0103-97332004000400022.pdf.txt: 13097 bytes, checksum: 41e6751a54fdae567496162132c6b3bb (MD5) Made available in DSpace on 2014-05-20T15:15:41Z (GMT). No. of bitstreams: 2 S0103-97332004000400022.pdf: 163059 bytes, checksum: cd009132f334dd2586e6a14554357af3 (MD5) S0103-97332004000400022.pdf.txt: 13097 bytes, checksum: 41e6751a54fdae567496162132c6b3bb (MD5) Previous issue date: 2004-06-01 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação para o Desenvolvimento da UNESP (FUNDUNESP) In this work we investigated the optical and structural properties of GaAs/GaInP quantum wells (QW) grown by Chemical Beam Epitaxy (CBE). The samples were characterized by photoluminescence (PL), photoluminescence excitation (PLE) and transmission electron microscopy (TEM). Simulations of the quantum well potential profiles, using the Van de Walle-Martin model, supplemented by our experimental results, allowed us to associate the interface properties with the growth procedures. We concluded that a thin GaP layer grown at the interface improves its quality and also that the observed broad emission band in the PL spectrum is related to quaternary Ga1-xIn xAs1-yPy. Universidade Estadual Paulista - UNESP Departamento de Física UNICAMP Instituto de Física Gleb Wathagin Laboratório Nacional de Luz Sincroton - LNLS Universidade Estadual Paulista - UNESP Departamento de Física

Submitted by Guilherme Lemeszenski (guilherme@nead.unesp.br) on 2013-08-22T19:01:19Z No. of bitstreams: 1 S0103-97332002000200017.pdf: 180353 bytes, checksum: 2194adc05325048b8fbc21a686c29502 (MD5) Made available in DSpace on 2013-08-22T19:01:19Z (GMT). No. of bitstreams: 1 S0103-97332002000200017.pdf: 180353 bytes, checksum: 2194adc05325048b8fbc21a686c29502 (MD5) Previous issue date: 2002-06-01 Made available in DSpace on 2013-09-30T20:01:14Z (GMT). No. of bitstreams: 2 S0103-97332002000200017.pdf: 180353 bytes, checksum: 2194adc05325048b8fbc21a686c29502 (MD5) S0103-97332002000200017.pdf.txt: 13279 bytes, checksum: 38c5a5f948ee92fd54c3dc16a3694924 (MD5) Previous issue date: 2002-06-01 Submitted by Vitor Silverio Rodrigues (vitorsrodrigues@reitoria.unesp.br) on 2014-05-20T15:15:39Z No. of bitstreams: 2 S0103-97332002000200017.pdf: 180353 bytes, checksum: 2194adc05325048b8fbc21a686c29502 (MD5) S0103-97332002000200017.pdf.txt: 13279 bytes, checksum: 38c5a5f948ee92fd54c3dc16a3694924 (MD5) Made available in DSpace on 2014-05-20T15:15:39Z (GMT). No. of bitstreams: 2 S0103-97332002000200017.pdf: 180353 bytes, checksum: 2194adc05325048b8fbc21a686c29502 (MD5) S0103-97332002000200017.pdf.txt: 13279 bytes, checksum: 38c5a5f948ee92fd54c3dc16a3694924 (MD5) Previous issue date: 2002-06-01 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Photoluminescence (PL) and excitation PL measurements have been performed at different temperatures in a number of lattice-matched GaAs/In0.49Ga0.51P quantum wells, where the uctuations of the potential energy are comparable with the thermal energy of the photocreated carriers. Two samples with different well widths allow to observe a series of anomalous e ects in their optical response. The observed effects are related to the disorder in the interface, characterizing uctuations in the confinement potential energy. It is proposed that the carrier relaxation processes occur either at the local minima or at the absolute minimum of the confinement potential, depending on the ratio of the thermal energy and the magnitude of the potential fluctuations. Universidade Estadual de Campinas Instituto de Física 'Gleb Wataghin' Laboratório Nacional de Luz Síncrotron Universidade de São Paulo Instituto de Física Universidade Estadual Paulista Departamento de Física Universidade Estadual Paulista Departamento de Física

We determined by means of photoluminescence measurements the dependence on temperature of the transition energy of excitons in GaAs/AlxGa1−xAs quantum wells with different alloy concentrations (with different barrier heights). Using a fitting procedure, we determined the parameters which describe the behavior of the excitonic transition energy as a function of temperature according to three different theoretical models. We verified that the temperature dependence of the excitonic transition energy does not only depend on the GaAs material but also depends on the barrier material, i.e. on the alloy composition. The effect of confinement on the temperature dependence of the excitonic transition is discussed.

In this paper, the optical properties of a Si doped In0.53Ga0.47As/In0.52Ga0.24 Al0.24As superlattice (SL) are investigated by using the photoluminescence (PL) technique in the temperature range of 9?300?K. The origins of the optical transitions observed in the spectra are attributed through the analysis of the transition behaviour with temperature and excitation power. The linewidths obtained at 9 and 300?K are smaller than those previously reported in the literature, and the blueshift observed with increasing temperature has a small magnitude, indicating the excellent quality of the sample. The experimental results are compared with theoretical calculations based on the envelope function formalism, with an excellent accordance. The excitonic emission was obtained in the range of 9?300?K, and the applicability of the Varshni, Vi?a, and P?ssler (p-type and ?-type) models, usually used to describe the variation of the excitonic energy transition as a function of temperature in semiconductor materials, was also analysed.

We identify quasi-donor-acceptor pair transitions in the photoluminescence spectra of GaAsSb and AlGaAsSb layers, lattice matched to InP, and grown by molecular-beam epitaxy. These alloys show compositional inhomogeneity due to phase separation resulting from miscibility gaps. The presence of Al in the quaternary alloy increases the fluctuation of the electrostatic potential in the epitaxial layer, increasing the variation of recombination energy as a function of intensity excitation in the range of low temperatures.

In this paper we investigate the optical properties of thin films produced through the layer-by-layer (LbL) technique with a poly (p-phenylene vinylene) (PPV) precursor and the Congo Red (CR) azodye. Some significant changes are observed when the PPV/CR film is irradiated with polarized laser. An increase of about 10 times in the PPV emission intensity was observed, with an irradiation time of 100 min. Photoinduced birefringence measurements revealed that a high birefringence was produced in the region of the film submitted to photo-irradiation, while a photobleaching phenomenon was observed in the same region. However, such phenomena were not detected in measurements performed at low temperatures. The results indicate that a degradation process of the CR molecules is occurring with photo-irradiation, and the changes observed in the photoluminescence spectra are correlated with the combined effects of degradation and energy transfer between the PPV light-emitting conjugated segments and CR azochromophores.

In this work the P3MT was synthesized electrochemically on transparent conductive substrates of fluorine tin oxide (FTO) using the electrolyte Et 4 NBF 4 in acetonitrile. To dedope the material, the samples were reduced electrochemically and chemically (in NH 4 OH solution). The films were characterized by electron paramagnetic resonance spectroscopy (EPR) and infrared spectroscopy (FTIR) and their optical properties examined by ultraviolet–visible absorption spectroscopy (UV–vis) and photoluminescence (PL). The FTIR spectra show the characteristic features of P3MT and, together with the results of EPR, allowed to evaluate the charge carriers and to identify benzene and quinones in the formation of chains. The UV–vis absorption spectra helped to obtain the average gap energy of P3MT and presented the π – π * transition band and a bipolaron band. In the PL spectra, with variation of excitation power and temperature, studies were performed through the fit with Gaussian functions, given the strong probability of the existence of two distinct contributions in the formation of the emission spectra. These two contributions were assigned to the emission of mixed chains (Gaussian centered at higher energy) and emission of benzenoid chains (Gaussian of lower energy) in the formed polymeric material.