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Made available in DSpace on 2022-04-28T19:49:32Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-11-01 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Monitoring glucose levels is critical for diabetes management and might be a key step in the development of individualized treatment strategies. In this scenario, tracking salivary glucose has been recognized as a promising strategy due to its merits of ease sampling and non-invasive nature. In this paper, we report on the development of an electrical impedance-based biosensor array to distinguish glucose at different concentrations in saliva. The enzymatic biosensors were made of gold interdigitated electrodes coated with pristine electrospun zinc oxide nanofibers (NFZ) and NFZ combined with graphene-based nanomaterials (i.e., reduced graphene oxide - rGO and graphene quantum dots - GQDs), on which a layer of glucose oxidase (GOx) enzyme was adsorbed. Electrical impedance measurements indicate that the NFZ-GQDs@GOx and NFZ-rGO@GOx platforms presented good linear relationship with glucose concentration in the range of 0.1 to 6 mM. The highest sensitivity was reached for NFZ-rGO@GOx with a detection limit (LOD) of 14 μM, while the LOD was 32 μM for NFZ-GQDs@GOx. Both biosensors were also capable of detecting glucose in artificial saliva using aliquots of 10 μL, with recovery between 87.3 and 106.8%. Furthermore, the three sensing units (NFZ@GOx, NFZ-rGO@GOx and NFZ-GQDs@GOx) were employed to build a bioelectronic tongue. Using Principal Component Analysis (PCA) technique to project the electrical impedance data of all sensing units allowed the discrimination of the different glucose concentrations and interferents. This study reveals the applicability of the developed bioelectronic tongue as non-invasive glucose sensors, which approach could also be pottentially adapted to detect other disease biomarkers present in saliva. Institute of Chemistry Federal University of Bahia (UFBA), BA Nanotechnology National Laboratory for Agriculture (LNNA) Embrapa Instrumentação, SP PPGQ Department of Chemistry Center for Exact Sciences and Technology Federal University of São Carlos (UFSCar), SP Physics Department Institute of Geosciences and Exact Sciences São Paulo State University (UNESP), SP Physics Department Institute of Geosciences and Exact Sciences São Paulo State University (UNESP), SP FAPESP: 2016/23793-4 FAPESP: 2017/10582-8 FAPESP: 2017/12174-4 FAPESP: 2018/22214-6

Made available in DSpace on 2020-12-12T02:43:31Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-05-01 Universidade Estadual Paulista Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Aluminum-oxide (Al2O3) is a low cost, easily processable and high dielectric constant insulating material that is particularly appropriate for use as the dielectric layer of thin-film transistors (TFTs). Growth of aluminum-oxide layers from anodization of metallic aluminum films is greatly advantageous when compared to sophisticated processes such as atomic layer deposition (ALD) or deposition methods that demand relatively high temperatures (above 300 °C) such as aqueous combustion or spray-pyrolysis. However, the electrical properties of the transistors are highly dependent on the presence of defects and localized states at the semiconductor/dielectric interface, which are strongly affected by the manufacturing parameters of the anodized dielectric layer. To determine how several fabrication parameters influence the device performance without performing all possible combination of factors, we used a reduced factorial analysis based on a Plackett-Burman design of experiments (DOE). The choice of this DOE permits the use of only 12 experimental runs of combinations of factors (instead of all 256 possibilities) to obtain the optimized device performance. The ranking of the factors by the effect on device responses such as the TFT mobility is possible by applying analysis of variance (ANOVA) to the obtained results. School of Technology and Sciences São Paulo State University-UNESP Scholl of Electronic Engineering Bangor University Institute of Geosciences and Exact Sciences São Paulo State University-UNESP School of Technology and Sciences São Paulo State University-UNESP Institute of Geosciences and Exact Sciences São Paulo State University-UNESP FAPESP: 14/13904-8 FAPESP: 16/03484-7 FAPESP: 19/01671-2 FAPESP: 19/05620-3 FAPESP: 19/08019-9

Made available in DSpace on 2020-12-12T01:53:47Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-04-01 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Polymer light-emitting electrochemical cells (PLECs) are organic electronic devices which operating mechanism depends on the injection and transport of electronic charge carrier and the electrochemical doping of the organic semiconductor. The details of the interactions between the salt (or its ions) and the semiconducting polymer composing the device active layer provide important information about the electronic processes associated to the device operation in steady-state. In this context, the present paper proposes a study where theoretical results from Density Functional Theory (DFT) were obtained for three different steady-state operational regimes: i) without external voltage, in which the undissociated salt molecules interact with uncharged semiconducting polymer; ii) for applied voltages lower than the device turn-on (VEg/e), in which the dissociated ions interact with charged semiconducting polymer. In addition, the theoretical results have been confronted with experimental results of PLECs fabricated using different salt concentrations. For both theoretical and experimental approaches, we considered lithium triflate as the salt compound and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-vinylenephenylene)] (F8PV) as the semiconducting polymer. We observed substantial changes in the electronic structure of the systems at the different operating regimes, which were interpreted in terms of the electronic charge injection from the electrodes and the electrochemical doping of the semiconducting polymer. São Paulo State University (UNESP), Posmat São Paulo State University (UNESP) Institute of Geosciences and Exact Sciences Physics Department University of São Paulo São Carlos Institute of Physics, São Carlos São Paulo State University (UNESP) School of Sciences Physics Department São Paulo State University (UNESP) Campus of Itapeva São Paulo State University (UNESP), Posmat São Paulo State University (UNESP) Institute of Geosciences and Exact Sciences Physics Department São Paulo State University (UNESP) School of Sciences Physics Department São Paulo State University (UNESP) Campus of Itapeva CNPq: 133770/2017-6 FAPESP: 2007/58991-1 FAPESP: 2013/24461-7 FAPESP: 2014/20410-1 CAPES: 23038.004680/2015-01 CNPq: 420449/2018-3 CNPq: 448310/2014-7

In the present study, we analyze the influence of temperature and active layer thickness on the electrical properties of electroluminescent devices comprising a polymeric conductive blend (poly(3,4 ethylenedioxythiophene):polystyrene sulfonate, PEDOT:PSS), an inorganic electroluminescent material (manganese doped zinc orthosilicate, Zn2SiO4:Mn) and an organosilicon material (3-glicidoxypropyltrimethoxysilane, GPTMS), manufactured at different weight ratios of the component materials. The devices were obtained by depositing the active layer by drop-casting onto ITO-coated (RF-sputtering) glass substrates and thermally evaporating gold top electrodes in high vacuum. The results show that 90 wt% Zn2SiO4:Mn is required to observe high electroluminescence from the fabricated devices and that the optimum performance (turn-on voltage of 33 V, luminous efficacy of 24 cd/A and maximum luminance of almost 2000 cd/m2) was achieve for a (9.5/0.5/90) (GPTMS/PEDOT:PSS/Zn2SiO4:Mn) weight ratio. The device turn-on voltage found to be as proportional to the thickness of the active layer, indicating that the electroluminescence occurs by a field-effect mechanism. The temperature variation in the 100–300 K range allowed us to develop a theoretical model for the device operation, where the charge carrier transport in the active layer is well described by the variable range hopping model, with luminous efficacy nearby independent of the temperature.

In the current work, we evaluate the influence of the processing parameters on the electrical properties of aluminium zinc oxide (AZO) thin films produced by airbrush spray-pyrolysis deposition technique. Spray-deposited AZO thin-films were produced with Al:Zn molar ratios varying from 0 % (pure ZnO) up to 30 %, using aluminium acetate and zinc acetate as organic precursors and water as solvent. Thermogravimetric analysis (TGA) and infrared spectroscopy (FTIR-ATR) were used to monitor the metal-oxide formation from the organic precursors as a function of the temperature. The results show that a temperature of 400 °C is necessary to completely degrade the organic phase and to obtain the desired inorganic metal-oxides films. The electrical properties of the TMOs were evaluated by d.c. current-voltage (I-V) analysis using planar thermally evaporated Al electrodes on top of the TMO layer, with different aspect ratios (1/18, 2/9, 5/13, 5/9 and 8/9). The lowest sheet resistance was obtained for AZO films at a molar Al concentration of 5 %. We also observed that, after carrying out a post-annealing treatment (30 mbar, 150 °C) the samples presented a decrease on the sheet resistance superior to 60 %, in comparison to the samples before the treatment.

Metal oxides like zinc oxide (ZnO) are promising materials for the active layer of thin-film transistors (TFTs) used in the drive circuit of next-generation large-area active matrix displays due to their high electronic mobility, high transmittance in the optical visible range and processability. Traditional deposition techniques employ RF sputtering or pulsed-laser deposition (PLD), which are relatively sophisticated techniques. The deposition of very thin (less than 50 nm thick) layers of ZnO using soluble organic precursors have been extensively investigated recently as an alternative to traditional deposition methods. Solution-based deposition processes include simple and affordable techniques like dip-coating, spin-coating, spray-pyrolysis and ink-jet printing. Spray-pyrolysis is particularly interesting due to the high film uniformity, low cost and high device performance. We carried out several experiments analyzing the performance of ZnO based devices using zinc acetate as organic precursor to confirm that spray pyrolysis deposition is a suitable technique for production of high-performance and reproducible TFTs. Moreover, we observed that device performance can significantly vary with little modifications on the deposition parameters, even for the same active layer composition and pyrolysis temperature. Electrical parameters, as the electrical mobility and the on/off ratio, varied several orders of magnitude, whereas the threshold voltage varied up to 20 V for the tested devices. Deposition parameters as the nozzle height during the deposition, nozzle air pressure and deposition time were varied until we obtained devices with optimum electrical performance. Optimized devices presented mobilities in the order of 1 cm2.V-1.s-1, on/off ratio of about 106 and relatively low operation voltages. A statistical analysis of a great number of devices manufactured using the same deposition parameters was also carried out to assure the reproducibility of the deposition technique.

Made available in DSpace on 2022-04-28T19:40:24Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-10-01 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Volatile organic compounds (VOCs) are environmental pollutants that pose risks to the human health even at very low concentrations. Therefore, fast and sensitive analytical methods capable to discriminate VOCs are highly demanded. Herein, we have successfully synthetized and characterized TiO2 nanofibers (NF) by electrospinning and TiO2 nanoparticles (NP) by sol-gel method to be employed in an electronic nose (e-nose) for monitoring VOCs. Electrical comparison between TiO2-NF and TiO2-NP indicated that the former presented better electrical response, which can be attributed to the better charge transfer along the nanofiber framework. The TiO2 nanostructures were combined with three different polymers, namely poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), polypyrrole (PPy), and polystyrene sulfonate (PSS), which obtained nanocomposites were deposit by drop casting onto gold interdigitated electrodes and used as sensing units of the e-nose. Electrical impedance spectroscopy measurements were employed to collect the e-nose electrical resistance data, which were treated by Principal Component Analysis (PCA), revealing the system was able to discriminate the three VOCs. Our results indicate that the e-nose system has potential to be employed as a rapid and simple alternative in the detection of VOCs. Nanotechnology National Laboratory for Agriculture (LNNA) Embrapa Instrumentação PPGQ Department of Chemistry Center for Exact Sciences and Technology Federal University of São Carlos (UFSCar) Institute of Chemistry Federal University of Bahia (UFBA) Institute of Geosciences and Exact Sciences São Paulo State University (UNESP) Institute of Geosciences and Exact Sciences São Paulo State University (UNESP) FAPESP: 2016/23793-4 FAPESP: 2017/12174-4 FAPESP: 2018-22214-6 FAPESP: 2018/08012-1

Made available in DSpace on 2020-12-12T01:28:46Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-10-01 Instituto Nacional de Ciência e Tecnologia em Eletrônica Orgânica The electrical properties of polymeric films/blends comprising polyaniline (PANI) and cashew gum (CG) blends were studied by impedance spectroscopy in the temperature range from 300 K to 353 K. Protonation promoted by sulfuric acid was responsible for changing the oxidation state from undoped emeraldine base (EB) to doped emeraldine salt (ES). The low-frequency conductivity of the films varied more than 6 orders of magnitude by doping. In (PANI- EB)/CG blends, the presence of cashew gum acts a dispersing agent, lowering the electrical conductivity contrasting to (PANI-ES)/CG blends, where the electrical conductivity increases with CG concentration up to 20% (w/w). Moreover, undoped films presented a thermally-activated electrical conductivity, whereas doped films presented a decrease in the conductivity by increasing temperature. An equivalent circuit model is proposed to analyze the data by considering the system being composed of highly conductive regions separated by insulating/semiconducting ones. Instituto de Física de São Carlos Universidade de São Paulo Instituto Federal do Maranhão - IFMA Campus de São João dos Patos Universidade Estadual Paulista - UNESP Departamento de Física Universidade Federal do Piauí - UFPI Departamento de Física Universidade Estadual Paulista - UNESP Departamento de Física

The present study reports a two-level multivariate analysis to optimize the production of anodized aluminum oxide (Al2O3) dielectric films for zinc oxide thin-film transistors (TFTs). Fourteen perf...

Solution-processed zinc oxide (ZnO) thin-film transistors (TFTs) obtained via hydrolysis/pyrolysis of an organic precursor present an excellent technique to obtain high performance electronic devices with low manufacturing cost. In the current work, we propose the use of an alternative deposition method, based on a polymeric precursor route (known as Pechini), to obtain solution-processed ZnO compact films as the active layer of TFTs. The elimination of the organic phase and the formation of inorganic thin-films was carried out by thermal treatment at different temperatures (ranging from 200oC to 500oC) and at different times (from 5 min to 2 hours), being monitored by UV-vis and infrared (IR) optical absorption spectroscopy. It was observed that, for temperatures above 400oC and treatment times superior to 30 min, the organic phase was completely eliminated, remaining only the inorganic (metal oxide) phase. The optical bandgap of the resulting ZnO films, determined from UV-vis absorption, is about 3.4 eV. The electrical characteristics (output and transfer curves) of the obtained devices demonstrate the feasibility of Pecchini method to build solution-processed metal oxide TFTs. The results for the electrical mobility of the majority charge-carriers (electrons) and for the threshold voltage were 0.39 cm2.V-1.s-1 and 0.45 V, respectively.

Made available in DSpace on 2018-12-11T17:36:53Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-07-01 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) A number of solution-processed electronic materials have been developed for application in printed devices, which are normally built by piling up several coated layers one on the top of the other. To achieve low-cost and simple to manufacture printed devices, stable and easy to process layers are mandatory. Here we introduce a mixture of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) with 3-glycidoxypropyltrimethoxysilane (GPTMS) as solution-processed material that becomes insoluble in water after film processing. We performed morphological, optical and electrical characterization in the proposed composite and highlight its advantages as a stable transparent electrode in light-emitting diodes as well as in electrochromic devices. Physics Department Institute of Geosciences and Exact Sciences São Paulo State University – UNESP Sao Carlos Institute of Physics University of São Paulo, PO Box 369 Brazilian Synchrotron Light Laboratory (LNLS) Brazilian Center for Research in Energy and Materials (CNPEM), Campinas Physics Department Institute of Biosciences Language and Exact Sciences São Paulo State University – UNESP Physics Department Institute of Geosciences and Exact Sciences São Paulo State University – UNESP Physics Department Institute of Biosciences Language and Exact Sciences São Paulo State University – UNESP CAPES: 2015/25069-9 FAPESP: 312025/2016-5

The transport properties (conductivity and mobility) of holes and electrons in poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) light-emitting diodes were investigated using direct current–voltage curves (I–V) and time of flight measurements (TOF) as a function of electric field and temperature. TOF results revealed that the transport of holes in the bulk follows a non-dispersive behavior at room temperature (300 K), exhibiting a progressive transition to a dispersive behavior as the temperature decreases down to 220 K. The dispersive transport characteristics were interpreted in the framework of carrier hopping in an exponential density of states. On the other hand, the analysis of the negative photocurrent transients indicated that the transport of negative charge carriers (electrons) is strongly dominated by trapping in the entire range of temperature studied. The I–V curves presented remarkable temperature dependence, being analyzed in terms of the classic Fowler–Nordheim tunneling, Richardson–Schottky thermionic emission and trap-controlled transport.

Poly(1-methoxy-4-(2-ethyl-hexyloxy)- p -phenylene vinylene) (MEH-PPV) films spin-cast from chloroform solution were characterized by cyclic voltammetry, in situ spectroelectrochemistry, and electrochemical quartz crystal microbalance (EQCM) measurements. The MEH-PPV films exhibited two well-defined redox waves during p- and n-electrochemical doping, and a mass gain at the end of each cycle that is characteristic of the movement of ions and solvent into and out the film. The electronic energy gap of MEH-PPV was estimated as 2.35 eV from the anodic and cathodic onset potentials obtained by cyclic voltammetry, in good agreement with the value of optical energy gap determined from the absorption edge, 2.21 eV. The films also showed a reversible and stable electrochromic transition at a positive range of applied potentials from reddish-orange (neutral form) to brownish-green (oxidized form).

In this work, the electrical properties of polymeric light-emitting diodes (LEDs) based on poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV, were studied by dc current–voltage (I–V) curves and impedance/admittance spectroscopy, at different temperatures. ITO/polymer/metal (Al and Au) structures were used to study the effect of the barrier height on the charge injection at the polymer/metal interfaces. For ITO/MEH-PPV/Al devices, strong temperature influence on the dc conductivity was observed in the forward direction whereas, in the reverse direction, the conductivity was almost temperature independent. On the other hand, ITO/MEH-PPV/Au devices presented temperature dependent conductivities for both polarities. This indicates that, for low barrier height at the interfaces (

We have used electrically detected magnetic resonance (EDMR) to study the spin dependent processes on the operation of diodes based on poly(2-metoxy-5-(2'-etil-hexiloxy)-1,4-phenylene vinylene) (MEH-PPV) and on tris-8-hydroxyquinoline-aluminum (Alq3). In MEH-PPV based diodes, the EDMR signal is found to be composed of two lines, a Lorentzian with ΔHpp=5 G and a Gaussian with ΔHpp=24 G. The g-factor of both components is about 2.0022. The signal is quenching, typically 10-5 in amplitude, and is assigned to the spin-dependent fusion of two like-charged polarons to spinless bipolarons. The signal dependence on temperature and on injected current is investigated. For bipolar Alq3 diodes the EDMR signal can be decomposed in at least two Gaussians, depending on the bias and temperature of measurements. The two components are characterized by g-factors close to 2.004 and a peak-to-peak linewidth (ΔHpp) of 12 and 24 G, respectively. The broad line is attributed to the resonance in Alq anions, while the 12 G is attributed to cationic states

Made available in DSpace on 2018-11-26T16:16:26Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-03-01 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) INEO nBioNet, Brazil In this study we propose the use of a blend, comprising poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) and polyvinyl alcohol (PVA), to be used as flexible, semitransparent and highly conductive material for electronic and optoelectronic devices. The electrical conductivity and optical transmittance of spray-deposited films of the blend, in different thicknesses and at different weight ratios, were studied in order to determine the most appropriate composition for optimal device performance and cost. We propose a direct method to determine the most appropriate blend composition to produce polymeric films with a desired sheet resistance, optical transmittance and cost, by using a diagram which considers the influence of blend composition and film thickness in a light-transmittance versus sheet resistance plot, mediated by the material cost per unit area. In this way, the present study establishes an empirical method to determine the PEDOT: PSS/PVA blend composition to be used as electrode material in optoelectronic devices which satisfies the desired conditions of optical transmittance, sheet resistance and cost-effectiveness. Univ Estadual Paulista UNESP, Dept Fis, BR-13506900 Rio Claro, Brazil Univ Estadual Paulista UNESP, Dept Fis, BR-15054000 Sao Jose Do Rio Preto, Brazil Univ Sao Paulo, Inst Fis Sao Carlos, BR-13560250 Sao Carlos, SP, Brazil Univ Estadual Paulista UNESP, Dept Fis, BR-13506900 Rio Claro, Brazil Univ Estadual Paulista UNESP, Dept Fis, BR-15054000 Sao Jose Do Rio Preto, Brazil FAPESP: 2008/57706-4 FAPESP: 2012/19005-0 FAPESP: 2012/01624-5

In this work the electron spin resonance (ESR) and electron detected magnetic resonance (EDMR) are applied in MH-PPV and MEH-PPV, respectively. The ESR measurements revealed that there is a change of lineshape and a clear reduction of spin density on MH-PPV degraded by light and air. This result is in agreament with the fact that the spin density resides predominantly on vinylic carbon. The EDMR measurements in MEH-PPV LEDs provided evidences that there is a spin-dependent fusion of two polarons to spinless bipolaron.

The optical properties of poly( p -phenylene vinylene) (PPV) derivatives with ion-coordinating side groups (ICSG) films and respective blends with poly(ethylene oxide) (PEO) complexed with lithium triflate are studied by means UV-visible absorption and photoluminescence (PL) spectroscopy. These PPV-derivatives were synthesized with the objective to improve the physical properties of PPV-derivative: PEO: lithium salt blends to be used in light emitting electrochemical cells (LECs). We have synthesised two homopolymers: the poly(2-methoxy, 5-(triethoxymethoxy)-1,4-phenylene vinylene) (MTEM-PPV) and the poly(2-methoxy,5-(2'ethylhexyloxy)-l,4-phenylene vinylene (MEH-PPV); and a statistical copolymer: the poly(2-methoxy, 5(2'ethylhexyloxy)-co-2-methoxy, 5-(triethoxy-methoxy) -1,4-phenylene vinylene) (MEH-co-MTEM)-PPV. The analysis of PL spectra of PPV derivatives blended with salt or PEO-salt showed that PPVs with higher amount of ICSG present higher quenching effect on the PL efficiency in the presence of lit...

A reversible electrochromical response of thin poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) films spin-coated on ITO substrates was observed during spectroelectrochemical experiments. The polymer layer changes its coloration from reddish-orange (neutral form) to brownish-green (oxidized form) upon varying the applied potential. The oxidized form showed to be stable even after removing the film from the electrochemical cell, returning to its original form only after electrochemical reduction. The color transitions at different potentials were studied by UV-vis-NIR absorption spectroscopy.

The efficiency of the charge-carrier photogeneration processes in poly(2,5-bis(3′,7′-dimethyl-octyloxy)-1,4-phenylene vinylene) (OC1OC10-PPV) has been analyzed by the spectral response of the photocurrent of devices in ITO/polymer/Al structures. The symbatic response of the photocurrent action spectra of the OC1OC10-PPV devices, obtained for light-excitation through the ITO electrode and for forward bias, has been fitted using a phenomenological model which considers that the predominant transport mechanism under external applied electric field is the drift of photogenerated charge-carriers, neglecting charge-carrier diffusion. The proposed model takes into account that charge-carrier photogeneration occurs via intermediate stages of bounded pairs (excitonic states), followed by dissociation processes. Such processes result in two different contributions to the photoconductivity: The first one, associated to direct creation of unbound polaron pairs due to intrinsic photoionization; and the second one is associated to secondary processes like extrinsic photoinjection at the metallic electrodes. The results obtained from the model have shown that the intrinsic component of the photoconductivity at higher excitation energies has a considerably higher efficiency than the extrinsic one, suggesting a dependence on the photon energy for the efficiency of the photogeneration process.

Spectral photoconductivity measurements were carried out in ITO/Polymer/Metal structures using three different polymers as the active layer: poly(2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV), poly(2-methoxy-5-decyloxy-pphenylene vinylene) (OC10-PPV) and a phenyl-substituted PPV copolymer. In order to observe the influence of the work-function of the cathode on the photoconductivity spectra, different metallic electrodes (Cu, Al and Mg) were deposited on the top of the device structure. The optical properties of the polymeric films were analyzed by means of absorption and stationary fluorescence spectra. The photoconductive behavior of the devices was investigated from the current-voltage (I-V) curves under different illumination wavelengths and from the steady-state photocurrent spectra in the dc regime. The devices were illuminated either through the ITO an orde or the metallic cathode to verify the influence of the penetration profile of light on the photoconductivity. The photocurrent spectral data under different experimental conditions were adjusted by using a model developed taking into account both the intrinsic and the extrinsic components of the photoconductivity. From this model, it was possible to obtain relevant information about the polymers used as the active layers, such as the nature of the major recombination mechanism and the internal efficiency of carrier photogeneration in the bulk and at the device interfaces.

The transport properties of holes and electrons in poly (2-methoxy,5-(2-ethyl-hexyloxi)-1-4 phenylene vinylene) (MEH-PPV) prepared on a p type gallium arsenide (p-GaAs) substrate are investigated by the time of flight technique (TOF). The results are presented in function of applied field, and the laser incident wavelength. Tail broadening parameter has been used to analyze the dispersion of the experimental results.

Complex resistivity measurements in dark and under vacuum were carried out to study the electrical behavior of luminescent polymer diodes (LEDs) having ITO or gold as hole injector electrodes, aluminium as the cathode, and poly[(2-methoxy-5-hexyloxy)-p-phenylenevinylene] - MH-PPV as the luminescent layer. The results obtained were analysed by a model based on resistor-capacitor circuits, which made possible to distinguish between bulk from interface effects, and the contribution of each phenomenon to the electrical current. It was also shown that bulk properties already exist in a film of 0.4 /spl mu/m thick, and from the theoretical fittings it was obtained the dielectric constant and resistivity of MH-PPV.

Photo-stimulated discharge (PSD) spectroscopy of polytetrafluoroethylene (PTFE) and polyethylene terephthalate (PETP) films volume- and surface-charged by either corona discharge or a monoenergetic electron beam was performed in order to investigate the mechanism(s) of charge storage and to determine trap depths. The samples were irradiated with monochromatic light from a Xenon arc lamp (photon energies of 3 - 6.2 eV) at constant temperature. Experimental results indicate that the threshold energy for the generation of a photocurrent depends on factors like film material, contact between sample and electrodes, and electrode material. Space-charge profiles determined with the piezoelectrically generated pressure-step (PPS) method show quantitative depopulation of near-surface charges. Partial detrapping of volume charge was also observed. Experimental results indicate that apparent detrapping can sometimes be attributed to photo-injection and exciton generation with subsequent charge compensation.

Light-emitting electrochemical cells (LECs) made of electroluminescent polymers were studied by d.c. and transient current-voltage and luminance-voltage measurements to elucidate the operation mechanisms of this kind of device. The time and external voltage necessary to form electrical double layers (EDLs) at the electrode interfaces could be determined from the results. In the low- and intermediate-voltage ranges (below 1.1 V), the ionic transport and the electronic diffusion dominate the current, being the device operation better described by an electrodynamic model. For higher voltages, electrochemical doping occurs, giving rise to the formation of a p-i-n junction, according to an electrochemical doping model.

Light emitting electrochemical cells (LECs) were prepared with poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly(ethylene oxide) (PEO) complexed with lithium triflate (CF 3 SO 3 Li) in different blend compositions. Atomic force microscopy (AFM) images were performed to observe the influence of the blend composition in the film morphology. UV-Vis absorption and photoluminescence spectra were used to characterize the blends, and current-voltage curves and electroluminecent measurements were carried out to study the device performance. A correlation between the blend morphology and the electrical performance of the devices were also studied. Optical and electrical results of LEDs made of pure MEH-PPV were presented for comparison with the LEC devices.

This paper presents the synthesis of homo- and copolymers obtained from 1,4-bis(chloromethyl)-2-methoxy,5-(triethoxymethoxy)benzene (MEH-BCM) and 1,4-bis(chloromethyl)-2-methoxy,5-(2'-ethyl-hexiloxy)benzene (MTEM-BCM). While poly(p-phenylene vinylene) (PPV) derivatives are extensively used as active layers in light-emitting electrochemical cells (LECs), copolymers are responsible for a considerable improvement of the performance of these devices Morphological and optical studies of PPV-derivatives and also of PPV derivatives,/poly(ethylene oxide)-salt blends films were carried out by atomic force microscopy (AFM) technique and by UV-Vis absorption and photoluminescence spectroscopy.

Despite the long-time use of space-charge electrets in various commercial applications, the mechanism of charge storage is still poorly understood. To study the energy distribution of deep traps, photostimulated discharge spectra of electron-beam and corona-charged polytetrafluoroethylene (PTFE), polyethylene terephthalate (PETP) and cellular polypropylene (PP) films were recorded for wavelengths between 200 and 400 nm. In addition, space-charge profiles were measured using the piezoelectrically generated pressure-step (PPS) method in order to monitor the detrapping process. Strong discharge currents of up to 15 pA/cm/sup 2/ were found in PETP around the UV absorption edge near 310 nm. Initial results show a quantitative depopulation of near-surface charges, whereas only partial detrapping could be achieved for bulk charges. Three distinct trapping levels could be identified in PP films.