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The increasing demand for food production has necessitated the development of sensitive and reliable methods of analysis, which allow for the optimization of storage and distribution while ensuring food safety. Methods to quantify and monitor volatile and biogenic amines are key to minimizing the waste of high-protein foods and to enable the safe consumption of fresh products. Novel materials and device designs have allowed the development of portable and reliable sensors that make use of different transduction methods for amine detection and food quality monitoring. Herein, we review the past decade?s advances in volatile amine sensors for food quality monitoring. First, the role of volatile and biogenic amines as a food-quality index is presented. Moreover, a comprehensive overview of the distinct amine gas sensors is provided according to the transduction method, operation strategies, and distinct materials (e.g., metal oxide semiconductors, conjugated polymers, carbon nanotubes, graphene and its derivatives, transition metal dichalcogenides, metal organic frameworks, MXenes, quantum dots, and dyes, among others) employed in each case. These include chemoresistive, fluorometric, colorimetric, and microgravimetric sensors. Emphasis is also given to sensor arrays that record the food quality fingerprints and wireless devices that operate as radiofrequency identification (RFID) tags. Finally, challenges and future opportunities on the development of new amine sensors are presented aiming to encourage further research and technological development of reliable, integrated, and remotely accessible devices for food-quality monitoring. Made available in DSpace on 2022-09-28T20:05:36Z (GMT). No. of bitstreams: 1 P-Recent-Progress-in-Amine-Gas-Sensors-for-Food-Quality-Monitoring.pdf: 4506930 bytes, checksum: afd8070c0df98dba888f9ab63a3b7a3e (MD5) Previous issue date: 2022

The implementation of the succession process as a strategy of mapping, development and retention of talent is gaining space in organizations. This article aimed to verify whether organizations have a structured process for retaining professionals identified as potential successors and whether there is differentiation according to the level of maturity of the succession process. A sample of companies seeking to differentiate themselves in terms of people management, consisting of 396 companies, was analyzed. For analysis, the Phi coefficient was used, and the survey results showed that a small number of organizations have a process of retaining professionals identified as potential successors, and that organizations that are more mature with respect to their succession process are more likely to work on retaining their talents in a structured manner, and the aspects that lead to most voluntary dismissals of professionals in the organizations surveyed are linked to compensation, career and development actions.

Made available in DSpace on 2021-06-25T10:27:43Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-06-01 Wireless communication employing radio-frequency identification (RFID) tags is progressively more widespread in a variety of industrial and commercial sectors. Aspects as demanding specific needs regarding reading distance, production cost, and processability are requested for commercial applications. Nowadays, printing of optical barcodes is the most economically technology for short distance (up to ~30 cm) reading applications. However, the large-scale and low-cost printing of RFID tags would be a competitive alternative to barcode technology because it allows quicker identification with much less reading errors. Here we demonstrate a semi-transparent, screen printed RFID antennas using a non-metallic and environmentally friendly conductive paste. The variation of the film thickness (from 2.5 up to 15 µm) permitted the fabrication of antennas with reading range (0.84–3.29 m), sheet resistance (6.2–36 Ω/sq), and optical transmittance (0–32.3%), which are compatible to commercial or industrial applications aiming short to moderate reading distances. Indústria de Tintas Condutivas TICON Flextronics Instituto de Tecnologia (FIT) Institute of Geosciences and Exact Sciences Department of Physics São Paulo State University (Unesp) Institute of Geosciences and Exact Sciences Department of Physics São Paulo State University (Unesp)

Made available in DSpace on 2020-12-12T02:46:10Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-07-15 A comparison of Zinc Oxide (ZnO) phototransistors prepared by spray and sputter coating process is presented. The work shows that spray coated layers provide significant advantages in sensor response over ZnO phototransistors made by physical vapour deposition and we show that spray deposited ZnO phototransistors can exhibit state-of-the-art performances for UV photodetectors. Topographic images of the samples surface shows that there is increase in surface roughness in spray coated samples indicating increasing grain sizes, which is considered the source of the greater sensor responsivity. X-ray photoelectron spectroscopy (XPS) is also used to understand the root cause of the greater UV responsivity. It was observed that sprayed ZnO TFTs are more sensitive to UV radiation due to higher adsorption of oxygen level. Responsivity and external quantum efficiency (EQE) of the sprayed and sputtered ZnO TFTs are also evaluated. School of Electronics Bangor University School of Technology and Sciences São Paulo State University (Unesp) Institute of Geosciences and Exact Sciences São Paulo State University (Unesp) Rio Claro Faculty of Science and Engineering University of Chester School of Technology and Sciences São Paulo State University (Unesp) Institute of Geosciences and Exact Sciences São Paulo State University (Unesp) Rio Claro

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 2019-10-06T17:10:15Z (GMT). No. of bitstreams: 0 Previous issue date: 2019-01-01 Conductive composites based on cross-linked poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) blends have been widely applied to produce electronic devices, such as organic electrochemical transistor, biosensors and electrochromic devices, to name a few. Notwithstanding, fundamental details regarding the influence of the cross-link agent on the charge transport mechanism of PEDOT:PSS based composites is still not completely understood. Herein, we present a study regarding the influence of the cross-link (3-glycidyloxypropyl) trimethoxysilane (GPTMS) on the charge transport mechanism of PEDOT:PSS/GPTMS composites. We have verified that the charge transport is dominated by hopping into percolated PEDOT:PSS-rich clusters for composites with a PEDOT:PSS content above 40%-wt. Interestingly, by analyzing the conductivity data by the variable range hopping model we observed that the hopping temperature, electronic localization length and charge carriers mobility remains practically independent on the PEDOT:PSS concentration. As for non-percolated composites, (i.e. below 40%-wt PEDOT:PSS), however, the charge transport is dominated by charge carrier hopping between unconnected PEDOT:PSS clusters, in which the electrical conductivity and the hopping temperature are strongly influenced by the cross-link agent content, and characterized by shorter carrier localization lengths. 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 Physics Department Institute of Geosciences and Exact Sciences São Paulo State University – UNESP

Made available in DSpace on 2022-04-28T19:41:05Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-06-25 Quality control in the production and processing of raw meat is currently one of the biggest concerns for food industry and would benefit from portable and wireless sensors capable of detecting the onset of spoilage. Raw meat is a natural source of biogenic and volatile amines as byproducts of decarboxylation reactions, and the levels of these compounds can be utilized as quality control parameters. We report herein a hybrid chemiresistor sensor based on inorganic nanofibers of SiO2:ZnO (an n-type material) and single-walled carbon nanotubes functionalized with 3,5-dinitrophenyls (a p-type material) with dosimetric sensitivity ∼40 times higher for amines than for other volatile organic compounds, which also provides excellent selectivity. The hybrid nanomaterial-based chemiresistor sensory material was used to convert radio-frequency identification tags into chemically actuated resonant devices, which constitute wireless sensors that can be potentially employed in packaging to report on the quality of meat. Specifically, the as-developed wireless tags report on cumulative amine exposure inside the meat package, showing a decrease in radio-frequency signals to the point wherein the sensor ceased to be smartphone-readable. These hybrid material-modified wireless tags offer a path to scalable, affordable, portable, and wireless chemical sensor technology for food quality monitoring without the need to open the packaging. Department of Chemistry Institute for Soldier Nanotechnologies Massachusetts Institute of Technology Cambridge, 77 Massachusetts Avenue Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge, 77 Massachusetts Avenue Nanotechnology National Laboratory for Agriculture (LNNA) Embrapa Instrumentação, São Paulo Institute of Geosciences and Exact Sciences São Paulo State University (UNESP), São Paulo Institute of Geosciences and Exact Sciences São Paulo State University (UNESP), São Paulo

Made available in DSpace on 2021-06-25T10:33:17Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-01-01 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Instituto Nacional de Ciência e Tecnologia em Eletrônica Orgânica Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) The UV photocurrent response of thin films of wide bandgap semiconductors such as zinc oxide (ZnO) can be applied to a great number of electronic devices aiming applications in environmental sensing or UV-detection. Electronic devices like thin-film transistors or Schottky diodes commonly present multiple parameters of electrical characteristic, which can be beneficially exploited to provide more information than sensors based on purely resistive or capacitive response. We manufactured Schottky diodes using spray-coated ZnO and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as an easy, simple and low-cost method for producing multiparametric UV-photodetectors. The diode parameters presented rectification ratios (RR) as high as 104 and ideality factors as low as 1.3, and their characteristic curves were analyzed by Cheung's method to determine the effect of UV irradiation on the ideality factor, series resistance and Schottky barrier height. The study of the photocurrent response from spray-coated ZnO films as a function of geometric parameters and UV intensity demonstrated a transition from bimolecular to monomolecular recombination process at higher irradiance values, as a result of the adsorption/desorption dynamics of molecular oxygen at the semiconductor/air interface. São Paulo State University – UNESP Faculty of Science and Technology (FCT) Physics Department São Paulo State University – UNESP Institute of Geosciences and Exact Sciences (IGCE) Physics Department São Paulo State University – UNESP Faculty of Science and Technology (FCT) Physics Department São Paulo State University – UNESP Institute of Geosciences and Exact Sciences (IGCE) Physics Department FAPESP: 2018/04169-3 FAPESP: 2019/01671-2 FAPESP: 2019/08019-9

Esta pesquisa teve como objetivo estudar a distribuição espacial do Aedes aegypti em escolas públicas do município de Bezerros-PE. Com o intuito de avaliar áreas vulneráveis a transmissão de arboviroses, no período de junho a novembro de 2018. Foram utilizadas armadilhas de oviposição para avaliar os quantitativos de ovos e coletados um total de 3.277 ovos, sendo estes 2.898 viáveis e 379 inviáveis. Após esse levantamento, foi possível correlacionar os dados climáticos, índices entomológicos e socioeconômicos e classificar a área em risco, alerta e controle para a transmissão de arboviroses.

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

Zinc oxide (ZnO) is a n-type transparent semiconductor which can be processed by low cost techniques (such as spray-pyrolysis and spin-coating) and can be applied as the active layer of thin-films transistors (TFTs). The electrical properties of ZnO films are strongly affected when the device is exposed to room conditions and/or UV-light, suggesting possible applications as UV or/and gas sensors. Atmospheric oxygen molecules adsorbed on ZnO surface act as charge traps, decreasing the material conductivity. The incidence of UV-light causes an increase of the material conductivity due to the photogeneration of electron-hole pairs via direct band-to-band transitions (classic photoconductivity process) and due to the desorption of oxygen molecules, which presents a relatively slower response and is a less understood mechanism. In the current paper, we study the influence of environmental parameters, such as temperature, humidity and UV-light intensity, on the electrical properties of spin-coated ZnO thin films to understand the role of the desorption mechanism on the photoconductivity process. The analysis of the device current vs. time curves shows the existence of two light-induced desorption mechanisms: i) one which increases the electrical conductivity of the ZnO film (desorption-like process) and ii) a second one which decreases the conductivity (adsorption-like process). A Plackett-Burman design of experiment (DOE) was used to study the influence of characterization factors like UV intensity, temperature and humidity on electrical parameters obtained from the experimental curves. We observed that the desorption-like process is a first order mechanism, exhibiting desorption rate proportional to n(t), where n(t) represents the adsorbate concentration as a function of the time, whereas the adsorption-like mechanism exhibits a desorption rate proportional to the forth power of n(t).

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 2020-12-10T20:06:07Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-01-01 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Recent studies have shown spray-pyrolysis is a low-cost, simple and efficient technique for deposition of metal oxide semiconductors (such as zinc oxide) as the active layer of thin-film transistors (TFTs). However, to allow the translation from laboratory scale to industry the reproducibility of such method needs to be evaluated. We present herein a representative study concerning the reproducibility and uniformity of spray-coated ZnO TFTs using several devices, from different production batches, following the same fabrication protocol. We demonstrate that it is possible to obtain transistors with high performance and reproducibility by controlling the most relevant deposition factors, corroborated by a low deviation rate (below 10%) from the characteristic TFT parameters (mobility in saturation, threshold voltage and on/off ratio), which is compatible to commonly available commercial electronic devices. Sao Paulo State Univ UNESP, Phys Dept IBILCE, BR-15054000 Sao Jose Do Rio Preto, SP, Brazil Sao Paulo State Univ UNESP, Phys Dept IGCE, Av 24A,1515, BR-13506900 Rio Claro, SP, Brazil Sao Paulo State Univ UNESP, Phys Dept IBILCE, BR-15054000 Sao Jose Do Rio Preto, SP, Brazil Sao Paulo State Univ UNESP, Phys Dept IGCE, Av 24A,1515, BR-13506900 Rio Claro, SP, Brazil FAPESP: 2019/08019-9 CAPES: 001

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...

Made available in DSpace on 2018-12-11T17:02:29Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-08-01 We present a study of the electrical properties of electrochemically doped conjugated polymers using polymeric light-emitting electrochemical cells (PLECs) and interpreting the results according to a phenomenological model (PM) which assumes that, above the device turn-on voltage, the bulk transport properties of the doped organic semiconductor are responsible for the main contribution to the whole device conductivity. To confirm the predictions of this model, the dependence of the conductivity of PLECs with different parameters is evaluated and compared with the behavior expected for a doped semiconducting polymeric material. The organic semiconductor doping level, the blend concentration of organic semiconducting molecules, the device thickness, the charge carrier mobility, and the temperature are the parameters varied to perform this analysis. We observed that the device conductivity is independent of the active layer thickness, weakly dependent on the temperature, but strongly dependent on the semiconductor doping level, on the semiconductor fraction in the blend, and on the intrinsic charge carrier mobility. These results were well described by the variable range hopping (VRH) model, which has been widely employed to describe the charge transport in doped semiconducting polymeric materials, confirming the prediction of the phenomenological model. The current analysis demonstrates that PLECs are a suitable system for studying, in situ, the electrochemical doping of semiconducting polymers, permitting the evaluation of material properties as, for instance, the density of electronic charge carriers (and, consequently, the ionic charge carrier concentration) necessary to achieve the maximum electrochemical doping level of the organic semiconductor. Departamento de Física Instituto de Geociências e Ciências Exatas Universidade Estadual Paulista – UNESP Instituto de Física de São Carlos Universidade de São Paulo Departamento de Física Instituto de Biociências Letras e Ciências Exatas Universidade Estadual Paulista – UNESP Departamento de Física Instituto de Geociências e Ciências Exatas Universidade Estadual Paulista – UNESP Departamento de Física Instituto de Biociências Letras e Ciências Exatas Universidade Estadual Paulista – UNESP

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 2019-10-06T16:38:15Z (GMT). No. of bitstreams: 0 Previous issue date: 2019-01-01 Thin-film transistors (TFTs) with the active layer composed by zinc oxide (ZnO) deposited via spray-pyrolysis present several advantages such as high electrical performance, high optical transmittance in the visible spectrum, low production cost and the ability to cover large areas. Besides the traditional application in electronic/optoelectronic circuits, ZnO TFTs can also be used in sensing devices due to its responsivity to UV-light. In the present work, we performed a bi-level full multifactorial analysis of TFT performance parameters exposed to UV-light. Characterization conditions like UV-light irradiance and time after UV exposure, as well as processing parameters such as annealing temperature were varied simultaneously, allowing the application of analysis of variance (ANOVA) to investigate the effect of these factors on the electrical performance of the devices. Field-effect mobility, threshold voltage, on/off current ratio and the device intrinsic current were among the parameters used as the responses in the factorial analysis. ANOVA was used to determine the ranking of significance of each factor on the different response parameters by the evaluation of the factor effects. Moreover, the results from ANOVA permitted the construction of linear functions used to predict the device responses in the whole range of the experimental conditions, which were confirmed by independent experimental results. The influence of factor interactions and of the linearization of some response parameters was also studied to improve the accuracy of TFT response prediction. Physics Department/IBILCE UNESP – São Paulo State University Physics Department/IGCE UNESP – São Paulo State University, Av. 24A, 1515 Physics Department/IBILCE UNESP – São Paulo State University Physics Department/IGCE UNESP – São Paulo State University, Av. 24A, 1515

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 electrical properties of organic electronic devices based on poly(2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,-phenylene vinylene) as the active layer were studied under light-excitation at different wavelength ranges and intensities, with particular focus on the frequency-response of the complex capacitance. To interpret the obtained results, an adaptation of abrupt cut-off frequency analysis of the capacitance data was developed considering the displacement of the quasi-Fermi levels due to the incidence of light. The application of such formalism allowed the determination of the position of the bulk Fermi level, the Debye length associated to the photo-generated charge-carriers and the displacement on the quasi-Fermi levels due to illumination.

In this paper, a detailed study of the capacitance spectra obtained from Au/doped-polyaniline/Al structures in the frequency domain (0.05 Hz–10 MHz), and at different temperatures (150–340 K) is carried out. The capacitance spectra behavior in semiconductors can be appropriately described by using abrupt cut-off models, since they assume that the electronic gap states that can follow the ac modulation have response times varying rapidly with a certain abscissa, which is dependent on both temperature and frequency. Two models based on the abrupt cut-off concept, formerly developed to describe inorganic semiconductor devices, have been used to analyze the capacitance spectra of devices based on doped polyaniline (PANI), which is a well-known polymeric semiconductor with innumerous potential technological applications. The application of these models allowed the determination of significant parameters, such as Debye length (≈20 nm), position of bulk Fermi level (≈320 meV) and associated density of states (≈2×1018 eV−1 cm−3), width of the space charge region (≈70 nm), built-in potential (≈780 meV), and the gap states’ distribution.

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.

We report on light-emitting devices based on a green-phosphor compound (Mn-doped zinc silicate, Zn2SiO4:Mn) dispersed in a conductive polymeric blend (poly-o-methoxyaniline/polyvinylene fluoride, POMA/PVDF-TrFE). The devices exhibited high luminance in the green, good stability and homogeneous brilliance over effective areas up to 5 cm2. The electroluminescence (EL) spectrum presented essentially the same characteristics as the photoluminescence (PL) and cathodoluminescence spectra, indicating that the light emission originates from decay of the same excited species, regardless of the excitation source. Operating characteristics were analyzed with current density–voltage (J–V ) and luminance–voltage (L–V ) curves to investigate the nature of the electroluminescence of the active material, which is still not completely understood.

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).

Esta tese de doutorado é o resultado de um estudo dos mecanismos de operação de dispositivos poliméricos emissores de luz, com um particular enfoque nos processos de injeção e transporte de portadores de carga em polímeros derivados do poli(p-fenileno vinileno), PPV. Para tanto, se fez necessário o domínio de todas as etapas de produção e caracterização, desde a síntese química dos polímeros até a fabricação propriamente dita dos dispositivos, as quais são brevemente descritas neste trabalho. Basicamente, dois tipos diferentes de dispositivos foram caracterizados: diodos poliméricos emissores de luz (PLEDs), nos quais a camada ativa é composta por um filme fino (100-500 nm de espessura) do polímero eletroluminescente puro, e células eletroquímicas emissoras de luz (LECs), compostas por blendas do polímero conjugado com um eletrólito polimérico condutor iônico. As propriedades ópticas dos dispositivos foram analisadas através dos espectros de absorção óptica na região do ultravioleta/visível e de emissão (foto e eletroluminescência) enquanto as propriedades de injeção e de transporte de carga foram exploradas através de medidas elétricas de corrente-voltagem (I-V), espectroscopia de impedância no domínio da freqüência (condutividade ac), espectroscopia de fotocorrente e ressonância magnética detectada eletricamente (EDMR). A influência de parâmetros como a estrutura dos dispositivos, os metais utilizados como eletrodos e a temperatura permitiram uma análise mais detalhada de alguns modelos teóricos utilizados na interpretação dos resultados experimentais, fornecendo, dessa forma, um maior conhecimento das propriedades físicas dos materiais estudados. This PhD thesis is an extensive study of the operation mechanisms of polymeric light-emitting devices, with a particular focus on the injection and transport properties of charge-carriers in poly(p-phenylene vinylene), PPV, derivatives. Therefore, it was necessary to dominate all the processes of fabrication and characterization of such devices, from the chemical synthesis of the polymers to the device fabrication, which are briefly described along this work. Two different kinds of devices were studied: polymeric ligh-emitting diodes (PLEDs) composed by a single thin layer (100-500 nm thick) of the pure electroluminescent polymer, and light-emitting electrochemical cells (LECs), which active layers are formed by a blend of the conjugated polymer and an ionic conductive polymeric electrolyte. The optical properties of the devices were analyzed by optical absorption in the ultraviolet-visible range and emission (photo- and electroluminescence) spectra. The charge injection and transport properties were studied by electrical measurements like current-voltage (I-V) curves, impedance spectroscopy in the frequency domain (ac conductivity), photocurrent spectroscopy and electrically detected magnetic resonance (EDMR). The influence of parameters like the device structure, the electrodes work function and the temperature allowed a detailed analysis of some theoretical models commonly used in the interpretation of the experimental results, providing more information about the physical properties of the studied conjugated polymers.

We have obtained the photoconductivity (PC) excitation spectrum for a stretch-oriented poly(paraphenylene vinylene) film over a wide spectral range (up to 5 eV). The measurements were performed in the surface cell configuration with the electric field parallel or perpendicular to the stretch direction. Although the sample had a stretch ratio of ∼4, the dark conductivity and the steady-state photoconductivity were both about 40 and 20 times higher with the electric field parallel to the average chain direction, respectively. However, the shape of the PC excitation spectrum was independent of field direction and did not show a significant rise in the ultraviolet, as is usually observed for measurements in the photodiode configuration. The implications of these results to the charge photogeneration mechanism in conjugated polymers are discussed.

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.

A phenomenological model has been developed to account for the results of impedance/admittance spectroscopy measurements from light-emitting electrochemical cells (LECs) comprising a polymer electrolyte and two different conjugated polymers used as organic semiconductor. The application of a d.c. offset bias superimposed to the a.c. modulation voltage was used to observe the transition from the behavior prior to device operation and after the formation of the electrochemical p-i-n junction. The analysis of the whole device “conductivity” as a function of the applied bias and of the frequency was used to support the assumptions considered to develop the model. The results show that the device, after the p-i-n junction formation, can be considered as composed by two highly conductive electrochemically doped (n and p) regions and a thin (few tens nanometers), insulating layer, where the electrical current is dominated by electronic charge carrier injection via tunneling through a rectangular energy barrier. Before the p-i-n junction formation, there is no doping of semiconductor material, and the device electrical properties are dominated by the intrinsic electronic charge carriers in the organic semiconductor. Results from devices made of organic semiconductors with different band gap energy and different layer thicknesses are used to corroborate the proposed model.

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.

In this work, electrically detected magnetic resonance (EDMR) at X-band is used to study the electronic properties of poly(2-metoxy-5-(2 0 -etil-hexiloxy)-1,4-phenylene vinylene) (MEH-PPV) light-emitting diodes (LEDs). The EDMR signal from MEH-PPV LEDs is found to be composed of two lines, a Lorentzian with peak-to-peak linewidth of 5 G, and a Gaussian with peak-to-peak linewidth of 24 G. The g-factor of both the components is about 2.002. The EDMR signal amplitude is typically 10 � 5 , and only observed at forward bias, for V > 10 V. The signal is a quenching, and is assigned to the spin-dependent fusion of two like-charged polarons to spinless bipolarons. The Lorentzian component is attributed to positive polarons fusion, and the Gaussian to negative polarons. The EDMR signal is found to depend on the process of carrier injection, polaron mobility, temperature and indirectly on bipolarons. r 2001 Elsevier Science B.V. All rights reserved.

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.

Understanding the microscopic origin of the dielectric properties of disordered materials has been a challenge for many years, especially in the case of samples with more than one phase. For polar dielectrics, for instance, the Lepienski approach has indicated that the random free energy barrier model of Dyre must be extended. Here we analyse the dielectric properties of a polymer blend made up with the semiconducting poly(o-methoxyaniline) and poly(vinylidene fluoride-trifluorethylene) POMA/P(VDF-TrFE), and of a hybrid composite of POMA/P(VDF-TrFE)/Zn2SiO4:Mn. For the blend, the Lepienski model, which takes into account the rotation or stretching of electric dipoles, provided excellent fitting to the ac impedance data. Because two phases had to be assumed for the hybrid composite, we had to extend the Lepienski model to fit the data, by incorporating a second transport mechanism. The two mechanisms were associated with the electronic transport in the polymeric matrix and with transport at the interfaces between Zn2SiO4:Mn microparticles and the polymeric matrix, with the relative importance of the interfacial component increasing with the percentage of Zn2SiO4:Mn in the composite. The analysis of impedance data at various temperatures led to a prediction of the theoretical model of a change in morphology at 190 ± 40 K, and this was confirmed experimentally with a differential scanning calorimetry experiment.

Photogenerated charge carrier profiles in poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) light-emitting diodes were determined from steady-state photocurrent spectra and transient photocurrent decay measurements. The observation that the photocurrent spectra behavior is strongly dependent on the bias polarity and amplitude suggests the existence of an intrinsic electric field, determined by the difference in the work function of the metallic electrodes, as well as a field dependence on the free charge carrier generation rates. The obtained results reveal built-in voltages of +0.2 and −0.6 V for Au and Al electrodes, respectively. The photocurrent spectra can be semi-quantitatively explained using a simple model which takes into account the internal built-in electric field of the device structure, the band gap energy and the migration/diffusion of the photogenerated charge carriers through the polymeric film. The drift mobility of carriers was investigated by the time-of-flight technique (TOF). The value for the mobility of holes was obtained from the change of slopes in the double logarithmic plot of the transient current and was independent of the applied field. The TOF transients for electrons showed no change of slopes in the double logarithmic plot and the drift mobility of electrons could not be calculated.

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.

Charge transport and storage phenomena were analyzed in Teflon FEP, polyethylene (PE), and poly(vinylidene fluoride) (PVDF) foils irradiated with partially penetrating electron beams, in a Split Faraday Cup (SFC) arrangement. Measurements of electrical current were performed during the electron irradiation. The irradiated front electrode was grounded while the rear electrode was positively biased. The experimental results were fitted by models which considered two different media: the irradiated region (I) and the non-irradiated one (NI). In the later, NI-region, the intrinsic electrical properties of the material were considered. In the I-region, on the other hand, effects of irradiation, as carrier generation, recombination effect and trapping-detrapping phenomenon play a fundamental role in the transport process. Beside the explanation of the experimental behavior observed in each material, the theoretical treatment provided important electrical parameters.