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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 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

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

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

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

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 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.

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.

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

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.

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.

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.

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.

Fast transient current decay was recorded on POMA samples during current pulses (in the order of milliseconds) provided by a low energy electron beam under an applied field. The characteristic time decay depends on the electron beam energy and on the bias polarity. The results were explained taking into account the effect of space charge, the intrinsic conductivity of the non-irradiated region of the sample and the radiation-induced conductivity of the thin irradiated region. Fitting parameters may provide the value of both intrinsic and radiation-induced conductivities and the average electron range.

Recently, piezoelectric cellular polypropylene (PP) was proposed as a new type of quasi-ferroelectric. The observed hysteresis of the charge density as a function of the electric field could be explained as field-dependent charging inside the gas-filled voids. Interestingly enough, the measurable poling behavior of the macroscopic "dipoles" formed by charges that are trapped at the internal void surfaces is phenomenologically completely identical to the cooperative poling behavior of microscopic molecular dipoles in ferroelectric polymers. Therefore, it can be assumed that charge separation (or charge redistribution) and subsequent trapping in cellular PP is a rather fast "switching" process. In order to examine the poling dynamics, we developed an experimental setup for pulsed poling. High-voltage pulses with a duration of 45 /spl mu/s (FWHM) were applied in direct contact to two-side metallized cellular PP films. The pulsed poling yields piezoelectricity in the cellular PP. We study and discuss the dependence of the resulting piezoelectricity on the poling field. We also characterize the charge separation during application of higher electric poling fields of up to -10 kV in direct contact to the two-side metallized films for longer times.