RESUMO

Detecting volatile organic compounds is a fundamental step in water quality analysis. Methylisoborneol (MIB) provides a lousy odor to water, whereas geosmin (GEO) is responsible for its sour taste. A widely-used technique for their detection is gas-phase chromatography. On the other hand, an electronic nose from organic thin-film transistors is a cheaper and faster alternative. Poly(2,5-bis(3-tetradecyl-thiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-C14) features semiconducting properties suitable for organic electronics. However, in order to expose the active layer in a bottom-gate transistor structure with photolithographically patterned electrodes, a cross-linked dielectric such as poly(4-vinyl phenol) (PVP) is necessary. In this work, the cross-linking was demonstrated using FTIR and Raman spectroscopies, as well as high-k capacitors with a dielectric constant of 5.3. The presence of enhanced crystallinity with terrace formation in the semiconducting film was confirmed with UV-visible spectrophotometry, atomic force microscopy, and X-ray diffraction. Finally, for the first time, a PBTTT-C14 transistor on cross-linked PVP was shown to respond to isoborneol with a sensitivity of up to 6% change in mobility per ppm. Due to its similarity to MIB, a system comprising these sensors must be investigated in the future as a tool for sanitation companies in real-time water quality monitoring.

RESUMO

A new benzotrithiophene-based small molecule, namely 2,5,8-Tris[5-(2,2-dicyanovinyl)-2-thienyl]-benzo[1,2-b:3,4-b':6,5-b″]-trithiophene (DCVT-BTT), was successfully synthesized and subsequently characterized. This compound was found to present an intense absorption band at a wavelength position of ∼544 nm and displayed potentially relevant optoelectronic properties for photovoltaic devices. Theoretical studies demonstrated an interesting behavior of charge transport as electron donor (hole-transporting) active material for heterojunction cells. A preliminary study of small-molecule organic solar cells based on DCVT-BTT (as the P-type organic semiconductor) and phenyl-C61-butyric acid methyl ester (as the N-type organic semiconductor) exhibited a power conversion efficiency of 2.04% at a donor: acceptor weight ratio of 1:1.

RESUMO

Three photobicyclized benzodithienoquinolizinium tetrafluoroborates (BPDTQBF4) were prepared and evaluated by UV-Vis and fluorescence spectral, electrochemical analysis, and by theoretical calculations as possible organic n-type semiconductors. Evaluation and comparison of their LUMO levels, HOMO-LUMO energy gaps as monomeric and π-stacked dimers with those of other materials, suggest their potential as organic n-type semiconductors. Calculations of their relative charge carrier mobilities confirmed this potential for one derivative with a long (C-14) alkyl chain appended to the polycyclic planar π-system.

RESUMO

Organic thin-film transistors (OTFTs) are miniaturized devices based upon the electronic responses of organic semiconductors. In comparison to their conventional inorganic counterparts, organic semiconductors are cheaper, can undergo reversible doping processes and may have electronic properties chiefly modulated by molecular engineering approaches. More recently, OTFTs have been designed as gas sensor devices, displaying remarkable performance for the detection of important target analytes, such as ammonia, nitrogen dioxide, hydrogen sulfide and volatile organic compounds (VOCs). The present manuscript provides a comprehensive review on the working principle of OTFTs for gas sensing, with concise descriptions of devices' architectures and parameter extraction based upon a constant charge carrier mobility model. Then, it moves on with methods of device fabrication and physicochemical descriptions of the main organic semiconductors recently applied to gas sensors (i.e., since 2015 but emphasizing even more recent results). Finally, it describes the achievements of OTFTs in the detection of important gas pollutants alongside an outlook toward the future of this exciting technology.

RESUMO

The combination of organic and inorganic materials at the nanoscale to form functional hybrid structures is a powerful strategy to develop novel electronic devices. The knowledge on semiconductor thin-film polarization brings direct benefits to the hybrid organic/inorganic electronics, becoming primordial for the development of devices such as electromechanical logic gates, solar cells, miniaturized valves, organic diodes, and molecular supercapacitors, among others. Here, we report on the dielectric polarization of ultrathin organic semiconducting films-ca. 5 nm thick metal phthalocyanine ensembles (viz., CuPc, CoPc, F16CuPc)-employed to build up hybrid metal/oxide/molecule heterojunctions. Such hybrid heterostructures are fully integrated into self-rolled nanomembrane-based capacitors and further investigated by impedance spectroscopy measurements as a function of temperature (from 6 to 300 K). The dielectric polarization of the metal phthalocyanines is found to be thermally activated above a specific threshold temperature, which depends on the molecular structure. Below this threshold, the current leakage across the system is suppressed, thus evidencing intrinsic-like polarization mechanisms. The temperature-independent permittivities of the ultrathin molecular films are found to be strongly dependent on the organic/inorganic hybrid interfaces, while the calculated relaxation times are more likely related to each single-molecule polarization. Beyond the advances in determining the temperature dependence of the permittivity for ultrathin phthalocyanine films integrated within solid-state electronics, our results also support the deterministic design of novel functional devices based on nanoscale hybrid organic/inorganic heterojunctions.

RESUMO

In organic molecular crystals, the polaronic hopping model for the charge transport assumes that the carrier lies at one or a small number of molecules. Such a kind of localization suffers the influence of the non-local electron-phonon (e-ph) interactions associated with intermolecular lattice vibrations. Here, we developed a model Hamiltonian for numerically describing the role played by the intermolecular e-ph interactions on the stationary and dynamical properties of polarons in a two-dimensional array of molecules. We allow three types of electron hopping mechanisms and, consequently, for the nonlocal e-ph interactions: horizontal, vertical, and diagonal. Remarkably, our findings show that the stable polarons are not formed for isotropic arrangements of the intermolecular transfer integrals, regardless of the strengths of the e-ph interactions. Interestingly, the diagonal channel for the e-ph interactions changes the transport mechanism by sharing the polaronic charge between parallel molecular lines in a breather-like mode.

RESUMO

This study refers to the doping of organic semiconductors by a simple reaction between copper phthalocyanine and tetrathiafulvalene or tetracyanoquinodimethane. The semiconductor films of copper phthalocyanine, doped with tetrathiafulvalene donor (CuPc-TTF) and tetracyanoquinodimethane acceptor (CuPc-TCNQ) on different substrates, were prepared by vacuum evaporation. The structure and morphology of the semiconductor films were studied with infrared (IR) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The absorption spectra for CuPc-TTF, recorded in the 200⁻900 nm UV⁻vis region for the deposited films, showed two peaks: a high energy peak, around 613 nm, and a second one, around 695 nm, with both peaks corresponding to the Q-band transition of the CuPcs. From the spectra, it can also be seen that CuPc-TTF has a B-band at around 330 nm and has a bandgap of approximately 1.4 eV. The B-band in the CuPc-TCNQ spectrum is quite similar to that of CuPc-TTF; on the other hand, CuPc-TCNQ does not include a Q-band in its spectrum and its bandgap value is of approximately 1.6 eV. The experimental optical bandgaps were compared to the ones calculated through density functional theory (DFT). In order to prove the effect of dopants in the phthalocyanine semiconductor, simple devices were manufactured and their electric behaviors were evaluated. Devices constituted by the donor-acceptor active layer and by the hollow, electronic-transport selective layers, were deposited on rigid and flexible indium tin oxide (ITO) substrates by the vacuum sublimation method. The current⁻voltage characteristics of the investigated structures, measured in darkness and under illumination, show current density values of around 10 A/cm² for the structure based on a mixed-PET layer and values of 3 A/cm² for the stacked-glass layered structure. The electrical properties of the devices, such as carrier mobility (µ) were obtained from the J⁻V characteristics. The mobility values of the devices on glass were between 1.59 × 108 and 3.94 × 1010 cm²/(V·s), whereas the values of the devices on PET were between 1.84 × 108 and 4.51 × 108 cm²/(V·s). The different behaviors of the rigid and flexible devices is mainly due to the effect of the substrate.

RESUMO

The influence of graphene and retinoic acid (RA) - a π-conjugated organic semiconductor - interface on their hybrid system is investigated. The physical properties of the interface are assessed via scanning probe microscopy, optical spectroscopy (photoluminescence and Raman) and ab initio calculations. The graphene/RA interaction induces the formation of a well-organized π-conjugated self-assembled monolayer (SAM) at the interface. Such structural organization leads to the high optical emission efficiency of the RA SAM, even at room temperature. Additionally, photo-assisted electrical force microscopy, photo-assisted scanning Kelvin probe microscopy and Raman spectroscopy indicate a RA-induced graphene doping and photo-charge generation. Finally, the optical excitation of the RA monolayer generates surface potential changes on the hybrid system. In summary, interface-induced organized structures atop 2D materials may have an important impact on both design and operation of π-conjugated nanomaterial-based hybrid systems.

RESUMO

We present an extensive study of the optical properties of Myrcia sylvatica essential oil with the goal of investigating the suitability of its material system for uses in organic photovoltaics. The methods of extraction, experimental analysis, and theoretical modeling are described in detail. The precise composition of the oil in our samples is determined via gas chromatography, mass spectrometry, and X-ray scattering techniques. The measurements indicate that, indeed, the material system of Myrcia sylvatica essential oil may be successfully employed for the design of organic photovoltaic devices. The optical absorption of the molecules that compose the oil are calculated using time-dependent density functional theory and used to explain the measured UV-Vis spectra of the oil. We show that it is sufficient to consider the α-bisabolol/cadalene pair, two of the main constituents of the oil, to obtain the main features of the UV-Vis spectra. This finding is of importance for future works that aim to use Myrcia sylvatica essential oil as a photovoltaic material.

Assuntos

Myrtaceae/química , Óleos Voláteis/química , Cromatografia Gasosa , Espectrometria de Massas , Difração de Raios X

RESUMO

The polaron stability in organic molecular crystals is theoretically investigated in the scope of a two-dimensional Holstein-Peierls model that includes lattice relaxation. Particularly, the investigation is focused on designing a model Hamiltonian that can address properly the polaron properties in different model oligoacene crystals. The findings showed that a suitable choice for a set of parameters can play the role of distinguishing the model crystals and, consequently, different properties related to the polaron stability in these systems are observed. Importantly, the usefulness of this model is stressed by investigating the electronic localization of the polaron, which provides a deeper understanding into the properties associated with the polaron stability in oligoacene crystals.

RESUMO

Polymer-based organic light-emitting diodes (OLEDs) with the structure ITO / PEDOT:PSS / MDMO-PPV / Metal were prepared by spin coating. It is known that electroluminescence of these devices is strongly dependent on the material used as cathode and on the deposition parameters of the polymer electroluminescent layer MDMO-PPV. Objective. In this work the effect of i) the frequency of the spin coater (1000-8000 rpm), ii) the concentration of the MDMO-PPV: Toluene solution, and iii) the material used as cathode (Aluminium or Silver) on the electrical response of the devices, was evaluated through current-voltage (I-V) measurements. Materials and methods. PEDOT:PPS and MDMO-PPV organic layers were deposited by spin coating on ITO substrates, and the OLED structure was completed with cathodes of aluminium and silver. The electric response of the devices was evaluated based on the I-V characteristics. Results. Diodes prepared with thinner organic films allow higher currents at lower voltages; this can be achieved either by increasing the frequency of the spin coater or by using concentrations of MDMO-PPV: Toluene lower than 2% weight. A fit of the experimental data showed that the diodes have two contributions to the current. The first one is attributed to parasitic currents between anode and cathode, and the other one is a parallel current through the organic layer, in which the carrier injection mechanism is mediated by thermionic emission. Conclusions. The results of the fitting and the energy level alignment through the whole structure show that PPV-based OLEDs are unipolar devices, with current mainly attributed to hole transport.

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Se fabricaron diodos orgánicos emisores de luz (OLEDs) con la estructura ITO / PEDOT:PSS / MDMO-PPV / Metal mediante la técnica de spin coating. Es ampliamente conocido que la electroluminiscencia de estos diodos depende fuertemente del material usado como cátodo y también de los parámetros de crecimiento de la capa del polímero electroluminiscente MDMO-PPV. Objetivo. En este trabajo el efecto de i) la frecuencia del spin coater (1000-8000 rpm), ii) la concentración de la solución MDMO-PPV: Tolueno y iii) el material usado como cátodo (plata o aluminio) sobre la repuesta eléctrica de los dispositivos, fue evaluado a través de medidas de corriente-voltaje (I-V). Materiales y métodos. Películas delgadas de los materiales orgánicos PEDOT:PSS y MDMO-PPV fueron depositados por spin coating sobre sustratos de ITO, y la estructura del OLED fue terminada con cátodo de plata y aluminio. La respuesta eléctrica de los dispositivos fue evaluada a través de su característica I-V. Resultados. Los diodos fabricados con películas orgánicas más delgadas son los que suministran mayores corrientes a menores voltajes. Esto puede lograrse ya sea incrementando la frecuencia de rotación del spin coating o usando concentraciones de MDMO-PPV: Tolueno menores al 2% en peso. Un ajuste de los datos experimentales demostró que los diodos poseen contribuciones de una corriente parásita entre ánodo y cátodo, y otra corriente paralela en donde el mecanismo predominante de inyección de portadores a la capa orgánica es a través de emisión termoiónica. Conclusiones. El ajuste de los datos experimentales, junto con la posición de niveles de energía a través de la heteroestructura, demuestra que los OLEDs basados en derivados de PPV son dispositivos unipolares, en el que la corriente se atribuye principalmente a transporte de huecos.

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Foram fabricados diodos orgânicos emissores de luz (OLEDs) com a estrutura de ITO / PEDOT: PSS / MDMO-PPV / metal, pela técnica de spin coating. É amplamente conhecido que a eletroluminescência destes diodos depende fortemente do material utilizado como cátodo, e também dos parâmetros de crescimento da camada de polímero eletroluminescente MDMO-PPV. Objetivo. Neste trabalho o efeito de i) a freqüência do spin coater (1000-8000 rpm), ii) a concentração da solução MDMO-PPV: Tolueno e iii) o material utilizado como cátodo (prata ou alumínio) sobre a resposta elétrica dos dispositivos, foi avaliado por medidas de corrente-voltagem (I-V). Materiais e métodos. Películas finas de materiais orgânicos PEDOT: PSS e MDMO-PPV foram depositadas por spin coating sobre substratos de ITO e a estrutura do OLED foi terminada com cátodo de prata e de alumínio. A resposta elétrica dos dispositivos foi avaliada pela sua característica I-V. Resultados. Os diodos feitos de películas orgânicas finas fornecem maiores correntes a menores voltagens. Isto pode ser conseguido, quer através do aumento da velocidade de rotação do spin coating ou usando concentrações de MDMO-PPV: Tolueno menores de 2% em peso. Um ajuste dos dados experimentais mostrou que os diodos têm uma contribuição de uma corrente parasita entre anodo e catodo, e outra corrente paralela, onde o principal mecanismo da injeção de portadores da camada orgânica é através da emissão termiônica. Conclusões. O ajuste dos dados experimentais, juntamente com a posição dos níveis de energia através da heteroestrutura, mostra que os OLEDs baseados em derivados de PPV são dispositivos unipolares, onde a corrente é atribuída principalmente ao transporte de ocos.