Regioregular poly(3-hexyl-thiophene) helical self-organization on carbon nanotubes

Mixtures of Regioregular Poly(3-hexyl-thiophene) (rrP3HT) and multi wall carbon nanotubes have been investigated by Scanning Tunneling Microscopy in Ultra High Vacuum. Carbon nanotubes covered by rrP3HT have been imaged and analyzed, providing a clear evidence that this polymer self assembles on the nanotube surface following geometrical constraints and adapting its equilibrium chain-to-chain distance. Largely spaced covered nanotubes have been analyzed to investigate the role played by nanotube chirality in the polymer wrapping, evidencing strong rrP3HT interactions along well defined directions.

Poly(3-hexyl-thiophene) coil-wrapped single wall carbon nanotube investigated by scanning tunneling spectroscopy

Scanning Tunneling Spectroscopy was performed on a (15,0) single wall carbon nanotube partially wrapped by Poly(3-hexyl-thiophene). On the bare nanotube section, the local density of states is in good agreement with the theoretical model based on local density approximation and remarkably is not perturbed by the polymer wrapping. On the coiled section, a rectifying current-voltage characteristic has been observed along with the charge transfer from the polymer to the nanotube. The electron transfer from Poly(3-hexyl-thiophene) to metallic nanotube was previously theoretically proposed and contributes to the presence of the Schottky barrier at the interface responsible for the rectifying behavior.

Evidence of multiwall carbon nanotube deformation caused by poly(3-hexylthiophene) adhesion

We show that when a soft polymer like Poly(3-hexyl-thiophene) wraps multiwall nanotubes by coiling around the main axis, a localized deformation of the nanotube structure is observed. High resolution transmission electron microscopy shows that radial compressions of about 4% can take place, and could possibly lead to larger interlayer distance between the nanotube inner walls and reduce the innermost nanotube radius. The mechanical stress due to the polymer presence was confirmed by Raman spectroscopic observation of a gradual upshift of the carbon nanotube G-band when the polymer content in the composites was progressively increased. Vibrational spectroscopy also indicates that charge transfer from the polymer to the nanotubes is responsible for a peak frequency relative downshift for high P3HT-content samples. Continuously acquired transmission electron microscopy images at rising temperature show the MWCNT elastic compression and relaxation due to polymer rearrangement on the nanotube surface.

Cycling performance of lithium metal polymer cells assembled with ionic liquid and poly(3-methyl thiophene)/carbon nanotube composite cathode

A poly(3-methylthiophene) (PMT)/multi-walled carbon nanotube (CNT) composite is synthesized by in situ chemical polymerization. The PMT/CNT composite is used as an active cathode material in lithium metal polymer cells assembled with ionic liquid (IL) electrolytes. The IL electrolyte consists of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and LiBF4. A small amount of vinylene carbonate is added to the IL electrolyte to prevent the reductive decomposition of the imidazolium cation in EMIBF4. A porous poly(vinylidene fluoride-co-hexafluoropropylene) (P(VdF-co-HFP)) film is used as a polymer membrane for assembling the cells. Electrochemical properties of the PMT/CNT composite electrode in the IL electrolyte are evaluated and the effect of vinylene carbonate on the cycling performance of the lithium metal polymer cells is investigated. The cells assembled with a non-flammable IL electrolyte and a PMT/CNT composite cathode are promising candidates for high-voltage–power sources with enhanced safety.

Evidence of room temperature charge-density wave behavior and glass-like states in pressed pellets of lightly doped poly (3-methyl thiophene)

We show room temperature charge-density wave (CDW) characteristics in d.c. and a.c. electric data in pressed pellets of lightly doped poly(3-methylthiophene). The possibility of a Peierls glass is discussed and metastables states are observed. D.C. and A.C. data also show a state with negative differencial resistance.

Estimation Of Charge Transport Parameters And Equivalent Circuit For Poly Alkyl Thiophene Field-Effect Transistors

The small signal ac response is measured across the source-drain terminals of organic field-effect transistors (OFET) under dc bias to obtain the equivalent circuit parameters of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and poly(3-hexyl thiophene) (P3HT) based devices. The numerically simulated response based on these parameters is in good agreement with the experimental data for PBTTT-FET except at low frequencies, while the P3HT-FET data show significant deviations. This indicates that the interface with the metal electrode is rather complex for the latter, involving additional circuit elements arising from contact impedance or charge injection processes. Such an investigation can help in identifying the operational bottlenecks and to improve the performance of OFETs.

Eletrodos nanoestruturados de 'TI' 'O IND.2' aplicados na degradação fotoeletrocatalítica de aminas aromáticas e desenvolvimento de dispositivo fotovoltaico 'TI 'O IND.2' / 'SB IND.2' 'S IND.3' / 'P3' 'HT'

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Investigation of electron acceptor states in poly(3-methylthiophene)

Both the DC and AC admittance of Schottky barrier diodes formed at the interface of aluminium and poly(3-methyl thiophene) have been investigated in some detail. The capacitance-voltage plots for the devices suggest the presence of two acceptor states, one shallow and one deep. The total concentration of acceptor states, 10 24-10 26 m -3 depending on the degree of undoping, agrees well with estimates from the reverse I-V characteristics assuming image force lowering of the interfacial potential barrier.

Investigation of electron acceptor states in poly(3-methylthiophene)

Both the DC and AC admittance of Schottky barrier diodes formed at the interface of aluminium and poly(3-methyl thiophene) have been investigated in some detail. The capacitance-voltage plots for the devices suggest the presence of two acceptor states, one shallow and one deep. The total concentration of acceptor states, 10 24-10 26 m -3 depending on the degree of undoping, agrees well with estimates from the reverse I-V characteristics assuming image force lowering of the interfacial potential barrier.

Temperature and electric field dependent mobility in poly„3-hexylthiophene diodes

Current-voltage (I-V) curves of Poly(3-hexyl-thiophene) (P3HT) diodes have been collected to investigate the polymer hole-dominated charge transport. At room temperature and at low electric fields the I-V characteristic is purely Ohmic whereas at medium-high electric fields, experimental data shows that the hole transport is Trap Dominated - Space Charge Limited Current (TD-SCLC). In this regime, it is possible to extract the I-V characteristic of the P3HT/Al junction showing the ideal Schottky diode behaviour over five orders of magnitude. At high-applied electric fields, holes’ transport is found to be in the trap free SCLC regime. We have measured and modelled in this regime the holes’ mobility to evaluate its dependence from the electric field applied and the temperature of the device.

Polyvinylbutyral based hybrid organic/inorganic films as a moisture barrier material

Flexible and thermally stable, freestanding hybrid organic/inorganic based polymer-composite films have been fabricated using a simple solution casting method. Polyvinylbutyral and amine functionalized mesoporous silica were used to synthesize the composite. An additional polyol-''tripentaerythritol''-component was also used to increase the -OH group content in the composite matrix. The moisture permeability of the composites was investigated by following a calcium degradation test protocol. This showed a reduction in the moisture permeability with the increase in functionalized silica loadings in the matrix. A reduction in permeability was observed for the composites as compared to the neat polymer film. The thermal and mechanical properties of these composites were also investigated by various techniques like thermogravimetric analysis, differential scanning calorimetry, tensile experiments, and dynamic mechanical analysis. It was observed that these properties detonate with the increase in the functionalized silica content and hence an optimized loading is required in order to retain critical properties. This deterioration is due to the aggregation of the fillers in the matrix. Furthermore, the films were used to encapsulate P3HT (poly 3 hexyl thiophene) based organic Schottky structured diodes, and the diode characteristics under accelerated aging conditions were studied. The weathered diodes, encapsulated with composite film showed an improvement in the lifetime as compared to neat polymer film. The initial investigation of these films suggests that they can be used as a moisture barrier layer for organic electronics encapsulation application.

Controlled phase separation for efficient energy conversion in dye/polymer blend bulk heterojunction photovoltaic cells

Low-cost photovoltaic energy conversion using conjugated polymers has achieved great improvement due to the invention of organic bulk heterojunction. in which the nanoscale phase separation of electron donor and acceptor favors realizing efficient charge separation and collection. We investigated the polymer photovoltaic cells using N, N'-bis(1-ethylpropyl)-3,4,9,10-perylene bis(tetracarboxyl diimide)/poly(3-hexyl thiophene) blend as an active layer. It is found that processing conditions for the blend films have major effects on its morphology and hence the energy conversion efficiency of the resulting devices. By optimizing the processing conditions, the sizes of donor/acceptor phase separation can be adjusted for realizing efficient charge separation and collection. The overall energy conversion efficiency of the photovoltaic cell processed with optimized conditions increases by nearly 40% compared to the normally spin-coated and annealed cell.

Design de nouvelles fonctionnelles en théorie de la fonctionnelle de la densité et conception de polymères pour application à la photovoltaïque organique

La présente thèse porte sur les calculs utilisant la théorie de la fonctionnelle de la densité (DFT) pour simuler des systèmes dans lesquels les effets à longue portée sont importants. Une emphase particulière est mise sur les calculs des énergies d’excitations, tout particulièrement dans le cadre des applications photovoltaïques. Cette thèse aborde ces calculs sous deux angles. Tout d’abord, des outils DFT déjà bien établis seront utilisés pour simuler des systèmes d’intérêt expérimental. Par la suite, la théorie sous-jacente à la DFT sera explorée, ses limites seront identifiées et de nouveaux développements théoriques remédiant à ceux-ci seront proposés. Ainsi, dans la première partie de cette thèse, des calculs numériques utilisant la DFT et la théorie de la fonctionnelle de la densité dépendante du temps (TDDFT) telles qu’implémentées dans le logiciel Gaussian [1] sont faits avec des fonctionnelles courantes sur des molécules et des polymères d’intérêt expérimental. En particulier, le projet présenté dans le chapitre 2 explore l’utilisation de chaînes latérales pour optimiser les propriétés électroniques de polymères déjà couramment utilisés en photovoltaïque organique. Les résultats obtenus montrent qu’un choix judicieux de chaînes latérales permet de contrôler les propriétés électroniques de ces polymères et d’augmenter l’efficacité des cellules photovoltaïques les utilisant. Par la suite, le projet présenté dans le chapitre 3 utilise la TDDFT pour explorer les propriétés optiques de deux polymères, le poly-3-hexyl-thiophène (P3HT) et le poly-3-hexyl- sélénophène (P3HS), ainsi que leur mélange, dans le but d’appuyer les observations expérimentales indiquant la formation d’exciplexe dans ces derniers. Les calculs numériques effectués dans la première partie de cette thèse permettent de tirer plusieurs conclusions intéressantes, mais mettent également en évidence certaines limites de la DFT et de la TDDFT pour le traitement des états excités, dues au traitement approximatif de l’interaction coulombienne à longue portée. Ainsi, la deuxième partie de cette thèse revient aux fondements théoriques de la DFT. Plus précisément, dans le chapitre 4, une série de fonctionnelles modélisant plus précisément l’interaction coulombienne à longue portée grâce à une approche non-locale est élaborée. Ces fonctionnelles sont basées sur la WDA (weighted density approximation), qui est modifiée afin d’imposer plusieurs conditions exactes qui devraient être satisfaites par le trou d’échange. Ces fonctionnelles sont ensuite implémentées dans le logiciel Gaussian [1] et leurs performances sont évaluées grâce à des tests effectués sur une série de molécules et d’atomes. Les résultats obtenus indiquent que plusieurs de ces fonctionnelles donnent de meilleurs résultats que la WDA. De plus, ils permettrent de discuter de l’importance relative de satisfaire chacune des conditions exactes.

A four-directional non-fullerene acceptor based on tetraphenylethylene and diketopyrrolopyrrole functionalities for efficient photovoltaic devices with a high open-circuit voltage of 1.18 V

Through the conjunction of tetraphenylethylene and diketopyrrolopyrrole functionalities, a novel four-directional non-fullerene electron acceptor (denoted as 4D) was designed, synthesized and characterized. The new chromophore is highly soluble (for instance >30 mg mL(-1) in o-dichlorobenzene), thermally stable, and exhibits energy levels matching those of the conventional and routinely used donor polymer poly(3-hexyl thiophene). A power conversion efficiency of 3.86% was obtained in solution-processable bulk-heterojunction devices with a very high open circuit voltage of 1.18 V.