Estudio químico-físico de propiedades intra- e intermoleculares que afectan al transporte de carga en transistores orgánicos de efecto campo

Fecha de lectura de Tesis Doctoral 18 de enero 2019.En la presente Tesis Doctoral se lleva a cabo un estudio químico-físico de propiedades que afectan al transporte de carga en transistores orgánicos de efecto campo (OFETs). Para ello, se estudian seis familias de semiconductores orgánicos, basados fundamentalmente en sistemas dador-aceptor (D-A). Estas familias se han dividido en cuatro capítulos en función del efecto de la propiedad intra- o intermolecular a estudio en el transporte de carga. En el Capítulo I se estudia el efecto del solapamiento de orbitales moleculares de dos familias basadas en derivados de oligotiofenos-naftalimidas. En el Capítulo II se estudia cómo influye el perfil de conjugación de tres semiconductores basados en uniones de dicetopirrolopirrol y tiofeno, pasando de un perfil lineal a un perfil trigonal. En el Capítulo III se analiza el efecto del isomerismo en el transporte de carga con el estudio de dos polímeros isoméricos basados en amidas cíclicas y grupos etilendioxitiofeno. Para concluir, en el Capítulo IV se estudia el efecto de la elongación del esqueleto conjugado en una familia de oligotiofeno-perilenimida y en una familia de sistemas de tipo escalera basados en bitiofenoimidas. El uso combinado de técnicas espectroscópicas, electroquímicas y métodos químico-cuánticos nos permite establecer relaciones entre estructura y propiedad de estos sistemas, lo que nos ayuda a guiar racionalmente los esfuerzos sintéticos encaminados a la obtención de compuestos con propiedades mejoradas para su uso en OFETs

Del estudio de la molécula al dispositivo

Los materiales orgánicos pi-conjugados han destacado en las últimas décadas como alternativas prometedores al silicio en su uso en dispositivos electrónicos, como por ejemplo transistores orgánicos de efecto campo (OFETs) o células solares orgánicas (OPVs). En estos materiales, el comportamiento del dispositivo está principalmente determinado por su esqueleto conjugado. Por ello, sistemas orgánicos con alto grado de planaridad, eficiente deslocalización pi-electrónica y adecuadas interacciones intermoleculares resultan de gran interés para semiconductores moleculares y poliméricos. En nuestro grupo de investigación, haciendo uso de diferentes técnicas espectroscópicas, electroquímicas, espectroelectroquímicas y de cálculos teóricos químico-cuánticos DFT, tratamos de llegar a una mejor comprensión del comportamiento fisicoquímico de estos compuestos, con el fin de buscar relaciones existentes entre estructura y propiedad. Estos estudios resultan primordiales para guiar los esfuerzos sintéticos encaminados a la obtención de compuestos orgánicos pi- conjugados con propiedades mejoradas, obteniendo así dispositivos electrónicos con mayores rendimientos.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Organic ambipolar semiconductors for TFT applications

In the last years we have devoted some effort to the search of new high-mobility semiconductors with ambipolar performances, good processability and environmental stability. Our approach, which is one of the most widely used, consists in the combination of donor and acceptor moieties in the conjugated skeleton, which allows fine tuning of the frontier molecular orbitals. For OTFT applications, low-lying HOMOs are essential to resist air oxidation and thus increase device stability. However, if the HOMO energy is too low, the resulting barrier to hole injection may compromise the transistor performance. Thus, a delicate balance between these two effects is needed. In particular, we have combined naphthaleneimide-derived moieties as electron accepting groups with electron-rich oligothiophene fragments. In these materials, we have found that the presence of ambipolar transport in these planar molecules can be understood on the basis of three interrelated properties: (i) the absence of skeletal distortions allows closer intermolecular pi-pi stacking and enhanced intramolecular pi-conjugation, (ii) increased pi-conjugation raises the HOMO energy, which approaches the Fermi level of common used electrodes; and (iii) more planar structures translate into lower Marcus reorganization energies. However, one of the limitations of these types of semiconductors is the presence of a molecular dipole moment, which forces the molecules to pack with pairwise intermolecular interactions orienting the naphthaleneimide cores in opposite directions, decreasing in some cases molecular orbitals overlapping. In recent contributions, we have devoted our efforts to analyze the effect of molecular interactions, through chemical modifications in order to induce parallel and antiparallel molecular packing, on the electronic properties of ambipolar semiconductors.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Understanding the structure-property correlations of n-type organic semiconductors in OFETs

In the organic electronic research field, the development of high-performance unipolar n-type semiconductors is still challenging. Here we present an in-depth study of a series of ladder-type semiconductors, which due to their all-acceptor backbones, exhibit unipolar n-type transport in OTFTs. It is well know that the performance of organic semiconductors is governed not only by their molecular structures but also by their intermolecular assembly in the solid state. Thus, highly planar backbones are beneficial for a good molecular packing and film ordering leading to good charge transport characteristics. In this contribution, we study a series of BTI small molecules and polymers, both from a molecular and from a supramolecular point of view, in order to establish useful structure-property relationships that may guide the rational synthesis of new and improved materials. To carry out this study, we make use of different spectroscopic techniques, supported by quantum theoretical calculations at the DFT level.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Understanding organic materials performance in field-effect transistors

Comunicación oralIn the last years, much of our effort has been devoted to the search and study of new high-mobility semiconductors for organic thin film transistors. The approach used for the materials design has been two-fold: (i) the combination of donor and acceptor moieties in the pi-conjugated skeleton, which allows fine tuning of the frontier molecular orbitals, being this necessary for achieving electron/hole or ambipolar transport and ambient stability; and (ii) rational selection of the type and positioning of specific solubilizing substituents ensuring processability, which is essential to make these materials scalable to industry. However, material processability should be attained minimizing a negative effect on charge transport. Therefore, proper energy levels, planar molecular conformations, close intermolecular pi-pi stacking and adequate thin film crystallinity need to be maintained upon alkyl substitution. In this communication, several examples of molecular and polymeric materials are shown. A rational design, guided by experimental and theoretical evidences, has prompted modifications on their conjugated skeletons, donor/acceptor subunits ratio and/or selection of proper alkyl solubilizing chains, which induce noticeable changes in their electronic performances. The main aim of these studies is the basic understanding of charge transport in organic materials. For this end, we will use Raman spectroscopy and DFT quantum-chemical calculations as important tools for materials characterization.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Understanding polymer orientation at the interface by SERS

In the organic electronic research field, it is well known that the largest contribution to charge transport occurs within the first few nanometers of the semiconductor near the dielectric interface. Surface Enhanced Raman spectroscopy (SERS) appears as an easy and straightforward technique to analyze this buried interface and to provide useful information on molecular orientation at the device active layer. Here we present the study of the molecular orientation of the widely studied P(NDI2OD-T2) polymer at the semiconductor/dielectric and semiconductor/metal interfaces using SERS and DFT calculations. Our first SERS results show a relative intensification of selected normal modes, which indicates that the orientation of the polymer changes from a face-on (before annealing treatment) to and edge-on disposition after melt annealing, being this in good agreement with the previous results gathered from other techniques (Figure 1).Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Transporte de carga de sistemas tipo escalera (ladder-type) en transistores orgánicos de efecto campo (ofets)

Las moléculas π-conjugadas tipo escalera con estructura plana están recibiendo un enorme interés como semiconductores moleculares o como unidades constituyentes de otro tipo de estructuras más grandes o polímeros para dispositivos electrónicos. Sin embargo, la mayoría de las moléculas de tipo escalera exhiben características de tipo p, siendo un gran desafío conseguir análogos deficientes en electrones. En este sentido, los arenos funcionalizados con imidas son materiales altamente prometedores. Entre ello, los bitiofenoimida (BTI) tipo escalera, presentan excelentes propiedades fisicoquímicas y electrónicas. Por ello, haciendo uso de técnicas espectroscópicas, cálculos químico-cuánticos DFT y caracterización eléctrica en transistores orgánicos de efecto campo (OFETs), establecemos en este estudio relaciones estructura-propiedad que nos ayudan a entender el transporte de carga de estos sistemas.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. RSEQ Sigma Aldric

Understanding charge transport in organic field effect transistors

The organic electronics research field has advanced tremendously in the last decades, having already led to field-effect mobilities able to compete with their inorganic counterparts. However, many fundamental aspects of this field remain still unclear and need to be clarified before its final blossoming, which would probably come with the complete understanding of the charge transport mechanism in organic materials. It is well-known that the performance of organic semiconductors is governed not only by their molecular structures but also by their intermolecular assembly in the solid state. Therefore, analyzing organic materials from both a molecular and supramolecular point of view is highly desirable. For this end, Raman spectroscopy is a rapid, non invasive technique able to gather information on molecular and supramolecular levels, thus being greatly useful in the organic electronics research field. Analyzing buried interfaces, such as the semiconductor-dielectric interface in organic field effect transistors (OFETs) is fundamental, since the largest contribution to charge transport occurs within the first few nanometers of the semiconductor near the dielectric interface. Surface Enhanced Raman Spectroscopy (SERS) appears as an easy and straightforward technique to carry out this task and to provide useful information on molecular orientation at the device active layer. In this communication, some examples will be presented in which several spectroscopic techniques, conventional Raman and SERS, supported by DFT quantum chemical calculations have been used to shed light on the mechanism of charge transport in OFETs.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

SpectroscopicTechniques and DFT Calculations to Understand Charge Transport Mechinisms in OFETs

The organic electronics research field has advanced tremendously in the last decades, but there is still an incomplete understanding of the main mechanisms governing charge injection and transport in such devices. The performance of organic semiconductors is governed not only by their molecular structures but also by their intermolecular assembly in the solid state. Here we use a combination of Raman spectroscopy and charge modulation spectroscopy (CMS) to gather information on molecular and supramolecular levels, of organic semiconductors [1,2] (Figure 1) [3]. This last one is an optical-spectroscopy technique conducted on a real OFETs, that allows us to study in situ the charge carriers present at the semiconductor-dielectric interface, where the largest contribution to charge transport occurs. [3] In this communication we will present the study of the bithiophene imide (BTIn) molecules which exhibit encouraging electron mobilities in OFETs [1,2], by using the spectroscopic techniques presented above, supported by DFT quantum chemical calculations in order to shed light on the mechanism of charge transport in OFETs.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Tuning of the electronic levels of oligothiophene-naphthalimide semiconductors: in the search of ambipolar behaviour

Ambipolar organic field-effect transistors (OFETs), which can efficiently transport both holes and electrons, using a single type of electrode, are currently of great interest due to their possible applications in complementary metal oxide semiconductor (CMOS)-like circuits, sensors, and in light-emitting transistors. Several theoretical and experimental studies have argued that most organic semiconductors should be able to transport both types of carrier, although typically unipolar behavior is observed. One factor that can compromise ambipolar transport in organic semiconductors is poor solid state overlap between the HOMO (p-type) or LUMO (n-type) orbitals of neighboring molecules in the semiconductor thin film. In the search of low-bandgap ambipolar materials, where the absence of skeletal distortions allows closer intermolecular π-π stacking and enhanced intramolecular π-conjugation, a new family of oligothiophene-naphthalimide assemblies have been synthesized and characterized, in which both donor and acceptor moieties are directly conjugated through rigid linkers. In previous works we found that oligothiophene-napthalimide assemblies connected through amidine linkers (NDI derivates) exhibit skeletal distortions (50-60º) arising from steric hindrance between the carbonyl group of the arylene core and the sulphur atom of the neighbored thiophene ring. In the present work we report novel oligo- and polythiophene–naphthalimide analogues NAI-3T, NAI-5T and poly-NAI-8C-3T, in which the connections of the amidine linkage have been inverted in order to prevent steric interactions. Thus, the nitrogen atoms are directly connected to the naphthalene moiety in NAI derivatives while they were attached directly to the thiophene moiety in the previously investigated NDI-3T and NDI-5T. The calculated molecular structure of NAI-3T together shows how the steric interactions are not present in the novel NAI derivative. The planar skeletons in these new family induce higher degree of crystallinity and the carrier charge transport can be switched from n-type to ambipolar behaviour. The highest FET performance is achieved for vapor-deposited films of NAI-3T with mobilities of 1.95x10-4cm2V-1s-1 and 2.00x10-4cm2V-1s-1 for electrons and holes, respectively. Finally, these planar semiconductors are compared with their NDI derivates analogues, which exhibit only n-type mobility, in order to understand the origin of the ambipolarity in this new series of molecular semiconductors.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec