Semiconductores para electrónica orgánica: el papel de la espectroscopía vibracional

Seminario especializado invitadoLa búsqueda de nuevas tecnologías capaces de reemplazar a la tecnología actual basada en silicio representa uno de los campos de investigación científica más dinámico en la actualidad. La electrónica y fotónica orgánica aparecen como alternativas viables, usando materiales orgánicos como componentes activos de los dispositivos. Estos materiales presentan propiedades deseables tales como bajo coste, excelente procesabilidad y compatibilidad con sustratos plásticos, lo que permite la fabricación de circuitos y células solares mecánicamente flexibles mediante el uso de técnicas económicas como la impresión. En este seminario nos centraremos, en primer lugar, en determinar cuáles son algunas de las características fundamentales que debemos perseguir en el diseño de semiconductores orgánicos para transistores de efecto campo. A su vez, se mostrarán ejemplos de cómo la espectroscopia vibracional puede servirnos como herramienta muy útil para caracterizar las distintas partes/propiedades de dichos dispositivos.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Materiales moleculares para electrónica orgánica

Se impartió un seminario especializado sobre materiales moleculares para electrónica orgánica. En el seminario, principalmente se revisaron las características deseables en los semiconductores orgánicos para obtener un buen funcionamiento en transistores de efecto de campo.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Fused Quinoidal Oligothiophenes Imides with High Electrical Conductivity

Organic diradicals are molecules containing two unpaired electrons, which are usually highly reactive.1-2 Although these organic diradicals present a wide range of potential applications, their air stability still remains as a major obstacle.3 In order to overcome this, new organic diradicals based on quinoidal oligothiophenes-derivatives (QOT) have been synthesized, i.e. BTICN, ISOCN and QTICN (see Figure 1). These new molecules present high stability and electrical conductivity, which have been achieved by employing imide-bridged fused molecular frameworks. The combination of strong electron-withdrawing imide with tetracyano groups in the conjugated skeletons also enabled extremely deeply aligned LUMO levels and large diradical character assisted by cross-conjugation.4 Here we use different experimental techniques and DFT calculations to provide new insights into the electron conduction mechanism of QOT diradicaloids, in order to demonstrate the great potential of fused quinoidal oligothiophene imides in developing stable organic diradicals and high-performance doping-free n-type conductive materials.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

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 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

Organic semiconductors: The effect of small modifications on device performance

Invited oral presentationIn the search on new high-mobiliity semiconductors with ambipolar performances, good processability and excellent environmental stability, diverse synthetic strategies have been approached. One of the most widely used consists in the alternation 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. Furthermore, high performance solution-processable materials require the correct selection and positioning of the specific solubilizing substituents in order to achieve proper HOMO and LUMO energy levels, planar molecular conformations, close intermolecular pi-pi stacking, and proper thin film crystallinity. In this communication, several examples of molecular and polymeric materials where modifications on their conjugated skeletons, donor/acceptor subunits ratio and/or the selection of proper alkyl solubilizing chains induce noticeable changes in their electronic performances.Universidad de Málaga, Campus de Excelencia Internacional Andalucía Tec

Analyzing thin film morphology by Resonance Raman spectroscopy

Polymeric organic thin film transistors (OFETs) and all-polymer bulk heterojunction solar cells (all-PCS), which are composed of a polymer donor and a polymer acceptor, have attracted considerable attention in the last years. The interest of these polymeric materials present various advantages versus small molecular counterparts, including strong light absorption, excellent mechanical flexibility and durability, and great potential in printing applications due to their great processability. In OFETs and bulk heterojunction solar cells, the morphology and crystallinity control of the neat polymer or blended donor-acceptor polymer films is essential in order to improve device performance. In this communication, we present a Resonance Raman spectroscopy study directed to disentangle the film morphology of a series of all-acceptor and donor acceptor polymers for OFETs and all-PCS applications.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 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