New organic semiconductors based on the carbazole core: synthesis and application in optoelectronic devices

Abstract

[eng] The increasing demand on next-generation displays encourages an exhaustive research on the development of new and enhanced semiconductors. Apart from the conventional inorganic materials, organic-based semiconductors can also fulfil this role while affording unique features. Indeed, organic materials claim coveted properties such as electronic tunability and the feasibility to fabricate flexible displays with large areas at a lower cost, thus finding enormous applicability in different optoelectronic devices. The capability to transport charges in organic compounds is strongly related to the molecular design, which determines key factors such as the electronic profile and the disposition in the solid state. In this context, the aim of this Thesis is focalized on the synthesis and application of new organic semiconductors designed from a common building block: the carbazole heterocycle. The envisioned applications go from Organic Thin-Film Transistors (OTFTs) to Organic Light-Emitting Diodes (OLEDs). As an initial approach towards p-type semiconductors, the π-system of the carbazole nucleus has been extended to furnish three ladder-type constructions, namely diindolocarbazole, bisbenzothienocarbazole and diphenanthrocarbazole. The main molecular backbones have been tailored with the inclusion of different alkylation patternings. The integration of the resulting compounds in OTFT devices led to hole mobility values ranging from 10−6 up to 10−3 cm2 V–1 s–1, accentuating the importance of both the molecular and the device architecture. The most promising materials arise from the homogeneous N-alkylation of diindolocarbazole with short-to-medium chains and the incorporation of peripheral alkyl chains to the diphenanthrocarbazole core. The availability of these cores is facilitated by accessible synthetic procedures, which conclude with a microwave-assisted Cadogan reaction in the former and the Scholl reaction in the latter. As corroborated by means of X-ray diffraction, both cores arrange in a favorable gamma packing in the solid state that justify these enticing results. Remarkably, all the studied compounds feature an extraordinary air-stability, with some of the devices featuring a shelf lifetime that surpass the milestone of 1000 days. As a second strategy, the carbazole core has been substituted by its sulfurated analog dibenzothiophene as the main building block. By means of sequential Suzuki-Miyaura and Scholl reactions, diverse diphenanthro[9,10-b:9',10'-d]thiophene derivatives have been synthesized from the commercially available tetrabromothiophene. The envisioned synthetic route could provide not only homogeneous structures, but also heterogeneous ones in a two-step procedure. The latter systems also behaved as p-type semiconductors, with hole mobility values up to 6 × 10−5 cm2 V–1 s–1 in OTFTs. The redesign of the triindole core, which is a well-known carbazole-based semiconductor, constituted another part of this Thesis. Specifically, it included the synthesis and physical characterization of triindole analogs, featuring: carbazole moieties attached to different positions, oxygen or sulfur substituting the nitrogen as heteroatom and the inclusion of peripheral alkyl chains. The collected results prompt further study in this direction. Finally, the promising optical properties of the 3-(phenylethynyl)-9H-carbazole unit have been exploited in the OLED technology. The two analyzed constructions, i.e. 6,6′-bis(phenylethynyl)-9H,9′H-3,3′-bicarbazole and 1,3,5-tris((9H-carbazol-3-yl)ethynyl)-benzene, exhibited blue emission in the solid state and in solution-processed OLEDs. The hexylated derivative of the first system, apart from emitting in the sought deep-blue region, also stood out as the most appropriate to perform as host for iridium complexes in white OLEDs. The emission of the fabricated devices covered a wide range of white hues, depending on the composition and the applied voltage.

[cat] Aquesta Tesi té com a objectiu la síntesi de semiconductors orgànics de tipus p basats en l’heterocicle 9H-carbazole i la seva aplicació en diferents dispositius electrònics, com poden ser els transistors orgànics de capa prima (OTFTs) i els díodes emissors de llum orgànics (OLEDs). Per assolir aquest objectiu, s’ha treballat en primer lloc en l’extensió lineal del sistema π de l’anell de carbazole. També s’ha fet èmfasi en la incorporació de cadenes alquíliques a les estructures dissenyades, ja que són fonamentals per modular la disposició en l’estat sòlid. Cal destacar especialment els nuclis derivats diindolocarbazole i difenantrocarbazole, ja que no només impliquen una ruta sintètica accessible, si no que a més han proporcionat mobilitats que arriben a 10−3 cm2 V−1 s−1 en dispositius OTFT i una estabilitat que pot superar els 1000 dies. En segon lloc s’ha considerat el nucli dibenzotiofè, que és l’anàleg sulfurat del carbazole, com a building block. A través de les reaccions de Suzuki-Miyaura i Scholl s’han format l’anell de difenantrotiofè i compostos derivats que inclouen diferents grups i extensions aromàtiques. La integració d’alguns d’aquests compostos en dispositius OTFT ha demostrat la seva viabilitat com a semiconductors. En vista de l’èxit de l’anell de triindole en treballs anteriors, s’han aplicat els coneixements adquirits al llarg de la tesi per modificar la seva estructura i intentar millorar els resultats com a semiconductor orgànic. Els canvis estructurals estudiats impliquen l’enllaç d’anells de carbazole suplementaris, la substitució dels nitrògens del nucli per oxígens o sofres i l’annexió de cadenes alquíliques perifèriques. Per últim, les propietats òptiques que aporta la unitat 3-(feniletinil)-9H-carbazole s’han aprofitat per dissenyar una sèrie de compostos derivats emissors en el blau com a components per a dispositius OLED. A més, la incorporació de diferents cadenes alquíliques ha permès millorar-ne la solubilitat i facilitar la fabricació dels dispositius en solució, fet que abarateix molt el cost de producció. Els compostos també s’han integrat juntament amb complexos d’iridi en OLEDs blancs. Factors com la composició i el voltatge aplicat han permès modular la temperatura de la llum, des del blanc pur fins a tonalitats molt més càlides.