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Design, Synthesis and Characterization of molecular semiconductors tailored to couple with vacuum field

par Meena, Rahul
Président du jury Losada Perez, Patricia
Promoteur Geerts, Yves
Publication Non publié, 2024-03-13

Thèse de doctorat

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Résumé :

Over the past two decades, organic electronics have grown using π-conjugated molecules and macromolecules as semiconductors. Compared to silicon, a widely used inorganic semiconductor, organic molecules-based semiconductors offer numerous advantages such as cost-effectiveness, ease of implementation, and mechanical flexibility. The primary parameter to decide the performance of a semiconductor material is its charge carrier mobility (μ), which for organic semiconductors denotes the efficiency of charge carriers moving within π-conjugated materials. Despite the development of various organic semiconductors, their mobility values have not surpassed a specific upper limit.Organic semiconductors face challenges due to their soft structure, held together by weak van der Waal forces. These forces render organic structures vulnerable to disorders caused by molecular and lattice vibrations in crystal structures. Vibrations in crystal structure hinder the charge carrier movement and reduce charge transport. Dimensionality corresponds to the number of dimensions of space (1D, 2D, 3D) in which the charge carrier can move. Smaller dimensionality means crystal structures are more sensitive to vibrations and defects and exhibit poor charge transport. Controlling the dimensionality of charge transport by modulating molecular and crystal structure in organic solids is crucial for high-performance organic electronic devices. While synthesizing new molecular structures with clever molecular design has played a vital role in developing efficient organic semiconductors, this strategy only partially encounters limitations. Thus, a new strategy or approach is required to surpass the charge carrier limits. This thesis presents a radically different approach based on light-matter coupling, which alters molecular properties. The light-matter coupling can occur even in the absence of light (photons) because of the vacuum electromagnetic field. Vacuum energy is an underlying background energy present throughout the universe that can be quantized under quantum confinement and hybridized with energy states of molecules. Ebbesen et al. have shown that coupling of states of confined vacuum electromagnetic fields with states of matter leads to hybridized states, which help to delocalize charge carriers and enhance charge carrier mobility of organic semiconductors. However, they have not examined a p-type molecular semiconductor. This leaves an opportunity for us to explore the behaviour of p-type semiconductors when coupled with vacuum field.Also, the material profile has not been optimized in this context, leaving ample room for research and development. Organic semiconductors, which are chromophores with intense absorption in the visible region, are necessary. This thesis attempts to build new materials more compatible with the vacuum electromagnetic field. This work is based on synthesizing and characterizing p-type molecular semiconductors based on rylene derivatives. The insertion of methyl substituents was employed to modulate the crystal structure to develop new semiconductors. Interestingly, the first results show that enhancement in the charge transport process was seen for the new materials, but the effect is inconclusive due to inconsistent measurements. Regrettably, the insertion of bulky substituents weakened intermolecular interactions, affecting organic semiconductors' intrinsic charge transport property and reducing their mobility.