Structure/charge transport relationships in molecular and polymeric materials for organic electronics through atomistic modeling

With the advancement of technology, miniaturized electronic devices are progressively integrated into our everyday lives, generating concerns about cost, efficiency and environmental impact of electronic waste. Organic electronics offers a tangible solution paving the way for low-cost, flexible, transparent and environment friendly devices. However, improving the functionalities of organic (opto) electronic devices such as light emitting diodes and photovoltaics still poses technological challenges due to factors like low efficiencies, performance stability, flexibility etc. Although more and more organic materials are being developed to meet these challenges, one of the fundamental concerns still arises from the lack of established protocols that correlate the inherent properties of organic materials like the chemical structure, molecular conformation, supra-molecular arrangement to their resulting charge-transport characteristics.In this context, this thesis addresses the prediction of charge transport properties of organic semiconductors through theoretical and computational studies at the atomistic scale, developed along three main axes :(I) Structure-charge transport relationships of crystalline organic materials and the role of energetic fluctuations in amorphous polymeric organic semiconductors. Kinetic Monte-Carlo (KMC) studies employing the Marcus-Levich-Jortner rate formalism are performed on ten crystalline Group IV phthalocyanine derivatives and trends linking the crystalline arrangement to the anisotropic mobility of electrons and holes are obtained. Subsequently, KMC simulations based on the simpler Marcus formalism are performed on an amorphous semiconducting fluorene-triphenylamine (TFB) copolymer, to highlight the impact of energetic fluctuations on charge transport characteristics. A methodology is proposed to include these fluctuations towards providing a semi-quantitative estimate of charge-carrier mobilities at reduced computational cost.(II) Impact of a mechanical strain on the electronic and charge transport properties of crystalline organic materials. Crystalline rubrene and its polymorphs, as well as BTBT derivatives (well studied high mobility organic materials) are subjected to mechanical strain and their electronic response is analyzed. Employing tools like Molecular Dynamic (MD) simulations and plane wave DFT (PW-DFT) calculations, unusual electro-mechanical coupling between different crystallographic axes is demonstrated, highlighting the role of inherent anisotropy that is present in the organic single crystals which translates in an anisotropy of their electro-mechanical coupling.(III) Protonation-dependent conformation of polyelectrolyte and its role in governing the conductivity of polymeric conducting complexes. Polymeric bis(sulfonyl)imide substituted polystyrenes are currently employed as counter-ions and dopants for conducting poly(3,4-ethylenedioxythiophene) (PEDOT), resulting in PEDOT-polyelectrolyte conducting complexes. Employing MD simulations and DFT calculations, inherent characteristics of the polyelectrolyte like its acid-base behavior, protonation state and conformation, are analyzed in conjunction with available experimental data and the role of these characteristics in modulating the conductivity of resulting PEDOT-polyelectrolyte conducting complexes is highlighted.The above studies, performed on different organic electronic systems, emphasize the importance of inherent characteristics of organic materials in governing the charge transport behavior in these materials. By considering the inherent characteristics of organic electronic materials and systematically incorporating them into simulation models, accuracy of simulation predictions can be greatly improved, thereby serving not only as a tool to design new, stable and high performance organic materials but also for optimizing device performances.