Computational Molecular Design of Polyhedral Oligomeric Silsesquioxane Based Organic-Inorganic Hybrid Semiconductors.

Abstract

Cubic silsesquioxanes (T8 SQs), with the formula of [RSiO1.5]8, enable advanced materials design. In this thesis, a computational materials science framework, including ab initio density functional theory (DFT) calculations, molecular dynamics (MD), and Monte Carlo (MC) simulations, was developed to perform computational molecular design and crystal engineering of SQ based diacene-SQ and then octa(halogenphenyl)-SQ molecular systems. The goal of this project was to identify novel molecular architectures, a priori, that exhibit targeted self-assembly behaviors and result in materials with improved electronic properties. First, existing force fields, including our in house charge transfer reactive (CTR) force field, and COMPASS, were evaluated for simulating cubic SQ systems. All force fields reproduced the experimental structure of SQ-based crystals very well. However, only the FLX force field reproduced the experimentally observed vibrational properties and thermodynamic behavior. Next, targeting materials performance, such as high electronic mobility, a series of diacene-SQ molecules were designed and their crystal structures predicted by following the computational molecular design recipe that accounts for transport theory, symmetry relationships, polymorph prediction procedures, and solid state electronic property evaluation methods. Computationally derived diacene-SQ crystals are predicted to exhibit advanced electronic properties, such as very small band gaps and parallel packing of the acene groups in crystal structures, indicating excellent transport properties, as well as improved thermal and mechanical properties. Finally, a series of new small-band gap octa(halogenphenyl)-SQ molecular systems were identified by computationally exploring alternative architectures and functionalization of recently synthesized octa(halogenphenyl)-SQ crystals. These hybrid molecular crystals also feature other unique properties, such as solution processability, cubic molecular symmetry, and the three-dimensional conjugation. The computationally designed octa(2,5-diiodophenyl)-SQ (ODIPS) shows a calculated conduction band structure similar to that of 1,4-diiodobenzene (DIB), whose high hole mobility is known from experiment. Electronic band structure calculations indicate that the SQ cages, which are by themselves insulators, contribute to the electronic transport process in these hybrid molecules, and enhance the intrinsic electronic properties of the organic semiconductor functional groups.