Ph.D Thesis


Ph.D StudentEkaterina Dovgolevsky
SubjectConjugated Polymers/Mesoporous Silica Guest-Host
Nanocomposites for Electronic Applications
DepartmentDepartment of Materials Science and Engineering
Supervisor Full Professors Frey Gitti
Full Thesis textFull thesis text - English Version


Abstract

Conjugated polymers have been the subject of extensive academic and industrial research efforts due to their promising potentials for use as active layers in electronic and opto-electronic devices, such as light-emitting diodes, field-effect transistors, lasers, photodiodes and solar cells. In these various applications it has been found that, in addition to the intrinsic chemical properties of the polymers and additives, the device performance is strongly dependent on the morphology of the film in which the polymer is cast. In this project we attain control over chemical composition, morphology and hierarchical structure of conjugated polymers by their incorporation into a suitable inorganic host matrix. The polymer-in-inorganic composites harness the good processability and low density of the polymer component, in combination with the high strength, stiffness, thermal stability, and mechanical durability of the inorganic component.

In this research a new class of self-organized nanocomposites, conjugated polymer-incorporated mesostructured silica, are designed, synthesized, characterized and integrated into optoelectronic devices. The self-organized nanocomposite thin films are prepared by introducing the pre-polymerized conjugated polymers into the ordered channels of the silica matrix during its formation. Using the evaporation-induced self-assembly technique 3D-, 2D- and 1D-silica mesophases are selectively obtained by varying the type and concentration of the structure-directing surfactant, and the film deposition conditions such as the relative humidity in the film deposition chamber. The structure and hierarchy of the films are characterized using X-ray Diffraction, Small Angle X-ray Scattering and Transmission Electron Microscopy.

The optical absorption and photoluminescence measurements of the nanocomposite films indicate that the confined polymers maintain their semiconducting properties upon incorporation into the inorganic host. Moreover, noticeable red shifts in both the absorption and emission spectra of the incorporated polymers compared to pristine polymers indicate an increase in conjugation length due to chain planarization in the channels of the silica matrix.

Finally, light-emitting diodes based on the nanocomposite mesophase with the interpenetrating 3D-continuous conjugated polymer network between the two electrodes showed diode characteristics and light emission. In contrast, devices based on the 1D-lamellar nanocomposite mesophases, showed weak or no diode characteristics due to poor through-film connectivity. The discrepancy in the performance of devices prepared from 2D- and 3D-mesophases indicates that the polymer is indeed incorporated in the ordered inorganic matrix and not in voids randomly arranged between silica grains. Furthermorte, the device performances demonstrate how self-assembly processes can be used to tailor hierarchical structures required for electronic and opto-electronic applications.