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theses

Integrating polymers with inorganic nanostructures is difficult due to wetting and surface energy considerations. We developed an electropolymerization method to grow conformal polymers on high aspect ratio nanostructures. Our method is shown to improve the polymer filling rate inside the nanostructures and can be used in the development of efficient hybrid solar cells. As an example, we have studied the hybrid system of electropolymerized polythiophene (e-PT) on a variety of conductive (Au and ITO) and semiconductive substrates (Si, Ge, ZnO). In particular, e-PT/ZnO hybrid structure can be further developed into organic photovoltaics (OPV). Although unsubstituted PT is not the ideal polymer material for high efficiency solar cells, it is an excellent choice for studying basic bonding and morphology in hybrid structures. We find that e-PT is covalently bound to the polar ZnO planar substrate via a Zn-S bond, adopting an upright geometry. By contrast, no strong covalent bonding was observed between e-PT and ZnO nanorods that consist of non-polar ZnO surfaces predominantly. Energy level alignment at interfaces is critical for fundamental understanding and optimization of OPV as band offsets of the donor and acceptor materials largely determine the open circuit voltage (Voc) of the device. Using ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPS), we examined the correlation between energy alignment and photovoltaic properties of a model hybrid solar cell structure incorporating undoped electrodeposited polythiophene (e-PT) films on ZnO planar substrates. The electrolyte anion (BF4-, PF6-, ClO4- or CF3SO3-) used in the electrodeposition solution was found to exert a strong influence on the neutral e-PT film morphology and adhesion, the band alignment at the interface, and ultimately the photovoltaic behavior. The interfacial dipole lowers polythiophene energy levels, increasing the theoretical and actual Voc in polythiophene/ZnO photovoltaics. Our electropolymerization approach to integrate the organic and inorganic phases aims at understanding the chemistry at the interface, and the electronic and morphological properties of the system. This work should be generally applicable to other conjugated polymers and nanostructures, and it contributes to an understanding of organic-inorganic interfaces and electronic structures that may be advantageous to a range of electronic/photonic applications.

ETD_5431

doi:10.7282/T30K26R2

Ph.D.

Includes bibliographical references

by Wenchun Feng

ETD doctoral

The author owns the copyright to this work.