The studies presented in this work are aimed towards a better understanding of the fundamental physics of the electrode/organic molecule interface in both the ground and excited state manifolds. Systematic investigations of single systems using two-photon photoemission (TPPE) and ultraviolet photoelectron spectroscopy (UPS) were undertaken in order to assess the evolution of the electronic structure and molecular organization at the interface. The adsorbate molecule vanadyl naphthalocyanine (VONc) was used whose properties are well-suited to this purpose. Interfacial electronic states of thin films of VONc were studied with two different substrates: highly ordered pyrolytic graphite (HOPG) and Au(111).The substrate of HOPG is a surface which does not possess reactive dangling bonds and the electron density close to the Fermi edge is very low, permitting high resolution spectroscopic band analysis of VONc and revealing subtle changes to the electronic structure. From interfacial studies of this weakly interacting substrate/ adsorbate system, it is shown in this work that molecular electronic levels in both the ground and excited state manifolds can shift independently of the vacuum level. Further, electron transfer between close lying electron donor and acceptor energy levels may be influenced by energy level shifts caused by depolarization effects as a function of dipole density.The VONc/Au(111) interface is investigated in order to examine energy level alignment in a system with the additional complexity of molecule/substrate interactions. The electron rich Au(111) surface leads to a strong interface dipole upon addition of VONc. Joint experimental and computational data is presented showing that the underlying cause of this interface dipole is Pauli repulsion. Additionally, investigations of energy level alignment in the excited state manifold are presented and the possibility of quantum interference is discussed.The interfacial electronic structure is quite different among these two model systems. The interfacial alignment observed in the HOPG/VONc system was largely due to depolarization of the intrinsic molecular dipole as a function of density, whereas the Au(111)/VONc interface is dominated by interfacial Pauli repulsion interactions.

The impetus of unsustainable consumption coupled with major environmental concerns has renewed our society's investment in new energy production methods. Solar energy is the poster child of clean, renewable energy. Its favorable environmental attributes have greatly enhanced demand resulting in a spur of development and innovation. Photovoltaics, which convert light directly into usable electrical energy, have the potential to transform future energy production. The benefit of direct conversion is nearly maintenance free operation enabling deployment directly within urban centers. The greatest challenge for photovoltaics is competing economically with current energy production methods. Lowering the cost of photovoltaics, specifically through increasing the conversion efficiency of the active absorbing layer, may enable the invisible hand to bypass bureaucracy. To accomplish the ultimate goal of increased efficiency and lowered cost, it is essential to develop new material systems that provide enhanced output or lowered cost with respect to current technologies. However, new materials require new understanding of the physical principles governing device operation. It is my hope that elucidating the dynamics and charge transfer mechanisms in novel photovoltaic material systems will lead to enhanced design principles and improved material selection. Presented is the investigation of electron dynamics in two materials systems that show great promise as active absorbers for photovoltaic applications: inorganic semiconductor quantum dots and organic semiconductors. Common to both materials is the strong Coulomb interaction due to quantum confinement in the former and the low dielectric constant in the latter. The perceived enhancement in Coulomb interaction in quantum dots is believed to result in efficient multiexciton generation (MEG), while discretization of electronic states is proposed to slow hot carrier cooling. Time-resolved two-photon photoemission (TR2PPE) is utilized to directly map out the hot electron cooling and multiplication dynamics in PbSe quantum dots. Hot electron cooling is found to proceed on ultrafast time scales (< 2ps) and carrier multiplication proceeds through an inefficient bulk-like interband scattering. In organic semiconductors, the strong Coulomb interaction leads to bound electron-hole pairs called excitons. TR2PPE is used to monitor the separation of excitons at the model CuPc/C₆₀ interface. Exciton dissociation is determined to proceed through "hot" charge transfer states that set a fundamental time limit on charge separation. TR2PPE is used to investigate charge and energy transfer from organic semiconductors undergoing singlet fission, an analog of multiple exciton generation. The dynamic competition between one and two-electron transfer is determined for the tetracene/C₆₀ and tetracene/CuPc interfaces. These findings allow for the formulation of design principles for the successful harvesting of hot or multiple carriers for solar energy conversion. / text

In this thesis, I present a comprehensive study of the interfacial electronic structure and thin film growth of two types of dipolar organic semiconductors on noble metals by employing a surface science approach, which underlines the critical role of surface electronic states in determining the interfacial electronic structure and self-assembly of organic semiconductors. I show that the electronic structure at organic/metal interfaces is complex and depends on important factors such as molecular adsorption configuration, surface/molecule coupling strength, reactivity of the substrate, molecular electrostatics, and local film structure. I demonstrate the fundamental capability of the image potential states and resonances in probing the local film environment, especially in systems consisting of inhomogeneous film structure. I also show that the presence of adsorbates on a surface allows one to investigate quantum mechanical interference effects otherwise not accessible on the bare surface. The dipolar organic semiconductors studied here are vanadyl naphthalocyanine (VONc) and chloroboron-subphthalocyanine (ClB-SubPc). The single crystals of gold and copper with hexagonal surface symmetry (111) were used to investigate the interfacial properties of VONc and ClB-SubPc, respectively. The fundamental understanding of self-assembly of large π-conjugated organic semiconductors on metals is a crucial step in controlling fabrication of supramolecular structures. Here, I provide a first step in this direction with a detailed and quantitative analysis of molecular nearest-neighbor distances that unravels the fundamental intermolecular interactions of organic semiconductors on transition metal surfaces. I additionally investigated the interfacial electronic structure of these organic semiconductors to examine the relation between molecular adsorption orientation and charge transfer across the interface.

Doctor of Philosophy / Department of Physics / Itzhak Ben-Itzhak / Patrick Richard / This work focuses on the ultrafast dynamics of electrons and phonons in graphitic materials. In particular, we experimentally investigated the factors which influence the transport properties of graphite and carbon nanotubes. In the first part of this dissertation, we used Time-resolved Two Photon photoemission (TR-TPP) spectroscopy to probe the dynamics of optically excited charge carriers above the Fermi energy of double-wall carbon nanotubes (DWNTs). In the second part of this study, time-resolved anti-Stokes Raman (ASR) spectroscopy is applied to investigating in real time the phonon-phonon interactions, and addressing the way the temperature affects the dynamics of single-wall carbon nanotubes (SWNTs) and graphite. With respect to the first part, we aim to deeply understand the dynamics of the charge carriers and electron-phonon interactions, in order to achieve an as complete as possible knowledge of DWNTs. We measured the energy transfer rate from the electronic system to the lattice, and we observed a strong non-linear increase with the temperature of the electrons. In addition, we determined the electron-phonon coupling parameter, and the mean-free path of the electrons. The TR-TPP technique enables us to measure the above quantities without any electrical contacts, with the advantage of reducing the errors introduced by the metallic electrodes. The second investigation uses time-resolved ASR spectroscopy to probe in real time the G-mode non-equilibrium phonon dynamics and the energy relaxation paths towards the lattice by variation of the temperature in SWNTs and graphite. The lifetime range of the optically excited phonons obtained is 1.23 ps to 0.70 ps in the lowest (cryogenic temperatures) and highest temperature limits, respectively. We have also observed an increase in the energy of the G-mode optical phonons in graphite with the transient temperature. The findings of this study are important since the non-equilibrium phonon population has been invoked to explain the negative differential conductance and current saturation in high biased transport phenomena.