Abstract Paul Rusu

This work aims at understanding and modeling dipole formation and charge injection barriers at interfaces between metal electrodes and organic materials. Such metal-organic interfaces (MOIs) are abundant in electronic devices such as light-emitting diodes, field-effect transistors or solar cells that are based upon organic semiconductors. MOIs have a strong influence on the performance of a device. The thesis encompasses a computational and theoretical study of MOIs using electronic structure calculations based upon density functional theory (DFT). MOI dipoles are localized foremost at the first adsorbed layer of molecules and can be extracted from the change in the surface work function produced upon deposition of an organic monolayer. In the first part of the thesis we focus on chemisorbed self-assembled monolayers (SAMs) of CH₃S, CH₃CH₂S, CF₃S and CF₃CH₂S molecules, adsorbed on noble metal (Ag, Au and Pt) (111) surfaces. Such molecules have a permanent dipole moment. Modifying the molecular tails allows one to tune the work function over a range of 2.5 eV. We show that there are two nearly independent contributions to the change in the work function. Bonds between the molecules and the metal surface generate a dipole, whose size is a function of the metal species. The molecular and bond dipoles can be added to determine the overall work function. The tunability of the metal work function by adsorption of a SAM can be used advantageously to lower the energy barrier for charge injection at a MOI and increase the device performance. Upon adsorption on a metal surface, even molecules that have a zero dipole moment and are relatively weakly bounded to the substrate can alter the surface dipole considerably. This is shown in the second part of the thesis by studying interfaces formed by p-conjugated molecules like PTCDA (C₂₂H₈O₆), perylene (C₂₀H₁₂) and benzene (C₆H₆) adsorbed on close packed metal surfaces of Ca, Mg, Al, Ag and Au. The work function changes are the result of two competing effects. In the presence of the molecule, electrons are pushed from the molecular region into the metal, which decreases the work function. If the resulting work function is sufficiently low with respect to the unoccupied states of the molecule, electrons are donated into these states, increasing the work function.

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Home News Events Strategic Orientations research groups Publications PhD students Education MESA+ NanoLab Starting entrepreneurs Tech Park Industrial Partners Vacancies Links Sitemap Search	Abstract Paul Rusu This work aims at understanding and modeling dipole formation and charge injection barriers at interfaces between metal electrodes and organic materials. Such metal-organic interfaces (MOIs) are abundant in electronic devices such as light-emitting diodes, field-effect transistors or solar cells that are based upon organic semiconductors. MOIs have a strong influence on the performance of a device. The thesis encompasses a computational and theoretical study of MOIs using electronic structure calculations based upon density functional theory (DFT). MOI dipoles are localized foremost at the first adsorbed layer of molecules and can be extracted from the change in the surface work function produced upon deposition of an organic monolayer. In the first part of the thesis we focus on chemisorbed self-assembled monolayers (SAMs) of CH₃S, CH₃CH₂S, CF₃S and CF₃CH₂S molecules, adsorbed on noble metal (Ag, Au and Pt) (111) surfaces. Such molecules have a permanent dipole moment. Modifying the molecular tails allows one to tune the work function over a range of 2.5 eV. We show that there are two nearly independent contributions to the change in the work function. Bonds between the molecules and the metal surface generate a dipole, whose size is a function of the metal species. The molecular and bond dipoles can be added to determine the overall work function. The tunability of the metal work function by adsorption of a SAM can be used advantageously to lower the energy barrier for charge injection at a MOI and increase the device performance. Upon adsorption on a metal surface, even molecules that have a zero dipole moment and are relatively weakly bounded to the substrate can alter the surface dipole considerably. This is shown in the second part of the thesis by studying interfaces formed by p-conjugated molecules like PTCDA (C₂₂H₈O₆), perylene (C₂₀H₁₂) and benzene (C₆H₆) adsorbed on close packed metal surfaces of Ca, Mg, Al, Ag and Au. The work function changes are the result of two competing effects. In the presence of the molecule, electrons are pushed from the molecular region into the metal, which decreases the work function. If the resulting work function is sufficiently low with respect to the unoccupied states of the molecule, electrons are donated into these states, increasing the work function.