Theoretical investigation of the C-60/copper phthalocyanine organic photovoltaic heterojunction

Molecular heterojunctions, such as the one based on copper phthalocyanine (CuPc) and carbon fullerene (C-60) molecules, are commonly employed in organic photovoltaic cells as electron donor-acceptor pairs. We have investigated the different atomic structures and electronic and optical properties of the C-60/CuPc heterojunction through first-principles calculations based on density functional theory (DFT) and time-dependent DFT. In general, configurations with the CuPc molecule "lying down" on C-60 are energetically more favorable than configurations with the CuPc molecule "standing up". The lying-down configurations also facilitate charge transfer between the two molecules, due to the stronger interaction and the larger overlap between electronic wavefunctions at the interface. The energetically preferred structure consists of CuPc placed so that the Cu atom is above a bridge site of C-60, with one N-Cu-N bond of CuPc being parallel to a C-C bond of C-60. We also considered the structure of a periodic CuPc monolayer deposited on the (001) surface of a face-centered cubic (fcc) crystal of C-60 molecules with the lying-down orientation and on the (111) surface with the standing-up configuration. We find that the first arrangement can lead to larger open circuit voltage due to an enhanced electronic interaction between CuPc and C-60 molecules.

Properties of copper (fluoro-)phthalocyanine layers deposited on epitaxial graphene

Efthimios Kaxiras, Sheng Meng, Jun Ren

We investigate the atomic structure and electronic properties of monolayers of copper phthalocyanines (CuPc) deposited on epitaxial graphene substrate. We focus in particular on hexadecafluorophthalocyanine (F16CuPc), using both theoretical and experimental (scanning tunneling microscopy STM) studies. For the individual CuPc and F16CuPc molecules, we calculated the electronic and optical properties using density functional theory (DFT) and time-dependent DFT and found a red-shift in the absorption peaks of F16CuPc relative to those of CuPc. In F16CuPc, the electronic wavefunctions are more polarized toward the electronegative fluorine atoms and away from the Cu atom at the center of the molecule. When adsorbed on graphene, the molecules lie flat and form closely packed patterns: F16CuPc forms a hexagonal pattern with two well-ordered alternating alpha and beta stripes while CuPc arranges into a square lattice. The competition between molecule-substrate and intermolecular van der Waals interactions plays a crucial role in establishing the molecular patterns leading to tunable electron transfer from graphene to the molecules. This transfer is controlled by the layer thickness of, or the applied voltage on, epitaxial graphene resulting in selective F16CuPc adsorption, as observed in STM experiments. In addition, phthalocyanine adsorption modifies the electronic structure of the underlying graphene substrate introducing intensity smoothing in the range of 2-3 eV below the Dirac point (E-D) and a small peak in the density of states at similar to 0.4 eV above E-D. (C) 2011 American Institute of Physics. [doi:10.1063/1.3590277]

American Institute of Physics2011

First-Principles Modeling of Optically Active Organic Molecules in Solar Cell Devices and Biological Environments

Jun Ren

With the increasing cognition of the importance of organic molecules, they are widely applied in printing, biological and pharmacological fields, because of their special capabilities of harvesting solar light, scavenging free radicals, and chelating metal ions. During the past decades, the unique photoelectronic and photochemical properties of organic molecules, such as phthalocyanine, cyanidin, and their relevant derivatives, attract tremendous attention, because they provide an excellent opportunity to solve the worldwide energy crisis by converting directly the solar light to electricity. The surface morphology and electronic interaction of these organic molecules with other molecules, surfaces or interfaces play a critical role in determining the performance of the electronic and optical devices based on organic molecules. In this thesis, we focus on the investigation of several selected organic molecules, and their interaction with molecules, inorganic semiconductors, and organic semi-conductors, by using first-principles electronic structure calculations based on density functional theory, and time-dependent density functional theory. Particular attention is paid to the atomic structure, electronic and optical properties of organic molecules and the corresponding interfaces. The focus of this thesis is on the following aspects of the organic molecules: (i) The complexation mechanism of flavonoids with metal ions. The most likely chelation site for Fe is the 3-hydroxyl-4-carbonyl group, followed by 4-carbonyl-5-hydroxyl group and the 3'-4' hydroxyl (if present) of quercetin. A complex of two quercetin molecules with a single Fe ion is energetically more stable, however, six orbitals of Fe in the three quercetin complex are saturated by three perpendicular molecules to form and octahedral configurations. Furthermore, the optical absorbance spectra serves as signatures to identify various complexes. (ii) The electronic coupling between a dye molecule (Cyanidin) and a TiO2 nanowire. Upon molecular adsorption on TiO2 [010]-wire, cyanidin will be deprotonated into the quinonoidal form. This results in its highest occupied molecular orbitals being located in the middle of TiO2 bandgap and its lowest occupied molecular orbitals being close to the TiO2 conduction band minimum, in turn enhancing visible light absorption. Moreover, the excited electrons are injected into TiO2 conduction band within a time scale of 50 fs with negligible electron-hole recombination. (iii) The atomic and electronic structure of copper (fluoro-)phthalocyanine and graphene. When adsorbed on graphene, F16CuPc molecules prefer to form a close-packed hexagonal lattice with two-ordered alternating α and β stripes, whereas CuPc would like to form a square lattice. In addition, phthalocyanine adsorption modifies the electronic structure of graphene introducing intensity smoothing at 2-3 eV below and a small peak at ∼0.4 eV above the Dirac point in the density of states. And finally, (iv) the electronic interaction between CuPc and fullerene. For CuPc/C60 molecular complex, CuPc prefers to lie flat on the C60 surface rather than taking the standing-up molecular orientation. The favorable adsorption site for CuPc is the bridge site of C60 with one N-Cu-N bond of CuPc being parallel to a C-C bond of C60. Based on the analysis of the molecular complex, we predict that CuPc/C60(001) thin film heterojunction adopting the lying-down molecular orientation should have a higher efficiency of charge transfer in comparison with the relevant CuPc/C60(111) heterojunction with the standing-up arrangement.

EPFL2011

FM-AFM constant height imaging and force curves: high resolution study of DNA-tip interactions

Livan Bladimir Alonso Sarduy, Andrea Cerreta, Giovanni Dietler, Dusan Vobornik

Interaction of the atomic force microscopy (AFM) tip with the sample can be invasive for soft samples. Frequency Modulation (FM) AFM is gentler because it allows scanning in the non-contact regime where only attractive forces exist between the tip and the sample, and there is no sample compression. Recently, FM-AFM was used to resolve the atomic structure of single molecules of pentacene and of carbon nanotubes. We are testing similar FM-AFM-based approaches to study biological samples. We present FM-AFM experiments on dsDNA deposited on 3-aminopropyltriethoxysilane modified mica in ultra high vacuum. With flexible samples such as DNA, the substrate flatness is a sub-molecular resolution limiting factor. Non-contact topographic images of DNA show variations that have the periodicity of the right handed helix of B-form DNA - this is an unexpected result as dehydrated DNA is thought to assume the A-form structure. Frequency shift maps at constant height allow working in the non-monotonic frequency shift range, show a rich contrast that changes significantly with the tip-sample separation, and show 0.2 to 0.4 nm size details on DNA. Frequency shift versus distance curves acquired on DNA molecules and converted in force curves show that for small molecules (height

Wiley-Blackwell2012

First-Principles Modeling of Optically Active Organic Molecules in Solar Cell Devices and Biological Environments

Jun Ren

EPFL2011