Session X16: Focus Session: Organic Electronics and Photonics: Electronic Structure and Interfaces

Theoretical spectroscopy of organic semiconductors: challenges and progress

The combination of density functional theory (DFT) with powerful spectroscopic tools, e.g., photoemission spectroscopy or absorption spectroscopy, is an important approach to elucidating the electronic structure of materials. In recent years, it has become a particularly popular tool for studying organic semiconductors and their interfaces with inorganic substrates - topics of great importance in organic electronics. Here I review our recent progress in understanding the strengths, limitations, and true predictive power of such analyses. In particular, the consequences of self-interaction and derivative discontinuity errors and the importance of long-range exchange and correlation are analyzed. I then show how this allows for the a priori selection and incorporation of the correct ``physical ingredients'' into (typically) orbital-dependent functionals. Finally, via judicious comparison with a variety of pertinent experimental results on prototypical organic electronic molecules and structures, I show that this approach results in quantitatively accurate calculations of properties often considered to be ``too difficult for standard DFT.''   

Electronic structure of metal-organic interfaces from first principles

Understanding electronic structure at hybrid metal-organic interfaces is crucial for the development and design of future nanoscale devices, particularly for many solar energy conversion applications. Using first-principles density functional theory and many-body perturbation theory within the GW approximation, we present calculations of molecular orbital energies of aromatic molecules covalently bonded to metal surfaces. Importantly, our parameter-free approach treats the metal slab and molecular adsorbate at the same level of theory, and includes exact exchange, as well as static and dynamical correlation effects. We explore the impact of variations in binding sites and molecular geometries for specific chemical link groups and surfaces, and compare directly with experiments where available.   

Describing both Dispersion and Electronic Structure Using Density Functional Theory: The Case of Metal-Phthalocyanine Dimers

Dispersion interactions often determine the structure of crystals and thin layers of organic semiconductors. However, treatment of dispersion is outside the reach of (semi)-local approximations to the exchange-correlation (xc) functional and of hybrid functionals based on semi-local correlation. We offer an approach that treats both dispersion and electronic structure within a computationally tractable scheme, without modifying the xc functional. This is based on adding the leading interatomic dispersion term via pair-wise ion-ion interactions to a suitable non-empirical hybrid functional, with dispersion coefficients and van der Waals radii determined from first-principles using the recently proposed ``TS-vdW'' scheme. The approach is demonstrated for the case of weakly bound metal-phthalocyanine dimers. It is additionally compared to the semi-empirical M06 functional. We find that both PBEh+vdW and M06 predict the electronic structure and the equilibrium geometry well, but with significant differences in the binding energy and in their different asymptotic behavior.   

Charge carrier dynamics at the sexithiophene/Au(111) interface

Time-resolved two-photon photoemission (2PPE) spectroscopy is employed to determine the electronic structure and charge carrier dynamics at the sexithiophene (6T)/Au(111) interface. The oligothiophene 6T, a $\pi $-conjugated chain-like planar molecule, has been widely investigated because it serves as a model system for the structural less controllable polythiophene. Furthermore it is used in organic field effect transistors and in bulk heterojunction photovoltaic cells. We found the HOMO to be located at -1.5 eV and two 6T-derived unoccupied states at 1.2 and 3.0 eV with respect to the Fermi level. The state at 1.2 eV can be assigned to a localized exciton, which possesses a lifetime of 500 fs. The state at 3.0 eV originates from the LUMO+1, having a lifetime of $\sim $ 150 fs.   

First-principles study of the electronic structure of organic semiconductors

Organic semiconductors are promising materials for next generation organic photovoltaics, with the characterization of their spectroscopic properties vital to improving the potential of such technologies. Here, we use density functional theory and many-body perturbation theory within the GW approximation to explore quantitatively the electronic structure of prototypical organic semiconductor crystals and compare directly with valence-band photoemission data. For pentacene and PTCDA, computed gas-phase ionization energies and electron affinities are compared with calculated crystal-phase quasiparticle band structures, and relationships between shifts in orbital energy with change of phase and static polarization of the bulk are discussed and compared with experiment. We acknowledge DOE, NSF, BASF, and ISF for financial support, and NERSC for computational resources.   

Organic semiconductor interfaces: low-lying lattice modes of pentacene monolayers

Highly uniform monolayers of pentacene that are grown on polymeric substrate of poly alpha-methylstyrene exhibit sharp and intense free exciton luminescence. Large enhancements of Raman scattering intensities at the free exciton resonance enable the first observations of low-lying lattice vibration modes in films reaching the single monolayer level.\footnote{Rui He, et al. Appl. Phys. Lett. 94, 223310 (2009).} The low- lying modes display characteristic changes when going from a single monolayer to two layers, revealing that a phase akin to a thin film phase of pentacene already emerges in structures of only two monolayers. A simple analysis of mode splittings offers estimates of the strength of inter-layer interactions. The results demonstrate novel venues for ultra-thin film characterization and studies of interface effects in organic molecular semiconductor structures.   

Energy level alignment of Zn-tetraphenylporphyrins derivatives adsorbed on wide band gap semiconductor oxides

Metalloporphyrins play an essential role in photosynthetic mechanisms and therefore are natural candidates for electron transfer mediator in light harvesting devices. The Zn(II) tetraphenylporphyrins (ZnTPP) derivatives have similar electron injection and charge recombination properties as the important standard ruthenium dye N3 for dye sensitized solar cells, and have shown reasonable performances using TiO$_{2}$ or ZnO as substrates. This study compares three different ZnTPP derivatives, for which the selective functionalization of the meso-phenyls, allows to control the adsorption geometry as well the molecule/molecule interaction onto a rutile TiO$_{2}$(110) and a wurtzite ZnO(11-20) surface. Valence band states have been probed using ultra-violet photoemission spectroscopy, while the conduction band has been obtained from inverse photoemission spectroscopy. Complementary insights on the adsorption geometries have also been obtained using scanning tunnel microscopy. The electronic structure determined experimentally compares well to the calculated density of states, allowing both a simple understanding of the adsorbate electronic properties, and a direct determination of the ZnTPP derivatives frontier orbitals with respect to the substrates band edges.   

Energy level alignments and photocurrents in crystalline Si - organic semiconductor heterojunction diodes

We investigate energy level alignment and photocurrent in crystalline silicon/organic/semitransparent metal diodes. Thin films of MEH-PPV, PFO, pentacene, and C$_{60}$ were deposited on n and p type Si wafers and diode structures were formed by depositing either an Au anode or Al cathode onto the organic film. The energy level alignment was assessed using built-in potential and capacitance-voltage measurements. In all cases, the results are consistent with near ideal vacuum energy level alignment between the organic and inorganic semiconductor. The current-voltage characteristics are consistent with the electronic structure results and, for MEH-PPV, could be described by an organic device model. With the exception of pentacene, the photocurrent results are consistent with the electronic structure and the properties of the individual materials. Photocurrents in Si/C$_{60}$ diodes exhibited both photoconductive gain and bias tunable wavelength response, demonstrating that these structures have potentially useful optoelectronic properties.   

Structural, electronic, and non-equilibrium charge transport properties of silicon-polymer interfaces

There is great interest in developing silicon-based electronic devices incorporating organic molecules as active components. In order to glean insights for applications of such devices in molecular electronics and photovoltaics contexts, we study paradigmatic organic-inorganic interfaces using first-principles calculations for pi-conjugated polymers in contact with H-passivated silicon surfaces. We describe the morphological and electronic properties of these hybrid structures, including interfacial charge transfer, band alignment, and non-equilibrium charge transport.   

Indium diffusion and the electronic energy structure in polymer layers on indium tin oxide

Using Kelvin force microscopy (KFM), coupled to photoelectron spectroscopy we have examined the effects of indium diffusion on the electronic structure of PEDOT:PSS layer deposited on InSb$_{x}$O$_{1-x }$(ITO). KFM shows a clear difference in contact potential difference between the PEDOT:PSS deposited on ITO and PEDOT:PSS deposited on glass. We show that indium diffuses through PEDOT:PSS and introduces new electronic states near the highest occupied molecular orbital of the polymer layer. We also observe that indium diffusion continues into an overlayer of a mixed of P3HT and PCBM. Numerical lineshape analysis of In 3d$_{5/2}$ core level emission rules out the presence of indium oxide or metallic indium clusters within the organic layers, and supports that indium/sulfur compounds are present within the organic layers.   

Charge injection mechanism through LiF layer in organic light emitting diodes: randomly oriented dipole model

Efficient charge injection at the metal/organic interfaces is essential in organic light emitting diodes and other organic electronic devices because it affects the efficiency of the devices significantly. To increase the electron injection a strategy of inserting a thin layer of lithium fluoride (LiF) or other alkali halides between a metal electrode and an active organic layer has been used widely, but the physical mechanism underlying the charge injection through the LiF layer remains unexplained. In order to explain the charge injection mechanism, it was assumed either the LiF dipoles were aligned statistically opposite to the applied electric field direction or the LiF molecules were dissociated and diffused into the organic layer. However, experimental evidences show that both presumptions are highly unlikely. Based on a randomly oriented dipole model, in conjunction with the local process of charge injection by the electric dipoles, we demonstrate that the randomly oriented LiF dipoles or any other dipoles can enhance the charge injection, which agrees well with the experimental results.   

Transparent Carbon Nanotube layers in OLED device structures

Organic Light Emitting diodes (OLEDs) have become well recognized as an important candidate for future lighting and display applications. Reported work on tandem OLED technology has displayed devices with increased, brightness, efficiency and lifetime. The developed tandem structure OLEDs is consisted of multiple electroluminescent units in series that require a complex interconnecting layer. Carbon nanotube (CNT) sheets are alternative interconnecting layers that simplify the fabrication process. CNT sheets are excellent candidates due to their transparency, simple processing, flexibility, electrical and mechanical properties[1]. We have shown earlier that transparent CNT can be used as effective three dimensional charge injectors in OLEDs[2]. Recently, SOLARNO and UTD have demonstrated growth of CNT sheets with improved conductivity. The true advantage of using the CNT sheets lies in flexible devices and new architectures which impossible with brittle ITO. We show that CNT can be used as efficient electrodes or interlayer in multi cell OLEDs with complimentary colors. [1] R.H.Baughman et al. Science, 297,787(2002).[2] C.D.Williams et al. APL 93, 183506(2008).