Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L29: Focus Session: Interface Controlled Organic Thin Films for Enhanced Device Performance |
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Sponsoring Units: FIAP Chair: Suchi Guha, University of Missouri-Columbia Room: C123 |
Tuesday, March 16, 2010 2:30PM - 3:06PM |
L29.00001: Very High Performance Polymer Solar Cells -- A step closer to reality Invited Speaker: Recently, together with our research partners, we have demonstrated polymer solar cell with a power conversion efficiency (PCE) of close to 8{\%}. This is achieved by several technical and scientific approaches. A new series of photovoltaic polymers based on a low bandgap polymer, poly[4,8-bis-substituted-benzo [1,2-b:4,5-b' ]dithiophene-2,6-diyl-alt-4-substituted-thieno[3,4-b]thio- phene-2,6-diyl] (PBDTTT), were designed and synthesized by chemists from University of Chicago and Solarmer Energy Inc., a start-up from UCLA. By adding different electron-withdrawing functional groups, the open circuit voltage (Voc) of polymers based on PBDTTT can be systematically increased, step by step. It was found that in this polymer system, the bandgap of the polymer can be maintained when the functional groups are added. As a result, the molecular energy levels of PBDTTT can be tuned without sacrificing the light harvesting. Together with the increased Voc, a polymer solar cell with efficiency as high as 7.7{\%} PCE was realized, bringing them one step closer to reality for practical application. [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:42PM |
L29.00002: Nanostructured Ultrathin Carbazole Polymer Layers for Improved Hole-Transport and Injection Properties on ITO Invited Speaker: This talk will focus on strategies to modify Indium Tin Oxide (ITO) surfaces, an important substrate for display and photovoltaic devices using macromolecular grafting, layer-by-layer, and electrodeposition methods of carbazole derivatives. An important consequence is the improved morphology of rough ITO surfaces and better interfacial adhesion. This involves the use of surface initiated polymerization (SIP) to directly modify and graft carbazole containing layers. Alternatively, layer-by-layer deposition of hole-transport and injecting layers allow for controlled layer thickness and layer ordering. Lastly, the use of electropolymerizable conjugated polymer network precursors allow for a highly cross-linked films with improved stability and charge carrier transport. The use of surface sensitive analytical methods is key to defining structure-property relationships. Several device configurations and performance will be described. [Preview Abstract] |
Tuesday, March 16, 2010 3:42PM - 4:18PM |
L29.00003: Theoretical Challenges Towards a Quantitative Description of Organic Thin Film Growth and Organic/(In)organic Interfaces Invited Speaker: The operation of organic opto-electronic devices is determined by the interface between the active organic layer with either the (non)metallic substrate or the metal electrode, but also on the morphology of the organic film. Thereby, the structural arrangements govern the electronic, optical, and transport properties. A reliable theoretical description of all these quantities from first principles is a prerequisite for understanding the physics behind and hence tuning the materials, however, presents serious challenges in all kinds of aspects. The weak van der Waals bonding requires the inclusion of this non-local correlation effect, which only recently became possible within density functional theory. For a controlled film growth one also needs to explore the relevant processes and the corresponding energy barriers, which -due to their complexity- can hardly be tackled with ab-initio methods. To determine the electronic band structure as well as the optical properties many-body effects have to be taken into account, which can be cumbersome - conceptually as well as numerically. Last but not least the interplay between electronic and vibrational excitations is crucial for the charge transport, where the proper methodology still has to be worked out. Nevertheless, great progress has been made during the last years. In this talk, I will show with several examples where we are, and what needs to be done to solve the problems. [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:30PM |
L29.00004: Theoretical Investigation of Interface Formation and Electronic Structure for Thiophenes on TiO$_{2}$ Marilia Caldas, Marcelo Alves-Santos Atomistic-level understanding of photovoltaic processes for hybrid polymer-oxide structures is a great challenge. The mechanisms of charge transfer across the interface are not known, the electronic characteristics of the two components are very different, and for the polymer depend strongly on the film morphology. The experimental analysis of the interface is difficult, so theoretical simulations can be of great value, but the study carries severe difficulties. We investigated the formation of the interface between thiophene oligomers and the TiO$_{2}$(101)-anatase surface, starting from classical molecular dynamics simulations for oligomer deposition, and proceeding to ab initio calculations based on Density Functional Theory (DFT) for electronic properties. We observed the formation of rough thiophene films, with a majority of molecules oriented almost normal to the surface, also molecules lying-down in the first interfacial layer. The DFT study of an infinite thiophene polymer lying-down on the surface, in that same orientation, indicates electronic coupling through a state at the oxide valence-band top, extended through both materials, which favors charge transfer. We acknowledge support from FAPESP and CNPq, Brazil [Preview Abstract] |
Tuesday, March 16, 2010 4:30PM - 4:42PM |
L29.00005: TEM Phase-Contrast Imaging of the nanomorphology of the Polymer/Fullerene Bulk Heterojunction Christopher Takacs Measurement and, ultimately, control of the nanoscale morphology is of central importance in high-performance polymer electronics. Direct measurement of the morphology is critical to characterization but few techniques are capable of imaging on the nanometer length scale required to resolve the phase separation in the BHJ. The TEM is well-suited in accessing this length scale; however, in-focus images of the current generation of high-performance polymers give little contrast due to small density differences between phase-separated regions. Invasive techniques such as staining with heavy elements can increase contrast but may also have substantial effects on morphology. Phase-Contrast Imaging with the TEM provides a non-invasive way to increase the measurement sensitivity to phase-separated regions in the BHJ. In this method, we defocus the image and transition into a regime where the TEM can be quantitatively described as a near-field scattering instrument. From analysis of the data, we characterize the morphology of the BHJ for systems of P3HT/PCBM and PCDTBT/PCBM. [Preview Abstract] |
Tuesday, March 16, 2010 4:42PM - 4:54PM |
L29.00006: Applications of interface controlled pulsed-laser deposited polymer films in field-effect transistors Danish Adil, Ndubuisi Ukah, Suchi Guha, Ram Gupta, Kartik Ghosh Matrix assisted pulsed laser evaporation, a derivative of pulsed laser deposition (PLD), is an alternative method of depositing polymer and biomaterial films that allows homogeneous film coverage of high molecular weight organic materials for layer-by-layer growth without any laser induced damage. Polyfluorene (PF)-based conjugated polymers have attracted considerable attention in organic field-effect transistors (FETs). A co-polymer of PF (PFB) was deposited as a thin film using matrix assisted PLD employing a KrF excimer laser. Electrical characteristics of FETs fabricated using these PLD grown films were compared to those of FETs using spin-coated films. We show that threshold voltages, on/off ratios, and charge carrier motilities are significantly improved in PLD grown films. This is attributed to an improved dielectric-polymer interface. [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L29.00007: A non-contact measurement of local charge density in organic semiconductor monolayer Xiaohui Qiu, Yeping Jiang, Qiong Qi, Rui Wang, Qikun Xue, Chen Wang, Chao Jiang The dynamic response of charge carriers in a monolayer film of pentacene grown on SiO2 surface has been investigated using an electric force microscope (EFM). The method characterized the electrostatic interactions between an EFM tip and the pentacene islands of various sizes that were formed by in-situ partitioning a coalesced pentacene polycrystalline film into separate domains. The observed dependence of EFM frequency-shift on the size of pentacene islands strongly suggests the existence of mobile charges in the single layer of pentacene on SiO2 substrate. The characteristic of the capacitance-voltage profile in pentacene monolayer film was observed, and the carrier concentration of the pentacene film was derived. The approach avoids the electrostatic complexity in conventional measurements using fixed metal electrodes, providing a general method for directly investigating the behavior of local carrier in organic thin films on dielectric substrates. [Preview Abstract] |
Tuesday, March 16, 2010 5:06PM - 5:18PM |
L29.00008: Structural and Electrical Characterization of Flip Chip Laminated $\omega $-functionalized thiols Mariona Coll, Nadine Gergel-Hackett, Curt A. Richter, Christina A. Hacker The ability to electrically contact organic materials remains one of the key issues to build high performance organic-based electronic devices. The use of organic molecules as active components in electronics offers a promising alternative for `more than Moore' devices. Molecules exhibit small size, advanced functionality and can form covalent bonding with semiconductors ensuring more robust devices and also speed integration of molecule-based devices with conventional CMOS technology. However, direct metal vapor deposition of the top contact leads to multiple metal filaments which diffuse through the organic monolayer and can dominate the electrical response. Flip Chip Lamination allows us to fabricate reliable silicon-based molecular electronic structures by forming highly ordered self-assembled bifunctional monolayers on ultrasmooth metal surfaces. Then, these molecular substrates are flipped and bonded to H-Si(111) enabling a detailed characterization of molecules confined between two electrodes. Using polarized backside reflection absorption infrared spectroscopy (pb-RAIRS) we are able to study the geometry, chemical and conformational changes at the interfaces and within the monolayers. Electrical behavior of molecular devices has been evaluated by transition voltage spectroscopy. [Preview Abstract] |
Tuesday, March 16, 2010 5:18PM - 5:30PM |
L29.00009: Single Molecule Study of Charge Transfer in 6T-TBrPP-Co Molecular Complex Y. Zhang, U.G.E. Perera, S.-W Hla When two molecules having tendency to donate or accept electronic charge are put together, charge transfer between the molecules can take place. By a suitable selection of donor and acceptor molecules, it is possible to engineer an entire class of materials having metallic, semiconducting, insulating, or even superconducting properties. Here, we present a low temperature scanning tunneling microscopy and spectroscopy study of single molecule level charge transfer process between $\alpha $-sexithiophene(6T) and TBrPP-Co molecules on a Cu(111) surface. We form molecular clusters composed of both molecular species on Cu(111). The charge transfer between the molecules is directly evident in the tunneling spectroscopy data, which reveal the shift of 6T HOMO towards the surface Fermi level indicating donation of charge from 6T to TBrPP-Co. [Preview Abstract] |
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