Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y44: Focus Session: Organic Electronics and Photonics -- Exciton and charge separation physics |
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Sponsoring Units: DMP DPOLY Chair: Bryan Boudouris, University of California, Berkeley Room: A309 |
Friday, March 25, 2011 8:00AM - 8:36AM |
Y44.00001: Charge transport studies in organic semiconductors using carrier extraction by linearly increasing voltage (CELIV) technique Invited Speaker: Organic optoelectronic devices, such as solar cells, light emitting diodes and transistors, share a common feature: their performance critically depends on the efficiency with which charge carriers (electrons and/or holes) move in the material. Understanding and improving the charge transport is the main goal when improving the device performance or designing novel organic compounds through chemical engineering. Due to low carrier mobility in disordered films, as well as due to its time, electric field and carrier density dependence, standard measurement technique like Hall effect and Time-of-Flight are either inapplicable or limited in applicability. Charge Carrier Extraction by Linearly Increasing Voltage (CELIV) technique has become a world standard used by many scientific groups to measure charge transport and recombination in inorganic and organic semiconductors. The method can be used to study the charge carrier mobility dependence on time, carrier concentration, electric field, temperature, film thickness and morphology directly in the operational devices. However, the latest research have shown that CELIV current transients and extraction maximum used for mobility evaluation is strongly dependent on experimental conditions such as carrier density, light absorption profile and electric field. Procedure, allowing estimating the correction factor in mobility relation will be presented. In contrast to inorganic crystalline semiconductors, the long-range disorder in the films of organic devices makes the charge transport properties strongly dependent on the degree of disorder and nanomorphology of the films. Carrier density, electric field and temperature dependent mobility in disordered organic semiconductors is shown to obey Arrhenius-type, Poole-Frenkel-type, Meyer-Neldel rule, and Gill's law. Stochastic transport theories are used to describe charge carrier hopping within localized Density-Of-States as opposed to delocalized band-transport in the crystals. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 8:48AM |
Y44.00002: Direct determination of energy level alignment of organic-organic bulk heterojunction: cases of the P3HT:PCBM and P3HT:FLN-i blend Zelei Guan, Antoine Kahn, Jong Bok Kim, He Wang, Yueh-Lin Loo Using photoemission spectroscopy (UPS{\&}IPES) combined near edge X-ray absorption fine structure method, we have determined the surface compositions and electronic alignments of the blend films comprising poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester. Given the fact that the surface of the blend film is a nearly pure P3HT wetting layer, we use a lift-off method to access the originally buried surface, which is rich in both P3HT and PCBM and thus representative of the BHJ. We show that the donor/acceptor LUMO-HOMO gap is 1.46 eV, implying a 0.5-0.6 eV interface dipole barrier between the two materials. As far as we know, this is the first report of the direct determination of electronic structure of the blend. The combined measurement and lift-off method are standard and can be applied to other organic blend films, like P3HT and FLN-i. [Preview Abstract] |
Friday, March 25, 2011 8:48AM - 9:00AM |
Y44.00003: Direct Determination of Energy Level Alignment and Charge Transport at Metal/Alq$_3$ Interfaces via Ballistic-Electron-Emission Spectroscopy (BEES) J.S. Jiang, J.E. Pearson, S.D. Bader In organic electronic devices, the difference between the electrode work function and the organic lowest unoccupied molecular orbital (LUMO) or highest occupied molecular orbital (HOMO) is a crucial parameter in determining the nature of charge transport. However, experimental determination of LUMO is challenging.\footnote{ J. C. Scott, J. Vac. Sci. Tech, A {\bf21}, 521 (2003).} For the archetypal electroluminescent organic semiconductor tris-(8-hydroxyquinoline) aluminum (Alq$_3$), various techniques gave significantly different HOMO-LUMO gap values.\footnote{I. H. Campbell, D. L. Smith, Appl. Phys. Lett. {\bf74}, 561 (1999); I. G. Hill \emph{et al.} Chem. Phys. Lett. {\bf327}, 181 (2000); S. F. Alvarado \emph{et al.} IBM J. Res. Dev. {\bf45}, 89 (2001).} Using BEES, we directly determined the energy barrier for electron injection at clean interfaces of Alq$_3$ with Al and Fe to be 2.1 eV and 2.2 eV, respectively. We quantitatively modeled the sub-barrier BEES spectra with an accumulated space charge layer, and found that the transport of non-ballistic electrons is consistent with random hopping over the injection barrier. Supported by U.S. DOE Office of Science Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Friday, March 25, 2011 9:00AM - 9:12AM |
Y44.00004: ABSTRACT WITHDRAWN |
Friday, March 25, 2011 9:12AM - 9:24AM |
Y44.00005: Band alignment optimization of bulk rrP3HT/C$_{60}$ heterojunction Arnaud Maillard, Alain Rochefort Organic solar cells could overcome the cost limitation of traditional solar cells by using large-scale fabrication techniques associated with polymers. However, a better understanding of the bulk heterojunctions (BHJs) electronic properties used in these devices is required to reach an efficiency of 10\%. DFT and GW computations were used to study BHJs formed by the inclusion of C$_{60}$ in a regioregular poly(3- hexylthiophene) polymer (rrP3HT) crystal. An increasing packing density in the BHJ extent the energy separation between the C$_{60}$-LUMO and the rrP3HT-HOMO, which is proportional to the open circuit voltage of the device (Voc). This trend is consistent with the induced dipole moment variation observed at a pentacene- C$_{60}$ junction upon reduction of the intermolecular distance [1]. In contrast, an increasing size of rrP3HT crystal domain leads to decrease both Voc and rrP3HT bandgap, in a similar fashion than upon the formation of rrP3HT crystallite along the annealing of BHJs [2]. \\[4pt] [1] M. Linares, et al., J. Phys. Chem. C, 114 (2010), 3215.\\[0pt] [2] G. Dennler, et al., Adv. Mater., 21 (2009), 1323. [Preview Abstract] |
Friday, March 25, 2011 9:24AM - 9:36AM |
Y44.00006: Strong interface p-doping and band bending in C$_{60}$ Irfan Irfan, Huanjun Ding, Yongli Gao, Minlu Zhang, Ching Tang C$_{60}$ is a strongly n-type material with its lowest unoccupied molecular orbital very close to the Fermi level, and p-doping C$_{60}$ has been a challenging issue. We measured the electronic energy level evolution of C$_{60}$ on molybdenum oxide (MoO$_{x})$/ conducting indium tin oxide (ITO) interfaces with ultra-violet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES) and atomic force microscopy (AFM). We found that MoO$_{x}$ strongly p-dopedC$_{60}$at the interface, resulting in an inversion layer in C$_{60}$. The energy levels of C$_{60}$relax gradually as the thickness of C$_{60}$ increases, and the band bending region is observed to be greater than 400 {\AA} in C$_{60}$. The root mean square (RMS) roughness measured with AFM of 581 {\AA} thick C$_{60}$ film was 68 {\AA}, slightly increased from that of the ITO substrate of 55 {\AA}. We have also investigated the effect of exposing the MoO$_{x}$ air, and found that it eliminated the doping effect. [Preview Abstract] |
Friday, March 25, 2011 9:36AM - 10:12AM |
Y44.00007: Understanding the role of interfaces in small molecule organic photovoltaics Invited Speaker: This abstract not available. [Preview Abstract] |
Friday, March 25, 2011 10:12AM - 10:24AM |
Y44.00008: Photophysics of Poly(3-dodecylthinylenevinylene) with Controlled Regioregularity Evan Lafalce, Xiaomei Jiang Poly(thienylene vinylene) (PTV) is a conductive polymer with potential applications for use in photovoltaics owing to its low-band gap, good hole-mobility and low oxidation potential. It is generally considered a non-luminescent material and reports suggest its emissive properties are highly dependent upon the excitation, conjugation length, alkyl side group and regio-regularity, complicating the interpretation of the non-radiative decay routes for photo-generated excitations. Better understanding of this behavior could explain the low efficiencies so far observed in PTV based solar cells and lead to improved performance. We have studied photoluminescence of Poly(3-dodedyl -2,5-thienylene vinylene) as a function of regio-regularity in thin films and solutions. By varying the excitation, temperature, and also by controlling the morphology through the use of different solvents, concentrations, and film preparation techniques, we hope to deduce the physical mechanisms competing with radiative recombination. Complimentary characterization of films through XRD and electro-absorption yield detailed information about the semi-crystalline structure and electronic levels, respectively. [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 10:36AM |
Y44.00009: Magneto-Optical Studies of Internal Photovoltaic Processes in Organic Solar Cells Huidong Zang, Zhihua Xu, Bin Hu It has been found that exciton dissociation inevitably forms electron-hole pairs, namely charge-transfer (CT) complexes, at donor-acceptor interfaces due to Coulomb attraction in organic solar cells. In particular, the dissociation of CT complexes is a critical process that is accountable for the generation of photocurrent. However, it is a challenging issue to study the CT complexes formed at donor-acceptor interfaces. Here, we use magneto-optical measurements: magnetic field effects of photocurrent (MFE$_{PC})$ and light-assisted dielectric response (LADR) as effective experimental tools to experimentally examine the formation of CT complexes and the related photovoltaic processes. Our studies reveal that internal electrical drifting and local Coulomb interaction can largely change the binding energy and dissociation probability of CT complexes through intrinsic electrical polarization in donor-acceptor interpenetrating network. This experimental finding indicates that intrinsic electrical polarization plays an important role in controlling charge dissociation, transport, and collection in organic solar cells. [Preview Abstract] |
Friday, March 25, 2011 10:36AM - 10:48AM |
Y44.00010: Photoluminescence Influenced by Chain Conformation in Thin Conjugated Polymer Films by Spin Coating and Dewetting P.W. Lee, W.C. Li, Y. Chien, G. Reiter, A.C.-M. Yang Motivated by recent observations of photoluminescence (PL) enhancement by molecular constraints, the chain conformation effect was explored. It was found that PL efficiency decreased with film thickness under a constant spin speed but increased under a constant solution concentration indicating that prolonged solvent evaporation, and hence more open entangled coils, improved PL efficiencies. Strong substrate dependence was observed in the ultrathin regime, revealing the role of substrate-polymer interactions during the condensation process. Upon annealing, the thin film dewetted and resulted multi-fold PL enhancement. As revealed by micro-PL spectroscopy, the PL efficiency was about 10 times greater in the residual layer ($\sim $3 nm) than that in the droplets and demonstrated independence of substrate quenching effect, unveiling important optoelectronic features of the molecular constrained states. [Preview Abstract] |
Friday, March 25, 2011 10:48AM - 11:00AM |
Y44.00011: ABSTRACT WITHDRAWN |
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