Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N25: Focus Session: Organic Photovoltaics |
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Sponsoring Units: DPOLY DMP Chair: Max Shtein, University of Michigan Room: Baltimore Convention Center 322 |
Wednesday, March 15, 2006 8:00AM - 8:36AM |
N25.00001: Organic photovoltaics - towards high performance low bandgap polyfluorene/fullerene bulk heterojunction devices Invited Speaker: We use alternating copolymers of polyfluorene (APFOs) in polymer/fullerene blends as used in plastic solar cells. APFO-3/PCBM devices typically give power conversion efficiencies up to 3.4 {\%} (AM1.5, 100 mW/cm$^{2})$. The APFO's are stable, have high mobility and may be fashioned for liquid crystalline phases, as well as for broad optical absorption. By chemical design it is possible to move the optical absorption edge out to 1000 nm, and also to extract this absorption in photocurrent generation out to 1000 nm. As polymer bandgap is reduced, LUMO and HOMO orbitals shift. This requires the use of modified fullerene acceptors, with shifted orbitals, necessary to give the conditions for photoinduced charge transfer with the low bandgap polymers. The APFO polymers therefore give the necessary variability to catch a larger fraction of the solar spectrum. We have developed full optical and electrical model, predicting device performance for multilayer cells and for tandem cells, and using as input empirical determination of optical and electronic transport properties. We find the hole mobility to be a limiting parameter for device function. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N25.00002: High Performance Organic Light-Emitting Diodes Based on Intramolecular Charge Transfer Emission from Donor-Acceptor Molecules Abhishek Kulkarni, Xiangxing Kong, Samson Jenekhe A clear understanding of the key factors governing the electroluminescence (EL) efficiency of emissive donor-acceptor (D-A) molecules in OLEDs is currently lacking, but is essential to a rational molecular design of future emissive materials. In this study, OLEDs based on intramolecular charge transfer emission from 3,7-[bis(4-phenyl-2-quinolyl)]-10-methylphenothiazine (BPQ-MPT) and 3,6-[bis(4-phenyl-2-quinolyl)]-9-methylcarbazole (BPQ-MCZ) had device performances that differ by orders of magnitude. High performance (44360 cd/m$^{2}$, 21.9 cd/A, 5.78{\%} external quantum efficiency (EQE) at 1140 cd/m$^{2})$ green OLEDs were achieved from BPQ-MPT which has a HOMO level at 5.09 eV and a non-planar geometry. In contrast, diodes with far lower performance (2290 cd/m$^{2}$, 1.4 cd/A, 1.7{\%} EQE) were obtained from BPQ-MCZ which has a HOMO level of 5.75 eV and a planar geometry. These results highlight the pronounced influence of the electron donor strength and molecular geometry on the EL efficiency of D-A molecules. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N25.00003: Reversible Persistence and Effects of Oxygen on the Photoconductivity of Porphyrin Nanorods E. A. Muller, V. H. Joines, W. F. Smith, A. D. Schwab, J. C. de Paula, D. E. Johnston, A. T. Johnson Tetrakis(4-sulfonatophenyl) porphine (TPPS$_{4})$ self assembles$^{1}$ into well-defined nanorods with intriguing photoelectronic properties.$^{2}$ New experiments show that illumination under Ar for several hours induces a change to persistent behavior, i.e. conductivity decays slowly when light is removed, rather than dropping to zero. After resting 24 hours, the sample recovers non-persistent behavior. The dark conductivity of TPPS$_{4}$ aggregates formed by a different technique is sensitive to O$_{2}$.$^{3}$ We find that the conductivity under illumination of nanorod aggregates decreases when 0.2{\%} O$_{2}$ is added, but this change is reversible. By contrast, if the sample is exposed to 21{\%} O$_{2}$ shortly after light is removed, the photoconductivity is permanently lowered. These effects may be due to a combination of O$_{2}$-mediated quenching of excited state porphyrin and oxidation. $^{1}$A.D. Schwab \textit{et al.}, J. Phys. Chem. B \textbf{107}, 11339 (2003). $^{2}$A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004). $^{3}$Y. Otsuka \textit{et al.}, Nanotechnology \textbf{15}, 1639 (2004). [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N25.00004: Photoinduced Charge Transport Spectra for Porphyrin and Naphthalene Derivative-based Dendrimers J.H. Park, Y. Wu, J.R. Parquette, A.J. Epstein Dendrimers are important chemical structures for harvesting charge. We prepared model dendrimers using two porphyrin derivatives and a naphthalene derivative. Films of these porphyrin derivatives have a strong Soret band ($\sim$430nm) and four significant Q-bands; the naphthalene derivative has strong absorption at 365 and 383nm. Two kinds of photovoltaic cell structures [ITO/BaytronP/(thick or thin) dendrimer/Al] are constructed to investigate the optical response spectra of dendrimers under electric potential($V$) on the cell (range from -1V to 2V). To obtain pure optical responses, incident light is modulated with an optical chopper and a lock-in amplifier is used to measure current ($I_{AC}$) and phase ($\theta$). For the excitation of the Soret band, $I_{AC}$ and $\theta$ do not change substantially with change of sign and amplitude of $V$. For Q-bands and naphthalene absorption bands, $\theta$ nearly follows the polarity of V on the cells and $I_ {AC}$ is linear with $V$. Hence, $I_{AC}$ is nearly ohmic for Q- band although there are shifts due to built-in-potential. $I_ {AC}$ for Soret band is almost same for thick and thin active layer cells. In contrast, $I_{AC}$ increases with thickness increase for Q bands. Mechanisms of photogeneration and charge transport will be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N25.00005: Carbon nanotube sheets as transparent charge injectors in organic light-emitting diodes Christopher Williams, Mei Zhang, Raquel Ovalle, Krutarth Trivedi, Alexander Kuznetsov, Sergey Lee, Ray Baughman, Anvar Zakhidov Carbon nanotubes (CNTs) have been recognized for their potential in many applications ranging from high strength materials and fibers to true nanoscale electronics. Recently a method for making strong and transparent CNT sheets has been developed, producing free-standing multiwall nanotube sheets which are easy to process [1]. Their mechanical and electrical properties allow them to meet the needs of a wide range of applications, particularly in optoelectronics. We show here the potential for using these thin, flexible CNT sheets in the development of flexible organic light-emitting diode (OLED) displays. The high transparency of the sheets, the high degree of orientation of tubes and the high work function of the material make them suitable hole injectors for typical hole transport materials used in OLEDs and polymeric LEDs (PLEDs). We show that CNT sheets can be used as anodes for both PLEDs and molecular OLEDs. We also introduce a method for producing inverted OLEDs on existing drive electronics for active matrix displays and a design for a transparent display using CNT sheets as both the electron and hole injector. [1] M. Zhang, S. Fang, A. Zakhidov, S. Lee, A. Aliev, C. Williams, K. Atkinson, R. Baughman, \textit{Science} \textbf{309}, 1215 (2005) [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N25.00006: Dissociation Processes of Singlet and Triplet Excitons in Organic Photovoltaic Cells Zhihua Xu, Yue Wu, Bin Hu The dissociation processes of singlet and triplet excitons were studied based on single-layer photovoltaic cells using: fluorescent Aluminum (III) 8-hydroxyquinoline (Alq$_{3})$ and phosphorescent \textit{fac }tris (2-phenylpyridine) iridium (Ir(ppy)$_{3})$ molecules. We found that triplet exciton dissociation leads to a more efficient photovoltaic response as compared to singlet excitons. The short-circuit photocurrent action spectra suggest that the triplet excitons dissociate mainly at the metallic electrode interface while the singlet excitons exhibit bulk dissociation. This interface dissociation of triplet excitons forms a mechanism for phosphorescent organic materials to show efficient photovoltaic responses. Therefore, control of singlet-to-triplet exciton ratio presents a new pathway to enhance photovoltaic response from organic materials. [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N25.00007: Multilayer polymer devices: light emitting diode and vertical hot carrier transistor Hsin-Fei Meng, Sheng-Fu Horng, Chain-Shu Hsu, Shin-Ron Tzeng, Yu-Chiang Chao, Syuan-Lin Yang Two new devices based on polymer multilayers are presented:charged-balanced LED and hot carrier transistor. 1. An intermediate liquid buffer layer is introduced to overcome the dissolution problem of solution-processed multilayer polymer light-emitting diodes. This method can be applied to arbitrary combinations of polymers with no restriction on solvents. As an example, a hole-blocking layer is successfully spin-coated on the emissive polymer layer. Three typical p-type polymers, The electron-hole balance is improved by the addition of hole-blockinglayer. The electroluminescence efficiency can be increased up to 5 times, while the luminance up to 7 times. Electron-blocking layer is applied to blue polyfluorene copolymer and the brightness is as high as 30,000 cd/m$^{2}$ while the yield is 4 cd/A. 2. Metal-base hot-carrier transistor with conjugated polymer emitter and collector is demonstrated. The device is fabricated by multiple spin-coating with the metal base sandwiched between two polymers. A thin insulating layer of LiF is inserted between emitter and base to enhance the hot carrier kinetic energy and reduce the mutual dissolution. Using poly(9-vinylcarbazole) as the emitter, Al as base, and poly(3-hexylthiophene) as the collector, common-emitter current gain of 26 is obtained with operation voltage as low as 5V. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N25.00008: Single-crystal films of a combination of materials (co-crystal) involving DAST and IR-125 for electro-optic applications A. Narayanan, J. Titus, H. Rajagopalan, P. Vippa, M. Thakur Single-crystal film of DAST (4'-dimethylamino-N-methyl-4-stilbazolium tosylate) has been shown [1] to have exceptionally large electro-optic coefficients (r$_{11} \quad \sim $ 770 pm/V at 633 nm). In this report, single crystal film of a combination of materials (co-crystal) involving DAST and a dye molecule IR-125 will be discussed. Modified shear method was used to prepare the co-crystal films. The film has been characterized using polarized optical microscopy, optical absorption spectroscopy and x-ray diffraction. The optical absorption spectrum has two major bands: one at about 350--600 nm corresponding to DAST and the other at about 600-900 nm corresponding to IR-125. The x-ray diffraction results show peaks involving the presence of DAST and IR-125 within the co-crystal film. Since the co-crystal has strong absorption at longer wavelengths it is expected to show higher electro-optic coefficients at longer wavelengths. Preliminary measurements at 1.55 $\mu $m indicate a high electro-optic coefficient of the co-crystal film. [1] Swamy, Kutty, Titus, Khatavkar, Thakur,\textit{ Appl. Phys. Lett.} 2004, $85$, 4025; Kutty, Thakur, \textit{Appl. Phys. Lett.} 2005, 87, 191111. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N25.00009: Highly efficient third-order optical nonlinearities and their frequency dependence in donor-substituted cyanoethynylethene molecules. Joshua C. May, Ivan Biaggio, Tsuyoshi Michinobu, Fran\c{c}ois Diederich We report on a new class of organic molecules with record efficiency for application in third-order nonlinear optics (NLO). The third-order polarizability, $\gamma$, of several donor-substituted cyanoethynylethene molecules was determined at the off-resonant wavelength of 1.5 microns using four-wave mixing. The nonlinearities were found to be extraordinarily large relative to the small molecular masses and were found to be within 50 times Kuzyk's fundamental limit,\footnote{M. G. Kuzyk, Opt. Lett. 25, 1218 (2000)} with $53\times10^{-48}$ m$^{5}$V$^{-2}$ as the highest $\gamma$ value. Select molecules were further investigated at wavelengths on and surrounding their two-photon (TP) absorption peaks, revealing large TP cross sections and the resonant influence on the real and imaginary parts of $\gamma$. Several members of this molecular family can be vapor-deposited and are likely candidates for third-order NLO devices. When considering their small mass, the \emph{specific} $\gamma$ ($\gamma$ per molecular mass) for this family (off resonance, at 1.5$\mu$m) is up to $6.5\times10^{-23}$ m$^{5}$V$^{-2}$Kg$^{-1}$, approximately one order of magnitude larger than previously known large $\gamma$ molecules.\footnote{J. C. May et al, Opt. Lett. 30, 3057 (2005)} [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N25.00010: A Time-Dependent Density Functional Theory Study of One-and Two-Photon Absorption: Donor-Acceptor Chromophores. Ruth Pachter, Paul N. Day, Kiet A. Nguyen We report time-dependent density functional theory (TDDFT) calculations of one-photon, and two-photon absorption spectra, for a series of compounds, in which electron donating and accepting groups are attached to a core having a delocalized pi-electron structure, such as stilbene or fluorene. We find that the calculated excitation energies are in better agreement with experimental data upon the application of (x-c) functionals that take into account long-range interactions, and also by the inclusion of solvent effects. Furthermore, two-photon absorption cross-section predictions are improved with the application of quadratic response TDDFT, in comparison to experiment. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N25.00011: Two Photon Absorption in a Novel Nano-optical Material Based on the Nonconjugated Conductive Polymer, Poly(beta-pinene) Jitto Titus, Mrinal Thakur As recently reported, the electrical conductivity of the nonconjugated polymer, poly(beta-pinene) increases by more than ten orders of magnitude upon doping with iodine [1]. The FTIR, optical absorption and EPR measurements have shown that radical cations are formed upon doping and charge-transfer involving the isolated double-bond in poly(beta-pinene). In this report, exceptionally large two-photon absorption in iodine-doped poly(beta-pinene) will be discussed. The linear absorption spectrum of medium-doped poly(beta-pinene) have peaks at about 4 eV and 3.1 eV. The first peak is due to the radical cation and the second due to the charge-transfer between the double bond and the dopant. The two-photon absorption of the medium-doped polymer has been measured at 730-860 nm using open-aperture z-scan with 150 femtosecond pulses from a Ti:Sapphire laser. A two-photon peak at about 1.5 eV with a magnitude of more than 1 cm/MW has been observed. The large magnitude of the two-photon absorption coefficient which is proportional to the imaginary part of the third order susceptibility has been attributed to the special structure of the radical cation and the confinement within a sub-nanometer dimension. [1] Vippa, Rajagopalan and Thakur, J. Poly. Sci. Part B: Poly. Phys., 43, 3695 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 10:36AM - 10:48AM |
N25.00012: Temperature dependent electrical and optical characterization of polyfluorene based organic light-emitting-diodes Mohammad Arif, S. Guha, M.S. Yun, S. Gangopadhyay Polyfluorene (PF) conjugated polymers have received widespread attention due to their strong blue emission, high charge mobility and excellent chemical and thermal stability which creates great prospect for optoelectronic device applications. Efficient and well balanced injection of charge carriers and transport capabilities are of paramount importance for high luminescence efficiency of organic light emitting diodes (OLEDs). The maximum current flowing through metal/semiconductor is limited by available space and trapped charges, barrier heights, applied electric field and mobility of the carriers. In this work we present detailed current- voltage (I-V) measurements as a function of temperature from 2- ethylhexyl substituted PF (PF2/6) based OLEDs. PF2/6 is characterized by Tg of ~80 $\rm ^o$C and a nematic liquid crystalline phase above 150 $\rm ^o$C. Barrier heights for current injection were calculated as a function of thermal cycling. The characteristic I-V measurements were fitted with ideal space charge limited conduction (SCLC) with traps to calculate carrier mobilities and trap concentration. Preliminary studies of Raman scattering from these working devices will be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 10:48AM - 11:00AM |
N25.00013: Pi-Conjugated Dendrimers for Organic Photovoltaics Sean Shaheen, William Mitchell, Nikos Kopidakis, Joseph Bozell, Garry Rumbles Polymer-based organic photovoltaic (OPV) devices are promising candidates for low-cost solar cell fabrication. The operation of such devices is known to be strongly dependent upon the morphology and carrier mobility of the polymer. Here we discuss the use of pi-conjugated dendrimers in OPV devices. Dendrimers have a precisely defined molecular weight, in contrast to pi-conjugated polymers, which leads to a well-defined morphology. This morphology can be highly ordered owing to strong pi-electron interactions between dendrimers. We have synthesized a family of phenyl-cored thiophene dendrimers with a variable number of arms and and variable number of thiophenes in each arm. The optical band gaps of these materials in thin film form range from 2.3 to 2.6 eV. Time-resolved microwave conductivity measurements of the dendrimers showed a power-law dependence of lifetimes extending into the millisecond regime, indicative of a very pure material. Preliminary OPV devices fabricated by blending the dendrimers with a soluble fullerene yielded maximum open-circuit-voltages of 900 mV and external quantum efficiencies of 22\%. A reduced band gap dendrimer was also synthesized by adding strong electron withdrawing groups onto the phenyl core, resulting in an optical band gap of 1.82 eV. This material show good molecular ordering as evidenced by x-ray diffraction. [Preview Abstract] |
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