Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W22: Focus Session: Organic Photovoltaics and LEDs |
Hide Abstracts |
Sponsoring Units: DMP DPOLY Chair: Chang Ryu, Rensselaer Polytechnic Institute Room: Morial Convention Center 214 |
Thursday, March 13, 2008 2:30PM - 2:42PM |
W22.00001: Probing Photoconductivity in Phthalocyanines by Terahertz Spectroscopy Chen Xia, Brian Kubera, Volodimyr Duzhko, Hefei Shi, Kenneth Singer, Jie Shan Liquid crystals (LCs) are a relatively new class of photoconductors. Surprisingly high carrier mobilities have lately been reported in LCs. Although the high molecular order in these systems has been recognized to play an essential role in the high carrier mobilities, the mechanism of charge transport and carrier photogeneration are still not well understood. In this work, we investigate phthalocyanine (Pc) derivatives belonging to a family of discotic LCs as a model system. Optical pump/terahertz probe spectroscopy was employed to measure frequency dependence of the photoconductivity in Pc from 0.2 to 2.5THz. Photoconductivity appears within $\sim $1ps (limited by time resolution of setup) after photoexcitation. It is followed by a fast decay of a few ps and a slow decay of 10's ps. Distinct frequency dependences were observed in the polycrystalline and liquid crystalline phases. The mechanism of charge transport and free carrier generation and recombination in the material will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W22.00002: Solution Processed Carbon Nanotube /PMMA Nano Composite Infrared Photodetectors Yi Liu, Liwei Liu, Paul Stokes, Qun Huo, Saiful I. Khondaker Solution processable nanostructured materials are of great interest for electronic and optical devices because of their enhance functionality, easy processibility, flexibility, and low cost of fabrication. We tested multi walled carbon nanotube networks dispersed in poly (methyl methacrylate) (PMMA) matrix for use as the infrared (IR) photodetectors at room temperature in ambient condition. Our study reveals both negative and positive infrared response depending upon the dark conductivity of the composite. The temperature dependence of resistance, photo intensities and bias voltages dependence of IR response will be presented. This work shows promising novel route for the fabrication of infrared bolometric photo detector based on solution-processed carbon nanotube/PMMA composites. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W22.00003: Triplet excitons in a ladder-type conjugated polymer: application in organic optoelectronics K. Yang, M. Arif, S. Guha Trace concentrations of metallic impurities present at the ppm level in conjugated polymers allow photophysical studies of triplet states at room temperature. We present temperature- dependent photoinduced absorption (PIA) and photoluminescence studies of diaryl (diphenyl)-substituted ladder-type poly (paraphenylene) (PhLPPP) containing a trace concentration of covalently bound Pd atoms. The T$_1$-T$_N$ peak ($\sim$1.3 eV) observed in the PIA blue shifts with increasing temperatures at a rate higher compared to the blue shift of singlet excitons in the sample with increasing temperatures. The temperature shift of the PIA signal arises both due to the temperature dependence of the triplet mobility as well as its mean free path. Our observations suggest that with increasing temperatures the triplet excitons remain localized on smaller chain segments. A triplet lifetime of $\sim$12 ms was determined at room temperature. Furthermore, we will discuss the application of triplet state enhanced polymers in photovoltaic applications. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W22.00004: Photovoltaic Effect in a Composite involving the Nonconjugated Conductive Polymer, Poly($\beta $-pinene) and C$_{60}$ Aditya Kumar Palthi, Ananthakrishnan Narayanan, Mrinal Thakur Photovoltaic effect in a composite involving a nonconjugated conductive polymer, poly($\beta $-pinene) and C$_{60}$ will be reported. The photovoltaic cell was fabricated using indium tin oxide coated glass as one electrode and aluminum as the other, with a poly($\beta $-pinene)-C$_{60}$ composite film sandwiched between the electrodes. Nitrogen laser (emission at 325 nm) and illuminant white light bulbs (emission at 300-700nm) were used as the light sources and the photo-voltage produced was recorded for different light intensities. The photo-voltage produced had a linear dependence on the light intensity. About 100 mV was generated for an intensity of $\sim $ 4mW/cm$^{2}$. Pristine poly($\beta $-pinene) has a photoluminescence peak at 360 nm for excitation at 280 nm. As we have observed, this photoluminescence is quenched when C$_{60}$ is added to poly($\beta $-pinene) to form the composite. Therefore, the observed photovoltaic effect appears to be a result of excited-state electron transfer from poly($\beta $-pinene) to C$_{60}$. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W22.00005: Femtosecond and CW transient studies of photoinduced charge transfer in donor/acceptor blends for organic solar cells Josh Holt, Sanjeev Singh, Tomer Drori, Alexandre Ndobe, Z. Valy Vardeny Current developments in organic solar cells ($\sim $6{\%} power conversion efficiency) require understanding and control of photoinduced charge transfer of donor-acceptor pairs. In this work we provide and compare evidence that poly(2-methoxy-5(2'-ethyl)hexoxy-phenylenevinylene) (MEH-PPV, donor) blended with 2,4,7-trinitrofluorenone (TNF, strong acceptor) form a below-gap charge transfer complex (CTC) state that extends absorption into the near infrared (NIR). Transient PA measurements show direct photoexcitation into the CTC state where significant charge species are initially photogenerated, the majority of which geminately recombine within $\sim $10 ps, but the few that escape geminate recombination are subsequently captured in long-lived traps. Polarons could also be photogenerated with high efficiency \textit{at NIR excitation}, with similar fate. This shows that a CTC state exists below the MEH-PPV optical gap, but with low dissociation efficiency, which leads to poor photovoltaic effect. We compare our results to those in blends of MEH-PPV/C$_{60}$, where apparently the obtained CTC state has a much higher dissociation efficiency. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W22.00006: Quantum efficiency in organic phototransistors William Hammond, Jiangeng Xue Organic optoelectronic devices hold a prominent role in current applied physics research. Although these devices inherently suffer from lower carrier mobility than inorganic devices, their mechanically flexible nature and low material costs enable new and interesting applications. Organic phototransistors, for example, may enable simplified circuits for large area and flexible sensors. Here we report the spectral dependence of the external quantum efficiency of organic phototransistors (OFETs) based on pentacene and C$_{60}$. Furthermore, we explore the gain mechanism in these devices and the effect of transistor structure on internal photomultiplication. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W22.00007: Ultrafast dynamics in blends of $\pi$-conjugated polymers/fullerenes Sanjeev Singh, Minghong Tong, ChuanXiang Sheng, Zeev Vardeny We have studied the ultrafast dynamics of photogenerated charges and excitons in a variety of $\pi$-conjugated polymer/fullerene blends using the transient pump-probe photomodulation (PM) spectroscopy with $\sim $ 100 fs resolution. These composites serve as active layers in organic photovoltaic devices with high power conversion quantum yield, due to the existence of a photoinduced charge transfer (PCT) reaction between the polymer and the fullerene molecules. Our transient PM spectrum spans a broad energy range from 0.1-2.4 eV, and this allows us to monitor the transient behavior of the various photoinduced absorption (PA) bands of polarons and excitons in the PM spectrum; as well as the transient exciton stimulated emission, and photobleaching (PB) of the ground state. The PB dynamics reflect the ground state recovery; hence, it can be used to determine the long-lived polaron photogeneration quantum efficiency in these systems. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W22.00008: Spin Response in Organic Light Emitting Diodes Fujian Wang, Cungeng Yang, Tomer Drori, Z. Valy Vardeny To understand the origin of the magnetic field effect in OLEDs, we studied the large magnetoresistance (MR) and magnto-electroluminescence (MEL) of OLEDs based on pristine MEH-PPV polymer, as well as MEH-PPV doped with various concentrations of radical impurities and C$_{60}$ molecules. In contrast to OLED based on pristine MEH-PPV that show MR and MEL up to 12{\%} at room temperature, we found in MEH-PPV:C$_{60}$ based OLED the MR and MEL decrease substantially with increasing C$_{60}$ molecule concentration. For MEH-PPV:C$_{60}$ devices with C$_{60}$ concentration of 50{\%}, the MR effect is less than 0.3{\%} at room temperature. In MEH-PPV: radical devices the MR and MEL effects again differ substantially from those found in pristine and C$_{60}$ doped MEH-PPV devices. At 50{\%} radical concentration the MR and MEL effects are about 1{\%} and 3.5{\%}, respectively. The results are discussed with existing models for the magnetic field effects in OLEDs. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:42PM |
W22.00009: Organic Semiconductors: devices, growth and ordered assembly Invited Speaker: Organic semiconductors are employed in devices such as field-effect transistors (FETs), light-emitting diodes and photovoltaic cells. Besides their technological interest, these devices are model systems to study physical processes in organic semiconductors [1]. Light-emitting field effect transistors (OLEFETs) based on organic semiconducting films are a novel class of devices integrating the transistor function with the light emission [2]. I will discuss LEFETs based on oligothiophenes [3] and oligoacenes [4] [5], in particular their optoelectronic properties and the films growth physics. A unique property of organic semiconductors is the ability to form ordered assemblies at surfaces that can be studied by scanning tunneling microscopy (STM). I will discuss the adsorption and self-assembly on different facets of copper of two organic semiconductors: the linear and planar quinquethiophene [6] and the branched non-planar rubrene [7]. These studies show the ability of organic semiconductors to form fascinating self-assembled motifs and are of paramount importance to understand the early stages of growth of organic films. Organic electrochemical transistors (OECTs) are expected to play a key role in future organic electronics. OECTs are ideal candidates for biosensing applications thanks to their low driving voltage and their ability to operate in aqueous environment. A great deal of work is needed to understand the device physics of OECTs and optimize their performance. I will discuss advances in the field drawing examples from studies on devices based on the conducting polymer PEDOT:PSS. \newline [1] G. G. Malliaras, R. H. Friend, Phys. Today 58, 53, 2005; \newline [2] F. Cicoira, C. Santato Adv. Funct. Mater. 17, 3421, 2007; \newline [3] F. Cicoira et al. Adv. Mater. 18, 169, 2006; \newline [4] F. Cicoira et al. Adv. Funct. Mater. 15, 375, 2005; \newline [5] F. Cicoira et al. J. Mater. Chem. in press; \newline [6] F. Cicoira et al. Small 2, 1366, 2006; \newline [7] F. Cicoira et al. J. Phys. Chem. A in press. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W22.00010: Spin injection effects on exciton distributions in conjugated organic semiconductors Mohammad Yunus, P. Paul Ruden, Darryl Smith Conjugated organic semiconductors are under rapid development as the active material in organic light emitting diodes (OLEDs). Electrons and holes injected into the organic semiconductor form bound singlet or triplet excitons. Singlet excitons may recombine radiatively giving rise to light emission whereas triplet excitons do not recombine radiatively. Thus the quantum efficiency of OLEDs is limited by the fraction of singlet excitons, \textit{$\chi $}$_{S}$. In this work, we explore theoretically an approach to control \textit{$\chi $}$_{S}$ through spin-polarized electron and hole injection from ferromagnetic contacts. Conventional ferromagnetic transition metals and half-metallic materials, such as LSMO, are considered as candidate electrode materials. Electron and hole transport in the organic semiconductor is treated through the conventional device equations with the formation of excitons described by a Langevin process. Once formed, the excitons may recombine or diffuse. Triplet excitons have a lower recombination probability and hence a longer diffusion length. The model calculations yield steady state spatial profiles for singlet and triplet exciton densities in the organic semiconductor. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W22.00011: Electrically detected coherent spin manipulation of polaron pairs in an MEH-PPV OLED Heather Seipel, Dane McCamey, Seoyoung Paik, Manfred Walter, Nick Borys, John Lupton, Christoph Boehme Understanding of spin relaxation in organic light emitting diodes (OLEDs) is important for determining the maximum device efficiencies, due to the spin dependence of electronic transitions in organic materials. Here, we present an experiment demonstrating that coherent spin motion of polaron spin pairs can influence the current through an OLED fabricated using the prototypical conjugated polymer poly[2-methoxy-5-(2$^{\prime }$-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV). We observe a change in the zero-bias photocurrent transient through an OLED device following a short, coherent microwave pulse resonant with polaron pair spin transitions. The shape of the current transient provides information about the recombination rates of polaron pairs in both the singlet and triplet configurations. By varying the length of this pulse, coherent Rabi oscillations are detected, which reveal that the polaron pairs responsible for the signal remain coherent for $>$0.5$\mu $s. Due to these extremely long coherence times, we conclude that spin mixing processes are unable to significantly influence the spin state of the polaron pairs. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W22.00012: Tunable and White Light Emitting Diodes of Single Component Fluorinated Benzoxazole Graft Copolymers Shih Jung Bai, Chien-Chang Wu Coil-like graft copolymers of poly(A$_{m}$-\textit{co}-B$_{(1-m)})$ containing identical heterocyclic aromatic benzoxazole with trifluoromethyl-ethyl as the backbone and pendants of \textit{mono}-hydroxl (A$_{m})$ and/or \textit{bi}-decyloxyl (B $_{(1-m)})$ on their phenylene ring were studied for luminescence properties. The copolymers were synthesized with molar fraction $m$ ranging from 0, 0.25, 0.5, 0.75 to 1, and then dissolved and spun onto Spectrosil$^{\mbox{{\textregistered}}}$ quartz slide or indium-tin-oxide (ITO) substrate. The fluorescence properties of copolymers were investigated by ultraviolet-visible absorption covering 185 nm to 800 nm and photoluminescence (PL) emission excited at 363 nm. The PL results exhibited an excellent chromatic tuning range from green to white emission as $m$ decreased. Aluminum electron injectors were evaporated onto the copolymer/ITO unit making it into \textit{mono-}layer light emitting diodes for current-voltage and electroluminescence (EL) responses. An emission threshold voltage of 6 V was achieved for all the \textit{mono}-layer copolymer devices. The Commission Internationale de l'Eclairage chromaticities of the EL emission were from (0.25, 0.53) to (0.24, 0.31) covering a wide visible range including white light emission. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700