Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P24: Focus Session: Organic Heterojuncture Photovoltaics |
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Sponsoring Units: DPOLY DMP Room: Colorado Convention Center 201 |
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P24.00001: Organic Semiconductors: A Molecular Picture of the Charge-Transport and Energy-Transport Processes. Invited Speaker: Conjugated organic oligomer and polymer materials are being increasingly considered for their incorporation as the active semiconductor elements in devices such as photo-voltaic cells, light-emitting diodes, or field-effects transistors. In the operation of these devices, electron-transfer and energy-transfer processes play a key role, for instance in the form of charge transport (in the bulk or across interfaces), energy transport, charge separation, or charge recombination [1]. Here, we provide a theoretical description of electron-transfer phenomena based on electron-transfer theory, which allows us to provide a molecular, chemically-oriented understanding. In this presentation, we focus on the parameters that impact the mobility of charge carriers [2], that is the electronic coupling within chains and between adjacent chains and the reorganization energy of the chains upon ionization. Materials under study include conjugated oligomers such as oligoacenes, oligothiophene-acenes, oligothiophenes, and oligothienacenes. \newline \newline [1] J.L. Br\'{e}das, D. Beljonne, V. Coropceanu, and J. Cornil, ``Charge-Transfer and Energy-Transfer Processes in pi-Conjugated Oligomers and Polymers'', Chemical Reviews, 104, 4971-5004 (2004). \newline [2] V. Coropceanu, J. Cornil, D.A. da Silva Filho, Y. Olivier, R. Silbey, and J.L. Br\'{e}das, ``Charge Transport in Organic Semiconductors'', Chemical Reviews, 107, xxx (2007). [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P24.00002: Predicting structure/property relations in polymeric photovoltaic devices. Gavin Buxton, Nigel Clarke Plastic solar cells are attractive candidates for providing cheap, clean and renewable energy. However, such devices are critically dependent on the internal structure, or morphology, of the polymer constituents. We have developed a model that enables us to predict photovoltaic behaviour for arbitrary morphologies, which we also generate from numerical simulations. We illustrate the model by showing how diblock copolymer morphologies can be manipulated to optimise the photovoltaic effect in plastic solar cells. In this manner, we can correlate photovoltaic properties with device structure and hence guide experiments to optimise polymer morphologies to meet photovoltaic needs. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P24.00003: Synthesis and Application of Conducting Block Copolymers in Organic Photovoltaics Bryan W. Boudouris, Marc A. Hillmyer, C. Daniel Frisbie Recent advances in the fabrication and post-processing of polymer -- fullerene bulk heterojunction solar cells have allowed for devices with power conversion efficiencies up to 5{\%} to be generated. An understanding of how the internal morphology of the active layer affects device performance would facilitate optimization and ultimately lead to higher efficiencies. Block copolymers have been shown to self-assemble into well-structured, microphase-separated domains on the order of the diffusion length ($\sim $ 10 nm) of an exciton (bound electron-hole pair) in thin films. In an effort to make a nanostructured active layer morphology we have synthesized block copolymers where the conducting moiety is either poly(3-hexylthiophene) or poly(3-dodeclythiophene) and the second, etchable block is polylactide. Hydroxyl-terminated polythiophene molecules were synthesized via the McCullough method and used as macroinitiators for the ring-opening polymerization of D,L-lactide. AFM images of spin-coated block copolymer films show separation between the polythiophene and polylactide segments. After subjecting the samples to a dilute aqueous base for short periods of time, we have selectively etched the polylactide segments to create pits in the semicrystalline polythiophene matrices. In addition to these findings, preliminary device results will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P24.00004: Fluorescence of Dendrons based on Donors and Accepter with Different Linkages J.H. Park, Y. Wu, D.A. Modarelli, J.R. Parquette, A.J. Epstein Earlier indirect studies utilizing wavelength and bias spectra of photocurrent in simple photovoltaic cells demonstrated charge transfer (CT) in 1st generation dendritic macromolecules prepared using two different donor (tetraphenylporphyrin) groups bound to an accepter (naphthalenediimide) group. We report here fluorescence for solid-state films and solutions of these donor and dendrons. Using 460nm excitation, fluorescence (660nm, 715nm) in solution samples can be observed for both donor and dendron but fluorescence in the solid state can be observable only in donor sample due to fluorescence quenching within the dendron. This demonstrates intermolecular CT from donor to accepter. Fluorescence lifetime measurements (460nm 1.5nsec FWHM pulse excitation) of donor and dendron solutions show that it depends on length of the linkage between donor and accepter. This shows a direct relaxation path from donor to accepter (intramolecular CT). The separation of the exciton to separate electron and on the donor and acceptor portions of the dendron would open the potential for its use in photovoltaic application. Supported in part by DOE \#DE-FG02-01ER45931 [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P24.00005: The interplay of morphology and carrier recombination in dendrimer-based organic photovoltaics. Sean Shaheen, Nikos Kopidakis, William Mitchell, William Rance, Jao van de Lagemaat, Garry Rumbles Pi-conjugated dendrimers provide an alternative to polymers in organic photovoltaic devices that allow for systematic study of how the molecular structure affects the morphology of the donor and acceptor components and subsequently how the device operates. The degree of mixing and specific geometry of the donor-acceptor blend play a determining role in the rate of exciton dissociation as well as the efficacy of charge transport out of the active layer. We find that pi-conjugated dendrimers are more miscible with the fullerene-derivative acceptor than their polymeric counterparts, which leads to smaller domains than are commonly found in polymer-fullerene blends. Here we discuss how these differing morphologies affect exciton dissociation, carrier transport, and carrier recombination in the devices. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P24.00006: Below gap external quantum efficiency of organic solar cells. Alexandre Ndobe, Valy Vardeny We fabricated a variety of organic bulk hetero-junction photovoltaic (PV) solar cells based on blends of regio-regular polythiophene (RR-P3HT) and MEHPPV with the fullerene molecules C60- and C70- PCBM. We found, surprisingly that the organic devices show a photovoltaic effect even when excited with light having photon energy below the optical gap of the polymers. This implies that organic solar cells efficiencies can be improve by considering material other than PCBM that have higher infrared absorption but still can serve as a good acceptors for the polymers. To complement this finding we measured the excitation dependence of various PV parameters such as the PV fill-factor, open-circuit voltage, and external quantum efficiency. The interesting excitation spectra reveal the device structure geometry as will be discussed in detail. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P24.00007: Nanocrystalline organic solar cells. Fan Yang, Kai Sun, Stephen Forrest Donor/acceptor (DA) heterointerfaces effectively dissociate excitons into carriers in organic solar cells. Unfortunately, the low carrier mobility of amorphous DA blends limits the active layer thickness to $\sim $25 nm, resulting in low solar absorption. Solar cells made from blends of organic and inorganic semiconductor nanorods overcome the low charge mobility in disordered organic films but have disadvantages due to the mismatch between the nanorods and organic material properties. Here we demonstrate organic solar cells in which both DA materials grow into an extended nanocrystalline network. Structural analysis confirms the existence of crystalline phases of the constituent donor molecule, copper phthalocyanine (CuPc), and the acceptor, C$_{60}$. The structure has a power conversion efficiency of 6.2$\pm $0.3{\%} at 1 sun, AM1.5 simulated solar illumination. This cell shares many of the merits of all organic DA blends and organic/inorganic nanorod cells without many of their disadvantages. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P24.00008: Fabrication and characterization of photovoltaic devices based on `self corralled' CdSe nanorods functionalized with polythiophene Suresh Gupta, Qingling Zhang, Ali Cirpan, Frank Karasz, Todd Emrick, Thomas P. Russell It has been shown that the CdSe nanorods can be oriented normal to the surface by employing an electric field and a polymer matrix where nanorods phase separate. The nanorods close pack with orientation normal to surface in a thin film when the CdSe nanorods are functionalized with alkane and poly(methyl methacrylate) or poly(3-hexyl thiophene)(P3HT) is the matrix. The film is drop-cast under electric field. The phase separation of the nanorods in polymer matrix can be directed by using a patterned surface. The patterned surface was prepared by soft-lithography. Further, the nanorods functionalized with P3HT are `self corralled' under electric field by using a polymer matrix and photovoltaic devices are fabricated. The devices are characterized and the results for devices with normally oriented nanorods are compared to the devices with nanorods parallel to surface. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P24.00009: Femtosecond transient studies of photoinduced charge transfer in~polymers doped with strong acceptor molecules; applications~for organic solar cells Josh Holt, Tomer Drori, Chuanxiang Sheng, Z. Valy Vardeny Current developments in organic solar cells ($\sim $5{\%} efficiency nowadays)~require understanding and control of photoinduced charge carrier transfer and electronic state dynamics of donor-acceptor pairs. One current drawback to organic solar cell efficiency is negligible absorption in the near infrared region of the solar spectrum. We provide and compare evidence that poly(2-methoxy-5(2'-ethyl)hexoxy-phenylenevinylene) (MEH-PPV) and regio-regular poly-3-hexyl thiophene (RR-P3HT) doped with 2,7-dinitrofluorenone (DNF) or 2,4,7-trinitrofluorenone (TNF) form below-gap charge transfer complex state that can extend absorption into the near infrared. Using fs transient and CW spectroscopies we found that the photoluminescence and~mid-ir photoinduced absorption (PA)~band of excitons~are simultaneously quenched, when excited in the visible/uv or near ir. We compare our results to those of comparable systems using C$_{60}$ as acceptor molecules. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P24.00010: Photoinduced charge transfer from polymers to fullerene molecules revisited. Tomer Drori, Chuanxiang Sheng, Alex Ndobe, Cungeng Yang, Minghong Tong, Valy Vardeny We study the process of photoinduced charge transfer (PCT) between conjugated polymers and fullerene molecules as electron acceptors, using the technique of picosecond transient, and steady state photomodulation at various modulation frequencies and temperatures. The polymers studied were MEH-PPV and regio-regular P3HT [RR-P3HT], which are some of the common polymers that are used in organic photovoltaic, as well as polyfluorene [PFO] with optical gap in the blue spectral range; whereas the fullerene molecules where C$_{60}$, C$_{70}$ and their PCBM variations. In all cases we found PCT as evident by the formation of strong photoinduced absorption (PA) polaron bands in the mid ir spectral range. Surprisingly we also found PCT with photon energy below the polymer optical gap. This below-gap PCT process will be discussed and compared with the more usual PCT process with above gap excitation. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P24.00011: Nanoscale Composition and Efficiency of Conjugated Polymer Based Photovoltaic Devices Benjamin Watts, Chris McNeill, Lars Thomsen, Warwick Belcher, Harald Ade, Neil Greenham, Paul Dastoor Organic solar cells based on thin blend films of conjugated polymers and/or fullerene derivatives promise significant advantages in flexibility and low-cost fabrication over conventional, silicon based devices. However, these polymer systems tend to display complex segregation of the component materials during film formation, with the degree of segregation observed shown to depend on parameters such as spincasting spin-speed and solvent type. Many studies in recent years have demonstrated a link between film morphology and device performance and subsequent changes in fabrication methods have resulted in improved device efficiencies that now approach 5\% total power conversion. Here, we present studies providing further details on the morphology-efficiency relationship through the application of scanning transmission X-ray Microscopy (STXM) to generate quantitative composition maps of conjugated polymer blend films and comparison to the measured efficiency of photovoltaic devices incorporating corresponding blend film active layers. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P24.00012: Optimization of the Negative Electrode in Organic Photovoltaic Devices Matthew Reese, Matthew White, Garry Rumbles, David Ginley, Sean Shaheen A blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C$_{61}$-butyric acid methyl ester (PCBM) is used as the active layer in a series of bulk heterojunction organic solar cells. This polymer blend serves as a test-bed to explore the significant effects on device performance of using low work function metals and/or alkali metal halides as the top, negative electrode. Work function values reported in the literature are compared with those measured for our thin films. A series of contact materials are investigated including Al, Ca/Al, Ba/Al, LiF/Al; many devices are prepared with each contact type to validate the statistical significance of the results. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P24.00013: Time-Resolved Microwave Photoconductivity study of the Photophysics of Bulk Heterojunction Organic Photovoltaic Devices Nikos Kopidakis, Andrew Ferguson, Sean Shaheen, Garry Rumbles Bulk heterojunctions composed of a blend of the polymer poly(3-hexylthiophene) (P3HT) and the acceptor fullerene derivative [6,6]-phenyl C$_{61}$-butyric acid methyl ester (PCBM) are the prototypical organic photovoltaic devices. The photophysical processes that take place in these structures involve exciton generation and quenching, and free carrier transport, trapping and recombination. To probe these processes we have performed contactless Time-Resolved Microwave Photoconductivity measurements in pure polymer films and in bulk heterojunctions with varying PCBM concentration. We compare our results with various models for free carrier generation in the pure polymer and in the bulk heterojunction and develop a kinetic scheme to describe free carrier generation and recombination that is consistent with our experimental data. We show that exciton quenching in the presence of the acceptor (PCBM) involves first and second order processes that become prevalent at low and high light intensities, respectively. [Preview Abstract] |
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