Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session M33: Focus Session: Organic Electronics and Photonics - Excited State Dynamics for Photovoltaics |
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Sponsoring Units: DMP Chair: Mike Arnold, University of Wisconsin-Madison Room: 341 |
Wednesday, March 20, 2013 8:00AM - 8:12AM |
M33.00001: Time resolved energy transfer in polymers doped with heavy atom molecule Ella Olejnik, Yaxin Zhai, Zeev Vardeny We used the technique of pump/probe transient photomodulation (PM) spectroscopy with high intensity and low repetition rate in the spectral range of 1.2 -- 2.5 eV and 100 fs time resolution for studying the excitons dynamics properties of solid state mixtures of few {\%} (X) heavy metal organic molecules in a pi-conjugated polymer host up to 2 ns. We found that the photobleaching (PB) spectrum contains two components; an instantaneous component due to the direct excitation of the heavy metal molecule guests, and a slower component due to energy transfer from the host polymer chains to the guest molecules. The PM spectrum also contains a built-up of a photoinduced absorption band at $\sim$1.5 eV that we assigned as due to excitons in the guest molecules, that has the same dynamics as that of the PB. [Preview Abstract] |
Wednesday, March 20, 2013 8:12AM - 8:24AM |
M33.00002: Femtosecond optical study of chemically induced polaron states in polythiophene films Hideo Kishida, Takaki Fujii, Tomoya Uchida, Takeshi Koyama, Arao Nakamura We performed femtosecond pump-probe measurements in poly(3-hexylthiophenes) (P3HT) doped with ionic liquid (OMIM/BF$_{\mathrm{4}})$. We fabricated an electrochemical cell with glass/ITO/P3HT (regioregular oriented film)/ionic liquid/ITO/glass structure. By applying the voltage between the electrodes, we electrochemically control the doping level. By increase of the applied voltage, the two polarized absorption peaks due to polaron states appear within the optical gap. The dynamics of the photoexcited states were studied by two-color femtosecond pump-probe measurements, in which the photon energies of the pump and probe pulses correspond to the $\pi $-$\pi $* transition and the polaron absorption band, respectively. At lower voltage, the increase of the near infrared absorption is observed, which is assigned to the photoinduced polaron absorption. On the other hand, at higher voltage, photoinduced bleaching is observed. The increase of the applied voltage reduces the lifetime of the excited states. These facts suggest that the photoexcitation in the chemically induced polaron states changes the electronic states and induces the new photoexcited species. The detailed origins of the new states are discussed by comparison with the femtosecond pump-probe spectra in PEDOT/PSS. [Preview Abstract] |
Wednesday, March 20, 2013 8:24AM - 8:36AM |
M33.00003: Transient picosecond studies of pristine and DIO doped PTB7/PC71BM blend for photovoltaic applications Uyen Huynh, Tek Basel, Valy Vardeny Recently there have been reports of a significant increase of power conversion efficiency in organic solar cells upon mixing the donor-acceptor blend with various additives. We studied the photoexcitation dynamics in thin films of pristine PTB7 (a low band-gap polymer), and pristine and doped PTB7/PCBM blend with DIO additives, using the pump/probe photoinduced absorption technique with probe in the mid-IR spectral range. We found that the photogenerated charges in DIO doped blend is more efficient than in the pristine blend. Specifically we found that the charge polarons in the DIO doped blend are photogenerated instantaneously, simultaneously with photogenerated singlet excitons. The excitons decay into charge-transfer (CT) excitons at the D-A boundaries within $\sim$1ps.The CT excitons may geminately recombine or dissociate into free charge polarons; where the dissociation time constant was found to be $\sim$450 ps. [Preview Abstract] |
Wednesday, March 20, 2013 8:36AM - 8:48AM |
M33.00004: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 8:48AM - 9:00AM |
M33.00005: Managing thermal effects in z-scan measurements on PTCDA films Niranjala Wickremasinghe, Xiaosheng Wang, Heidrun Schmitzer, Hans Peter Wagner We study the two-photon absorption in micrometer thick polycrystalline PTCDA (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride) films using the open aperture z-scan technique. The films were grown by organic molecular beam deposition on Pyrex substrate and have been excited with 150 fs high repetition rate laser pulses at a wavelength of 820 nm. The pulses are focused onto the sample with a 10 x or a 20 x long distance microscope objective lens. The excitation intensities have been kept the same in both cases. To study the influence of sample heating the laser repetition rate has been varied from 4 MHz to 50 kHz by an acousto-optic pulse selector. At laser repetition rates larger than 200 kHz and 1 MHz for the 10 x and 20 x microscope lenses, respectively, we observe a reduction of the z-scan transmission dip. This reduction is attributed to a counteracting thermal effect due to film heating in the focus area. The reduced thermal effect using a 20 x microscope lens is attributed to faster thermal diffusion from the smaller focus area into the unexcited film. At lower repetition rates the z-scan dip is independent of the repetition rate and the two-photon absorption coefficient in PTCDA films was determined to be approximately 4 cm/GW. [Preview Abstract] |
Wednesday, March 20, 2013 9:00AM - 9:12AM |
M33.00006: Excitonic Properties of Novel $\pi $-conjugated Polymers for Organic Electronics Evan Lafalce, Xiaomei Jiang, Cheng Zhang We compare the photophysics of different derivatives of the $\pi $-conjugated polymer Poly(thienylenevinylene) (PTV) by photoluminescence (PL) and electro-absorption (EA) spectroscopy. The binding energy of the primary excitonic excited state is obtained from EA and is found to be related to the quantum efficiency for PL. In particular, both quantities are determined by the energies of the first optically allowed state and the first optically forbidden state above the ground state. In most PTV derivatives, the optically forbidden state lies below the optically allowed state and the PL is efficiently quenched by internal conversion. When the order of excited states is reversed, PL is observable with an efficiency that scales with the binding energy of the exciton as determined by EA. Thus, the chemical structure governs the interplay between $\pi $-conjugation delocalization and electron correlation that determines the ordering of excitonic states. This ordering then in turn dictates the effectiveness of a $\pi $-conjugated polymer for both emission and exciton dissociation and therefore dictates a material's suitability for either Organic Light-emitting Diodes or Organic Photovoltaic devices. This information then may be useful in the design of novel materials for application in these devices. [Preview Abstract] |
Wednesday, March 20, 2013 9:12AM - 9:48AM |
M33.00007: External quantum efficiency exceeding 100{\%} in a singlet-exciton-fission-based solar cell Invited Speaker: Marc Baldo Singlet exciton fission can be used to split a molecular excited state in two. In solar cells, it promises to double the photocurrent from high energy photons, thereby breaking the single junction efficiency limit. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, the peak external quantum efficiency is (109$+$/-1){\%} at $\lambda =$ 670 nm for a 15-nm-thick pentacene film. The corresponding internal quantum efficiency is (160$+$/-10){\%}. Independent confirmation of the high internal efficiency is obtained by analysis of the magnetic field effect on photocurrent, which determines that the triplet yield approaches 200{\%} for pentacene films thicker than 5~nm. To our knowledge, this is the first solar cell to generate quantum efficiencies above 100{\%} in the visible spectrum. Alternative multiple exciton generation approaches have been demonstrated previously in the ultraviolet, where there is relatively little sunlight. Singlet exciton fission differs from these other mechanisms because spin conservation disallows the usual dominant loss process: a thermal relaxation of the high-energy exciton into a single low-energy exciton. Consequently, pentacene is efficient in the visible spectrum at $\lambda =$ 670 nm because only the collapse of the singlet exciton into \textit{two }triplets is spin-allowed. [Preview Abstract] |
Wednesday, March 20, 2013 9:48AM - 10:00AM |
M33.00008: An Electric Field Stimulated Spin Crossover Transition in a Molecular Adsorbate Xin Zhang, Tatiana Palamarciuc, Patrick Rosa, Jean-Fran\c{c}ois L\'etard, Eduardo V. Lozada, Fernand Torres, L.G. Rosa, Bernard Doudin, Peter A. Dowben We have investigated the occupied and unoccupied electronic structure of ultra thin films of the spin crossover [Fe(H$_2$B(pz)$_2)_2$(bipy)] complex (with H$_2$B(pz)$_2 =$ bis(hydrido)bis(1H-pyrazol-1-yl)borate and bipy $=$ 2,2'-bipyridine) by ultraviolet photoelectron spectroscopy (UPS), inverse photoemission (IPES) and X-ray absorption spectroscopy (XAS). A bandgap of 2-3 eV is deduced from combined UPS and IPES measurements of the films on Au substrates. The shift of the unoccupied density of states seen in IPES is consistent with the thermally induced spin crossover transition for molecules deposited on the organic ferroelectric copolymer polyvinylidene fluoride with trifluoroethylene (PVDF-TrFE). Perhaps more significant is the fact that the spin crossover transition, and certainly the unoccupied electronic structure, is influenced by the ferroelectric polarization direction of PVDF-TrFE substrates at temperatures in the vicinity of the thermally driven spin crossover transition. [Preview Abstract] |
Wednesday, March 20, 2013 10:00AM - 10:12AM |
M33.00009: Exploration of Excitonic States in Dilute Magnetic Organic Semiconductors Lane Manning, Naveen Rawat, Cody Lamarche, Lauren Paladino, Ishviene Cour, Randall Headrick, Madalina Furis The electronic and excitonic properties of mixed dilute metal/metal-free phthalocyanine crystalline thin films are explored. The immediate focus is on molecular systems containing Cobalt and Copper phthalocyanines in ratios to the metal-free phthalocyanines ranging from 1:1 to 1:10. The molecular thin films samples are deposited using a novel hollow pen-writing technique\footnote{R. Headrick et al, APL 92 063302 (2008)} that produce millimeter sized grains with long range macroscopic order. Electronic and excitonic states are investigated using temperature dependent absorption/transmission and photoluminescence spectroscopy. All optical characterization indicates a very uniform mixing of the species is achieved in films without loss of long range order previously observed in individual species. At low temperatures, a novel high energy state is observed. Its intensity is directly related to the ratio of metal to metal-free Phthalocyanine. In addition, a unique linear dichroism mapping is performed on these thin film samples, giving insight into electronic states both close to and far from grain boundaries. [Preview Abstract] |
Wednesday, March 20, 2013 10:12AM - 10:24AM |
M33.00010: Harvesting singlet fission for solar energy conversion: one versus two-electron transfer electron transfer from the quantum superposition state Wai-Lun Chan, John Tritsch, Xiaoyang Zhu Singlet fission (SF) is being explored to increase the efficiency of organic photovoltaics. A key question is how to effectively extract multiple electron-hole pairs from multiple excitons with the presence of other competing channels such as electron transfer from the singlet state. Recent experiments on the pentacene and tetracene show that a quantum superposition of the singlet (S$_{1}$) and multiexciton (ME) state is formed during SF. However, little is known about the kinetics of electron transfer from this quantum superposition. Here, we apply time-resolved photoemission spectroscopy to the tetracene/C$_{60}$ interface to probe one and two electron transfer from S$_{1}$ and ME states, respectively. Because of the relatively slow (~7 ps) SF in tetracene, both one- and two-electron transfer are allowed. We show evidence for the formation of two distinct charge transfer states due to electron transfer from photo-excited tetracene to the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 levels in C$_{60}$. Kinetic analysis shows that ~60\% of the quantum superposition transfers one electron through the S$_{1}$ state to C$_{60}$ while ~40\% undergoes two-electron transfer through the ME state. [Preview Abstract] |
Wednesday, March 20, 2013 10:24AM - 11:00AM |
M33.00011: Charge Transfer and Triplet States in High Efficiency OPV Materials and Devices Invited Speaker: Vladimir Dyakonov The advantage of using polymers and molecules in electronic devices, such as light-emitting diodes (LED), field-effect transistors (FET) and, more recently, solar cells (SC) is justified by the unique combination of high device performance and processing of the semiconductors used. Power conversion efficiency of nanostructured polymer SC is in the range of 10{\%} on lab scale, making them ready for up-scaling. Efficient charge carrier generation and recombination in SC are strongly related to dissociation of the primary singlet excitons. The dissociation (or charge transfer) process should be very efficient in photovoltaics. The mechanisms governing charge carrier generation, recombination and transport in SC based on the so-called bulk-heterojunctions, i.e. blends of two or more semiconductors with different electron affinities, appear to be very complex, as they imply the presence of the intermediate excited states, neutral and charged ones [1-3]. Charge transfer states, or polaron pairs, are the intermediate states between free electrons/holes and strongly bound excitons. Interestingly, the mostly efficient OLEDs to date are based on the so-called triplet emitters, which utilize the triplet-triplet annihilation process. In SC, recent investigations indicated that on illumination of the device active layer, not only mobile charges but also triplet states were formed [4]. With respect to triplets, it is unclear how these excited states are generated, via inter-system crossing or via back transfer of the electron from acceptor to donor. Triplet formation may be considered as charge carrier loss channel; however, the fusion of two triplets may lead to a formation of singlet excitons instead. In such case, a generation of charges by utilizing of the so far unused photons will be possible. The fundamental understanding of the processes involving the charge transfer and triplet states and their relation to nanoscale morphology and/or energetics of blends is essential for the optimization of the performance of molecular photovoltaic devices. I will present the state of the art in this field and discuss the mechanisms of polaron pair generation and recombination in the novel low band gap polymer-fullerene blends as well as in high-efficiency SC.\\[4pt] [1] C. Deibel, T. Strobel, V. Dyakonov, Phys. Rev. Lett. 103, 036402 (2009).\\[0pt] [2] C. Deibel, T. Strobel, and V. Dyakonov, Adv. Mater. 22, 4097 (2010).\\[0pt] [3] C. Deibel, and V. Dyakonov, Rep. Prog. Phys. 73, 096401 (2010).\\[0pt] [4] M. Liedtke, et al., JACS 133, 9088 (2011). [Preview Abstract] |
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