Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T20: Focus Session: Organic Electronics and Photonics - Photophysics |
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Sponsoring Units: DMP DPOLY Chair: Sahar Sharifzadeh, Lawrence Berkeley National Laboratory Room: 405 |
Thursday, March 6, 2014 11:15AM - 11:51AM |
T20.00001: Exciton fission and solar energy conversion beyond the limit Invited Speaker: Xiaoyang Zhu The absorption of one photon by a semiconductor material usually creates one electron-hole pair, but this general rule breaks down in a few organic semiconductors, such as pentacene and tetracene, where one photon absorption may result in two electron-hole pairs in a process called singlet exciton. Recent measurements in our group by time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton state. More importantly, population of the multiexciton state is found to rise concurrently with that of the singlet state on the ultrafast time scale upon photo excitation. This observation provides an experimental foundation for a quantum coherent mechanism in which the electronic coupling creates a quantum superposition of the singlet and the multiexciton state immediately following optical excitation. We demonstrate the feasibility of harvesting the multiexciton state for multiple charge carriers and the implementation of singlet fission for solar energy conversion beyond the Shockley-Queisser limit. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T20.00002: Probing inter- and intrachain coupling in P3HT using time resolved spectroscopy Bhoj Gautam, Robert Younts, Cong Mai, Harald Ade, Christoph Hellmann, Natalie Stingelin, Kenan Gundogdu The change in relative intensity of 0-0 to 0-1 peaks and the considerable shift in the absorption /emission spectra have been taken as the tool to distinguish the H-like and J-like aggregation (i.e., to address the interchain and intrachain coupling). In order to elucidate whether or not only linear absorption or photoluminescence (PL) can account for this, we studied the absorption, PL and the time resolved spectroscopy of P3HT in two different forms: pristine film and films blended with polar additive PEO. The red shift in 0-0 absorption peak and significant change in the relative intensity of 0-0 to 0-1 peaks upon blending indicates the change in aggregation from H-like to J-like. Surprisingly, we did not see a significant change in either the relative intensity of 0-0 to 0-1 peaks in the PL spectrum or the long time decay dynamics of the time resolved PL, suggesting both samples show H-like behavior. Here we address these contradictory observations by time resolved absorption measurements. We observed that the electronic coupling is a dynamics process evolving from J-like to H-like within a few hundred picoseconds. Our observations indicate that charge separation dynamics in P3HT is mostly governed by the very early dynamics. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T20.00003: Charge transfer in rare earth oxide hybrid solar cells revealed through ultrafast spectroscopic measurement Bill Pandit, Kasun Fernando, Bruce Alphenaar, Jinjun Liu Hybrid inorganic-organic solar cells typically combine a transition metal oxide (such as TiO$_{2})$ and organic dye or polymer absorber to form the donor acceptor pair. Here, Oxidized neodymium (Nd$_{2}$O$_{3})$ particles are combined with [6,6]-Phenyl C$_{61}$ butyric acid methyl ester (PCBM) to form the active layer of a bulk heterojunction solar cell. The addition of the Nd$_{2}$O$_{3}$ results in an enhancement in the short circuit current and open circuit voltage compared to pure PCBM. We also studied the ultrafast dynamics of photoexcitation in pristine PCBM film, and their blends with the rare earth oxide neodymium particles using the pump-probe photomodulation (PM) spectroscopy with $\sim $30 fs time resolution. Our transient PM spectrum covers spectral range of 430 nm to 730 nm. Although the spectra of Nd$_{2}$O$_{3}$/PCBM are very similar with pristine PCBM, the recombination kinetics of photogenerated excitons decay rate increases with the addition of Nd$_{2}$O$_{3}$, and ground state photobleaching is also observed. Taken together this provides evidence for the charge transfer between the organic and rare earth inorganic components. Supported by the DOE-EPSCoR fund DOE BES (DE-FG02-07ER46375) at University of Louisville. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T20.00004: Charge Photogeneration (CPG) in Low-Band-Gap (LBG) Donor-Acceptor (D-A) Copolymers: Higher Efficiency in LBG Polymer-Fullerene Solar Cells Karan Aryanpour, Sumit Mazumdar LBG copolymers (bandgap $\sim$ 1.5 eV) of alternating D-A moieties have attracted substantial interest in photovoltaics. Power conversion efficiency over 10\% has been reported for tandem LBG copolymer-fullerene solar cells [1]. Understanding CPG in pristine LBG copolymers is a key step towards higher efficiency in LBG copolymer-fullerene solar cells. We present correlated-electron calculations within the Pariser-Parr-Pople model for excited states in LBG copolymers thieno[3,4-b]thiophene/benzodithiophene (PTB7) and poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:2',3'-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothia diazole)] (PDTP-DFBT). The goals are to understand ground state absorption, electroabsorption, and most importantly photoinduced absorptions in experiments. Of interest is the possible role of triplet excitons within the LBG donor domains in the CPG of LBG copolymers. Experiments present evidence on the high energy excited states as possible triplet-triplet (TT) combinations. While TT states in ``ordinary'' commonplace polymers may not play significant roles in photoinduced charge-transfer, they can possibly provide additional paths to CPG in the LBG copolymers other than the optical exciton and states close to it. [1] J. You et. al., Nat. Comm. 4, 1446 (2013) [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 1:03PM |
T20.00005: Exciton dynamics in organic molecular crystals and nanostructures Invited Speaker: Chris Bardeen The photophysical behavior of organic semiconductors is governed by their excitonic states. In this talk, we classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge-transfer) typically encountered in organic semiconductors. The availability of several different exciton bands provides the possibility of interband processes. One such process is singlet fission, where an initially excited singlet exciton can spontaneously split into a pair of spin-entangled triplet excitons. We analyze this phenomenon in detail, emphasizing the role of spin state coherence and magnetic fields in studying singlet $\leftarrow \to $ triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge-transfer) interact to generate unique nonlinear excitonic processes in molecular systems. These processes may be useful for applications like solar energy conversion, where the generation of two excitons per absorbed photon could lead to significant enhancements in the efficiency of single junction photovoltaic cells. Finally, we will briefly describe how excitons can also be used to initiate photochemical reactions in molecular crystal nanostructures, resulting in large shape changes and deformations. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T20.00006: Poly-(3-hexylthiophene) Aggregate Formation in Binary Solvent Mixtures: An Excitonic Coupling Analysis David Boucher, Calynn Johnson We have studied the aggregation behavior of P3HT [M$_{n} \approx $ 28.2 kDa, regioregularity \textgreater 96 {\%}, PDI $\approx$ 1.3] in 96 solvent mixtures is studied using UV-Vis absorption spectroscopy. We used Hansen solubility parameters (HSPs) and Spano excitonic coupling analyses to identify correlations between the properties of the solvent mixtures and the extent of structural order of the aggregates. It is clear that the identity of the poor solvent used to drive aggregation has a significant impact on the excitonic coupling behavior and, hence, the structural order of the P3HT aggregates. However, solubility parameter theory does not account nor provide a predictive theory for the observed trends. Instead, qualitative arguments based on the nature of the interactions between the solvents and the polythiophene and hexyl side chain motifs are used to rationalize the kinetics of formation and the observed excitonic coupling characteristics of the P3HT aggregates. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T20.00007: How the structures of salts involve in the optical properties of Pyrene (C$_{16}$H$_{10})$? Hyun-Sook Jang, Jing Zhao, Mu-Ping Nieh Pyrene (Py), due to its specific optical properties (i.e., long life time, excimer, polarity), has been used as a variety of sensors. It has reported that the high vapor pressure in processing the films is an important factor for the enhanced Py optical properties [1]. In this paper, the effects of a series of tetraalkylammonium salts (with a variety of chain lengths and anions) on Py optical properties are investigated in order to identify the controlling parameters of the Py fluorescence quenching in the binary system from the solution to solid state [2]. Several experimental approaches including steady-state fluorescence spectroscopy, $^{13}$C-NMR, and time-dependent fluorescence decay are employed in order to seek for the fundamental understanding of the optical properties of Py. The result shows that cation chain length of tetrabutylammonium (TBA$+)$ and hexafluorophosphate (PF$_{6}$-) anion play an important role in the Py optical properties. These interaction between Py and salts is mainly governed by dynamic quenching processes [2]. The knowledge obtained in this study provides insights to the design of the molecular self-assembly for the development of sensors with high performance.\\[4pt] [1] Jang, H.-S et al. J. Phys. Chem. C 2012, 117, 1428-1435.\\[0pt] [2] Jang, H.-S et al. submitted. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T20.00008: Polymer Films with Enhanced Light Emission Adam Thomas, Zachary Barcikowski, Marian Tzolov We present results on improving the photoluminescence quantum efficiency (PLQE) of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) films by addition of polyfluorene derivatives poly(9,9-di(ethyl-hexyl)fluorine) (PFO) or poly(9,9-di(2-(2-(2-methoxy-ethoxy)ethoxy)ethyl)fluorenyl-2,7-diyl)(PFO-EO3). We have compared the optical absorption, photoluminescence emission and excitation spectra of the starting solutions with those of the films. We report the PLQE of the films upon excitation with different wavelengths. The PLQE of MEH-PPV films is enhanced from 12{\%} to 17 {\%} after addition of polyfluorenes when excited with light which is not absorbed by the polyfluorenes. We present results suggesting that the interpenetration of the polyfluorene chains in the MEH-PPV network leads to this improvement. Our comparison with the solutions allows us to conclude that the main mechanism for energy transfer from the PFO to MEH-PPV is the F\"{o}rster transfer. We have applied the concept of interpenetrating structures in polymer light emitting devices. The polymer devices show a significant improvement in efficiency and light emission over the single film of MEH-PPV. The devices also hold their original color for the MEH-PPV enhanced polymer. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T20.00009: Interaction of Poly(3-hexylthiophene) (P3HT) with NiO (100) Surface: A First-Principles Study Longhua Li, Oleg Kontsevoi, Arthur Freeman Recent experiments show that NiO outperforms PEDOT:PSS as a hole transport layer in organic photovoltaic (OPV) cells; they also demonstrate that the device performance strongly depends on the composition of the NiO surface, e.g., O$_{\mathrm{2}}$-plasma treated NiO exhibits higher performance than as-deposited NiO. Thus, the polymer/NiO interfacial atomic structure plays a critical role for improving OPVs performance. We model the P3HT/NiO(100) interface by employing DFT calculations to explore the structural and electronic properties and the role of O at the interface. Our results show that in the most energetically favorable interfacial structure the P3HT backbone is aligned along the Ni-O direction. The different roles of the P3HT backbone and side-chain at the interface are presented. Our calculations suggest that side-chains could be used to enhance the interaction of polymer and NiO surface due to a significant contribution to the adsorption energy from the P3HT side-chains. A strong electronic coupling is found between carbon from the P3HT backbone and oxygen of the NiO(100) surface; such C-O coupling may be a possible reason why O$_{\mathrm{2}}$-plasma treatment of NiO results in enhanced device performance. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T20.00010: Electronic Structure Investigation of Doping C$_{60}$ with Metal Oxide Chenggong Wang, Yongli Gao Fullerene (C$_{60})$ has been used extensively as an acceptor material in organic photovoltaic (OPV) cells. Other applications including n-channel organic thin film transistors (OTFT) and C$_{60}$ based organic superconductors have been reported more than a decade ago. We have investigated p-doping of C$_{60}$ with molybdenum oxide (MoO$_{\mathrm{x}})$ with ultra-violet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES) and atomic force microscopy (AFM). Both surface doping and bulk doping by MoO$_{\mathrm{x}}$ are studied. It was found that the thermally evaporated MoO$_{\mathrm{x}}$ inter-layer substantially increased the surface workfunction. This increased surface workfunction strongly attract electrons towards the MoO$_{\mathrm{x}}$ layer at the C$_{60}$/MoO$_{\mathrm{x}}$ interface, resulting in strong inversion of C$_{60}$. Energy levels of C$_{60}$ relax gradually as the thickness of C$_{60}$ increases. An exceptionally long (greater than 400 Angstrom) band bending is observed during this relaxation in C$_{60}$. Such a long band bending has not been observed for other organic/MoO$_{\mathrm{x}}$ interface. For the bulk doping, MoO$_{\mathrm{x}}$ doping ratios from 1{\%} to over 100{\%} were investigated. The saturation occurs at approximately 20 {\%}, when the highest occupied molecular level (HOMO) of C$_{60}$ starts to be pinned at the Fermi level. These studies demonstrate effective ways to manipulate the electronic structures of the fullerene. [Preview Abstract] |
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