Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J41: Focus Session: Organic Electronics and Photonics, Optical and Electrical Properties |
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Sponsoring Units: DPOLY Chair: Michael Chabinyc, University of California, Santa Barbara Room: 214A |
Tuesday, March 3, 2015 2:30PM - 3:06PM |
J41.00001: BREAK |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J41.00002: Measuring Exciton Diffusion in Conjugated Polymer Films with Super-resolution Microscopy Samuel Penwell, Lucas Ginsberg, Rodrigo Noriega Manez, Naomi Ginsberg Conjugated polymers are highly tunable organic semiconductors, which can be solution processed to form thin films, making them prime candidates for organic photovoltaic devices. One of the most important parameters in a conjugated polymer solar cell is the exciton diffusion length, which depends on intermolecular couplings, and is typically on the order of 10 nm. This mean exciton migration can vary dramatically between films and within a single film due to heterogeneities in morphology on length scales of 10's to 100's nm. To study the variability of exciton diffusion and morphology within individual conjugated polymer films, we are adapting stimulated emission depletion microscopy. STED is typically used in biology with well-engineered fluorescent labels or on NV-centers in diamond. I will, however, describe how we have demonstrated STED in conjugated polymer films of MEH-PPV and CN-PPV by taking care to first understand the film's photophysical properties. This new approach provides a way to study exciton diffusion by utilizing subdiffraction optical excitation volumes. In this way, we will obtain a spatiotemporal map of exciton distributions that will help to correlate the energetic landscape to film morphology at the nanoscale. [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J41.00003: Probing the exciton coherent size in organic crystals and the effect of polar bonds Wai-Lun Chan, Ti Wang Exciton transport in organic crystals is commonly described by a series of incoherent hopping. This is no longer valid if the transport range is on the order of the exciton coherent size. Recently, it has been proposed that exciton delocalization is responsible for ultrafast charge separation found in bulk heterojunction (BHJ) photovoltaics. However, the coherent exciton transport range has not been measured. Here, by using time-resolved photoemission and zinc phthalocyanine crystals as a model system, we observe a transition from coherent to incoherent transport while the exciton coherent size is decreasing. The electron-vibration interaction is found to be the main driving force that reduces the exciton coherent size. Furthermore, by using phthalocyanine molecules with a polar bond, e.g. titanyl phthalocyanine, we observe an upshift in electron energy as a function of time after photo-excitation. Our observation can be explained by spontaneous exciton dissociation after photoexcitation, which produces a more delocalized charge transfer state. This would explain why BHJ made by `push-pull' polymers or small molecules, which consist of electron-rich and electron- deficient units, often have a better charge separation efficiency. [Preview Abstract] |
Tuesday, March 3, 2015 3:30PM - 3:42PM |
J41.00004: Fully three dimensional calculations of c-AFM current flow patterns, including space charge effects, traps and fibrous morphologies Kanokkorn Pimcharoen, Phillip Duxbury Organic semiconductors are promising materials for many optoelectronic devices due to their versatile applications and low-cost fabrication, including organic photovoltaics (OPV), light-emitting diodes (OLED), and thin-film transistors (OTFT). The performance of these devices are controlled by charge transport which primarily depends on the nanoscale morphology. Unlike other microscopies, conductive Atomic Force Microscope (c-AFM) is capable of characterizing both nanoscale morphology and local electronic properties simultaneously. With this unique ability, c-AFM has been used extensively to characterized these organic semiconductor devices in the past decade, however the spreading of current from the tip geometry in the presence of traps, which are ubiquitous in these materials, is not well understood. We have developed a fully three dimensional device simulation tool enabling treatment of inhomogeneous systems including c-AFM tip geometry, spatially varying trap distributions, and fibrous morphologies. Results characterizing charge transport in the fibrous morphologies and in the presence of traps will be discussed, including the effect of traps and space charge effects on current spreading from the c-AFM tip. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J41.00005: Interface structure of P3HT/SWNT blend and charge separation process on it Katsuhiko Nishimra, Ryota Jono, Mikiya Fujii, Koichi Yamashita We investigated the mechanism that suppression of charge recombinations takes place in blends of regioregular Poly-3-HexylThiophene (rrP3HT) and semiconducting Single Walled Carbon Nanotube (scSWNT) only if excess P3HT exists. The rrP3HT/scSWNT blend seems to be suitable for OPV application because rrP3HT is common acceptor material and also can be used for purifying scSWNT and removing metallic one. Then the suppression of charge recombinations is attributed to unique helical supramolecular structure at P3HT/SWNT interfaces.\footnote{S. D. Stranks et. al., {\bf Nano. Lett.} 11, 66} However, the detailed mechanism of the suppression has not been clarified yet. In this presentation, we show that side chains of P3HT are important in formation of the helical structure rather than alignment of main chains with graphene lattices of SWNT by using semi-empirical quantum chemistry method. Moreover, HOMO levels of P3HT molecules at the interfaces estimated to be lower than those in crystalline domain because of disordered stacking due to formation of the helical structure. This difference in HOMO levels can act as the driving force for escape of charge carriers from the interfaces and can result to the suppression of charge recombinations. [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J41.00006: Charge transfer type excitons at donor/acceptor interfaces of organic solar cells Azusa Muraoka, Koichi Yamashita The conversion of excitons into charge within organic solar cells is complicated by bound electronhole pairs, or charge transfer states at donor/acceptor interfaces. The solar cell requires generating an efficient current. Thus it is necessary that charge transfer is further separated into free charge carriers to be transported to electrode. We focus on the improved the conversion efficiency of Bulk-heterojunction organic solar cells. We use dependent density functional theory with CAM-B3LYP/6-31G(d) to study the oscillator strengths, electronic structure, HOMO-LUMO band gap and energy level in several polymer (donor) : fullerene (acceptor) blends, such as MDMO-PPV, PCDTBT, PCPDTBT, PBB3, PTB7 and PTBF2 with PC70BM. To determine the effective physical factor in light energy conversion, we consider ,(i) charge transfer type excitation generated directly by photoinduced electron transition in the donor/acceptor interface (ii) the factors for controlling the conversion efficiency such as short-circuit current density and closed circuit voltage. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J41.00007: On the theory of Carriers's Electrostatic Interaction near an Interface Michael Waters, Hossein Hashemi, John Kieffer Heterojunction interfaces are common in non-traditional photovoltaic device designs, such as those based small molecules, polymers, and perovskites [1]. We have examined a number of the effects of the heterojunction interface region on carrier/exciton energetics using a mixture of both semi-classical [2] and quantum electrostatic methods, \textit{ab initio} methods, and statistical mechanics. Our theoretical analysis has yielded several useful relationships and numerical recipes that should be considered in device design regardless of the particular materials system. As a demonstration, we highlight these formalisms as applied to carriers and polaron pairs near a C60/subphthalocyanine interface [1]. On the regularly ordered areas of the heterojunction, the effect of the interface is a significant set of corrections to the carrier energies, which in turn directly affects device performance. \\[4pt] [1] S.E. Morris et al, Organic Electronics, Vol. 15, 12, 2014\\[0pt] [2] J. D. Jackson, Classical Electrodynamics, 1962. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:54PM |
J41.00008: Combining and Correlating DC, Modulated, and Transient Measurement Techniques to Disentangle and Quantify Key Physical Properties for Organic Semiconductor Devices Invited Speaker: David Gundlach Organic thin film electronics offer the potential to significantly impact how humans interface with their surroundings and society in general. Substantial contributions over the past two decades in this highly multidisciplinary area of research have led to significant improvements in discrete device performance and several impressive advanced technology demonstrations. However, fundamental understanding and quantification of the physical properties and processes that govern device operation remains limited compared to conventional semiconductors, such as silicon. In this presentation I will discuss our recent development and application of combined and correlated optical-electrical measurement methods to obtain a more nuanced understanding and quantification of the critical properties and fundamental processes relevant to device operation. In particular, I will discuss the use of steady state and pulsed light techniques combined with modulated and DC electrical measurements tailored to the specific operating regimes and device structures of organic diodes (solar cells and light emitters) and transistors to provide greater understanding of charge injection, transport, lifetime, density, and recombination kinetics. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J41.00009: Special Effect of Polystyrene on Quenching Pyrene in presence of Nitro-Aromatics Explosives David Uhrig, Hyun-Sook Jang, Hyun-Seok Cho, John W. Van Zee, Mu-Pingh Nieh The pyrene (Py) excimer fluorescence and its time-dependent quenching by 2,4-DNT in Py/polystyrene (PS) binary thin films are investigated regarding different architectures (i.e., linear, centipede and 4-arm star) and molecular weights (i.e., 2.5K, 35K, 192K, 350K and 900K) of PS. Both molecular architecture and molecular weight do not show significant effect on either Py excimer fluorescence or its quenching. However, the presence of PS in Py evidently enhances the excimer quenching efficiency under the exposure of nitroaromatic molecules (2,4 dinitrotoluene, DNT). The results of cyclic voltammetry and UV-vis absorption suggest that PS can effectively reduce the lowest unoccupied molecular orbit (LUMO) level of Py/PS than that of Py alone and thus decrease the conduction band with 2,4-DNT --facilitating the photoinduced electron transfer (PET) from Py/PS to 2,4-DNT for fluorescence quenching. This interaction between Py and PS in confirmed by F\"{o}ster Resonance Energy Transfer (FRET) results in the enhanced S$_{0} \to $ S$_{2}$ excitation of Py in the range between 300 and 350 nm. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J41.00010: Transient magneto-photoinduced absorption study of singlet fission in low band gap copolymers Uyen Huynh, Z. Valy Vardeny We have observed the existence of singlet fission in thin films of low band gap (LBG) copolymers, PDTP-DFBT and PTB7, using the ultrafast optical pump/probe spectroscopy, probed at the energy range from IR to MIR. The singlet fission is the dissociation of a singlet exciton into two triplets through an intermediate triplet pair state (TT pair) in an overall singlet configuration; in the studied copolymers, it was observed to be very fast, in femtosecond time domain. The intermediate TT state, which dissociates into two separated triplets at later time, or recombines to the ground state appears instantaneously with the singlet exciton formation using our laser system that has $\sim$ 150 fs time resolution. The interplay between the rate of singlet fission into sTT pairs, triplet fusion back to singlet excitons and relaxation between the TT spin sublevels explains the obtained opposite pattern of the transient magnetic field response on the dynamics of singlet excitons and TT pairs. [Preview Abstract] |
(Author Not Attending)
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J41.00011: On the Origins of Reduced Nongeminate Recombination in Organic Semiconductor Blends Michael Heiber, Vladimir Dyakonov, Carsten Deibel Despite vast research on organic semiconducting materials and devices over the last two decades, significant gaps in fundamental understanding exist in key areas. A detailed understanding of charge carrier recombination is particularly important to promote efficient radiative recombination in light emitting diodes, minimize power conversion efficiency losses in photovoltaics, and enhance sensitivity in photodiodes. Bimolecular charge recombination in these devices is commonly described using the Langevin model, which assumes an encounter-limited process. However, blends for organic photovoltaics have often been measured to have major deviations from the Langevin model, most significantly a recombination rate that is several orders of magnitude less than expected. Here, transient experimental and computational simulation results are presented to help elucidate the origins of the unexpected nongeminate recombination dynamics, focusing on the role of morphology, charge carrier mobility, density of states, and charge-transfer state properties. We demonstrate that greatly reduced recombination rates are not an inherent property of phase separated systems and show how charge-transfer state properties can have a critical impact on the recombination dynamics. [Preview Abstract] |
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