Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C56: Organic Electronics and Photonics I: Charge TransportFocus
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Sponsoring Units: DPOLY DMP Chair: Elizabeth Von Hauff, Vrije Universiteit Amsterdam Room: LACC 515B |
Monday, March 5, 2018 2:30PM - 3:06PM |
C56.00001: Time-dependent Behavior of Molecular Transport Invited Speaker: Mark Ratner Single molecule electron transport has become a very large part of molecular study. In this talk, I will provide a time-dependent description of molecular transport. Almost all of the work in this area is based on energy dependence. The difference between energy and time dependence is striking. The behavior in the first 50 femtoseconds is striking–there are several time-dependent behaviors that I wouldn’t have imagined, especially those related to time-dependent quantum interference. The talk will describe how to control interference utilizing so-called Butikker probes, and how to understand the dynamics of interference in molecular transport. We will describe a wide set of molecular structures, and their interactions and correlations. |
Monday, March 5, 2018 3:06PM - 3:18PM |
C56.00002: Solid-State Polarization, Charge-Transfer Excitations and Transport Levels at Organic Interfaces from a Screened Range-Separated Hybrid Functional Leeor Kronik, Zilong Zheng, David Egger, Jean-Luc Bredas, Veaceslav Coropceanu We present a robust approach for the description of the energetics of charge-transfer (CT) excitations and transport levels at organic interfaces based on a screened range-separated hybrid (SRSH) functional.[1] We show that SRSH functionals correctly capture the effect of solid-state electronic polarization on the renormalization of the transport gap and on screening of the electron–hole interaction. Compared to calculations based on nonscreened optimally-tuned RSH (long-range corrected) functionals, the SRSH-based calculations can be performed for both isolated molecular complexes and systems embedded in a dielectric medium with the same range-separation parameter. This allows a clear physical interpretation of the results in terms of solid-state polarization without further perturbation of the molecular electronic structure. Results obtained with this approach for CT-state energies of the weakly interacting donor/acceptor complexes of pentacene/C60 and poly-3-hexylthiophene (P3HT)/PCBM compare very well with experimental data. |
Monday, March 5, 2018 3:18PM - 3:30PM |
C56.00003: Polaron Formation Mechanisms in Conjugated Polymers Joel Bombile, Michael Janik, Scott Milner In semiconducting polymers, polarons can form when excess charges are able to induce deformations of the surrounding medium, including local vibrational modes or dielectric polarization. These deformations then interact attractively with the charge, tending to localize it, and potentially affect charge transport. First, we investigate vibrational polaron formation in poly(3-hexylthiophene) [P3HT], with a tight-binding model for charges hopping between adjacent rings, coupled to ring distortions. We use density functional theory calculations to determine the “spring constant” for ring distortions and the coupling to the charge carrier. On single chains, we find only broad, weakly bound polarons by this mechanism. In 2d crystalline layers of P3HT, even rather weak transverse hopping between chains destabilizes this polaron. Then, we consider polarons stabilized by dielectric polarization, described semiclassically with a polarizable continuum interacting with the carrier wavefunction. In contrast to vibrational polarons, we find dielectrically stabilized polarons in P3HT are narrower, more strongly bound, and stable in 2d crystalline layers. The predicted stabilization energy matches charge modulation spectroscopic data, which are not well explained by vibrational polarons alone. |
Monday, March 5, 2018 3:30PM - 3:42PM |
C56.00004: Using Momentum Resolved Spectroscopies to Quantify Organic Semiconductor-Surface Plasmon Polariton Coupling in a Drastically Reorientable Small Molecule System Ryan DeCrescent, Steven Brown, David Nakazono, Sam Willenson, Niva Ran, Xiaofeng Liu, Guillermo Bazan, Thuc-Quyen Nguyen, Jon Schuller Momentum-resolved spectroscopies have proven to be extremely effective techniques for quantifying optical anisotropies intrinsic to organic semiconductor films. Such optical anisotropies profoundly impact light absorption and emission rates in photovoltaic and light-emitting devices. To date, a molecular system that can controllably adopt both “in-plane” (molecular backbones parallel to substrate interface) and “out-of-plane” molecular orientations has been elusive. Here, we use a combination of momentum-resolved spectroscopies and ellipsometry to demonstrate that small-molecule p-SIDT(FBTTh2)2 can controllably adopt both orientations when spin-cast from solution. We show, by comparing SPP dispersion measurements with theory, that this combination of techniques allows extraction of anisotropic optical constants with remarkable precision. Finally, we show that this molecular reorientation yields a two-fold increase in PL output for thin films on plasmonic substrates, and that this improvement agrees precisely with theory. These results highlight the importance of molecular orientation in the context of plasmonic device architectures. |
Monday, March 5, 2018 3:42PM - 3:54PM |
C56.00005: Multifunctional Electronic Materials for Thin Film Devices Sang Ha Yoo, Enrique Gomez Organic electronics have garnered a wide range of attention for their potential advantages such as bio-compatiblility, large area compatiblility, and multifunctionality. Herein, we present a polymer blend thin film with semi-conductive, adhesive, and water-proof characteristics that can be utilized as an interfacial layer in three-dimensional organic-based thin film electronic devices for bioelectronics applications. By mixing a semiconducting polymer with conjugated backbone and an adhesive polymer with strong underwater adhesion, we were able to fabricate phase-separated thin films that possess a strong bonding force and exhibit semiconducting behavior applicable to organic thin film transistors (OTFT). Electrical testing demonstrated that blend films of 50 wt% semiconductor and 50 wt% adhesive retains ~ 85% of average saturation mobility of that of 100 wt% semiconductor films. In addition, a 73% reduction in average hysteresis was observed in the blend films compared to that of the neat semiconductor films. Characterizations under spectroscopic ellipsometry and secondary ion mass spectroscopy suggest a vertical separating behavior between the semiconducting layer and the adhesive layer, which allows for multifunctionality of the blend film. |
Monday, March 5, 2018 3:54PM - 4:06PM |
C56.00006: Correlation between microstructural changes and electrical properties in organic semiconductors using in operando X-ray scattering Elayne Thomas, Michael Brady, Hidenori Nakayama, Eunhee Lim, Rachel Segalman, Michael Chabinyc Some transport models in doped organic semiconductors describe electrical conductivity using a static density of states. It is important to consider changes in the DOS due to both carrier/counter-ion Coulombic interactions and microstructural variation at high doping levels. We have explored the structural evolution of poly(3-hexylthiophene), a p-type organic semiconductor, in a thin-film transistor geometry using a polymeric ionic liquid (PIL) dielectric. PILs comprise of charged polymer sidechains and a mobile counter-ion, allowing only the counter-ion to diffuse on experimentally relevant timescales. We observe through in operando X-ray scattering (GIWAXS) that changes in domain structure of P3HT are dependent on the sign and magnitude of the applied gate voltage; bias less than |1.2| V, or 3x1019 cm–3, do not cause structural changes, while larger bias swells the polymer crystallites due to ion infiltration. Electrical conductivity follows a similar trend, increasing dramatically at |1.2| V from 5x10–3 S/cm to 1.5 S/cm. This work shows that substantial differences exist between a doped polymer and its insulating state, signifying the importance of integrating doping-induced disorder into organic semiconductor transport models. |
Monday, March 5, 2018 4:06PM - 4:18PM |
C56.00007: Ultrathin C8-BTBT Films for 2D Organic Transistors Polina Pechnikova, Sara Pazoki, Daniel Dougherty
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Monday, March 5, 2018 4:18PM - 4:30PM |
C56.00008: Improved Static Charge Stability from Fluorinated Polystyrenes in Multilayer Polystyrene Electrets Evan Plunkett, Qingyang Zhang, Daniel Reich, Howard Katz Thin film polymer electrets are of considerable interest for applications in organo-electronic systems such as piezoelectrics for energy harvesting and nonvolatile field effect transistors as memory, sensing, and logic elements. Copolymerized combinations of substituted styrenes can be used to produce multifunctional polymers with tunable electronic and structural properties. In multilayered dielectric stacks, these modified polymers demonstrate stable and controlled effects on adjacent semiconductors. Poly(3-trifluoromethylstyrene) and related copolymers exhibit layer position-dependent effects on the initial performance of thin film pentacene-based transistors, more consistent performance compared with unmodified polystyrene dielectrics, and large threshold voltage shifts (ΔVth= O(30 V)) after charging by application of static electric fields. In-situ dynamic measurements of transistor properties show these trifluoromethylated materials confer greater stability of electronic effects in terms of the magnitude of effect, biasing from device operation, and response to multiple charging and discharging cycles. |
Monday, March 5, 2018 4:30PM - 4:42PM |
C56.00009: Correlating charge transport to structure in deconstructed diketopyrrolopyrrole molecules: A case study of a monomer Alec Pickett, Tushita Mukhopadyay, Andreas Lauritzen, Amrit Laudari, Mika Torkkeli, Jakob Kjelstrup-Hansen, Satish Patil, Matti Knaapila, Suchismita Guha Organic co-polymer semiconductors based on a diketopyrrolopyrrole (DPP) core have attracted a lot of attention due to their high p-type as well as n-type carrier mobilities in organic field-effect transistors (FETs) and high power conversion efficiencies in solar cell structures. FETs were fabricated from both dropcasted and evaporated n-dialkyl side-chain substituted thiophene DPP (Ph-TDPP-Ph) films; the device performance is sensitive to the film thickness as well as the film deposition method. Some of the best FET performances are with evaporated films with p-type carrier mobilities on the order of 0.1 cm2/Vs and on/off ratios of 106. As measured from grazing incidence X-ray diffraction (GIXRD) in bottom-gate, bottom-contact FETs, Ph-TDPP-Ph thin film reveals a triclinic crystal structure with pi-conjugated stacking direction orientated in-plane. Depending on processing, the unit cell comprises either two or one monomer and may thus signal a shift from herringbone to coplanar orientation of the molecules. |
Monday, March 5, 2018 4:42PM - 4:54PM |
C56.00010: Polarization Modulation in Ferroelectric Organic Field-Effect Transistors Amrit Laudari, Alessandro R. Mazza Mazza, Alexander Daykin, Soma Khanra, Kartik Ghosh, Franscious Cummings, Theophillus Muller, Paul Miceli, Suchismita Guha The effect of polarization modulation of the gate dielectric on the performance of metal-oxide-semiconductor field-effect transistors has been investigated for more than a decade. However, there are no comparable studies in the area of organic field-effect transistors (FETs) using polymer ferroelectric dielectrics. We demonstrate the effect of polarization rotation in a relaxor ferroelectric dielectric, poly(vinylidene fluoride trifluorethylene (PVDF-TrFE), on the performance of small molecule based organic FETs. The subthreshold swing and other transistor parameters in organic FETs can be controlled in a reversible fashion by switching the polarization direction in the PVDF-TrFE layer. X-ray diffraction and electron microscopy images from PVDF-TrFE reveal changes in the ferroelectric phase and domain size, respectively, upon rotating the external electric field by 90°. The structural changes corroborate density-functional theoretical studies of an oligomer of the ferroelectric molecule in the presence of an applied electric field. The strategies enumerated here for polarization orientation of the polymer ferroelectric dielectric are applicable for a wide range of polymeric and organic transistors. |
Monday, March 5, 2018 4:54PM - 5:06PM |
C56.00011: Intrinsic Conduction Mechanism in Polymer Nanofibers Yung Park, K. H. Kim, H. Kang, Samuel Lara-Avila, Sergey Kubatkin The I-V characteristics on pristine polymer nanofibers and carbonized polymer nanofibers show the similar power law scaling behavior despite the structural differences: the former quasi one-dimensional and the latter quasi-amorphous carbon networks of two- or three- dimensional. The power law scaling in carbonized polymer nanofibers is interpreted by the Efros-Shklovskii variable range hopping (ES-VRH), which we suggest as the main transport mechanism of pristine polymer nanofibers[1]. The magneto resistance (MR), however, shows distinct behavior between the two materials. The pristine polyacetylene nanofibers show zero magneto resistance (ZMR) in high electric field, E > 3 x 104 V/cm. But the MR of carbonized polyacetylene nanofibers remains positive which is typical for the VRH conduction. The ZMR observed in polyacetylene nanofibers in high electric field is explained with the de-confined conduction of spinless charged soliton which is a 1-D topological insulator. |
Monday, March 5, 2018 5:06PM - 5:18PM |
C56.00012: Interface-engineered electrical transport properties in benzenedithol self-assembled monolayer molecular junctions using chemically p-doped graphene electrodes Yeonsik Jang, Sung-Joo Kwon, Jaeho Shin, Hyunhak Jeong, Wang-Taek Hwang, Junwoo Kim, Jeongmin Koo, Gunuk Wang, Tae-Woo Lee, Takhee Lee In this study, we fabricated molecular junctions consisting of self-assembled monolayers of benzenedithiol (BDT) using p-doped multi-layer graphene electrode. The p-type doping of graphene film was done by treating pristine graphene (work function of ~4.40 eV) with trifluoromethanesulfonic (TFMS) acid, resulting in an increased work function (~5.23 eV). The chemically p-doped graphene-electrode molecular junctions statistically showed an order of magnitude higher current density with a lower charge injection barrier height than those of pristine graphene-electrode molecular junctions, as a result of interface engineering. This enhancement is due to the increased work function of TFMS-treated p-doped graphene electrode in highest occupied molecular orbital (HOMO)-mediated tunneling molecular junctions. The validity of these results was proven by theoretical analysis based on coherent transport model considering asymmetric couplings at electrode-molecule interfaces. |
Monday, March 5, 2018 5:18PM - 5:30PM |
C56.00013: Revealing Spin-Orbit Coupling Effects in π-Conjugated Polymers by Multifrequency Electrically Detected Magnetic Resonance Gajadhar Joshi, Mandefro Teferi, Richards Miller, Shirin Jamali, Douglas Baird, Johan Tol, Hans Malissa, John Lupton, Christoph Boehme Charge carriers in organic semiconductors typically have weak spin-orbit coupling (SOC), however, for some magneto-opto-electronic materials properties, SOC is non-negligible. To investigate charge carrier SOC effects in π-conjugated polymer thin films, we carried out electrically detected magnetic resonance (EDMR) spectroscopy on bipolar injection devices made of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), the related poly-phenylenevinylene SY-PPV, polyfluorene (PFO) and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) over a broad magnetic field range [1]. We observed strong broadening of the charge carrier EDMR lines with increasing magnetic field, reflecting the increasing influence of SOC induced g-factor distributions. The analysis of this broadening allowed to test theoretical density functional theory predictions of charge carrier wave functions and an assignment of electrons and holes to the different observed lines. Furthermore, access to these g-factor distributions also allows for predictions of the magneto-resistance characteristics of the studied materials. [1] G. Joshi et al., Appl. Phys. Lett. 109, 103303 (2016). |
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