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Abstract: In this work, we experimentally studied the influence of photoexcitation energy influence on the charge transport in organic semiconductors. Organic semiconductors were small molecules like corannulene, perylene and pentacene derivatives, polymers such as polythiophene and benzothiophene derivatives, and graphene, along with combinations of these materials in heterojunctions or composites. The first part of this study is focused on the photoexcitation energy influence on the transient photoconductivity of non-crystalline curved π-conjugated corannulene layers. The enhanced photoconductivity, in the energy range where optical absorption is absent, is deduced from theoretical predictions of corannulene gas-phase excited state spectra. Theoretical analysis reveals a consistent contribution involving transitions to Super Atomic Molecular Orbitals (SAMOs), a unique set of diffuse orbitals typical of curved π-conjugated molecules. More, the photoconductivity of the curved corannulene was compared to the π-conjugated planar N,N′-1H,1H- perfluorobutyldicyanoperylene-carboxydi-imide (PDIF-CN2), where the photoexcitation energy dependence of photocurrent closely follows the optical absorption spectrum. We next characterized charge transport in poly(3-hexylthiophene) (P3HT) layers deposited from solution. Our results indicate that time-of-flight (TOF) mobility depends on the photoexcitation energy. It is 0.4× 10 −3 cm 2 /Vs at 2.3 eV (530 nm) and doubles at 4.8 eV (260 nm). TOF mobility was compared to field-effect (FET) mobility of P3HT field-effect transistors (OFETs). The FET mobility was similar to the 2.3 eV excitation TOF mobility. In order to improve charge mobility, graphene nanoparticles were blended within a P3HT solution before the deposition. We found that the mobility significantly improves upon the addition of graphene nanoparticles of a weight ratio as low as 0.2 %. FET mobility increases with graphene concentration up to a value of 2.3× 10 −2 cm 2 /Vs at 3.2 %. The results demonstrate that phase segregation starts to influence charge transport at graphene concentration of 0.8 % and above. Hence, the graphene cannot form a bridged conduction channel between electrodes, which would cancel the semiconducting effect of the polymer composite. An alternative approach to enhance mobility is to optimize the molecular ordering of organic semiconductors. For that purpose, we studied an innovative nanomesh device. Free-standing nanomesh devices were used to form nanojunctions of N,N′- iiDioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) nanowires and crystalline bis(triisopropylsilylethinyl)pentacene (TIPS-PEN). We characterized the photocurrent response time of this novel nanomesh scaffold device. The photoresponse time depends on the photon energy. It is between 4.5 − 5.6 ns at 500 nm excitation wavelength and between 6.7 − 7.7 ns at 700 nm excitation wavelength. In addition, we found that thermal annealing reduces charge carrier trapping in crystalline nanowires. This confirms that the structural defects are crucial to obtaining high photon-to-charge conversion efficiency and subsequent transport from pn junction in heterostructured materials. Structural defects also influence the power conversion efficiency of organic heterostructured photovoltaics (OPVs). Anticipating that polymers with different backbone lengths produce different level of structural defects, we examined charge transport dependence on the molecular weight of poly[4,8-bis(5-(2- ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2- ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl] (PTB7-Th) from 50 kDa to 300 kDa. We found p-type hopping transport in PTB7-Th, characterized by 0.1 – 3× 10 −2 cm 2 /Vs mobility, which increases with temperature and electric field. The polymer molecular weight exhibits a non-trivial influence on charge transport. FET mobility in the saturation regime increases with molecular weight. A similar trend is observed in TOF mobility and FET mobility in the linear regime, except for the 100kDa polymer, which manifests in the highest mobility due to reduced charge trapping. The lowest trapping at the dielectric interface of OFET is observed at 200 kDa. In addition, the 200 kDa polymer exhibits the lowest activation energy of the charge transport. Although the 100 kDa polymer indicates the highest mobility, OPVs using the 200 kDa polymer exhibit the best performance in terms of power conversion efficiency.
Keywords: organic semiconductors, optical absorption spectroscopy, time-of-flight photoconductivity, transient photocurrent spectroscopy, organic thin film transistors, atomic force microscopy, superatomic molecular orbitals, pn heterojunction, organic nanowires, graphene, composites, charge mobility, charge trapping, temperature dependence, photodetector, photovoltaic, solar cell, organic electronics
Published in RUNG: 08.10.2018; Views: 5473; Downloads: 164
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Abstract: Galaxies, and clusters of galaxies, can act as gravitational lenses and magnify the light of objects behind them. The effect enables observations of very distant supernovae, that otherwise would be too faint to be detected by existing telescopes, and allows studies of the frequency and properties of these rare phenomena when the universe was young. Under the right circumstances, multiple images of the lensed supernovae can be observed, and due to the variable nature of the objects, the difference between the arrival times of the images can be measured. Since the images have taken different paths through space before reaching us, the time-differences are sensitive to the expansion rate of the universe. One class of supernovae, Type Ia, are of particular interest to detect. Their well known brightness can be used to determine the magnification, which can be used to understand the lensing systems. I will also report our discovery of the first resolved multiply-imaged gravitationally lensed supernova Type Ia. I will also show the expectations of search campaigns that can be conducted with future facilities, such as the James Webb Space Telescope (JWST) or the Wide-Field Infrared Survey Telescope (WFIRST).
Keywords: Strong lensing, Hubble constant, measuring expansion history with time delays, lensed supernovae
Published in RUNG: 09.07.2018; Views: 3550; Downloads: 139
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Abstract: We demonstrate that the degree of charge delocalization has a strong impact on polarization energy and thereby on the position of the transport band edge in organic semiconductors. This gives rise to long-range potential fluctuations, which govern the electronic transport through delocalized states in organic crystalline layers. This concept is employed to formulate an analytic model that explains a negative field dependence coupled with a positive temperature dependence of the charge mobility observed by a lateral time-of-flight technique in a high-mobility crystalline organic layer. This has important implications for the further understanding of the charge transport via delocalized states in organic semiconductors.
Keywords: organic semiconducotrs, time of flight, transport
Published in RUNG: 13.09.2017; Views: 4281; Downloads: 0
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Abstract: We study systems on time scales that are generalizations of classical differential or difference equations and appear in numerical methods. In this paper we consider linear systems and their small nonlinear perturbations. In terms of time scales and of eigenvalues of matrices we formulate conditions, sufficient for stability by linear approximation. For non-periodic time scales we use techniques of central upper Lyapunov exponents (a common tool of the theory of linear ODEs) to study stability of solutions. Also, time scale versions of the famous Chetaev’s theorem on conditional instability are proved. In a nutshell, we have developed a completely new technique in order to demonstrate that methods of non-autonomous linear ODE theory may work for time-scale dynamics.
Keywords: Time scale system, Linearization, Lyapunov functions, Millionschikov rotations, Stability
Published in RUNG: 15.03.2017; Views: 4009; Downloads: 160
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Keywords: Cherenkov Telescope Array (CTA), algorithms for the CTA real-time analysis, parallel waveform extraction
Published in RUNG: 16.02.2017; Views: 4217; Downloads: 248
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