51. Characterisation of charge carrier transport in thin organic semiconductor T layers by time-of-flight photocurrent measurementsGvido Bratina, Egon Pavlica, 2019, pregledni znanstveni članek Opis: The paper reviews recent advances in characterisation of charge carrier transport in organic semiconductor layers by time-of-flight photocurrent measurements, with the emphasis on the measurements of the samples with co-planar electrodes. These samples comprised an organic semiconductor layer whose thickness is on the order of a μm or less, and thus mimic the structures of organic thin film transistors. In the review we emphasise the importance of considering spatial variation of electric field in these, essentially two-dimensional structures, in interpretation of photocurrent transients. We review the experimental details of this type of measurements and give examples that demonstrate exceptional sensitivity of the method to minute concentration of electrically active defects in the organic semiconductors as well as the capability of probing charge transport along the channels of different mobility that reside in the same sample. Ključne besede: organic semiconductors, time of flight, mobiulity Objavljeno v RUNG: 24.10.2018; Ogledov: 3918; Prenosov: 0 Gradivo ima več datotek! Več... |
52. PHOTO-EXCITATION ENERGY INFLUENCE ON THE PHOTOCONDUCTIVITY OF ORGANIC SEMICONDUCTORSNadiia Pastukhova, 2018, doktorska disertacija Opis: 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. Ključne besede: 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 Objavljeno v RUNG: 08.10.2018; Ogledov: 5719; Prenosov: 166 Celotno besedilo (4,56 MB) |
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54. Prospects for lensed supernovae behind galaxy clusters with the James Webb Space TelescopeTanja Petrushevska, 2018, objavljeni povzetek znanstvenega prispevka na konferenci Opis: 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). Ključne besede: Strong lensing, Hubble constant, measuring expansion history with time delays, lensed supernovae Objavljeno v RUNG: 09.07.2018; Ogledov: 3777; Prenosov: 139 Celotno besedilo (4,14 MB) |
55. Negative field‐dependent charge mobility in crystalline organic semiconductors with delocalized transportFei Tong, Andrey Kadashchuk, Egon Pavlica, Gvido Bratina, 2018, izvirni znanstveni članek Opis: Charge-carrier mobility has been investigated by time-of-flight (TOF) transient photocurrent in a lateral transport con- figuration in highly crystalline thin films of 2,7-dioctyl[1]benzothieno [3,2-b][1] benzothiophene (C8-BTBT) grown by a zone-casting alignment technique. High TOF mobility has been revealed that it is consistent with the delocalized nature of the charge transport in this material, yet it featured a positive temperature dependence at T ≥ 295 K. Moreover, the mobility was surprisingly found to decrease with electric field in the high-temperature region. These observations are not compat- ible with the conventional band-transport mechanism. We have elaborated an analytic model based on effective-medium approximation to rationalize the puzzling findings. The model considers the delocalized charge transport within the energy landscape formed by long-range transport band-edge variations in imperfect organic crystalline materials and accounts for the field-dependent effective dimensionality of charge transport percolative paths. The results of the model calculations are found to be in good agreement with experimental data. Ključne besede: time of flight, organic semiconductors, single crystals Objavljeno v RUNG: 07.05.2018; Ogledov: 5488; Prenosov: 0 Gradivo ima več datotek! Več... |
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57. Introduction to Electronic Properties and Dynamics of Organic Complexes as Self‐Assembled MonolayersMaddalena Pedio, 2017, samostojni znanstveni sestavek ali poglavje v monografski publikaciji Opis: Self‐assembled monolayers (SAMs) of organic‐conjugated transition metal complexes on surfaces is a focus of both device engineering and basic science, since it is a key factor in nearly all important aspects of device performances, including operation voltages, degradation, and efficiency. The huge amount of literature results related to the first monolayer, and reorganization and self‐assembling processes are due to the general accepted result that structural and chemical properties of the first monolayer are the key parameters for controlled thin film growth. Optical and magneto‐electronic properties are intimately connected, and the accurate determination of electronic levels, excitation, and relaxation dynamics is mandatory for the optimization of electronic, photovoltaic, and opto‐electronic devices. Quite a number of electronic states is generated by the interaction of light with
complex organic molecules. Time‐resolved spectroscopies are a new investigation tool
that gives the possibility of correctly addressing their origin and life time. Examples of prototypical systems are presented and discussed. We review on complementary techniques, trying to single out how different approaches are fundamental to fully characterize
these complex systems. Ključne besede: self‐assembled monolayer (SAM), surface structures molecular layers, nanotechnology, electronic properties, spectroscopies, time resolved Objavljeno v RUNG: 12.06.2017; Ogledov: 4768; Prenosov: 208 Celotno besedilo (5,78 MB) |
58. Stability by linear approximation for time scale dynamical systemsSergey Kryzhevich, Alexander Nazarov, 2017, izvirni znanstveni članek Opis: 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. Ključne besede: Time scale system, Linearization, Lyapunov functions, Millionschikov rotations, Stability Objavljeno v RUNG: 15.03.2017; Ogledov: 4197; Prenosov: 160 Povezava na datoteko |
59. Real-Time Analysis sensitivity evaluation of the Cherenkov Telescope ArrayValentina Fioretti, Andrej Filipčič, Gašper Kukec Mezek, Samo Stanič, Serguei Vorobiov, Lili Yang, Gabrijela Zaharijas, Danilo Zavrtanik, Marko Zavrtanik, 2015, objavljeni znanstveni prispevek na konferenci Ključne besede: Cherenkov Telescope Array (CTA), CTA Real-Time Analysis (RTA), RTA sensitivity evaluation Objavljeno v RUNG: 16.02.2017; Ogledov: 4420; Prenosov: 201 Celotno besedilo (278,05 KB) |
60. Parallel waveform extraction algorithms for the Cherenkov Telescope Array Real-Time AnalysisAndrea Zoli, Andrej Filipčič, Gašper Kukec Mezek, Samo Stanič, Serguei Vorobiov, Lili Yang, Gabrijela Zaharijas, Danilo Zavrtanik, Marko Zavrtanik, 2015, objavljeni znanstveni prispevek na konferenci Ključne besede: Cherenkov Telescope Array (CTA), algorithms for the CTA real-time analysis, parallel waveform extraction Objavljeno v RUNG: 16.02.2017; Ogledov: 4357; Prenosov: 248 Celotno besedilo (151,69 KB) |