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1.
High charge carrier mobility in thin films of quasi-two-dimensional polyacetylenes with sulphuric inter-chain linkers
Nadiia Pastukhova, Gvido Bratina, Egon Pavlica, 2023, objavljeni povzetek znanstvenega prispevka na konferenci

Opis: Quasi-two-dimensional conjugated polymers (q2DCP) have been described and recognised as crystalline, one- or two-layer polymer nanosheets prepared by arranging linear conjugated polymer chains in a 2D plane via non-covalent interchain interactions.[1,2] The extension of polymer dimensionality to two dimensions improves the alignment of individual polymer layers and overcomes the limitations associated with charge carrier hopping between polymer chains in one-dimensional and crosslinked polymers [3] Compared to other two-dimensional materials such as graphene or transition metal dichalcogenides, q2DCPs offer a high degree of flexibility in chemical design and are compatible with liquid-based processing methods. Various q2DCPs have been synthesized by surface active monolayer-assisted interfacial synthesis (SMAIS) [5] The photoreaction of these materials is of particular interest due to their tunable properties such as band gap and associated wavelength-dependent photoexcitation, which enables a wide range of applications in optoelectronic devices. Using time-of-flight photoconductivity measurements (TOF-PC) [4], we investigate the charge transport properties of 2D polyacetylene prepared by the SMAIS method. A typical TOFP measurement of q2D polyacetylene is shown in Figure 1, using a focused nanosecond pulse laser at 325 nm and an electrode spacing of 250 µm. From the polarity of the bias voltage and the duration of the photocurrent, we can determine the polarity, velocity, and mobility of the photo-excited carriers as a function of the applied bias voltage and excitation wavelength. and observed electron mobility of 250 cm2/Vs, which is in the range of the most advanced organic single-crystal small-molecule semiconductors and almost an order of magnitude higher than linear polymeric semiconductors. We investigated the optical absorption and transmission on a lateral scale using scanning near-field optical microscopy (SNOM).
Ključne besede: 2D polymers, organic semiconductors, q2DPA, SNOM, time-of-flight photoconductivity
Objavljeno v RUNG: 25.03.2024; Ogledov: 520; Prenosov: 1
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Organic and flexible : lecture at Ženske v znanosti in za znanost, Ljubljana, 12. 2. 2024
Nadiia Pastukhova, 2024, prispevek na konferenci brez natisa

Opis: The development of organic semiconductors (OS) opens a new perspective and possibilities for novel device architectures including flexible and wearable electronics. One of the key parameters is the charge carriers' mobility. In OSs, it is affected by many factors, including molecular stacking, chemical impurities, temperature, pressure, electric field, and charge carrier density. These factors can affect the structural or energetic disorder. To overcome limitations, new polymers that extend the π-conjugation to two dimensions were developed. The two-dimensional network structure provides a high degree of structural stability and tunability of properties, while the organic molecules can be engineered to exhibit specific chemical and physical properties such as large surface area, pore size, and electronic properties. I will present our recent research, where quasi-2D polyacetylene (q2DPA) demonstrates high electron mobility along the direction of the layer, measured by the lateral time-of-flight photoconductivity (TOF) method.
Ključne besede: 2D polymers, time-of-flight photoconductivity, TOF, organic semiconductors
Objavljeno v RUNG: 22.03.2024; Ogledov: 597; Prenosov: 2
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Characterization of organic semiconductor thin layers by transient photocurrent spectroscopy
Nadiia Pastukhova, Egon Pavlica, Gvido Bratina, 2019, objavljeni povzetek znanstvenega prispevka na konferenci

Ključne besede: organic semiconductors, thin layers, spectroscopy
Objavljeno v RUNG: 10.02.2020; Ogledov: 3318; Prenosov: 0
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7.
Characterization of organic semiconductor thin layers by transient photocurrent spectroscopy
Nadiia Pastukhova, Egon Pavlica, Gvido Bratina, 2019, objavljeni povzetek znanstvenega prispevka na konferenci

Ključne besede: organic semiconductors, thin layers, spectroscopy
Objavljeno v RUNG: 10.02.2020; Ogledov: 3387; Prenosov: 0
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8.
The role of charge transfer at reduced graphene oxide/organic semiconductor interface on the charge transport properties
Gvido Bratina, Egon Pavlica, 2019, izvirni znanstveni članek

Opis: The effect of 1-pyrenesulfonicacid sodium salt (1-PSA), tetracyanoethylene (TCNE) and tetrafluoro- tetracyanoquinodimethane (F4-TCNQ) on charge transport properties of reduced graphene oxide (RGO) is examined by measuring the transfer characteristics of field-effect transistors and co-planar time-of-flight photocurrent technique. Evidence of p-type doping and a reduction of mobility of electrons in RGO upon deposition of these materials is observed. Time-resolved photocurrent measurements show a reduction in elec- tron mobility even at submonolayer coverage of these materials. The variation of transit time with different coverages reveals that electron mobility decreases with increasing the surface coverage of 1-PSA, TCNE and F4- TCNQ to a certain extent, while at higher coverage the electron mobility is slightly recovered. All three molecules show the same trend in charge carrier mobility variation with coverage, but with different magnitude. Among all three molecules, 1-PSA acts as weak electron acceptor compared to TCNE and F4-TCNQ. The additional fluorine moieties in F4-TCNQ provides excellent electron withdrawing capability compared to TCNE. The experimental results are consistent with the density functional theory calculations.
Ključne besede: organic semiconductors, reduced graphene oxide, time-resolved photocurrent measurements, organic thin film transistors
Objavljeno v RUNG: 28.10.2019; Ogledov: 3888; Prenosov: 1
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9.
Characterisation of charge carrier transport in thin organic semiconductor T layers by time-of-flight photocurrent measurements
Gvido 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: 3947; Prenosov: 0
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10.
PHOTO-EXCITATION ENERGY INFLUENCE ON THE PHOTOCONDUCTIVITY OF ORGANIC SEMICONDUCTORS
Nadiia 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: 5767; Prenosov: 166
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