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Graphene/organic semiconductor systems
JINTA MATHEW, 2017, doctoral dissertation

Found in: osebi
Published: 15.09.2017; Views: 4806; Downloads: 101
.pdf Fulltext (41,84 MB)

Characterisation of charge carrier transport in thin organic semiconductor T layers by time-of-flight photocurrent measurements
Gvido Bratina, Egon Pavlica, 2019, review article

Abstract: 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.
Found in: osebi
Keywords: organic semiconductors, time of flight, mobiulity
Published: 24.10.2018; Views: 2524; Downloads: 0
.pdf Fulltext (2,53 MB)

Enhancement of Charge Transport in Polythiophene Semiconducting Polymer by Blending with Graphene Nanoparticles
Egon Pavlica, Gvido Bratina, 2019, original scientific article

Abstract: This paper describes a study on the charge transport in a composite of liquid‐exfoliated graphene nanoparticles (GNPs) and a polythiophene semiconducting polymer. While the former component is highly conducting, although it consists of isolated nanostructures, the latter offers an efficient charge transport path between the individual GNPs within the film, overall yielding enhanced charge transport properties of the resulting bi‐component system. The electrical characteristics of the composite layers were investigated by means of measurements of time‐of‐flight photoconductivity and transconductance in field‐effect transistors. In order to analyze both phenomena separately, charge density and charge mobility contributions to the conductivity were singled out. With the increasing GNP concentration, the charge mobility was found to increase, thereby reducing the time spent by the carriers on the polymer chains. In addition, for GNP loading above 0.2 % (wt.), an increase of free charge density was observed that highlights an additional key role played by doping. Variable‐range hopping model of a mixed two‐ and three‐dimensional transport is explained using temperature dependence of mobility and free charge density. The temperature variation of free charge density was related to the electron transfer from polythiophene to GNP, with an energy barrier of 24 meV.
Found in: osebi
Keywords: grafen, polimeri, transport električnega naboja, časovno odvisna fotoprevodnost
Published: 23.08.2019; Views: 1863; Downloads: 0
.pdf Fulltext (1,78 MB)

The role of charge transfer at reduced graphene oxide/organic semiconductor interface on the charge transport properties
Gvido Bratina, Egon Pavlica, 2019, original scientific article

Abstract: 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.
Found in: osebi
Keywords: organic semiconductors, reduced graphene oxide, time-resolved photocurrent measurements, organic thin film transistors
Published: 28.10.2019; Views: 2166; Downloads: 1
.pdf Fulltext (2,22 MB)

Elucidation of Donor:Acceptor Phase Separation in Nonfullerene Organic Solar Cells and Its Implications on Device Performance and Charge Carrier Mobility
Sebastian F. Hoefler, Georg Haberfehlner, Thomas Rath, Andreas Keilbach, Mathias Hobisch, Alexander Dixon, Egon Pavlica, Gvido Bratina, Gerald Kothleitner, Ferdinand Hofer, Gregor Trimmel, 2019, original scientific article

Abstract: In bulk-heterojunction solar cells, the device performance strongly depends on the donor and acceptor properties, the phase separation in the absorber layer, and the formation of a bicontinuous network. While this phase separation is well explored for polymer:fullerene solar cells, only little is known for polymer:nonfullerene acceptor solar cells. The main hurdle in this regard is often the chemical similarity of the conjugated polymer donor and the organic nonfullerene acceptor (NFA), which makes the analysis of the phase separation via atomic force microscopic (AFM) phase images or conventional transmission electron microscopy difficult. In this work, we use the donor polymer PTB7-Th and the small molecule acceptor O-IDTBR as the model system and visualized the phase separation in PTB7-Th:O-IDTBR bulk-heterojunctions with different donor:acceptor ratios via scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) based elemental mapping, which resulted in a good contrast between the donor and the acceptor despite very low differences in the chemical composition. AFM as well as grazing-incidence wide-angle X-ray scattering (GIWAXS) investigations support the electron microscopic data. Furthermore, we elucidate the implications of the phase separation on the device performance as well as charge carrier mobilities in the bulk-heterojunction layers, and a high performance of the solar cells was found over a relatively broad range of polymer domain sizes. This can be related to the larger domain sizes of the acceptor phase with higher amounts of O-IDTBR in the blend, while the polymer donor phase still forms continuous pathways to the electrode, which keeps the hole mobility at a relatively constant level.
Found in: osebi
Keywords: nanomorphology bulk-heterojunction scanning transmission electron microscopy organic photovoltaics charge carrier mobility
Published: 15.01.2020; Views: 1847; Downloads: 63
.pdf Fulltext (6,22 MB)

Characterization of organic semiconductor thin layers by transient photocurrent spectroscopy
Nadiia Pastukhova, Egon Pavlica, Gvido Bratina, 2019, published scientific conference contribution abstract

Found in: osebi
Keywords: organic semiconductors, thin layers, spectroscopy
Published: 10.02.2020; Views: 1944; Downloads: 0
.pdf Fulltext (865,49 KB)

Characterization of organic semiconductor thin layers by transient photocurrent spectroscopy
Nadiia Pastukhova, Egon Pavlica, Gvido Bratina, 2019, published scientific conference contribution abstract

Found in: osebi
Keywords: organic semiconductors, thin layers, spectroscopy
Published: 10.02.2020; Views: 1932; Downloads: 0
.pdf Fulltext (626,90 KB)

Molecular alignment on graphene surface determines transport properties of graphene/organic semiconductor transistors
Gvido Bratina, Egon Pavlica, Vadym Tkachuk, 2020, original scientific article

Abstract: Graphene field-effect transistor structures were used to investigate the role of molecular alignment on charge transport properties of heterostructures comprising a single-layer graphene and variable thickness of N,N′-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bisdicarboximide (PDI8-CN2) - an n-type organic semiconductor. Our atomic force microscopy data show that under selected growth conditions PDI8-CN2 grows in a layer-by-layer fashion up to a second monolayer. The first layer comprises flat-lying molecules, whereas the molecules in the second layer orient themselves in an upright orientation. Transconductance measurements show that the flat-lying molecules have little effect on the position of the Fermi level in graphene. Upright oriented molecules in the second layer instead, have a strong effect as to neutralize native p-type doping of graphene and cause a shift of charge-neutrality level towards the Dirac point. We interpret such behavior in terms of different orientation of the surface dipole on layers with different molecular orientations. At the same time the overall mobility of the charge carriers reaches values exceeding 3000 cm2/Vs.
Found in: osebi
Keywords: organski polprevodniki, grafen, transport električnega naboja
Published: 13.10.2020; Views: 1438; Downloads: 0
.pdf Fulltext (2,64 MB)

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