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Time-of-flight photoconductivity investigation of high charge carrier mobility in ▫$Ti_3C_2T_x$▫ MXenes thin-film
Jurij Urbančič, Erika Tomsic, Manisha Chhikara, Nadiia Pastukhova, Vadym Tkachuk, Alex Dixon, Andraž Mavrič, Payam Hashemi, Davood Sabaghi, Ali Shaygan Nia, Gvido Bratina, Egon Pavlica, 2023, original scientific article

Abstract: Charge transport through a randomly oriented multilayered network of two-dimensional (2D) Ti3C2Tx (where Tx is the surface termination and corresponds to O, OH and F) was studied using time-of-flight photoconductivity (TOFP) method, which is highly sensitive to the distribution of charge carrier velocities. We prepared samples comprising Ti3C2Tx with thickness of 12 nm or 6-monolayers. MXene flakes of size up to 16 μm were randomly deposited on the surface by spin-coating from water solution. Using TOFP, we have measured electron mobility that reached values up to 279 cm2/Vs and increase with electric-field in a Poole-Frenkel manner. These values are approximately 50 times higher than previously reported field-effect mobility. Interestingly, our zero-electric-field extrapolate approaches electron mobility measured using terahertz absorption method, which represents intra-flake transport. Our data suggest that macroscopic charge transport is governed by two distinct mechanisms. The high mobility values are characteristic for the intra-flake charge transport via the manifold of delocalized states. On the other hand, the observed Poole-Frenkel dependence of charge carrier mobility on the electric field is typical for the disordered materials and suggest the existence of an important contribution of inter-flake hopping to the overall charge transport.
Keywords: charge transport, multilayered network, flakes, time-of-flight photoconductivity, MXene exfoliation, high-mobility solution-cast thin-film, semiconducting MXene
Published in RUNG: 31.03.2023; Views: 1448; Downloads: 4
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Charge transport characterization of P3HT thin-film organic semiconductor : Written report: in fulfilment of diploma seminar 1FAF29 requirement
Matija Filipčič, 2021, research project (high school)

Abstract: The focus of this diploma seminar is to describe different types of charge transport theory and models used for finding mobility in semiconductors, more specifically focused on P3HT thin-film organic semiconductor. It also describes the time-of-flight method, which was used to measure the experimental data for P3HT with time dependent current I(t) curve. Another data set was obtained by performing Kinetic Monte Carlo (KMC) simulations using Miller-Abrahams hopping formalism. KMC was used to determine the material disorder, simulate I(t) curve and transit time for every charge carrier. Simulated data, different transit times and mobilities were then finally compared with experimental, in order to find agreements between the two.
Keywords: charge transport, P3HT, thin-film organic semiconductor
Published in RUNG: 15.09.2021; Views: 2572; Downloads: 0
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5.
A pyrrolopyridazinedione-based copolymer for fullerene-free organic solar cells
Astrid-Caroline Knall, Samuel Rabensteiner, Sebastian F. Hoefler, Matiss Reinfelds, Mathias Hobisch, Heike M. A. Ehmann, Nadiia Pastukhova, Egon Pavlica, Gvido Bratina, Illie Hanzu, Shuguang Wen, Renqiang Yang, Gregor Trimmel, Thomas Rath, 2021, original scientific article

Abstract: The recent success of non-fullerene acceptors in organic photovoltaics also entails a change in the requirements to the polymer donor in terms of optical and morphological properties leading to a demand for novel conjugated polymers. Herein, we report on the synthesis of a 1,4-bis-(thiophene-2-yl)-pyrrolopyridazinedione based copolymer with 2-ethylhexyl substituents on the pyrrolopyridazinedione moiety. A 2D conjugated benzodithiophene (BDT) was chosen as comonomer. The resulting copolymer T-EHPPD-T-EHBDT showed a molecular weight of 10.2 kDa, an optical band gap of 1.79 eV, a hole mobility of 1.8 × 10−4 cm2 V−1 s−1 and a preferred face-on orientation with regard to the substrate. The comparably wide band gap as well as the determined energy levels (HOMO: −5.47 eV, LUMO: −3.68 eV) match well with the narrow band gap non-fullerene acceptor ITIC-F, which was used as the acceptor phase in the bulk heterojunction absorber layers in the investigated solar cells. The solar cells, prepared in inverted architecture, revealed power conversion efficiencies up to 7.4% using a donor:acceptor ratio of 1 : 1 in the absorber layer.
Keywords: non-fullerene solar cells, charge transport, charge mobility, power conversion efficiency
Published in RUNG: 27.01.2021; Views: 2786; Downloads: 0
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6.
The role of local potential minima on charge transport in thin organic semiconductor layers
Egon Pavlica, Raveendra Babu Penumala, Gvido Bratina, 2016, original scientific article

Abstract: We have performed a systematic study of dependence of time-resolved photocurrent on the point of charge excitation within the organic semiconductor channel formed by two coplanar metal electrodes. The results confirm that spatial variation of electric field between the electrodes crucially determines transport of photogenerated charge carriers through the organic layer. Time-of-flight measurements of photocurrent demonstrate that the transit time of photogenerated charge carrier packets drifting between the two electrodes decreases with increasing travelling distance. Such counterintuitive result cannot be reconciled with the spatial distribution of electric field between coplanar electrodes, alone. It is also in contrast to expected role of space-charge screening of external electric field. Supported by Monte Carlo simulations of hopping transport in disordered organic semiconductor layer, we submit that the space-charge screens the external electric field and captures slower charge carriers from the photogenerated charge carrier packet. The remaining faster carriers, exhibit velocity distribution with significantly higher mean value and shorter transit time.
Keywords: Charge transport, Organic semiconductors, Time of flight, Mobility, Traps
Published in RUNG: 23.12.2016; Views: 5607; Downloads: 0
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