A. Kadashchuk - Academia.edu (original) (raw)
Papers by A. Kadashchuk
Interplay between hopping and band transport in high-mobility disordered semiconductors at large carrier concentrations: The case of the amorphous oxide InGaZnO
Physical Review B, 2016
Defect states in Rubrene films grown by Hot Wall Epitaxy
Physical Review B, 2014
We have extended an effective medium approximation theory [Fishchuk, Kadashchuk, Genoe, Ullah, Si... more We have extended an effective medium approximation theory [Fishchuk, Kadashchuk, Genoe, Ullah, Sitter, Singh, Sariciftci, and Bässler, Phys. Rev. B 81, 045202 (2010)] to investigate how polaron formation affects the Meyer-Neldel (MN) compensation behavior observed for temperature-dependent charge-carrier transport in disordered organic semiconductors at large carrier concentrations, as realized in organic field-effect transistors (OFETs). We show that the compensation behavior in organic semiconductor thin films can be consistently described for both nonpolaronic and polaronic hopping transport in the framework of the disorder formalism using either Miller-Abrahams or polaron Marcus rates, respectively, provided that the polaron binding energy is small compared to the width of the density of states (DOS) distribution in the system. We argue that alternative models based on thermodynamic reasoning, like the multiexcitation entropy (MEE) model, which assumes charge transport dominated by polarons with multiphonon processes and ignores the energy disorder, are inherently not applicable to describe adequately the charge-carrier transport in disordered organic semiconductors. We have suggested and realized a test experiment based on measurements of the compensation behavior for the temperature-dependent conductivity and mobility in OFET devices to check the applicability of these models. We point out that the MN behavior observed in thin-film OFETs has nothing to do with the genuine MN rule predicted by the MEE approach, but rather it is an apparent effect arising as a consequence of the functional dependence of the partial filling of the DOS in a disordered system with hopping transport. This fact is fully supported by experimental results. The apparent MN energy was found to depend also on the shape of the DOS distribution and polaron binding energy.
Publisher's Note: Origin of Meyer-Neldel type compensation behavior in organic semiconductors at large carrier concentrations: Disorder versus thermodynamic description [Phys. Rev. B 90 , 245201 (2014)]
Physical Review B, 2015
Localized Trions as Metastable Charged States in Conjugated Polymers
Molecular Crystals and Liquid Crystals, 2007
ABSTRACT We have shown that a charged trion consisting of two on-chain polarons of the same sign ... more ABSTRACT We have shown that a charged trion consisting of two on-chain polarons of the same sign and a trapped polaron of the opposite sign can be metastable with respect to both dissociation and recombination. A trion can be created upon fusion of a free polaron with a neutral pair, which eliminates a high Coulomb potential barrier for fusion of two charge carriers into a bipolaron. We argue that trions can be created by photoexcitation of a conjugated polymer, in which illumination creates a high density of geminate pairs. Since the metastable trion is anchored by a deep-trapped charge, it is immobile and can be considered as a coulombically trapped bipolaron, although the last may not exist as a free particle. We employ the concept of localized trions in order to explain the reversible photoinduced fatigue of thermally stimulated luminescence in films of poly[methyl(phenyl)silylene].
Charge Transport in Disordered Organic Semiconductors
From Single Molecules to Single Crystals, 2005
Nondispersive polaron transport in disordered organic solids
Physical Review B, 2003
An analytical theory based on the effective medium approach is formulated to describe nondispersi... more An analytical theory based on the effective medium approach is formulated to describe nondispersive hopping charge transport in a disordered organic material where polaron effects are important. The treatment of polaron transport in solids with superimposed disorder and polaron effects is based on the Marcus jump rate equation, while the conventional Miller-Abrahams formalism is used to describe charge mobility in polaron-free systems. It is shown that the Poole-Frenkel-type field dependence of mobility ln mu∝(E) occurs for both the bare charge carrier and the polaron transport provided that energetic correlation effects have been taken into account. We show that our polaron model can quantitatively explain the observed magnitudes of temperature- and electric-field-dependent polaron mobilities assuming physically reasonable values of polaron binding energies and transfer integrals; it gives a background for the development of the method for estimation of polaron binding energy and the energetic disorder parameter from these dependences. The results of the calculations are found to be in good agreement with both experimental results obtained for some sigma-conjugated polysilylenes where polaron formation was straightforwardly demonstrated and recent computer simulations of polaron transport.
Spectroscopy of Defects in Epitaxially Grown Para-sexiphenyl Nanostructures
Springer Proceedings in Physics, 2009
We present a study of steady-state-and time-resolved photoluminescence of para-sexiphenyl (PSP) f... more We present a study of steady-state-and time-resolved photoluminescence of para-sexiphenyl (PSP) films on KCl grown by organic molecular beam epitaxy (OMBE). Using different OMBE growth conditions has enabled us to vary greatly the morphology of the ...
Physical Review B, 2001
An analytical model of thermally stimulated photoluminescence ͑TSPL͒ in a random hopping system i... more An analytical model of thermally stimulated photoluminescence ͑TSPL͒ in a random hopping system is formulated. The model is based on the assumption that TSPL originates from radiative recombination of sufficiently long geminate pairs of charge carriers created during photoexcitation of the sample at a low ͑helium͒ temperature. Since TSPL measurements are normally performed after some dwell time, the initial energy distribution of localized carriers is formed after low-temperature hopping relaxation of photogenerated carriers and, therefore, the first thermally assisted jumps of relaxed carriers are considered as the rate-limiting steps in the present model. Predictions of the model are found to be in good quantitative agreement with experimental data on molecularly doped polymers if a double-peak energy distribution of localized states is invoked for these materials. Comparing theoretical results with existing experimental data also reveals a somewhat slower low-temperature energy relaxation of charge carriers in these materials than predicted by the conventional theory of carrier random walk in random hopping systems. TSPL was also measured in a methyl-substituted ladder-type poly͑paraphenylene͒. Both fluorescence and phosphorescence were found to contribute to the TSPL spectrum whereas only the latter was observed in the long isothermal afterglow following photoexcitation of the sample at helium temperature. This implies that the binding energy of a short off-chain geminate pair is higher than the binding energy of a singlet excitation but lower than that of a triplet exciton. The experimentally observed TSPL curve reveals an unusually low-temperature peak with the maximum at around 40 K. Interpretation of the experiment based on the hopping model of TSPL yields an effective density of states width of 0.055 eV, in good agreement with the value of 0.050 eV estimated from transport measurements.
Polarons in Polysilanes: Theoretical Background and Experimental Detection
2005 12th International Symposium on Electrets, 2005
Charge carrier transport in disorder wide band-gap organics occurs by hopping. However, in many o... more Charge carrier transport in disorder wide band-gap organics occurs by hopping. However, in many organic solids polaron effects arc important. The strong electron-phonon coupling causes carrier self-trapping and creates a quasiparticle, a polaron, which can move to on adjacent molecule only by carrying along the associated molecular deformation. On the example of polysilanes it is shown how the temperature and electric field dependences of charge carrier mobility can be utilized for the distinguishing between polaron and polaron-free transport. Another prove follows from thermally stimulated luminescence - the test is based on the assumption that the energy relaxation of polarons is much slower than that of charge carriers in a similarly disordered but rigid hopping system.
Thermally Stimulated Luminescence In Electroluminescent Polymers
Towards highly-ordered thin crystalline organic films: interrelation between film morphology and PL properties
Triplet Excitation Scavenging in Solid-State Organic Materials
4. Electric field confinement effect on charge transport in polycrystalline organic field-effect transistors
Light-induced charge transfer in nominally pure and iron doped Sn 2 P 2 S 6: a study by combined EPR-optical absorption spectroscopy
ABSTRACT
Thermally stimulated luminescence in electroluminescent pi-conjugated polymers
INTERNATIONAL WINTERSCHOOL FOR BIOELECTRONICS BioEl 2015
Nonlinear Optics of Liquid and Photorefractive Crystals II, 1998
ABSTRACT The low-temperature thermoluminescense (TSL) technique has been applied for probing the ... more ABSTRACT The low-temperature thermoluminescense (TSL) technique has been applied for probing the energetic disorder of localized states in poly(methyl-phenylsilylene) (PMPSi) combining the TSL studies and transport properties as determined by photocurrent transients. The results are described in terms of the disorder model and the energetic relaxation of photogenerated charge carriers within a manifold of the states of Gaussian distribution. Analysis both of the energetic position of the TSL peak maximum and the shape of its high-energy wing allowed to extract a parameter characterized the intrinsic energetic disorder in PMPSi, the value of which coincided well with the width of the density-of-states determined from transport measurements. The interpretation of results leads to conclusion that although the disorder is the dominant factor, the polaron contribution should be taken into account for the description of the charge carrier transport in PMPSi. The problem of the photochemical stability of PMPSi as well as the appearance of new trapping centers, is briefly outlined.
AIP Conference Proceedings, 2014
We developed an analytical model to describe hopping conductivity and mobility in organic semicon... more We developed an analytical model to describe hopping conductivity and mobility in organic semiconductors including both energetic disorder and polaronic contributions. The model is based on the Marcus jump rates with a Gaussian energetic disorder, and it is premised upon a generalized Effective Medium approach yet avoids shortcoming involved in the effective transport energy or percolation concepts. The carrier concentration dependence becomes considerably weaker when the polaron energy increases relative to the disorder energy, indicating the absence of universality that is at variance with recent publications.
Highly Efficient Intrinsic Phosphorescence from a sigma-Conjugated Poly(silylene) Polymer
Journal of Physical Chemistry C, 2014
ABSTRACT We have observed highly efficient intrinsic phosphorescence of a neat σ-conjugated polym... more ABSTRACT We have observed highly efficient intrinsic phosphorescence of a neat σ-conjugated polymer, poly-(biphenyl-4-ylmethylsilylene) (PBMSi). At low temperatures, PBMSi solid films featured ∼15% phosphorescence quantum yield, which is unusually high for purely organic conjugated polymers and is comparable to that of organometallic polymers. Exciton dynamics in PBMSi was studied by ultrafast fluorescence and time-gated delayed emission measurements. It was shown that the phosphorescence of PBMSi originates from the radiative decay of triplets on the π-conjugated biphenyl group constituting the lowest triplet state, T1 , which is populated under the excitation of the σ-conjugated polymer backbone, i.e., with energy well below the lowest singlet excited state of the biphenyl group itself. The nature of the excited states in PBMSi was further investigated by performing quantum-mechanical calculations of the model compound. The calculations showed that the lowest singlet excited state has charge-transfer (CT) character involving different parts of the same macromolecule. Energetically this state lies very close to the CT triplet excited state. We argue that the intramolecular CT state is responsible for the strongly enhanced intersystem crossing (ISC) in PBMSi due to the small positive CT singlet−triplet energy splitting, which is itself a consequence of a weak exchange interaction of a spatially separated electron and hole in the CT state. This study suggests a new molecular-level engineering approach for enhancement of the ISC, enabling efficient conversion of primary excited singlets into triplets in conjugated polymers without involving a heavy atom effect while leaving the rate of radiative T1 → S0 transition virtually unaffected.
Interplay between hopping and band transport in high-mobility disordered semiconductors at large carrier concentrations: The case of the amorphous oxide InGaZnO
Physical Review B, 2016
Defect states in Rubrene films grown by Hot Wall Epitaxy
Physical Review B, 2014
We have extended an effective medium approximation theory [Fishchuk, Kadashchuk, Genoe, Ullah, Si... more We have extended an effective medium approximation theory [Fishchuk, Kadashchuk, Genoe, Ullah, Sitter, Singh, Sariciftci, and Bässler, Phys. Rev. B 81, 045202 (2010)] to investigate how polaron formation affects the Meyer-Neldel (MN) compensation behavior observed for temperature-dependent charge-carrier transport in disordered organic semiconductors at large carrier concentrations, as realized in organic field-effect transistors (OFETs). We show that the compensation behavior in organic semiconductor thin films can be consistently described for both nonpolaronic and polaronic hopping transport in the framework of the disorder formalism using either Miller-Abrahams or polaron Marcus rates, respectively, provided that the polaron binding energy is small compared to the width of the density of states (DOS) distribution in the system. We argue that alternative models based on thermodynamic reasoning, like the multiexcitation entropy (MEE) model, which assumes charge transport dominated by polarons with multiphonon processes and ignores the energy disorder, are inherently not applicable to describe adequately the charge-carrier transport in disordered organic semiconductors. We have suggested and realized a test experiment based on measurements of the compensation behavior for the temperature-dependent conductivity and mobility in OFET devices to check the applicability of these models. We point out that the MN behavior observed in thin-film OFETs has nothing to do with the genuine MN rule predicted by the MEE approach, but rather it is an apparent effect arising as a consequence of the functional dependence of the partial filling of the DOS in a disordered system with hopping transport. This fact is fully supported by experimental results. The apparent MN energy was found to depend also on the shape of the DOS distribution and polaron binding energy.
Publisher's Note: Origin of Meyer-Neldel type compensation behavior in organic semiconductors at large carrier concentrations: Disorder versus thermodynamic description [Phys. Rev. B 90 , 245201 (2014)]
Physical Review B, 2015
Localized Trions as Metastable Charged States in Conjugated Polymers
Molecular Crystals and Liquid Crystals, 2007
ABSTRACT We have shown that a charged trion consisting of two on-chain polarons of the same sign ... more ABSTRACT We have shown that a charged trion consisting of two on-chain polarons of the same sign and a trapped polaron of the opposite sign can be metastable with respect to both dissociation and recombination. A trion can be created upon fusion of a free polaron with a neutral pair, which eliminates a high Coulomb potential barrier for fusion of two charge carriers into a bipolaron. We argue that trions can be created by photoexcitation of a conjugated polymer, in which illumination creates a high density of geminate pairs. Since the metastable trion is anchored by a deep-trapped charge, it is immobile and can be considered as a coulombically trapped bipolaron, although the last may not exist as a free particle. We employ the concept of localized trions in order to explain the reversible photoinduced fatigue of thermally stimulated luminescence in films of poly[methyl(phenyl)silylene].
Charge Transport in Disordered Organic Semiconductors
From Single Molecules to Single Crystals, 2005
Nondispersive polaron transport in disordered organic solids
Physical Review B, 2003
An analytical theory based on the effective medium approach is formulated to describe nondispersi... more An analytical theory based on the effective medium approach is formulated to describe nondispersive hopping charge transport in a disordered organic material where polaron effects are important. The treatment of polaron transport in solids with superimposed disorder and polaron effects is based on the Marcus jump rate equation, while the conventional Miller-Abrahams formalism is used to describe charge mobility in polaron-free systems. It is shown that the Poole-Frenkel-type field dependence of mobility ln mu∝(E) occurs for both the bare charge carrier and the polaron transport provided that energetic correlation effects have been taken into account. We show that our polaron model can quantitatively explain the observed magnitudes of temperature- and electric-field-dependent polaron mobilities assuming physically reasonable values of polaron binding energies and transfer integrals; it gives a background for the development of the method for estimation of polaron binding energy and the energetic disorder parameter from these dependences. The results of the calculations are found to be in good agreement with both experimental results obtained for some sigma-conjugated polysilylenes where polaron formation was straightforwardly demonstrated and recent computer simulations of polaron transport.
Spectroscopy of Defects in Epitaxially Grown Para-sexiphenyl Nanostructures
Springer Proceedings in Physics, 2009
We present a study of steady-state-and time-resolved photoluminescence of para-sexiphenyl (PSP) f... more We present a study of steady-state-and time-resolved photoluminescence of para-sexiphenyl (PSP) films on KCl grown by organic molecular beam epitaxy (OMBE). Using different OMBE growth conditions has enabled us to vary greatly the morphology of the ...
Physical Review B, 2001
An analytical model of thermally stimulated photoluminescence ͑TSPL͒ in a random hopping system i... more An analytical model of thermally stimulated photoluminescence ͑TSPL͒ in a random hopping system is formulated. The model is based on the assumption that TSPL originates from radiative recombination of sufficiently long geminate pairs of charge carriers created during photoexcitation of the sample at a low ͑helium͒ temperature. Since TSPL measurements are normally performed after some dwell time, the initial energy distribution of localized carriers is formed after low-temperature hopping relaxation of photogenerated carriers and, therefore, the first thermally assisted jumps of relaxed carriers are considered as the rate-limiting steps in the present model. Predictions of the model are found to be in good quantitative agreement with experimental data on molecularly doped polymers if a double-peak energy distribution of localized states is invoked for these materials. Comparing theoretical results with existing experimental data also reveals a somewhat slower low-temperature energy relaxation of charge carriers in these materials than predicted by the conventional theory of carrier random walk in random hopping systems. TSPL was also measured in a methyl-substituted ladder-type poly͑paraphenylene͒. Both fluorescence and phosphorescence were found to contribute to the TSPL spectrum whereas only the latter was observed in the long isothermal afterglow following photoexcitation of the sample at helium temperature. This implies that the binding energy of a short off-chain geminate pair is higher than the binding energy of a singlet excitation but lower than that of a triplet exciton. The experimentally observed TSPL curve reveals an unusually low-temperature peak with the maximum at around 40 K. Interpretation of the experiment based on the hopping model of TSPL yields an effective density of states width of 0.055 eV, in good agreement with the value of 0.050 eV estimated from transport measurements.
Polarons in Polysilanes: Theoretical Background and Experimental Detection
2005 12th International Symposium on Electrets, 2005
Charge carrier transport in disorder wide band-gap organics occurs by hopping. However, in many o... more Charge carrier transport in disorder wide band-gap organics occurs by hopping. However, in many organic solids polaron effects arc important. The strong electron-phonon coupling causes carrier self-trapping and creates a quasiparticle, a polaron, which can move to on adjacent molecule only by carrying along the associated molecular deformation. On the example of polysilanes it is shown how the temperature and electric field dependences of charge carrier mobility can be utilized for the distinguishing between polaron and polaron-free transport. Another prove follows from thermally stimulated luminescence - the test is based on the assumption that the energy relaxation of polarons is much slower than that of charge carriers in a similarly disordered but rigid hopping system.
Thermally Stimulated Luminescence In Electroluminescent Polymers
Towards highly-ordered thin crystalline organic films: interrelation between film morphology and PL properties
Triplet Excitation Scavenging in Solid-State Organic Materials
4. Electric field confinement effect on charge transport in polycrystalline organic field-effect transistors
Light-induced charge transfer in nominally pure and iron doped Sn 2 P 2 S 6: a study by combined EPR-optical absorption spectroscopy
ABSTRACT
Thermally stimulated luminescence in electroluminescent pi-conjugated polymers
INTERNATIONAL WINTERSCHOOL FOR BIOELECTRONICS BioEl 2015
Nonlinear Optics of Liquid and Photorefractive Crystals II, 1998
ABSTRACT The low-temperature thermoluminescense (TSL) technique has been applied for probing the ... more ABSTRACT The low-temperature thermoluminescense (TSL) technique has been applied for probing the energetic disorder of localized states in poly(methyl-phenylsilylene) (PMPSi) combining the TSL studies and transport properties as determined by photocurrent transients. The results are described in terms of the disorder model and the energetic relaxation of photogenerated charge carriers within a manifold of the states of Gaussian distribution. Analysis both of the energetic position of the TSL peak maximum and the shape of its high-energy wing allowed to extract a parameter characterized the intrinsic energetic disorder in PMPSi, the value of which coincided well with the width of the density-of-states determined from transport measurements. The interpretation of results leads to conclusion that although the disorder is the dominant factor, the polaron contribution should be taken into account for the description of the charge carrier transport in PMPSi. The problem of the photochemical stability of PMPSi as well as the appearance of new trapping centers, is briefly outlined.
AIP Conference Proceedings, 2014
We developed an analytical model to describe hopping conductivity and mobility in organic semicon... more We developed an analytical model to describe hopping conductivity and mobility in organic semiconductors including both energetic disorder and polaronic contributions. The model is based on the Marcus jump rates with a Gaussian energetic disorder, and it is premised upon a generalized Effective Medium approach yet avoids shortcoming involved in the effective transport energy or percolation concepts. The carrier concentration dependence becomes considerably weaker when the polaron energy increases relative to the disorder energy, indicating the absence of universality that is at variance with recent publications.
Highly Efficient Intrinsic Phosphorescence from a sigma-Conjugated Poly(silylene) Polymer
Journal of Physical Chemistry C, 2014
ABSTRACT We have observed highly efficient intrinsic phosphorescence of a neat σ-conjugated polym... more ABSTRACT We have observed highly efficient intrinsic phosphorescence of a neat σ-conjugated polymer, poly-(biphenyl-4-ylmethylsilylene) (PBMSi). At low temperatures, PBMSi solid films featured ∼15% phosphorescence quantum yield, which is unusually high for purely organic conjugated polymers and is comparable to that of organometallic polymers. Exciton dynamics in PBMSi was studied by ultrafast fluorescence and time-gated delayed emission measurements. It was shown that the phosphorescence of PBMSi originates from the radiative decay of triplets on the π-conjugated biphenyl group constituting the lowest triplet state, T1 , which is populated under the excitation of the σ-conjugated polymer backbone, i.e., with energy well below the lowest singlet excited state of the biphenyl group itself. The nature of the excited states in PBMSi was further investigated by performing quantum-mechanical calculations of the model compound. The calculations showed that the lowest singlet excited state has charge-transfer (CT) character involving different parts of the same macromolecule. Energetically this state lies very close to the CT triplet excited state. We argue that the intramolecular CT state is responsible for the strongly enhanced intersystem crossing (ISC) in PBMSi due to the small positive CT singlet−triplet energy splitting, which is itself a consequence of a weak exchange interaction of a spatially separated electron and hole in the CT state. This study suggests a new molecular-level engineering approach for enhancement of the ISC, enabling efficient conversion of primary excited singlets into triplets in conjugated polymers without involving a heavy atom effect while leaving the rate of radiative T1 → S0 transition virtually unaffected.