Amorphous Molecular Semiconductors Research Articles

Modelling of the Electrostatic Image in Films of Photosensitive Amorphous Molecular Semiconductors

Modelling of the Electrostatic Image in Films of Photosensitive Amorphous Molecular Semiconductors

Peculiarities of the Process of Photogeneration of Charge Carriers in Amorphous Molecular Semiconductors

Peculiarities of the Process of Photogeneration of Charge Carriers in Amorphous Molecular Semiconductors

Formation of Periodic Nanostructures and Mechanism of Formation of Geometrical Relief on the Surface of Layers of Amorphous Molecular Semiconductors

Formation of Periodic Nanostructures and Mechanism of Formation of Geometrical Relief on the Surface of Layers of Amorphous Molecular Semiconductors

Full quantum treatment of charge dynamics in amorphous molecular semiconductors

We present a treatment of charge dynamics in amorphous molecular semiconductors that accounts for the coupling of charges to all intramolecular phonon modes in a fully quantum mechanical way. Based on ab initio calculations, we derive charge transfer rates that improve on the widely used semiclassical Marcus rate and obtain benchmark results for the mobility and energetic relaxation of electrons and holes in three semiconductors commonly applied in organic light-emitting diodes. Surprisingly, we find very similar results when using the simple Miller-Abrahams rate. We conclude that extracting the disorder strength from temperature-dependent charge transport studies is very possible but extracting the reorganization energy is not.

Effects of energy correlations and superexchange on charge transport and exciton formation in amorphous molecular semiconductors: An ab initio study

In this study, we investigate on the basis of ab initio calculations how the morphology, molecular on-site energies, reorganization energies, and charge transfer integral distribution affect the hopping charge transport and the exciton formation process in disordered organic semiconductors. We focus on three materials applied frequently in organic light-emitting diodes: $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{NPD}$, TCTA, and Spiro-DPVBi. Spatially correlated disorder and, more importantly, superexchange contributions to the transfer integrals, are found to give rise to a significant increase of the electric field dependence of the electron and hole mobility. Furthermore, a material-specific correlation is found between the HOMO and LUMO energy on each specific molecular site. For $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{NPD}$ and TCTA, we find a positive correlation between the HOMO and LUMO energies, dominated by a Coulombic contribution to the energies. In contrast, Spiro-DPVBi shows a negative correlation, dominated by a conformational contribution. The size and sign of this correlation have a strong influence on the exciton formation rate.

Тонкие пленки аморфных молекулярных полупроводников для создания поверхностных темплатов упорядоченных структур

Тонкие пленки аморфных молекулярных полупроводников для создания поверхностных темплатов упорядоченных структур

Ab initio modeling of steady-state and time-dependent charge transport in hole-only α-NPD devices

We present an ab initio modeling study of steady-state and time-dependent charge transport in hole-only devices of the amorphous molecular semiconductor α–NPD [N,N′-Di(1–naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine]. The study is based on the microscopic information obtained from atomistic simulations of the morphology and density functional theory calculations of the molecular hole energies, reorganization energies, and transfer integrals. Using stochastic approaches, the microscopic information obtained in simulation boxes at a length scale of ∼10 nm is expanded and employed in one-dimensional (1D) and three-dimensional (3D) master-equation modeling of the charge transport at the device scale of ∼100 nm. Without any fit parameter, predicted current density-voltage and impedance spectroscopy data obtained with the 3D modeling are in very good agreement with measured data on devices with different α-NPD layer thicknesses in a wide range of temperatures, bias voltages, and frequencies. Similarly good results are obtained with the computationally much more efficient 1D modeling after optimizing a hopping prefactor.

Ab initiocharge-carrier mobility model for amorphous molecular semiconductors

Accurate charge-carrier mobility models of amorphous organic molecular semiconductors are essential to describe the electrical properties of devices based on these materials. The disordered nature of these semiconductors leads to percolative charge transport with a large characteristic length scale, posing a challenge to the development of such models from ab initio simulations. Here, we develop an ab initio mobility model using a four-step procedure. First, the amorphous morphology together with its energy disorder and intermolecular charge-transfer integrals are obtained from ab initio simulations in a small box. Next, the ab initio information is used to set up a stochastic model for the morphology and transfer integrals. This stochastic model is then employed to generate a large simulation box with modeled morphology and transfer integrals, which can fully capture the percolative charge transport. Finally, the charge-carrier mobility in this simulation box is calculated by solving a master equation, yielding a mobility function depending on temperature, carrier concentration, and electric field. We demonstrate the procedure for hole transport in two important molecular semiconductors, $\ensuremath{\alpha}$-NPD and TCTA. In contrast to a previous study, we conclude that spatial correlations in the energy disorder are unimportant for $\ensuremath{\alpha}$-NPD. We apply our mobility model to two types of hole-only $\ensuremath{\alpha}$-NPD devices and find that the experimental temperature-dependent current density--voltage characteristics of all devices can be well described by only slightly decreasing the simulated energy disorder strength.

Giant magnetoresistance due to electron-hole pair mechanism in poly( N-vinylcarbazole)

An amorphous molecular semiconductor, poly( N-vinylcarbazole) (PVCz), exhibits negative giant magnetoresistance (MR) under ambient conditions. The application of a weak magnetic field of 10 mT to PVCz films doped with lumichrome, in which light excitation immediately creates triplet electron-hole (e-h) pairs that are precursors of photocarriers, causes a steep decrease in resistivity by more than 20% at ambient temperature. Further, the resistivity of the doped film gradually reduces by approximately one-half under a field of 1 T, equivalent to an MR ratio of −55%. In addition, anomalous spikes are also detected at 0.07, 0.30, and 9.0 T, indicative of an exponential dependence with a decay distance of 0.1 nm in the exchange interaction of the e-h pair. A quantum mechanical calculation based on the density operator formalism clarifies that the observed MR effect can be comprehensively understood by the spin-selective charge dynamics and the coherent and incoherent spin dynamics of the e-h pair in a quasi-one-dimensional lattice for photocarrier generation. Model calculations also indicate the importance of the spin-lattice relaxation for the giant MR effect in organic molecular semiconductors.

Carbazole-based bis(enamines) as effective charge-transporting amorphous molecular materials

A series of carbazole-based bis(enamines) as glass-forming hole transport materials has been synthesized and characterized. The amorphous films of the reported compounds demonstrate extraordinarily high hole-drift mobilities reaching 5 × 10 −2 cm 2/V s at high electric fields. To our knowledge this is one of the highest values observed to date for amorphous molecular p-type semiconductors. The reported compounds constitute materials with high thermal stability with five percent weight loss temperatures exceeding 370 °C as characterised by thermogravimetric analysis. They form glasses with the glass transition temperatures ranging from 83 to 126 °C as established by differential scanning calorimetry. Photoelectric properties of the materials were examined by electron photoemission and time of flight technique. The electron photoemission spectra of the layers revealed the ionization potentials as low as 4.98–5.15 eV.

Observation of intermediate-range order in a nominally amorphous molecular semiconductor film

Synchrotron X-ray diffraction from a nominally amorphous molecular semiconductor film reveals both the presence of intermediate-range order (IRO) corresponding to crystalline domains with an average size of a few nanometres, and the growth of these domains upon exposure of the film to moisture.

Hole-injection from a polar mono-molecular layer derivatised ultra-thin gold electrode into a triphenylamine derivative

This Letter, reports the hole-injection characteristics of an ultra-thin, transparent gold electrode derivatized with a thiolate polar monolayer into the amorphous molecular semiconductor, N, N′-bis(3-methylphenyl)- N, N′diphenyl-1,1′biphenyl- 4,4′-diamine (TPD). Thermionic emission with image force lowering gives an excellent description of the injection mechanism over a large range of temperatures and electric field strengths, yielding a zero field Schottky barrier height of ∼0.4 eV. This study demonstrates that derivatizing the surface of this novel gold electrode with a 4-nitrophenylthiolate monomolecular layer is an effective means of circumventing the abrupt negative vacuum level shift, which forms when TPD is thermally deposited onto gold electrodes.

Cover Picture: phys. stat. sol. (RRL) 1/2007

AbstractIn the Rapid Research Letter on p. R37 Musubu Ichikawa et al. report very high electron mobility above 10–3 cm2/Vs in the electron transporting material bipyridyl substituted oxadiazole (Bpy‐OXD). These favourable electrical properties make the amorphous molecular semiconductor promising for potential applications in organic light‐emitting diodes (OLEDs), flatpanel displays and lighting. The authors also give reasons – by means of computational chemistry – why this planar material forms stable amorphous solid films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photogeneration of holes and electrons in amorphous molecular semiconductors with neutral and ionic organic dyes

Photoconducting properties of amorphous molecular semiconductors based on polystyrene films doped with tetranitrofluorenone and merocyanine or an anionic polymethine dye, or with epoxypropylcarbazole and merocyanine or a cationic polymethine dye were studied. The former type of the films is characterized by electron conductivity, whereas the latter type by hole conductivity. The activation energy for photogeneration of mobile charge carriers increases on passing from a merocyanine dye to ionic dyes and decreases with a growth in quantum energy of excitation light for the films of the former type, but does not depend on the light quantum energy for the films of the latter type. It was concluded that the activation energy of photogeneration is determined by electrostatic interaction of a photogenerated charge carrier with the ionized photogeneration site or a counterion for neutral and ionic dyes, respectively. At low dissipation rates of the excess thermal energy of excited dye molecules via electron-nucleus interaction, photogenerated electrons have a possibility to travel over a long distance from the photogeneration site as compared with holes.

Photogeneration of electrons and holes in amorphous molecular semiconductors

This paper reports on the results of investigations into the photoconducting properties of amorphous molecular semiconductors based on films of two types: (i) poly(styrene) films doped with epoxypropylcarbazole (EPC) and a cationic polymethine dye (PD1) and (ii) poly(styrene) films doped with tetranitrofluorenone (TNF) and an anionic polymethine dye (PD2). Films of the first type possess p-type conductivity, whereas films of the second type exhibit n-type conductivity. It is found that, for films with n-type conductivity, unlike films with p-type conductivity, the activation energy of photogeneration of mobile charge carriers decreases with a decrease in the optical wavelength in the absorption range of the dyes. The possible mechanisms of the influence of the photoexcitation energy on the initial distance between charge carriers in electron-hole pairs are analyzed. The inference is made that, when the excess thermal energy of excited dye molecules dissipates at a low rate, the distance between the photogenerated electrons and photogeneration centers increases as compared to the distance between the photogenerated holes and photogeneration centers due to the electron-nucleus interaction.

Photoconductivity of polymer compositions with a high content of organic dyes

The photoconductivity of films of an amorphous molecular semiconductor based on poly-N-epoxypropylcarbazole with high contents of organic dye molecules and films of a polymer composition with organic dye aggregates is studied. It is found that the dependence of the photoconductivity activation energy on the external electric field disappears as one passes from polymer compositions of the former type to those of the latter. The experimental results are explained by assuming that carrier photogeneration and transport in polymer compositions with regions of quasi-ordered dye molecules, in contrast to amorphous molecular semiconductors, take place within such regions.

Electric Field Effects on Photoconductivity and Electronic Absorption Spectra of Photogeneration Sites in Amorphous Molecular Semiconductors

Field dependences of photogeneration quantum yields of charge carriers in films of amorphous molecular semiconductors at high external electric field strengths cannot be always explained in terms of the Onsager or the Poole–Frenkel model, which do not consider the effect of an electric field on basic parameters of these materials. It is assumed that an external electric field shifts the electron density in molecules from its equilibrium distribution, thus altering the probability of electronic transitions. The assumption has been verified in studying the effect of an external electric field on the absorption spectra of poly(N-epoxypropylcarbazole) films doped with a merocyanine dye. Correlation between the theoretical and experimental results has been obtained.

Charge carrier generation in photosensitive amorphous molecular semiconductors

Charge carrier generation in photosensitive amorphous molecular semiconductors

Electric and photophysical properties of holographic and electroluminescent media based on amorphous molecular semiconductors

Electric and photophysical properties of holographic and electroluminescent media based on amorphous molecular semiconductors

Photoluminescence and recombination luminescence in amorphous molecular semiconductors doped with organic dyes

Photoconductivity, photoluminescence (PL), and thermally stimulated luminescence of photoconductive poly-N-epoxypropylcarbazole and poly-N-vinylcarbazole films and non-photoconductive polyvinylbutyral, polyvinyl alcohol, polystyrene, and polyethylene films doped with cationic, anionic, and neutral dyes are studied. It is found that the PL of cationic dyes in photoconductive polymer films is enhanced in comparison to nonphotoconductive ones. The PL enhancement correlates with an increase in photoconductivity, with the quenching effect of an external electric field on the PL intensity, and with an increase in the intensity of the recombination luminescence. It is assumed that this enhancement is related to the presence of predimer traps for holes in the vicinity of dye ions in the films of carbazolyl-containing polymers. A model describing the trap formation upon the photoexcitation of holes into predimer states is suggested.