Characteristics Of Organic Semiconductors Research Articles

Techniques for characterization of charge carrier mobility in organic semiconductors

AbstractThe charge transport characteristics of organic semiconductors are one of the key attributes that impacts the performance of organic electronic and optoelectronic devices in which they are utilized. For improved performance in organic photovoltaic cells, light‐emitting diodes, and field‐effect transistors (FETs), efficient transport of the charge carriers within the organic semiconductor is especially critical. Characterization of charge transport in these organic semiconductors is important both from scientific and technological perspectives. In this review, we shall mainly discuss the techniques for measuring the charge carrier mobility and not the theoretical underpinnings of the mechanism of charge transport. Mobility measurements in organic semiconductors and particularly in conjugated polymers, using space‐charge‐limited current, time of flight, carrier extraction by linearly increasing voltage, double injection, FETs, and impedance spectroscopy are discussed. The relative merits, as well as limitations for each of these techniques are reviewed. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Electron mobility characterization in OLEDs from ac small signal optical modulation

This paper investigates the field dependence of electron mobility in tris(8-hydroxyquinolinato)aluminium (Alq3) and bathocuproine (BCP) through ac small signal optical modulation on green light (ITO/PEDOT/NPD/Alq3/Ba/Ag) and blue light (ITO/PEDOT/NPD/BCP/Alq3/Ba/Ag) OLED. The electroluminescence (EL) transient time delay for the blue light OLED is much longer than for the green one. The electron mobility in BCP was extracted based on a Poole–Frenkel-like equation and EL transient time delay measurement, which is in the range (7–9) × 10−8 cm2 V−1 s−1 at an external electric field of 1530–1830 (V cm−1)1/2, comparable to the results from other published reports (Muckl et al 2000 Synth. Met. 111–112 91; Barth et al 2001 J. Appl. Phys. 89 3711; Nakamura H et al 1996 Int. Symp. on Inorganic and Organic Electroluminescence ed R H Mauch and H-E Gumlich (Berlin: Wissenschaft und Technik) p 95; Xie et al 2002 Appl. Phys. Lett. 80 1477). The difference in EL transient time delay and electron mobility for green and blue light OLEDs was demonstrated by the results of direct modulation. The electron transit time shows similar field dependence in both Alq3 layers in green and blue OLEDs. Unlike Alq3, the field dependence of electron mobility in BCP did not fit the conventional organic semiconductor characteristics μ∞exp(βE1/2), and the excitons formation at the NPD/BCP interface for the blue OLEDs was demonstrated through the EL spectrum.

Magnetoresistance effect in an organic spin valve

Based on the spin diffusion theory and the Ohm’s law, we theoretically studied the magnetoresistance (MR) effect in an organic spin valve with structure of ferromagnetic/organic semiconductor/ferromagnetic system, which takes into account the special characteristics of organic semiconductors. Self-trapped states, such as spin polarons as well as spinless bipolarons are assumed to be the main carriers in organic semiconductors. From the calculation, it is found that MR ratio increases with the increasing of the polaron proportion and rapidly decreases with the increasing of the organic layer thickness. MR ratio can be enhanced remarkably when the interfacial resistances are spin related. Effects of the conductivity match and the spin polarization of the ferromagnetic layer on the MR are also discussed.

Optical, electrical and photovoltaic characteristics of organic semiconductor based on oxazine/n-Si heterojunction

In this work, the construction and photoelectrical characterization of p-type organic semiconductor oxazine (OXZ) in junction with n-type silicon semiconductor are presented. The Stokes shift between absorption and emission of oxazine was analyzed. The analysis of the spectral behavior of the absorption coefficient ( α) of OXZ, in the absorption region revealed a direct transition, and the energy gap was estimated as 1.82 eV. From the current–voltage, I– V, measurements of the Au/OXZ/n-Si/Al heterojunction in the temperature range 300–375 K, characteristic junction parameters and dominant conduction mechanisms were obtained. This heterojunction showed a photovoltaic behavior with a maximum open circuit voltage, V oc, of 0.42 V, short-circuit current density, J sc, of 3.25 mA/cm 2, fill factor, FF, of 0.35 and power conversion efficiency, η, of 3.2% under 15 mW/cm 2 white light illumination.

Alignment-Induced Epitaxial Transition in Organic-Organic Heteroepitaxy

We report the epitaxial growth of thin films of a small organic molecule (pentacene) on polymer substrates with controllable photoalignment over a wide range. The pentacene molecular plane exhibited a distinct orientational change from parallel to perpendicular relative to the polymer chain with increasing substrate polymer alignment. Each orientation consists of twinlike domains. Such characteristics reveal a unique alignment-induced epitaxial transition controlled by the subtle balance of weak interactions, showing a promising approach for tuning the characteristics of organic semiconductor based electronic devices.

Novel small molecules for organic field-effect transistors: towards processability and high performance

The processing characteristics of organic semiconductors make them potentially useful for electronic applications where low-cost, large area coverage and structural flexibility are required. This critical review gives a general introduction about the current standing in the area of OFETs focusing on the new processable small molecules that have been recently reported for their use as organic semiconductors. A general description of the OFETs device operation and the transport mechanisms that dominate organic semiconductors is provided, followed by an overview of the strategies and materials employed to fabricate p-type, n-type and ambipolar OFETs. Some new tendencies and applications that are currently being developed employing OFETs are also described, such as the preparation of electronic paper, sensors or light emitting transistors (85 references).

Current spin polarization in ferromagnetic/organic semiconductor/ferromagnetic system

Based on the spin diffusion theory and the Ohm's law, we theoretically studied the spin polarized injection and transport through the ferromagnetic/organic semiconductor/ferromagnetic system and obtained the current spin polarization, which takes into account the special characteristics of organic semiconductors. Self-trapped states, such as spin polarons as well as spinless bipolarons are assumed to be the main carriers in organic semiconductors. From the calculation, it is found that polarons are the effective spin carriers in spin dependent injection and transport in organic semiconductors. The effect of the spin related interfacial resistances, conductivity match and organic semiconductor length on the current spin polarization are also discussed respectively.

High mobility top-gated pentacene thin-film transistors

A common device geometry for measuring the electrical characteristics of organic semiconductors is the thin-film organic field-effect transistor (OTFT). Mostly for reasons of cost, convenience, and availability, this usually involves depositing the organic material on a prefabricated gate structure such as Si∕SiO2, the surface chemistry of which is often modified with self-assembled monolayers. The interactions between these surfaces and the deposited organic can have a profound effect on thin-film growth and the resulting electrical characteristics since most of the charge transport in these structures occurs near the organic-insulator interface. An alternative to this traditional technique is to assemble the transistor on top of the organic semiconductor. We have used chemical-vapor deposition of the polymeric dielectric material parylene to create pentacene OTFTs with gate electrodes both on top of and below the semiconductor film, with field-effect mobilities as high as 0.1cm2∕Vs and Ion∕Ioff ratios greater than 106 in the top-gated OTFTs. Comparing the electronic properties of top- and bottom-gate structures yields insight into the charge transport characteristics as well as the effects of device geometry, contacts, and surface roughness of the organic thin film.

Fabrication of combinatorial nm-planar electrode array for high throughput evaluation of organic semiconductors

We have fabricated a combinatorial nm-planar electrode array by using photolithography and chemical mechanical polishing processes for high throughput electrical evaluation of organic devices. Sub-nm precision was achieved with respect to the average level difference between each pair of electrodes and a dielectric layer. The insulating property between the electrodes is high enough to measure I–V characteristics of organic semiconductors. Bottom-contact field-effect-transistors (FETs) of pentacene were fabricated on this electrode array by use of molecular beam epitaxy. It was demonstrated that the array could be used as a pre-patterned device substrate for high throughput screening of the electrical properties of organic semiconductors.

Small‐signal characteristics of organic semiconductors with continuous energy distribution of traps

AbstractThe electrical characteristics of organic light emitting devices containing a continuous distribution of trap states in the forbidden gap are obtained by numerically solving the basic semiconductor equations for the steady state and under small‐signal conditions. The spatially‐dependent occupied trap states, which are described by an explicit density of states function, modify the charge distribution within the structure and the shape of the electric field and the carrier current densities. The effect of the modulation frequency, the applied voltage and the device temperature are studied for a hole conducting layer with a trap density of states consisting of a double gaussian profile and for a given set of microscopic parameters including the carrier mobilities and thermal velocities, the capture cross sections, and the residual shallow impurity concentration. The frequency‐dependent loss and capacitance curves are obtained for various experimental conditions, like temperature and applied steady‐state voltage. Effects of parameters describing electrical contacts and trap density of states are shown. Such results are particularly useful for the analysis of experimental electrical characteristics obtained by thermal admittance spectroscopy. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Energy‐level alignment at metal–organic and organic–organic interfaces

AbstractThis article reports on the electronic structure at interfaces found in organic semiconductor devices. The studied organic materials are C60 and poly (para‐phenylenevinylene) (PPV)‐like oligomers, and the metals are polycrystalline Au and Ag. To measure the energy levels at these interfaces, ultraviolet photoelectron spectroscopy has been used. It is shown how the energy levels at interfaces deviate from the bulk. Furthermore, it is demonstrated that the vacuum levels do not align at the studied interfaces. The misalignment is caused by an electric field at the interface. Several effects are presented that influence the energy alignment at interfaces, such as screening effects, dipole layer formation, charge transfer, and chemical interaction. The combination of interfaces investigated here is similar to interfaces found in polymer light‐emitting diodes and organic bulk heterojunction photovoltaic devices. The result, the misalignment of the vacuum levels, is expected to influence charge‐transfer processes across these interfaces, possibly affecting the electrical characteristics of organic semiconductor devices that contain similar interfaces. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2549–2560, 2003

Polymethacrylate Functionalized Polypyrrole Network Films on Indium Tin Oxide: Electropolymerization of a Precursor Polymer and Comonomer

The electropolymerization and deposition of conjugated polypyrrole network films on planar indium tin oxide electrode surfaces is described. The precursor polymer, a polymethacrylate functionalized with pyrrole, PPHMA2, was synthesized and then electropolymerized on ITO substrates. The possibility of inter- and intramolecular polymerization resulted in a cross-linked conjugated network microstructure. The pyrrole group was chosen because of its good electropolymerizablity and organic semiconductor characteristics. To induce greater electropolymerizability toward linear polypyrrole units and control of surface morphology, PPHMA2 was copolymerized with pyrrole monomer at various composition (feed) ratios. The spectroscopic, morphological, and composition properties of the films were characterized by FT-IR spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy, respectively. Interesting trends were observed on structure−property relationships with respect to film composition.

Electron-Phonon Coupling Spectrum in Photodoped Pentacene Crystals

The coupling between conduction charges and the vibrational modes of the molecular lattice plays a defining role in the transport characteristics of organic semiconductors. Using electron tunneling spectroscopy, we obtain the electron--optical-phonon coupling spectrum in photodoped pentacene crystals at energies <30 meV. Comparison of the tunneling spectrum to infrared absorption data on the optical phonon density of states yields the energy dependence of the electron-phonon scattering matrix element. The integrated spectral weight of the electron-phonon coupling shows that superconductivity in pentacene is likely of electron-phonon origin.