Comparative study of hole transport in poly(p-phenylene vinylene) derivatives (original) (raw)
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Hole Transport in Poly(p-Phenylene Vinylene)
physica status solidi (b), 2000
The influence of side chains on hole transport in poly(p-phenylene vinylene) is examined as a function of temperature T and electrical field E by means of current±voltage experiments, and impedance spectroscopy which probes the transit time of injected carriers. The data are analyzed using a model for hopping in a Gaussian site-energy distribution. Energetic disorder predominantly governs the conductive properties of the PPV derivatives.
Charge transport in poly (p-phenylene vinylene) at low temperature and high electric field
Charge transport in poly(2-methoxy, 5-(2 0 -ethyl-hexyloxy)-p-phenylene vinylene ) (MEH-PPV)-based hole-on ly diodes is investigated at high electric fields and low temperat ures using a novel diode architecture. Charge carrier densities that are in the range of those in a field-effect transistor are achieved, bridging the gap in the mobility versus charge carrier density plot between polymer-based light-emitting diodes and field-effect transistors. The extended field range that is accessed allows us to discuss the applicability of current theoretical models of charge transport, using numer ical simulations. Finally, within a simple approximation, we extract the hopping length for holes in MEH-PPV directly from the experimental data at high fields, and we derive a value of 1.0 ± 0.1 nm.
Journal of Materials Science, 2007
The polyfluorene/poly(p-phenylenevinylene) copolymer based hole-only devices are fabricated and the current-voltage characteristics are measured as a function of temperature. The hole current is fitted well with spacecharge limited and field-dependent mobility model, which provides a direct measurement of the hole mobility l as a function of electric field E and temperature. The mobility is fitted with existing Gill's model, Gaussian disorder model, correlated Gaussian disorder model and Brownian motion model. Energy hopping time and activation energy are obtained from Brownian motion model. Microscopic transport parameters are derived and a consistent picture of the influence of the molecular structure of the polymer on the charge transport is depicted. For the polyfluorene/ poly(p-phenylenevinylene) copolymer, although with a high degree of irregularity in structure and larger energetic disorder, the two bulky structure favors charge delocalization and remove defect sites, results in a higher mobility. The results suggest space-charge limited and field-dependent mobility model combine with various mobility model, include Brownian motion model, is a useful technique to study charge transport in thin films with thicknesses close to those used in real devices.
Journal of Applied Physics, 2005
We present a systematic study of the influence of the processing conditions on the charge-carrier mobility in hole-only diodes and field-effect transistors ͑FETs͒ based on alkoxy-substituted poly͑p-phenylene vinylene͒ ͑PPV͒. It is demonstrated that by chemical modification from asymmetrically to fully symmetrically substituted PPVs the mobility in both types of devices can be significantly improved. Furthermore, for symmetrical PPVs the mobility is strongly dependent on processing conditions, such as choice of solvents and annealing conditions. The increase in mobility is accompanied by a strong enhancement of the anisotropy in the charge transport. Ultimately, mobility of up to 10 −2 cm 2 / Vs in FETs and 10 −5 cm 2 / Vs in hole-only diodes have been achieved.
On the field and temperature dependence of hole mobility in molecularly doped polymer
Synthetic Metals, 2005
The charge transport properties of N,N-diphenyl-N,N-bis(3-methylphenyl)-(1,1-biphenyl)-4,4-diamine (TPD) doped polycarbonate (PC) have been measured as a function of field and temperature. Hole mobility is observed to follow log µ ∝ E 1/2 (i.e. Poole-Frenkel behavior) at relatively high field strengths while at low field strengths mobility initially decreased with increase in field strength. Mobility value undergoes minima at a particular field value for a given temperature. Minima in mobility are observed to occur at higher field strength as the temperature is lowered. Mobility data and transport parameters are analyzed using the formalism of Gaussian disorder model (GDM). The mobility behavior at low field and shift of mobility minima with temperature suggests large disorder in the film. Investigation of film morphology using photoluminescence, X-ray diffraction and scanning electron microscopy suggests that large positional disorder in the film is probably due to film morphology and TPD-PC interaction.
Thin Solid Films, 2007
The transport properties (conductivity and mobility) of holes and electrons in poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) light-emitting diodes were investigated using direct current-voltage curves (I-V) and time of flight measurements (TOF) as a function of electric field and temperature. TOF results revealed that the transport of holes in the bulk follows a non-dispersive behavior at room temperature (300 K), exhibiting a progressive transition to a dispersive behavior as the temperature decreases down to 220 K. The dispersive transport characteristics were interpreted in the framework of carrier hopping in an exponential density of states. On the other hand, the analysis of the negative photocurrent transients indicated that the transport of negative charge carriers (electrons) is strongly dominated by trapping in the entire range of temperature studied. The I-V curves presented remarkable temperature dependence, being analyzed in terms of the classic Fowler-Nordheim tunneling, Richardson-Schottky thermionic emission and trap-controlled transport.
Temperature and electric field dependent hole mobility in a polyfluorene copolymer
Lancet, 2011
Transport of holes in thin films of a low-bandgap alternating polyfluorene copolymer, APFO-Green5, was investigated by means of admittance spectroscopy as a function of field and temperature. The values of hole mobility were evaluated from the position of the maxima in the plots of the negative differential susceptance as a function of frequency. Hole mobility was found to be strongly field- and temperature-dependent. The charge transport parameters were extracted by analyzing the mobility data by the uncorrelated and the correlated Gaussian Disorder Models.
Comparison of hole mobility in thick and thin films of a conjugated polymer
Organic Electronics, 2010
The problem that conventional time of flight (TOF) mobility measurements are made on much thicker films than typically used in organic optoelectronic devices is investigated by comparing the mobility of thick and thin films of the poly(phenylenevinylene) based copolymer ''SuperYellow". We report for the first time mobility values perpendicular to the substrate for this widely used material and find them to be of the order of 10 À6 -10 À7 cm 2 /V s for both spin-coated and drop-cast films. A high photoluminescence quantum yield (PLQY) of 60 ± 5% is measured for both types of films. The results indicate a trade-off between PLQY and charge transport in poly(arylenevinylene)s. The insensitivity of the charge transporting and photophysical properties to the film preparation procedure and thickness is a desirable characteristic that may result from the bulky side groups of ''SuperYellow".
Wide Energy-Window View on the Density of States and Hole Mobility in Poly(p-Phenylene Vinylene)
Physical Review Letters, 2004
Using an electrochemically gated transistor, we achieved controlled and reversible doping of poly(p-phenylene vinylene) in a large concentration range. Our data open a wide energy-window view on the density of states (DOS) and show, for the first time, that the core of the DOS function is Gaussian, while the low-energy tail has a more complex structure. The hole mobility increases by more than 4 orders of magnitude when the electrochemical potential is scanned through the DOS.
Polymers for Advanced Technologies, 2006
A theoretical study and experimental evidence for a decrease of the charge carrier mobility in poly[2methoxy-5-(2(-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) under the influence of dipolar species are presented in this paper. The presence of polar species in the vicinity of an MEH-PPV chain modifies the on-chain site energies and consequently, due to random mutual positions and orientations, increases the width of the energy distribution of the chain transport states. The influence of energetic disorder of these states on the charge carrier mobility was modeled by means of the Monte Carlo method, based on a numerical solution of the time-dependent Schrö dinger equation in the tight-binding approximation. It was shown that the increasing disorder destroys the resonance between charge carrier energies on adjacent sites and, therefore, limits the diffusive charge carrier motion.