Simultaneous measurement of electron and hole mobilities in polymer light-emitting diodes (original) (raw)
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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.
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".
Organic Electronics, 2007
The charge transport properties in a novel electroluminescent poly{[2-(4 0 ,5 0 -bis(3 00 -methylbutoxy)-2 0 -p-methoxy-phenyl)phenyl-1,4-phenylene vinylene]-co-(9,9-dioctyl-2,7-fluorenylene vinylene)} (BPPPV-PF) have been studied using a time-of-flight (TOF) photoconductivity technique. The TOF transients for holes were recorded over a range of temperatures (207-300 K) and electric fields (1.5 · 10 5 -6.1 · 10 5 V/cm). The hole transport in this polymer was weakly dispersive in nature with a mobility at 300 K of 5 · 10 À5 cm 2 /V s at 2.5 · 10 5 V/cm. This increased to 8.4 · 10 À5 cm 2 /V s at 6.1 · 10 5 V/ cm. The temperature and field dependence of charge mobility has been analyzed using the disorder formalisms (Bässler's Gaussian disorder model (GDM) and correlated disorder model (CDM)). The fit with Gaussian disorder (GDM) model yielded the mobility pre-factor l 1 = 1.2 · 10 À3 cm 2 /V s, energetic disorder parameter r = 82 meV and positional disorder parameter R = 1.73. The average inter-site separation (a = 7 Å ) and the charge localization length (L = 3.6 Å ) was estimated by assuming the CDM type charge transport. The microscopic charge transport parameters derived for this polymer are almost identical to the reported values for fully conjugated polymers with high chemical purity. The results presented indicate that the charge transport parameters can be controlled and optimized for organic optoelectronic applications.
Temperature- and field-dependent electron and hole mobilities in polymer light-emitting diodes
Applied Physics Letters, 1999
We have studied the transport properties of electron-and hole-dominated MEH-PPV, poly͑2-methoxy,5-͑2Ј-ethyl-hexoxy͒-p-phenylene vinylene͒, devices in the trap-free limit and have derived the temperature-dependent electron and hole mobilities (ϭ 0 e ␥ͱE ) from the space-charge-limited behavior at high electric fields. Both the zero-field mobility 0 and electric-field coefficient ␥ are temperature dependent with an activation energy of the hole and electron mobility of 0.38Ϯ0.02 and 0.34Ϯ0.02 eV, respectively. At 300 K, we find a zero-field mobility 0 on the order of 1Ϯ0.5ϫ10 Ϫ7 cm 2 /V s and an electric-field coefficient ␥ of 4.8Ϯ0.3 ϫ10 Ϫ4 (m/V) 1/2 for holes. For electrons, we find a 0 an order of magnitude below that for holes but a larger ␥ of 7.8Ϯ0.5ϫ10 Ϫ4 (m/V) 1/2 . Due to the stronger field dependence of the electron mobility, the electron and hole mobilities are comparable at working voltages in the trap-free limit, applicable to thin films of MEH-PPV.
Comparative study of hole transport in poly(p-phenylene vinylene) derivatives
Physical Review B, 2000
The dc mobility of holes in four poly(p-phenylene vinylene) (PPV) derivatives (three fully conjugated polymers with different side chains and one partially conjugated PPV) is examined as a function of temperature T and applied electrical field E. In all cases the mobility μ follows the ...
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.
Polymer light-emitting diodes with doped hole-transport layers
physica status solidi (a), 2011
We demonstrate a solution processed bi-layer PLED based on poly(p-phenylene vinylene) derivatives using orthogonal solvents. To lower the voltage drop the hole transport layer (HTL) based on poly[2,5-bis(2 0-ethylhexyloxy)-co-2,5-bis(butoxy)-1,4-phenylenevinylene] (BEH/BB-PPV (1:3)) is doped with tetracyano-tetrafluoro-quinodimethane (F4TCNQ). The conductivity of BEH/BB-PPV (1:3) was observed to increase by two orders of magnitude upon doping with F4TCNQ. The doped HTL was observed to lower the operating voltage of a double layer PLED, but suffers from additional quenching by the dopant at higher voltages due to the lack of an electron blocking functionality.
Hole transport in blue and white emitting polymers
Journal of Applied Physics, 2008
Hole transport in a blue emitting polyspirobifluorene polymer and in a white emitting polymer consisting of a polyspirobifluorene backbone and two dyes ͑green and red͒ was studied. The hole mobility was measured using the time-of-flight method as a function of the electric field and temperature in the range 10 5 −10 6 V / cm and 285− 335 K, respectively. The observed temperature and electric field dependence of the hole mobility was analyzed in the framework of the Bässler disorder model. Also, steady-state current-voltage characteristics were measured over a wide range of electric fields and temperatures and the hole mobility was determined. Our measurements have shown that the hole mobility in the white emitting polymer is the same as in the blue emitting polymer. The performed disorder model analysis gives the same values for the effective energetic disorder ͑115 meV͒ and for the positional disorder ͑1.85͒ for both polymers. Therefore, we have concluded that the added green and red dyes do not act as hole traps as they have no influence on the hole mobility. It can therefore be concluded that their highest occupied molecular orbital ͑HOMO͒ levels are aligned with the HOMO level of the polyspirobifluorene backbone.