Thermally activated injection limited conduction in single layer N,N '-diphenyl-N,N '-bis(3-methylphenyl)1-1 '-biphenyl-4,4 '-diamine light emitting diodes

Citation
Aj. Campbell et al., Thermally activated injection limited conduction in single layer N,N '-diphenyl-N,N '-bis(3-methylphenyl)1-1 '-biphenyl-4,4 '-diamine light emitting diodes, J APPL PHYS, 86(9), 1999, pp. 5004-5011
Citations number
33
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Apllied Physucs/Condensed Matter/Materiales Science
Journal title
JOURNAL OF APPLIED PHYSICS
ISSN journal
0021-8979 → ACNP
Volume
86
Issue
9
Year of publication
1999
Pages
5004 - 5011
Database
ISI
SICI code
0021-8979(19991101)86:9<5004:TAILCI>2.0.ZU;2-Q
Abstract
Impedance, current-voltage-luminosity and spectral measurements have been c arried out on indium-tin-oxide/N,N'-diphenyl-N,N'-bis(3-methylphenyl)1-1'-b iphenyl-4,4'-diamine (TPD)/Al light emitting diodes. The devices have a blu e/violet emission with a spectrum peaked at 404 nm. Capacitance-voltage mea surements show that at zero bias the devices are fully depleted. The impeda nce measurements show that the devices can be modeled on a single, frequenc y-independent parallel resistor-capacitor RPCP circuit with a small series resistance R-S. R-P changes with applied bias and temperature, while C-P re mains constant. The values of C-P give epsilon(r)=3.0 +/- 0.3. Analysis of the current-voltage (J-V) characteristics show that the dominant conduction mechanism cannot be either ohmic, trap free space charge limited, or tunne ling injection. The temperature and thickness dependence indicate that it m ust be either thermionic emission or thermally assisted tunneling, the carr ier density varying from about 10(10)/10(11) to 3x10(13) cm(-3) over the me asured bias range. The EL efficiency increases 20 fold upon cooling but sho ws little variation with bias at all temperatures, indicating the same mech anism is responsible for the injection of both holes and electrons. Modelin g the results with thermionic emission suggests that image force lowering i s responsible for the variation of the current with applied bias, but the c alculated injection barrier height and Richardson constant are much smaller than expected. This cannot be explained by models based on a backflowing s urface recombination current due to the high carrier mobility found in TPD. (C) 1999 American Institute of Physics. [S0021-8979(99)01420-6].