Investigation of Charge Generation Effect in Tandem White-light Organic Light Emitting Diodes
Date Issued
2012
Date
2012
Author(s)
Chang, Tsao-Lun
Abstract
Tandem organic light emitting diodes (tandem OLEDs) are known for the outstanding color quality as well as the enduring operation lifetime and therefore exhibit a great potential to be the next generation display and solid state lighting. Tandem OLEDs have come a long way since their first introduction but there are still some issues that have yet been clarified or well explained.
In this thesis, we demonstrate a tandem white light OLED with a p-n junction charge generation layer (CGL). As for the physical mechanisms, we will first clarify the definition of turn-on voltage in a more physical way and then discover the relationship between the architectures of devices and their turn-on voltages. Interestingly, the turn-on voltages of tandem OLEDs are found to be the sum of those of their single OLED units. Besides, the interconnecting layers surprisingly exhibit selectivity in charge transportation and therefore play a very important role in determining the performance of those intermediate devices. Most importantly, by applying impedance measurement directly on tandem OLEDs, a unique transition in phase characteristics, which relates to the turn-on of luminance and the doping ratio in CGL, is revealed at a very first time to be the direct evidence of charge generation effect.
Generally speaking, tandem OLEDs seem to possess the features of connection in series to some extents by judging the large turn-on voltage and the greatly enhanced current efficiency which are both the sum of those of the OLED units. However, after the turn-on, tandem OLEDs behave like multilayer devices and show a slowly rising phase characteristics as compared to OLEDs connected in series with electric wires. This demonstrates the functions of charge generation layers, which are not just an electric connection, but provide the electron and hole pairs to the single OLED units connecting to them.
Subjects
Organic light emitting diodes
Tandem OLED
Charge generation
Impedance Measurement
White light
Type
thesis
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