Electrical doping is an important method in organic electronics to enhance device efficiency by controlling Fermi level, increasing conductivity, and reducing injection barrier from electrode. To understand the charge generation process of dopant in doped organic semiconductors, it is important to analyze the charge transfer complex (CTC) formation and dissociation into free charge carrier. In this paper, we correlate charge generation efficiency with the CTC formation and dissociation efficiency of n-dopant in organic semiconductors (OSs). The CTC formation efficiency of Rb2CO3 linearly decreases from 82.8% to 47.0% as the doping concentration increases from 2.5 mol% to 20 mol%. The CTC formation efficiency and its linear decrease with doping concentration are analytically correlated with the concentration-dependent size and number of dopant agglomerates by introducing the degree of reduced CTC formation. Lastly, the behavior of dissociation efficiency is discussed based on the picture of the statistical semiconductor theory and the frontier orbital hybridization model.
We present high efficiency orange emitting OLEDs with low driving voltage and low roll-off of efficiency using an exciplex forming co-host by (1) co-doping of green and red emitting phosphorescence dyes in the host and (2) red and green phosphorescent dyes doped in the host as separate red and green emitting layers. The orange OLEDs achieved a low turn-on voltage of 2.4 V and high external quantum efficiencies (EQE) of 25.0% and 22.8%, respectively. Moreover, the OLEDs showed low roll-off of efficiency with an EQE of over 21% and 19.6% at 10,000 cd/m2, respectively. The devices displayed good orange color with very little color shift with increasing luminance. The transient electroluminescence of the OLEDs indicated that both energy transfer and direct charge trapping took place in the devices.
We demonstrated that an organic p–n junction was successfully adapted to inverted organic light emitting diodes
(IOLEDs) as an electron injection layer (EIL). The organic p–n junction composed of a ReO3 doped copper
phthalocyanine (CuPc)/Rb2CO3 doped 4,7-diphenyl-1,10-phenanthroline (Bphen) layer showed very efficient
charge generation under a reverse bias reaching to 100 mA/cm2 at 0.3 V and efficient electron injection from
indium tin oxide (ITO) when adopted in IOLEDs. Moreover, the organic p–n junction resulted in the same
current density–voltage–luminance characteristics independent of the work function of the cathode, which is a
valuable advantage for flexible displays.
We reported a couple of methods to improve electron injection from the ITO electrode, thereby to fabricate efficient
inverted bottom emission organic light emitting diodes (IBOLEDs). The first method is to use an n-doped electron
transporting layer (ETL) as the electron injection layer. Electron only device characteristics and UPS measurements
confirmed that B3PYMPM homo-junction has the lowest injection barrier at the interface among three different ETLs,
resulting in the highest maximum EQE of 19.8% at low voltage in IBOLEDs. The energy barrier between n-ETL and
ETL is one of the most important factors for high performance inverted OLEDs. The second method is to use an organic
p-n junction as an electron injection layer, where the p-n junction generated electrons and holes under reverse bias,
which corresponds to the forward bias in the OLEDs. The organic p-n junction composed of a p-CuPc/n-Bphen layer
shows almost the same electron injection characteristics for the cathodes with different work functions whereas the
injection characteristics of the n-Bphen EIL significantly depend on the work function of the cathode. These facts
indicate that the organic p-n junction can be efficiently applied as an electron injection layer for high performance
flexible organic electronics, regardless of the electrodes.
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