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.
Organic light emitting diodes (OLEDs) are now widely commercialized in market due to many advantages such as possibility of making thin or flexible devices. Nevertheless there are still several things to obtain the high quality flexible OLEDs, one of the most important issues is the light extraction of the device. It is known that OLEDs have the typical light loss such as the waveguide loss, plasmon absorption loss and internal total reflection. In this paper, we demonstrate the one-step processed light scattering films with aluminum oxide nano-particles and polystyrene matrix composite to achieve highly efficient OLEDs. Optical characteristics and surface roughness of light scattering film was optimized by changing the mixing concentration of Al2O3 nano-particles and investigated with the atomic force microscopy and hazemeter, respectively.
We report the rate limiting step of charge generation in the charge generation units (CGUs) composed of a p-HTL,
HATCN and n-doped electron transporting layer (n-ETL) where TAPC was used as the HTL. Energy level alignment
determined by the capacitance-voltage (C-V) measurements and the current density-voltage characteristics of the
structure clearly showed that the electron injection at the HATCN/n-ETL junction limits the charge generation in the
CGUs rather than charge generation itself at the p-HTL/HATCN junction. Consequently, the CGUs with 30 mol%
Rb2CO3 doped BPhen formed with the HATCN layer generates charges very efficiently and the excess voltage required
to generate the current density of ±10 mA/cm2 was around 0.17 V, which is extremely small compared with the literature
values reported up to now.
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.
Novel p-dopants of ReO3 and CuI, and an n-dopant of Rb2CO3 have been developed. Among many other p-dopants, ReO3 possesses superior characteristics of low temperature deposition, efficient charge generation and increasing the device lifetime. The absorption intensity of charge transfer complexes and current-voltage characteristics revealed that charge generation in p-doped hole transporting layers is more effective when the work function of the dopant is larger. High performance OLEDs have been fabricated using the p- and n-dopants, including the low driving voltage p-i-n phosphorescent OLEDs, high power efficiency of tandem OLEDs using ReO3 doped NPB/ReO3 (1 nm)/Rb2CO3 doped Bphen as the interconnection unit, and top emission OLEDs using CuI doped NPB as the hole injection layer from Ag electrode.
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