**3. Results and discussion**

144 Organic Light Emitting Devices

[1]. The number of holes (or the hole current) is limited by the hole injection from anode to organic layer, which is controlled by the Schottky barrier height at the anode/organic interface. If pristine based organic hole transporters are used, for example, N,N'-bis-(1 naphthl)-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB), 4,4'-N,N'-dicarbazole-biphenyl (CBP), 4,4',4''-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA), because the Fermi level of the high-work function anode is pinned at about 0.5-0.6 eV above the energy level of highest occupied molecular orbital (HOMO) of organic hole transporters [2, 3], the injection barrier is always no less than 0.5 eV, leading to inefficient hole injection and thereby high driving voltage in such OLEDs. However, if the p-doped materials are utilized, an efficient hole injection can be achieved. In this case, athough the barrier height of hole injection remains almost the same and unchanged by the intervention of the p-doped layer, holes can easily tunnel into organic layer of an OLED through the very thin depletion zone formed at the interface of p-doped material and anode even at very low driving voltage [4]. In this chapter, we introduce a method of efficient hole current generation at the interface of electron donor and acceptor, in clear contrast to the above-mentioned hole injection

The active matrix displays based on OLEDs have been successfully applied to portable electronic devices, e.g., mobile phones and music players. In order to facilitate the large-scale commercialization for active matrix OLED displays, it is of great necessity to reduce their fabrication cost. Hence, the n-channel amorphous silicon thin-film transistor technology may preferably be utilized to drive the light emitting elements, but requires the OLED structure with inverted layer sequence [8, 9]. Compared to the regular OLEDs, higher driving voltages are always obtained in the inverted ones due to the poor electron injection as a consequence of the inefficient metal penetration into organics [10,11]. Thus, the n-doped electron transport layers (n-ETLs) are adopted to enhance the electron current in inverted OLEDs [4], e.g., lithium doped bathcuproine (BCP). However, the electron injection from the cathode into n-ETL in the inverted OLED is found always less efficient than that in the regular OLED. Thomschke et al. [12] improved the electron injection in a n-i-p OLED via inserting an interlayer of Bphen double-doped with cesium and silver between the silver cathode and cesium-doped Bphen, followed by the thermal annealing of the whole device. Though this method led to almost the same electrical properties in the inverted OLEDs as those in the regular OLEDs, it is relatively complicated and therefore unsuitable for use in the mass production of the active matrix OLED displays. Here, we introduce the increased electron injection in inverted bottom-

technologies, and discuss its potential applications in OLEDs [5-7].

emission OLEDs (IBOLEDs) via using the combination of two n-ETLs [13-16].

100-nm-thick indium tin oxide (ITO) thin film coated glass substrates were commercially bought with a sheet resistance of 10-30 per square, used as the anode in the conventional OLEDs and as the cathode in the inverted OLEDs. After being cleaned in acetone, alcohol,

**1.3. Electron injection in inverted OLEDs** 

**2. Experimental methods** 
