**1.2. Hole injection in OLEDs**

The light emission from an electrically driven OLED occurs due to the recombination of positive and negative charge carriers, hereafter called as holes and electrons, respectively

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

The Advanced Charge Injection Techniques Towards the Fabrication of High-Power Organic Light Emitting Diodes 145

and de-ionized water sequentially by an ultrasonic horn, the patterned ITO substrates were blown dry by nitrogen gun and then treated in UV-ozone for 15 min. The base vacuum

Copper phthalocyanine (CuPc, electron donor) and 3, 4, 9, 10 perylenetetracarboxylic dianhydride (PTCDA, electron acceptor) were used to form the interfaces of generating holes and electrons. Molybdenum oxide (MoO3) and NPB were adopted as hole injection and transport materials, respectively. Tris(8-quinolinolato) aluminum (Alq3) and BCP were chosen as the emissive and electron transport materials, respectively. The lithium carbonate was selected as an n-dopant to the PTCDA and BCP. 1,4,5,8-naphthalene-tetracarboxylic-

The current versus voltage (I-V) characteristics of the devices were measured by a programmable Keithley 2400 sourcemeter or a Keithley electrometer 617, and the device luminance was recorded by an ST-86LA spot photometer. The optical absorption spectra of organic thin films were obtained using a Cary 300 spectrophotometer or an UV-3100 spectrophotometer. The X-ray diffraction (XRD) measurements were performed on an X-ray

**3.1. Hole generation at the interfaces of electron donor (CuPc) and acceptor** 

*3.1.1. The hole generation at the planar CuPc/PTCDA interface* 

Device 1: ITO/ CuPc 5 nm/ NPB 75 nm/ Alq3 60 nm/ Mg:Ag/ Ag; Device 2: ITO/ PTCDA 10 nm/ NPB 70 nm/ Alq3 60 nm/ Mg:Ag/ Ag; Device 3: ITO/ PTCDA 20 nm/ NPB 60 nm/ Alq3 60 nm/ Mg:Ag/ Ag;

Device 4: ITO/ PTCDA 10 nm/ CuPc 5 nm/ NPB 65 nm/ Alq3 60 nm/ Mg:Ag/ Ag; Device 5: ITO/ PTCDA 20 nm/ CuPc 5 nm/ NPB 55 nm/ Alq3 60 nm/ Mg:Ag/ Ag;

Note that, the devices 4 and 5 are fabricated with planar PTCDA/ CuPc interfaces for hole

We firstly fabricated the following light emitting devices:

Pairs of organic donors and acceptors have been applied in organic solar cells to realize the efficient photo-to-electricity conversion since 1986 [17]. The underlying mechanism for organic photovoltaic devices is the photoinduced electron transfer from donor to acceptor, generating free holes in donor and electrons in acceptor. The donors and acceptors in organic photovoltaic devices constitute either planar interfaces or mixed interfaces. Recently, the planar donor/acceptor interfaces have been successfully used to realize the ambipolar transport in organic field effect transistors [18, 19]. The recent work shows that the planar and mixed interfaces of donor and acceptor can generate very efficient hole

pressures of the thin film and device fabrications were 110-5 to 410-4 Pa.

dianhydride (NTCDA) was also chosen to form an n-doped material with ITO.

diffractometer (D/max-RB).

**(PTCDA) in OLEDs** 

current in OLEDs [5-7].

injection.

**3. Results and discussion** 
