Rubrene OLEDs

By optimization of device structures and by using different hole and electron injection or transport materials, Chen et al. achieved an excellent red OLED with a very high efficiency based on the DCJTB molecule. The OLED structure is glass (0.7 mm)/SiO2(20 nm) ITO/CFx/NPD(l 10 nm)/Alq3 5% rubrene 2%DCJTB(30 nm)/Alq3(55 nm)/LiF(0.1... 

This material has been used extensively as the HTL. However, its glass transition temperature Tg is arelatively low 65° C (see Table 1.1). Hence, it causes a failure of OLEDs as it recrystrallizes (see Section 1.9 below). The recrystallization may be suppressed and the device lifetime greatly enhanced by adding a guest molecule such as rubrene. However, in that case carriers may recombine on the rubrene, resulting in red EL from that guest molecule.69... 

As mentioned above, rubrene is a prominent red-emitting molecule, as its PL quantum yield is 100% in dilute solution, but that emission is strongly suppressed in the solid state due to fission of the l1 to two triplets. Hence, it yields bright red OLEDs when incorporated as a guest in hosts such as TPD.69... 

Fig. 3.4. The SV plot at 23°C of a PdOEP-based gas-phase sensor excited by a rubrene-doped Alqs OLED [17]. The sensing film was prepared by drop casting. The SV plot is linear up to 40% O2. Copyright 2006, with permission from Elsevier...

First, OLEDs fabricated by vacuum process are discussed. Four kinds of OLEDs fabricated by OMBD are discussed and the emission characteristics are summarized. There are two types of organic materials for OLEDs one for undoped type such as a-NPD and Alqg, and the other is for doped type such as rubrene and porphine derivative (TPP), which are doped in a carrier transporting material. In case of a device in which a-NPD is... 

Emission characteristics of OLED with rubrene doped in Alqj emissive layer, (a) Emission spectrum and the molecular structure of rubrene. (From Ohmori, Y. et al., IEEE J. Selected Top. Quantum Electron., 10, 70,2004. With permission.) (b) Current-voltage and EL intensity-voltage characteristics of OLEDs, and (c) EL output signal at lOOMHz directly modulated by pulsed voltage application. (From Ohmori, Y. et al., IEEE. Selected Top. Quantum Electron., 10, 70, 2004. With permission.)... 

The SV plot (see Equation 16.1) for gas-phase Oj at 23°C, obtained from a drop-cast PdOEPrPS film excited by a rubrene-doped Alqs OLED. Note that the SV plot is linear only up to -40% Oj. (From Shinar, R. et al.. Anal. Chim. Acta, 568,190,2006. With permission.)... 

To monitor O2 levels accurately and reliably over the entire 0%-l(X)% O2 range, PtOEP- and PdOEP-based sensors would be used simultaneously. Alternatively, by using different films prepared under different conditions (e.g., PS dye ratio, film thickness), different SV plots and detection sensitivities are expected. For example, a simple array could consist of 2-4 Alqj OLED pixels exciting a 1 10 PtOEP PS film, which exhibits near linear SV plot over the whole 0%-100% range (see Figure 16.4), and 2-4 rubrene-doped Alqj OLED pixels exciting a PdOEPiPS film, that is very sensitive to low levels of O2 and exhibits a linear behavior up to -40% O2 (Figure 16.3). °... 

In order to investigate the referenced inkjet-printed film in an OLED, some inkjetted PEDOT-PSS films were used as the anode. On top of the inkjet-printed anode, the hole transport layer (HTL) solution (TPD, [M, M, -bis(3-methylphenyl)-N JV dimethyl benzidine] 67.6 wt.%, polycarbonate (PC) 29.0 wt.%, rubrene 3.4 wt.%, 10.35 mg/ml chloroform) was spin-coated at 1000 rpm for 1 min in a class 100 cleanroom. A 60 nm layer of tris-(8-hydroxyquinoline)-aluminum (Alq3) was then thermally deposited under the high vacuum at the rate of 0.7 A/s. Then, a 300 nm layer of Mg Ag (magnesium -silver) was thermally coevaporated at the ratio of 10 1 on the top of electron transport layer (ETL) layer (Figure 3.10). The thickness of spin-coated layers was matched to one of the inkjet-printed layers (L = 0). Additionally, Figure 3.10 shows results of OLED characteristics with the same layer configuration except that ITO is used as the anode layer. 

---