Double-layer device

Figure 16-42. Semi log ploi of current density (0) and luminance (O) of an ITO/Oocl-OPV5 (ISO nm)/Oocl-OPV5-CN" (45 nm)/AI double-layer device as a function of bias voltage. Inset double-layer electroluminescence spectrum.

Hanack and coworkers [161] reported related cyano-substituted naphthalene vinylene derivatives 126 and 127. Interestingly, replacing the phenylene unit in CN-PPV 113 with naphthalene in polymers 126 and 127 results in significant blue shift of the emission maxima from 710 to 595 nm (for 126a) and 500 nm (for 127). In addition, the efficiency, tested for double-layer device ITO/l/127/Mg Al(3 97), was rather low (3>el = 0.017%). 

Polybenzobisthiazoles 608 and polybenzobisoxazole 609 have been used as efficient electron transport materials in PLEDs [71] (Chart 2.143). Although these polymers show poor fluorescence quantum yields in thin films likely due to excimer formation [700], double-layer devices ITO/PEDOT/polymer/ETL/Al with PPV or MEH-PPV as emissive polymers and... 

A new branched carbazole derivative with phenyl ethylene moieties attached, l,3,5-tris(2-(9-ethylcarbazyl-3)ethylene)benzene (TECEB, 41) (Scheme 3.15), was prepared as a HTM for OLEDs [86], TECEB has a HOMO energy level of —5.2 eV and hole-drift mobility of 1(T 4 cm2/(V s), comparable to NPD. The device performance (maximum luminance of about 10,000 cd/m2 and current efficiency of 3.27 cd/A) in a standard HTL/tris-(8-hydroxyquino-line) aluminum double-layer device is also comparable to NPD, but TECEB has a higher Tg (130°C) and its ease of synthesis is superior to NPD. Distyryl units linked to a TPD derivative, A, A"-bis(4-(2,2-diphenylethenyl)-phenyl)-jY,jV -di(p-tolyl)-bendidine (DPS, 42) (Scheme 3.15), reported by Yamashita and coworkers, showed good hole transport properties and improved thermal stability compared with the parent TPD [87]. 

Y. Liu, J. Guo, H. Zhang, and Y. Wang, Highly efficient white organic electroluminescence from a double-layer device based on a boron hydroxyphenylpyridine complex, Angew. Chem. Int. Ed., 41 182-184 (2002). 

Figure 11.19 PL of [Tb(acac-azain)3]2 in PVK and EL of the double-layer device [58]. (Reprinted with permission from R.Y. Wang et al., Syntheses, structures, and electroluminescence of Lu2(acac-azain)4(p.-acac-azain)2 [acac-azain= l-(Af-7-azaindolyl)-1,3-butanedionato, Ln = Tb(lll) and Y(lll)], Inorganic Chemistry, 41, 5187-5192, 2002. 2002 American Chemical Society.)...

The biggest challenge today for the EL devices containing RE + /3-diketonates is the increase of efficiency and stability of the devices. Double layer devices, such as ito/tpd/[Eu /3-dike ton ate]/Al, have demonstrated the ability of the RE + /3-diketonates to act also as electron transporters, despite the large electron energy barrier between the Al electrode Fermi level and the RE + /3-diketonate LUMO level. 

Carter and coworkers studied how side-chain branching in PFs affects device performance with and without an additional HTL of cross-linkable polymer 2 [ 19]. They found that the device efficiency is affected more by the position of the exciton recombination zone than by variations of polymer morphology induced by side-chain branching, which mainly controls the relative emission between vibrational energy levels and has a minimal effect on polymer charge transport properties. For double-layer devices (ITO/PEDOT PSS/2/3,4, or 5/Ca), a typical brightness of 100 cdm 2 at 0.8 MV cm-1, maximum luminance of 10 000 cd m-2 at 1.5 MV cm x, and device efficiencies between 1.3 and 1.8 cd A 1 for 3 and 5 branching can be achieved. 

Liu Y., Guo J.H., Zhang H.D., Wang Y., "Highly Efficient White Organic Electroluminescence from a Double-Layer Device Based on a Boron Hydroxyphenylpyridine Complex", Angeiv. ChertL Int. Ed., 41,2002,182... 

Fig. 5.2 Device configuration and working principle of OLEDs. (a) a triple-layer device showing a hole-transporting layer (HTL), emissive layer (EML) and electron-transporting layer (ETL) sandwiched between two electrodes (b) a double-layer device. An energy diagram showing hopping transport of holes and electrons in (c) a triple-layer device and (d) a double-layer device. Light comes out upon radiative decay of excitons.

Copolymers with fluorene and 1,3,4-oxadiazole show highly efficient photoluminescence. A double layer device consisting of PVK and an alternating copolymer of 9,9 -didodecylfluorene-2,7-diyl and (l,4-bis-( 1,3,4-oxadiazole)-2,5-di(2-ethylhexyloxy)phenylene)-5,5 -diyl exhibits a narrow blue electroluminescence with a maximum at 430 nm. Electrochemical analysis of the pol5miers using cycUc voltametry suggests that they can be used both as electron transport materials and as blue emission materials for LEDs. 

Figure 13 shows the optical properties of a typical copolymer 34e (PMA-DSB-PBD) and its electroluminescence in a double layer device using PPV as hole-injection layer. The device has good shelf stability and an internal quantum efficiency (0.04%). It begins to emit blue light (475 nm) at a forward bias potential of 17 V (Figure 14). 

Figure 8. Current density-luminance curve of single- and double-layer device using RO-PPV.

Figure 9. EL spectrum of a double-layer device using RO-PPV and Alq3 together with PL spectra of RO-PPV and Akjj...

While polyphenylsilane (1) and poly(l-methyl-2-phenyldisilane) (2) rather deteriorate the diode behavior polyaminophenylsilane (3) strongly reduces the onset voltage of the devices and more than doubles the power efficiency and brightness compared to the single-layer LEDs. (Figs. 2, 3) These results compare favorably to the data Kido et al. reported for double-layer devices using Alqs as emissive layer and polymethylphenylsilane (PMPS) as HTL [2]. In this case a luminescence of 115 cd/m was measured at a current of 10 mA. For the device Al/Alqs/polyaminophenylsilane/lTO, 130 cd/m were obtained at the same current density. 

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