Hole-transport layer

There is one added layer which deserves special mention, namely a thin copper phthalocyaninc layer, which has been placed [103] between an 1TO anode and the hole transport layer. It is not an injection layer in the sense just discussed, because its HOMO is not well aligned with the 1TO Fermi energy and it slightly raises the operating voltage of the structure. It does, however, dramatically improve the stability of the device and appears to act as an adhesion layer for the organic materials above it. The inechanism(s) for these improvements is not yet well understood. 

Using a stable dopant as the emissive dye has been shown to greatly enhance the lifetime of small molecule LEDs. Rubrene doped into the Alq, electron transport layer ] 184] or into the TPD hole transport layer 1185] can extend the lifetime by an order of magnitude. Similarly, dimclhylquinacridone in Alq has a beneficial effect ]45 ]. The likely mechanism responsible for this phenomenon is that the dopant acts as a trap for the excilon and/or the charge. Thus, molecules of the host maLrix are in their excited (cationic, anionic or cxcitonic) states for a smaller fraction of the time, and therefore have lower probability to undergo chemistry. 

In contrast with conjugated polymers, such as PPV, devices employing CN-PPV 47 as the emissive layer can achieve respectable internal efficiencies (ca. 0.2%) with both calcium and aluminum electrodes. EL efficiency may be further improved by employing a hole-transporting layer such as PPV in conjunction with... 

According to that model, the net current flow in the device therefore can be increased in bilayer structures using a hole-transport layer, which possess higher hole mobility than the active polymer layer and which changes the height of the potential barrier at the interface transport layer/hole injection contact [81],... 

Figure 9-28. Trap-limited current (low ills (solid lines) lo the experimental (symbols) l/V characteristics of two typical devices with a 200 nin and 600 nm thick hole-transport layer and Alq3. Inset shows l/V curves for various different Alq3-lhicknesses. Reproduced front Ref. 82. ...

A typical multilayer thin film OLED is made up of several active layers sandwiched between a cathode (often Mg/Ag) and an indium-doped tin oxide (ITO) glass anode. The cathode is covered by the electron transport layer which may be A1Q3. An emitting layer, doped with a fluorescent dye (which can be A1Q3 itself or some other coordination compound), is added, followed by the hole transport layer which is typically a-napthylphenylbiphenyl amine. An additional layer, copper phthalocyanine is often inserted between the hole transport layer and the ITO electrode to facilitate hole injection. 

L. Ding, F.E. Karasz, Z. Lin, M. Zheng, L. Liao, and Y. Pang, Effect of Forster energy transfer and hole transport layer on performance of polymer light-emitting diodes, Macromolecules, 34 9183-9188,2001. 

D.C. Muller, T. Braig, H.-G. Nothofer, M. Amoldi, M. Gross, U. Scherf, O. Nuyken, and K. Meerholz, Efficient blue organic light-emitting diodes with graded hole-transport layers, Chem-physchem., 1 207-211, 2000. 

The simplest manifestation of an OLED is a sandwich structure consisting of an emission layer (EML) between an anode and a cathode. More typical is an increased complexity OLED structure consisting of an anode, an anode buffer or hole injection layer (HIL), a hole transport layer (HTL), a light-emitting layer, an electron transport layer (ETL), a cathode... 

Electron and hole migration through the electron and hole transport layers... 

Q. Huang, J. Cui, H. He, J.G.C. Veinot, and T.J. Marks, Small molecule organic light-emitting diodes can exhibit high performance without conventional hole transport layers, Appl. Phys. Lett., 81 3528-3530 (2002). 

A. Yamamori, C. Adachi, T. Koyama, and Y. Taniguchi, Doped organic light emitting diodes having a 650-nm-thick hole transport layer, Appl. Phys. Lett., 72 2147-2149 (1998). 

C. Giebeler, H. Antoniadis, D.D.C. Bradley, and Y. Shirota, Influence of the hole transport layer on the performance of organic light-emitting diodes, J. Appl. Phys., 85 608-615 (1999). 

K. Yamashita, T. Mori, T. Mizutani, H. Miyazaki, and T. Takeda, EL properties of organic light-emitting-diode using TPD derivatives with diphenylstylyl groups as hole transport layer, Thin Solid Films, 363 33-36 (2000). 

C. Liao, M. Lee, C. Tsai, and C.H. Chen, Highly efficient blue organic light-emitting devices incorporating a composite hole transport layer, Appl. Phys. Lett., 86 i.d. 203507, 3 pages (2005). 

Y. Wang, Dramatic effects of hole transport layer on the efficiency of iridium-based organic light-emitting diodes, Appl. Phys. Lett., 85 4848-4850 (2004). 

FIGURE 7.1 A two-layer vapor-deposited OLED first demonstrated by Tang et al. [4], The diamine acts as the hole transporting layer, Alq3 acts as the electron transporting or emitting layer. The external quantum efficiency was 1%. 

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