A series of poly(norbornenes) with pendant triarylamine (TPA) groups has been synthesized by ring-opening metathesis polymerization and investigated as hole transport materials in organic two-layer light-emitting diodes (LEDs). Efficient device fabrication through spin casting of the hole transport layer (HTL) was possible, since the polymers exhibited excellent film formation properties. LEDs of the form ITO/poly(norbornene)-TPA/Alq3/Mg (ITO = indium tin oxide, Alq3 = tris(8-quinolinato)aluminum) showed bright green emission with external quantum efficiencies of up to 0.77% (1.30 lm/W) for 20 nm thick HTL films. The length and polarity of the linker between the triarylamine functionality and the polymer backbone were varied systematically. The device performance was found to depend strongly on these structural differences. Substitution of ester groups by less polar ether functionalities greatly enhances external quantum efficiencies, lowers the operating voltage, and improves the stability of the device. Further improvement of the device characteristics is achieved by reducing the length of the alkyl linker. The HTL can be conveniently cross-linked by UV irradiation. Cross-linking was found to decrease device performance. A maximum external quantum efficiency of 0.37% was achieved for an Alq3-LED with cross-linked HTL.

A series of soluble arylamine-based hole-transporting polymers with glass transition temperatures in the range of 130−150 °C have been synthesized. The synthetic methodology allows facile substitution of the aryl groups on the amine with electron-withdrawing and electron-donating moieties, which permits tuning of the redox potential of the polymer. These polymers have been used as hole-transport layers (HTLs) in two-layer light-emitting diodes ITO/HTL/Alq/Mg [ITO = indium tin oxide, Alq = tris(8-quinolinato)aluminum]. The maximum external quantum efficiency of the device increases if the redox potential of the HTL is increased to facilitate reduction of the positive charge carriers at the HTL/Alq interface. A fluorinated hole-transport polymer with a relatively large redox potential (390 mV vs ferrocenium/ferrocene) yielded the device with the highest external quantum efficiency of 1.25% photons/e-. The device stability, however, follows the opposite trend. The device with the most electron-rich HTL exhibited the best performance after prolonged usage.

We have used triphenyldiamine side-group polymers as hole transport layers in multilayer organic light-emitting diodes using 8-hydroxyquinoline aluminum (Alq 3 ) as an emission layer. The device efficiency systematically increases as the ionization potential of the hole transport layer is shifted further from the work function of the indium-tin-oxide anode. We attribute this trend to better balance of hole and electron charges in the device. An optimized device consisting of a fluorinated version of the polymer as the hole transport layer, quinacridone doped Al as the emission layer, and a LiF/Al cathode results in a peak external luminous efficiency of 20 lm/W. © 1999 American Institute of Physics. ͓S0003-6951͑99͒02421-3͔Research interest in organic light-emitting diodes ͑OLEDs͒ 1,2 continues to grow as their performance approaches a commercially viable level for applications such as low-cost, flat-panel displays. In order to be useful, these devices must have high brightness and efficiency, while requiring a low operating voltage. Multilayer devices consisting of thermally deposited hole transport layer ͑HTL͒ and emission layers have been shown to have high performance and good operational stability. 3,4 The HTL typically consists of a triphenyldiamine ͑TPD͒ or similar compound which is known to have high hole mobility. TPD also has an ionization potential ͑IP͒ which is well positioned between the work function of indium-tin-oxide ͑ITO͒ (ϳ4.7 eV) and the IP of many emission materials. Initial studies addressing the effects of varying the IP of the HTL on the device performance have led to differing results. 5,6 However, more recent studies have shown that the device quantum efficiency increases as the difference between the IP of the HTL and the emission layer is decreased. 7,8 These studies have generally been done using thermally deposited small-molecule hole transport materials. One disadvantage to this approach is that the morphological properties of the HTL film are affected by the particular molecular design. Possible crystallization of the hole transport material and poor interfacial contact with the ITO anode result in decreased device performance. In this study, we use a series of functionalized polymers with TPD derivative side groups as the HTL. The IP of these polymers can be controlled to provide a systematic way to investigate the importance of the IP of the HTL to the device performance while maintaining a consistent film morphology.The IP of TPD has been measured to be 5.38 eV using ultraviolet photoelectron spectroscopy. 9 This value can be systematically decreased ͑shifted toward the vacuum level͒ by adding an electron-donating moiety, such as p-OCH 3 , or increased ͑shifted further from the vacuum level͒ by adding an electron-withdrawing moiety, such as m-F. This principle is demonstrated by the three polymer TPD derivatives shown in Fig. 1, P1-P3, that have an IP that ranges from 5.06 to 5.56 eV. In this study, we used polymers P1-P3 as the HTL in double-layer OLEDs with a thermally evapora...