barrier between the anode and the I p of the HTL is the primary criterion for efficient hole injection. This points to a more complicated relationship between carrier injection than is predicted simply from energy level offset considerations. Other explanations for efficient hole injection are based on the importance of good adhesion between the organic layers and ITO. Defect states at the interface between the organic layer and the electrode have also been shown to be useful for enhancing injection over energy barriers. [14,15] Conversely varying the work function of ITO and hence the injection barrier into the HTL, was not found to lead to significant change in Z. [16] Thus we observe that injection of carriers can not be fully controlled by simply varying the energy offset barrier at the electrode. It should also be noted that no correlation between device efficiency and the size of the electron-blocking barrier (the difference between the LUMO levels of Alq 3 and the HTL) at the HTL/Alq 3 interface was observed.The most useful of the novel HTLs are ISB and PPD, [7] which maintain the device performance of TPD and a-NPD while significantly increasing the T g . HTLs with a T g equivalent to ISB but lower than PPD have been published previously. [12,13] Adachi et al. [12] reported a series of HTLs with a maximum luminance of 790 cd/m 2 for T g = 110 C at a current density of 30 mA/cm 2 for devices comparable to structure 2. At an equivalent current density, PPD and ISB have luminances of >1000 cd/m 2 and 880 cd/m 2 , respectively. Okutsu et al., for hole transport materials with triphenylamine and fluorene structures, reported a T g between 100 C and 112 C and luminance efficiencies of »2.0 cd/A, [13] for devices also similar in design to structure 2. For ISB and PPD, we find an almost equivalent efficiency at 5 mA of »1.8 cd/A. However Okutsu's devices have a significantly lower power efficiency, with a brightness of only 1 cd/m 2 at 6.0±9.3 V, in comparison to 3.8 V and 5.8 V for PPD and ISB, respectively. Thelakkat et al. reported other triphenylamine derivative HTLs having T g £ 123 C. However, OLEDs based on these materials have a twenty times smaller maximum luminance output than reported here. [17] An HTL with T g = 165 C, which has a quantum efficiency of 1.3 % when incorporated into a device architecture comparable to structure 3, has been patented. [18] Complete information regarding the device characteristics have, to our knowledge, not yet been reported. Other work has used a starburst amine molecule with a T g » 200 C [19] in a double-layer HTL structure. However, device performance comparable to that reported in this work was only achieved by using a second lower T g HTL in the device.In conclusion we have fabricated a series of devices utilizing high glass transition temperature HTLs. Two materials, PPD and ISB, have excellent device characteristics coupled to a high T g , demonstrating that high-T g HTLs can be used to fabricate efficient and stable OLEDs. We have found no relationship...

We identify Stoner ferromagnetism in fcc C60H(n) (n=odd) by using a local density approximation in the framework of the density functional theory. Hydrogen chemisorption on fullerenes creates quasilocalized π electrons on the fullerene surface, overlapping of their wave functions giving rise to a narrow half filled impurity band in the fcc C60H(n). The Stoner-type ferromagnetic exchange between the itinerant electrons leads to spin-split impurity bands. The magnetic moment per C60H(n) molecule is 1  μ(B) (for n=odd) or 0 (for n=even, including zero), only one of the hydrogens contributing to the spin-split states. Direct overlapping of the quasilocalized π-electron orbitals is essential for the ferromagnetism.

Hydrogen shallow donors in undoped and doped ZnO systems have been studied by means of the electron paramagnetic resonance and nuclear magnetic resonance measurements. Experimental evidence is given in this work for coupling of hydrogen shallow donors and Mn ions in Mn-doped ZnO mediating short-range ferromagnetic spin-spin interaction.

Electrical conductivity and photoconductivity measurements were carried out on semiconducting vanadium oxide nanowire networks synthesized through the polycondensation of vanadic acid. The temperature-dependent conductivity was described well by Mott’s optical multiphonon assisted hopping model at high temperatures and by the variable-range hopping mechanism at low temperatures. The photoconductivity showed a weak temperature dependence. Besides, quasipersistent photocurrent with a single-exponential decay behavior was observed right after switching off the light and was explained in terms of the random local-potential fluctuation model.

In the case of colossal magnetoresistance in the perovskite manganites, "double exchange" mediated by the itinerant spins is believed to play a key role in the ferromagnetism. In contrast, the conventional "Heisenberg" interaction, i.e., direct (unmediated) interaction between the localized spins produced by the proton irradiation, is identified as the origin of proton irradiation-induced ferromagnetism in graphite.