P-type Doping of Organic Charge Transport Materials with Tungsten Oxide
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P-type Doping of Organic Charge Transport Materials with Tungsten Oxide X. M. Li, R. Y. Yang, and X. A. Cao Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506, USA ABSTRACT The effects of WO3 doping in 4,4’-bis-9-carbozyl biphenyl (CBP) were studied through detailed electrical device characterization. A series of hole-only devices have been fabricated, where the doping level was varied from 10-40mol% and the doped CBP thickness was varied from 5-40 nm. It was found that, to achieve effective doping for improved hole injection and transport, the doping level should be greater than 20mol% and the doped layer should be at least 10 nm thick. It was also found that an energy barrier exists at the doped and undoped CBP interface, resulting in an additional voltage drop. This finding was explained by a large downward shift of the Fermi level in WO3-doped CBP, which causes band bending and depletion at the interface. Finally, simplified green phosphorescent organic light-emitting diodes (OLEDs) with CBP as the hole transport and host material were fabricated. With a WO3-doped hole transport layer, the OLEDs attained brightness above 105 cd/m2 at 20 mA/cm2, and exhibited an improved reliability under constant-current stressing as compared to undoped OLEDs. INTRODUCTION Electrical doping has been developed to dramatically increase the free carrier density in organic materials by several orders of magnitude and thus improve their electrical conductivity. Organic or inorganic dopants have been successfully introduced into the charge transport layers of organic light-emitting diodes (OLEDs) to enhance their charge transport capability, leading to the development of high-efficiency OLEDs based on a simple p-i-n structure with low operation voltages [1-13]. Doping can also remarkably reduce the interfacial energy barriers between the electrodes and charge transport layers [1], and therefore eliminate the need for additional charge injection layers. The resulting structural simplification improves not only the device manufacturability, but also the device performance as many adverse effects associated with heterointerfaces are suppressed. Effective doping is especially important for short-wavelength phosphorescent OLEDs (PhOLEDs) emitting green and blue light, whose host and charge-transport materials typically have a wide bandgap and a deep-lying highest occupied molecular orbital (HOMO) level [14,15]. Doping provides an important means to overcoming the charge injection and transport problems. Given that the ionization energies of wide bandgap organic materials are very large, above 6 eV in many cases, the choices for appropriate p-type dopants are very limited. Earlier studies using showed that transition metal oxides such as MoO3, WO3, and ReO3 have a large electron affinity above 6.0 eV [16]. They are thus well suited for p-type doping in organic materials with a deep HOMO level. Effective p-type doping in 4, 4’-N,N’-dicarbazolebiphenyl (CBP), N, N`-bis(Inaphthyl)-N,N`-dip
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