Doped organic semiconductors: Physics and application in light emitting diodes

In this paper, we discuss recent experiments which prove that evaporated organic films can be efficiently doped by co-evaporation with organic dopant molecules. Key advantages for devices are the high conductivity and the formation of ohmic contacts despite large energetic barriers. For p-type doping, efficient doping is possible for a variety of polycrystalline and amorphous materials. Despite the differences in the microscopic behavior, all basic effects known from doped inorganic semiconductors are found in organics as well. However, efficient n-type doping with stable molecular dopants is still a challenge. Organic light emitting diodes (OLED) with conductivity doped transport layers show significantly improved properties: For instance, we have achieved a brightness of 100 cd/m(2) already at a voltage of 2.55 V, well below previous results for undoped devices. The advantages of doping are even more pronounced for top-emitting, inverted OLED structures: Due to the ohmic contacts nearly independent of the contact properties, it is possible to realize inverted top-emitting devices with parameters comparable to standard devices. Our doping technology is thus a significant advantage for active-matrix OLED displays and other displays on opaque substrate. (C) 2003 Elsevier B.V. All rights reserved.