ABSTRACT
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .186
functions between 4 and 5 eV. e compounds that have HOMO levels matching these values are air stable weak electron donors. A typical example of such compounds would be an aromatic heterocycle containing nitrogen heteroatoms, such as triarylamine or carbazole derivatives. Indeed, the rst heterostructure OLED employed 1,1′-bis[(di4-tolylamino)phenyl]cyclohexane (TAPC), in which the hole transport was provided by two tolylamine functionalities. TAPC has fairly high hole mobility for an organic compound, 2.9 × 10−4 cm2/Vs (Muellen and Scherf 2006; Aonuma et al. 2007). In addition to sucient charge mobility, TAPC has high enough triplet exciton energy (2.87 eV, [Goushi et al. 2004]) to provide exciton connement in the emissive layer even for phosphorescent blue-emitting devices. Finally, since the lowest singlet excited state energy of an organic molecule is higher than the lowest triplet. TAPC possesses a shallow LUMO as well, making it a good electron blocker for most device structures. All these desirable characteristics of TAPC mean that, in fact, this rst hole transport material (HTM) ever used in a modern OLED has been the material to be included in some of the most ecient blue devices (Chopra et al. 2010) to date. However, as an OLED component, TAPC has one crucial issue that keeps the search for new HTLs an ongoing problem: its stability in an operating device is insucient due to chemical degradation via bond breaking at the saturated carbon site (Kondakov 2008). Stability continues to be an issue for many materials; chemical degradation and crystallization are both known to lead to loss of performance. Some of the materials reviewed further in this chapter achieved outstanding charge transport characteristics, others-excellent exciton and charge con-ning properties, others still showed very long operational life times. But a material that combines all these desired properties has not yet been found at the time of writing. First, this chapter will focus on methods to characterize HTL materials, followed by a discussion of the technologically relevant issue of achieving high charge conductivity in thick layers of organic lms. Finally, we will conclude with a review of the current state of material development and remaining challenges in HTMs to enable wider OLED commercialization.
