(722b) A Modular Approach to Organic Light-Emitting Materials with Tunable Charge Injection and Transport Properties

Since the discovery of efficient electroluminescence at a relatively low voltage using organic materials, including low-molar-mass fluorescent dyes and p-conjugated polymers, intensive efforts have been devoted to improving the efficiency and lifetime of organic light-emitting diodes (OLEDs). While low-molar-mass materials can be deposited as thin films by sublimation, conjugated polymers can be readily processed into large-area thin films by spin-coating from dilute solutions. In principle, electrons and holes are injected from the cathode and anode, respectively, for the formation of excitons in the emissive layer where radiative decay takes place. To achieve a high quantum yield with long device lifetime, it is crucial that charge injection and transport be balanced and that the recombination zone be spread out in space. Four strategies have been reported in the literature: (i) using a low work function metal as the cathode, such as Ca or Ba capped with Al or Ag, (ii) adding an injection, a buffer, and/or a charge-transport layer, (iii) physical blending of an emissive material with a charge-transporting material, and (iv) chemical modification of an emissive material with charge-transporting moieties. Of the four strategies, chemical modification appears to be the most versatile, and hence has been the most intensively pursued. Low-molar-mass evaporable materials have been constructed by bonding electron- or hole-conducting moieties to light-emitting conjugated molecules through p-conjugation, inevitably affecting individual functionalities. In most conjugated polymers, holes are preferentially transported over electrons. Electron transport has been improved by incorporating p-electron-deficient moieties, such as oxadiazole, triazole, triazine, and quinoxaline, in the polymer backbone, as the pendant, or as the end-cap. In the case of blue OLEDs, hole injection is also a limiting factor because of the high ionization potentials of most blue-emitting materials. This difficulty can be overcome in part by adding a layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate), PEDOT:PSS, between the indium tin oxide, ITO, anode and the emissive polymer layer. Because of its acidic nature, PEDOT:PSS was found to etch ITO, causing device instability. This problem has been addressed using an alternative hole-injection material or a self-assembled monolayer on the ITO anode.

Because of the cost advantage organic materials amenable to solution processing into noncrystalline films are of particular interest. While conjugated polymers hold enormous potential in this regard, it is believed that monodisperse conjugated oligomers with a relatively low molecular weight are advantageous from both the scientific and technological perspectives. Monodisperse conjugated oligomers are characterized by a well-defined and uniform molecular structure as well as superior chemical purity acquired through recrystallization and/or column chromatography. Relatively short and uniform chains are also conducive to the formation of monodomain glassy-nematic films without grain boundaries through thermal annealing under mild conditions. These intrinsic merits are imperative to furnishing fundamental insight into structure-property relationships and to improving OLED device performance, as traces of impurities could result in exciton quenching and device failure. In contrast to polymers, oligomers are less likely to undergo glass transition to form a morphologically stable glassy film. Furthermore, few monodisperse conjugated oligomers are capable of both liquid crystalline mesomorphism and glass transition with an elevated glass transition temperature, Tg, to enable the processing of morphologically stable glassy liquid crystalline films that resist crystallization under ambient conditions. Recently, we have reported the first examples of monodisperse glassy-nematic conjugated oligomers for the demonstration of linearly polarized, full-color and white-light OLEDs. Both the luminance yield and polarization ratio are the best of all polarized OLEDs reported to date. Nevertheless, it was also recognized that charge injection and transport must be varied as desired to further improve device performance. Liquid-crystal conjugated oligomers capable of in-situ polymerization represent a viable alternative to preserving molecular order in solid films; however, heat, UV-irradiation and/or initiators and inhibitors are needed for film processing.

The goal of this study is to create multifunctional molecular materials capable of forming glassy isotropic and liquid crystalline films using light-emitting conjugated oligomers and charge injection and transport moieties as the building blocks. A flexible spacer, such as an alkyl chain, connecting the two building blocks serves to permit light emission and charge injection/transport to be incorporated without mutual interference and to prevent crystallization while encouraging glass formation of the hybrid system. Although the merits of a multilayer device structure are well documented, multifunctional materials will offer devices comprising fewer layers, thereby reducing the fabrication costs and operating voltages while improving device performance. Major findings are as summarized in what follows.

Novel molecular systems are designed to incorporate the following features: (1) an electron- and a hole-conducting core, (2) terfluorene and pentafluorene pendants for blue emission (3) a flexible spacer attaching pendants to cores, thereby enabling independent functions of the two structural elements, (4) the ability to form glassy isotropic and liquid crystalline films by solution processing, and (5) tunable charge injection and transport properties while emitting unpolarized and polarized light. Four representative materials have been synthesized and demonstrated to be capable of forming glassy-isotropic and glassy-nematic films with a glass transition temperature and clearing point close to 140 and 250oC, respectively, an orientational order parameter of 0.75, a photoluminescence quantum yield in neat film up to 51 %, and the HOMO and LUMO levels intermediate between blue-emitting oligofluorenes and the electrodes commonly used in OLEDs (e.g. ITO and Mg:Ag). Work is in progress to exploit these promising materials for the fabrication of highly efficient OLEDs with long-term stability, and the results will be the subject of a future publication.