ABSTRACT

Another significant milestone in OLED device technology involves the discovery of phosphorescent emitters in OLEDs, which was first reported by S. Forrest and M. Baldo in 1998.2,3 These phosphorescent emitters provide a significant boost in device efficiency and, as a result, have gradually become an indispensable emissive material for the flat-panel and portable display industry. Very recently in 2012, C. Adachi’s group developed new emitters based on thermally activated delayed fluorescence (TADF)3 to significantly lower the cost of the device, while maintaining efficiencies as high as those achieved from phosphorescent emitter-based OLEDs. More interestingly, the same concept of TADF has been further exploited as the basis for building efficient host layers using either a single TADF material or two materials deposited simultaneously (cohost) to yield record performance OLEDs. The very first commercial OLED product was introduced by Pioneer Corporation in 1997, which was a passive matrix organic light-emitting diode (PMOLED) display for car audio screens. However, it was not until a decade later in 2007 that Samsung Mobile Display introduced the first commercial AMOLED display, which remains to be the screen of choice for portable smart phones and tablets. As a general light source, the first white OLED was reported in 1995 by mixing emitters of the three primary colors into a single OLED device to produce white light.4 The panel efficiency of white OLEDs has just reached ~130 lm/W at 1000 cd/m2, which well exceeds the performance of a standard fluorescent tube (~70 lm/W). Although the performance of OLED panels has been greatly improved, there is still much room (nearly 120 lm/W) for improvement in efficacy, when considering a theoretical limit of ~250 lm/W. Currently, OLED for lighting remains an active target globally with key challenges revolving around extending the lifetime of the blue emitters, reducing fabrication steps (and cost), improving light extraction, and increasing device stability for high-brightness operations under a continuous electrical drive on both flexible and rigid substrates. Fundamentally, an OLED is an EL device made up of layers of functional organic materials with tens of nanometers thickness that are stacked between two electrodes, an anode and a cathode. In order for light to escape out of the device, one of the electrodes must be transparent. To avoid water vapor and oxygen exposure

that react with the organics to form light-quenching centers, these organic layers are typically deposited by thermal evaporation under ultrahigh vacuum (<1 × 10-7 Torr) environment. A variety of substrates including glass, plastic, Si, or even stainless steel could be used for the OLEDs to be deposited on as long as they are sufficiently smooth (under ~30 nm in roughness). Figure 1.2 shows an illustration of the OLED working principle. Under an applied bias voltage that induces an electric field, holes (electrons) are injected from the anode (cathode) and migrated through the hole (electron) transport layer to the emissive layer (EML) at the center. Since organic molecules are held together by weak van der Waals forces, the migration of charges is a random, molecule-to-molecule, hopping process. The applied voltage is large enough such that the difference in quasi-Fermi levels formed between the two electrodes exceeds the energy gap of the host (typically >2.5 eV) in order to be able to supply charge carriers with sufficient energy and density into the host layer. Once inside the EML, electrons and holes pair up to form tightly bound pairs called excitons due to Coulomb interaction.5 Here, the EML typically consists of a host-dopant combination where the dopant has a lower energy corresponding to the visible spectral range and is incorporated in small fractions into the host matrix. In this way, once high-energy excitons are formed in the host, they are able to reach thermodynamic equilibrium by performing excitonic energy transfer, either of a Förster6 or Dexter7 type, to form lower-energy excitons on the dopant molecules facilitated by a close spatial proximity and a strong spectral overlap between the host (donor) and the dopant molecules (acceptor). These initially formed excitons on the dopant molecules then quickly relax to the lowest singlet (S1) and triplet (T1) states following Kasha’s rule5 as a result of the much faster internal conversion (IC) rate than the intersystem crossing (ISC) rate, before finally releasing the energy either radiatively to produce vibrant, visible colors, or nonradiatively as heat via molecular vibrations and other quenching processes. After radiative emission, the absorption by the same type of dopant or other lower-energy dopants in the device is minimal since the dopants are present only in small fractions within the relatively thin EML layer. In general, the nonradiative relaxation rate of the lowest-energy excitons on the dopant molecules follows the energy gap law:8

knr = 1013 e –aEg, (1.1)where α is a proportionality constant depending on the nature of the molecule, and Eg represents the energy gap as determined from the dopant molecule’s lowest singlet-exciton energy level. This implies that the lower the emission energy (longer emission wavelength), the higher the likelihood of nonradiative processes occurring, hence the less efficient the dopants are.