Work OLEDS

Studies of double carrier injection and transport in insulators and semiconductors (the so called bipolar current problem) date all the way back to the 1950s. A solution that relates to the operation of OLEDs was provided recently by Scott et al. [142], who extended the work of Parmenter and Ruppel [143] to include Lange-vin recombination. In order to obtain an analytic solution, diffusion was ignored and the electron and hole mobilities were taken to be electric field-independent. The current-voltage relation was derived and expressed in terms of two independent boundary conditions, the relative electron contributions to the current at the anode, jJfVj, and at the cathode, JKplJ. 

The materials used as the electron and hole injecting electrodes play a crucial role in the overall performance of the device and therefore cannot be neglected even in a brief review of the materials used in OLEDs. The primary requirements for the function of the electrodes is that the work function of the cathode be sufficiently low and that of the anode sufficiently high, to enable good injection of electrons and holes, respectively. In addition, at least one electrode must be sufficiently transparent to permit the exit of light from the organic layer. 

Pumfrey, Stephen. "The spagyric art or, the impossible work of separating pure from impure Paracelsianism." In Paracelsus the man and his reputation, ed. Ole Peter Grell, 21-51. Leiden Brill, 1998. 

The phenomenon of organic EL was first demonstrated using a small-molecule fluorescent emitter in a vapor-deposited OLED device. The Kodak group first used metal oxinoid materials such as the octahedral complex aluminum tris-8-hydroxyquinoline (Alq3) (discussed above as an ETM) as the fluorescent green emitter in their pioneering work on OLED architectures [167],... 

The concept of using HBMs in OLEDs started with the pioneering work of Kijima et al. when they were trying to get pure blue emission from an EL device with Alq3 as ETM and NPD as an EML [346], An undesired green emission color from Alq3 was suppressed when a thin layer of BCP was added between the NPD and Alq3 layers. 

Other work by Tsuboyama et al. reported a very highly efficient red PHOLED with power efficiency of 8.0 lm/W at 100 cd/m2 using Ir(piq)3 as a dopant [362], Most exciting, however, is the relatively recent demonstration of exceptional lifetimes for these materials in OLED devices where work from UDC has claimed a 14 cd/A red CIE (0.65, 0.35) with a lifetime of 25,000 h at 500 nit. Such performance promises much for phosphorescent red emitters in commercial devices and even higher efficiencies have been realized in systems that compromise the chromaticity toward the deep red with CIE (0.67, 0.33) and lifetimes >100,000 h at 500 cd/m2 [363],... 

Blue fluorescent emitters based on fused polyaromatic ring systems have long been known and systematic work has steadily improved the efficiencies and colors, while pushing the limits of stability in an operational device. A sky blue based on styrylamine doped 2-methyl-9,10-di(2-naphthyl)anthracene OLED was reported to provide the highest efficiency device (Scheme 3.99) [365],... 

Very recently there has been a report about bridged triarylamine helicenes exhibiting CPL [131], These molecules preferentially emit and absorb CP light without the help of an LC matrix. Currently, there seems to be ongoing work to further increase the efficiency of these types of CPL materials and to develop first devices of polarized OLEDs. 

The J-V, L-V, and E V characteristics, measured for the OLEDs made with a commercial ITO anode and an AZO anode are plotted in Figure 6.18a and b, respectively. The current density measured for an OLED with an AZO anode is lower than that obtained for a device made with an ITO anode at the same operating voltage. A slight high turn-on voltage observed in the OLED using an AZO anode is attributed to its lower work function compared... 

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