Mobilities 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. 

Another issue that can be clarified with the aid of numerical simulations is that of the recombination profile. Mailiaras and Scott [145] have found that recombination takes place closer to the contact that injects the less mobile carrier, regardless of the injection characteristics. In Figure 13-12, the calculated recombination profiles arc shown for an OLED with an ohmic anode and an injection-limited cathode. When the two carriers have equal mobilities, despite the fact that the hole density is substantially larger than the electron density, electrons make it all the way to the anode and the recombination profile is uniform throughout the sample. 

The analytic theory outlined above provides valuable insight into the factors that determine the efficiency of OI.EDs. However, there is no completely analytical solution that includes diffusive transport of carriers, field-dependent mobilities, and specific injection mechanisms. Therefore, numerical simulations have been undertaken in order to provide quantitative solutions to the general case of the bipolar current problem for typical parameters of OLED materials [144—1481. Emphasis was given to the influence of charge injection and transport on OLED performance. 1. Campbell et at. [I47 found that, for Richardson-Dushman thermionic emission from a barrier height lower than 0.4 eV, the contact is able to supply... 

Due to the relatively high mobility of holes compared with the mobility of electrons in organic materials, holes are often the major charge carriers in OLED devices. To better balance holes and electrons, one approach is to use low WF metals, such as Ca or Ba, protected by a stable metal, such as Al or Ag, overcoated to increase the electron injection efficiency. The problem with such an approach is that the long-term stability of the device is poor due to its tendency to create detrimental quenching sites at areas near the EML-cathode interface. Another approach is to lower the electron injection barrier by introducing a cathode interfacial material (CIM) layer between the cathode material and the organic layer. The optimized thickness of the CIM layer is usually about 0.3-1.0 nm. The function of the CIM is to lower... 

The most commonly used HTL materials are triarylamine compounds. These compounds were developed as HTMs for photoconductive applications such as xerography [69]. They naturally have been selected as HTMs for OLED applications largely because of their ready availability and their good electrochemical and thermal stabilities. The hole mobilities of these materials are also adequate for OLED applications. In addition, high purity, so as to ensure low hole-trap contamination, is believed necessary for long-lived OLED performance and such materials may often be train sublimed to very high purity. 

A new branched carbazole derivative with phenyl ethylene moieties attached, l,3,5-tris(2-(9-ethylcarbazyl-3)ethylene)benzene (TECEB, 41) (Scheme 3.15), was prepared as a HTM for OLEDs [86], TECEB has a HOMO energy level of —5.2 eV and hole-drift mobility of 1(T 4 cm2/(V s), comparable to NPD. The device performance (maximum luminance of about 10,000 cd/m2 and current efficiency of 3.27 cd/A) in a standard HTL/tris-(8-hydroxyquino-line) aluminum double-layer device is also comparable to NPD, but TECEB has a higher Tg (130°C) and its ease of synthesis is superior to NPD. Distyryl units linked to a TPD derivative, A, A"-bis(4-(2,2-diphenylethenyl)-phenyl)-jY,jV -di(p-tolyl)-bendidine (DPS, 42) (Scheme 3.15), reported by Yamashita and coworkers, showed good hole transport properties and improved thermal stability compared with the parent TPD [87]. 

Methods to determine or justify the utility of the electron transport properties of ETM are TOF electron mobility and electron-only diode device measurement as well as the overall OLED performance. 

Triazines are well-known compounds with high thermal stability and higher EA than 1,3,4-oxadiazoles (PBD) and 1,2,4-triazoles (TAZ, 92). Schmidt et al. studied a series of dimeric 1,3,5-triazine ethers for application as ETMs for OLEDs [150], However, despite their high EA, the efficiency of the OLEDs improved only modestly. One possible explanation is due to their rather poor electron mobilities. 

Without using Alq3 as the ETM, BCP has been used as an ETL and was demonstrated in DPVBI-based blue OLEDs [347], BCP has superior electron transport properties and its electron mobility is around 5.2 x 10-4 cm2/(V s) (5.5 x 105 V/cm) as measured by the TOF method [348], The concept of using BCP has been extended into doped OLEDs. 

To facilitate good charge transport in an OLED, the organic materials must satisfy three key requirements they must have a high mobility for either electrons or holes, a good injection efficiency from the contact electrode, and suitable band offsets with other organic layers within the device. These processes are discussed in detail by, for example, Kalinowski [73] and Greenham and Friend [74],... 

The criteria for good electron transport materials are that they should transport electrons, block holes, and have a small barrier to electron injection from the metal cathode. The most commonly used ETL in vacuum-deposited OLEDs is tris-(8-hydroxyquinoline) aluminum (Alq3), as shown in Figure 7.7. Alq3 for example, has a LUMO energy level of 3 eV [65] and an electron mobility of 5x 10 5 cm2/(V s) [66]. 

One of the most obvious markets for thin-film vapor-deposited organic materials is in flat panel displays [123], a market currently dominated by LCDs. Over the last two decades, a great improvement in the lifetime and efficiency of OLEDs have been achieved. OLED displays can already be found in simple applications such as automobile stereos, mobile phones, and digital cameras. However, to exploit the advantages of the technology fully, it is necessary to pattern the OLEDs to form monochrome, or more preferentially, full-color displays. This section will consider the difficulties involved in addressing such displays (either passively or actively) and the variety of patterning methods that can be used to produce full-color displays. 

TFT field-effect mobility compensation OLED threshold voltage compensation... 

Important electrical informations about OLEDs, such as charge transport, charge injection, carrier mobility, etc., can be obtained from bias-dependent impedance spectroscopy, which in turn provides insight into the operating mechanisms of the OLED [14,15,73,75 78]. Campbell et al. reported electrical measurements of a PLED with a 50-nm-thick emissive layer [75], Marai et al. studied electrical measurement of capacitance-voltage and impedance frequency of ITO/l,4-Mv-(9-anthrylvinyl)-benzene/Al OLED under different bias voltage conditions [76], They found that the current is space-charge limited with traps and the conductivity exhibits power-law frequency dependence. 

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