Materials for the electron transport layer (ETL) have been investigated intensively and several families of candidate materials are known. Intermolecular transport occurs by electron hopping, and a LUMO level that is similar in energy to the workfunction of the cathode and the electron-conducting level in the EIL is required, as shown in Figure 6.14.
The ETL should have a mobility of at least 10-6 cm2V-1s-1, which is one to two orders of magnitude smaller than the mobility range of HTL materials. Improving this low mobility has been one key target of the intensive investigation of these materials. Insufficient electron mobility in the ETL means that in many cases holes that enter the light emitting layer (LEM) will not encounter electrons and will therefore continue until they reach the ETL before they recombine. Since the ETL is not optimized for high recombination efficiency, a lower device efficiency can result. ETL is also often oxidized by hole conduction, in which electron loss and subsequent degradation of the ETL material occur due to holes that enter the material. This is a major degradation mechanism for small-molecule OLEDs.
The crystallization temperature of the ETL should be high enough to retain the amorphous structure during device operation at the operating temperature. Generally a glass transition temperature should be above 120°C.
The ability of the ETL to withstand long-term exposure to the applied electric field is essential. Since the ETL has a lower mobility and therefore lower conductivity than the HTL a larger voltage drop and hence a larger electric field drops across the ETL. The molecules in the ETL should not lose multiple electrons by field ionization. However, they need to be able to permit the flow of one electron at a time and reversibly change charge state by a single electron charge as the electron enters and leaves a given molecule.
Finally, the ETL material must be able to be processed and coated with good interface stability and with good layer uniformity and quality.
By far the most common and most successful ETL is Alq3 as shown in Figure 6.11. This is an example of a metal chelate material. An Al3+ ion at the centre of the molecule is surrounded by three side-groups called quinolines. Alq3 has a glass transition temperature of over 172°C and an electron mobility of 1.4 x 10-6 cm2V-1s-1. The LUMO level is -3.0 eV below the vacuum level, which is a good match to the cathode workfunction of 3.6-3.8 eV in the case of LiF/Al.
The concern regarding the low mobility of Alq3 is lessened since Alq3 also functions well as a LEM. This means that an oversupply of holes can penetrate a relatively thick ETL and recombine radiatively within this combined LEM/ETL. The emission wavelength in this material has been extensively studied and various substitutions may be made to modify the emission characteristics while retaining the electron transport properties. Examples of substitutions will be discussed in the context of light emitting materials discussed in the next section.
Figure 6.20 TPBI, ATZL and TPQ are members of imine-based molecules which are candidate electron transport layer (ETL) materials as well as light emission materials. Other candidate ETL materials include C60. See Section 6.16. Chemical Structure reproduced from Organic light – emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis
Another important class of ETL materials is the group of oxadiazoles. In Figure 6.11 the molecule PBD is an example of an oxadiazole having a LUMO level of — 2.16 eV, which permits a high device efficiency. Unfortunately the materials have low glass transition temperatures of about 60°C, although this can be increased by making larger molecules that resemble groups of two or four PBD molecules connected to each other forming a new molecule with a linear or a star shape respectively. The most serious difficulty associated with the use of oxadiazoles, however, is the tendency of the excited states of the molecules to be unstable resulting in short device lifetimes.
Other potential ETL materials include various molecules containing double-bonded C=N groups, which are known as imines. These include TPBI, ATZL and TPQ, as shown in Figure 6.20. TPBI has an electron mobility in the range of 10—6 to 10—5 cm2V—1s—1, which is slightly higher than Alq3. It has a LUMO level of —2.7 eV. TPQ has even higher mobility, of 10—’4cm2V—1s—1 and good thermal stability.