The basic requirement of the HTL is good hole conductivity. In conjugated polymers hole conductivity arises through conjugated bonding, and in small-molecule hole transport
Liq LiMeq Liph LiOXD
Figure 6.17 Organo-metallic complexes may also be used for the electron injection layer. Examples are shown consisting of some lithium-quinolate complexes. Liq, LiMeq, Liph and LiOXD. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis
materials the same mechanism applies, combined with the transfer of charge between HTL molecules. As shown in Figure 6.11, TPD and NPD are popular hole conductors consisting of small molecules containing six-carbon rings with conjugated bonds allowing intramolecular hole transport. TPD and NPD are members of a family of compounds known
Triarylamenes were developed for xerography in the 1970s and are well-developed photoconductive materials. Here the electrical conductivity is controlled by the density of mobile charge carriers that are generated by illumination of the triarylamine.
Both TPD and NPD are commonly applied to OLEDs due to their modestly high hole mobilities in the range of 10-3 to 10-4cm2/Vs. A significant challenge is their low- temperature crystallization, which progresses slowly at typical device operation temperatures of 30-40°C. This causes the materials to become mechanically unstable and device stability is compromised.
Another group of triarylamenes include the hole conductors triphenylamine (TPA) and TPTE, shown in Figure 6.18. OLEDs employing these materials may be operated continuously at temperatures of 140°C without breakdown since they do not crystallize readily. A number of other triarylamenes are being studied also. One additional key requirement
Figure 6.18 Two further examples of hole-conducting triarylamenes include TPA (triphenylamine) and TPTE (a tetramerofTPA). TPTE enables high-temperature OLED operation without crystallization. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis
for efficient OLED devices using them is the size of the energy barrier at the interface of the HTL and the HIL, which must be small enough to result in an efficient OLED. Hence the HOMO level of the HTL should be within a fraction of an electron volt from the anode energy band. In Figure 6.14 the conduction band of ITO is shown at 4.7 eV below a vacuum reference level and the HOMO levels of TPD and NPD are suitable, being close to 5 eV below the vacuum level.
In addition to the triarylamines, another family of hole transport materials consists of the phenylazomethines. Four examples of phenylazomethines are shown in Figure 6.19.
Upon mixing these phenylazomethines with metal ions, such as Sn ions, the resulting metal complexes can form good HTL materials with both thermal stability and high-efficiency charge injection of holes. These complexed materials exhibit HOMO levels in the range of -5.2 eV to -5.4 eV (5.2 to 5.4 eV below the vacuum level), which accounts for their good efficiency.