Low Molecular Weight Materials: Electron-Transport Materials
Since the first report of practical OLED by Tang and VanSlyke in 1987, metal complex, 8-hydroxyquinoline aluminum (Alq) is widely used for fluorescent OLEDs. However, Alq cannot be used in second- and third-generation green and blue OLEDs due to its low triplet energy (ET) of 2.15 eV. Therefore, researchers have been exploring a novel ETM with high ET in this decade. In this chapter, we summarized and demonstrated representative wide-energy-gap ETMs for use in second- and third-generation OLEDs, and some typical strategies for high-performance ETMs are demonstrated. Further, future challenges are briefly discussed.
KeywordsElectron-transport material Wide-energy gap Low-operating-voltage Electron mobility Molecular orientation Weak hydrogen bond
1 Historical Overview of the Development of Electron-Transport Materials
Since the first report of a practical OLED by Tang and VanSlyke in 1987, metal complex, silole, oxadiazole, and triazole derivatives have been mainly used as an ETM (Tang and VanSlyke 1987; Kulkarni et al. 2004; Hughes and Bryce 2005; Sasabe and Kido 2011, 2013a, b; Xiao et al. 2011; Yook and Lee 2012; Chen et al. 2014). ETMs perform several important functions. The key requirements for multifunctional ETMs are (1) good electron-injection properties, (2) high electron mobility, (3) high hole-blocking ability, and (4) sufficient triplet energy for exciton blocking. As a representative example, 8-hydroxyquinoline aluminum (Alq) is a well-known material that is widely used for fluorescent OLEDs; however, Alq cannot be used in second- and third-generation green or blue OLEDs due to its low ET of 2.15 eV (Montes et al. 2007). Therefore, researchers have spent the last decade searching for novel ETMs with high ET.
The phenylpyridine derivative TpPyPB was developed by Su and Kido and has a superior μe of 7.8 × 10−3 cm2 V−1 s−1, as measured by the time-of-flight method, and a relatively high ET of 2.57 eV (Su et al. 2008). The bipyridine derivative 1,3-bisbipyridyl-5-terpyridylbenzene (BBTB) was developed by Ichikawa and has a superior μe of ~10−3 cm2 V−1 s−1 and a relatively high ET of 2.53 eV (Ichikawa et al. 2012). 3-(4-Biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ, ET = 2.67 eV) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, ET = 2.63 eV) are well-known wide-energy-gap materials (Sasabe and Kido 2013a). However, these materials have a lower ET than emitters in second- and third-generation blue OLEDs, so they are not suitable ETMs for these types of OLEDs. A phosphine oxide derivative, 2,6-bis(4-(diphenylphosphoryl)phenyl)pyridine (BM-A11) developed by Padmaperuma has a high ET of 2.7 eV, and a FIrpic-based OLED with BM-A11 has a maximum power efficiency ηp,max of 48.4 lm/W and an external efficiency ηext of 14.9% (Von Ruden et al. 2010). 1,3-bis(4-(diphenylphosphoryl)phenyl)benzene (BPOPB) developed by Pu and Kido has a high ET of 2.79 eV and is used in solution-processed multilayer small-molecule OLEDs (Aizawa et al. 2014). A polyboryl-functionalized triazine derivative, 2,4,6-tris(m-dimesitylborylphenyl)-1,3,5-triazine (B3T), was developed by Wang and has a very high ET of 3.07 eV. An Ir(ppy)3-based OLED with B3T showed a maximum current efficiency of 68.9 cd/A and an ηext of 19.8% (Sun et al. 2011). A poly(diphenylphosphoryl)phenyl-functionalized triazine derivative, 2,4,6-tris(3-(diphenylphosphoryl)phenyl)-1,3,5-triazine (PO-T2T) developed by Wong has a very high ET of 2.99 eV (Lee et al. 2015; Hung et al. 2014). A FIrpic-based OLED with a combination of PO-T2T and N,N2-dicarbazolyl-3,5-benzene (mCP) as exciplex host has an ηp,max of 66 lm/W and an ηext of 30.3% (Lee et al. 2015; Hung et al. 2014).
2 Effect of Chemical Structure of Electron-Transport Materials in OLEDs
To date, a large variety of ETMs and device structures has been reported. An OLED is a multilayered device that typically consists of HTM, EML, ETM, electron-injection layer, and metal cathode. The number of combinations of materials and thicknesses is unlimited. Therefore, a fundamental question is how the ETM influences the OLED performance and what kind of chemical structure leads to high-performance OLEDs; in other words, the relationship between chemical structure, physical properties, and device performance should be investigated and understood. To address this question, Sasabe and Kido systematically investigated the effect of the ETM in phosphorescent green OLEDs (Sasabe and Kido 2011).
3 Effect of Chemical Structure on Physical Properties of Electron-Transport Materials
4 Boosting OLED Performance Through Sophisticated Manipulation of Weak Intra- and Intermolecular Interaction(s)
The performance of an amorphous solid film can be improved through sophisticated manipulation of weak intra- and intermolecular interaction(s). A representative example is the weak CH/n (n: lone pair) hydrogen bond (H-bond) (i.e., CH/O and CH/N). The binding energy of this weak CH/N H-bond is estimated to be 10–20 kJ/mol, which is about half the energy of a typical H-bond (20–30 kJ/mol) (Sasabe and Kido 2013b).
An alternative is to use 3- and/or 4-pyridine derivative(s) of BPyMPM derivatives, which take advantage of the intermolecular hydrogen-bond network. In 2011, Yokoyama and Sasabe reported the molecular orientation and intermolecular CH/N H-bonds in BPyMPM derivatives by using a variable-angle spectroscopic ellipsometry (VASE) and infrared (IR) spectrometry analyses (Yokoyama et al. 2011). They pointed out that B3PyMPM and B4PyMPM give highly oriented films that are likely driven by CH/N hydrogen bonding. In 2015, Sasabe and Kido developed triphenyltriazine-based BPyPTZ derivatives with a combination of intramolecular and intermolecular hydrogen bonds that result in a strongly horizontal orientation and a low operating voltage in OLEDs (Watanabe et al. 2015). A planar molecular orientation can be actively manipulated by the synergetic effect of intra- and intermolecular CH/N H-bonds.
Optical and physical properties of ETMs
μe (cm2 V−1 s−1)
4.0 × 10−5
2.8 × 10−4
(Ichikawa et al. 2012)
7.9 × 10−3
(Su et al. 2008)
(Su et al. 2008)
3.1 × 10−3
(Ichikawa et al. 2007)
(Xiao et al. 2011)
(Xiao et al. 2011)
(Sasabe et al. 2011)
(Sasabe et al. 2008a)
(Xiao et al. 2009)
(Von Ruden et al. 2010)
(Aizawa et al. 2014)
(Sun et al. 2011)
(Zhang et al. 2014)
- Sasabe H, Chiba T, Su SJ, Pu YJ, Nakayama K, Kido J (2008b) 2-Phenylpyrimidine skeleton-based electron-transport materials for extremely efficient green organic light-emitting devices. Chem Commun (Camb) (44):5821–5823Google Scholar