Low-molecular-weight hole injection materials (HIMs) are categorized as small molecules with a strong acceptor and metal oxides, and halogen derivatives. Since the relationship between the substrate work function and the ionization energy of an organic layer is important for considering the hole injection barrier, not only the HIM-dependent OLED characteristics but also the energy diagram between the anode and HIM has been extensively studied using a Kelvin probe and by ultraviolet photoelectron spectroscopy. The hole injection barrier is defined as the energy difference between the Fermi level of the anode and the highest occupied molecular orbital level of the organic layer on the anode. In this chapter, both the improvement of device characteristics using HIMs and the detailed hole injection mechanisms are summarized.
Hole injection Electron transfer Surface work function
As the anode of an OLED, materials with a high work function (WF) have been used to ensure effective hole injection into the organic semiconductor. Transparent conducting oxides, typified by indium tin oxide (ITO), are widely used in bottom-emission or transparent OLEDs. Although the WF of ITO can be up to 5 eV (Mason et al. 1999), a hole injection barrier of about 0.5 eV still exists between the ITO anode and most hole-transporting materials (HTMs) (Lee et al. 1999). In the case of energy-level alignment, where the vacuum level is aligned, the hole injection barrier can be over 0.5 eV, since the ionization energy (IE) of an HTM, typified by N,N′-diphenyl-N,N′-bis(1-naphthyl phenyl)-1,1′-biphenyl-4,4′-diamine (NPB), is about 5.5 eV. Such a large hole injection barrier between the anode and the organic semiconductor exists even on a gold (Au) surface, although the surface WF of a clean Au surface is about 5.4 eV (Wan et al. 2005). To minimize the hole injection barrier from the anode to the HTMs, hole injection materials (HIMs) are often employed.
In the 1990s, many small molecules, the IEs of which are smaller than those of HTMs, were mainly used as HIMs for stepwise energy level alignment, examples of which are shown in Fig. 1. For instance, the star arylamines, typified by 4,4′,4″-tris(3-methylphenyl phenylaminotriphenylamine (m-TDATA) and 4,4,4-tris N,N-diphenyl-amino triphenylamine (TDATA) (Shirota et al. 1994; Wang et al. 2008; Shirota and Kageyama 2007), and the phthalocyanines, typified by copper phthalocyanine (CuPc), were generally used (Van Slyke et al. 1996). The IE of these HIMs is about 5 eV, which is smaller than those of typical HTMs (Lee et al. 1999; Shirota et al. 1994) (see Fig. 2). Thus, the device performances were improved by inserting such an HIM between the anode and the HTM. VanSlyke et al. inserted a thin CuPc layer between the anode and the hole-transporting layer (HTL), and achieved a low-voltage and highly stable OLED with optimized thickness (Van Slyke et al. 1996). Wang et al. demonstrated that the current efficiency of OLEDs is enhanced by inserting m-TDATA between the ITO and the NPB (Wang et al. 2008). Although driving voltage and efficiency were moderately improved by employing these small molecules, the driving voltage of the OLEDs was not markedly reduced.
In the 2000s, small molecules with a strong acceptor property were beginning to be reported as HIMs, the most notable examples being tetrafluoro-tetracyano-quinodimethane (F4-TCNQ) and 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT-CN), as shown in Fig. 3. In the beginning, both the reduction in the driving voltage of OLEDs by F4-TCNQ-doped HTL and the reduction in the hole injection barrier by F4-TCNQ doping were discussed (Pfeiffer et al. 1998; Zhou et al. 2001; Gao and Kahn 2001). Pfeiffer et al. reported that the conductivity and hole injection property of phthalocyanine films are improved by adding a small amount of F4-TCNQ (Pfeiffer et al. 1998). In addition, the reduction in the driving voltage of OLEDs and the improvement of the OLED efficiency were achieved by doping F4-TCNQ into an HIM consisting of TDATA (Zhou et al. 2001). In similar studies, the electronic structure of an F4-TCNQ-doped zinc phthalocyanine (ZnPc) film, which was studied by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy, was also reported by Gao et al. (Gao and Kahn 2001) In this study, the hole injection barrier between the Au electrode and the ZnPc was studied by UPS with increasing the thickness of a ZnPc film from 0 to 128 Å on a clean Au surface, as shown in Fig. 4. The energy difference between the Fermi level of Au and the highest occupied molecular orbital (HOMO) level of ZnPc, which corresponds to the hole injection barrier, is 0.9 eV. In contrast, the hole injection barrier at the Au/organic layer interface was reduced to 0.18 eV in the F4-TCNQ-doped ZnPc film. The observed significant reduction in the hole injection barrier originates from the efficient charge transfer due to the excellent match between the IE of ZnPc and the electron affinity (EA) of F4-TCNQ, which was detected by inverse photoemission spectroscopy (Gao and Kahn 2003). This charge transfer is defined to be p-type doping in an organic semiconductor and thus the conductivity of the organic semiconductor film can also be improved significantly. Gao et al. also reported that p-type doping is effective for improving the hole injection property and conductivity of an HTM film using NPB, which is a typical HTM (See Fig. 5) (Gao and Kahn 2003). Such p-doped HIMs are widely used in OLEDs not only for reducing the driving voltage but also for improving efficiency and operational stability (Walzer et al. 2007; Lüssem et al. 2013a).
The hole injection barrier at the metal/organic layer interface can also be reduced by preparing a thin F4-TCNQ layer on the electrode surface. Koch et al. precisely examined the formation of the interface dipole at Au/F4-TCNQ monolayer interface (Koch et al. 2005). During the formation of an organic layer on the metal surface, the substrate WF of a typical metal generally decreases since the tailing part of the electronic cloud into vacuum is pushed back by repulsion with the electron cloud in the adsorbate (Ishii et al. 1999; Witte et al. 2005). In the case of F4-TCNQ on the metal substrate, on the other hand, F4-TCNQ forms a charge transfer complex on a Au substrate, resulting in the substrate WF becoming larger (Koch et al. 2005) (Fig. 6). An electric dipole layer with the vacuum side negatively charged was formed by the electron transfer from Au to the adsorbed F4-TCNQ, which was detected by UPS and X-ray photoelectron spectroscopy. The hole injection barrier between the F4-TCNQ-modified Au substrate and the organic layer was much smaller than that between the Au substrate and the organic layer. Similar electronic structures of F4-TCNQ, where the electrons transfer from the substrate to the adsorbed F4-TCNQ, have been reported on several substrates such as other metal electrodes (Rana et al. 2012) and diamond (Qi et al. 2007). Moreover, the interdiffusion of F4-TCNQ was reported by Duhm et al. (2009). In this study, F4-TCNQ is deposited on pristine layers of TDATA, and then the diffusion of F4-TCNQ toward the Au substrate is detected. The formation of a charge transfer complex on Au modifies the buried metal surface potential, leading to a realignment of the energy levels of the organic overlayer (Duhm et al. 2009). As discussed earlier, F4-TCNQ is one of the most well-known acceptor materials, and the related phenomena caused by its strong acceptor property have been intensively investigated. More recently, a similar molecule, 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F6-TNAP), the acceptor property of which is stronger than that of F4-TCNQ, has also been reported (Lüssem et al. 2013b).
One of the most used acceptor materials in recent years has been HAT-CN, which was reported by LG Chem, Ltd. in 2004 (Son et al. 2004). It is a strongly electron-withdrawing molecule because of its six nitrile groups. Kim et al. reported that the hole injection barrier at the Au/NPB interface is significantly reduced by inserting HAT-CN between the Au substrate and the NPB (Kim et al. 2009) (Fig. 7). While the surface WF of a thick HAT-CN film deposited on Au is about 6.1 eV, it remains a good conducting molecular layer. It was found that the lowest unoccupied molecular orbital (LUMO) of HAT-CN is very close to the Fermi level of Au, and the HOMO level of NPB is located only about 0.3 eV below the Fermi level of Au at this molecular interface. It has been suggested that this arrangement allows the electrons from the NPB HOMO to be easily excited to the HAT-CN LUMO, forming a charge carrier generation interface. The reason why HAT-CN is one of the most used acceptor materials is that it is an electrically/chemically stable material. Thus, HAT-CN is used for demonstrating operationally stable OLEDs (Gao et al. 2013). In addition, HAT-CN is used as an HIM in top-emitting inverted OLEDs owing to its relatively high resistance to sputtering (Park et al. 2015).
The first reported metal oxide HIMs were vanadium oxide (VOx), molybdenum oxide (MoOx), and ruthenium oxide (RuOx), which were reported by Tokito et al. in 1996 (Tokito et al. 1996). The driving voltages of the OLEDs using metal oxides as HIMs are significantly smaller than those of OLEDs without metal oxides. In particular, MoOx has been widely used up to the present owing to its high hole injection property and relatively low sublimation temperature. In 2006, Morii et al. reported that MoO3 is applicable as an HIM in an inverted OLED consisting of only air-stable materials (Morii et al. 2006). Matsushima et al. reported that a device with a 0.75-nm-thick MoO3 HIM could achieve Ohmic hole injection from the ITO anode to NPB (Matsushima et al. 2007). The Ohmic hole injection was attributed to electron transfer from ITO and NPB to MoO3. In 2009, Kröger et al. reported the detailed electronic structure at ITO/MoO3/NPB interfaces using UPS, as shown in Fig. 8 (Kröger et al. 2009). The observed phenomena were similar to the case of Au/HAT-CN/NPB interfaces. The enhancement of hole injection by inserting MoO3 was concluded to be due to the very high WF and the deep-lying unoccupied states, i.e., the large EA, of the transition metal oxide. Kröger et al. also provided solid experimental evidence for hole injection via electron extraction from the NPB HOMO through the MoO3 conduction band. Nakayama et al. reported the origin of the improved hole-injection efficiency by the deposition of MoO3 on a polymeric semiconductor in an inverted OLED (Nakayama et al. 2009). Although there have been many reports on the hole injection mechanism in a conventional bottom-emitting OLED, little has been discussed regarding the hole injection mechanism and the related electronic structure in an inverted OLED. More recently, the enhancement of hole injection from graphene to an HTL has also been demonstrated by inserting MoO3 between a graphene anode and an HTL (Meyer et al. 2014). The surface WF of the graphene/MoO3 surface reaches 6.6 eV, which is caused by the electron transfer from graphene to MoO3 (Fig. 9). In addition to VOx, MoOx, and RuOx, some other metal oxides suitable as HIMs have been reported. In 2002, NiO2 was reported to enhance hole injection from ITO to TPD (Chan et al. 2002). On the other hand, WO3 has been used as an HIM in both conventional and inverted OLEDs (Meyer et al. 2007; Kim et al. 2011).
The substrate surface WF can be enlarged by forming halogen derivatives, the electronegativity of that is high, on the surface of the electrode. However, its mechanism is different from that of acceptor molecules such as F4-TCNQ, HAT-CN, and metal oxides. An electric dipole layer with the vacuum side negatively charged is formed by forming halogen derivatives on the surface due to the large electronegativity of halogen derivatives (Campbell et al. 1996). A typical material is a fluorocarbon (CFx) layer formed by the plasma polymerization of CHF3 (Hung et al. 2001). A CFx layer formed on ITO has been widely used since it is effective not only in enhancing hole injection but also in demonstrating operationally stable OLEDs (Tang et al. 2004; Okumoto et al. 2006a, b; Kanno et al. 2007). Some halogen derivatives suitable for forming self-assembled monolayers (SAMs) have also been reported (Choi et al. 2001). In 2011, Helander et al. succeeded in coating the surface of an ITO electrode. The surface WF of Cl-coated ITO is over 6.1 eV (Helander et al. 2011).
In summary, low-molecular-weight HIMs are categorized as small molecules, metal oxides, and halogen derivatives. The small molecules, the IEs of which are smaller than those of HTMs, are mainly used for stepwise energy level alignment. On the other hand, the small molecules with a strong acceptor property can not only act as a p-type dopant but also enlarge the surface WF of the anode. The role of the metal oxides is similar to that of the small molecules with a strong acceptor property. Halogen derivatives also enlarge the surface WF of the anode by forming an electric dipole layer with the vacuum side negatively charged.
Campbell IH, Rubin S, Kress JD, Martin RL, Smith DL, Barashkov NN, Ferraris JP (1996) Controlling Schottky energy barriers in organic electronic devices using self-assembled monolayers. Phys Rev B 54:14321(R)ADSCrossRefGoogle Scholar
Chan I-M, Hsu T-Y, Hong FC (2002) Enhanced hole injections in organic light-emitting devices by depositing nickel oxide on indium tin oxide anode. Appl Phys Lett 81:1899–1901ADSCrossRefGoogle Scholar
Choi B, Rhee J, Lee HH (2001) Tailoring of self-assembled monolayer for polymer light-emitting diodes. Appl Phys Lett 79:2109–2111ADSCrossRefGoogle Scholar
Duhm S, Salzmann I, Bröker B, Glowatzki H, Johnson RL, Koch N (2009) Interdiffusion of molecular acceptors through organic layers to metal substrates mimics doping-related energy level shifts. Appl Phys Lett 95:093305ADSCrossRefGoogle Scholar
Gao W, Kahn A (2001) Controlled p-doping of zinc phthalocyanine by coevaporation with tetrafluorotetracyanoquinodimethane: a direct and inverse photoemission study. Appl Phys Lett 79:4040–4042ADSCrossRefGoogle Scholar
Gao W, Kahn A (2003) Controlled p doping of the hole-transport molecular material N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine. J Appl Phys 94:359–366ADSCrossRefGoogle Scholar
Gao C-H, Zhu X-Z, Zhang L, Zhou D-Y, Wang Z-K, Liao L-S (2013) Comparative studies on the inorganic and organic p-type dopants in organic light-emitting diodes with enhanced hole injection. Appl Phys Lett 102:153301ADSCrossRefGoogle Scholar
Helander MG, Wang ZB, Qiu J, Greiner MT, Puzzo DP, Liu ZW, Lu ZH (2011) Chlorinated indium tin oxide electrodes with high work function for organic device compatibility. Science 332:944–947ADSCrossRefGoogle Scholar
Hung LS, Zheng LR, Mason MG (2001) Anode modification in organic light-emitting diodes by low-frequency plasma polymerization of CHF3. Appl Phys Lett 78:673–675ADSCrossRefGoogle Scholar
Ishii H, Sugiyama K, Ito E, Seki K (1999) Energy level alignment and interfacial electronic structures at organic/metal and organic/organic interfaces. Adv Mater 11:605–625CrossRefGoogle Scholar
Kanno H, Ishikawa K, Nishio Y, Endo A, Adachi C, Shibata K (2007) Highly efficient and stable red phosphorescent organic light-emitting device using bis[2-(2-benzothiazoyl)phenolato]zinc(II) as host material. Appl Phys Lett 90:123509ADSCrossRefGoogle Scholar
Kim Y-K, Kim JW, Park Y (2009) Energy level alignment at a charge generation interface between 4,4′-bis(N-phenyl-1-naphthylamino)biphenyl and 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile. Appl Phys Lett 94:063305ADSCrossRefGoogle Scholar
Kim YH, Kwon S, Lee JH, Park SM, Lee YM, Kim JW (2011) Hole injection enhancement by a WO3 interlayer in inverted organic light-emitting diodes and their interfacial electronic structures. J Phys Chem C 115:6599–6604CrossRefGoogle Scholar
Koch N, Duhm S, Rabe JP (2005) Optimized hole injection with strong electron acceptors at organic-metal interfaces. Phys Rev Lett 95:237601ADSCrossRefGoogle Scholar
Kröger M, Hamwi S, Meyer J, Riedl T, Kowalsky W, Kahn A (2009) Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films. Appl Phys Lett 95:123301ADSCrossRefGoogle Scholar
Lee ST, Wang YM, Hou XY, Tang CW (1999) Interfacial electronic structures in an organic light-emitting diode. Appl Phys Lett 74:670–672ADSCrossRefGoogle Scholar
Lüssem B, Tietze ML, Kleemann H, bach CH, Bartha JW, Zakhidov A, Leo K (2013b) Doped organic transistors operating in the inversion and depletion regime. Nat Commun 4:2775ADSCrossRefGoogle Scholar
Mason MG, Hung LS, Tang CW, Lee ST, Wong KW, Wang M (1999) Characterization of treated indium–tin–oxide surfaces used in electroluminescent devices. J Appl Phys 86:1688–1692ADSCrossRefGoogle Scholar
Matsushima T, Kinoshita Y, Murata H (2007) Formation of Ohmic hole injection by inserting an ultrathin layer of molybdenum trioxide between indium tin oxide and organic hole-transporting layers. Appl Phys Lett 91:253504ADSCrossRefGoogle Scholar
Meyer J, Hamwi S, Bülow T, Johannes H-H, Riedl T, Kowalsky W (2007) Highly efficient simplified organic light emitting diodes. Appl Phys Lett 91:113506ADSCrossRefGoogle Scholar
Meyer J, Kidambi PR, Bayer BC, Weijtens C, Kuhn A, Centeno A, Pesquera A, Zurutuza A, Robertson J, Hofmann S (2014) Metal oxide induced charge transfer doping and band alignment of graphene electrodes for efficient organic light emitting diodes. Sci Rep 4:5380ADSCrossRefGoogle Scholar
Morii K, Ishida M, Takashima T, Shimoda T, Wang Q, Nazeeruddin MK, Grätzel M (2006) Encapsulation-free hybrid organic-inorganic light-emitting diodes. Appl Phys Lett 89:183510ADSCrossRefGoogle Scholar
Nakayama Y, Morii K, Suzuki Y, Machida H, Kera S, Ueno N, Kitagawa H, Noguchi Y, Ishii H (2009) Origins of improved hole-injection efficiency by the deposition of MoO3 on the polymeric semiconductor poly(dioctylfluorene-alt-benzothiadiazole). Adv Funct Mater 19:3746–3752CrossRefGoogle Scholar
Okumoto K, Kanno H, Hamada Y, Takahashi H, Shibata K (2006a) Green fluorescent organic light-emitting device with external quantum efficiency of nearly 10%. Appl Phys Lett 89:063504ADSCrossRefGoogle Scholar
Okumoto K, Kanno H, Hamada Y, Takahashi H, Shibata K (2006b) High efficiency red organic light-emitting devices using tetraphenyldibenzoperiflanthene-doped rubrene as an emitting layer. Appl Phys Lett 89:013502ADSCrossRefGoogle Scholar
Park CH, Lee HJ, Hwang JH, Kim KN, Shim YS, Jung S-G, Bae BH, Park CH, Lee DJ, Park YW, Ju BK (2015) High-efficiency hybrid buffer layer in inverted top-emitting organic light-emitting diodes. SID 2015 Digest 46:1647–1649CrossRefGoogle Scholar
Pfeiffer M, Beyer A, Fritz T, Leo K (1998) Controlled doping of phthalocyanine layers by cosublimation: a systematic seebeck and conductivity study. Appl Phys Lett 73:3202–3204ADSCrossRefGoogle Scholar
Qi D, Chen W, Gao X, Wang L, Chen S, Loh KP, Wee ATS (2007) Surface transfer doping of diamond (100) by tetrafluoro-tetracyanoquinodimethane. J Am Chem Soc 129:8084–8085CrossRefGoogle Scholar
Rana O, Srivastava R, Chauhan G, Zulfequar M, Husain M, Srivastava PC, Kamalasanan MN (2012) Modification of metal–organic interface using F4-TCNQ for enhanced hole injection properties in optoelectronic devices. Phys Status Solidi A 209:2539–2545ADSCrossRefGoogle Scholar
Shirota Y, Kageyama H (2007) Charge carrier transporting molecular materials and their applications in devices. Chem Rev 107:953–1010CrossRefGoogle Scholar
Shirota Y, Kuwabara Y, Inada H, Wakimoto T, Nakada H, Yonemoto Y, Kawami S, Imai K (1994) Multilayered organic electroluminescent device using a novel starburst molecule, ,4′,4′-tris(3-methylphenylphenylamino)triphenylamine, as a hole transport material. Appl Phys Lett 65:807–809ADSCrossRefGoogle Scholar
Son S-H, Jang J-G, Jeon S-Y, Yoon S-H, Lee J-C, Kim K-K (2004) Electroluminescent devices with low work function anode. WO Patent 2004054326 A2Google Scholar
Tang JX, Li YQ, Zheng LR, Hung LS (2004) Anode/organic interface modification by plasma polymerized fluorocarbon films. J Appl Phys 95:4397–4403ADSCrossRefGoogle Scholar
Tokito S, Noda K, Taga Y (1996) Metal oxides as a hole-injecting layer for an organic electroluminescent device. J Phys D Appl Phys 29:2750–2753ADSCrossRefGoogle Scholar
Van Slyke SA, Chen CH, Tang CW (1996) Organic electroluminescent devices with improved stability. Appl Phys Lett 69:2160–2162ADSCrossRefGoogle Scholar
Walzer K, Maennig B, Pfeiffer M, Leo K (2007) Highly efficient organic devices based on electrically doped transport layers. Chem Rev 107:1233–1271CrossRefGoogle Scholar
Wan A, Hwang J, Amy F, Kahn A (2005) Impact of electrode contamination on the α-NPD/Au hole injection barrier. Org Electron 6:47–54CrossRefGoogle Scholar
Wang H, Klubek KP, Tang CW (2008) Current efficiency in organic light-emitting diodes with a hole-injection layer. Appl Phys Lett 93:093306ADSCrossRefGoogle Scholar
Witte G, Lukas S, Bagus PS, Wöll C (2005) Vacuum level alignment at organic/metal junctions: “Cushion” effect and the interface dipole. Appl Phys Lett 87:263502ADSCrossRefGoogle Scholar
Zhou X, Pfeiffer M, Blochwitz J, Werner A, Nollau A, Fritz T, Leo K (2001) Very-low-operating-voltage organic light-emitting diodes using a p-doped amorphous hole injection layer. Appl Phys Lett 78:410–412ADSCrossRefGoogle Scholar