Small molecule host materials for solution processed phosphorescent organic light-emitting diodes.

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Small Molecule Host Materials for Solution Processed Phosphorescent Organic Light-Emitting Diodes Kyoung Soo Yook and Jun Yeob Lee* 100% internal quantum efficiency.[3] The triplet excitons cannot be utilized for light Solution processed phosphorescent organic light-emitting diodes (OLEDs) emission in common organic emitting have been actively developed due to merits of high quantum efficiency of materials because of non-radiative decay phosphorescent materials and simple fabrication processes of solution of triplet excitons via internal conversion processed OLEDs. The device performances of the solution processed phosprocess. The radiative transition from the triplet excited state to the singlet ground phorescent OLEDs have been greatly improved in the last 10 years and the state is a forbidden transition, but the progress of the device performances was made by the development of small transition can be allowed in organomemolecule host materials for solution processes. A hybrid host of polymer tallic complexes with heavy metals such and small molecules, a single small molecule host and a mixed host of small as Ir, Pt and Os due to spin-orbit coumolecule hosts have effectively enhanced the quantum efficiency of the solupling.[4] Therefore, the triplet excitons can be used for light emission and an tion processed phosphorescent OLEDs. Therefore, this paper reviews recent internal quantum efficiency of 100% can developments in small molecule host materials for solution processed phosbe obtained in phosphorescent OLEDs. phorescent OLEDs and provides future directions for the development of the Since the pioneering work of Forrest, small molecule host materials. Thompson and Ma groups on phosphorescent OLEDs,[4,5] there have been many studies to achieve 100% internal quantum efficiency in OLEDs and most studies on high efficiency OLEDs 1. Introduction have been focussed on developing organic materials and device structures for phosphorescent OLEDs. Currently, almost 100% Electroluminescence of organic materials was first demoninternal quantum efficiency was already reported in red, green, strated by Bernanose et al. in the 1950s by applying highand blue phosphorescent OLEDs by several research groups voltage alternating current to the acridine orange thin film.[1] using vacuum thermal evaporation processes.[6–8] After that there was remarkable progress in organic light-emitting diodes (OLEDs) made by Tang et al. Tris(8-hydroxyquinoliAlthough vacuum thermal evaporation has been generally nato)aluminum (Alq3) was sandwiched between an anode and used as a fabrication process of OLEDs, the vacuum deposition method has several problems such as inefficient use of a cathode by a vacuum thermal deposition process and green material, poor scalability, high equipment cost, high vacuum emission was obtained by applying a voltage to the device.[2] pressure, and complicated color patterning processes. These The device structure of the OLEDs was evolved from a single problems can be solved by introducing solution coating prolayer device structure with only one emitting layer to a multicesses such as spin coating, ink-jet printing, or nozzle printing layer structure with charge transport materials and an emitting instead of the vacuum evaporation process. The material usage layer to improve the device performances of the OLEDs. can be minimized in the solution coating process because all At first, fluorescent materials were used as emitting materials liquid inks can be coated inside the pixel and large size printing of OLEDs, but the intrinsic low internal quantum efficiency of is simple because of easy scalability of the ink-jet printing or 25% of the fluorescent emitting materials limited the applicanozzle printing. In addition, the printing equipment is cheap tion of fluorescent OLEDs. It has been known that the ratio because the solution printing process does not require high of singlet excitons to triplet excitons is 1:3 from spin statistics vacuum pressure and color patterning processes and is simple and the use of triplet excitons for light emission can achieve compared with the vacuum evaporation process because fine metal masks are not required in the solution printing process. Moreover, the solution printing process is comparable to the K. S. Yook, Prof. J. Y. Lee vacuum deposition process in terms of film density, film uniDepartment of Polymer Science and Engineering Dankook University, 126 formity, and surface roughness[9] if the process condition and Jukjeon-dong, Suji-gu, Yongin ink formulation are properly controlled.[10] Gyeonggi 448-701, Korea Initial studies about the solution processed OLEDs were E-mail: [email protected] focussed on polymer OLEDs due to the high film density and low surface roughness.[11] However, it is sophisticated to control DOI: 10.1002/adma.201306266

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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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2. Basic Device Structure of Solution Processed Phosphorescent OLEDs The vacuum deposition process is effective to fabricate a multilayer device structure because each organic layer can be easily stacked without any damage to the underlying organic layer. In general, hole injection layer, hole transport layer, emitting layer, and electron transport layer were stacked on a transparent anode by a vacuum thermal evaporation process. However, the multilayer structure was difficult to be formed by a solution process due to intermixing of organic materials during wet coating process. Therefore, a simple device structure with only two organic layers, a hole injection layer, and an emitting layer was initially used to fabricate solution processed phosphorescent OLEDs. Typically, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) was used as the hole injection layer and a triplet emitter solution was spin coated on the PEDOT:PSS followed by cathode deposition.[16] However, triplet excitons of triplet emitters were quenched by the PEDOT:PSS and the cathode, so a high triplet energy hole transport layer and an electron transport layer for triplet exciton blocking were introduced into the device structure afterwards. An orthogonal solvent should be used to form the multilayer structure, but most organic materials are soluble in aromatic solvents which hinders the multilayer structure formation by solution process. Typically, aqueous dispersions of conducting organic materials were coated as the hole injection layer, while organic solvent resistant materials, such as crosslinked polymers[17,18] or high molecular weight polymers[19] were deposited onto the hole

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Prof. Kyoung Soo Yook is a research professor at the department of polymer science and engineering of Dankook University. He received his Ph. D. from Dankook University, in 2012. After that he did a postdoctoral research at University of Michigan(2012–2013). His research focuses on the development of device structures for solution and vacuum deposited organic lightemitting diodes and organic solar cells.

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the molecular weight of the polymer materials and to obtain high purity due to the difficult purification of the polymer materials. Additionally, it was difficult to achieve high quantum efficiencies in polymer OLEDs due to the low triplet energy of the polymer materials although solution processed phosphorescent polymer OLEDs have been developed.[12,13] Therefore, recent development of solution processed phosphorescent OLEDs is being directed to improve the quantum efficiency using small molecule materials instead of polymer materials. In particular, small molecule host and phosphorescent dopant materials which can be processed from solution are being applied in solution processed phosphorescent OLEDs. A previous review about small molecules for solution processed OLEDs was focussed on fluorescent materials and polymer based phosphorescent materials.[14] Recently, there was a great progress of device performances of small molecule based phosphorescent OLEDs. In this work, recent developments of small molecule host materials for solution processed phosphorescent OLEDs was reviewed. Small molecule soluble triplet host materials which have been reported in the literature were covered. Dendrimers are a kind of small molecule host materials for solution processed phosphorescent OLEDs,[15] but they are not included in this work because our main focus is on small molecule host materials which are compatible both in vacuum and solution processes. In addition, future direction and prospects for the development of small molecule materials for solution processed phosphorescent OLEDs are proposed.

Prof. Jun Yeob Lee received his Ph.D degree from Seoul National University, Korea in 1998. After a postdoc at Rensselaer Polytechnic Institute (1998 ∼ 1999) he joined Samsung SDI and developed active matrix organic light emitting diodes for 6 years. After that, he has been working as assistant professor at the department of polymer science and engineering of Dankook University. His main research areas are synthesis of organic electronic materials and development of novel device structures for organic electronic devices.

injection layer by solution process. The solution processed hole injection layer was insoluble in organic solvent (solvent for hole transport layer) and orthogonal films could be coated on the substrate. The emitting layer could be formed on the hole transport layer without intermixing when crosslinked polymer was used as the hole transport material or alcohol soluble emitting layer were coated on the hole transport layer. However, small molecule or high molecular weight polymer hole transport materials were intermixed with the emitting layer processed from aromatic solution. In the case of the electron transport layer, it was difficult to avoid intermixing between the emitting material and electron transport material due to dissolution of the emitting layer by the electron transport material solution although some alcohol or water soluble electron transport materials were orthogonally spin coated on the emitting layer derived from polymer materials.[20,21] In most cases, the electron transport materials were deposited on the solution processed emitting layer by vacuum thermal evaporation process. Therefore, the device structure of solution processed phosphorescent OLEDs was anode/hole injection layer (solution coating/polymer)/hole transport layer (solution coating/polymer)/emitting layer (solution coating/polymer or small molecule)/electron transport



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Figure 1. (a) Device fabrication process and (b) device structure of the solution processed phosphorescent OLEDs.

layer (vacuum deposition/small molecule)/cathode. Device fabrication process and device structure of the solution processed phosphorescent OLEDs are shown in Figure 1.

3. Small Molecule Host Materials for Solution Processed Phosphorescent OLEDs Host materials for solution processed phosphorescent OLEDs should satisfy several requirements to obtain high quantum efficiencies. Firstly, stable and uniform film morphology should be secured after solution coating process. Surface roughness less than 1.0 nm is required to prevent current leakage and stable film morphology up to 100 °C should be guaranteed because the thermal annealing temperature can be as high as 100 °C when aromatic solvents are used to dissolve the emitting materials. Secondly, high triplet energy for efficient energy transfer from the host to triplet emitters is also important to achieve high quantum efficiency. Triplet exciton quenching of the triplet emitter by the host materials should be blocked to increase the quantum efficiency and the triplet energy of the host materials should be higher than that of triplet emitters. Thirdly, bipolar charge transport properties are needed to balance holes and electrons in the emitting layer. Recombination efficiency in the emitting layer depends on the charge balance and the charge balance is improved by using bipolar host materials instead of hole transport or electron transport type host materials. The use of the bipolar charge transport materials also decreases efficiency roll-off of the phosphorescent OLEDs. Fourthly, high glass transition temperature helps to improve the quantum efficiency by stabilizing the morphology of the emitting layer. Crystallization of the coated film is induced above the glass transition temperatures of the matrix host materials, which can be easily managed when the glass

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transition temperature is high (>100 °C). Finally, good solubility in alcohol or aromatic solvents is necessary to obtain high quantum efficiency via management of solution coating process. Most phosphorescent emitting materials are soluble only in alcohol or aromatic solvents, so the host materials should also have good solubility in alcohol or aromatic solvents. Poor solubility of the host materials makes it difficult to control the thickness of the emitting layer and gives rise to a problem of non-uniform surface morphology such as aggregation of the host or dopant materials. A concentration of at least 1 wt% is desired to coat a several tens of nm thick emitting layer. Other than these, compatibility of the host materials with the dopant materials and a large overlap of PL emission of the host materials with the absorption of the triplet emitters are also important parameters for the high quantum efficiency of the solution processed phosphorescent OLEDs. In the early study of solution processed phosphorescent OLEDs, only polymer based phosphorescent OLEDs were developed because no small molecule host materials for solution process were available. However, the polymer host materials could not satisfy the requirements of soluble triplet host materials such as high triplet energy and good charge balance, therefore hybrid host materials of polymers and small molecule triplet host materials began to be used in soluble phosphorescent OLEDs. After that, various small molecule host materials which can form stable film morphology after solution coating have been synthesized, which enabled the fabrication of small molecule solution processed phosphorescent OLEDs.

3.1. Polymer/Small Molecule Hybrid Host Since the first demonstration of phosphorescent OLEDs by a vacuum evaporation process,[4] solution processed phosphorescent


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An oxadiazole derivative, 2-(4-biphenylyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole (PBD), was also used as the electron transport type host mixed with PVK. The PBD host was added to PVK to facilitate electron transport and injection of PVK and was applied as the host material for phosphorescent dopants, iridium(III) compound, iridium(III) bis(2-phenylpyridinato-N,C2′) (acetylacetonate) (Ir(ppy)2(acac)), and platinum(II) compound.[12,16] The PVK:PBD mixed host enabled the fabrication of single layer phosphorescent OLEDs by solution process and an external quantum efficiency of 3.4% was reported. Further optimization of the device structure improved the external quantum efficiency to 8.5%.[29] In other work, tris[9,9′dihexyl-2-(pyridinyl-2′)fluorene] iridium (Ir(DPF)3) was doped into the PVK:PBD host and a high quantum efficiency close to 10% was demonstrated in green phosphorescent OLEDs.[30,31] The PVK:PBD host was also applied as the host material for white OLEDs, doped with osmium(II)(tri(3-trifluoromethyl5-(4-tert-butyl-2-pyridyl)triazolate)) (Os(bpftz)) and 4,4'-bis(2(4-(N,N-diphenylamino)phenyl)vinyl)biphenyl (DPAVBi), and a maximum external quantum efficiency of 6.1% with a color coordinate of (0.33,0.34) was obtained.[32] The efficiency of the white phosphorescent OLEDs could be improved to 42.9 cd/A by doping FIrpic and a yellow triplet emitter 1.[33] Optimization of host composition of the PVK:OXD-7 host gave maximum power efficiency of 20.3 lm/W and the power efficiency was still 16.8 lm/W at a luminance of 1000 cd/m2 due to facile electron injection and charge balance in the emitting layer. The comparison of OXD-7 and PBD revealed that OXD-7 was better than PBD as an electron transport component of PVK based host material to improve the quantum efficiency of blue phosphorescent OLEDs.[34] In addition to the oxadiazole based electron transport host materials, a phosphine oxide type SPPO1 host was also mixed with PVK and optimization of device structure gave a current efficiency of 11.9 cd/A in blue device due to little aggregation of FIrpic dopant, smooth surface morphology and good charge balance.[35] In general, electron transport type host materials were blended with PVK, but several PVK host systems mixed with hole transport or bipolar type host materials were also reported. 4,4′–bis(N-carbazolyl)biphenyl (CBP) was added to the PVK host to improve the quantum efficiency of the PVK device. Although CBP is a hole transport type host material, moderate electron transport properties of the CBP host enhanced the quantum efficiency of the PVK device. Green phosphorescent OLEDs with PVK:CBP:tris(2-(p-methylphenyl)pyridine)iridium (Ir(mppy)3) emitting layer provided a highcurrent efficiency of 33.7 cd/A using 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) hole transport layer.[36] A ternary blend of PVK and small molecules was also developed as the host material for phosphorescent OLEDs. In the ternary blend system, PVK was a matrix host material and the PVK was mixed with strong hole transporting host and an strong electron transporting host. The hole transport type host was added to the PVK host to facilitate hole transport in the PVK emitting layer and the electron transport type host was blended to compensate for the poor electron transport properties of the PVK host. Cho et al. fabricated green phosphorescent OLEDs by screen printing process using PVK host blended with electron transport type PBD and hole transport



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OLEDs have been widely studied. At first, polymer host materials have been widely used in solution processed phosphorescent OLEDs due to high film density and stable film morphology. The polymer host materials have entangled chain structure and can form stable amorphous film during thermal annealing after spin-coating. The entanglement of the polymer chains suppresses the crystallization of the polymer materials and the chains are closely packed by the thermal annealing without crystallization. The most widely used polymer host material for solution processed phosphorescent OLEDs was poly(N-vinylcarbazole) (PVK).[22] Although the PVK host has been widely used as the soluble host materials for phosphorescent OLEDs, the good hole transport properties and poor electron transport properties limited the quantum efficiency of the phosphorescent OLEDs due to poor charge balance in the emitting layer. Additionally, the unbalanced charge transport properties decreased the maximum luminance of the device and induced serious efficiency roll-off at high luminance. Another problem of the PVK host material was incompatibility of the PVK with phosphorescent dopant materials, which induced aggregation of the phosphorescent dopant materials. Therefore, a hybrid host made up of PVK and an electron transport type small molecule host was developed. The addition of the electron transport type host can solve the problem of imbalance of holes and electrons in the emitting layer and improve the efficiency roll-off of the device. The most widely used hybrid type host was a blend of PVK and OXD-7. OXD-7 is an electron transport type host with a high triplet energy of 2.7 eV.[23] The OXD-7 host is suitable as a host material for soluble hybrid host due to good solubility and film morphology by tert-butyl unit and good electron transport properties by the oxadiazole unit. Several groups studied the device performances of the triplet emitter doped PVK:OXD-7 devices. So group reported high power efficiency of 14 lm/W and current efficiency of 22 cd/A by optimizing the OXD-7 content in the emitting layer.[24] Jenekhe group further improved the efficiency of the PVK:OXD-7:FIrpic device using solution processed electron transport layer spin coated from formic acid solution.[25] High external quantum efficiency of 16.0% and current efficiency of 30.5cd/A were demonstrated in the PVK:OXD-7 device. The PVK:OXD-7 host was evaluated as the host material for solution processed yellow devices doped with tris(2-(9,9-dioctyl9H-fluoren-2-yl)pyridinato-C3,N) iridium(III) (Ir(FP)3) and high current efficiency of 41.7 cd/A was achieved using a solution processed small molecular electron transport layer composed of 1,3,5-tris(N -phenylbenzimidazol-2-yl)benzene (TPBI), 3-(biphenyl-4-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ) and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB).[26] The mixing of three electron transport materials solved the problem of thickness limitation and poor film-forming ability of soluble electron transport materials. In the solution processed green phosphorescent OLEDs, a high external quantum efficiency of 16.1% was achieved using the PVK:OXD-7 host with solution processed Bphen electron transport layer.[21] Other than these, there have been many papers about PVKOXD-7 host material for solution processed phosphorescent OLEDs.[27,28]

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type bis(N-(1-naphthyl)-N-phenyl)benzidine (NPB).[37] The PVK:PBD:NPB device doped with Ir(ppy)3 showed a very high efficiency of 63.2 cd/A due to balanced charge density in the emitting layer although the driving voltage of the device was high. The PVK was also mixed with PBD and N,N'-diphenylN,N′-(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) to develop high efficiency green phosphorescent OLEDs. A uniform dispersion of PVK, PBD and TPD was prepared by solution process and gave high external quantum efficiency of 10.2% in the green device using a phosphorescent dopant p-G1-Ir.[38] Similarly, So et al. reported high power efficiency of 41 lm/W using the PVK:PBD:TPD blend doped with Ir(mppy)3 using nanoscale interfacial layer between PEDOT:PSS and the green emitting layer.[39] Orange emitting phosphorescent OLEDs were also developed with the PVK:PBD:TPD host and a current efficiency of 20 cd/A was obtained.[40] In other works, red phosphorescent OLEDs was developed using the same host material system and a current efficiency of 17 cd/A was demonstrated using the PVK:PBD:TPD host doped with tris(2-(4-nhexyl-phenyl)quinoline)iridium(III) and tris((4-nhexylphenyl) isoquinoline))iridium(III).[41] In general, the ternary blend system was better than binary blend system because relatively poor hole transport properties of the PVK host could be improved by a hole transport type host as well as the electron transport properties by an electron transport type host. Device performances of the hybrid type host materials are summarized in Table 1 and the chemical structures of small molecule host materials for the hybrid host materials are shown in Figure 2. Although high quantum efficiency was achieved in the solution processed phosphorescent OLEDs using the hybrid host of PVK and small molecule host, the PVK type host materials

suffer from high driving voltage due to low hole mobility of PVK and poor lifetime. It is difficult to overcome the intrinsic problems of low purity, molecular weight distribution, weak molecular backbone structure and low triplet energy of polymer materials which degrade the quantum efficiency and lifetime of solution processed phosphorescent OLEDs. The low purity of polymer materials does not greatly affect the quantum efficiency of the phosphorescent OLEDs, but it is critical to the lifetime because the impurity in the polymer materials induces luminance quenching, charge accumulation, charge trapping and electrochemical degradation of the polymer materials. The molecular weight distribution also affects the lifetime of the device due to poor lifetime of low molecular weight component of the polymer materials. The weak molecular backbone structure and the low triplet energy are closely related each other. The PVK polymer has a backbone structure made up of carboncarbon single bond which has low bonding energy compared to carbon-carbon double bond. Therefore, the backbone structure can be easily cleaved by chemical degradation and the device lifetime is significantly degraded.[42] Although aromatic backbone structure can be adopted in the molecular design without conjugation breaking units such as silicon or sp3 carbon, it is difficult to obtain high triplet energy from the polymer materials. Additionally, in the case of the hybrid of polymers and small molecules, phase separation may occur either after some time of operation or immediately following fabrication due to differences between polymers and small molecules such as viscosity and boiling point, which result in the degradation of the device performances of the hybrid of polymer and small molecules. Therefore, the polymer materials or a hybrid of polymer and small molecules are not suitable as the host materials for solution processed phosphorescent OLEDs and small molecule

Table 1. Device performances of the hybrid type host materials. Polymer

Host

Dopant

E.Q.E. (%) a)

C.E. (cd/A) a)

PVK

OXD-7

Ir(ppy)3

16.1

PVK

OXD-7

FIrpic

16.0a)

30.5a)

PVK

OXD-7

Ir(FP)3

12.7a)

41.7a)

PVK

OXD-7

FIrpic 1

PVK

PBD

Btp2Ir(acac)

PVK

PBD

Os(bpftz)

PVK

PBD

1

19.1

a),18.6b)

53.8

42.9a),41.7b)

P.E. (lm/W) a)

13.3

a)

Ref.

chlorobenzene

[21]

chlorobenzene

[25]

chlorobenzene:chloroform

[26]

20.3a),16.8b)

1.6a) 18.7a)

Solvent

[33] chlorobenzene

[31]

45.0a)

[32]

40.4a)

[33]

a)

PVK

PBD

Ir(ppy)2(acac)

3.4

12.3

chloroform

[16]

PVK

PBD

Ir(ppy)3

8.5a)

30.1a)

chlorobenzene

[29]

PVK

PBD

Ir(DPF)3

9.1a)

36a)

PVK

PBD

Ir(mppy)3

PVK

PBD

Os(bpftz) DPAVBi

PVK

SPPO1

FIrpic

PVK

CBP

Ir(mppy)3

PVK

PBD NPD

Ir(ppy)3

PVK

PBD TPD

p-G1-Ir

PVK

PBD TPD

Ir(mppy)3

PVK

PBD TPD

PO-01-Hex

2.5a)

23.0a) 6.12a)

chlorobenzene

13.2a)

33.7a),

32.0b)

63.2a)

benzene

20

[35]

19.6a),15.9b)

chlorobenzene

[36]

12.2a)

chlorobenzene

[37]

chlorobenzene

[38]

chlorobenzene

[39]

chlorobenzene

[40]

40.0a) 50a)

[31] [32]

11.9b)

10.8a)

[30]

41a)

a)

a)

Maximum efficiency; b)Efficiency at 1000 cd/m2.

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O

O

N P O

N N

N N

OXD-7

PBD

SPPO1 CH3

N N

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N O

N

H3C

N

N

N

CBP

NPB

TPD

Figure 2. Chemical structures of small molecule host materials for the hybrid host materials.

host materials are being actively developed to secure high quantum efficiency and long-term stability in the solution processed phosphorescent OLEDs.

3.2. Small Molecule Host Materials Several demerits of the hybrid type host materials prompted the development of small molecule host materials for solution processed phosphorescent OLEDs. The uses of the small molecule host materials are advantageous in terms of material purity, chemical stability, triplet energy, and lifetime. The purity level of small molecules can be as high as above 99.9% because small molecules can be purified by vacuum train sublimation which cannot be applied for polymer materials. The high purity is critical to secure long lifetime in the phosphorescent OLEDs and long lifetime can be obtained using the highly pure small molecule host materials. Chemical stability of the small molecule host materials can also be better than that of hybrid host materials due to aromatic character of the backbone structure of the small molecule host materials. In most small molecules, the molecular structure is made up of the combination of aromatic units with high bonding energy, which is beneficial to the chemical stability of the host materials. It is easy to get high triplet energy using the small molecules as the conjugation length of the small molecules is shorter than that of polymer materials. The triplet energy can be managed from 2.0 eV to 3.0 eV by simple manipulation of the conjugation length of the small molecule materials. Therefore, in recent years, the small molecule host materials are being widely studied to improve the device performances of phosphorescent OLEDs. However, compared with the polymer materials, small molecules can be easily crystallized by solution coating process because there is no entanglement of the small molecules. In particular, small molecule materials with low glass transition temperature can be crystallized during thermal annealing process to remove residual solvent. Therefore, small molecule materials for solution process were designed to suppress the crystallization by attaching flexible chains, by adopting an asymmetric molecular structure and by substituting bulky side groups. Alternatively, a mixed host of a hole transport type host and an electron transport type host was adopted for stable film morphology without crystallization.

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3.2.1. Small Molecule Single Host There are several design rules for the soluble host materials of the solution processed phosphorescent OLEDs. Firstly, a donor-acceptor molecular structure is preferred to balance holes and electrons in the emitting layer because the donoracceptor type host can have bipolar charge transport properties. Secondly, asymmetric molecular structure is better than symmetric molecular structure for uniform film morphology. Symmetric molecules can be easily crystallized by regular molecular packing, but asymmetric molecules are resistant to crystallization due to irregular molecular packing. Thirdly, rigid materials can be rendered to be soluble in coating solvent by attaching flexible chains. The flexible chains can prevent regular molecular packing of the host materials, stabilizing the morphology of the spin coated film. Fourthly, bulky side groups can also help the formation of smooth film with little crystallization due to hindered molecular packing. However, the flexible chains and bulky side groups have negative effect on the driving voltage of the device due to low carrier mobility of the host material caused by long intermolecular distance. Carbazole has been the most popular moiety for small molecule soluble host materials due to high triplet energy and rigid molecular structure for thermal stability. Initially, common carbazole derived high triplet energy hosts applied in vacuum thermal evaporation were tested as the soluble host materials for Ir triplet emitters. CBP, TCTA and N,N′-dicarbazolyl-3,5benzene (mCP) were doped with red, green and blue triplet emitters to fabricate white OLEDs and the small molecule host materials performed better than PVK as the host materials. In particular, CBP performed best in terms of luminance and efficiency due to low energy barrier for charge injection.[43] The CBP host also gave moderate device performances as the soluble host material in other phosphorescent OLEDs. Wong et al. developed high efficiency orange phosphorescent OLEDs using the CBP host doped with a carbazole or fluorene modified triplet emitters. The current and external quantum efficiencies of the orange device doped with iridium complex 2 reached 30 cd/A and 10%, respectively.[44] The same group also reported 7.65% external quantum efficiency in red phosphorescent OLEDs using a iridium complex 3[45] and fabricated red and yellow phosphorescent OLEDs using the CBP host material and Ir-2 and Pt-1 phosphorescent dopants.[46] Although CBP



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was widely adopted as the soluble host material, it was difficult to balance holes and electrons, which lead to just moderate quantum efficiency in the phosphorescent OLEDs. The improvement of the quantum efficiency of the CBP type host material was enabled by inserting cyclohexanyl or adamantyl groups between two phenyl rings of CBP, Cy-Cz and Ad-Cz, by Watanabe et al.[47] Although the LUMO energy level of the host materials was not optimally adjusted, the green PHOLEDs showed high external quantum efficiency of 11.0% at the Ir(ppy)3 doping concentration of 24%. Carbazole derivatives have been modified in various ways to obtain bipolar charge transport properties and good filmforming properties. N,N′-dicarbazolyl-3,5-benzene (mCP) was used as a high triplet energy host material in vacuum processed phosphorescent OLEDs, but it could not be used in solution process owing to crystallization during spin coating caused by low glass transition temperature and symmetric molecular structure of mCP. Therefore, the mCP was modified with a bulky side group such as triphenylsilyl unit to increase the glass transition temperature and secure smooth film morphology.[48] 3,5-Bis(9-carbazolyl) tetraphenylsilane (SimCP) was reported as the mCP derivative with a high glass transition temperature of 101 °C and low surface roughness of 0.6 nm compared to 55 °C and 5.7 nm of mCP. The high glass transition temperature and low surface roughness enhanced the quantum efficiency of FIrpic doped blue phosphorescent OLEDs and high external quantum efficiency of 11.5% was obtained compared to 3.6% of the mCP device. However, the bulky triphenylsilyl substituent decreased the hole and electron mobilities of SimCP and had negative effect on the driving voltage. Further modification of the SimCP host using two additional carbazole units (3,5-di(9H-carbazol-9-yl)tetraphenylsilane (SimCP2)) increased the hole and electron mobility of the soluble host material and improved the driving voltage and quantum efficiency of the blue phosphorescent OLEDs. The maximum external quantum efficiency of the SimCP2 device was 19.8% and an additional electron blocking layer (1,1-Bis(4(N,N′-di(ptolyl)amino)phenyl)cyclohexane (TAPC)) further improved the external quantum efficiency up to 21.0% in spite of intermixing of the emitting layer with TAPC. That is one of the best efficiency data in solution processed blue phosphorescent OLEDs. A diphenylphosphine oxide modified bipolar host material was also derived from mCP as the soluble host material for blue phosphorescent OLEDs. (9-(3-(9H-carbazol-9-yl)phenyl)9H-carbazol-3-yl)diphenylphosphine oxide (mCPPO1) had a mCP backbone structure and one carbazole of mCP was substituted with a strong electron withdrawing diphenylphosphine oxide unit.[49] The substitution of the diphenylphosphine oxide unit maintained the high triplet energy of the mCP core and improved thermal stability, film-forming properties and electron transport properties.[50] The mCPPO1 host was doped with a deep blue emitting bis((3,5-difluoro-4-cyanophenyl)pyridine)iridium picolinate (FCNIrpic) dopant and high external quantum efficiency of 22.1% with a color coordinate of (0.14, 0.19) was reported. Compared to PVK:FCNIrpic device, the external quantum efficiency was doubled and this is the best efficiency in solution processed deep blue phosphorescent OLEDs.


One of the best efficiency data in blue PHOLEDs was obtained using asymmetric 9-(3-(dibenzo(b,d)furan-2-yl) phenyl)-9H-carbazole (CzDBF) as the soluble host material.[51] The CzDBF had a carbazole derived backbone structure modified with a dibenzofuran moiety. The CzDBF host showed low surface roughness of 0.33 nm and bipolar charge transport properties for high recombination efficiency, resulting in high external quantum efficiency of 23.9% in blue phosphorescent OLEDs doped with FIrpic. Similar to CzDBF, 3-(3-(carbazole9-yl)phenyl)benzofuro(2,3-b)pyridine (PCz-BFP) was synthesized as a soluble bipolar host material possessing an electron transporting benzofuropyridine modified carbazole backbone structure.[52] The PCz-BFP host had an asymmetric molecular structure and showed an amorphous film morphology in spite of low glass transition temperature. Although the external quantum efficiency of the solution processed device was lower than that of vacuum processed device, high external quantum efficiency of 18.0% was achieved in the blue phosphorescent OLEDs. Oxadiazole modification of carbazole was another approach to develop the soluble bipolar host materials. 2,5-Bis(4-(9-(2ethylhexyl)-9H-carbazol-3-yl)phenyl)-1,3,4-oxadiazole (CzOXD) incorporating both electron- and hole-transport functionalities also worked well as a bipolar host material for solution processed OLEDs. A smooth and homogeneous film was obtained by spin coating of bis(2-(4′-(2′′-phenylpyridinyl))N-(2-ethylhexyl)carbazole)(acetylacetonate)iridium (Ir(2-PhPyCz)2(acac)) doped CzOXD from a chloroform solution and current efficiencies of 20 cd/A and 4.6 cd/A were achieved in yellow and red emitting OLEDs, respectively.[53] Triazine modification of carbazole also produced bipolar host materials for potential use in solution processed green phosphorescent OLEDs. The hole transporting carbazole and electron transporting triazine were combined via flexible linkages and nonconjugated bipolar hybrids were prepared as the host materials, Cz(MP)2, TRZ1Cz(MP)2 and TRZ-3Cz(MP)2. Ir(mppy)3 doped TRZ-3Cz(MP)2 device showed a maximum current efficiency of 32 cd/A and the driving voltage was reduced according to the content of triazine units.[54] Gong et al. modified single-silicon-bridge host material and synthesised a series of oxadiazole/carbazole or arylamine hybrids, 4,4′-(1,3,4-oxadiazole-2,5-diylbis(4,1phenylene(diphenylsilanediyl)))bis-(N,N-diphenylaniline) (pOXDDSiPA), 9,9′-(1,3,4-oxadiazole-2,5-diylbis(4,1phenylene(diphenylsilanediyl)-4,1-phenylene))-bis(3,6-ditert-butyl-9H-carbazole) (pOXDDSiCz), 3,3′-(1,3,4-oxadiazole-2,5-diylbis(3,1-phenylene(diphenylsilanediyl)))bis-(N,Ndiphenylaniline) (mOXDDSiPA), 9,9′-(1,3,4-oxadiazole2,5-diylbis(3,1-phenylene(diphenylsilanediyl)-3,1-phenylene))bis(3,6-di-tert-butyl-9H-carbazole) (mOXDDSiCz), by integrating double-silicon-bridge between the electron-donating arylamine group and electron-accepting oxadiazole moiety. Those host materials possessed good solution processability, high glass transition temperatures and morphological stability compared with single-silicon-bridge host molecules. The mOXDDSiPA based FIrpic doped blue phosphorescent OLEDs showed high external quantum efficiency of 10.7%.[55] Pyridine modified carbazole compound, 6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine(26DCzPPy), was also reported as


Adv. Mater. 2014, 26, 4218–4233


Adv. Mater. 2014, 26, 4218–4233

hole transporting diphenylamine unit and electron transporting 1,3,4-oxadiazole unit to obtain bipolar charge transport properties. The TPO host was doped with Ir(ppy)3 or bis(2,4-diphenylquinolyl-N,C2')iridium(acetylacetonate) ((PPQ)2Ir(acac)) and gave high current efficiencies of 56.8 cd/A and 13.3 cd/A in green and red phosphorescent OLEDs.[62] The aromatic amine core was also modified with pyridine and quinolone for better electron transport properties. 4,4′-Bis(N-(1-naphthyl)-N-(3pyridinylamino)biphenyl (NPyB) and 4,4′-bis (N-(3-quinolinyl)N-phenylamino]biphenyl (QuPB) were synthesized as aromatic amine based soluble host materials and red emitting bis(2phenylquinoline(acetylacetonate)iridium (Ir(phq)2acac) doped NPyB device showed high current efficiency and power efficiency of 21.0 cd/A and 11.3 lm/W, respectively.[63] Electron transport type host materials were applied as the host materials for solution processed phosphorescent OLEDs. Bis(9,9'-spirobifluorene-2-yl)phenylphosphineoxide (M) was used as the electron transport type host in green phosphorescent OLEDs doped with fac-tris(2-(3-p-xylyl)phenyl)pyridine iridium (III) (TEG). The M host showed good electron transporting properties and a triplet energy of 2.58 eV for energy transfer to the green emitting TEG dopant. The M:TEG emitting layer was spin coated on a crosslinked hole transport layer generated by crosslinking reaction of oxetane terminal groups and high current efficiency of 59 cd/A with a low driving voltage of 3.2 V was obtained without any electron transport layer.[64] In general, organic host materials are soluble in aromatic solvents, but the organic host materials can become soluble in alcohol solvents as well as in aromatic solvents if highly polar functional groups like diphenylphosphine oxide or pyridine are introduced in the molecular structure. Alcohol soluble host materials are effective to improve the device performances of solution processed phosphorescent OLEDs because they can be stacked on common high triplet energy hole transport material such as PVK without any intermixing at the interface. In particular, common blue triplet emitters such as FIrpic and FCNIrpic are soluble in 2-propanol due to polar picolinic acid ancillary ligand and alcohol soluble emitting layer could be used in the blue phosphorescent OLEDs. Discrete interface can be generated by spin coating of alcohol soluble emitting layer on high molecular weight PVK (Mw ∼1,100,000) and multilayer device structure to confine triplet excitons could be fabricated. 2,7-bis(diphenylphosphoryl)-9,9-spirobi(fluorene) (SPPO13) was the first alcohol soluble host material used to fabricate multilayer device structure in solution processed phosphorescent OLEDs.[65] An emitting layer of SPPO1:FIrpic was processed from 2-propanol and was spin coated on the PVK hole transport layer at a surface roughness of 0.15 nm. The multilayer blue phosphorescent OLEDs developed using the SPPO13:FIrpic emitting layer gave high external quantum efficiency of 14.1 % with SPPO13 electron transport layer and 13.2 % even without SPPO13 electron transport layer. Following the pioneering work to fabricate multilayer device structure using the alcohol soluble SPPO13 emitting layer, several alcohol soluble host materials were developed for high efficiency in sky blue and deep blue phosphorescent OLEDs. As SPPO13 was an electron transport type host, bipolar host materials with the diphenylphosphine oxide functional groups were synthesized as the alcohol soluble host materials.



REVIEW

soluble bipolar host materials for phosphorescent OLEDs. The 26DCzPPy host was originally developed as a vacuum evaporable host material[56] and was effective to improve the quantum efficiency of blue phosphorescent OLEDs. Good electron transport properties of the 26DCzPPy host was demonstrated by Kido et al. and high current efficiency of 25.1 cd/A in blue device and a current efficiency of 34.2 cd/A in solution processed white phosphorescent OLEDs doped with blue phosphorescent dopant FIrpic and orange phosphorescent dopant iridium(III) bis (4-(4-t-butylphenyl) thieno(3,2-c)pyridinato-N,C2′)acetylacetonate (PO-01-TB) were reported by Meng et al.[28] Particularly, the 26DCzPPy host was coated by blade coating on a small molecule hole transport layer and methanol soluble TPBI electron transport layer was additionally blade coated on the emitting layer. They confirmed that the 26DCzPPy host is better than PVK:OXD-7 mixed host in terms of current efficiency. The same host material was also tested as the host material for red, green and blue phosphorescent OLEDs by blade coating process.[57] Current efficiencies of 40 cd/A for orange, 32 cd/A for green and 20 cd/A for blue phosphorescent OLEDs were obtained using the 26DCzPPy host. Carbazoleand fluorene-based solution processable host material, 1,4-bis(9-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)9H-fluoren-9-yl)benzene (DTCPFB), was developed by Liu et al. Phenyl-substitutedfluorene dimer core was functionalized with twocarbazole groups because of high triplet energy of carbazole. DTCPFB showed high triplet energy of 2.85 eV, excellent miscibility with the dopant, and morphological stability as well as thermal and chemical stability. Maximum current efficiency of 24 cd/A was obtained for solution processed phosphorescent OLEDs using DTCPFB host and FIrpic dopant.[58] Aromatic amine based bipolar host materials are another class of soluble host materials for solution processed phosphorescent OLEDs. Two solution-processable bipolar materials, tris(3′-(1-phenyl-1H-benzimidazol-2-yl)biphenyl-4-yl)amine (TBBI) and tris(2-methyl-3'-(1-phenyl-1H-benzimidazol-2-yl) biphenyl-4-yl)amine (Me-TBBI), bearing hole-transporting triphenylamine and electron-transporting benzimidazole moieties were synthesized as the soluble host materials. Star-shaped configuration and meta-linked molecular structure allowed for high solubility and excellent thermal stability with high glass transition temperature above 140 °C. The two host materials were tested in single layer device structure doped with Ir(ppy)3 and produced highly efficient green phosphorescent OLEDs with a current efficiency of 27.4 cd/A.[59,60] (4-((4-(5-(4-tertbutylphenyl)-4-phenyl-4 H -1,2,4-triazol-3-yl)phenyl)(diphenyl)silyl)phenyl)diphenylamine (p–TAZSiTPA) was another aromatic amine derived soluble host material for deep blue OLEDs. Electron donating diphenylamine group and electron accepting 1,2,4-triazole group were incorporated in the molecular structure through tetraarylsilane linkage and provided high triplet energy, good film-forming properties and bipolar charge transport properties. A moderate external quantum efficiency of 6.3% was achieved in the deep blue device doped with iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6), which was higher than that of PVK device.[61] Ma et al. synthesized 2-(3,5-bis(4′-(diphenylamino)phenyl)phenyl)-5-(4tert-butylphenyl)-1,3,4-oxadiazole (TPO) as the aromatic amine derived bipolar host material for solution process. TPO had

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4226

2,7-Bis(diphenylphosphoryl)-9-phenyl-9H-carbazole (PPO27) was a representative alcohol soluble bipolar host material for blue PHOLEDs.[66] The PPO27 host was soluble in 2-propanol and possessed a high triplet energy of 2.81 eV for application as a host for sky blue (FIrpic) and deep blue (FCNIrpic) triplet emitters. A smooth surface morphology with a low surface roughness less than 0.30 nm was observed in the PPO27, PPO27:FIrpic and PPO27:FCNIrpic films spin coated from 2-propanol solution. High external quantum efficiencies of 14.6% and 13.4% were achieved in the PPO27:FIrpic and PPO27:FCNIrpic devices, respectively. 9-(3-(9H-carbazole-9-yl)phenyl)-3,6-bis(diphenylphosphoryl)9H-carbazole (CPBDC) was also developed as the 2-propanol soluble bipolar host material.[67] The two diphenylphosphine oxide units of CPBDC made it soluble in 2-propanol and imparted bipolar charge transport properties. The CPBDC:FCNIrpic emitting layer exhibited a low surface roughness of 0.23 nm and the CPBDC:FCNIrpic device showed a high external quantum efficiency of 13.8% with a color coordinate of (0.14,0.22). The CPBDC:FCNIrpic emitting layer was further doped with a 2-propanol soluble bis(2-phenylquinolinato) iridium picolinate(Ir(pq)2pic) dopant and two color white OLEDs doped with alcohol soluble blue and orange triplet emitters were grown by solution process. White EL spectra were controlled by Ir(pq)2pic doping concentration and the maximum external quantum efficiency of the white OLEDs was 15.1%. The alcohol soluble blue emitting layer was also adopted to develop white phosphorescent OLEDs with stacked two emitting layers.[68] The double emitting layer structure was difficult to fabricate by solution process because of intermixing of two emitting layers during spin coating. However, the intermixing of two emitting layers could be avoided by spin coating of the alcohol soluble emitting layer on aromatic solvent based emitting layer. An alcohol soluble SPPO13:FIrpic emitting layer was spin coated on toluene soluble PVK:Ir(ppy)3:Ir(phq)2acac emitting layer and the EL spectra of the white device were managed by optimizing the thickness of the SPPO13:FIrpic emitting layer and by changing the doping concentration of Ir(ppy)3. The white phosphorescent OLEDs with the double emitting layer structure yielded high current efficiency of 17.8 cd/A and a white color coordinate of (0.31,0.46). However, no literature reporting double emitting layer structure made up of small molecule host materials in both emitting layers is available because of intermixing of the emitting layers. Device performances of the phosphorescent OLEDs with small molecule hosts are summarized in Table 2. As described above, various small molecule host materials were effective to improve the quantum efficiency of phosphorescent OLEDs and chemical structures of small molecule host materials are shown in Figure 3. Among the various designs of the host materials, the best molecular design in terms of quantum efficiency is the molecular structure with a core unit asymmetrically substituted with electron donating and electron withdrawing groups because of charge balance and stable film morphology. Although many host materials have already been developed, there are still many ways to design new soluble host materials for solution processed phosphorescent OLEDs. Diphenylphosphine oxide, pyridoindole, benzofuropyridine,


benzothiopyridine, pyridofuropyridine, benzimidazole, imidazole, triazole, pyrazole, oxazole, oxadiazole, thiazole, thiadiazole, tetrazole, pyridine, pyrimidine and triazine units can be included as electron withdrawing moieties, while carbazole, diphenylamine, ditolylamine, indole, indolocarbazole, benzofurocarbazole, and benzothiocarbazole can be introduced as electron donating moieties. Molecular design strategy to develop soluble host materials is summarized in Figure 4. However, the use of flexible chains or alkyl side groups should be avoided to obtain long lifetime in solution processed phosphorescent OLEDs and bulky side groups should also be excluded from the molecular structure to reduce the driving voltage of the device due to mobility decrease by the bulky side groups although those moieties can guarantee better solubility and uniform film formation. Instead, short alkyl chains or short alkoxy chains combined with asymmetric molecular design which hinders close molecular packing may be better to obtain good film-forming properties and long lifetime in the solution processed OLEDs.

3.2.2. Small Molecule Mixed Host The use of mixed host instead of single host is advantageous in that charge transport properties can be freely manipulated by changing the composition of the mixed host and crystallization can be handled by restricted molecular packing of the host materials. The molecular packing of each host material is hindered by the other host material due to intermixing of two host materials. Therefore, the mixed host structure has been commonly used as the host material for solution processed phosphorescent OLEDs. However, the host materials for the mixed host system should be carefully chosen because some host materials are immiscible and shows poor solubility in the solution. A mixed host of NPB and TPBI was the first mixed host material for solution processed red phosphorescent OLEDs and was used as the host material tris(1-phenylisoquinoline)iridium (Ir(piq)3). The mixed host composition, and thickness of the emitting layer and electron transport layer were optimized for high efficiency and the maximum external quantum efficiency of the deep red device was 15.1% with a color coordinate of (0.66,0.33), which was comparable to that of vacuum deposited device. In addition, the surface roughness of the spin coated mixed host emitting layer was also similar to that of vacuum deposited emitting layer.[69] Therefore, the NPB:TPBI mixed host was effective as the soluble host material for red phosphorescent OLEDs. In addition to the NPB:TPBI, (4,4′,4′′-tris(N-(2naphthyl)-N-phenyl-amino)triphenylamine) (2-TNATA):TPBI and m-(4,4′,4′′-tri-(N-3-methylphenyl-N-phenylamino)triphenylamine) (MTDATA):TPBI were also reported as the soluble mixed host materials for red or orange phosphorescent devices.[70] As the triplet energy of the hole transport type host material used in red phosphorescent OLEDs was not high enough for application in green phosphorescent OLEDs, high triplet energy hole transport type materials were mixed with electron transport type host materials in the green device. Methoxy substituted 1,3,5-tris(4-(diphenylamino)phenyl)benzene (TDAPB)


Adv. Mater. 2014, 26, 4218–4233


Host

Dopant

E.Q.E.(%)

C.E. (cd/A)

P.E. (lm/W)

Solvent

Ref.

CBP

2

9.58a)

29.77a)

13.35a)

chloroform

[44]

CBP

3

7.64a)

6.18a)

1.07a)

a)

CBP

Ir-2

7.04

CBP

Pt-1

9.55a)

CzOXD

Ir(2-PhPyCz)2(acac)

a)

[45]

21.40

a)

2.92

chloroform

3.36a)

2.31a)

chloroform

[46]

20.0a)

5.0a)

chloroform

[53]

13a)

40a)

20c) a)

[46]

chloroform

[57]

TBBI

Ir(ppy)3

4.3 , 0.8

1.1 , 0.3c)

chloroform

[59]

Me-TBBI

Ir(ppy)3

27.4a), 26.8c)

4.5a), 4.4c)

chloroform

[59]

TIBN

Ir(ppy)3

0.64a),

Me-TIBN

Ir(ppy)3

14.6a), 5.8c)

DM-TIBN

Ir(ppy)3

TPO

Ir(ppy)3

26DCzPPy

PO-01-TB

M

a)

15a)

TEG 15.8c)

0.55c)

0.27a),

0.21c)

[60]

3.7a), 2.0c)

[60]

27.3a), 10.3c)

7.3a), 3.6c)

56.8a), 52.8c)

31.3a), 28.1c)

chlorobenzene

[62]

59a), 55c)

58a), 49c)

tetrahydrofuran

[64]

toluene

[48]

41.2a),

31.1c)

[60]

SimCP2

FIrpic

SimCP

FIrpic

22.1c)

toluene

[48]

FIrpic

6.9c)

toluene

[48]

28.4a), 23.9c)

toluene

[50]

toluene

[51]

mCP mCPPO1

21.0

a),

c)

FCNIrpic

22.1a), 18.9c)

FIrpic

23.9a), 17.7c)

CzDBF

a),

17.6c)

PCz-BFP

FIrpic

toluene

[52]

mOXDDSiPA

FIrpic

10.7a)

23.4a), 17.1c)

10.2a)

chlorobenzene

[55]

26DCzPPy

FIrpic

10.8a)

25.1a)

9.3a)

chlorobenzene

[28]

26DCzPPy

FIrpic

20a)

chlorobenzene

[57]

FIrpic

a)

a)

[58]

18.0

c)

24.0 , 19.3

8.8

chlorobenzene

FIr6

6.3a)

12.5a), 11.4c)

6.2a)

chlorobenzene

[61]

SPPO13

FIrpic

14.1a), 11.0c)

30.4a), 24.0c)

2-propanol

[65]

PPO27

FIrpic

14.6a)

28.2a)

15.8a)

2-propanol

[66]

a)

21.0a)

12.2a)

2-propanol

[66]

2-propanol

[67]

chlorobenzene

[28]

DTCPFB p-TAZsiTPA

PPO27

FCNIrpic

13.4

CPBDC

FCNIrpic

13.8a),13.4c)

FIrpic PO-01-TB

11.6a)

26DCzPPy

34.2a)

12.0a)

REVIEW

Table 2. Device performances of the phosphorescent OLEDs with small molecule hosts.

a)

Maximum efficiency; b)Efficiency at 100 cd/m2; c)Efficiency at 1000 cd/m2.

was mixed with PBD for efficient energy transfer to green emitting Ir(ppy)3 and complete energy transfer from the TDAPB:PBD mixed host to Ir(ppy)3 was observed. The methoxy unit of the TDAPB stabilized amorphous film morphology of TDAPB and meta- linked phenyl units increased the triplet energy of TDAPB. Moderate external quantum efficiency of 8.2% was realized in the solution processed TDAPB:PBD mixed host device.[71,72] In the blue phosphorescent OLEDs, a mixed host composed of high triplet energy hole and electron transport type host materials was popular. A mixed host of TCTA and SPPO13 was used as a universal mixed host for red (bis(2methyldibenzo(f,h)quinoxaline)(acetylacetonate) iridium(III) (Ir(MDQ)2acac)), green (Ir(mppy)3) and blue (FIrpic) phosphorescent OLEDs and high external quantum efficiencies of 10.8%, 16.3% and 15.2% were achieved for red, green and blue OLEDs, respectively, in inverted OLED structure. Hole transport type TCTA and electron transport type SPPO13

Adv. Mater. 2014, 26, 4218–4233

balanced charge density in the emitting layer and smoothened the film morphology of the emitting layer.[73] Similarly, a mixed host of TCTA and 26DCzPPy was evaluated as the soluble host material for FIrpic based blue OLEDs. Although 26DCzPPy was a bipolar host material, the mixing of TCTA and 26DCzPPy improved the device performances of the solution processed blue phosphorescent OLEDs. A low driving voltage of 5.4 V at 1000 cd/m2 and maximum external quantum efficiency of 14.6% was achieved due to smooth film morphology and high carrier mobility of the mixed host. Star shaped carbazole compounds, 9-(5′,5′′′-diphenyl[1,1′:3′,1′′:3′′,1′′′:3′′′,1″″-quinquephenyl]-5′′-diyl)-9H-carbazole (DQC) and 9,9′-(5′-phenyl(1,1′:3′,1′′-terphenyl)-3,5-diyl)bis9H-carbazole (PTC), were synthesized as high triplet energy soluble host materials. The nonplanar star shaped design was helpful to form amorphous film and to get high glass transition temperature. Moderate external quantum efficiencies of 9.2%



4227

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REVIEW


N N

N

N

N N

N

N O N N

CBP

Ad-Cz

N

Si

N

P O N

Si N

N

CzOXD

Cy-Cz

N

N

N

N

N

N

SimCP

N

SimCP2

mCP

Cz(MP)2

mCPPO 1

N O N

N

N

N O

N

CzDBF

N

PCz-BFP

N

N

N

TRZ-1Cz(MP)2

N N

N

N N

N

N

N

N

N

Si

N

N

N

TRZ-3Cz(MP)2

26DCzPPy

N

p-TAZsiTPA

N

N

N Si

Si

O

Si N

Si

O

N N

N

N

mOXDDSiPA

pOXDDSiPA

N

N

N Si

Si

Si

O

N

Si

O N

N N

pOXDDSiCz

N

mOXDDSiCz N

N N

N N

N

N

N

N

N

DTCPFB

N

N

N

N

N

N

N

N N

N

Me-TIBN

DM-TIBN

O

N N

N

N

N

N

N

N

QuPB

O

P

SPPO13

O P

N

PPO27

Figure 3. Chemical structures of small molecule host materials.


CPBDC

O

P

O

M

O

N

NPyB

P

N

P

N

TPO

P

N

N

4228

N

N

N

N

TIBN

N

N

N

N

N

N

N

Me-TBBI

N

N

O

N

TBBI

N

N

N

N

P O

and 11.9% were reported in the DQC and PTC devices, respectively, by mixing with OXD-7.[74] A tert-butyl substituted high triplet energy host, 9,9-bis(4-(3,6-di-tert-butylcarbazol-9-yl)phenyl)fluorene (TBCPF), was also mixed with the electron transport type OXD-7 host, producing homogeneous film by spin coating. The tert-butyl modification resulted in better solubility in solvent, suppressed self-quenching and stable amorphous film formation by spin coating in spite of high driving voltage. The TBCPF:OXD-7 mixed host doped with FIrpic showed a maximum current efficiency of 12.5 cd/A.[75] The current efficiency of white OLEDs fabricated with the same TBCPF:OXD-7 mixed host doped with FIrpic, Ir(mppy)3 and iridium bis(1-phenylisoquinoline) (acetylacetonate) (Ir(piq)2acac) was 29.6 cd/A.[76] TAPC was also used as the hole transport type host in combination with 26DCzPPy to develop high efficiency white phosphorescent OLEDs. Red, green and blue triplet emitters were doped into the TAPC:26DCzPPy mixed host and high external quantum efficiency of 15.7% was obtained. In particular, the efficiency roll-off of the white OLEDs was greatly reduced and the color coordinate of the white OLEDs remained stable with little change according to the luminance of the device due to suppressed exciton quenching and charge trapping.[77] In most studies about the mixed hosts for solution process, a hole transport type host was mixed with an electron transport type host. However, Chen et al. demonstrated that a mixed host of hole transport hosts, TCTA:TAPC, reduced the driving voltage and improved the efficiency of blue phosphorescent OLEDs. The TCTA:TAPC host induced facile hole injection and transport as well as suppression of triplet exciton quenching. A maximum current efficiency of 32.0 cd/A and power efficiency of 25.9 lm/W were achieved in the optimized blue device. White OLEDs also exhibited high current efficiency of 37.1 cd/A and power efficiency of 32.1 lm/W. In particular, the TAPC:TCTA mixed host was effective to reduce the efficiency roll-off.[78] The same group also improved the lifetime of solution processed small molecule phosphorescent OLEDs.[79] A ternary blend system comprised of three host materials were also used as the soluble host material for OLEDs. Shinar et al. developed green phosphorescent OLEDs with a ternary blend of CBP, PBD and TPD.[80] CBP was the matrix material, while PBD and TPD were blended to improve the charge balance in the emitting layer. The device performances of the CBP:PBD:TPD host were compared with those of PVK:PBD:TPD host using Ir(mppy)3 dopant. The PBD and TPD reduced the build-up of space charge in the device, and improved the charge


Adv. Mater. 2014, 26, 4218–4233


REVIEW

Figure 4. Molecular design strategy to develop soluble host materials.

transport and balance in the emitting layer due to high carrier mobility.[16,17,30,81] The Ir(mppy)3 doped CBP:PBD:TPD device showed maximum luminous efficiency, luminous power efficiency and external quantum efficiency of 69 cd/A, 60 lm/W and 22%, respectively. In particular, the CBP based ternary blend host exhibited much longer lifetime than PVK based ternary blend host, proving that small molecule host is better than polymeric host in terms of device stability. Although this work reported improved lifetime in the solution processed OLEDs, the lifetime level of the device was inferior to that of vacuum deposited OLEDs. Most long lifetime data were reported from industry such as Merck, DuPont and UDC. Merck reported red lifetime of 120 000 h and green lifetime of 320 000 h at 1000 cd/m2 using a mixed host of small molecules although exact molecular structure of the host materials was not revealed.[82] UDC also demonstrated long lifetime of 100 000 h at 500 cd/m2, 63 000 h at 1000 cd/m2 and 6000 h at 500 cd/m2 in red, green and blue phosphorescent OLEDs and ink-jet compatibility of the small molecule based phosphorescent emitting

layer.[83,84] It was reported that purity of solvent, mixed host formulation and solvent type were critical to the long-term stability of the solution processed OLEDs and the lifetime of the solution processed OLEDs is close to that of the vacuum processed OLEDs by optimizing the ink formulation and coating process. Device performances of the phosphorescent OLEDs with the small molecule mixed hosts are summarized in Table 3. The chemical structures of hole transport type and electron transport type small molecule host used in the mixed hosts are described in Figure 5. As explained above, the mixed host of small molecule host materials were effective to enhance the quantum efficiency and lifetime of solution processed phosphorescent OLEDs. The mixed host of a hole transport type host and an electron transport type host balanced holes and electrons in the emitting layer due to balanced charge density and bipolar charge transport properties, which improved the recombination efficiency in the emitting layer. The better charge injection and charge transport properties also reduced the driving voltage of the device, resulting in

Table 3. Device performances of the phosphorescent OLEDs with the small molecule mixed hosts. Host 2

Dopant

E.Q.E. (%)

C.E. (cd/A)

NPB

Host 1

TPBi

Ir(piq)3

15.1a)

12.7a),9.5b)

NPB

TPBi

Ir(phq)2acac

P.E. (lm/W)

12.3b

6.6b

17.4a),17.4b)

10.4a),10.1b)

Solvent

Ref.

1,2-dichloroethane

[69]

1,2-dichloroethane

[70]

m-MTDATA

TPBi

Ir(phq)2acac

9.9a)

1,2-dichloroethane

[70]

2-TNATA

TPBi

Ir(phq)2acac

10.2a)

17.8a),17.8b)

11.3a),11.2b)

1,2-dichloroethane

[70]

SPPO13

Ir(MDQ)2acac

10.8a),8.9b)

12.0a),9.8b)

7.9a),3.5b)

chlorobenzene

[73[

29a)

17.3b)

1,2-dichloroethane

[71]

60a)

chlorobenzene

[80]

1,2-dichloroethane

[74]

1,2-dichloroethane

[74]

chlorobenzene

[73]

chlorobenzene

[75] [77]

TCTA

PBD

Ir(ppy)3

8.2a)

CBP

PBD TPD

Ir(mppy)3

22a)

69a)

DCQ

OXD-7

FIrpic

9.2a)

21.7a)

TDAPB

a)

PTC

OXD-7

FIrpic

11.9

TCTA

SPPO13

FIrpic

15.2a),13.7b)

TBCPF

OXD-7

FIrpic

25.7a) 34.3a),30.8b)

20.5a),13.4b)

12.5a) a),33.0b)

22.8a),19.1b)

chlorobenzene

TAPC

26DCzPPy

FIrpic

33.7

TCTA

TAPC

FIrpic

32.0a),29.6b)

25.9a),19.1b)

chlorobenzene

[78]

TCTA

OXD-7

FIrpic

18.3a,18.1b

14.4a,12.8b

chlorobenzene

[78]

TBCPF

OXD-7

FIrpic Ir(mppy)3 Ir(piq)2acac

15.7a)

29.6a),28.2b)

15.6a)

FIrpic Ir(ppy)2acac Ir(MDQ)2acac

15.7a)

36.5a)

23.4a)

TAPC

a)Maximum

26DCzPPy

[76] chlorobenzene

[77]

efficiency; b)Efficiency at 1000 cd/m2.

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CH 3

N

N

H3 C

N

N

O

O

N N

NPB

TPD

N N

OXD-7

N

N N

N N

N

O

N

N

TAPC

TCTA

N

PBD

N N

N

N

N N

N

N

N

N

2-TNATA

N

N

m-MTDATA

TPBi

O P

N

O P

N

N

PTC

DQC O

CH 3

SPPO13 H 3C

N N

N

N

O

N

O

O

CH 3

CH 3

N

N

N

N H3 C

TBCPF

O

O

CH 3

TDAPB

26DCzPPy

Figure 5. Chemical structures of hole transport type and electron transport type small molecule host used in the mixed hosts.

high power efficiency as well as high quantum efficiency in the phosphorescent device. In addition, the mixed host stabilized the morphology of the triplet emitter doped emitting layer because crystallization of the host materials was suppressed by mixing two different host materials. The better charge balance, better charge injection and better morphological stability of the mixed host enhanced the quantum efficiency and lifetime of the mixed host device. Although the device fabrication process of the mixed host structure is sophisticated in the vacuum evaporation process because three materials should be deposited at the same time, it is easy to fabricate the mixed host structure in solution process because the solution of the mixed can be simply prepared by dissolving the two host materials and a triplet emitter in the solvent. Therefore, the small molecule mixed host structure is promising as a host structure for solution processed phosphorescent

4230


OLEDs. Current device performances of the vacuum deposited and solution processed phosphorescent OLEDs using small molecule host materials are summarized in Table 4.

4. Summary and Outlook In summary, we have discussed small molecule host materials for application in solution processed phosphorescent OLEDs and discussed the device performances of the small molecule based phosphorescent OLEDs. The small molecule host materials have been used as a single host, a mixed host, and a hybrid host of polymer and small molecules. Although the hybrid host of polymer and small molecules have been typically used as the host material for solution processed phosphorescent OLEDs, it


Adv. Mater. 2014, 26, 4218–4233


Color Red

Solution process

Efficiency

Lifetime

Ref.

12.7 cd/A

120 000 h (1000 cd/m2)

[82]

20.0 cd/A Vacuum evaporation

Green

Solution process

Vacuum evaporation

Blue

Solution process

Vacuum evaporation

[85]

24.6 %

–

[7]

81.3 cd/A

320 000 h (1000 cd/m2)

[82]

69 cd/A

–

[62,80]

85 cd/A

400 000 h (1000 cd/m2)

[85]

106 cd/A

–

[86]

19 cd/A

6000 h (500 cd/m2)

[84]

28.4 cd/A

–

[50]

50 cd/A

20 000 (1000 cd/m2)

[85]

53.6 cd/A

–

[8]

600 000 h (1000 cd/m

is difficult to overcome the demerits of the hybrid host, such as low triplet energy, incompatibility with small molecule hosts and triplet emitters, low purity level, poor stability, and short lifetime. Therefore, the hybrid type host materials may not be applicable in the phosphorescent OLEDs. Instead, small molecule host materials are promising as host materials for solution processed phosphorescent OLEDs owing to their high purity, high triplet energy, compatibility with triplet emitters, chemically stable molecular structure and long lifetime in spite of the easy crystallization of the small molecules compared to polymers. However, the critical problems of morphological stability can be easily solved by applying a mixed host structure which combines a hole transporting host and an electron transporting host. Therefore, the mixed host of several small molecule host materials is the most promising host structure for solution processed phosphorescent OLEDs. Although the mixed host of small molecule host materials may enhance the device performances of solution processed phosphorescent OLEDs, there are still several device parameters to be considered. Firstly, the lifetime should be improved further because the lifetime of the solution processed OLEDs is still too short compared to that of vacuum processed OLEDs. Even though some companies reported long lifetimes in solution devices, further improvement of the lifetime via material, device structure and coating process developments are required. In particular, new host and triplet emitters compatible with the solution process are necessary. Secondly, high quantum efficiency comparable to that of vacuum processed devices should be obtained in the solution processed devices. Despite tremendous progresses in quantum efficiencies reported in previous years, the quantum efficiencies of solution processed OLEDs are still below the ones of the vacuum processed OLEDs. Therefore, efficiency improvement is important to meet the power consumption specification of OLED panels. Finally, the driving voltage of the solution processed device should be lowered further. As low film density of the solution processed device is responsible for the high driving voltage, high film density should be realized by optimizing the processing conditions of the film deposition from

Adv. Mater. 2014, 26, 4218–4233

Received: December 24, 2013 Revised: February 10, 2014 Published online: May 7, 2014

[53] 2)

30 cd/A

solution. The development of organic materials, device structure, and process may overcome the hurdles of the solution processed OLEDs and will commercialize the solution processed phosphorescent OLEDs in the OLED panels.

REVIEW

Table 4. Summary of the state-of-the-art device performances of solution and vacuum processed phosphorescent OLEDs using small molecule host materials.

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Adv. Mater. 2014, 26, 4218–4233


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REVIEW

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Molecular design of triazine and carbazole based host materials for blue phosphorescent organic emitting diodes.

An easy route to red emitting homoleptic IrIII complex for highly efficient solution-processed phosphorescent organic light-emitting diodes.

Highly efficient, solution processed electrofluorescent small molecule white organic light-emitting diodes with a hybrid electron injection layer.

m2.

Rational design of host materials for phosphorescent organic light-emitting diodes by modifying the 1-position of carbazole.

A low bandgap asymmetrical squaraine for high-performance solution-processed small molecule organic solar cells.

Morphological effects on the small-molecule-based solution-processed organic solar cells.

A solution processed flexible nanocomposite electrode with efficient light extraction for organic light emitting diodes.

Tuning Charge Balance in Solution-Processable Bipolar Triphenylamine-diazafluorene Host Materials for Phosphorescent Devices.

Vacuum nanohole array embedded phosphorescent organic light emitting diodes.

Unsymmetrical Donor-Acceptor-Acceptor-π-Donor Type Benzothiadiazole-Based Small Molecule for a Solution Processed Bulk Heterojunction Organic Solar Cell.

An electron transporting unit linked multifunctional Ir(III) complex: a promising strategy to improve the performance of solution-processed phosphorescent organic light-emitting diodes.

Solution processed, white emitting tandem organic light-emitting diodes with inverted device architecture.

Small molecule-based tandem solar cells with solution-processed and vacuum-processed photoactive layers.

Solution-processed transparent blue organic light-emitting diodes with graphene as the top cathode.

Deep Blue and White Phosphorescent Organic Light-Emitting Diodes (PhOLEDs) Hosted by a Polysiloxane Derivative with Pendant mCP (1,3-bis(9-carbazolyl)benzene).

Ideal bipolar host materials with bis-benzimidazole unit for highly efficient solution-processed green electrophosphorescent devices.

Bipolar host with multielectron transport benzimidazole units for low operating voltage and high power efficiency solution-processed phosphorescent OLEDs.

All-solution-processed inverted quantum-dot light-emitting diodes.

Understanding the charge-transfer state and singlet exciton emission from solution-processed small-molecule organic solar cells.

Impact of the electron-transport layer on the performance of solution-processed small-molecule organic solar cells.

Mobility guidelines for high fill factor solution-processed small molecule solar cells.

Mulifunctional Dendritic Emitter: Aggregation-Induced Emission Enhanced, Thermally Activated Delayed Fluorescent Material for Solution-Processed Multilayered Organic Light-Emitting Diodes.

Electronic excitations in solution-processed oligothiophene small-molecules for organic solar cells.