All-solution-processed blue small molecular organic light-emitting diodes with multilayer device structure

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Letter

All-solution-processed blue small molecular organic light-emitting

diodes with multilayer device structure

Jia-Da You

b

, Shin-Rong Tseng

a

, Hsin-Fei Meng

a,*

, Feng-Wen Yen

c

, I-Feng Lin

c

, Sheng-Fu Horng

b

a_{Institute of Physics, National Chiao Tung University, Hsinchu 300, Taiwan, ROC}

b_{Department of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC}

c_{Luminescence Technology Corporation, 2F, No. 21 R&D Road II, Science-Based Industrial Park, Hsinchu 300, Taiwan, ROC}

a r t i c l e

i n f o

Article history: Received 20 March 2009

Received in revised form 21 June 2009 Accepted 21 June 2009

Available online 18 July 2009 PACS:

85.60.Jb Keywords:

Organic light-emitting diodes Solution process

Multilayer Blue Blade coating

a b s t r a c t

All-solution-processed multilayer blue small molecular organic light-emitting diodes are fabricated by blade coating method. Fluorescent blue host,1-(7-(9,90

-bianthracen-10-yl)-9,9-dioctyl-9H-ﬂuoren-2-yl)pyrene, and blue dopant, 4,40_-(1E,10_E)-2,20

-(naphthalene-2,6-diyl)bis(ethene-2,1-diyl)bis(N,N-bis(4-hexylphenyl)aniline), are used to achieve good solubility and pinhole-free thin ﬁlm by solution process. The multilayer device structure with hole/electron transport layer is achieved by blade coating method without the disso-lution problem between layers. The efﬁciency of the all-sodisso-lution-processed device is 4.8 cd/ A at 1200 cd/m2_{, close to that by thermal deposition in high vacuum chamber. The device}

Organic emitting diodes (OLED) and polymer light-emitting diodes (PLED) have been regarded as the candidate for the next generation light source and flat panel display. Due to the well developed materials and multilayer design of device structure, some OLED products have come into the market[1]. In general the small molecules are deposited by thermal evaporation in high vacuum chamber to form multilayer structure, including carrier transport layers, carrier blocking layers, and at least one emissive layer. The carriers can rapidly transport to emissive layer and then be confined by the blocking layers, resulting in high device efficiency. On the other hand, PLED with single layer device structure is fabricated by solution process, which can be made in large area with low cost but low device efficiency. The synthesis and purification of the polymers however are more complicated than those of small molecules. There

would be great benefits to apply solution process to fabri-cate OLED, which combines the simple materials of OLED and cheap process of PLED[2–6]. However most of them are single layer or double layer device structure with a vac-uum deposited electron transport layer. There are remain-ing two problems in fabricatremain-ing OLED with multilayer device structure by solution process. Firstly, most small molecules could not form a thin film by solution process due to the fact that there is no chain entanglement of the small molecules. Secondly, it is difficult to form multilayer structure by solution process because of the dissolution problem between layers. The former layer would be re-dis-solved by the later solution. Some modifications of the small molecules are required to form a pinhole-free layer, such as the addition of side chains or the formation of larger mole-cules. In this work, blue small molecular host, 1-(7-(9,90

-bianthracen-10-yl)-9,9-dioctyl-9H-ﬂuoren-2-yl)pyrene (LT-492), and dopant, 4,40_-(1E,10_E)-2,20

-(naphthalene-2,6-diyl)bis(ethene-2,1-diyl)bis(N,N-bis(4-hexylphenyl)aniline)

* Corresponding author. Tel.: +886 35731955. E-mail address:[email protected](H.-F. Meng).

Contents lists available atScienceDirect

Organic Electronics

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(LT-632) with side chains have been used in order to get uniform thin ﬁlms. In addition to the molecular modiﬁca-tion, many efforts have been made to fabricate multilayer structure by solution process, such as synthesis of cross-linked materials[3], water/methanol soluble materials[7], lamination method[8], liquid buffer layer[9], and blade coating method[10]. All-solution-processed OLEDs in mul-tilayer device structure, hole transport layer (HTL)/emissive layer (EML)/electron transport layer (ETL), have been fabri-cated by blade coating without applying any cross-linked materials. In such devices the LiF/Al is used as the cathode, without any low work function metals like Ca, Ba, or Mg. The performance of the device by all-solution process is close to that by evaporation but the cost would be reduced dramatically due to the cheap solution process.

All-solution-processed multilayer blue light OLED with high device efﬁciency has been compared with its counter-part by vacuum deposition. Three devices with device struc-ture of ITO/PEDOT:PSS/HTL/EML/ETL/LiF/Al were made to compare the different fabrication processes, including the device by all evaporation process (device A), the device with solution-processed HTL and evaporated EML/ETL (device B), and the device by all-solution process (device C). Four devices by all-solution process with different annealing temperatures of ETL layer, 20 °C (device C-1), 50 °C (device C-2), 70 °C (device C-3), and 90 °C (device C-4) for 10 min were made to optimize the device performance. Further-more a device without ETL by solution process was made as comparison (device D). ITO is indium tin oxide and

PED-OT:PSS is poly-(3,4-ethylenedioxythiophene):poly-(sty-renesulfonate) (CLEVIOSTM_{P VP CH 8000, purchased from}

HC Starck). PEDOT:PSS was spin coated to form a 40 nm thin ﬁlm on pre-cleaned the ITO substrate and then was baked at 100 °C for 40 min. N,N0_{-Bis(naphthalen-1-yl)- N,N}0_-bis

(phenyl)- 9,9-dimethyl-ﬂuorene (DMFL-NPB, from

Luminescence Technology Corporation), acting as HTL, was dissolved in chlorobenzene to form a 20 nm film by blade and spin coating. LT-492 and LT-632 provided by Luminescence Technology Corporation, act as blue host and blue dopant individually. The 2 wt% LT-632 in the host LT-492 were dissolved in chlorobenzene and blade coated to form a 50 nm film on top of DMFL-NPB. Blade and spin coating is the process to use first blade coating to form the wet film and then spin coating until dried film is formed. Blade coating is the process only to use the blade coating without any spin coating. The DMFL-NPB and LT-492:LT-632 layers were baked at 120 °C for 10 min and 125 °C for 5 min in vacuum (103 _{torr). The 2,2}0_,200

-(1,3,5-Ben-zinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi, from Luminescence Technology Corporation), acting as the ETL and hole blocking layer, was dissolved in methanol to form a 20 nm ﬁlm by blade and spin coating. The ﬁlm thickness by blade coating can be controlled by the solution

concen-Fig. 1. (a) Chemical structures of the organic materials, and (b) schematic energy proﬁle of the multilayer device structure design in this work. The numbers are in eV.

Fig. 2. Device performance of device A (solid square), device B (empty square), device C-2 (solid circle), and device D (empty circle). (a) The current efﬁciency. Inset is the electroluminescent spectra. (b) The luminance. Inset is the current density.

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tration and the gap between substrate and blade coater. The film thickness by blade and spin coating can be controlled by the spin rate in addition to the solution concentration and the gap. The speed of blade coating is fixed at 10 cm/ s. The film thicknesses are measured by a Kosaka ET4000 Surface Profiler.Fig. 1shows the material chemical

struc-tures and schematic energy proﬁle of the multilayer device structure. All the solution and annealing processes were performed in the glove box with low humidity (<5 ppm) and oxygen (<5 ppm). In device A all the organic layers were made by conventional thermal evaporation in the high vac-uum chamber (107_{torr) without any annealing process. All}

Fig. 3. AFM images of the (a) HTL, (b) HTL/EML, (c) HTL/EML/ETL by evaporation, and (d) HTL, (e) HTL/EML, (f) HTL/EML/ETL by solution process. The solution-processed ETL (TPBi) is annealed at 50 °C.

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the devices were coated with LiF(1 nm)/Al(100 nm) cathode and packaged in a glove box. Ionization potential is mea-sured by cyclic voltammetry and electron afﬁnity is calcu-lated by Ionization potential plus band gap determined by the ultraviolet absorption spectrum. The electrolumines-cence (EL) spectra and current–luminance–voltage (I–L–V) characteristics are measured by a Photo Research PR650 spectrophotometer integrated with Keithley 2400 multi-meter.

Fig. 2shows the device performances of device A (all

evaporation), device B (only HTL by solution process),

de-vice C-2 (all-solution process), and dede-vice D (all-solution process, without TPBi). While operating at 10 mA/cm2_,

the device efﬁciency is 6.3 cd/A of device A, 5.6 cd/A of de-vice B, 4.8 cd/A of dede-vice C-2, and 0.01 cd/A of dede-vice D.

The maximum luminance is 7054 cd/m2 _{of device A,}

11,460 cd/m2 _{of device B, 3677 cd/m}2_{of device C-2, and}

26 cd/m2of device D. The maximum luminance of the de-vices is defined as the luminance that can be achieved of the device. The luminance will decay after this maximum luminance even the bias increases. There are some dam-ages occurred of the devices after the maximum lumi-nance. The device performance of device D without TPBi as ETL is obviously poor, indicating TPBi layer is required to offer efficient electron injection. As can be seen in I–V– L relation, the current and luminance of all-solution-pro-cessed device C-2 at low bias before 9 V are about the same as those of device B and are higher than those of all-evap-orated device A. At higher bias larger than 10 V, the current and luminance of device C become saturated and lower than those of device A and device B. Moreover the electro-luminescent (EL) spectra of the devices with solution-pro-cessed EML (device C-2 and device D) show red-shift compared with the devices with evaporated EML (device A and device B). This may be attributed to the slight disso-lution between the HTL and EML during blade coating, which might cause exciplex formation in such mixing area. The dissolution has been verified to be less than 10 nm by measuring the total film thickness. In order to study the detail of the different processes, the microscopic morphol-ogies of each layer of device A and device C-2 have been checked by AFM. The results are shown in Fig. 3. The roughness of the evaporated layers is 1.04 nm for DMFL-NPB, 0.87 nm for LT-492 LT-632, and 1.01 nm for TPBi. The roughness of the solution-processed layers is 0.42 nm for DMFL-NPB, 0.46 nm for LT-492:LT-632, and 1.39 nm for TPBi. The morphologies of each layer by evap-oration are about the same. The uniformity of the solution-processed DMFL-NPB and LT-492:LT-632 are better than the evaporated ones but a few spots still can be seen in the solution-processed DMFL-NPB. There are even more spots shown in the solution-processed TPBi layer due to the fact that there is no side chains in the TPBi molecules. We speculate that crystallization occurs in the solution-processed DMFL-NPB and TPBi and such crystallization could be regarded as one kind of impurity, which would quench the excitons. That is the reason why the efficiency decreases in device B and even more in device C-2. On the

Fig. 4. Device performance of the all-solution-processed devices with different annealing temperature of TPBi, 20 °C (solid square), 50 °C (empty square), 70 °C (solid circle) and 90 °C (empty circle). (a) The current efﬁciency. Inset is the electroluminescent spectra. (b) The luminance. Inset is the current density.

Table 1

Performance of OLEDs in this work.

Label HTL (DMFL-NPB) EML (LT-492:LT-632) ETL (TPBI) Max. efﬁciency (cd/A) Max. luminance (cd/m2

) Device A Ea E E 7.4(4V) 7054(11.5V) Device B Sb E E 5.6(8V) 11,460(11.5V) Device C-1 S S S(20 °C) 2.9(10V) 2125(13V) Device C-2 S S S(50 °C) 4.8(8V) 3677(11.5V) Device C-3 S S S(70 °C) 2.3(10V) 856(13V) Device C-4 S S S(90 °C) 1.8(10V) 315(13V) Device D S S 0.1(11.5V) 26(11.5V) a E: evaporation. b_{S: solution process.}

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other hand, the crystallization in DMFL-NPB may cause higher mobility in device B and device C-2, therefore the current density is higher than that of device A at lower bias range (below 9 V). While the bias increasing, the emission zone will move to the interface between EML and TPBi due to the carrier blocking effect and the excitons would be quenched by the large crystallization of TPBi. Therefore the luminance of device C-2 becomes lower at higher bias range (after 10 V). In order to optimize the all-solution-processed OLED, devices with different annealing temper-atures of TPBi layer have been made, which are device device C-1 (20 °C), C-2 (50 °C), device C-3 (70 °C), and device C-4 (90 °C). The results are shown inFig. 4. The efﬁ-ciencies while operating at 10 mA/cm2 _{are 2.9 cd/A for}

device C-1, 4.8 cd/A for device C-2, 2.1 cd/A for device C-3, and 1.7 cd/A for device C-4. The maximum luminance are 2125 cd/m2for device C-1, 3677 cd/m2for device C-2, 856 cd/m2 _{for device C-3, and 315 cd/m}2_{for device C-4.}

The device performance of device C-1 is lower than that of device C-2, indicating that the crystallization of device C-2, as can be seen in Fig. 3f, may increase the carrier mobility and help the electron transporting. In device C-3 and device C-4 large crystallization occurs and obvious spots can be seen by naked eyes, therefore the device per-formances are poor due to the exciton quenching effect. the crystallization of organic ﬁlm is detrimental to the de-vice and should be avoided. The case of TPBi ﬁlms annealed at 70 °C and 90 °C indeed shows the crystal formation at the scale of

l

m or larger in organic films which can be ob-served by naked eyes. The performance of the devices with TPBi at 70 °C or 90 °C shows poor efficiency. On the other hand the TPBi film at 50 °C has very small spots (much less than

l

m). We speculate that the ﬁlms at 50 °C just start packing and cause higher carrier mobility than that an-nealed at 20 °C but less harmful than that with real crys-tals. Furthermore, according to the AFM image the morphology and roughness of HTL/EML (annealed at 125 °C, 5 min) seems to be about the same as those of HTL (annealed at 120 °C, 10 min). However the morphol-ogy and roughness changed after adding the ETL (annealed at 20 °C, 50 °C, 70 °C, or 90 °C for 10 min). Because the annealing temperature of ETL is less than that of HTL or

HTL/EML, we think the roughness and morphology change should be resulted from the crystallinity change of TPBi layer, not from other layers (HTL, EML). We suggest that vapor-quality devices from solution process can be ob-tained with all the small molecules including hole/electron transport and emissive materials modiﬁed with side chains to get a uniform thin ﬁlm without crystallization. All the device performances are summarized inTable 1.

In conclusion we have demonstrated all-solution-pro-cessed multilayer blue OLEDs by blade coating without any cross-linked materials. This kind of device combines the advantages of small molecular organic materials and large area manufacture process with low cost. Blue organic host and guest have been used with side chains to get a uniform thin ﬁlm by solution process. The performance of the solution-processed device with multilayer structure can be very close to the evaporated one, and the cost of the solution-processed can be reduced dramatically.

Acknowledgement

This work is supported by the National Science Council of Taiwan under Grant numbers NSC96-2120-M-007-007 and NSC96-2112-M-009-036.

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