The Effect of Orthogonal Solvent of Colloidal Quantum Dots on QD-LED device

Yohan Kim1, Se Min Kim1, Hanho Park2, Dae-gyu Moon2, Jungwon Kang2 and Chul Jong Han1*

1Flexible Display Research Center, Korea Electronics Technology Institute, Seongnam-si, Gyeonggi-do, 463-816, Korea

2Department of Electronic and Electrical Engineering, Dankook University, Yongin-si, Gyeonggi-do, 448-701, Korea

3Department of Materials Engineering, Soonchunhyang University, Asan-si, Chungnam, 336-745, Korea

Abstract We investigated the orthogonal solvent of CdSe/ZnS quantum dots in order to deposit poly-TPD hole transport layer without damages and enhance the performances of QD-LEDs. Using hexane and heptane as nonpolar organic solvent of QDs does not affect poly-TPD layer has found. 3 mg/ml in hexane shows highest luminance and current efficiency but creates non-uniform QD-layer because lower boiling point of hexane. The last we found that even different concentration of QDs in heptane show similar device performance.

Author Keywords Colloidal quantum dots; orthogonal solvent; QD-LEDs; poly-TPD

1. Introduction In the flat panel display industry, colloidal quantum dots (QDs) have earned much attention because of its superb properties such as tunable color spectra, narrow emission bandwidth, broad absorption, and cost-effective solution based processing [1,2]. These unique optical/electrical properties of colloidal QDs, which is conventionally synthesized with various ligands and ranged from 2 to 12nm in diameter, is determined by quantum confinement effects with hetero structure of core/shell [3]. This gives high quantum yield of colloidal QDs in organic solvent and novel emitter layer in organic and inorganic hybrid QD-LED structure. Since the first report on quantum dot light emitting diodes (QD-LEDs) in 1994 [4], many research groups have investigated to enhance the device performances [5-8]. Efficient EL in and QD-LEDs requires not only balanced injection of but also similar mobility value for electrons and holes. In order to accomplish efficient carrier balance, a bi-layer QD-LEDs by ETL and HTL layer is needed. Energy band structure of the device is designed to give quantum confinement effect on emissive layer, which confines the radiative decay of exciton. Therefore, the thickness of the layer is also important to balance the carriers in the device. Colloidal quantum dot is usually dispersed in nonpolar organic solvent such as toluene, chloroform; therefore it is important to find the hole transport materials which are chemically and physically stable against those solvent to deposit the desired thickness of hole transport layer.

Since various researches on QD-LEDs performances were performed with poly-TPD hole transport layer which is known as chemically stable material to nonpolar organic solvent [9], here, we investigated the effects of orthogonal solvent of CdSe/ZnS quantum dots in order to deposit poly-TPD hole transport layer without damages and enhance the performances of QD-LEDs. Toluene, hexane and heptane were investigated as nonpolar organic solvent of QDs when we deposit the QD layer. Also, the

concentration of quantum dots in organic solvent was changed during the deposition process in order to make the homogeneous QD layer and enhance the performances of QD-LEDs

2. Device structure and fabrication The QD-LEDs structure is shown in Figure 1. and energy band diagram for QD-LEDs is shown Figure 2. The ITO (Indium-Tin Oxide) coated substrates, which is ITO-glass (Samsung Corning

Precision Materials, Korea, 10 Ω/sq), were used as rigid

substrates. The device consists of ITO as the anode, PEDOT:PSS [Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)] (Heraeus, Clevios PH 1000) as the hole injection layer, poly-TPD [Poly(4-butylphenyl-diphenyl-amine)] (ADS254BE, M.W83000)

Figure 1. (a) A schematic of QD-LEDs device and (b) suggested energy diagram for QD-LEDs.

as the hole transport layer, CdSe/ZnS QDs [NSQ, Korea] as the

(a)

(b)

P-70 / Y. Kim

1322 • SID 2012 DIGEST ISSN 0097-966X/12/4303-1322-$1.00 © 2012 SID

emission layer, TPBi[1,3,5-tris(N-phenylbenzimidazol-2,yl)benzene] (Withlchem, Korea) as the electron transport layer and LiF/Al metallic cathode. Our structure was designed to achieve efficient hole and electron valance in hetero interface of organic and inorganic layer and avoid color contamination caused by luminescence of organic layers, which were studied prior experiments [10, 11]. The emitting area is 6 mm × 4 mm, which is defined by the crossing overlap of patterned anode and cathode.

The fabrication of QD-LEDs begins with cleaning patterned ITO substrate by sonication sequentially in acetone, methanol and SC1. After that, clean ITO substrates were dried on hot-plate at

120℃ and exposed to Oxygen plasma for for 5 min (150 W) to remove the surface hydrocarbon contamination and increase the work function of ITO contact with employed PEDOT:PSS HIL, which also changes the ITO surface to be hydrophilic. The

PEDOT:PSS solution filtered with 0.8 μm cellulose acetate filter to remove residual gel or dried particulates was spin-coated on the ITO substrate at a spin rate of 3500 rpm for 20 sec and dried in vacuum condition. Poly-TPD dissolved in chlorobenzene (3.5 mg/ml, 40 nm) was also deposited by spin-coating after filtering

the solution with 0.5 μm PTFE membrane filter on the PEDOT:PSS layer and followed by vacuum dry. Before deposit QD film on poly-TPD layer, the thickness difference of poly-TPD layer was investigated after spin-coating the toluene, hexane and heptane solvent in order to find orthogonal solvent during the process. Green emission (560 nm) of the CdSe/ZnS QDs washed by purification procedure (dispersing in toluene, precipitating with excess methanol, repeating 4 times) were dispersed in each nonpolar organic solvent and spin-coated on the ITO/PEDOT:PSS/poly-TPD layer with different concentration of QD solution. After finishing solution process, TPBi organic electron transport layer of desired thickness of 70 nm and the LiF/Al (0.5 nm/150 nm) cathode were deposited by thermal evaporation on top of QD layer under a base pressure of 3*10-6 torr. After deposition of cathode layer, the devices were encapsulated with an encapsulation glass using UV sealant in a nitrogen glove box.

A spectroradiometer (Minolta CS1000) was employed for measurements of the electroluminescence (EL) spectrum and various electrical characteristics were measured with an experimental set-up consisting of a Keithley 2400 source meter and calibrated with fast silicon photodiode at ambient condition. The luminance and current efficiency were calculated from the photocurrent of photodiode and compensated precisely with the luminance detected from the spectroradiometer. Device performances were measured in the atmosphere.

3. Results and Discussion We investigated the effects of orthogonal solvent of CdSe/ZnS quantum dots in order to deposit poly-TPD hole transport layer without damages and enhance the performances of QD-LEDs. We found toluene organic solvent decease the thickness of poly-TPD layer during the deposition of QD layer and if molecular weight (M.W) of poly-TPD is smaller, the damage of poly-TPD layer is getting worse. However, using hexane and heptane as organic solvent of QDs does not affect poly-TPD layer, which has found by measuring the thickness of poly-TPD layer after spin-coating the hexane and heptane.

Figure 2. (a) Images of the green emission from 6 mm×4 mm pixel of QD-LEDs based on the different concentration of QDs in hexane solvent. (b) Spin-coating vs. spin-casting method.

Figure 3. (a) Compared current efficiency as a function of current density and (b) Luminance-voltage characteristic by changing concentration of QDs in hexane. (Inset: Current-Voltage characteristic)

(a)

(b)

(a)

(b)

P-70 / Y. Kim

SID 2012 DIGEST • 1323

Figure 2 shows the EL emission from 6 mm×4 mm pixel of the QD-LEDs by based on the different concentration of QDs in hexane solvent. We can see the emitting areas are not homogeneous from 5 mg/ml to 3 mg/ml of QD solution. This non-uniformity of emitting areas was generated during spin-coating

process of QD layer. Boiling point of hexane is 69℃ and it is

lower than other organic solvent (B.P of toluene: 110℃, heptane:

98.42℃), therefore hexane solvent evaporated so fast during the spin-coating process and left the stain on QD layer. However, the uniformity of QD surface was getting better according as the concentration of QDs was reduced and disappeared under 2 mg/ml of QD solution.

In Figure 3, the performances of QD-LEDs with different concentration in hexane solvent show that the device with 3 mg/ml of QD solution has highest current efficiency (~2.5 cd/A) and luminance (6500 cd/m2), but still has non-uniform emission form active area. When we applied the spin-casting method (drop the solution while the substrate is spinning), the uniformity was modified, however the device performance was deceased. (need more investigation)

Figure 4. (a) Compared current efficiency as a function of

current density and (b) Luminance-voltage characteristic by changing the organic solvent of QDs. (Inset: Current-Voltage characteristic)

When we apply heptane as organic solvent for QDs, we didn’t see any stain from the emitting area because B.P of heptane was low

enough compare to the hexane. The device performances with different organic solvent for QDs are shown in figure 4. Here, we found the device with QDs dispersed in heptane solvent shows better performances especially comparing the luminance of the devices (~ 6000 cd/m2).

In addition, from the characteristic results of changing the concentration of QDs in heptane solvent in figure 5, we found that the concentration of QDs in heptane solvent changes the device performance within just 10 %, which could reduce the errors of results generated by different concentration of QD solution.

Figure 5. (a) Compared current efficiency as a function of current density and (b) Luminance-voltage characteristic by changing concentration of QDs in heptane. (Inset: Current-Voltage characteristic)

4. Conclusion Colloidal quantum dot is usually dispersed in nonpolar organic solvent such as toluene and chloroform. Therefore, orthogonal process during spin-coating the QD layer on poly-TPD hole transport layer is very important. Here, we found toluene could damage poly-TPD layer even though it is known as stable materials against toluene. However, the hexane and heptane didn’t damage poly-TPD layer; in particular, heptane is more suitable nonpolar organic solvent for fabrication of QD-LEDs in regards to uniformity of QD emissive layer and fabrication error generated by inaccurate concentration of QD solution.

(a)

(b)

(a)

(b)

P-70 / Y. Kim

1324 • SID 2012 DIGEST

5. Acknowledgment This research was supported by QD-LED Project of International Cooperation Program funded by the Ministry of Knowledge and Economy.

6. Reference [1] Q. Sun, Y. A. Wang, L. S. Li, D. Wang, T. Zhu, J. Xu, C.

Yang and Y. Li, Nature Photon., 1, 717 (2007).

[2] V. Wood and V. Bulović, Nano Reviews, 1, 5202 (2010).

[3] X. Peng et al., J. Am. Chem. Soc., 119, 7019 (1997)

[4] V. L. Colvin, M. C. Schlamp, A. P. Alivisatos, Nature 370, 354 (1994)

[5] V. Bulovic´, Nature, 420, 800-803 (2002)

[6] S. Coe-Sullivan1, J. S. Stecke, W.K. Woo, M. G. Bawendi and V. Bulović, Adv. Funct. Mater, 15, 1117-1124 (2005)

[7] P. O. Anileeva, K. E. Halpert, M. G. Bawendi and V. Bulović1, Nano Leteer, 9, 2532-2536 (2009)

[8] Z. Tan, J. Xu, and E. Mohney, J. Appl.Phys. 105, 034312 (2009)

[9] B. Friedel, P. E. Keivanidis and N. C. Greenham, Macromolecules, 42, 6741–6747 (2009)

[10] S. M. Kim, Y. H. Kim , J. W. Kang, C. J. Han , IWFPE, Flexible, P88, (2010)

[11] Y. H. Kim, S. M. Kim, J. W. Kang, C. J. Han , IWFPE, Lighting, PL07, (2011)

P-70 / Y. Kim

SID 2012 DIGEST • 1325