Bright and Efficient Full-Color Colloidal Quantum Dot Light-Emitting

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Bright and Efficient Full-Color Colloidal Quantum Dot Light-Emitting Diodes Using an Inverted Device Structure Jeonghun Kwak,†,∇ Wan Ki Bae,‡,∇ Donggu Lee,∥ Insun Park,⊥ Jaehoon Lim,# Myeongjin Park,∥ Hyunduck Cho,∥ Heeje Woo,# Do Y. Yoon,⊥ Kookheon Char,*,# Seonghoon Lee,*,⊥ and Changhee Lee*,∥ †

Department of Electronic Engineering, Dong-A University, Busan 604-714, Korea Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ∥ School of Electrical and Computer Engineering, Inter-University Semiconductor Research Center, ⊥Department of Chemistry, and # School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea ‡

S Supporting Information *

ABSTRACT: We report highly bright and efficient inverted structure quantum dot (QD) based light-emitting diodes (QLEDs) by using solution-processed ZnO nanoparticles as the electron injection/transport layer and by optimizing energy levels with the organic hole transport layer. We have successfully demonstrated highly bright red, green, and blue QLEDs showing maximum luminances up to 23 040, 218 800, and 2250 cd/m2, and external quantum efficiencies of 7.3, 5.8, and 1.7%, respectively. It is also noticeable that they showed turn-on voltages as low as the bandgap energy of each QD and long operational lifetime, mainly attributed to the direct exciton recombination within QDs through the inverted device structure. These results signify a remarkable progress in QLEDs and offer a practicable platform for the realization of QD-based full-color displays and lightings. KEYWORDS: Quantum dots, quantum dot light-emitting diodes, inverted device structure, electroluminescence, zinc oxide nanoparticles ∼ −3 eV, respectively), there is a large potential energy barrier (more than 1 eV) for the injection of holes from the adjacent organic layer into QDs, while electrons can easily migrate into QDs. Another issue is the difficulty in the use of organic hole transport layer (HTL) materials in the conventional QLED structure constrained by the physical damage of HTL during the solution-based QD deposition processing. Several attempts have been introduced to resolve the hole injection problem in a conventional QLED structure with systematic engineering of the HTL based on conjugated polymers and organic molecules or developing the QD deposition method, for instance, phasesegregation method2 or using a robust HTL such as crosslinkable polymer or inorganic HTL.3,7−11 The previous approaches improve the hole injection into QDs, but they result in undesired drawbacks. Phase-segregation method is one of the easiest ways to form a organic/QD bilayer at a single process, but it sometimes makes an unintended direction of phase-separation (e.g., QD/organic/QD).7 The use of robust and stable HTL requires either additional advances in chemistry for the synthesis of cross-linkable polymers or complicated and expensive processing steps (i.e., sputtering or chemical vapor deposition) for the inorganic film deposi-

C

olloidal quantum dots (QDs) have very unique and attractive characteristics for optoelectronic devices, such as wide absorption and narrow emission spectral bandwidth. In addition, the emission wavelength can be easily controlled by changing the size of the QD during the synthesis, so it is considered as one of the most practical candidates for QDbased light-emitting diodes (QLEDs) and solid-state lightings.1−6 Ever since the first QLEDs were demonstrated,1 the device performances have rapidly improved in terms of the brightness (>10 000 cd/m2), the external quantum efficiency (EQE > 2%), and device stability, as a result of multilateral efforts based on the fundamental understanding in the device physics as well as the advanced engineering in materials and device architectures.2−12 Despite these recent progresses, there are still remaining issues in QLEDs for the applications toward displays or solid-state lightings: high turn-on voltages, low device efficiency in the practicable brightness region, and nonnegligible parasitic electroluminescence (EL) emission from the adjacent conjugated organic layers or surface-trap states of QDs, mainly due to inefficient carrier injection into the QDs and resultant poor electron−hole balance. Since QDs intrinsically possess lower valence band (VB) (∼ −6 to −7 eV) and conduction band (CB) (∼ −4 eV) energy levels as compared with the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of typical conjugated organic molecules or polymers (∼ −5.5 and © 2012 American Chemical Society

Received: January 26, 2012 Revised: March 28, 2012 Published: April 2, 2012 2362

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tion,8−10 which, in turn, deteriorates the advantages of QLEDs in terms of the cost-efficiency and the easy processability. Herein, we introduce easily processable, high-performance, red, green, and blue (RGB) QLEDs with the inverted device structure by adopting the solution-processed ZnO nanoparticle film as an electron injection layer (EIL) and an electron transporting layer (ETL). The ZnO nanoparticle film in the inverted device architecture presents three important advantages: (1) It acts as a common EIL/ETL for RGB QD emitters since it has efficient electron injection and transport property; (2) it provides a robust platform for the consecutive QD deposition; and (3) it enables the systematic engineering of HTLs with conventional organic materials with proven performances. Thus, independent optimization of charge injection, transport, and light emission for efficient QLEDs becomes possible. We used various types of colloidal QDs, CdSe/CdS/ZnS, CdSe@ZnS, and Cd1−xZnxS@ZnS as the red, green, and blue emitters, respectively, possessing high photoluminescence (PL) quantum yields (70−80%). These QDs are synthesized and prepared in the method of our previous reports,14,15 and the detailed information of QDs is available in Supporting Information. By using the common device structure and process, we can fabricate color-saturated red (EL λmax. = 637 nm), green (EL λmax. = 520 nm), and blue (EL λmax. = 437 nm) QLEDs with high efficiencies (7.3, 5.8, and 1.7% in EQE) and luminances (23 040, 218 800, and 2250 cd/m2) and low turn-on voltages (1.8, 2.4, and 3.0 V, respectively), which directly correspond to the optical bandgap of each QDs and enhanced operational stability. The significant improvement in the injection of electrons and holes into QDs enables the balanced exciton formation and efficient exciton recombination within QD active layers in the inverted QLEDs and leads to high efficiency and brightness compared to previous reports with normal device structure;2−10,16 in particular, the green QLED shows an unprecedentedly high brightness over 200 000 cd/m2, which is the highest value ever and comparable to the brightness of the best phosphorescent organic lightemitting diodes. A schematic diagram and cross-sectional transmission electron microscopy (TEM) image of the inverted QLED with a multilayered structure of the patterned ITO (cathode), ZnO nanoparticle film as the EIL/ETL (50 nm), QDs as the emitting layer (∼2 monolayers), organic conjugated molecules as the HTL (40 nm), MoO3 as the HIL (10 nm), and Al (anode, 100 nm), and its energy band diagram are shown in Figure 1. The band diagram under biased condition is also displayed in Supporting Information for the better understanding. In contrast to the conventional QLED structure, electrons and holes are injected from ITO and Al, and transported to QDs through the ZnO nanoparticle film (EIL/ ETL) and organic conjugated molecules (HTL), respectively. ZnO nanoparticles were synthesized by modifying the previously reported method,17 and their sizes were 3−5 nm (see Supporting Information). Spin-coated ZnO film was smooth and uniform (root-mean-square roughness below 5 nm, see Figure S2, Supporting Information) and adopted as the EIL/ETL due to their transparency in the visible spectral region, good electron mobility (μe ∼ 1.3 × 10−3 cm2/(V s), measured by using ZnO thin film transistor, see Figure S2, Supporting Information) compared to typical organic electron transport materials, the solution processability, and the film robustness to the subsequent solution process.18 Due to the low CB and the VB edge energy levels (−4.02 and −7.47 eV,

Figure 1. (a) A schematic representation (left) and cross-sectional TEM image (right) of the inverted QLED and (b) its energy band diagram in the unbiased condition. Contrary to a conventional device structure, the electrons are injected from ITO, while holes are injected from Al in this inverted structure. In this study, four HTL materials possessing different HOMO energy levels in the range of 5.1−6.0 eV are used for investigating the dependence of device performances on the hole injection ability into the QD emissive layer (see Table S1, Supporting Information).

respectively) of the ZnO films, the electron injection and transport from ITO into QDs are facilitated while the hole leakage current from the HTL to the ETL through the QD layer can be efficiently suppressed. Moreover, ZnO nanoparticles hardly quenches excitons formed in adjacent QDs, while the devices using other metal-oxides formed by sputter or thermal annealing of precursor materials quenches excitons severely by nonradiative energy transfer to metal-oxide layer.19−21 As mentioned above, since the ZnO layer can inject and transport electrons efficiently, the device performances of inverted QLEDs are mainly affected by the HTL. In order to examine the dependence of device performances on the hole injection ability, we characterized QLEDs by changing the HTL materials possessing different HOMO energy levels: N,N′diphenyl-N,N′-bis-[4-(phenyl-m-tolyl-amino)-phenyl]-biphenyl-4,4′-diamine (DNTPD, −5.1 eV), N,N′-bis(naphthalen-1-yl)N,N′-bis(phenyl)-benzidine (α-NPD, −5.4 eV), 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA, −5.7 eV), and 4,4′bis(carbazole-9-yl)biphenyl (CBP, −6.0 eV). Figure 2a shows the maximum EQEs RGB QLEDs as a function of HOMO energy level of HTLs. As can be seen, for all colors, QLEDs employing HTL with lower HOMO energy levels show higher EQEs: QLEDs employing CBP as HTL outperformed other devices, while the devices with DNTPD exhibited low efficiencies, meaning that the HOMO energy levels of hole transport materials should be low enough to inject holes into corresponding VB edges of QDs. The current density−voltage and luminance−voltage characteristics of blue QLEDs according to various HTLs are plotted in Figure 2b,c, which clearly shows their differences in driving voltages, brightness, and turnon voltages due to the large energy barrier between QDs and each HTL. This consistent trend in device performance against 2363

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Figure 2. (a) Maximum EQE of RGB QLEDs using various HTLs having different HOMO energy levels. Higher EQEs were obtained by using HTLs with lower HOMO energy levels. (b) Current−voltage and (c) luminance−voltage characteristics of blue QLEDs with various HTLs.

Figure 3. Characteristics of inverted QLEDs using CBP as the HTL in terms of (a) luminance−voltage and (b) EQE−current density. (c) Normalized PL spectra of QDs (dashed line) and EL spectra of QLEDs (solid line) with pure emission from QD layers. (d) The CIE color coordinates for RGB QLEDs are (0.70, 0.29), (0.14, 0.74), and (0.17, 0.02), respectively, which can express 110% and 167% of the original NTSC 1953 and 1987 color gamut (dashed and dotted lines), respectively. (e) Photographs of RGB QLEDs (the emitting area of 1.2 × 1.2 cm2) at applied voltages of 2.6−3.3 V presented in inset.

Table 1. Summary of the Optical and Electrical Properties of QDs and QLEDs

a

color

PL λmax (nm)

EL λmax (nm)

Von (V)a

max. EQE (%)

max. LE (cd/A)

max. luminance (cd/m2)

CIE index (x, y)

red green blue

628 515 433

637 520 437

1.8 2.4 3.0

7.3 5.8 1.7

5.7 19.2 0.4

23 040 218 800 2250

(0.70, 0.29) (0.14, 0.74) (0.17, 0.02)

Von is the applied voltage when the luminance is detected with photomultiplier tube.

19.2, and 0.4 cd/A in luminous efficiencies, respectively. To the best of our knowledge, these values (i.e., the brightness and efficiency) are the highest among the recent reports on QLEDs.2−16,23−26 Furthermore, there was no parasitic emission from the neighboring organic layers or surface states of QDs over almost the entire range of driving voltages (see Figure S3, Supporting Information). We also found that QDs can be patterned on ZnO with a simple and conventional transfer printing technique (Figure S4, Supporting Information), which implicates that the inverted QLEDs demonstrated in this paper can be also applicable to full-color displays. The Commission Internationale de l’Eclairage (CIE) color coordinates of each color were (0.70, 0.29), (0.14, 0.74), and (0.17, 0.02), so that 110% of National Television System Committee (NTSC) 1953

the HOMO energy level of HTL clearly shows that HTLs with lower HOMO energy facilitate the hole injection into QDs and improve the charge carrier balance within the QD active layers. The devices with CBP showed the best performances in all colors primarily owing to its low HOMO energy level, and we believe that high hole mobility of CBP (μh ∼ 1 × 10−3 cm2/(V s))22 also contributed to the superior device performance. Figure 3 illustrates the performances of QLEDs using CBP in terms of luminance−voltage, EQE, and PL and EL spectra, and these performances are summarized in Table 1. The luminances reached 23 040, 218 800, and 2250 cd/m2 at the maximum in red-, green-, and blue-emitting devices, respectively, built in the same inverted device architecture. In addition, the EQEs of the RGB devices were 7.3, 5.8, and 1.7%, corresponding to 5.7, 2364

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channel thin-film transistor (TFT) backplanes, such as amorphous silicon (α-Si) or metal-oxide based TFTs, for manufacturing active matrix displays.29−31 In contrast to the conventional device structure with the bottom anode, the transparent bottom cathode of the inverted QLEDs can be directly connected to the drain line of n-type TFTs, which allows the direct programming of the TFT gate-source voltage independent of the electrical characteristics of QLEDs. For that reason, we believe that this inverted QLED structure exhibiting high brightness, high efficiency, low turn-on voltage, and enhanced operational stability offers a practicable platform for the realization of active matrix-driven QD displays using the current TFT technology. In conclusion, we demonstrated highly bright and efficient, RGB-emitting inverted QLEDs in the same device structure by introducing solution-processable ZnO nanoparticles as the electron injection and transport layer. The luminance of the devices presented here is significantly higher than that of previously reported QLEDs and is comparable to the brightness of the phosphorescent organic light-emitting diodes. Low turnon voltages of QLEDs close to the bandgap of each QD are also achieved owing to the exciton recombination mainly within the QD layers by engineering carrier transport layers. We believe that the device structure and process presented in this paper can be applicable to displays and lighting devices.

(or 167% of NTSC 1987) color gamut can be achieved without additional optical outcoupling techniques (Figure 3d). It is also noteworthy that inverted QLEDs exhibited very low turn-on voltages of 1.8, 2.4, and 3.0 V for red, green, and blue devices, corresponding to the bandgap of each QD. We consider that these significantly improved performances (i.e., low turn-on voltage similar with the optical bandgap of QDs, no parasitic emission from adjacent organic layers over wide range of driving voltage, and high EQE) result from the direct and efficient carrier recombination within QD layers owing to the facilitated charge carrier injection into QDs from ZnO or CBP (i.e., HOMO = −6.0 eV) as well as the enhanced charge carrier balance within the devices. Photographs of RGB QLEDs (the pixel size of 1.2 × 1.2 cm2) in Figure 3e show that this process and device structure can be applicable to large-sized applications. Despite the significant advances in material designs and device fabrication, the stability of QLEDs has not improved from the early research stage (i.e., a lifetime of tens of hours at most).6,27,28 The poor operational stability of standard QLEDs employing charge transport/injection materials with proven stability is considered as a result of charge carrier imbalance within the devices, particularly due to the insufficient hole injection into QDs. As shown in Figure 4, the QLEDs with

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic method and characteristics of QDs (UV−vis and PL spectra and TEM images) and ZnO nanoparticles (UV−vis spectra, TEM/SEM/AFM images, TFT characteristics), properties of the hole transporting materials, the device fabrication and characterization methods, normalized EL spectra at various current densities, QD patterns on ZnO layer, and the energy band diagram under biased condition. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 4. The operational lifetime characteristics of red-emitting QLEDs with standard and inverted structure. The luminance of QLEDs was measured during the continuous operation at a constant current density corresponding to an initial luminance of 500 cd/m2. Both types of QLEDs were fabricated with same QDs and characterized under the same conditions (i.e., encapsulated in an inert condition, same initial brightness, and same humidity and temperature).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]; [email protected]. kr Author Contributions ∇

standard device structure (i.e., ITO/poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (40 nm)/poly-(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine) (poly-TPD) (50 nm)/QDs/1,3,5-tris(Nphenylbenzimidazol-2,yl)benzene (TPBI) (40 nm)/LiF (0.5 nm)/Al (100 nm))23 showed rapid deterioration within couple of hours of continuous operation at a constant current density corresponding to an initial luminance of 500 cd/m2. By contrast, QLEDs with inverted structure displays a half-lifetime of ca. 600 h, which is comparable to or better than recent reports by other groups.6,16 The significant improvement in the device stability, in agreement with low turn-on voltage and high EQE, is a consequence of facilitated hole injection from systematically engineered hole injection layer with lower HOMO level into QDs, which enhances the charge carrier balance within the devices during operation. In addition to these high performances, inverted structure has great advantages of the easy fabrication based on solution process with ZnO nanoparticles and the use of low cost n-

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) through the Acceleration Research Program (R0A-2008-000-20108-0), the grant (NRF-2009-C1AAA0012009-0093282), the National Creative Research Initiative Center for Intelligent Hybrids (No. 2010-0018290), the Basic Science Research Program (2011-0022716), and the Leading Foreign Research Institute Recruitment Program (20110030065) funded by Korean government (MEST).

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