Highly Efficient All-Solution Processed Inverted Quantum Dots Based

Jan 24, 2018 - In all-solution processed inverted quantum dots based light emitting diodes (QLEDs), the solvent erosion on the quantum dot (QD) layer ...
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Highly Efficient All-Solution Processed Inverted Quantum Dots Based Light Emitting Diodes Yu Liu, Congbiao Jiang, Chen Song, Juanhong Wang, Lan Mu, Zhiwei He, Zhenji Zhong, Yangke Cun, Chaohuang Mai, Jian Wang,* Junbiao Peng, and Yong Cao Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: In all-solution processed inverted quantum dots based light emitting diodes (QLEDs), the solvent erosion on the quantum dot (QD) layer prevents devices from reaching high performance. By employing an orthogonal solvent 1,4-dioxane for the hole transport layer (HTL) poly(9-vinlycarbazole) (PVK), the external quantum efficiencies (EQE) of red QLED is increased 4fold, while the luminous efficiencies (LE) of blue QLED is enhanced by 25 times, compared to the previous devices’ record. To further improve the device efficiency and reduce the efficiency roll-off, solution processed PVK/poly [(9,9dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(p-butylphenyl))diphenylamine)] (TFB) double-layer HTL is introduced to facilitate hole injection with stepwise energy level. By reducing the hole injection barrier, the turn-on voltage of QLEDs decreases from 3.4 to 2.7 V for red, from 5.1 to 2.7 V for green, and from 5.3 to 4.1 V for blue. The peak LE reach 22.1 cd/ A, 21.4 cd/A, and 1.99 cd/A, while the maximum EQE reach 12.7%, 5.29%, and 5.99%, for red, green, and blue QLEDs, respectively. To the best of our knowledge, the red and blue QLEDs exhibit the best device performance among all the allsolution processed inverted QLEDs. In addition, the blue QLED is the champion among all the inverted QLEDs, including the devices fabricated by thermal evaporation. KEYWORDS: light emitting diodes, quantum dots, solution process, orthogonal solvent, hole transport layer large area uniformity, and low fabrication cost.23,24 To integrate QLEDs with n-type TFT, QLEDs with inverted device structure are preferred, because the cathode of inverted QLED can directly connect to the drain of TFT, thereby reducing the pixels’ driving voltage and stabilizing the devices.24−26 However, most inverted QLEDs’ HTLs are small molecules, thermally deposited in vacuum.27,28 Compared to the costly vacuum process, solution process, such as inkjet printing and blade coating, is more economical and efficient.10,29,30 Recently, all-solution processed inverted QLEDs have been developed.19,31,32 The challenge facing all-solution processed inverted QLED is the solvent erosion of the HTL on the QD emission layer. Since QD can be dispersed in common organic solvents, such as chlorobenzene (CB) and chloroform (CF), it is easily damaged by the HTL layer coated above. A couple of

S

ince its invention, quantum dots based light emitting diodes (QLEDs) have attracted great attention in both academy and industry, due to quantum dots (QDs)’ narrow emission bandwidth, wide emission spectral window in the visible region, and low-cost synthesis based on solution process.1−5 The features make QLEDs a promising candidate in the display and solid-state lighting applications.5−10 By optimizing QDs’ structure and device architecture, as well as incorporating inorganic electron transport layer (ETL) and organic hole transport layer (HTL), the QLEDs’ performance has been significantly enhanced.11−18 The state of arts external quantum efficiencies (EQE) of red, green, and blue (RGB) QLED with regular device structure are 20.5%, 15.4%, and 12.2%, respectively, and with inverted device structure are 18%, 15.6%, and 4.0%, respectively.14,16,19−22 To drive large size and high-resolution display, active-matrix driving scheme is necessary. N-type metal oxides are an attractive class of semiconductors to fabricate thin film transistor (TFT) in active-matrix back-panel, due to their high electron mobility, © 2018 American Chemical Society

Received: November 16, 2017 Accepted: January 24, 2018 Published: January 24, 2018 1564

DOI: 10.1021/acsnano.7b08129 ACS Nano 2018, 12, 1564−1570

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Figure 1. (a) PL Spectra of various films. Film 1: ITO/ZnO/QDs. Film 2: ITO/ZnO/QDs/1,4-diaoxane rising. Film 3: ITO/ZnO/QDs/PVK (7 nm). Film 4: ITO/ZnO/QDs/PVK (7 nm)/p-Xy rinsing. Film 5: ITO/ZnO/QDs/PVK (15 nm). Film 6: ITO/ZnO/QDs/PVK (15 nm)/p-Xy rinsing. (b) Cross-sectional highresolution STEM image of red QLED. (c) Line scan analysis of elements distribution. (d) EDS mapping image of various elements in QLED.

approaches have been developed to address such problem. Heeyeop Chae et al. added cross-linkable agent hexamethyldisilazane (HDMS) into green QDs to prevent the solvent erosion. As the result, the inverted green QLED achieves a maximum EQE of 11.6%. Afterward, the same group inserted an interfacial polymeric surface modifier, polyethylenimine ethoxylated (PEIE), between the QD layer and HTL to defend against the solvent’s attack. The inverted green QLED reaches a peak luminous efficiency (LE) of 65.3 cd/A and a maximum EQE of 15.6%. Since PEIE and HDMS are insulator, the driving voltage increases as the thickness of PEIE and the amount of HDMS added into QDs increase. Currently, the best EQEs of allsolution processed inverted QLEDs are respectively 2.72% and 15.6%.19,31 For blue QLED, no EQE is reported. The best LE is merely 0.06 cd/A.32 In our contribution, the polar solvent 1,4-dioxane as an orthogonal solvent is employed as the solvent of HTL to prevent solvent erosion. Photoluminescence (PL) spectrum and crosssectional scanning transmission electron microscope (STEM) images confirm that there is no solvent erosion and intermixing between QD layer and HTL. As a result, the peak LEs of 19.5 cd/ A, 15.5 cd/A, and 1.55 cd/A, the EQEs of 11.2%, 3.83%, and 4.69% have been achieved for all-solution processed inverted RGB QLEDs with single-layer HTL of poly(9-vinlycarbazole) (PVK), respectively. The EQE of red QLED is increased 4-folds, while the LE of blue QLED is enhanced by 25 times. To further enhance the electroluminescence (EL) performance of the devices, a double-layer HTL, PVK/poly [(9,9-dioctylfluorenyl-

2,7-diyl)-co-(4,4′-(N-(p-butylphenyl))diphenylamine)] (TFB), has been developed to provide a stepwise hole injection. In consequence, all-solution processed inverted RGB QLED exhibit a turn-on voltage of 2.7, 2.7, and 4.1 V, a peak LE of 22.1 cd/A, 21.4 cd/A, and 1.99 cd/A, a maximum EQE of 12.7%, 5.29%, and 5.99%, and a maximum luminance of 9.80 × 104 cd/m2, 8.00 × 104 cd/m2, and 4.14 × 103 cd/m2. To the best of our knowledge, the red device’s performance is the best for all-solution processed inverted red QLEDs, while the blue device’s performance is the best not only for all-solution processed, but also for thermally evaporated inverted blue QLEDs.

RESULTS AND DISCUSSION Orthogonal Solvent. Preventing solvent erosion and intermixing of multilayer device is critical to achieve high performance in all-solution processed inverted QLEDs. Utilizing orthogonal solvents is a viable approach to keep each layer intact after the subsequent layer is deposited.33,34 In QLED, the QD emission layer is only about a few tens of nanometers thick, which is easily damaged by solvent. Redispersion and washing off would happen if the solvents are not orthogonal. The popular solvents of PVK are CB and CF, which would dissolve the QD layer underneath as well. To avoid the solvent erosion, the orthogonal solvent 1,4-dioxane is selected. To test the solvent resistance of the red QD layer, 40 μL 1,4-dioxane was spin-coated (35 s at 3000 rpm) onto the 27 nm thick red QD thin film. The degree of solvent erosion was evaluated by intensity variation of PL spectrum.31 With excitation wavelength of 520 nm, the PL 1565

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Figure 2. (a) Schematic illustration of the device structure of all-solution processed inverted RGB QLEDs. (b) J−V−L characteristics. (c) The dependence of LE on the current density. (d) The dependence of EQE on the current density. Red QLED: red circle. Green QLED: green diamond. Blue QLED: blue triangle.

Table 1. Device Performance of All-Solution Processed Inverted QLEDs voltage (V) Red Green Blue

devices

VT

at 150 mA/cm

Single-layer HTL Double-layer HTL Single-layer HTL Double-layer HTL Single-layer HTL Double-layer HTL

3.3 2.7 5.1 2.7 5.3 4.1

15.1 6.9 15.4 6.1 16.8 8.6

luminous efficiency (cd/A) 2

intensities of the red QD film before and after the solvent washing are identical, as shown in Figure 1a, confirming that the red QD film has perfect resistance to the polar solvent 1,4dioxane. Moreover, the PL spectrum of the red QD film after 7 and 15 nm PVK coated on top was measured. The PL intensity is as same as that of the red QD film before PVK deposition (Figure 1a), showing that PVK HTL with 1,4-dioxane solvent does not have any negative effect on the red QD emission layer. In the following sections, a double-layer HTL, PVK/TFB, is introduced. Since QDs could be dispersed in p-xylene (p-Xy) which is the solvent of TFB, the solvent proof capability of PVK layer is examined by spin-coating 40 μL p-Xy (35 s at 3000 rpm) on top of PVK layer. PVK layers with two different thickness, 7 and 15 nm, are examined. As shown in Figure 1a, the identical PL spectra before and after p-Xy rinsing verify that PVK layer prevents p-Xy from permeating through, thereby protecting the

max

at 150 mA/cm

19.6 22.1 15.5 21.4 1.55 1.99

13.2 21.9 11.6 20.6 0.90 1.62

EQE (%) 2

luminance (cd/m2)

max

max

11.2 12.7 3.83 5.29 4.69 5.99

2.27 × 104 9.80 × 104 1.99 × 104 8.00 × 104 1.28 × 103 4.14 × 103

underneath QD layer from the solvent’s attack. On top of the HTL, hole injection layer (HIL) phosphomolybdic acid hydrate (PMAH) which has been widely used as solution processed HIL in OLED, QLED, and organic solar cells,19,35 was spin-coated. PMAH is dissolved in IPA in which the QDs, PVK, and TFB cannot be dissolved.36,37 Hence, no solvent’s erosion takes place during HTL deposition. To further verify that the layers in the all-solution processed inverted QLED device do not intermix with each other, a crosssectional high-resolution STEM image, along with line scan analysis of elements distribution, and energy dispersive spectrum (EDS) compositional mapping image, were taken on the device with the structure of ITO/ZnO/QDs/PVK/TFB/PMAH/Al, shown in Figure 1b,c,d. As shown in Figure 1b, each functional layer has clear and clean boundaries, confirming that there’s no intermixing between adjacent layers. Figure 1c,d exhibit elements 1566

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and Table S1, S2, S3 in Supporting Information (SI), the best device performance is achieved by 15 nm PVK combined with 27 nm TFB for red QLED, 7 nm PVK combined with 27 nm TFB for green QLED, and 20 nm PVK combined with 17 nm TFB for blue QLED, as shown in Table S1, S2, and S3, respectively. The J−V−L characteristics, and the dependence of LE and EQE on the current density of the double-layer HTL devices are shown in Figure 4. With PVK/TFB double-layer HTL, the VT of RGB QLEDs are 2.7, 2.7, and 4.1 V, respectively, a significant reduction compared to single-layer HTL devices, which are 3.3, 5.1, and 5.3 V, respectively. In addition, the driving voltage of the devices with double-layer HTLs is reduced substantially at high current density. At the current density of 150 mA/cm2, the driving voltages are 6.9, 6.1, and 8.6 V, respectively for RGB double-layer HTL devices, while the driving voltages are 15.1 V, 15.4, and 16.8 V for single-layer HTL devices. The reduction of the driving voltage is a strong evidence that the stepwise energy level of the double-layer HTL reduces the hole injection barrier. The charge transport of the devices is analyzed by fitting the J− V characteristics with power law J ∝ Vm. As shown in Figure S4, the charge transport in both single-layer HTL and double-layer HTL devices exhibits two distinct regions. Under low operation voltage, the charge transport is close to ohmic with the exponent around 1.3. At high operation voltage, the charge transport is trap-limited conduction with the exponent larger than 2.8 In traplimited conduction region, the exponent of double-layer HTL device is larger than that of single-layer HTL device, which is attributed to the improved charge transport in device with double-layer HTL.8 Due to the balanced carriers, the device performance is enhanced substantially as listed in Table 1. The peak brightness of all the QLEDs increases 3 to 4 times with the double-layer HTL. As illustrated in Figure 4b,c, the maximum LEs are further improved to 22.1 cd/A, 21.4 cd/A, and 1.99 cd/A, while the maximum EQEs are improved to 12.7%, 5.29%, and 5.99%, respectively for RGB QLEDs with double-layer HTL. At current density of 150 mA/cm2, the LEs of RGB QLEDs are 21.9 cd/A, 20.6 cd/A, and 1.62 cd/A. The efficiency roll-off of red, and blue QLED are only 0.9%, and 18.6%, compared to 32.3% and 41.9% for QLEDs with single-layer HTL. At 150 mA/cm2, the green QLED’s LE has yet to reach its peak value. Significant reduction on the efficiency roll-off is another proof that charge carriers are more balanced in the double-layer HTL devices than in the single-layer HTL devices. To compare the inverted devices’ performance with the regular devices’ performance based on the same set of QDs, regular structured QLED devices were fabricated by reverting the device structure of the inverted device. By optimizing the thickness of each functional layer, the best device performance is achieved by the device structure of ITO/PMAH (7 nm)/TFB (27 nm)/PVK (7 nm)/QD (27 nm)/ZnO (50 nm)/Al (120 nm) for all red, green, and blue QDs. The dependence of luminous efficiency on current density is illustrated in Figure S5, and the peak LEs are summarized in Table S4. For green QLEDs, the regular device’s performance is on par with the inverted device, though at high current density, the inverted device maintains its high efficiency. For both red and blue QLEDs, the inverted device’s performance is significantly higher than the regular device, suggesting that the inverted device structure has better charge balance than the regular device structure. Owning to balanced charge carriers and low operation voltage, the double-layer HTL device has a much longer lifetime than the single-layer HTL device, even driven at higher current density, as

distribution of each layer, which further confirms the layers do not intermix with each other, and each layer has good thickness uniformity. It is well-known that clear interface and uniform thin films help devices to achieve high performance.19 Single-Layer HTL. All-solution processed inverted QLEDs with the device structure of ITO/ZnO/QDs/PVK/PMAH/Al were fabricated. All layers except of Al anode were deposited by spin-coating. The current density (J)−voltage (V)−luminance (L) characteristics, and the dependence of LE and EQE on the current density are illustrated in Figure 2. The EL performance is summarized in Table 1. With PVK single-layer HTL, the turn-on voltage VT (defined as the voltage at the luminance of 1 cd/m2) are 3.3, 5.1, and 5.3 V, and the peak brightness are 2.27 × 104 cd/ m2, 1.99 × 104 cd/m2, and 1.28 × 103 cd/m2 for RGB QLEDs, respectively. In addition, the RGB QLEDs exhibit peak LEs of 19.6 cd/A, 15.5 cd/A, and 1.55 cd/A, and maximum EQEs of 11.2%, 3.83%, and 4.69%, respectively. The red and blue QLEDs show the best EL performance of all-solution processed inverted QLEDs. As shown in Figure 2c, at high current density of 150 mA/cm2, the LEs drop to 13.2 cd/A, 11.6 cd/A, and 0.90 cd/A, respectively for RGB QLEDs. The efficiency roll-off are respectively 32.3%, 25.2%, and 41.9%. Carriers imbalance plays a critical role in efficiency roll-off.38 Generally in QLEDs, the hole density is less than the electron density due to the large hole injection barrier, resulting from deep valence band of QDs.18 Since QLEDs are electron-dominant devices, to suppress efficiency roll-off, either the excessive electrons have to be reduced by inserting a thin insulating layer between QD emission layer and ETL,16,39 or the injected holes have to be enhanced by doping engineering or employing double-layer HTL with stepwise energy level.40,41 In the following section, doublelayer HTL is introduced to further improve the device efficiency and suppress the efficiency roll-off. Double-Layer HTL. To reduce the hole injection barrier, thereby achieving carrier balance, double-layer HTL is developed. By inserting an additional HTL TFB between PVK and HIL PMAH, a stepwise hole-injection is realized. As energy level diagram shown in Figure 3, the highest occupied molecular orbital (HOMO) energy level of PVK is 5.9 eV, while PMAH’s lowest unoccupied molecular orbital (LUMO) level is 5.4 eV. The HOMO energy level of TFB is 5.5 eV, right between PVK’s and PMAH’s. To achieve best device performance, the thickness of PVK and TFB has been optimized. As shown Figure S1, S2, S3,

Figure 3. Schematic illustration of the energy levels of all-solution processed inverted double-layer HTL QLEDs. 1567

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Figure 4. (a) J−V−L characteristics. (b) The dependence of LE on the current density. (c) The dependence of EQE on the current density. (d) EL spectra of RGB QLEDs recorded at 100 mA/cm2. Red QLED: red circle. Green QLED: green diamond. Blue QLED: blue triangle.

shown in Figure S6. To the best of our knowledge, the red QLED’s efficiency is the champion among the all-solution processed inverted QLEDs, while the blue QLED’s efficiency is the champion among all the inverted blue QLEDs. For green QLED, our device’s performance has not surpassed the previous record. It is probably due to the difference in QD’s core−shell structures. Though the CdSe/ZnS green QDs’ ligands are the same, the green QDs used in Heeyeop Chae’s group have an average size of about 13.5 nm, and a peak wavelength of 512 nm, while the green QDs used in this work have an average size of around 11.6 nm, and a peak wavelength of 528 nm.19,42 It is known that thick shells benefit high efficiency.43 The EL spectra of the QLEDs are shown in Figure 4d. The peak wavelengths are 618 nm for red, 528 nm for green, and 454 nm for blue with full width at half max (fwhm) of 28, 29, and 21 nm, respectively. Hole Dominant Device. It is proposed that the double-layer HTL provides a stepwise energy level to reduce the hole injection barrier, thereby injecting more holes into the device. As the result, the balanced carriers significantly improve the device performance. To verify the enhancement of the holes, hole dominant device with the structure of ITO/PEDOT/HTL/ PMAH/Al was fabricated.31,44 The J−V characteristics of the hole dominant devices are shown in Figure 5. It is observed that with double-layer HTL, the hole current density increases substantially. The enhancement of the hole current comes from the stepwise energy level of double-layer HTL, and high hole mobility of TFB (about 1.0 × 10−2 cm2 V−1 s−1).40,45 Balanced

Figure 5. J−V characteristics of hole dominant devices with the structure of ITO/PEDOT/HTL/PMAH/Al. Singlelayer HTL device: brown triangle. Double-HTL device: red circle.

charge carriers not only improve the efficiency, but also stabilize the efficiency, as discussed in prior section.

CONCLUSION In summary, record performance of all-solution processed inverted RGB QLEDs have been realized by employing an orthogonal solvent for HTL, and introducing a double-layer PVK/TFB HTL. The orthogonal solvent 1,4-dioxane prevents 1568

DOI: 10.1021/acsnano.7b08129 ACS Nano 2018, 12, 1564−1570

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AUTHOR INFORMATION

HTL from attacking the QD layer, guaranteeing the interface between adjacent layers clean and intact. The double-layer PVK/ TFB HTL provides a stepwise energy level to facilitate the hole injection, achieving balanced carriers. As the result, the red and blue QLEDs exhibit a peak LE of 22.1 cd/A and 1.99 cd/A, and a maximum EQE of 12.7% and 5.99%, respectively. The red QLED’s performance is the highest among all-solution processed inverted QLEDs, while the blue QLED’s performance is the highest among all inverted QLEDs including thermally evaporated devices.

Corresponding Author

*E-mail: [email protected]. ORCID

Jian Wang: 0000-0001-5629-6793 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are deeply grateful to the National Key Basic Research and Development Program of China (973 program, Grant No. 2015CB655004) founded by MOST, National Key Research and Development Program of China (2016YFB0401400), National Nature Science Foundation of China (51573056, 51373057), and Science and Technology Project of Guangdong Province (2015B090915001) for their financial supports.

EXPERIMENTAL SECTION Materials. Zinc oxide (ZnO) nanoparticles were purchased from Guangdong Poly OptoElectronics Co. Ltd. Red and green QD solution with CdSe/ZnS core/shell structure were purchased from Mesolight Inc. Blue-QD solution was purchased from Guangdong Poly OptoElectronics Co. Ltd. PVK and PMAH were purchased from Sigma-Aldrich. TFB was purchased from American Dye Source, Inc. PEDOT:PSS Clevios P VP AI 4083 was purchased from Heraeus Electronic Materials Division. All solvents with purity of 99.9% were purchased from Sigma-Aldrich. Device Fabrication. ITO-coated glass (purchased from China Southern Glass Holding Corp.) with a sheet resistance of 15−20 Ω per square is used as the substrate. Prior to device fabrication, the ITOsubstrates were thoroughly cleaned in sequence in ultrasonic bath of acetone, isopropanol, detergent, deionized water, isopropanol, and dried in a vacuum baking oven, followed by 10 min oxygen plasma. ZnO dissolved in ethanol with a concentration of 30 mg/mL was deposited on the substrate with a thickness of 50 nm, followed by baking at 100 °C for 11 min in a nitrogen filled glovebox. QDs dissolved in octane with a concentration of 20 mg/mL was spin-casted on ZnO layer with a thickness of 27, 25, and 27 nm, for RGB emission layer, respectively. The QD layer was annealed at 100 °C for 11 min in the nitrogen filled glovebox. The HTL was deposited on top of the QD layer by spin coating. For single-layer HTL, PVK dissolved in 1,4-dioxane with a concentration of 8 mg/mL was spin-coated with a thickness of 35 nm, followed by annealing at 100 °C for 11 min in the nitrogen filled glovebox. For double-layer HTLs, the first HTL layer PVK was deposited with the same spin-coating process as for single-layer HTL. The PVK thickness are respectively 7, 15, and 20 nm for RGB devices. The second HTL layer TFB dissolved in p-Xy with a concentration of 8 mg/mL was spin-coated on PVK layer with a thickness of 27, 27, and 21 nm, for RGB devices, followed by annealing at 100 °C for 11 min in the nitrogen filled glovebox. On top of HTL, PMAH dissolved in isopropanol with a concentration of 5 mg/mL was spin-coated with a thickness of 8 nm, followed by annealing at 100 °C for 11 min in the nitrogen filled glovebox. To complete the device, a 120 nm thick layer of Al was thermally evaporated in a vacuum chamber at 7 × 10−5 Pa. Device Characterization. The current density−voltage−luminance characteristics were measured by a Keithley 2400 source meter and a Konica Minolta Chroma Meter CS-200. The electroluminescence spectra were taken by a USB 2000+ (Ocean Optics) spectrometer. The cross-sectional scanning transmission electron microscope images were measured by FEI Titan Themis 200. The photoluminescence spectra were taken by FL3C-111 (HORIBA Instruments Inc.). The thickness measurement was carried out by Dektak XT stylus profiler.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b08129. Optimization on the thickness of hole transporting layers; Charge transport characteristics; Comparison of device performance between regular and inverted device; Devices’ operation stability (PDF) 1569

DOI: 10.1021/acsnano.7b08129 ACS Nano 2018, 12, 1564−1570

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DOI: 10.1021/acsnano.7b08129 ACS Nano 2018, 12, 1564−1570