Research Article www.acsami.org
Inkjet-Printed Quantum Dot Light-Emitting Diodes with an Air-Stable Hole Transport Material Zhenhua Xing,†,‡ Jinyong Zhuang,*,† Changting Wei,† Dongyu Zhang,† Zhongzhi Xie,§ Xiaoping Xu,*,‡ Shunjun Ji,‡ Jianxin Tang,§ Wenming Su,*,† and Zheng Cui† †
Printable Electronics Research Center, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, People’s Republic of China ‡ Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, and § Institute of Functional Nano & Soft Materials, Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, 199 Ren-ai Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, People’s Republic of China S Supporting Information *
ABSTRACT: High-efficiency quantum dot light-emitting diodes (QLEDs) were fabricated using inkjet printing with a novel cross-linkable hole transport material N,N′-(9,9′-spirobi[fluorene]-2,7-diylbis[4,1-phenylene])bis(N-phenyl-4′-vinyl[1,1′-biphenyl]-4-amine) (SDTF). The cross-linked SDTF film has excellent solvent resistance, high thermal stability, and the highest occupied molecular orbital (HOMO) level of −5.54 eV. The inkjet-printed SDTF film is very smooth and uniform, with roughness as low as 0.37 nm, which is comparable with that of the spin-coated film (0.28 nm). The SDTF films stayed stable without any pinhole or grain even after 2 months in air. All-solution-processed QLEDs were fabricated; the maximum external quantum efficiency of 5.54% was achieved with the inkjet-printed SDTF in air, which is comparable to that of the spin-coated SDTF in a glove box (5.33%). Electrical stabilities of both spin-coated and inkjet-printed SDTF at the device level were also investigated and both showed a similar lifetime. The study demonstrated that SDTF is very promising as a printable hole transport material for making QLEDs using inkjet printing. KEYWORDS: quantum dots, cross-linkable, hole transport material, air-stable, inkjet printing
1. INTRODUCTION Quantum dots (QDs) have unique photophysical properties, namely, size-dependent emitting colors, narrow spectral emission bandwidths, a wide color gamut, and high photoluminescence quantum yields, which have attracted great attention both in academic and in industrial communities.1−3 Because of their high color purity and efficient optical properties, QDs are very promising in display and solid-state lighting. Currently, downconversion QD devices containing green and red QD emitters are already being used as backlighting sources in high-color-quality liquid-crystal displays, which makes the displays more vivid. However, electrically driven QDs have also developed very fast since their invention.1 Recently, quantum dot light-emitting diodes (QLEDs) have achieved over 20% external quantum efficiency (EQE), and green and red QLEDs with a long lifetime have been reported,4,5 whose performance is comparable to that of the state-of-art OLEDs and the superior color gamut property.6 In addition, QDs are solution processable, which potentially leads to their low cost and large-scale manufacturing using printing. So far, QLEDs have been fabricated by a combined solution/ vacuum processing and complete solution processing.7−17 © 2017 American Chemical Society
Although the solution-processing technology is very costeffective, it can cause corrosion and intermixing of solvents at the interfaces, which degrades the device performance. Orthogonal solvent systems are commonly used in solutionprocessed OLEDs, which effectively solves the issue.18−21 However, it is difficult to find suitable orthogonal solvents for a specific pair of materials because most materials are soluble in organic solvents. Cross-linking, which starts with a soluble precursor and then turns into insoluble material, seems to be a plausible way to solve the corrosion and intermixing problems at the interfaces. Functional cross-linkable materials have been successfully used in multilayer solution-processed light-emitting diodes and solar cells.22−31 In this work, we have designed and synthesized a new styrene-based cross-linkable hole-transporting material N,N′(9,9′-spirobi[fluorene]-2,7-diylbis[4,1-phenylene])bis(N-phenyl-4′-vinyl-[1,1′-biphenyl]-4-amine) (SDTF), which consists of spiro-bifluorene and triphenylamine groups. The photoReceived: January 13, 2017 Accepted: April 18, 2017 Published: April 18, 2017 16351
DOI: 10.1021/acsami.7b00615 ACS Appl. Mater. Interfaces 2017, 9, 16351−16359
Research Article
ACS Applied Materials & Interfaces Scheme 1. Synthesis of SDTF
physical, thermal properties and molecular dynamics of SDTF were investigated. Excellent solvent resistance of the crosslinked films was achieved after thermal curing without any initiator. The highest occupied molecular orbital (HOMO) level was calculated to be −5.54 eV for the cross-linked SDTF. The inkjet-printed SDTF film is very smooth and uniform, with a roughness as low as 0.37 nm, which is comparable to that of the spin-coated film (0.28 nm). The SDTF films stayed stable without any pinhole or grain even after 2 months in air. QLEDs were fabricated by spin coating and inkjet printing; the maximum EQE of 5.33% was obtained based on the spincoated SDTF in a glove box. The inkjet-printed device in air achieved an EQE of 5.54%, which is comparable to that of the spin-coated one. The electrical stability of the devices was investigated for both the spin-coated (in a glove box) and the inkjet-printed (in air) SDTF. Their lifetime is comparable, indicating the high air stability of SDTF. This work demonstrates that the novel cross-linkable SDTF is an efficient hole transport material (HTM) for inkjet printing of QLEDs.
Figure 1. UV−vis absorption and photoluminescence spectra of SDTF before and after cross-linking in films.
turn into single bonds for polymerization after cross-linking, which will decrease the π-conjugation system of the crosslinked SDTF. Thus, the triplet energy of SDTF slightly increased after cross-linking. The thermal stability of organic semiconductor materials is very important to the device fabrication and lifetime. The thermal stability of SDTF was investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The TGA and DSC curves are shown in Figure 2. The cross-linkable HTM of SDTF showed the decomposition
2. RESULTS AND DISCUSSION The synthetic route to SDTF is demonstrated in Scheme 1. The compound is constructed with spiro-bifluorene and triphenylamine groups, which are both commonly used groups in HTMs. SDTF was obtained through a two-step Suzuki coupling reaction, shown in Scheme 1. The chemical structures of the intermediates and SDTF were confirmed by proton nuclear magnetic resonance (1H NMR), carbon 13 NMR (13C NMR), and high-resolution mass spectrometry. The detailed synthetic procedures and material characterization are described in the Experimental Section. The UV−vis absorption and photoluminescence (PL) spectra of SDTF were measured for the films both before and after cross-linking. As shown in Figure 1, there are two absorption peaks at approximately 275 and 380 nm for both of the films; the peak at 380 nm is the typical absorption of the 2,7-substituted spiro-bifluorene derivatives.32,33 The emission peaks for the films before and after cross-linking are 441 and 459 nm, respectively. A red shift was observed for the emission of the cross-linked film, and the emission band was wider in comparison with that of the uncross-linked film. The phosphorescence spectra of the films before and after crosslinking were measured at 77 K (Figure S5). On the basis of the highest-energy vibronic subband of the phosphorescence spectra, the triplet energy (ET) values of SDTF before and after cross-linking films were calculated as 2.11 and 2.12 eV, respectively. The double bonds of the monomer of SDTF will
Figure 2. TGA and DSC curves of SDTF with a heating rate of 10 °C min−1 under nitrogen. 16352
DOI: 10.1021/acsami.7b00615 ACS Appl. Mater. Interfaces 2017, 9, 16351−16359
Research Article
ACS Applied Materials & Interfaces temperature (Td) of 325 °C (corresponding to 5% weight loss), indicating its high thermal stability. As shown in Figure 2, the material exhibited a melting endothermic peak at 150 °C in the first DSC scan curve followed by a polymerization exothermic peak at 211 °C. The melting state before the cross-linking is beneficial to a high degree of cross-linking, which makes the film more compact after cross-linking.24,30 As for the second and third heating scans, no obvious peak was observed in the temperature ranging from 30 to 270 °C, which indicates the excellent thermal stability of SDTF after cross-linking. These results imply that the cross-linked film can endure the thermal annealing or drying process after the deposition of the upper functional layer, which is important for the device performance. Molecular dynamics simulation was carried out to study HOMO and the lowest unoccupied molecular orbital (LUMO) levels of the material on the molecular scale. The geometric and electronic properties of SDTF were obtained at the B3LYP/631G* level using the Gaussian 09 program package. The spatial distribution of HOMO and LUMO levels is showed in Figure 3. HOMO was mostly distributed on the triphenylamine
eV, respectively, which were calculated from the HOMO level and the Eg value in the film state. The cross-linked film has a suitable HOMO level of −5.53 eV, which is helpful for the hole injection from the anode. On the contrary, a high LUMO level of −2.63 eV will block and confine the electron in the emitting layer. The physical parameters of SDTF were summarized in Table 1. Table 1. Physical Parameters of SDTF
before crosslinking after crosslinking
a λmax abs (nm)
a λmax em (nm)
HOMOb/LOMOc (eV)
Egd (eV)
ETe (eV)
275/383
441
−5.49/−2.52
2.97
2.11
274/381
459
−5.53/−2.63
2.90
2.12
Measured in thin film by spin coating. bObtained from UPS measurements. cLUMO values were calculated by using the optical band gap and HOMO values from UPS measurements. dObtained from phosphorescence peaks in films at 77 K. eObtained from the phosphorescence peaks at 77 K. a
The solvent resistance of the functional materials is very important for the performance of the multilayer solutionprocessed optoelectronic devices. Here, organic solvents, such as chlorobenzene, toluene, 3,5-dimethylanisole, and indan, were used to test the solvent resistance of the cross-linked films on quartz by UV−vis absorption. Chlorobenzene and toluene are often used in spin-coated optoelectronic devices, whereas highboiling-point solvents of 3,5-dimethylanisole and indan are used in inkjet printing.36−40 As shown in Figure 5, each absorption spectrum of the films after rinsing with the solvents precisely overlaps with that of the pristine films, which implies the excellent solvent resistance of the films after cross-linking. Poly[N,N′-bis(4-butylphenyl)-N,N′-bisphenylbenzidine] (polyTPD) is commonly used as a hole transport layer (HTL) in solution-processed QLEDs, and the solvent resistance of polyTPD (annealed at 110 °C for 20 min4,41) was also investigated. The absorption spectra showed that more than 90% ± 3% and 88% ± 3% of the films were washed away by chlorobenzene and toluene, respectively (Figure S6). On the contrary, the poly-TPD film stayed stable with the solvent of hexane (Figure S6). These results indicate that the application of poly-TPD in multilayered solution-processed optoelectronic devices should be done very carefully as the commonly used organic solvent (chlorobenzene and toluene) can cause serious damage to the poly-TPD film (Table 1). To investigate the film-forming property of SDTF, the morphologies of films fabricated by spin coating and inkjet printing were investigated using atomic force microscopy (AFM). The AFM images of the spin-coated (Figure 6a−c) and inkjet-printed (Figure 6d−f) films are shown in Figure 6. The films are all smooth and uniform, and the root-mean-square surface roughness of the spin-coated and inkjet-printed films after rinsing with chlorobenzene are only 1.08 and 1.16 nm, respectively. The AFM data of the SDTF films rinsed with toluene, 3,5-dimethylanisole, and indan are also shown in Figure S7. The film stability of SDTF was also investigated; the films still stayed stable without any pinhole or grain even after two months in air (Figure S8). On the basis of these results, a decent device performance can be expected for the spin-coated and inkjet-printed HTL of SDTF. Wetting property is essential to the film morphology for solution-processed technology. In this work, contact angle test
Figure 3. HOMO and LUMO orbital distributions and energy levels obtained from the molecular simulation of SDTF.
groups and also the fluorene moiety in the core of the compound. As for LUMO, the distribution was located on the fluorene group. Because of the sp3-hybridized carbon atom, there are no distributions on the non-substituted fluorene moiety. Ultraviolet photoemission spectroscopy (UPS) was performed to evaluate the energy levels for both pristine and crosslinked films (Figure 4). The HOMO levels of SDTF before and
Figure 4. UPS spectra of SDTF in films before and after cross-linking.
after cross-linking were −5.49 and −5.53 eV, respectively. The double bonds on the molecule of SDTF were turned into single bonds after cross-linking (polymerized); therefore, the πconjugated system of the molecule decreased. Thus, the deeper HOMO level for SDTF after cross-linking could be attributed to the limited π-conjugation system and electron delocalization of the materials.34,35 The LUMO levels of SDTF before and after the thermal cross-linking process were −2.52 and −2.63 16353
DOI: 10.1021/acsami.7b00615 ACS Appl. Mater. Interfaces 2017, 9, 16351−16359
Research Article
ACS Applied Materials & Interfaces
Figure 5. UV−vis absorption spectra of the cured films of SDTF before and after rinsing with chlorobenzene, toluene, 3,5-dimethylanisole, and indan.
Figure 6. AFM topographic images (2 × 2 μm2) of SDTF thin films on the ITO substrates. (a−c) Spin coated and (d−f) inkjet printed. (a,d) Before cross-linking; (b,e) after cross-linking; and (c,f) cross-linked film after rinsing with chlorobenzene (inkjet-printed samples were rinsed thrice).
ITO/PEDOT:PSS (30 nm)/HTL (30 nm)/MoO3 (10 nm)/ Al. The hole transport ability of SDTF before and after crosslinking was investigated. As shown in Figure 8, the current density of SDTF before cross-linking is higher than that after cross-linking at a voltage lower than 1.5 V. Because the HOMO level of SDTF before cross-linking is a little higher than that after cross-linking, this phenomenon could be attributed to the different hole injection barrier from PEDOT:PSS to SDTF.42−44 For comparison, a poly-TPD-based HOD was also fabricated. As shown in Figure 8, the current density of the poly-TPD-based HOD is higher than that of the SDTF-based device at a voltage ranging from 0 to 5 V. At a higher voltage, the current density of SDTF and poly-TPD are almost the same. The HOMO level of SDTF is −5.53 eV, which is deeper
was performed to investigate the property of both the spincoated and the inkjet-printed films. As shown in Figure 7, the contact angles for the spin-coated and inkjet-printed films are 93.6° and 94.2°, respectively. After rinsing with chlorobenzene, the contact angle decreased from 93.6° to 80.0° for the spincoated film. As for the inkjet-printed film, the contact angle was a little different. After the first rinsing, the contact angle was 85.7°, and after rinsing twice, it was 84.2°. The contact angle dropped to 82.8° after rinsing with chlorobenzene thrice. The differences in the contact angles between the spin-coated and the inkjet-printed films could be attributed to the effects of molecular arrangements for spin coating and inkjet printing. First, the hole transport ability of SDTF was investigated. We fabricated the hole-only device (HOD) with the structure of 16354
DOI: 10.1021/acsami.7b00615 ACS Appl. Mater. Interfaces 2017, 9, 16351−16359
Research Article
ACS Applied Materials & Interfaces
Figure 7. Images of the contact angle of H2O on treated SDTF films. (a,b) Spin coated and (c−f) inkjet printed. (a) After-cross-linking; (b) crosslinked film after rinsing with chlorobenzene; (c) after cross-linking; (d) cross-linked film after rinsing with chlorobenzene; (e) cross-linked film after rinsing with chlorobenzene twice; and (f) cross-linked film after rinsing with chlorobenzene thrice.
the device without an HTL (device D). As shown in Figure 9, the maximum EQE of 2.85% was achieved for the poly-TPDbased device, which is comparable to the reported result.48 As for the device without an HTL, the device performance dramatically decreased, as shown in Figure 9. Also, only an EQE of 0.34% was obtained, which may be because of the quenching at the interface between the QD layer and PEDOT:PSS. The HTL of SDTF was also fabricated by inkjet printing. On the basis of the optimized HTL, an ink with a concentration of 10 mg mL−1 (leading to a 30 nm layer) was prepared. The ink formula consists of two solvents (indan and butyl phenolate), which is helpful in suppressing the coffee ring effect and obtaining a smooth and uniform film morphology (AFM images in Figure 6). SDTF was inkjet printed onto the PEDOT:PSS in air with a DMP-2831 printer. As shown in Figure 9, the maximum EQE of 5.54% was achieved for the inkjet-printed device (device B), which is comparable to the spin-coated one. The maximum CE and PE of 22.82 cd A−1 and 15.32 lm W−1 were achieved, respectively. The electroluminescence (EL) spectra of the devices are shown in Figure 9 (d, inset). All of the devices constructed with HTL (SDTF and poly-TPD) exhibited the same emission peak of 528 nm, except for the device without the HTL (532 nm). Also, the fullwidth half maximum of the EL spectra is approximately 30 nm. The detailed data are summarized in Table 2. Besides, the lifetimes of the electroluminescent devices fabricated by the spin coating and inkjet printing of SDTF were 0.99 and 1.06 h, respectively, which indicates a high air stability of SDTF (Figure S11). Phosphorescent OLED devices for red, orange, and blue were also fabricated using trilayered solution processing, which are shown in the Supporting Information.
Figure 8. Current density−voltage curves of HODs based on SDTF (before and after cross-linking) and poly-TPD.
than that of the poly-TPD, of −5.4 eV.45 The current density of the HOD is directly connected to the transport ability and the injection barrier. In this case, the lower current density of the SDTF-based HOD could be partly attributed to the higher injection barrier from the PEDOT:PSS (HOMO: −5.0 to 5.2 eV) layer.46,47 Further, all-solution-processed QLEDs were fabricated using the HTM of SDTF. The structure of the device consists of ITO/PEDOT:PSS (30 nm)/HTL (30 nm)/QDs/ZnO/Al, as shown in Figure 9a (device A: HTL = spin-coated SDTF; device B: HTL = inkjet-printed SDTF; device C: HTL = polyTPD; and device D: w/o HTL). All of the functional layers were solution processed for the green QLEDs. PEDOT:PSS was used as the hole injection layer (HIL) by spin coating. The HTL of SDTF was deposited both by spin-coating and inkjet printing. The green-emitting core−shell CdSe/ZnS QDs were used as the emissive layer. The ZnO layer was used as the electron transport layer by spin coating from ZnO nanoparticles (NPs) in 2-ethoxyethanol solution. The HTL of QLEDs based on SDTF was optimized. The thickness of HTL was tuned from the solution concentration. We fabricated the devices with different solution concentrations ranging from 3 to 10 mg mL−1 (Figure S9). The device based on SDTF with a concentration of 7 mg mL−1 (30 nm) achieved the maximum EQE of 5.33% (device A). Also, the maximum current efficiency (CE) and power efficiency (PE) were 22.0 cd A−1 and 15.0 lm W−1, respectively. For comparison, the device with poly-TPD (device C) as the HTL was fabricated and also
3. CONCLUSIONS We have designed and synthesized a novel cross-linkable HTM of SDTF, which is constructed with spiro-bifluorene and triphenylamine groups. The compound showed good thermal stability and a suitable HOMO level of −5.53 eV. Excellent solvent resistance was obtained after thermal curing, and both the spin-coated and the inkjet-printed films showed good morphology. The cross-linked SDTF films stayed stable without any pinhole or grain after 2 months in air. QLED devices were fabricated by spin coating and inkjet printing of SDTF. The maximum EQEs of 5.33 and 5.54% were achieved with spin-coated SDTF (in a glove box) and inkjet-printed 16355
DOI: 10.1021/acsami.7b00615 ACS Appl. Mater. Interfaces 2017, 9, 16351−16359
Research Article
ACS Applied Materials & Interfaces
Figure 9. (a) Schematic diagram of QLEDs. (b) Current density/luminance vs voltage (J−V−L). (c) Current efficiency/power efficiency vs luminance (CE−L−PE). (d) External quantum efficiency (EQE) characteristics. Inset: EL spectra.
Table 2. EL Data of QLEDs performance at 100 cd m−2
performance at 1000 cd m−2
device
Von (V)a
L (cd m−1)b
CE (cd A−1)c
PE (lm W−1)d
EQE (%)e
V (V)
CE (cd A−1)
PE (lm W−1)
EQE (%)
V (V)
CE (cd A−1)
PE (lm W−1)
EQE (%)
A B C D
2.5
