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quantum-dots hybrid white light emitting diodes
Thin Solid Films 669 (2019) 34–41
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Inverted polymer/quantum-dots hybrid white light emitting diodes
T
Zhiwei He, Congbiao Jiang, Chen Song, Juanhong Wang, Zhenji Zhong, Gancheng Xie, ⁎ Jian Wang, Junbiao Peng , Yong Cao Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, State Key Laboratory of Luminescent Materials and Devices, Guangzhou 510640, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Inverted structure device Polymer/quantum-dot hybrid White light emitting diode Solution process Interface modified
Inverted polymer/quantum-dots (QDs) hybrid white light emitting diodes with high efficiency and color stability were fabricated. The white devices show high luminous efficiency of 5.3 cd/A, external quantum efficiency of 3.0%, by using polyetherimide (PEI) to modify zinc oxide layer as the electron injection layer and blue emitting polymer of poly (dibenzothiophene-S,S-dioxide-co-9,9-dioctyl-2,7-fluorene) (PFSO) doped with green and red quantum-dots (G-QDs, R-QDs) as the emitting layer. Pure white emission at Commission Internationale de L'Eclairage coordinates of (0.31, 0.32) was achieved by adjusting weight ratio of PFSO: G-QDs: R-QDs to be 10.0: 3.6: 2.2. In addition, the electroluminescence (EL) spectra of the white device are quite stable in the wide brightness range between 1.0 × 102 cd/m2 and 1.0 × 104 cd/m2. The possible reason was that the PEI could reduce the energy barrier height between the cathode and emitting layer, and facilitate electron injection into the emitting layer. It's confirmed that both Forest energy transfer and charge trapping mechanisms work in the EL process and the latter is dominant.
1. Introduction Light emitting diodes based on colloidal quantum dots (QLEDs) have attracted tremendous attentions in both academy and industry, because of their narrow emission band, high efficiency, and long operation stability [1–6]. The electroluminescence (EL) performances of QLEDs have been significantly enhanced by improving quantum dots' structure and EL device architecture. So far, the external quantum efficiency (EQE) for red QLEDs is over 20%, and its operation lifetime reaches > 10,0000 h at initial brightness of 100 cd/m2 [7]. White QLEDs have potential for applications in lighting and backlight of thin film transistor liquid crystal displays. To obtain white light emission, an alternative method is a mixture of red, green, and blue (RGB) emissions. The white QLED based on mixed RGB quantum dots (QDs) was reported by Prof. Bulović's group [8]. By adjusting the ratio of RGB QDs, the EQE of the white QLED reached 0.36% with Commission Internationale de L'Eclairage (CIE) coordinates of (0.31, 0.45). The relatively low EL efficiency was owing to low quantum yield of the QDs films. In 2015, Yang's group reported a white QLED based on mixed RGB QDs with a peak luminous efficiency (LE) of 21.8 cd/A, and the maximum EQE of 10.9% [9]. Another approach to achieve a white QLED is to utilize polymer/QDs hybrid structure. Compared to blue QDs, blue polymer materials could provide higher
⁎
quantum efficiency and better device stability [10–13]. The polymer/QDs hybrid white QLED was reported by Lee et al. [14]. The blue polymer of poly[(9,9-dihexyloxyfluoren-2,7-diyl)-alt-co(2-methoxy-5-{2-ethylhexyloxy}phenylen-1,4-diyl)], blending with red QDs as a part of the emitter. The green emission is from metal chelate complex of 8-hydroxyquinoline aluminum, which also serves as the electron injection and transporting layer. Energy transfer and charge transfer between the blue polymer and the red QDs are principal mechanism for tuning the EL spectrum. The pure white emission at CIE coordinates of (0.30, 0.33) was obtained. However, the device's EQE is only 0.24%. To achieve high EL efficiency, green phosphorescent material of tris[2–4(4-toltyl) phenylpyridine]iridium [Ir(mppy)3] was introduced. The device consisted of the blue polymer of 2,2′,7,7′-tetrakis (2,2-diphenylvinylspiro-9,9′-bifluorene) doped with Ir(mppy)3 and red QDs as the emitter, exhibited a maximum EQE of 2.1% with peak brightness of 10,200 cd/m2 [15]. Most recently, J. S. Lee and coworkers employed the blue polymer of poly[9,9-dioctylfluorenyl-2,7-diyl]-end capped with N,N-Bis(4-methylphenyl)-aniline, doped with blue, green and red QDs as the emitter, combined with zinc oxide (ZnO) as the electron injection layer, to make a white QLED with a peak luminance of 15,950 cd/m2 and a maximum LE of 1.43 cd/A [16]. However, ZnO is an inefficient electron injection layer in this device. Because it has a deep conducting band at about −4.3 eV which doesn't match the blue
Corresponding author. E-mail address: [email protected] (J. Peng).
https://doi.org/10.1016/j.tsf.2018.10.028 Received 30 June 2018; Received in revised form 22 October 2018; Accepted 23 October 2018 Available online 23 October 2018 0040-6090/ © 2018 Published by Elsevier B.V.
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Fig. 1. Architecture and energy level diagram of the EL device.
2. Experiments
film by using spin-coating technique followed by annealing on a hot plate at 120 °C for 20 min in a nitrogen filled glove box. And then, a 15 nm thick PVK layer was formed by spin-coating from the solution of 1,4-dioxane in the concentration of 4 mg/mL and annealed at 140 °C for 10 min on a hot plate in a nitrogen filled glove box. The devices were completed by thermally evaporating 8 nm thick MoO3 in a vacuum chamber at the vacuum of 2 × 10−4 Pa followed to deposit 200 nm thick Al at vacuum of 5 × 10−5 Pa. The emission area of the devices is defined by the overlap of the anode and the cathode. The active area is about 2 × 2 mm2. The PL and EL spectra were recorded by FL3C-111 (HORIBA Instruments Inc), and a fiber optic spectrometer (Ocean Optics USB 2000), respectively. The thickness was characterized by Dektak XT surface profiler (Bruker Corp.). The current-voltage-luminous (J-V-L) characteristics and CIE coordinates were measured by a source meter (Keithley 2400) and a Konica Minolta Chroma Meter (CS-200). The interface analysis was performed by using an Ultraviolet Photoelectron Spectrometer (UPS) with an unfiltered He I (21.22 eV) gas-discharge lamp. The PL lifetimes of films were measured by a fluorescence lifetime spectrometer (Hamamatsu C11367).
2.1. Materials
3. Results and discussion
Indium tin oxide (ITO) coated glass with a sheet resistance of 15–20 Ω/□ was purchased from China Southern Glass Holding Corp. ZnO nanoparticles solution was purchased from Guangdong Poly Optoelectronics Co., Ltd. Red and green QDs solutions with CdSe/ZnS core/shell structure were purchased from Mesolight Inc.. The red and green QDs with alloyed core-shell CdSe/ZnS, capped with oleic acid ligand are purchased from Mesolight Inc. The diameter is about 11 ± 1.5 nm for red QD and 10 ± 1.3 nm for green QD, and quantum yield is about 80% for red QD and 85% for green QD, respectively. PFSO was synthesized in our group [17]. Poly (9-vinylcarbazole) (PVK), PEI, molybdenum oxide (MoO3) powders, and all chemical solvents with purity of 99.9% were purchased from Sigma-Aldrich.
3.1. EL performances of white emission devices without PEI layer
polymer's the lowest unoccupied molecular orbital at around −2.0 eV. As the result, a large electron injection barrier between ZnO and the blue polymer blocks the device to gain a high EL efficiency. In our contribution, the blue polymer of poly(dibenzothiophene-S,Sdioxide-co-9,9-dioctyl-2,7-fluorene) (PFSO) synthesized in our group was utilized to fabricate a polymer/QDs hybrid white QLED devices. The emitting layer (EML) consists of PFSO doped with red and green QDs. Polyetherimide (PEI) was introduced to reduce the electron injection barrier height between ZnO and PFSO layers. The white emission was achieved with a peak LE of 5.3 cd/A, and EQE of 3.0% at CIE coordinates of (0.32, 0.33). The device EL efficiency is the highest ever reported for polymer/QDs hybrid white QLEDs. Moreover, the color stability of the white device is significantly improved. The CIE coordinates only slightly shift from (0.30, 0.29) to (0.32, 0.34), with increasing the brightness from 1.0 × 102 to 1.0 × 104 cd/m2. The photoluminescence (PL) and EL spectra and a transient fluorescence lifetime show that both energy transfer and charge trapping mechanisms work in the EL process and the latter mechanism is dominant.
Inverted polymer/QDs hybrid white light emitting diodes with blue polymer PFSO doped with red and green QDs as a single EML were fabricated in the configuration of ITO/ZnO/EML/PVK/MoOx/Al. Blue PFSO, G-QDs and R-QDs have relatively high efficiency and stable EL spectrum [18,19]. The device architecture and energy level diagram are illustrated in Fig. 1. In the device structure, MoO3/Al is the anode, PVK serves as the hole transport layer, while ITO is the cathode, and ZnO nanoparticles layer is the electron injection layer (EIL). By adjusting the ratio of green and red QDs in the blue polymer, the emission color of the device is easily tuned. The Current density-Voltage-Luminous (J-VL) characteristics, the dependence of LE on the current density, and the normalized EL spectra at different current density are illustrated in Fig. 2a and b. With 10.0: 2.0: 1.2 weight ratio of PFSO: R-QDs: G-QDs, a white color emission with CIE coordinates of (0.32, 0.33) at the current density of 125 mA/cm2 was achieved. Unfortunately, the device performances are not promising. The peak brightness of the device is only 1.5 × 103 cd/m2, and the maximum LE is 3.2 cd/A. The turn-on voltage exceeds 4.4 V, suggesting that the electron injection from ZnO into PFSO is inefficient, although ZnO was demonstrated to be an efficient EIL in QLEDs [3,9,20–22]. However, as illustrated in Fig. 1b, the electron injection barrier from ZnO into PFSO is around 1.0 eV, too large for efficient electron injection at low voltages. Thus, the blue emission rapidly increases with increasing the drive voltage compared to the red and green emissions [23–25], as shown in Fig. 2c, d, e and f, implying that the electron injection into PFSO is more efficient than
2.2. Device fabrication and characterization Prior to fabrication of devices, the glass/ITO substrates were cleaned in sequence in ultrasonic bath of acetone, isopropanol, detergent, deionized water, and isopropanol, followed by being dried in a vacuum baking oven. Before depositing ZnO, the substrates were treated with 10 min oxygen plasma. 40 nm thick ZnO film was prepared by spin-coating method followed by 10 min annealing on a hot plate at 150 °C in a nitrogen filled glove box. Subsequently, about 5 nm thick PEI layer was spin-coated on the top of ZnO film from ethyl-alcohol solution with a concentration of 0.3 mg/mL. The EML solution was formulated by mixing different ratios of PFSO (10 mg/mL in p-Xylene), red QDs (20 mg/mL in n-octane), and green QDs (20 mg/mL in n-octane) solutions. The EML with 40 nm thick was prepared on the ZnO 35
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Fig. 2. The characteristics of the device without PEI. (a) J-V-L (b) LE-J; The normalized EL spectra with different blend ratios at various current densities. (c) PFSO: GQDs: R-QDs = 10.0: 2.0: 1.2; (d) PFSO: G-QDs: R-QDs = 10.0: 2.6: 1.6; (e) PFSO: G-QDs: R-QDs = 10.0: 3.6: 2.2; (f) The ratio of blue emission intensity to normalized red emission intensity of the devices with different EMLs at various current densities. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
operating voltage increases, and reduced the injection difficulty. Relatively, the QDs emission is stronger than PFSO based on the energy diagram as shown in the EL spectrum at a low operating voltage.
those into QDs with increasing the drive voltage. With increasing the ratio of the green and red QDs, the blue emission and the ratios of blue to the red emission intensity are decreasing, implying that a significant energy transfer process may occur from PFSO to QDs. For the injection of hole into QDs, some reports mentioned that the injection of holes from PVK into QDs is efficient. Possible reasons could be that although the hole injection barrier is 0.5 ev form PVK into QDs, a triangular injection barrier is formed between the PVK and PFSO layer as the
3.2. EL performances of devices with PEI layer PEI was considered as an electron injection material and introduced between ZnO and PFSO to reduce the electron injection barrier at the 36
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Fig. 3. The J-V-L characteristics of the monochrome devices with the emitter: (a) PFSO; (c) R-QDs; and (e) G-QDs; The LE-J characteristics of the monochrome devices with the emitter: (b) PFSO; (d) R-QDs; and (f) G-QDs.
QDs emitter is slightly improved even though with PEI modification layer, implying that the electron injection is not significantly improved by using PEI layer for the QDs devices. The J-V-L and LE-J characteristics of the white EL devices with PEI are shown in Fig. 4 (Also exhibiting the data of the device without PEI for comparison). Both devices with and without PEI layer have the same EML with PFSO: G-QDs: R-QDs weight ratio of 10.0: 2.0: 1.2. The EL spectra at current density of 125 mA/cm2 are inserted in Fig. 4b. With PEI layer, the turn-on voltage decreases from 4.8 V to 3.4 V, the brightness increases from 2.0 × 102 to 7.9 × 103 cd/m2 at bias of 10 V,
interface of ZnO/PFSO [26–29]. It was found that through Lewis acidbase interactions between the amino groups of PEI and Zn ions, a dipole layer could be formed at the interface of ZnO/PFSO and can effectively reduce the electron injection barrier [28,30]. The monochromic devices with and without PEI layer were fabricated to comparatively evaluate the functions of PEI, as shown in Fig. 3, the peak efficiency of the OLED with pure PFSO emitter and PEI modification layer is enhanced from 0.77 cd/A to 4.6 cd/A, and the maximum luminance increases from 1.7 × 103 to 8.7 × 103 cd/m2, meanwhile the turn-on voltage is reduced from 4 V to 3.2 V. However, the LE of device with R-QDs or G-
37
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Fig. 4. Performances of the devices with and without PEI. (a) The J-V-L characteristics; (b) The LE-J characteristics, Insert: EL spectra at current density of 125 mA/ cm2.
At the weight ratio of 10.0: 3.6: 2.2, the pure white emission with CIE coordinates of (0.31, 0.32) was obtained. The EL spectra of the device under different current densities are illustrated in Fig. 5b, exhibiting stable EL spectra than those of the devices without PEI layer because of the efficient electron injection from ZnO into PFSO layer. Therefore, when electrons are injected into the EML, electrons can be simultaneously injected into PFSO, G-QDs and R-QDs, which gives rise to a stable EL spectrum and good device performance. The dependence of
and the maximum LE increases from 3.2 to 4.3 cd/A. As demonstrated in the monochromic devices, it is more efficient that PEI could improve electron injection from ZnO into PFSO than into QDs. Therefore, in the EL spectra of the white device with PEI, the intensity of blue emission is relatively much enhanced, resulting the CIE coordinates shift from (0.32, 0.33) to (0.21, 0.35) at current density of 125 mA/cm2. To achieve pure white emission from the device with PEI layer, the weight ratio of PFSO: G-QDs: R-QDs was optimized, as shown in Fig. 5a.
Fig. 5. (a) The EL spectra of the devices with different weight ratios at current density of 125 mA/cm2; (b) The EL spectra of the device with PFSO: G-QDs: R-QDs weight ratio of 10.0: 3.6: 2.2 under different current densities. Insert: the dependence of the CIE color coordinates on the brightness; (c) The J-V-L characteristics of the devices with different weight ratios; (d) The LE-J characteristics of the devices with different weight ratios. 38
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Table 1 Device performances of the polymer/QDs hybrid devices with and without PEI layer and at difference weight ratio of PFSO: G-QDs: R-QDs. Devices
PFSO:G-QDs:R-QDs
ZnO/EML ZnO/EML ZnO/EML ZnO/PEI/EML ZnO/PEI/EML ZnO/PEI/EML ZnO/PEI/EML
10.0:2.0:1.2 10.0:2.6:1.6 10.0:3.6:2.2 10.0:2.0:1.2 10.0:3.6:3.6 10.0:3.0:2.2 10.0:3.6:2.2
Von (V)
4.8 4.7 4.4 3.3 3.4 3.4 3.3
Lmax (cd/m2)
3
5.8 × 10 6.4 × 103 7.5 × 103 1.5 × 104 1.9 × 104 1.7 × 104 1.8 × 104
Performance @ 1000 cd/m2
LEmax (cd/A)
2.8 3.0 3.2 4.0 5.7 4.3 5.3
J (mA/c m2)
V (V)
LE (cd/A)
CIE
50.4 54.0 65.9 20.2 19.8 23.0 19.0
13.0 12.2 12.0 7.0 6.6 7.7 6.8
1.8 1.9 2.2 4.1 5.6 3.8 5.1
(0.30, (0.33, (0.36, (0.22, (0.35, (0.30, (0.31,
0.31) 0.37) 0.38) 0.28) 0.34) 0.31) 0.32)
between PFSO and the R-QDs. Fig. 7b shows the PL decay of the PFSO film, R-QDs film, and PFSO/R-QDs mixed film with the weight ratio of 10.0: 2.0. The PL decay characteristics can be fitted with a bi-exponential decay function as follows,
the CIE coordinates on the brightness is shown in the insert of Fig. 5b. With increasing the brightness from 1.0 × 102 cd/m2 to 1.0 × 104 cd/ m2, the color coordinates slightly shift from (0.30, 0.29) to (0.32, 0.34), suggesting the device with PEI layer possess stable white EL spectra under large range of the brightness. The J-V-L, and LE-J characteristics of the devices with different weight ratios of PFSO: G-QDs: R-QDs are shown in Fig. 5c and d. For all the devices, the turn-on voltage located around 3.2–3.4 V, much less than those of the devices without PEI layer. At the weight ratio of 10.0: 3.6: 2.2, the device has a peak LE of 5.3 cd/A, maximum brightness of 1.8 × 104 cd/m2, EQE of 3.0%. The performances of all devices with and without PEI layer are summarized in Table 1. As demonstrated, the EL performances of the devices with PEI layer are greatly enhanced.
t
t
f (t ) = A1 e− τ1 + A2 e− τ2
τavg =
A1 τ12 + A2 τ22 A1 τ1 + A2 τ2
(1)
(2)
where τ1 and τ2 are radiation lifetime and non-radiation lifetime, respectively, A1 and A2 are the corresponding pre-exponential factors. Amplitude-weighted average lifetime τavg is used as a representative measure of exciton lifetime [16,32–34]. By fitting the PL decay with Eq. (1), the exciton lifetime of PFSO film is 1.6 ns, while the exciton lifetime of R-QDs film is 9.9 ns. For the PFSO/R-QDs mixed film, the exciton lifetime of PFSO decreases to 1.4 ns, while the exciton lifetime of R-QDs increases to 12.6 ns. The decrease of the exciton lifetime of PFSO and the increase of the exciton lifetime of R-QDs confirm the existence of Forster energy transfer between PFSO and the R-QDs [34–37]. In addition, charge trapping is another possible mechanism in a host-guest system. Steady-state PL and EL spectroscopies of PFSO/RQDs hybrid film with a weight ratio of 10.0:2.0 were investigated. Blue emission intensity is much stronger than that of the red emission, as shown in Fig. 7c of the PL spectrum. However, contrary to the PL spectrum, the EL emission intensity of the red emission is stronger than that of the blue one. The difference between the PL and EL spectra suggests that charge trapping mechanism exists in the EL process [38,39].
3.3. The work mechanism of white hybrid devices To investigate the mechanism of PEI function, the energy-level alignment was analyzed. The work function of ITO/ZnO film is reduced from 3.82 eV to 3.07 eV when using PEI modification layer, as UPS data indicated in Fig. 6. The lowered ITO/ZnO's work function reduces the electron injection barrier height between ZnO and PFSO from 1.02 eV to 0.27 eV. The work function shift caused by PEI layer may be due to the formation of interface dipole [21,28,31]. To understand the work mechanism of the polymer/QDs hybrid white emission device, absorption and emission spectra of PFSO film, RQDs and G-QDs films were tested respectively, as shown in Fig. 7a. It can be seen the overlap area between PL spectrum of PFSO and absorption spectrum of R-QDs is larger than that between the PL spectrum of PFSO and absorption spectrum of G-QDs, PL spectrum of G-QDs and absorption spectrum of R-QDs, implying that energy transfer between PFSO and G-QDs, G-QDs and R-QDs is almost negligible. A PL transient lifetime was tested to analyze the process of Forster energy transfer
4. Conclusions In summary, white emitting diode with inverted polymer/QDs
Fig. 6. UPS spectra, (a) Secondary-electron cut-off region for ZnO and ZnO/PEI films; (b) Valence band edge region for ZnO and ZnO/PEI films. 39
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Fig. 7. (a) Absorption and emission spectra of PFSO film, R-QDs film, R-QDs film; (b) Transient PL fluorescence lifetime of PFSO film, R-QDs film, and PFSO/R-QDs hybrid film with the weight ratio of 10.0: 2.0; (c) PL and EL spectra of PFSO/R-QDs hybrid film with the weight ratio of 10.0: 2.0.
hybrid structure was fabricated. The diode with ZnO/PEI as the ETL and the PFSO: G-QDs: R-QDs weight ratio of 10.0: 3.6: 2.2 as the EML shows high EL performances. The white device exhibits a peak LE of 5.3 cd/A, EQE of 3.0%, and CIE coordinates of (0.31, 0.32) at current density of 125 mA/cm2. The white EL spectrum is stable in a wide range of brightness increasing from 1.0 × 102 to 1.0 × 104 cd/m2. The PEI layer in the white QLED can reduce the work function of ITO/ZnO, thereby lowering the electron injection barrier and facilitating the electron injection from the cathode to PFSO layer. As a result, the turnon voltage of the white emission device is decreased from 4.8 V to 3.3 V. Both Forster energy transfer and charge trapping mechanisms work in the EL process and the later one is dominant.
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Acknowledgements 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 Nature Science Foundation of China (61574061), and Science and Technology Project of Guangdong Province (2017A050503002), Guangzhou Science and Technology Project (201804020033). References [1] X. Dai, Y. Deng, X. Peng, Y. Jin, Quantum-dot light-emitting devices for large-area displays: towards the dawn of commercialization, Adv. Mater. 29 (2017) 1607022, https://doi.org/10.1002/adma.201607022. [2] E. Jang, S. Jun, H. Jang, J. Lim, B. Kim, Y. Kim, White-light-emitting diodes with quantum dot color converters for display backlights, Adv. Mater. 22 (2010) 3076–3080, https://doi.org/10.1002/adma.201000525. [3] B.S. Mashford, M. Stevenson, Z. Popovic, C. Hamilton, Z. Zhou, C. Breen, J. Steckel, V. Bulovic, M. Bawendi, S. Coe-Sullivan, P.T. Kazlas, High-efficiency quantum-dot light-emitting devices with enhanced charge injection, Nat. Photonics 7 (2013) 407–412, https://doi.org/10.1038/nphoton.2013.70. [4] J.S. Steckel, J. Ho, C. Hamilton, J. Xi, C. Breen, W. Liu, P. Allen, S. Coe-Sullivan, Quantum dots: the ultimate down-conversion material for LCD displays, J. Soc. Inf. Disp. 23 (2015) 294–305, https://doi.org/10.1002/jsid.313. [5] Q. Sun, Y.A. Wang, L.S. Li, D. Wang, T. Zhu, J. Xu, C. Yang, Y. Li, Bright, multicoloured light-emitting diodes based on quantum dots, Nat. Photonics 1 (2007) 717–722, https://doi.org/10.1038/nphoton.2007.226. [6] J.F. Van Derlofske, J.M. Hillis, A. Lathrop, J. Wheatley, J. Thielen, G. Benoit, Illuminating the value of larger color gamuts for quantum dot displays, Digest of Technical Papers - SID International Symposium, 45 2014, pp. 237–240, , https:// doi.org/10.1002/j.2168-0159.2014.tb00065.x. [7] X. Dai, Z. Zhang, Y. Jin, Y. Niu, H. Cao, X. Liang, L. Chen, J. Wang, X. Peng, Solution-processed, high-performance light-emitting diodes based on quantum dots, Nature 515 (2014) 96–99, https://doi.org/10.1038/nature13829. [8] P.O. Anikeeva, J.E. Halpert, M.G. Bawendi, V. Bulović, Electroluminescence from a mixed red-green-blue colloidal quantum dot monolayer, Nano Lett. 7 (2007) 2196–2200, https://doi.org/10.1021/nl0703424. [9] K.H. Lee, C.Y. Han, H.D. Kang, H. Ko, C. Lee, J. Lee, N.S. Myoung, S.Y. Yim, H. Yang, Highly efficient, color-reproducible full-color electroluminescent devices based on red/green/blue quantum dot-mixed multilayer, ACS Nano 9 (2015) 10941–10949, https://doi.org/10.1021/acsnano.5b05513. [10] C.-Y. Huang, Y.-K. Su, R.W. Chuang, Y.-C. Chen, T.-S. Huang, C.-T. Wan, Tetrachromatic hybrid white light-emitting diodes and the energy transfer between conjugated polymers and CdSe/ZnS quantum dots, J. Electrochem. Soc. 156 (2009) H625, https://doi.org/10.1149/1.3138462.
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Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Inverted polymer/quantum-dots hybrid white light emitting diodes
T
Zhiwei He, Congbiao Jiang, Chen Song, Juanhong Wang, Zhenji Zhong, Gancheng Xie, ⁎ Jian Wang, Junbiao Peng , Yong Cao Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, State Key Laboratory of Luminescent Materials and Devices, Guangzhou 510640, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Inverted structure device Polymer/quantum-dot hybrid White light emitting diode Solution process Interface modified
Inverted polymer/quantum-dots (QDs) hybrid white light emitting diodes with high efficiency and color stability were fabricated. The white devices show high luminous efficiency of 5.3 cd/A, external quantum efficiency of 3.0%, by using polyetherimide (PEI) to modify zinc oxide layer as the electron injection layer and blue emitting polymer of poly (dibenzothiophene-S,S-dioxide-co-9,9-dioctyl-2,7-fluorene) (PFSO) doped with green and red quantum-dots (G-QDs, R-QDs) as the emitting layer. Pure white emission at Commission Internationale de L'Eclairage coordinates of (0.31, 0.32) was achieved by adjusting weight ratio of PFSO: G-QDs: R-QDs to be 10.0: 3.6: 2.2. In addition, the electroluminescence (EL) spectra of the white device are quite stable in the wide brightness range between 1.0 × 102 cd/m2 and 1.0 × 104 cd/m2. The possible reason was that the PEI could reduce the energy barrier height between the cathode and emitting layer, and facilitate electron injection into the emitting layer. It's confirmed that both Forest energy transfer and charge trapping mechanisms work in the EL process and the latter is dominant.
1. Introduction Light emitting diodes based on colloidal quantum dots (QLEDs) have attracted tremendous attentions in both academy and industry, because of their narrow emission band, high efficiency, and long operation stability [1–6]. The electroluminescence (EL) performances of QLEDs have been significantly enhanced by improving quantum dots' structure and EL device architecture. So far, the external quantum efficiency (EQE) for red QLEDs is over 20%, and its operation lifetime reaches > 10,0000 h at initial brightness of 100 cd/m2 [7]. White QLEDs have potential for applications in lighting and backlight of thin film transistor liquid crystal displays. To obtain white light emission, an alternative method is a mixture of red, green, and blue (RGB) emissions. The white QLED based on mixed RGB quantum dots (QDs) was reported by Prof. Bulović's group [8]. By adjusting the ratio of RGB QDs, the EQE of the white QLED reached 0.36% with Commission Internationale de L'Eclairage (CIE) coordinates of (0.31, 0.45). The relatively low EL efficiency was owing to low quantum yield of the QDs films. In 2015, Yang's group reported a white QLED based on mixed RGB QDs with a peak luminous efficiency (LE) of 21.8 cd/A, and the maximum EQE of 10.9% [9]. Another approach to achieve a white QLED is to utilize polymer/QDs hybrid structure. Compared to blue QDs, blue polymer materials could provide higher
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quantum efficiency and better device stability [10–13]. The polymer/QDs hybrid white QLED was reported by Lee et al. [14]. The blue polymer of poly[(9,9-dihexyloxyfluoren-2,7-diyl)-alt-co(2-methoxy-5-{2-ethylhexyloxy}phenylen-1,4-diyl)], blending with red QDs as a part of the emitter. The green emission is from metal chelate complex of 8-hydroxyquinoline aluminum, which also serves as the electron injection and transporting layer. Energy transfer and charge transfer between the blue polymer and the red QDs are principal mechanism for tuning the EL spectrum. The pure white emission at CIE coordinates of (0.30, 0.33) was obtained. However, the device's EQE is only 0.24%. To achieve high EL efficiency, green phosphorescent material of tris[2–4(4-toltyl) phenylpyridine]iridium [Ir(mppy)3] was introduced. The device consisted of the blue polymer of 2,2′,7,7′-tetrakis (2,2-diphenylvinylspiro-9,9′-bifluorene) doped with Ir(mppy)3 and red QDs as the emitter, exhibited a maximum EQE of 2.1% with peak brightness of 10,200 cd/m2 [15]. Most recently, J. S. Lee and coworkers employed the blue polymer of poly[9,9-dioctylfluorenyl-2,7-diyl]-end capped with N,N-Bis(4-methylphenyl)-aniline, doped with blue, green and red QDs as the emitter, combined with zinc oxide (ZnO) as the electron injection layer, to make a white QLED with a peak luminance of 15,950 cd/m2 and a maximum LE of 1.43 cd/A [16]. However, ZnO is an inefficient electron injection layer in this device. Because it has a deep conducting band at about −4.3 eV which doesn't match the blue
Corresponding author. E-mail address: [email protected] (J. Peng).
https://doi.org/10.1016/j.tsf.2018.10.028 Received 30 June 2018; Received in revised form 22 October 2018; Accepted 23 October 2018 Available online 23 October 2018 0040-6090/ © 2018 Published by Elsevier B.V.
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Fig. 1. Architecture and energy level diagram of the EL device.
2. Experiments
film by using spin-coating technique followed by annealing on a hot plate at 120 °C for 20 min in a nitrogen filled glove box. And then, a 15 nm thick PVK layer was formed by spin-coating from the solution of 1,4-dioxane in the concentration of 4 mg/mL and annealed at 140 °C for 10 min on a hot plate in a nitrogen filled glove box. The devices were completed by thermally evaporating 8 nm thick MoO3 in a vacuum chamber at the vacuum of 2 × 10−4 Pa followed to deposit 200 nm thick Al at vacuum of 5 × 10−5 Pa. The emission area of the devices is defined by the overlap of the anode and the cathode. The active area is about 2 × 2 mm2. The PL and EL spectra were recorded by FL3C-111 (HORIBA Instruments Inc), and a fiber optic spectrometer (Ocean Optics USB 2000), respectively. The thickness was characterized by Dektak XT surface profiler (Bruker Corp.). The current-voltage-luminous (J-V-L) characteristics and CIE coordinates were measured by a source meter (Keithley 2400) and a Konica Minolta Chroma Meter (CS-200). The interface analysis was performed by using an Ultraviolet Photoelectron Spectrometer (UPS) with an unfiltered He I (21.22 eV) gas-discharge lamp. The PL lifetimes of films were measured by a fluorescence lifetime spectrometer (Hamamatsu C11367).
2.1. Materials
3. Results and discussion
Indium tin oxide (ITO) coated glass with a sheet resistance of 15–20 Ω/□ was purchased from China Southern Glass Holding Corp. ZnO nanoparticles solution was purchased from Guangdong Poly Optoelectronics Co., Ltd. Red and green QDs solutions with CdSe/ZnS core/shell structure were purchased from Mesolight Inc.. The red and green QDs with alloyed core-shell CdSe/ZnS, capped with oleic acid ligand are purchased from Mesolight Inc. The diameter is about 11 ± 1.5 nm for red QD and 10 ± 1.3 nm for green QD, and quantum yield is about 80% for red QD and 85% for green QD, respectively. PFSO was synthesized in our group [17]. Poly (9-vinylcarbazole) (PVK), PEI, molybdenum oxide (MoO3) powders, and all chemical solvents with purity of 99.9% were purchased from Sigma-Aldrich.
3.1. EL performances of white emission devices without PEI layer
polymer's the lowest unoccupied molecular orbital at around −2.0 eV. As the result, a large electron injection barrier between ZnO and the blue polymer blocks the device to gain a high EL efficiency. In our contribution, the blue polymer of poly(dibenzothiophene-S,Sdioxide-co-9,9-dioctyl-2,7-fluorene) (PFSO) synthesized in our group was utilized to fabricate a polymer/QDs hybrid white QLED devices. The emitting layer (EML) consists of PFSO doped with red and green QDs. Polyetherimide (PEI) was introduced to reduce the electron injection barrier height between ZnO and PFSO layers. The white emission was achieved with a peak LE of 5.3 cd/A, and EQE of 3.0% at CIE coordinates of (0.32, 0.33). The device EL efficiency is the highest ever reported for polymer/QDs hybrid white QLEDs. Moreover, the color stability of the white device is significantly improved. The CIE coordinates only slightly shift from (0.30, 0.29) to (0.32, 0.34), with increasing the brightness from 1.0 × 102 to 1.0 × 104 cd/m2. The photoluminescence (PL) and EL spectra and a transient fluorescence lifetime show that both energy transfer and charge trapping mechanisms work in the EL process and the latter mechanism is dominant.
Inverted polymer/QDs hybrid white light emitting diodes with blue polymer PFSO doped with red and green QDs as a single EML were fabricated in the configuration of ITO/ZnO/EML/PVK/MoOx/Al. Blue PFSO, G-QDs and R-QDs have relatively high efficiency and stable EL spectrum [18,19]. The device architecture and energy level diagram are illustrated in Fig. 1. In the device structure, MoO3/Al is the anode, PVK serves as the hole transport layer, while ITO is the cathode, and ZnO nanoparticles layer is the electron injection layer (EIL). By adjusting the ratio of green and red QDs in the blue polymer, the emission color of the device is easily tuned. The Current density-Voltage-Luminous (J-VL) characteristics, the dependence of LE on the current density, and the normalized EL spectra at different current density are illustrated in Fig. 2a and b. With 10.0: 2.0: 1.2 weight ratio of PFSO: R-QDs: G-QDs, a white color emission with CIE coordinates of (0.32, 0.33) at the current density of 125 mA/cm2 was achieved. Unfortunately, the device performances are not promising. The peak brightness of the device is only 1.5 × 103 cd/m2, and the maximum LE is 3.2 cd/A. The turn-on voltage exceeds 4.4 V, suggesting that the electron injection from ZnO into PFSO is inefficient, although ZnO was demonstrated to be an efficient EIL in QLEDs [3,9,20–22]. However, as illustrated in Fig. 1b, the electron injection barrier from ZnO into PFSO is around 1.0 eV, too large for efficient electron injection at low voltages. Thus, the blue emission rapidly increases with increasing the drive voltage compared to the red and green emissions [23–25], as shown in Fig. 2c, d, e and f, implying that the electron injection into PFSO is more efficient than
2.2. Device fabrication and characterization Prior to fabrication of devices, the glass/ITO substrates were cleaned in sequence in ultrasonic bath of acetone, isopropanol, detergent, deionized water, and isopropanol, followed by being dried in a vacuum baking oven. Before depositing ZnO, the substrates were treated with 10 min oxygen plasma. 40 nm thick ZnO film was prepared by spin-coating method followed by 10 min annealing on a hot plate at 150 °C in a nitrogen filled glove box. Subsequently, about 5 nm thick PEI layer was spin-coated on the top of ZnO film from ethyl-alcohol solution with a concentration of 0.3 mg/mL. The EML solution was formulated by mixing different ratios of PFSO (10 mg/mL in p-Xylene), red QDs (20 mg/mL in n-octane), and green QDs (20 mg/mL in n-octane) solutions. The EML with 40 nm thick was prepared on the ZnO 35
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Fig. 2. The characteristics of the device without PEI. (a) J-V-L (b) LE-J; The normalized EL spectra with different blend ratios at various current densities. (c) PFSO: GQDs: R-QDs = 10.0: 2.0: 1.2; (d) PFSO: G-QDs: R-QDs = 10.0: 2.6: 1.6; (e) PFSO: G-QDs: R-QDs = 10.0: 3.6: 2.2; (f) The ratio of blue emission intensity to normalized red emission intensity of the devices with different EMLs at various current densities. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
operating voltage increases, and reduced the injection difficulty. Relatively, the QDs emission is stronger than PFSO based on the energy diagram as shown in the EL spectrum at a low operating voltage.
those into QDs with increasing the drive voltage. With increasing the ratio of the green and red QDs, the blue emission and the ratios of blue to the red emission intensity are decreasing, implying that a significant energy transfer process may occur from PFSO to QDs. For the injection of hole into QDs, some reports mentioned that the injection of holes from PVK into QDs is efficient. Possible reasons could be that although the hole injection barrier is 0.5 ev form PVK into QDs, a triangular injection barrier is formed between the PVK and PFSO layer as the
3.2. EL performances of devices with PEI layer PEI was considered as an electron injection material and introduced between ZnO and PFSO to reduce the electron injection barrier at the 36
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Fig. 3. The J-V-L characteristics of the monochrome devices with the emitter: (a) PFSO; (c) R-QDs; and (e) G-QDs; The LE-J characteristics of the monochrome devices with the emitter: (b) PFSO; (d) R-QDs; and (f) G-QDs.
QDs emitter is slightly improved even though with PEI modification layer, implying that the electron injection is not significantly improved by using PEI layer for the QDs devices. The J-V-L and LE-J characteristics of the white EL devices with PEI are shown in Fig. 4 (Also exhibiting the data of the device without PEI for comparison). Both devices with and without PEI layer have the same EML with PFSO: G-QDs: R-QDs weight ratio of 10.0: 2.0: 1.2. The EL spectra at current density of 125 mA/cm2 are inserted in Fig. 4b. With PEI layer, the turn-on voltage decreases from 4.8 V to 3.4 V, the brightness increases from 2.0 × 102 to 7.9 × 103 cd/m2 at bias of 10 V,
interface of ZnO/PFSO [26–29]. It was found that through Lewis acidbase interactions between the amino groups of PEI and Zn ions, a dipole layer could be formed at the interface of ZnO/PFSO and can effectively reduce the electron injection barrier [28,30]. The monochromic devices with and without PEI layer were fabricated to comparatively evaluate the functions of PEI, as shown in Fig. 3, the peak efficiency of the OLED with pure PFSO emitter and PEI modification layer is enhanced from 0.77 cd/A to 4.6 cd/A, and the maximum luminance increases from 1.7 × 103 to 8.7 × 103 cd/m2, meanwhile the turn-on voltage is reduced from 4 V to 3.2 V. However, the LE of device with R-QDs or G-
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Fig. 4. Performances of the devices with and without PEI. (a) The J-V-L characteristics; (b) The LE-J characteristics, Insert: EL spectra at current density of 125 mA/ cm2.
At the weight ratio of 10.0: 3.6: 2.2, the pure white emission with CIE coordinates of (0.31, 0.32) was obtained. The EL spectra of the device under different current densities are illustrated in Fig. 5b, exhibiting stable EL spectra than those of the devices without PEI layer because of the efficient electron injection from ZnO into PFSO layer. Therefore, when electrons are injected into the EML, electrons can be simultaneously injected into PFSO, G-QDs and R-QDs, which gives rise to a stable EL spectrum and good device performance. The dependence of
and the maximum LE increases from 3.2 to 4.3 cd/A. As demonstrated in the monochromic devices, it is more efficient that PEI could improve electron injection from ZnO into PFSO than into QDs. Therefore, in the EL spectra of the white device with PEI, the intensity of blue emission is relatively much enhanced, resulting the CIE coordinates shift from (0.32, 0.33) to (0.21, 0.35) at current density of 125 mA/cm2. To achieve pure white emission from the device with PEI layer, the weight ratio of PFSO: G-QDs: R-QDs was optimized, as shown in Fig. 5a.
Fig. 5. (a) The EL spectra of the devices with different weight ratios at current density of 125 mA/cm2; (b) The EL spectra of the device with PFSO: G-QDs: R-QDs weight ratio of 10.0: 3.6: 2.2 under different current densities. Insert: the dependence of the CIE color coordinates on the brightness; (c) The J-V-L characteristics of the devices with different weight ratios; (d) The LE-J characteristics of the devices with different weight ratios. 38
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Table 1 Device performances of the polymer/QDs hybrid devices with and without PEI layer and at difference weight ratio of PFSO: G-QDs: R-QDs. Devices
PFSO:G-QDs:R-QDs
ZnO/EML ZnO/EML ZnO/EML ZnO/PEI/EML ZnO/PEI/EML ZnO/PEI/EML ZnO/PEI/EML
10.0:2.0:1.2 10.0:2.6:1.6 10.0:3.6:2.2 10.0:2.0:1.2 10.0:3.6:3.6 10.0:3.0:2.2 10.0:3.6:2.2
Von (V)
4.8 4.7 4.4 3.3 3.4 3.4 3.3
Lmax (cd/m2)
3
5.8 × 10 6.4 × 103 7.5 × 103 1.5 × 104 1.9 × 104 1.7 × 104 1.8 × 104
Performance @ 1000 cd/m2
LEmax (cd/A)
2.8 3.0 3.2 4.0 5.7 4.3 5.3
J (mA/c m2)
V (V)
LE (cd/A)
CIE
50.4 54.0 65.9 20.2 19.8 23.0 19.0
13.0 12.2 12.0 7.0 6.6 7.7 6.8
1.8 1.9 2.2 4.1 5.6 3.8 5.1
(0.30, (0.33, (0.36, (0.22, (0.35, (0.30, (0.31,
0.31) 0.37) 0.38) 0.28) 0.34) 0.31) 0.32)
between PFSO and the R-QDs. Fig. 7b shows the PL decay of the PFSO film, R-QDs film, and PFSO/R-QDs mixed film with the weight ratio of 10.0: 2.0. The PL decay characteristics can be fitted with a bi-exponential decay function as follows,
the CIE coordinates on the brightness is shown in the insert of Fig. 5b. With increasing the brightness from 1.0 × 102 cd/m2 to 1.0 × 104 cd/ m2, the color coordinates slightly shift from (0.30, 0.29) to (0.32, 0.34), suggesting the device with PEI layer possess stable white EL spectra under large range of the brightness. The J-V-L, and LE-J characteristics of the devices with different weight ratios of PFSO: G-QDs: R-QDs are shown in Fig. 5c and d. For all the devices, the turn-on voltage located around 3.2–3.4 V, much less than those of the devices without PEI layer. At the weight ratio of 10.0: 3.6: 2.2, the device has a peak LE of 5.3 cd/A, maximum brightness of 1.8 × 104 cd/m2, EQE of 3.0%. The performances of all devices with and without PEI layer are summarized in Table 1. As demonstrated, the EL performances of the devices with PEI layer are greatly enhanced.
t
t
f (t ) = A1 e− τ1 + A2 e− τ2
τavg =
A1 τ12 + A2 τ22 A1 τ1 + A2 τ2
(1)
(2)
where τ1 and τ2 are radiation lifetime and non-radiation lifetime, respectively, A1 and A2 are the corresponding pre-exponential factors. Amplitude-weighted average lifetime τavg is used as a representative measure of exciton lifetime [16,32–34]. By fitting the PL decay with Eq. (1), the exciton lifetime of PFSO film is 1.6 ns, while the exciton lifetime of R-QDs film is 9.9 ns. For the PFSO/R-QDs mixed film, the exciton lifetime of PFSO decreases to 1.4 ns, while the exciton lifetime of R-QDs increases to 12.6 ns. The decrease of the exciton lifetime of PFSO and the increase of the exciton lifetime of R-QDs confirm the existence of Forster energy transfer between PFSO and the R-QDs [34–37]. In addition, charge trapping is another possible mechanism in a host-guest system. Steady-state PL and EL spectroscopies of PFSO/RQDs hybrid film with a weight ratio of 10.0:2.0 were investigated. Blue emission intensity is much stronger than that of the red emission, as shown in Fig. 7c of the PL spectrum. However, contrary to the PL spectrum, the EL emission intensity of the red emission is stronger than that of the blue one. The difference between the PL and EL spectra suggests that charge trapping mechanism exists in the EL process [38,39].
3.3. The work mechanism of white hybrid devices To investigate the mechanism of PEI function, the energy-level alignment was analyzed. The work function of ITO/ZnO film is reduced from 3.82 eV to 3.07 eV when using PEI modification layer, as UPS data indicated in Fig. 6. The lowered ITO/ZnO's work function reduces the electron injection barrier height between ZnO and PFSO from 1.02 eV to 0.27 eV. The work function shift caused by PEI layer may be due to the formation of interface dipole [21,28,31]. To understand the work mechanism of the polymer/QDs hybrid white emission device, absorption and emission spectra of PFSO film, RQDs and G-QDs films were tested respectively, as shown in Fig. 7a. It can be seen the overlap area between PL spectrum of PFSO and absorption spectrum of R-QDs is larger than that between the PL spectrum of PFSO and absorption spectrum of G-QDs, PL spectrum of G-QDs and absorption spectrum of R-QDs, implying that energy transfer between PFSO and G-QDs, G-QDs and R-QDs is almost negligible. A PL transient lifetime was tested to analyze the process of Forster energy transfer
4. Conclusions In summary, white emitting diode with inverted polymer/QDs
Fig. 6. UPS spectra, (a) Secondary-electron cut-off region for ZnO and ZnO/PEI films; (b) Valence band edge region for ZnO and ZnO/PEI films. 39
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Fig. 7. (a) Absorption and emission spectra of PFSO film, R-QDs film, R-QDs film; (b) Transient PL fluorescence lifetime of PFSO film, R-QDs film, and PFSO/R-QDs hybrid film with the weight ratio of 10.0: 2.0; (c) PL and EL spectra of PFSO/R-QDs hybrid film with the weight ratio of 10.0: 2.0.
hybrid structure was fabricated. The diode with ZnO/PEI as the ETL and the PFSO: G-QDs: R-QDs weight ratio of 10.0: 3.6: 2.2 as the EML shows high EL performances. The white device exhibits a peak LE of 5.3 cd/A, EQE of 3.0%, and CIE coordinates of (0.31, 0.32) at current density of 125 mA/cm2. The white EL spectrum is stable in a wide range of brightness increasing from 1.0 × 102 to 1.0 × 104 cd/m2. The PEI layer in the white QLED can reduce the work function of ITO/ZnO, thereby lowering the electron injection barrier and facilitating the electron injection from the cathode to PFSO layer. As a result, the turnon voltage of the white emission device is decreased from 4.8 V to 3.3 V. Both Forster energy transfer and charge trapping mechanisms work in the EL process and the later one is dominant.
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