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Enhanced performance of green quantum dots light-emitting diodes: The case of Ag nanowires
Physica E: Low-dimensional Systems and Nanostructures 101 (2018) 11–15
Contents lists available at ScienceDirect
Physica E: Low-dimensional Systems and Nanostructures journal homepage: www.elsevier.com/locate/physe
Enhanced performance of green quantum dots light-emitting diodes: The case of Ag nanowires Yidong Zhang a, *, Zhiwei Li b a Key Laboratory for Micro-Nano Energy Storage and Conversion Materials of Henan Province, Institute of Surface Micro and Nano Materials, Xuchang University, Xuchang, 461000, PR China b College of Chemistry and Chemical Engineering, Zhoukou Normal University, Zhoukou, Henan Province, 466001, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ag nanowires (NWs) NiO nanocrystals (NCs) thin film Quantum dots light-emitting diodes (QLEDs)
In this work, The performance of green quantum dots light-emitting diodes (QLEDs) were enhanced by dispersing Ag nanowires (NWs) into NiO hole injection layer (HIL). Highly bright green QLEDs with a maximum luminance of 32780 cd m2 and current efficiency of 4.1 cd A1, exhibiting 50% improvement compared with the device without Ag NWs. The improved performance may be attributed to the significant increase in the conductivity and hole injection rate as a result of the introduction of Ag NWs and the good matching between the resonance frequency of the localized surface plasma resonance (LSPR) generated by NWs and QDs, as well as the suppressed Auger recombination of QDs layer due to the LSPR-induced near-field enhanced radiative recombination rate of excitons.
1. Introduction Colloidal quantum dots (QDs) are promising light-emitting materials thanks to their tunable wavelength and narrow full-width-at-halfmaximum (FWHM) [1]. Since the first demonstration by Allvisatos group [2], the performance of QLEDs has been enhanced greatly because of the optimization of the device architecture [3]. As one of the most widely applied hole injection material in QLEDs, NiO has caused great attention owing to its low cost, low surface roughness, high stability and high transparency [4]. However, NiO is a kind of Mott insulator with low conductivity and carrier mobility, which is the largest bottleneck for the balance of carrier transportation [5]. In order to improve the electric properties of NiO, monovalent atoms such as Liþ and Kþ were incorporated into NiO crystal lattice by magnetron sputtering method due to the increasing amount of nickel vacancies [6]. For example, the resistivity of NiO can be decreased to 1 Ω cm after doping a certain amount of Li and K atoms [7]. Yang et al. also fabricated transparent p-type conductive Ni0.9Cu0.1O thin films by pulsed plasma deposition (PPD) method, exhibiting the highest conductivity of 5.17 S cm1, with an average transmittance of 60% in the visible region [8]. Recently, nobel metal nanoparticles (Au, Ag) attracts more attention due to its excellent optical properties with localized surface plasmon resonance (LSPR) effect in the applications of QLEDs, solar cells and photocatalysis [9–11]. For
example, plasma-enhanced green QLEDs by incorporating solution processed Au NPs into device to obtain the doubled luminance and current efficiency [12]. Since LSPR strongly affects the kinetic characteristics of nearby molecules. Au or Ag NPs induce a strong enhancement of the local electromagnetic field intensity close to the NPs. The electric field vector decays exponentially with distance from the metal surface, with a decay length of the order of one half of the excitation wavelength. When a fluorescent emitter layer is placed within the proper range of the enhanced local electric field intensity, plasmonic interaction takes place, which can enhance the radiative decay rates of the fluorescent species [13]. Enlightened by the idea, for the first time, Ag nanowires (NWs) were introduced to the NiO NCs film as HIL in QLED to improve the device performance. (see Table 1) In this work, Ag NWs are introduced into the NiO HIL to enhance its conductivity, and the effect of LSPR triggered by Ag NWs on QLED is investigated. The lifetime of the QD sample emission band matching the LSPR frequency is shortened, and the overall device performance is enhanced, which is ascribed to the suppression of Auger recombination of QDs by LSPR induced by Ag NWs and enhanced holes transport ability. By using our present strategy, highly bright green QLED with peak luminance up to ~32780 cd m2, and current efficiency of 4.1 cd A1 are achieved successfully based on Ag NWs.
* Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Zhang). https://doi.org/10.1016/j.physe.2018.03.008 Received 18 January 2018; Received in revised form 23 February 2018; Accepted 7 March 2018 Available online 8 March 2018 1386-9477/© 2018 Elsevier B.V. All rights reserved.
Y. Zhang, Z. Li
Physica E: Low-dimensional Systems and Nanostructures 101 (2018) 11–15
2.4. Preparation of ZnO NCs
Table 1 Summary of the QLED performance with and without Ag NWs.
NiO Ag NWsNiO
L (cd m2)
J (mA cm2)
CE (cd A1)
EQE (%)
Lifetime (ns)
24540 32780
326 184
1.3 4.1
3.04 7.14
28.64 22.16
In a typical process, 0.66 g Zn(Ac)2 in 30 ml dimethyl sulfonate (DMSO) and 10 g tetramethylammonium hydroxide (TMAH) in 100 ml ethanol were mixed and stirred for 1 h in ambient atmosphere. The prepared product was collected by centrifugation and then washed with the mixture solution of ethanol and n-heptane (volume ratio 1: 4) three times. Finally, the precipitate was re-dispersed in ethanol to form a ZnO nanoparticles solution with a concentration of 30 mg ml1.
2. Experimental method
2.5. Fabrication of the QLED
All the reagents used in the experiments were in analytic grade (purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd) and used without further purification.
The patterned ITO substrates with a sheet resistance of ~20 Ω sq1 were ultrasonically cleaned subsequently with deionized water, acetone, and isopropanol for 15 min, respectively, followed by treating with ozone generated by ultraviolet light in air for 15 min. Then the prepared substrates were spinning-coated by Ag NWs- NiO NCs at a rotation speed of 3000 rpm, followed by an annealing process at 60 C for 15 min in air. Then the substrates were transferred to a N2-filled glove-box for spincoating of the TFB, CdS/CdSe/ZnS QDs, and ZnO NCs layers. The TFB hole transport layer (HTL) was spin-coated at a speed of 3000 rpm for 45 s, then annealing at 150 C for 30 min. The spin speed for QD emitter layer (15 mg ml1, toluene) and ZnO HTL (30 mg ml1, ethanol) were 2000 rpm and 3000 rpm, respectively, followed by annealing at 60 C for 30 min. These multilayer samples were then loaded into a custom highvacuum chamber (pressure, ~3 107 Torr) to deposit the top Al cathode (100 nm thick) patterned by an in situ shadow mask to form an active device area of 4 mm2.
2.1. Preparation of Ag nanowires Typically, 60 mL 0.001 mM NaCl in ethylene glycol was heated in oven at 160 C for 1 h. Then, 0.4 g poly-vinylpyrrolidone (PVP) was poured into the system followed by 0.25 g AgNO3 crystal addition. Thereafter, the solution is heated for 2 h, followed by separation and purification of Ag NWs using acetone and water, respectively. Finally, Ag NWs were suspended in water with the concentration of 2 wt.%.
2.2. Preparation of NiO nanocrystals NiO nanocrystals (NCs) were prepared according to the method reported by Liang et al.7 on the top of Typically, 0.5 mmol Ni(St)2, 0.2 mmol LiSt, 3 mmol ODA and 5 ml 1-octadecene (ODE) were loaded in a three-neck flask and degassed at 80 C for 30 min. The reaction mixture was then heated to 235 C and kept at this temperature for 3 h under argon flow. The products were precipitated out by adding a mixture of ethanol and ethyl acetate and further purified by dispersing/precipitating twice using the combination of hexane/ethanol. Finally, a certain amount of Ag NWs were dispersed into the NiO ethanol suspension (1 mg ml1).
2.6. Characterization and electrical luminance performance test The samples were characterized by atomic force microscopy (AFM) (Icon, Bruker Instruments Inc.). An Agilent 4255C equipped with a calibrated Newport silicon photodetector was used to measure the current-luminance-voltage characteristics. The electroluminescence spectra were recorded using an Ocean Optics high-resolution spectrometer (HR4000). The devices were measured under ambient conditions without encapsulation.
2.3. Preparation of QDs 3. Result and discussion For a typical synthesis of CdSe/ZnS QDs, 0.2 mmol of CdO, 4 mmol of zinc acetate and 5 ml of oleic acid (OA) were placed in a 100 ml threenecked bottle and heated to 150 C in flowing high-purity argon for 30 min. Then 15 ml of 1-octadecene (ODE) was added to the flask and the temperature was elevated to 300 C. A stock solution containing 0.2 mmol of Se and 4 mmol of S dissolved in 2 ml of trioctylphosphine (TOP) was quickly injected into the flask to react for 6 min. The resulting QDs were washed several times and finally dispersed in toluene form a suspension on the concentration of 15 mg ml1.
3.1. Characterization of Ag NWs, NiO NCs, ZnO and QDs Fig. 1a and b shows the TEM images of the prepared NiO NCs. It can be seen that the NiO NCs are uniform with the average diameter of 3 nm. High resolution TEM image is shown in the inset of Fig. 1a. The selected area electron diffraction (SAED) pattern obtained by focusing the electric beam on NiO particles is displayed in the inset of Fig. 1b, which shows diffused rings indicating that the NiO particles are constructed of NiO
Fig. 1. TEM images of NiO NCs: high magnification (a), HRTEM image (inset a), low magnification (b), and SAED pattern (inset b), AFM image of Ag NWs on quartz substrate: high magnification (c) and low magnification (d).
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Y. Zhang, Z. Li
Physica E: Low-dimensional Systems and Nanostructures 101 (2018) 11–15
Fig. 2. TEM image of ZnO NCs (a); QDs (b), PL and UV–vis absorption spectrums of the QDs (c), the digital light photo of the QDs with the irradiation of UV light (inset c), band gap of the QDs (d).
Fig. 3. Schematic diagrams of the Ag NWs NiO-based QLED (a), energy level alignment of the QLED (b).
3.2. Electroluminescence performance of QLEDs
NCs. Fig. 3c and d displays the AFM image of the prepared Ag NWs on quartz substrate, showing that the average diameter and length is 20 nm and 10 μm, respectively, with high aspect ratio. The average size of the QDs spin-coating of ZnO NCs and QDs is ~2.5 nm and ~6 nm, respectively, as shown in Fig. 2a and b, respectively. The typical absorption wavelength and emission wavelength of the QDs is 510 nm and 519 nm, respectively, as shown in Fig. 2c. The digital light photo of the QDs with the irradiation of UV light is shown in the inset of Fig. 2c. The optical band gap energy (Eg) of the green QDs was ~2.38 eV estimated by the extrapolation method, as shown in Fig. 2d.
The QLED architecture is schematically displayed in Fig. 3a. Specifically, the multilayer structure is consisted of patterned ITO, Ag-NiO, QDs, ZnO NCs and Al, in which serve as the anode, HIL, HTL, emitting layer, ETL, and cathode, respectively. Fig. 3b illustrates the corresponding energy level diagram of the QLED. Fig. 4a shows the J-V characteristic curve of the QLEDs, indicating that the leakage current will decreased after introducing Ag NWs in NiO layer due to the balanced carrier transportation, i.e. the relative carrier transportation balance can 13
Y. Zhang, Z. Li
Physica E: Low-dimensional Systems and Nanostructures 101 (2018) 11–15
Fig. 4. Current density-voltage (J–V) and luminance-voltage (L–V) characteristic curves of the QLEDs (a), the current efficiency as a function of the luminance (b), and the inset (b) is the photograph of the green light emission of the QLED at an applied voltage of 4.5 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. J–V characteristic curves of hole-only devices (a), PL decay curves of ITO/Ag-NiO/TFB/QDs films (b).
Time-resolved PL measurements were performed to further investigate the PL lifetime of QDs, as shown in Fig. 5b. The average PL lifetime of ITO/NiO/TFB/QDs film decreases from 28.64 to 22.16 ns after introducing AgNWs into HIL due to the LSPR induced by the Ag NWs in HIL. It indicates that adding Ag NWs into HIL shortens the lifetime of the excitons in the QD-emitting layer, enhances the radiative recombination rate of excitons, and thus reduces the Auger recombination caused by QDs charging effectively due to the decreased concentration of excitons in the emission layer [15]. When Ag NWs were dispersed in NiO as HTL, Ag NWs will induce a strong enhancement of the local electromagnetic field intensity close to the NiO NCs [16]. The electric field vector decays exponentially with distance away from the metal surface, with decay length of the order of one half of the excitation wavelength [17]. If the green QDs with PL peak at 519 nm is located within the proper range of the enhanced local electric field, plasmonic interaction will take place, leading to the shorten radiative lifetime of the QDs and enhance the radiative decay rates of the fluorescent species. Usually, the lifetime of 1 excitons is defined as: τ ¼ kr þk , where ‘kr’ and ‘knr’ is the radiative rate nr
be obtained. Also, the luminance can be greatly enhanced from 24540 to 37580 cd m2 after introducing Ag NWs in NiO layer. Further, we can observe that the threshold voltage decrease from 3.2 V to 2.2 V after introducing Ag NWs in NiO layer, indicating that the Ag NWs can decrease the hole injection barrier. The current efficiency (CE) as a function of the luminance is shown in Fig. 4b, obviously, the CE of QLED with Ag NWs is much higher than that of standard QLED, particularly, the device with Ag NWs shows low efficiency roll-off. The inset of Fig. 4b is the photograph of the Ag-NiO based QLED at an applied voltage of 4.5 V. In order further investigate the effect of Ag NWs on the device performance, the hole-only devices of Au/HTL/HIL/ITO structures were further characterized to investigate the holes injection and transport capability of the QLEDs. In this device structure, as shown in the inset of Fig. 5a, it is expected that the electron injection from the Au electrode to the TFB is well obstructed by the large injection barrier at TFB/Au interface. Thus, the holes injection and transport capability can be exclusively characterized by measuring the J-V characteristic curves. The J-V curves of hole-only devices presented different electrical features with that of QLEDs [14]. The Ag NWs-based QLED showed higher current density, indicating that the Ag NWs contributed to the high holes conduction capability, as shown in Fig. 5a. Since the hole is the minority carrier in our QLED device, the enhancement of hole currents in Ag NWs-based QLED will make a better balance between the hole and the electron injection, leading to a substantial luminous-efficiency increase compared to the standard device.
and the non-radiative rate, respectively. The ‘knr’ consists of two terms: one originates from the intrinsic nonradiative decay (ki-nr) in QD particles and the other (kt-nr) originates from the resonance coupling effect between QDs and Ag NWs. The coupling effect between Ag NWs and excitons results in an increase of kr and kt-nr, leading to a reduced lifetime of excitons in QDs. The quantum yield (Q) of QDs can be calculated ackr cording to the formula: Q ¼ kr þk . The increased ‘kr’ and ‘kt-nr’ will nr 14
Y. Zhang, Z. Li
Physica E: Low-dimensional Systems and Nanostructures 101 (2018) 11–15
Fig. 6. Impedance spectroscopy (a) and voltage-capacitance characteristics (b) of the QLEDs with and without Ag NWs.
enhance the ‘Q’ of QDs, i.e., the electron-hole recombination in QDs is improved [18]. An improved recombination implies that more photons will be generated in the QDs, resulting in the enhanced luminance of device. Further, the coupling between Ag NWs and QDs probably increase the light extraction efficiency so as to increase the luminance. To further explore the role of the Ag NWs on the electrical performance of QLEDs, we carried out impedance spectroscopy (IS) measurements, as shown in Fig. 6a. We can see that the impedance of the device without Ag NWs decreases dramatically when the bias is higher than 0.5 V. Combined with the luminance-voltage data shown in Fig. 4a, we can deduce that this decrease of the impedance is not a result of the recombination of the electrons and holes due to the low bias below the VT (~3 V), and it should be attributed to the leakage current across the devices. Also, the increase of the current phase is not from the electronhole recombination, but from the leakage of the carriers. Fortunately, this decreasing behavior of impedance is alleviated after introduction of Ag NWs, which demonstrates that the Ag NWs can suppress electron leakage into the NiO layer. Fig. 6b shows the voltage-capacitance (V-C) characteristics of the QLEDs Similarly, the capacitance of the QLED without Ag NWs decreases sharply when the bias voltage is more than 0.5 V, however, it will be more stable after introducing Ag NWs in NiO, further indicating that a large leakage current exists in QLEDs, which can be suppressed by Ag NWs.
References [1] L. Qian, Y. Zheng, J.G. Xue, P.H. Holloway, Stable and efficient quantum-dot lightemitting diodes based on solution-processed multilayer structures, Nat. Photon. 5 (2011) 543–548. [2] V.L. Colvin, M.C. Schlamp, A.P. Allvisatos, Light-emitting diodes made from cadmium selenide nanocrystals and a semiconductor polymer, Nature 370 (1994) 354–356. [3] K.H. Lee, J.H. Lee, H.D. Kang, B. Park, Y. Kwon, H. Ko, C. Lee, J. Lee, H. Yang, Over 40 cd/A efficient green quantum dot electroluminescent device comprising uniquely large quantum dots, ACS Nano 8 (2014) 4893–4901. [4] S.Y. Liu, R. Liu, Y. Chen, S. Ho, J.H. Kim, F. So, Nickel oxide hole injection/ transport layers for efficient solution processed organic light-emitting diodes, Chem. Mater. 26 (2014) 4528–4534. [5] J.M. Caruge, J.E. Halpert, V. Bulovic, M.G. Bawendi, NiO as an inorganic holetransporting layer in quantum-dot light-emitting devices, Nano Lett. 6 (2006) 2991–2994. [6] M. Yang, H.F. Pu, Q.F. Zhou, Q. Zhang, Transparent p-type conducting K-doped NiO films deposited by pulsed plasma deposition, Thin Solid Films 520 (2012) 5884–5888. [7] W.L. Jang, Y.M. Lu, W.S. Hwang, W.C. Chen, Electrical properties of Li-doped NiO films, J. Eur. Ceram. Soc. 30 (2010) 503–508. [8] M. Yang, Z. Shi, J.H. Feng, H.F. Pu, G.F. Li, J. Zhou, Q. Zhang, Copper doped nickel oxide transparent p-type conductive thin films deposited by pulsed plasma deposition, Thin Solid Films 519 (2011) 3021–3025. [9] X. Wu, Y. Yang, X. Chi, T. Han, M. Hanif, L. Liu, Z. Xie, X. Chen, Y. Ma, An ultra thin gold nanoparticle layer modified cathode for the enhanced performance of polymer light emitting diodes, J. Mater. Chem. C 3 (2015) 9928–9932. [10] Y. Lei, H.M. Jia, W.W. He, Y.G. Zhang, L.W. Mi, H.W. Hou, G.S. Zhu, Z. Zheng, Hybrid solar cells with outstanding short-circuit currents based on a room temperature soft-chemistry strategy: the case of P3HT: Ag2S, J. Am. Chem. Soc. 134 (2012) 17392–17395. [11] W.W. He, H.K. Kim, W.G. Wamer, D. Melka, J.H. Callahan, J.J. Yin, Photogenerated charge carriers and reactive oxygen species in ZnO/Au hybrid nanostructures with enhanced photocatalytic and antibacterial activity, J. Am. Chem. Soc. 136 (2014) 750–757. [12] W.Y. Ji, P.T. Jing, J.L. Zhao, Improving the efficiency and reducing efficiency rolloff in quantum dot light-emitting devices by utilizing plasmonic Au nanoparticles, J. Mater. Chem. C 1 (2013) 470–476. [13] W.L. Barnes, A. Dereux, T.W. Ebbesen, Surface plasmon subwavelength optics, Nature 424 (2003) 824–830. [14] Y.D. Zhang, S.J. Wang, L. Chen, Y. Fang, H.B. Shen, Z.L. Du, Solution-processed quantum dot light-emitting diodes based on NiO nanocrystals hole injection layer, Org. Electron. 44 (2017) 189–197. [15] E. Hao, G.C. Schatz, Electromagnetic fields around silver nanoparticles and dimers, J. Chem. Phys. 120 (2004) 357–366. [16] N. Calander, M. Willander, Theory of surface-plasmon resonance optical-field enhancement at prolate spheroids, J. Appl. Phys. 92 (2002) 4878–4884. [17] X. Gu, T. Qiu, W. Zhang, P.K. Chu, Light-emitting diodes enhanced by localized surface plasmon resonance, Nanoscale Res. Lett. 6 (2011) 1–12. [18] H. Nakanotani, C. Adachi, Organic light-emitting diodes containing multilayers of organic single crystals, Appl. Phys. Lett. 96 (2010) 053301–053303.
4. Conclusions In summary, Ag NWs-NiO NCs thin film was successful prepared as a HIL role in QLEDs. The Ag-NiO based QLED shows great enhancement on the performance such as luminance, leakage current, turn-on voltage and EQE, CE and lifetime. Particularly, the EQE has increased from 3.04% to 7.14% after introducing Ag NWs in NiO NCs. The significant improvement may be attributed to the good electrical properties of Ag NWs and the suppressed Auger recombination due to the enhanced exciton radiative rate in the emitting layer, which stems from the LSPR-induced nearfield enhancement with the incorporation of Ag NWs dopants. The Ag NWs is a promising HIL intensifier candidate in QLED device. Acknowledgements This work was financially supported by the Key Scientific Research Project of Henan Province Funded Plan (Grant no. 18A150052).
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Contents lists available at ScienceDirect
Physica E: Low-dimensional Systems and Nanostructures journal homepage: www.elsevier.com/locate/physe
Enhanced performance of green quantum dots light-emitting diodes: The case of Ag nanowires Yidong Zhang a, *, Zhiwei Li b a Key Laboratory for Micro-Nano Energy Storage and Conversion Materials of Henan Province, Institute of Surface Micro and Nano Materials, Xuchang University, Xuchang, 461000, PR China b College of Chemistry and Chemical Engineering, Zhoukou Normal University, Zhoukou, Henan Province, 466001, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ag nanowires (NWs) NiO nanocrystals (NCs) thin film Quantum dots light-emitting diodes (QLEDs)
In this work, The performance of green quantum dots light-emitting diodes (QLEDs) were enhanced by dispersing Ag nanowires (NWs) into NiO hole injection layer (HIL). Highly bright green QLEDs with a maximum luminance of 32780 cd m2 and current efficiency of 4.1 cd A1, exhibiting 50% improvement compared with the device without Ag NWs. The improved performance may be attributed to the significant increase in the conductivity and hole injection rate as a result of the introduction of Ag NWs and the good matching between the resonance frequency of the localized surface plasma resonance (LSPR) generated by NWs and QDs, as well as the suppressed Auger recombination of QDs layer due to the LSPR-induced near-field enhanced radiative recombination rate of excitons.
1. Introduction Colloidal quantum dots (QDs) are promising light-emitting materials thanks to their tunable wavelength and narrow full-width-at-halfmaximum (FWHM) [1]. Since the first demonstration by Allvisatos group [2], the performance of QLEDs has been enhanced greatly because of the optimization of the device architecture [3]. As one of the most widely applied hole injection material in QLEDs, NiO has caused great attention owing to its low cost, low surface roughness, high stability and high transparency [4]. However, NiO is a kind of Mott insulator with low conductivity and carrier mobility, which is the largest bottleneck for the balance of carrier transportation [5]. In order to improve the electric properties of NiO, monovalent atoms such as Liþ and Kþ were incorporated into NiO crystal lattice by magnetron sputtering method due to the increasing amount of nickel vacancies [6]. For example, the resistivity of NiO can be decreased to 1 Ω cm after doping a certain amount of Li and K atoms [7]. Yang et al. also fabricated transparent p-type conductive Ni0.9Cu0.1O thin films by pulsed plasma deposition (PPD) method, exhibiting the highest conductivity of 5.17 S cm1, with an average transmittance of 60% in the visible region [8]. Recently, nobel metal nanoparticles (Au, Ag) attracts more attention due to its excellent optical properties with localized surface plasmon resonance (LSPR) effect in the applications of QLEDs, solar cells and photocatalysis [9–11]. For
example, plasma-enhanced green QLEDs by incorporating solution processed Au NPs into device to obtain the doubled luminance and current efficiency [12]. Since LSPR strongly affects the kinetic characteristics of nearby molecules. Au or Ag NPs induce a strong enhancement of the local electromagnetic field intensity close to the NPs. The electric field vector decays exponentially with distance from the metal surface, with a decay length of the order of one half of the excitation wavelength. When a fluorescent emitter layer is placed within the proper range of the enhanced local electric field intensity, plasmonic interaction takes place, which can enhance the radiative decay rates of the fluorescent species [13]. Enlightened by the idea, for the first time, Ag nanowires (NWs) were introduced to the NiO NCs film as HIL in QLED to improve the device performance. (see Table 1) In this work, Ag NWs are introduced into the NiO HIL to enhance its conductivity, and the effect of LSPR triggered by Ag NWs on QLED is investigated. The lifetime of the QD sample emission band matching the LSPR frequency is shortened, and the overall device performance is enhanced, which is ascribed to the suppression of Auger recombination of QDs by LSPR induced by Ag NWs and enhanced holes transport ability. By using our present strategy, highly bright green QLED with peak luminance up to ~32780 cd m2, and current efficiency of 4.1 cd A1 are achieved successfully based on Ag NWs.
* Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Zhang). https://doi.org/10.1016/j.physe.2018.03.008 Received 18 January 2018; Received in revised form 23 February 2018; Accepted 7 March 2018 Available online 8 March 2018 1386-9477/© 2018 Elsevier B.V. All rights reserved.
Y. Zhang, Z. Li
Physica E: Low-dimensional Systems and Nanostructures 101 (2018) 11–15
2.4. Preparation of ZnO NCs
Table 1 Summary of the QLED performance with and without Ag NWs.
NiO Ag NWsNiO
L (cd m2)
J (mA cm2)
CE (cd A1)
EQE (%)
Lifetime (ns)
24540 32780
326 184
1.3 4.1
3.04 7.14
28.64 22.16
In a typical process, 0.66 g Zn(Ac)2 in 30 ml dimethyl sulfonate (DMSO) and 10 g tetramethylammonium hydroxide (TMAH) in 100 ml ethanol were mixed and stirred for 1 h in ambient atmosphere. The prepared product was collected by centrifugation and then washed with the mixture solution of ethanol and n-heptane (volume ratio 1: 4) three times. Finally, the precipitate was re-dispersed in ethanol to form a ZnO nanoparticles solution with a concentration of 30 mg ml1.
2. Experimental method
2.5. Fabrication of the QLED
All the reagents used in the experiments were in analytic grade (purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd) and used without further purification.
The patterned ITO substrates with a sheet resistance of ~20 Ω sq1 were ultrasonically cleaned subsequently with deionized water, acetone, and isopropanol for 15 min, respectively, followed by treating with ozone generated by ultraviolet light in air for 15 min. Then the prepared substrates were spinning-coated by Ag NWs- NiO NCs at a rotation speed of 3000 rpm, followed by an annealing process at 60 C for 15 min in air. Then the substrates were transferred to a N2-filled glove-box for spincoating of the TFB, CdS/CdSe/ZnS QDs, and ZnO NCs layers. The TFB hole transport layer (HTL) was spin-coated at a speed of 3000 rpm for 45 s, then annealing at 150 C for 30 min. The spin speed for QD emitter layer (15 mg ml1, toluene) and ZnO HTL (30 mg ml1, ethanol) were 2000 rpm and 3000 rpm, respectively, followed by annealing at 60 C for 30 min. These multilayer samples were then loaded into a custom highvacuum chamber (pressure, ~3 107 Torr) to deposit the top Al cathode (100 nm thick) patterned by an in situ shadow mask to form an active device area of 4 mm2.
2.1. Preparation of Ag nanowires Typically, 60 mL 0.001 mM NaCl in ethylene glycol was heated in oven at 160 C for 1 h. Then, 0.4 g poly-vinylpyrrolidone (PVP) was poured into the system followed by 0.25 g AgNO3 crystal addition. Thereafter, the solution is heated for 2 h, followed by separation and purification of Ag NWs using acetone and water, respectively. Finally, Ag NWs were suspended in water with the concentration of 2 wt.%.
2.2. Preparation of NiO nanocrystals NiO nanocrystals (NCs) were prepared according to the method reported by Liang et al.7 on the top of Typically, 0.5 mmol Ni(St)2, 0.2 mmol LiSt, 3 mmol ODA and 5 ml 1-octadecene (ODE) were loaded in a three-neck flask and degassed at 80 C for 30 min. The reaction mixture was then heated to 235 C and kept at this temperature for 3 h under argon flow. The products were precipitated out by adding a mixture of ethanol and ethyl acetate and further purified by dispersing/precipitating twice using the combination of hexane/ethanol. Finally, a certain amount of Ag NWs were dispersed into the NiO ethanol suspension (1 mg ml1).
2.6. Characterization and electrical luminance performance test The samples were characterized by atomic force microscopy (AFM) (Icon, Bruker Instruments Inc.). An Agilent 4255C equipped with a calibrated Newport silicon photodetector was used to measure the current-luminance-voltage characteristics. The electroluminescence spectra were recorded using an Ocean Optics high-resolution spectrometer (HR4000). The devices were measured under ambient conditions without encapsulation.
2.3. Preparation of QDs 3. Result and discussion For a typical synthesis of CdSe/ZnS QDs, 0.2 mmol of CdO, 4 mmol of zinc acetate and 5 ml of oleic acid (OA) were placed in a 100 ml threenecked bottle and heated to 150 C in flowing high-purity argon for 30 min. Then 15 ml of 1-octadecene (ODE) was added to the flask and the temperature was elevated to 300 C. A stock solution containing 0.2 mmol of Se and 4 mmol of S dissolved in 2 ml of trioctylphosphine (TOP) was quickly injected into the flask to react for 6 min. The resulting QDs were washed several times and finally dispersed in toluene form a suspension on the concentration of 15 mg ml1.
3.1. Characterization of Ag NWs, NiO NCs, ZnO and QDs Fig. 1a and b shows the TEM images of the prepared NiO NCs. It can be seen that the NiO NCs are uniform with the average diameter of 3 nm. High resolution TEM image is shown in the inset of Fig. 1a. The selected area electron diffraction (SAED) pattern obtained by focusing the electric beam on NiO particles is displayed in the inset of Fig. 1b, which shows diffused rings indicating that the NiO particles are constructed of NiO
Fig. 1. TEM images of NiO NCs: high magnification (a), HRTEM image (inset a), low magnification (b), and SAED pattern (inset b), AFM image of Ag NWs on quartz substrate: high magnification (c) and low magnification (d).
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Physica E: Low-dimensional Systems and Nanostructures 101 (2018) 11–15
Fig. 2. TEM image of ZnO NCs (a); QDs (b), PL and UV–vis absorption spectrums of the QDs (c), the digital light photo of the QDs with the irradiation of UV light (inset c), band gap of the QDs (d).
Fig. 3. Schematic diagrams of the Ag NWs NiO-based QLED (a), energy level alignment of the QLED (b).
3.2. Electroluminescence performance of QLEDs
NCs. Fig. 3c and d displays the AFM image of the prepared Ag NWs on quartz substrate, showing that the average diameter and length is 20 nm and 10 μm, respectively, with high aspect ratio. The average size of the QDs spin-coating of ZnO NCs and QDs is ~2.5 nm and ~6 nm, respectively, as shown in Fig. 2a and b, respectively. The typical absorption wavelength and emission wavelength of the QDs is 510 nm and 519 nm, respectively, as shown in Fig. 2c. The digital light photo of the QDs with the irradiation of UV light is shown in the inset of Fig. 2c. The optical band gap energy (Eg) of the green QDs was ~2.38 eV estimated by the extrapolation method, as shown in Fig. 2d.
The QLED architecture is schematically displayed in Fig. 3a. Specifically, the multilayer structure is consisted of patterned ITO, Ag-NiO, QDs, ZnO NCs and Al, in which serve as the anode, HIL, HTL, emitting layer, ETL, and cathode, respectively. Fig. 3b illustrates the corresponding energy level diagram of the QLED. Fig. 4a shows the J-V characteristic curve of the QLEDs, indicating that the leakage current will decreased after introducing Ag NWs in NiO layer due to the balanced carrier transportation, i.e. the relative carrier transportation balance can 13
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Physica E: Low-dimensional Systems and Nanostructures 101 (2018) 11–15
Fig. 4. Current density-voltage (J–V) and luminance-voltage (L–V) characteristic curves of the QLEDs (a), the current efficiency as a function of the luminance (b), and the inset (b) is the photograph of the green light emission of the QLED at an applied voltage of 4.5 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. J–V characteristic curves of hole-only devices (a), PL decay curves of ITO/Ag-NiO/TFB/QDs films (b).
Time-resolved PL measurements were performed to further investigate the PL lifetime of QDs, as shown in Fig. 5b. The average PL lifetime of ITO/NiO/TFB/QDs film decreases from 28.64 to 22.16 ns after introducing AgNWs into HIL due to the LSPR induced by the Ag NWs in HIL. It indicates that adding Ag NWs into HIL shortens the lifetime of the excitons in the QD-emitting layer, enhances the radiative recombination rate of excitons, and thus reduces the Auger recombination caused by QDs charging effectively due to the decreased concentration of excitons in the emission layer [15]. When Ag NWs were dispersed in NiO as HTL, Ag NWs will induce a strong enhancement of the local electromagnetic field intensity close to the NiO NCs [16]. The electric field vector decays exponentially with distance away from the metal surface, with decay length of the order of one half of the excitation wavelength [17]. If the green QDs with PL peak at 519 nm is located within the proper range of the enhanced local electric field, plasmonic interaction will take place, leading to the shorten radiative lifetime of the QDs and enhance the radiative decay rates of the fluorescent species. Usually, the lifetime of 1 excitons is defined as: τ ¼ kr þk , where ‘kr’ and ‘knr’ is the radiative rate nr
be obtained. Also, the luminance can be greatly enhanced from 24540 to 37580 cd m2 after introducing Ag NWs in NiO layer. Further, we can observe that the threshold voltage decrease from 3.2 V to 2.2 V after introducing Ag NWs in NiO layer, indicating that the Ag NWs can decrease the hole injection barrier. The current efficiency (CE) as a function of the luminance is shown in Fig. 4b, obviously, the CE of QLED with Ag NWs is much higher than that of standard QLED, particularly, the device with Ag NWs shows low efficiency roll-off. The inset of Fig. 4b is the photograph of the Ag-NiO based QLED at an applied voltage of 4.5 V. In order further investigate the effect of Ag NWs on the device performance, the hole-only devices of Au/HTL/HIL/ITO structures were further characterized to investigate the holes injection and transport capability of the QLEDs. In this device structure, as shown in the inset of Fig. 5a, it is expected that the electron injection from the Au electrode to the TFB is well obstructed by the large injection barrier at TFB/Au interface. Thus, the holes injection and transport capability can be exclusively characterized by measuring the J-V characteristic curves. The J-V curves of hole-only devices presented different electrical features with that of QLEDs [14]. The Ag NWs-based QLED showed higher current density, indicating that the Ag NWs contributed to the high holes conduction capability, as shown in Fig. 5a. Since the hole is the minority carrier in our QLED device, the enhancement of hole currents in Ag NWs-based QLED will make a better balance between the hole and the electron injection, leading to a substantial luminous-efficiency increase compared to the standard device.
and the non-radiative rate, respectively. The ‘knr’ consists of two terms: one originates from the intrinsic nonradiative decay (ki-nr) in QD particles and the other (kt-nr) originates from the resonance coupling effect between QDs and Ag NWs. The coupling effect between Ag NWs and excitons results in an increase of kr and kt-nr, leading to a reduced lifetime of excitons in QDs. The quantum yield (Q) of QDs can be calculated ackr cording to the formula: Q ¼ kr þk . The increased ‘kr’ and ‘kt-nr’ will nr 14
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Fig. 6. Impedance spectroscopy (a) and voltage-capacitance characteristics (b) of the QLEDs with and without Ag NWs.
enhance the ‘Q’ of QDs, i.e., the electron-hole recombination in QDs is improved [18]. An improved recombination implies that more photons will be generated in the QDs, resulting in the enhanced luminance of device. Further, the coupling between Ag NWs and QDs probably increase the light extraction efficiency so as to increase the luminance. To further explore the role of the Ag NWs on the electrical performance of QLEDs, we carried out impedance spectroscopy (IS) measurements, as shown in Fig. 6a. We can see that the impedance of the device without Ag NWs decreases dramatically when the bias is higher than 0.5 V. Combined with the luminance-voltage data shown in Fig. 4a, we can deduce that this decrease of the impedance is not a result of the recombination of the electrons and holes due to the low bias below the VT (~3 V), and it should be attributed to the leakage current across the devices. Also, the increase of the current phase is not from the electronhole recombination, but from the leakage of the carriers. Fortunately, this decreasing behavior of impedance is alleviated after introduction of Ag NWs, which demonstrates that the Ag NWs can suppress electron leakage into the NiO layer. Fig. 6b shows the voltage-capacitance (V-C) characteristics of the QLEDs Similarly, the capacitance of the QLED without Ag NWs decreases sharply when the bias voltage is more than 0.5 V, however, it will be more stable after introducing Ag NWs in NiO, further indicating that a large leakage current exists in QLEDs, which can be suppressed by Ag NWs.
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4. Conclusions In summary, Ag NWs-NiO NCs thin film was successful prepared as a HIL role in QLEDs. The Ag-NiO based QLED shows great enhancement on the performance such as luminance, leakage current, turn-on voltage and EQE, CE and lifetime. Particularly, the EQE has increased from 3.04% to 7.14% after introducing Ag NWs in NiO NCs. The significant improvement may be attributed to the good electrical properties of Ag NWs and the suppressed Auger recombination due to the enhanced exciton radiative rate in the emitting layer, which stems from the LSPR-induced nearfield enhancement with the incorporation of Ag NWs dopants. The Ag NWs is a promising HIL intensifier candidate in QLED device. Acknowledgements This work was financially supported by the Key Scientific Research Project of Henan Province Funded Plan (Grant no. 18A150052).
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