Brightness enhancement of a direct-current-driven electroluminescent device prepared with zinc-sulfide powder

Journal Pre-proof Brightness enhancement of a direct-current-driven electroluminescent device prepared with zinc-sulfide powder Wonhee Lee, Hong-Kun Lyu, Hui-Sup Cho, Seung Eui Lee, Byeongdae Choi PII:

S0022-2313(19)31306-7

DOI:

https://doi.org/10.1016/j.jlumin.2019.117015

Reference:

LUMIN 117015

To appear in:

Journal of Luminescence

Received Date: 1 July 2019 Revised Date:

26 December 2019

Accepted Date: 28 December 2019

Please cite this article as: W. Lee, H.-K. Lyu, H.-S. Cho, S.E. Lee, B. Choi, Brightness enhancement of a direct-current-driven electroluminescent device prepared with zinc-sulfide powder, Journal of Luminescence (2020), doi: https://doi.org/10.1016/j.jlumin.2019.117015. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Brightness enhancement of a direct-current-driven electroluminescent device prepared with zinc-sulfide powder Wonhee Leea, Hong-Kun Lyub, Hui-Sup Chob, Seung Eui Leea,*, and Byeongdae Choib,** a

Department of Advanced Material, Korea Polytechnic University, Si-heung 15073, Korea Division of Electronics and Information System, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Korea

b

E-mail: *[email protected], **[email protected], Keywords: electroluminescence, direct current, energy barrier, hole injection layer, ZnS powder Abstract In this paper, we report an improved luminous efficiency of a direct-current-driven electroluminescent (EL) device using ZnS powder by controlling electron and hole injection barriers. The emission layer was fabricated by screen-printing of an EL paste using Cu-coated ZnS particles on a transparent conductive indium–tin-oxide electrode. The device was completed by stacking an Al thin film on the emission layer by thermal evaporation. We applied various forming voltages to investigate the current–voltage–luminescence characteristics of the device. An abrupt change in current during the forming process was caused by variation of the electron-blocking barrier toward the anode. The hole-injection barrier from the anode to the emission layer increased with the forming voltage. The obtained results indicated that the hole-injection barrier to the emission layer can be lowered by inserting materials with a low highest-occupied-molecular-orbital (HOMO) energy level between the anode and emission layer. We achieved a brightness enhancement of 29% by employing a thin film of poly-N-vinyl carbazole with a HOMO level of -5.6 eV as a holeinjection layer on the anode. We believe that the presented results will guide further studies for efficiency improvement of EL devices using powder particles applicable to nextgeneration lighting and displays. 1. Introduction An electroluminescent (EL) device using powder particles, in which individual particles act as individual light-emitting sources, has the advantage of device characteristic preservation upon bending or stretching deformations. This property is one of the advantages of powderbased EL devices, promising in the field of lighting technology for next-generation devices such as artificial skin for a humanoid and deformable light-emitting device. [1–10] So far, studies on flexible EL using organic-inorganic hybrid structures have been actively conducted. Davies et al. [11] reported a low-cost binder material for alternative current electroluminescent (ACEL) lamps, and Silver et al. [13, 14] reported a flexible EL lamps applicable to a display. In addition, the printing technique can be applied to the manufacturing process. [15] Recently, an ultraviolet-(UV)-curing-based paste, which can provide device fabrication process at room temperature, has been developed, enabling cost competitiveness in the manufacturing process, compared to organic light-emitting diodes (OLEDs) based on thin films. [16, 17] However, powder-based EL devices are not widely used as they are driven by an alternating current (AC) of a high driving voltage; therefore, they are relatively inefficient compared to direct-current-(DC)-driven devices. In recent

years, various approaches, such as structure optimization of the AC-driven EL device, have been employed to overcome the drawbacks of a low luminous efficiency and high driving voltage. As a typical approach, introduction of a dielectric material into the device to maximize the discharge effect has been proposed. In addition, recently, it has been reported that the luminescence brightness can be improved by applying a retroreflective electrode. [18-20] Nevertheless, the performance of the EL device is still insufficient for practical use. From a fundamental point of view, the AC-driven EL devices have a limitation in achieving low-voltage driving and efficiency comparable to DC-driven devices such as light-emitting diodes (LEDs) and OLEDs.

Fig. 1. Comparison of device ELs with different driving voltage waveforms: (a) AC and (b) DC voltages. As shown in Fig. 1, which compares the luminescent characteristics of AC- and DC-driven EL devices, the AC-driven EL device emits light intermittently, only when the applied voltage is inverted (Fig. 1a). This occurs as light is generated when the ground-state electrons in the light-emitting body undergo excitation and transition to the ground state; therefore, when a large voltage capable of exciting the ground-state electrons is momentarily induced by reversing the voltage applied to the device, the device emits light. [21] Owing to this phenomenon, the emission intensity increases proportionally to the applied voltage and frequency; a high voltage and high-frequency AC are required to obtain a high brightness. In the case of the DC-driven EL device (Fig. 1b), as light is generated upon the combination of electrons and holes injected to phosphors, light is uniformly emitted during the voltage application. In principle, the DC driving provides continuous light emission and higher luminous efficiency than the AC driving. Therefore, development of an efficient DC-driving technique can be an excellent approach for the practical application of a powder-phosphor EL device. A research group has reported several issues important for applications, such as lifetimes and color conversion strategies. [22] One major problem with DC-driven powderbased EL devices using flexible substrate is that the device should be encapsulated to keep out atmospheric oxygen and water. Studies on thin film passivation technology have been being conducted to prevent degradation of organic light emitting device (OLED) from atmospheric oxygen and water, and the results can be used to solve the above-mentioned problem in DC powder EL devices. [23-26] On the other hand, ZnS is a wide bandgap semiconductor which is widely used as an electroluminescent material because it is easier to manufacture than a multicomponent fluorescent material and has excellent price competitiveness. In this study, we fabricated a DC-driven EL device using ZnS powder as the phosphor and controlled the energy barrier of electrons and holes to improve the luminous efficiency of the EL device. [27]

2. Experimental Section 2.1 EL powder preparation In order to prepare the 4-wt%-Mn-doped ZnS, we mixed 50 g of ZnS powder, 50 ml of deionized (DI) water, 3 g of MnCl2, and 50 g of zirconia balls (2 mm) in a ball mill pot and milled the mixture for 24 h. It was then dried in a convection oven for 24 h before annealing for 1 h at 1100 °C in N2 atmosphere. After addition of 0.1 g of CuSO4 and 0.1 g of ZnO, we performed another annealing for 2 h at 800 °C to induce the transition from wurtzite to sphalerite. In order to provide conductivity, the ZnS powder was reacted with a copper acetate solution prepared by mixing 10 ml of copper acetate and 20 ml of DI water, which was then washed and dried. 2.2 EL device fabrication The EL paste was prepared by mixing the ZnS powder with a binder (DOW chemical, Ethyl Cellulose 5%) at a ratio of 2:1. We fabricated the EL device with a thickness of 20 µm by screen-printing of the EL paste on an ITO-coated glass substrate. The PVK (Mn: 25000– 50000) solution was prepared by dissolving 0.3 g of PVK in 10 ml of toluene. The PVK film was formed by spin-coating. After dropping the solution onto the substrate, we performed the spin-coating at 500 rpm for 5 s and 2000 rpm for 15 s, followed by drying at 80 °C in a convection oven for 10 min to form an 800-nm-thick film on the ITO surface. The EL device with the PVK layer was fabricated by screen-printing of the EL paste and subsequent deposition of an Al thin film thereon. In order to stabilize of the luminescence properties of the prepared devices, the forming process was carried out; A set DC voltage was applied to device and held for 10 min. The forming process was performed at various DC voltages of 10, 20, 30, 70, 90, 110, and 130 V, respectively. 2.3 Characterization The crystalline structures of the powders were evaluated with an X-ray diffractometer (XRD Bruker, D2 PHASER, Cu Kα, λ = 0.15406 nm). The cross-sectional and surface images were obtained using a high-resolution field-emission scanning electron microscope (HR-FESEM SU8020, Hitachi). The PL properties were evaluated at room temperature (excitation wavelength: 300 nm) with a spectrofluorometer (JASCO FP-6500, light source: Xe lamp). The electroluminescent properties of the devices were evaluated by an I–V–L system equipped with a function generator (Rigol, DG4062), voltage amplifier (Pintek, HA405), high-performance digital multi-meter (Keithley, 2001), and spectrometer (Photo research, PR-655). 3. Results and discussion 3.1 Photoluminescence of ZnS:Mn powder The emission wavelengths of ZnS can be shifted from UV- to visible-light wavelengths through doping of transition metal ions. The central wavelength of light emission can be shifted from the blue- to the red-wavelength region depending on the ion types. [28–31] In this study, we doped ZnS powder with Mn ions (4 wt%) to shift the emission center to the yellow-wavelength region. In addition, as the luminous efficiency of sphalerite is higher, we transformed the structure of ZnS from wurtzite to sphalerite [32] through a second heat treatment to obtain improved luminous characteristics; the results are shown in Fig. 2 and 3.

Fig. 2. Characteristics of the Mn-doped ZnS powders for different concentrations of MnCl2. (a) X-ray diffraction patterns (b) Photoluminescence spectra.

Even though no significant phase change of ZnS particles by the doping with 4 wt% Mn was observed (Fig. 2a), the structure of the ZnS powder was transformed from wurtzite to sphalerite by the incorporation of ZnO and CuSO4, as shown in Fig. 3a and 3b. The XRD peaks of the (101), (002), (011), and (012) planes of wurtzite shown in Fig. 3a are significantly decreased, while the (111), (002), and (113) peaks of sphalerite became significant (Fig. 3b) after the heat treatment. The SEM image in Figure 3c shows the shape of the prepared particles. Fig. 3d and 3e present a digital photography of orange emission and PL spectra of ZnS:Mn powders before and after phase transition, respectively. The results show that the emission wavelength of ZnS was shifted to 585 nm by the doping of Mn ions, and that the luminance was improved over 60% by the phase transition, as intended.

Fig. 3. Characteristics of the Mn-doped ZnS powder after phase transition from wurtzite to sphalerite. X-ray diffraction (XRD) patterns of the Mn-doped ZnS powder (a) before annealing for phase transition and (b) after annealing at 800 °C. (c) Scanning electron microscopy (SEM) image of the Mn-doped ZnS powder after the annealing. (d) Photoluminescence (PL) of the Mn-doped ZnS powder after the annealing. (e) Comparison of the PL intensities of the Mn-doped ZnS powders of wurtzite and sphalerite.

3.2 DC-driven EL devices prepared by ZnS:Mn powder On the other hand, in order to emit light by DC, electrons and holes must be supplied to the phosphor host through the anode and cathode electrodes, respectively.

Fig. 4. Preparation of a Cu-coated ZnS powder and EL device. (a) Schematic illustration of the Cu coating on the ZnS powder and fabrication of the EL device. (b) Cross-sectional image of the powder-based EL device. However, as the phosphor host is a semiconductor, it is difficult to flow electrons and holes directly toward ZnS particles. In order to transfer electrons and holes, it is necessary to apply a conductive material to the surfaces of the phosphor particles. In the case of ZnS, copper can be applied to the surfaces of the particles to form electron and hole transport and injection layers. We reacted copper acetate with ZnS so that Cu is coated on the surface. Figure 4a illustrates the detailed procedure. First, copper acetate is mixed with deionized (DI) water to prepare a coating solution, and then heated at 100 °C to activate copper acetate. The ZnS powder is then added to the solution and reacted to precipitate Cu on the ZnS surface. After the reaction, the powders, which were washed and dried, are mixed with a binder to prepare an EL paste. A DC-driven EL device was fabricated by printing the EL paste on a glass substrate coated with indium tin oxide (ITO) and depositing an Al electrode thereon (Fig. 4b).

Fig. 5. Current–voltage–luminescence (I–V–L) curves of the EL device as prepared. Figure 5 shows the current and luminescence characteristics of the fabricated device as a function of the voltage. Contrary to the AC-driven EL, the current increases exponentially with the voltage and then significantly decreases after a certain value. This phenomenon

occurs in the forming process for light emission in a DC-driven EL device, attributed to resistance change due to movement of Cu ions. [33] However, it has not been fully explained; a detailed understanding of the relation between the current and EL in the device is crucial to control the luminescence properties of the device. In order to investigate the dependence of the EL on the current, we simultaneously observed the current and luminescence changes (Fig. 5a, closed triangles) of the device as a function of the voltage. The luminescence increases proportionally to the voltage regardless of the current drop. An ideal DC-driven light-emitting device should have an increase in brightness proportional to the current increase. The abrupt change in the current in our device implies that it does not simply occur owing to the combination of electrons and holes. The simplified energy band diagram in Fig. 6 shows that the device has a symmetric energy band structure centered on ZnS at equilibrium (Fig. 6c). When a forward bias is applied, the energy band on the anode is lowered, while the band on the cathode side is raised (Fig. 6d), so that electrons and holes move toward the light-emitting body. Simultaneously, some electrons are combined with holes emitting light, while the remaining electrons leak to the anode as the barrier at the anode side is lowered. Furthermore, when the specific voltage is reached, the conduction band level of Cu2S on the anode becomes lower than that of ZnS. A large amount of electrons then moves to the anode without combining with the holes at the light-emitting layer, which leads to an abrupt increase in the current. On the other hand, it is known that mass transfer of Cu ions occurs when a high voltage is applied to the device. Cu combines with S to form various intermetallic compounds including CuS, Cu1.96S, Cu1.75S, and Cu2S. The phase diagram of CuS–Cu2S in Figure 6a shows that solid solutions can be also formed at Cu composition ratios in the range of 1 to 2. [34] According to previous reports, the band-gap energy (Eg) of copper sulfide increases with the increase in the x value of the bulk copper sulfide (Fig. 6b); Eg = 1.2 eV for Cu2S, 1.35 eV for Cu1.96S, 1.75 eV for Cu1.8S, 2.11 eV for Cu1.75S, and 2.2 eV for CuS. [35, 36]

Fig. 6. Schematic illustrations of the energy barrier transition of Cu2-xS by migration of Cu ions. (a) Phase diagram of the Cu–S system. (b) Band-gap

transition according to the stoichiometry of Cu. (c) Band structure of the EL device at the equilibrium state. (d) Band structure at a forward bias. (e) Increase in the energy barrier blocking electron leak toward the anode by the migration of Cu ions. (f) Band structure of the EL device after the forming process. The mass transfer of Cu ions by the applied voltage induces composition unbalance of Cu2-xS at both electrodes. This causes the variation in the energy barrier of Cu2-xS at the anode. When Cu2-xS on the anode side is changed to CuS, the conduction band minimum (CBM) increases from -3.6 eV to -1.94 eV (see Figs. 6b, 6e, and 6f), so that CuS acts as a barrier layer of electrons moving to the anode beyond the ZnS layer.

Fig. 7. Electro-optical properties of the EL device. (a) L–V curves for different forming voltages. (b) The variation of Vth by the forming voltage. The validity of this explanation was verified by the emission characteristics of the device according to the forming voltage. Figure 7a shows the L–V characteristics of the DC EL devices observed after application of forming voltages of 70, 90, 110, and 130 V for 10 min. At a larger forming voltage, the L–V curve shifts to the right side. As shown in Fig. 7b, threshold voltage (Vth) for electroluminescence increases proportionally to the forming voltage. This is attributed to the resistance increase of the device due to the decrease in the Cu content in the Cu2-xS layers on the anode with the increase in the forming voltage. The resistivity of Cu2-xS significantly depends on the Cu content. [37] 3.3 Improvement of EL efficiency by modifying the electrode layer Then, the presented results suggest that Vth can be further decreased by lowering the forming voltage. As expected, when we performed forming processes at voltages of 10, 20, and 30 V, Vth shifted to the left, as shown in the inset of Fig. 8a. The L–V characteristics of the devices with different forming voltages are shown in Fig. 7 and 8a. Furthermore, the left-shift of Vth indicates that the emission intensity can be improved by the forming voltage. However, when the forming process is performed at a voltage lower than a certain value, we cannot anticipate efficiency improvement, as the mass transfer of Cu ions might be negligible and consequently the energy barrier preventing anode leakage of electrons cannot be formed. Another approach to improve the luminescence efficiency is to create an electron leakage barrier with CuS and, in addition, to insert a layer with a low highest-occupied-molecular-orbital (HOMO) energy level on the anode to lower the hole-injection barrier.

Fig. 8. Enhanced performance of the DC-driven EL device by controlling energy barriers. (a) L–V curves for different forming voltages. (b) and (c) Schematics of the EL devices and band diagrams without and with a poly-Nvinyl carbazole (PVK) layer, respectively. (d) L–V curves of the EL devices without and with the PVK layer. (e) Digital photography of the emission of the DC-driven EL device (7×7 passive type, 2 V/µm). This approach is illustrated in Fig. 8b and 8c as schematic diagrams of the device structures and energy band diagrams. Holes might be easily supplied to the light-emitting layer by inserting a material layer with a low HOMO energy value between the anode and CuS layer, as a direct hole tunneling from the hole-injection layer to the light-emitting layer can be induced. In order to confirm this approach, we fabricated an EL device using an ITO substrate coated with an 800-nm-thick PVK with a HOMO energy value of -5.6 eV. Figure 8d compares the L–V characteristics of the devices with and without the PVK layer as the hole-injection layer. The inset of Fig. 8d shows a cross-sectional SEM image of a spin-coated PVK film. The emission characteristics of the device according to the driving voltage exhibit an average improvement of 29% at 2.7 V/µm compared to the device without PVK. The proposed structure is fully compatible with the commercial device manufacturing technology. Figure 8e presents an emission image of the device in a passive matrix type of 7×7 fabricated by conventional coating techniques. 4. Conclusion In summary, we fabricated a high-brightness DC-driven powder-based EL device using a PVK layer as the hole-injection layer. The overcurrent and stabilization in the forming

processes of the EL devices were provided by the energy barrier change accompanying the mass transfer of Cu ions in Cu2-xS. The high voltage applied in the forming process increased the hole-injection barrier and shifted the L–V curve to the right. We applied a p-type semiconductor PVK with a low HOMO energy level as a hole-injection layer; consequently, the brightness was improved by approximately 29% in average at the driving voltage of 2.7 V/µm. The lifetime of a flexible DC-driven powder-based EL device is a significant issue in practical use, and we believe it can be solved by using polymer substrates with multilayer barrier thin films in which water and oxygen permeate less than 10-6g/m2 per day. We proposed a novel approach to improve the luminous efficiency of a powder-based EL device. Acknowledgments This work was partially supported by the Basic Research Program (19-NT-01) through the Daegu Gyeongbuk Institute of Science and Technology (DGIST), funded by the Ministry of Science, ICT, and future planning of Korea and Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1A6A1A03015562). Some of the results in this work were based on the thesis submitted by Wonhee Lee, Korea Polytechnic University, Si-heung, Korea.. References [1] C. Larson, B. Peele, S. Li, S. Robinson, M. Totaro, L. Beccai, B. Mazzolai, R. Shepherd, “Highly stretchable electroluminescent skin for optical signaling and tactile sensing,” Science 351, (2016) 1071–1073. [2] B. Hu, D. P. Li, O. Ala, P. Manandhar, Q. G. Fan, D. Kasilingam, and P. D. Calvert, “Textile-Based Flexible Electroluminescent Devices,” Adv. Funct. Mater. 21 (2011) 305– 311. [3] C. Yang, B. Chen, J. Zhou, Y. Chen, and Z. Suo, “Electroluminescence of Giant Stretchability,” Adv. Mater. 28 (2016) 4480–4484. [4] J. Wang, C. Yan, K. J. Chee, and P. S. Lee, “Highly stretchable and self-deformable alternating current electroluminescent devices,” Adv. Mater. 27 (2015) 2876–2882. [5] F. Stauffer, and K. Tybrandt, “Bright Stretchable Alternating Current Electroluminescent Displays Based on High Permittivity Composites,” Adv. Mater. 28 (2016) 7200–7203. [6] R.C. Webb, A.P. Bonifas, A. Behnaz, Y. Zhang, K.J. Yu, H. Cheng, M. Shi, Z. Bian, Z. Liu, Y.-S. Kim, W.-H. Yeo, J.S. Park, J. Song, Y. Li, Y. Huang, A.M. Gorbach, and J.A. Rogers, “Ultrathin conformal devices for precise and continuous thermal characterization of human skin,” Nat. Mater., 12 (2013) 938–944. [7] Z.-G. Wang, Y.-F. Chen, P.-J. Li, X. Hao, J.-B. Liu, R. Huang, and Y.-R. Li, Flexible graphene-based electroluminescent devices, ACS Nano 5 (2011) 7149–7154. [8] B. You, Y. Kim, B.-K. Ju, J.-W. Kim, “Highly stretchable and waterproof electroluminescence device based on superstable stretchable transparent electrode,” ACS Appl. Mater. Interfaces, 9 (2017) 5486–5494 [9] J. Wang, C. Yan, K.J. Chee, and P.S. Lee, “Highly stretchable and self-deformable alternating current electroluminescent devices”, Adv. Mater., 27 (2015) 2876–2882. [10] C. Schrage, and S. Kaskel, “Flexible and transparent SWCNT electrodes for alternating current electroluminescence devices”, ACS Appl. Mater. Interfaces, 1 (2009) 1640–1644. [11] D.A. Davies, A. Vecht, J. Silver, P. Titler and D.C. Morton, “A Novel Low Cost Binder for Flexible AC Electroluminescent Lamps”, SID Symp. Digest Tech. Papers 2001, 32, 395–397.

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Highlights




The electron blocking layer induces abnormal current drop during forming process.




Stoichiometry changes the band structure of Cu-coated ZnS powder-based EL devices.




Tunneling of holes through the hole injection layer improves luminous efficiency.

Author Statement Wonhee Lee: Investigation. Hong-Kun Lyu: Visualization Hui-Sup Cho: Writing- Reviewing and Editing. Seung Eui Lee: Investigation, Writing- Original draft preparation, Supervision and Byeongdae Choi: Investigation, Writing- Original draft preparation, Supervision

There are no conflicts of interest to declare.