Flexible alternating current electroluminescent devices (ACEL) are more and more popular and widely used in liquid-crystal display back-lighting, large-scale architectural and decorative lighting due to their uniform light emission, low power consumption and high resolution. However, presently how to acquire high brightness under a certain voltage are confronted with challenges. Here, we demonstrate an electroluminescence (EL) enhancing strategy that tetrapod-like ZnO whiskers (T-ZnOw) are added into the bottom electrode of carbon nanotubes (CNTs) instead of phosphor layer in flexible ACEL devices emitting blue, green and orange lights, and the brightness is greatly enhanced due to the coupling between the T-ZnOw and ZnS phosphor dispersed in the flexible polydimethylsiloxane (PDMS) layer. This strategy provides a new routine for the development of high performance, flexible and large-area ACEL devices.
© 2016 Optical Society of America
Lighting and display technologies have gained rapid developments in the 21st century due to global energy crisis. In these technologies, flexible optoelectronic devices with low processing costs and mechanical flexibility are highly attractive for many applications . ACEL as one of lighting and display technologies have attracted great attention in areas of liquid-crystal display , back-lighting, large-scale architectural and decorative lighting  due to their high resolution, good contrast and brightness, uniform light emission, thin profile and low power consumption . Traditional ACEL devices are composed of a phosphor layer, e.g. a Cu-doped zinc sulfide (ZnS:Cu) and a vertically sandwiched structure between two insulators contacted by electrodes. In a traditional ACEL device, when a sufficiently high voltage is applied between the electrodes, electrons trapped on interfaces between the layers are injected into the conduction band of the phosphor, then are accelerated by the field and excite the luminescent centers in the phosphor layer via impact excitation and ionization mechanisms [5, 6]. However, these traditional devices are generally rigid instead of flexible. Fortunately, flexible ACEL devices are fabricated based on the ZnS microparticle-embedded polydime-thylsiloxane (PDMS, ZnS@PDMS) structure, where the insulated flexible PDMS can simplify the device structure and make the ACEL with better flexibility and insulation characteristics in the meantime. This structure has two advantages. First, the insulating PDMS, in which the ZnS:Cu phosphor powders are well dispersed, can effectively prevent the catastrophic dielectric breakdown . Second, the ACEL device consisting of a phosphor layer of Cu-doped ZnS in a flexible PDMS (ZnS:Cu@PDMS) film, vertically sandwiched between two electrodes, have fair robustness and flexibility, long lifetime and high luminance with relatively low power consumption [8–10].
In the other hand, how to improve the brightness of ACEL devices has been the subject of considerable studies over the past decades, and significantly still a great challenge. Till now, various approaches have been investigated to enhance the brightness of the displays by adjusting the phosphor. For example, by using the addition of CNTs in the emission layer, Bae et al showed a considerable improvement of EL performance in the cathodoluminescent phosphor . In the most straight forward approach, Xu and associates added barium titanate (BaTiO3) particles to the emissive light layer [12, 13] to improve dielectric constants, where the electric field is focused on the EL particles, leading to a higher light emission . In another route, by incorporating SiC whiskers into phosphor material, Brandon et al. greatly enhanced the field near the tips of the whiskers to improve light emission . Even so, new routines should be developed to enhance the brightness of ACEL devices.
As stated above, flexible ACEL devices with enhanced electroluminescence are desired to meet application requests. Till now, no one used T-ZnOw to strength the brightness of a flexible ACEL, where the needle point of T-ZnOw is cutting edge and possesses good field emission performance [16–18]. In this study, three flexible ACEL devices with different dopant or dopant concentration in ZnS phosphor are fabricated to emit blue, green and orange lights, respectively. We demonstrate electroluminescence enhancement by adding T-ZnOw to bottom electrode of CNTs through coupling between the T-ZnOw and ZnS phosphor dispersed in the flexible PDMS layer. The optimal weight ratio of T-ZnOw to CNTs in bottom electrode layer is 1:1. The ACEL devices show high luminance and flexibility, working well even under a very large strain.
2.1 Reagents and materials
Phosphors (blue, ZnS:Cu; green, ZnS:Cu; orange, ZnS:Cu:Mn) were bought from ShenZhen Obest Technology Co., Ltd, CNTs powder from Beijing Boyu Gaoke New Material Technology Co., Ltd, polydimethylsiloxane (PDMS) from Sylgard 184, Dow Corning. ITO coated polyethylene terephthalate (PET) film (ITO-PET), sodium Dodecylbenzenesulfonate (SDBS) and Zn power were analytical grade.
2.2 Preparation of T-ZnOw
T-ZnOw were fabricated by chemical vapor deposition. A fixed amount (0.2 g) of Zn powder was placed in a quartz boat in the end of one side of the open quartz tube. The silicon slice was placed at a site 10 cm away from the Zn powder in the furnace. The open quartz tube was inserted into a two-zone horizontal tube furnace at 950 °C for 5 min under a normal atmospheric pressure. After the growth process, we took the reacted bases rapidly out from the 950 °C zone.
2.3 Process of fabricating ACEL
20 mg CNTs, 20 mg SDBS and 40 mL deionized water were added to a 50 ml glass bottle. The mixture was probe ultrasonicated for 30 min and then stirred for 10 min to form uniform solution at room temperature. Afterwards, T-ZnOw were added into the mixture and stirred for 1 hour. At last, the obtained materials were filtered on polytetrafluoroethylene (PTFE) film, washed with deionized water to form the deposited film and dried at 70 °C for 1 h. 2 ml liquid PDMS with a curing agent at a weight ratio of 10:1 were spin-coated on 6 cm culture dish to form film, and then dried at 70 °C for 10 min. The deposited film was pasted on the PDMS film, the PTFE film was peeled up, and the bottom electrode was obtained. The mixture (ZnS:Cu powder and PDMS with various concentrations) was spin-coated onto the bottom electrode and baked at 70 °C for 7 min, on which ITO-PET electrode was pasted, and the flexible ACEL device with a structure of PET electrode/ZnS:Cu@PDMS/T-ZnOw@CNTs embedded PDMS was obtained, as schematically shown in Fig. 1a. In the application, we package the device by using PDMS, as shown in Figs. 6(a) and 6(b).
The morphologies of the sample were observed and analyzed by a scanning electron microscope (SEM, FEI Nova Nano-SEM 450). The XRD patterns were recorded with X-ray diffraction-meter (XRD, PANalytical B.V. X’PertPRO). Emissions were gathered from the contact point by an optical fiber connected to a spectrometer (Ocean Optics QE65pro). Light emissions of the device were measured in the range of 300-800 nm. A function generator (Yokogawa FG 300) connected with a power amplifier (Trek PZD 2000) was used to apply alternating voltage for the ACEL devices. The bending test of the sample was operated on a home-made stretching stage at room temperature.
3. Results and discussion
Figure 1(a) shows a schematic illustration of the flexible ACEL devices (emitting blue, green and orange lights). The device is composed of the bottom and top electrodes with sandwiched ZnS@PDMS emissive layers. Due to the special structure of the T-ZnOw tip, the enhancing electron field on the tip of T-ZnOw will couple with the EL from ZnS phosphor, leading to its enhancement of EL. As well known, ZnS:Cu is a widely available ACEL material with well-studied emission behavior [19, 20]. The ZnS:Cu phosphor particles [Fig. 1(b)] have been well dispersed in the PDMS matrix without any bubbles and voids. The complete coverage of PDMS over the phosphor particles can effectively avoid the catastrophic dielectric breakdown [7, 21]. Therefore, it is unnecessary to insert another dielectric layer between the rear electrode and the light-emitting layer as usual. The mixture of CNTs and T-ZnOw [Fig. 1(c), (d)] with various proportions was vacuum-filtered onto PTFE and then transferred to PDMS to form the flexible bottom electrode. The T-ZnOw are entwined by CNTs [Fig. 1(e)]. The cross-sectional SEM image of the light-emitting device was acquired, as shown in Fig. 1(f). For the whole structure, the ZnS:Cu@PDMS composite layers can harvest the excellent flexibility of the PDMS matrix and the sustained functionality of the emissive material of ZnS:Cu, while the bottom electrode of T-ZnO@CNTs can enhance the emission of ZnS:Cu. Resultantly, a PDMS-based thick powdered ZnS:Cu@PDMS film between the T-ZnOw@CNTs bottom electrode and PET top electrode exhibits enhanced ACEL characteristics.
We mixed phosphor ZnS:Cu with PDMS at various weight ratios as the fluorescent layer, and got various devices. Figure 2(a) shows the EL spectra of the devices with various concentrations of the ZnS:Cu phosphor in PDMS. Compared to the device with lower ZnS:Cu concentration, the higher concentration (2g/ml) device exhibited higher EL intensities because of an increased number of EL-emitting ZnS particles in the film. However, the phosphor ZnS:Cu has a limited solubility in PDMS, and the maximum solubility is 2g/mL, which is the optimum for our EL-emission devices.
In the bottom electrode, the morphology of T-ZnOw entwined by CNTs plays a crucial role to improve the field emission properties [22, 23]. Generally, the surface charge density on a conductor in an electric field is proportional to the surface curvature, so the charges is densely piled up on the tip of T-ZnOw due to the largest curvature and the electric field of the tip is strengthened significantly [18, 24]. When the electric field intensity of the tip increases to a certain degree, the surrounding air molecules will be broken down and a point discharge occurs and may enhance the EL of ZnS phosphor. But it does not mean that more T-ZnOw are better. So, the effect of various weight ratio of T-ZnOw to CNTs in the bottom electrode on the emission performance should be examined. It is shown that the ratio is very important and an appropriate ratio can strengthen the EL to the most. Too high concentration of T-ZnOw will increase the sheet resistance, decrease the electrical conductivity of the bottom electrode, and lower the field emission properties [23, 25–27]. As shown in Fig. 2(b) (200 V, 1 kHz), the brightest luminescence is at the weight ratio of 1:1 of T-ZnOw to CNTs and the luminescence has been enhanced by 60% compared to the device without T-ZnOw addition.
By using the above obtained optimum conditions, we fabricated an ACEL device emitting blue light and accomplish the following experiments. EL spectra at varied applied voltages from 75V to 200 V at a frequency of 1kHz is shown in Fig. 2(c). The EL intensity increases with voltage and the emission spectrum was centered at 450 nm. The emission intensity of the flexible EL device increased with increasing frequency, as shown in Fig. 2(d). Initially, the luminescence flickers at low frequency and gradually gets stable at increasing high frequency. At a frequency of 100 kHz, the intensity increase is saturated and a maximum is obtained (~16 cd/m2) at 200 V [Fig. 2(e)]. As to the characteristics in Fig. 2, the carriers are accelerated by electrical field excitation, and cause excitation or ionization of the luminescent center generating electron-hole pairs. Subsequently, luminescence is produced as a result of the exciton relaxation through radiative recombination of the electron-hole pairs.
The device initiated light emission at the bias voltage of around 75 V after which the emission intensity increased rapidly. The relation between the bias voltages and EL intensity can be evaluated  by the equation: L = L0exp(-β/V1/2), where L is the luminance, V is the voltage, and L0 and β are the constants decided by the devices . After a certain bias voltage, the probability of electrons to be accelerated to a given energy and excite (or ionize) subsequently the luminescent centers will increase steeply, corresponding to the rapid increase in the luminescent intensity. The solid fitting curves in the inset of Fig. 2(e) show that the experiment data agree well with the equation as bias increases. In the other hand, when the frequency of the applied voltage increases, the accelerated times of electrons also increases in a unit time, thus lighting number of times increases. Hence, luminance is enhanced with the frequency increase. However, since the applied voltage is sinusoidal AC signal and the frequency of the applied voltage increases, the width of signal reduces, then the time for electron acceleration becomes short, and the energy of overheated electrons lowers. Therefore, when the frequency increases to a certain degree, the reduction of signal width plays a main role leading to the decrease of luminescent brightness. At last, the increasing frequency will result in an extreme EL brightness due to a balance of increase and decrease of EL brightness with frequency. The emitting blue light at 450 nm can be drawn in the CIE diagram with color coordinates (0.150, 0.747), as shown in Fig. 2(f).
Another ACEL device with green emission [Fig. 3(a)] based on ZnS:Cu phosphor (a different concentration of Cu dopant in ZnS from the above ACEL device with blue emission) are obtained at the above optimum conditions of 2g/ml of ZnS:Cu in PDMS, 1:1 of T-ZnOw to CNTs in the bottom electrode [Fig. 3(b)], the emission is also enhanced to the maximum). At a fixed frequency 1 kHz, the emission intensity (green light) against the applied voltage is plotted in Fig. 4(a). The ACEL device initiates light emission at 75 V and the emission intensity increases rapidly with the increasing applied voltage. Performance of the device at various frequencies under a fixed voltage of 200 V can be observed in Fig. 4b and c. The centre wavelength of the emission spectra exhibits a blue-shift, varying from 502 nm (2 kHz) to 450 nm (100 kHz) with increasing the frequency. The results demonstrate that the frequency not only influence the emission brightness of the device but also the center wavelength of spectrum. This phenomenon is similar to that reported in the literatures [30, 31]. With frequency increase, the emission brightness also increases quickly, then tends to a saturation, as illustrated in Fig. 4(c).
Because of their simple device architecture, low production cost and easily tunable emission colors by using different active dopants or dopant concentrations, ACEL devices, which have attracted persistent interest and been developed for display or lighting applications for decades , can be used to emit mixture light of blue and yellow lights, i.e. orange light by the mixture of Cu and Mn doping activators, i.e. ZnS:Cu and ZnS:Mn phosphors in an ACEL device. The third emission enhanced ACEL emitting orange light [Fig. 3(c)] was obtained, by using the above obtained optimum conditions (2g/ml phosphor in PDMS and 1:1 of ZnOw to CNTs in bottom electrode [Fig. 3(d)]. The emission is also enhanced to the maximum).
The emission intensity (orange light) against the applied voltage is plotted in Fig. 4(d). The light emission also initiates at 75 V and the intensity increases rapidly with the increasing applied voltage. Figure 4(e) and (f) show the performance of the device under various frequencies at a fixed voltage 200 V, where there are two peaks (the main peak located at 581 nm and minor peak located at 450 nm). The minor peak almost disappears at low frequencies. The emission intensity also increases with driving frequency increase, then tends to a saturation, as shown in Fig. 4(f).
Most studies on ZnS have reported single emission spectra with single colors determined by the types of dopants. However, studies of EL have shown tunable emission colors by varying the applied electrical frequency [32, 33]. As shown in Fig. 5(a) [corresponding to Fig. 3 (a) and (b), Fig. 4(a), (b) and (c)], the CIE coordinates vary from green (0.165, 0.320) to blue (0.121, 0.131) by increasing frequency. It is generally accepted that two emission bands of blue and green lights exist in ZnS phosphors, as seen in Fig. 5(b). The green emission may arise from the transition between conduction band and the impurity induced green center of Cu, whereas the blue emission may relate to the blue centers. At high excitation frequencies, a blue-colored EL occurs because of the higher energy excitation. In contrast, at low frequencies the emission is dominated by lower energy levels or green center because the low frequency is insufficient to activate blue emission centers.
Figure 5(c) shows the CIE color coordinates (0.514, 0.464) of the orange light from ZnS:Cu:Mn phosphor of our ACEL [corresponding to Fig. 3(c) and (d), Fig. 4(d), (e) and (f)]. The dominant peak of luminescence spectrum is 581 nm, and a minor shoulder can be found at 450 nm. These spectra features can be understood by the donor-acceptor (D-A) type emission process as schematically shown in Fig. 5(d). The sulfur vacancies (Vs) caused by the lattice dislocation and the Mn2+ substitution to Zn2+ site (MnZn) can separately generate two different donor levels below the conduction band (CB). The acceptor level above the valance band (VB) was generated by the Cu+ substitution to Zn2+ site (CuZn). Thus, the dominant peak at 581 nm (2.13 eV) can be assigned to the D-A recombination of MnZn-CuZn in the energy level diagram, and the shoulder band at 450 nm (2.76 eV) originates from the D-A recombination of VS-CuZn .
We have also investigated the flexibility of ACEL device emitting blue light based on PET coated with tin-doped indium oxide (ITO) thin films as the transparent electrode. The ACEL device is attached to a linear motor moving back and forth, as shown in Fig. 6(a) and (b). The light emitting situations at initial state and bending state can be seen in Fig. 6(c) and (d), respectively. The emission in the bending process remains stable, as shown in the movie (Supplementary Material, Visualization 1). The emission intensity at the 1st bending decreased slightly compared with the initial intensity [Fig. 6(e)]. Neither large decrease in EL intensity [Fig. 6(f)] nor obvious physical damages of the device is observed after 1000 bending cycles. These results indicate the robustness of our ACEL devices in both mechanical and EL emission properties after a long period of operations, making them reliable for practical applications.
In summary, we have demonstrated the fabrication and performance of flexible ACEL devices (blue, green, orange) showing an enhancement with the incorporation of T-ZnOw into the bottom CNTs electrode. With the frequency increase, the centre wavelength of the emission spectrum exhibit blue-shift, varying from 502 nm (2 kHz) to 450 nm (100 kHz) in green light emission. It is explained that emission bands of blue and green exist in ZnS phosphors. The low frequency is insufficient to activate blue emission centers, whereas the high electrical frequency can shift the excitation from the green to blue centers. The flexibility of ACEL device was also investigated, and the results clearly indicate the robustness of our ACEL devices in both mechanical and EL properties after a long period of operations, making them reliable for practical applications. Our study provides a routine to enhance the emission of flexible ACEL device by using T-ZnOw into the bottom CNTs electrode.
National Natural Science Foundation of China (NSFC) (11374110, 11204093, 11304106, 51371085, and 11674113)
Y.H.G. thanks Prof. Zhong Lin Wang for the support of experimental facilities in WNLO of HUST.
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