1. Introduction

Printed flexible and stretchable electronic devices have attracted a great deal of research interest in recent years because these functional devices are low cost, disposable and well suited to mobile and wearable applications [1]. Although many devices, such as field effect transistors, sensors, solar cells, batteries and supercapacitors have been demonstrated [2], the printed sheet light source is less well developed. A low-cost printed flexible white light source with high luminance, efficacy, large emission area, light weight and adequate color temperature and lifetime will have many applications such as signage, advertising and solid-state lighting. One device structure is the organic light-emitting diode (OLED) based on multiple evaporated layers of phosphorescent emitting layers and charge transport/blocking layers [3]. Although high performance white OLEDs have been reported [4], the costs of both organic semiconductor materials and the batch fabricating process remain high [5, 6]. Furthermore, organic materials are sensitive to moisture and oxygen and must be encapsulated by passivation films which add to the cost [7]. An alternative printed light source is the much earlier ACPEL device based on activated and encapsulated ZnS phosphor powders. At its simplest, the voltage driven ACPEL devices consist of just two layers sandwiched between two electrodes, one of which is transparent. Despite the lower material and processing costs, conventional ACPEL devices suffer from low luminance, low efficacy and a limited choice of colors [8–10]. ZnS phosphors emitting at longer wavelengths have been reported [11, 12]. However, their performance is even poorer than the standard blue and blue/green phosphors. This limitation is especially problematic for the realization of a white light source. Recent studies on ACPEL devices include the use of ultra-violet (UV) curing of resin at ambient temperature to enhance the luminance of blue ZnS:Cu phosphors [13]. In particular, Kim used spin-coated downshifting organic dyes, coumarin-6 and DCJTB to downshift blue light from ZnS:Cu,Cl phosphors to red light [14]. Both coumarin-6 and DCJTB are small molecular fluorescent dyes. This type of hybrid organic-inorganic phosphor represents a novel approach to overcome the lack of good red-emitting inorganic phosphors [8].

In this paper, we report the EL emission properties and aging of screen printed hybrid ACPEL devices containing ZnS based phosphors and two downshifting fluorescent organic dyes: F8BT and DCJTB. F8BT is a conjugated copolymer, which had previously been investigated for organic/inorganic LEDs [15]. We demonstrate the continuous tuning of the emission color from the hybrid ACPEL device by using these two downshifting organic dyes. Furthermore, the accessible CIE color coordinates enabled by this approach include those for white light.

2. Experimental details

2.1 Fabrication of forward and reversed ACPEL devices

Forward and reversed blue reference ACPEL devices were fabricated by low-cost screen-printing process using a Micro-tec MT-850 screen printer. Polyethylene terephthalate (PET) pre-coated with indium tin oxide (ITO) (~100 Ω/sq) was used as the substrate. For forward devices, blue ZnS phosphor powders (~20 µm in diameter, Gwent C2061027P15) was premixed with resin (Gwent R2070613P2) consisting of a film former polyurethane and epoxy. The phosphor paste was printed on top of the ITO-coated PET to form a 105 mm x 105 mm square emitting layer (~20 µm thick) using the printing speed of 300 mm/s. Thereafter, the devices were cured at 130 °C for 30 min. After the emitting layer was fully cured, a 105 mm x 105 mm dielectric layer (~35 µm thick) was printed over the emitting layer using barium titanate (BaTiO₃) nanoparticles dispersed in resin (~100 nm, Gwent D2070209P6) with the printing speed of 300 mm/s. This was followed by a second curing step at 130 °C for 30 min. Finally, Ag nanoparticle paste (Gwent C2131014D3) was printed as the back electrode (~10 µm thick) using the printing speed of 30 mm/s and pattern size of 100 mm x 100 mm. The forward ACPEL devices emit blue light from the PET side. For the reversed ACPEL devices, the dielectric layer was printed first followed by the emitting layer with the same printing speed and pattern size as the forward devices. The Ag electrode was replaced by the screen-printed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, CLEVIOS S V3 HV) (~200 nm thick) using the printing speed of 100 mm/s and pattern size of 100 mm x 100 mm. The reversed ACPEL device size is 100 mm x 100 mm and light is emitted from the PEDOT:PSS side.

2.2 Fabrication of hybrid ACPEL devices with a single organic dye

F8BT and DCJTB with the molecular structures shown in Fig. 11 in the Appendix were first incorporated individually with the ACPEL device to study the downshifting process. Initially, the dye solution was mixed directly with the resin containing the phosphor. However, as shown in Fig. 12 in the Appendix, the curing step was found to adversely affect the photoluminescence (PL) spectrum of the dye. As a result, the drop-casting process is adopted instead for organic dye deposition. Drop-casting is chosen because screen printing is not suitable for low viscosity solvents and the PET substrate used is too large for spin coating. In addition, drop-casting is straightforward and there is no waste of organic material. 15.6 mg of F8BT (Lumtec LT-S957) was dissolved in 4 mL of chloroform (CHCl₃, 99% Sigma-Aldrich) at room temperature. Forward hybrid ACPEL devices were fabricated by drop-casting one 20 µL drop of the above 3.9 mg/mL F8BT solution over a horizontal flat PET surface of forward ACPEL devices with the pipette placed about 2 cm above the sample. After drying at room temperature, the forward hybrid ACPEL devices exhibit the final structure as shown in Fig. 1(a). The typical diameter of the organic dye is ~1cm. As for the reversed hybrid devices, 20 µL of the above F8BT solution was drop-casted over the PEDOT:PSS of the reversed ACPEL devices as illustrated in Fig. 1(b) and dried at room temperature. The forward and reversed hybrid ACPEL devices incorporating DCJTB were fabricated by the same method using a 1.9 mg/mL DCJTB solution prepared by dissolving 11.4 mg of DCJTB (Lumtec LT-E704) in 6 mL of CHCl₃ at room temperature.

 

Fig. 1 Structures of (a) forward and (b) reversed hybrid ACPEL devices with a layer of organic dye.

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2.3 Fabrication of Hybrid ACPEL devices with F8BT and DCJTB mixture

In order to tune the emission color of hybrid ACPEL devices, a mixed F8BT- and DCJTB-CHCl₃ solution was drop-casted over forward and reversed ACPEL devices using the same process stated in section 2.2. The mixture is obtained by dissolving F8BT and DCJTB separately into CHCl₃ first and mixing the resulting solutions. Two sets of experiments were conducted in the present study. The first one involved varying the F8BT:DCJTB mass ratio. 18mg of F8BT was added into 4.5 mL of CHCl₃ to prepare a starting solution. By adding 1, 1.286, 1.8, 2.25, and 3 mL of the 2 mg/mL DCJTB-CHCl₃ solution, the F8BT:DCJTB mass ratios of 9:1, 7:1, 5:1, 4:1, and 3:1 were achieved respectively and the resulting concentrations of F8BT-DCJTB solution (final concentration) were 3.64, 3.56, 3.43, 3.33 and 3.2 mg/mL respectively. The final concentration is determined by dividing the total mass of F8BT and DCJTB by the total volume. The second experiment involved diluting the final mixture with the F8BT:DCJTB mass ratio of 3:1 from the first experiment to obtain different final concentrations. By adding 2.5 mL of CHCl₃ successively to the previous mixture up to 5 times, the final concentrations of 2.40, 1.92, 1.60, 1.37 and 1.20 mg/mL were obtained. As summarized in Table 1, 20 µL of the mixtures in these two sets of experiments was drop-casted over forward and reversed ACPEL devices to fabricate hybrid devices.

Table 1. Solution concentrations for the drop casting process

View Table

2.4 Characterization methods

PL emission spectra of the organic dyes in CHCl₃ solution and on PET films were acquired using a Fluorolog-3 PL system. The absorbance spectra were measured by a Shimadzu UV-2450 UV-VIS spectrometer from 350 nm to 800 nm. The dark field images of hybrid ACPEL devices were taken using an Olympus upright metallurgical microscope BX53M by exciting the EL devices at 110 V and 400 Hz and turning off the characterization light under the dark field mode. EL spectra, luminance, and CIE coordinates were measured using the SpectraWin software of the PR655 spectroradiometer from Photo Research. The measurement was performed at a 2° observer angle with a spot size about 5 mm in a dark enclosure at ambient conditions. The AC sinusoidal excitation voltages of the ACPEL devices were applied by a Pacific 105-AMX power source without any DC bias voltage. EL and white light aging were performed to study the durability of F8BT and DCJTB in hybrid ACPEL devices. In order to reduce experimental errors, 6 un-passivated samples drop-casted with the F8BT:DCJTB mass ratio of 9:1 were tested for each aging study. EL aging was carried out by operating the devices continuously at 110 V and 400 Hz in a dark room at 25 °C and 70% relative humidity. For white light aging, a fluorescent white light box with the radiance power of 4.3 Wsr⁻¹m⁻² was used to age the hybrid ACPEL device at 25 °C and 70% relative humidity in a dark room. The drop-casted F8BT:DCJTB film of the hybrid ACPEL device was placed directly over the white light box and AC voltage was not applied to the device during white light aging.

3. Results and discussion

3.1 Hybrid ACPEL devices using F8BT

Before incorporation into ACPEL devices, F8BT was dissolved in CHCl₃ and 20 µL of the 3.9 mg/mL solution was drop-casted over a PET film, followed by drying at room temperature. The PL emission and absorbance spectra of the F8BT-CHCl₃ solution and F8BT-PET film are shown in Fig. 2(a). The F8BT-CHCl₃ solution emits yellow light with a peak wavelength of 548 nm and a full width at half maximum (FWHM) of 67 nm while the F8BT-PET film experiences a slight blue shift to a peak wavelength of 543 nm. With the peak wavelengths of 468 nm and 478 nm, the absorbance spectra of the F8BT-CHCl₃ solution and F8BT-PET film overlap well with the EL spectra of the forward and reversed blue ACPEL devices (forward blue, reversed blue) as shown in Fig. 2(b). When excited by the voltage signal of 110 V and 400 Hz, both forward blue and reversed blue have a blue emission with the peak wavelength of about 460 nm and a small green shoulder at around 500 nm. The ACPEL emission mechanism was proposed by Fischer [8]. Briefly, Cu_xS precipitates form “needles” within the ZnS phosphor powder. Due to electric field enhancement at points of high curvature, these needles field-emit holes and electrons in one half of the AC cycle. As illustrated in Fig. 3, as a semiconductor with a wide bandgap of around 3.8 eV [16], ZnS has two crystal structures: cubic (sphalerite) at low temperature and hexagonal (wurtzite) at high temperature [17]. In low temperature cubic ZnS powders, doped Cu forms two trapping states contributing to green emission at the energy level of −5.41 to −5.51 eV (G-Cu) and blue emission at the energy level of −5.81 to −6.01 eV (B-Cu) [18]. When doped with Cl or Br, electron trapping sites are formed within the cubic ZnS with the energy level of −3.01 to −3.21 eV. At the heterojunction of cubic ZnS and Cu_xS needles, carriers are trapped in the acceptor and donor sites. In the following half-cycle green and blue lights are emitted when the electrons in donor sites recombine with the holes trapped in the G-Cu and B-Cu states. The light intensity of reversed blue is lower than that of forward blue because as shown in Fig. 13 in the Appendix, the transmittance of PEDOT:PSS is lower than ITO-coated PET.

 

Fig. 2 (a) PL emission and absorbance spectra of F8BT-CHCl₃ solution and dried F8BT-PET film; (b) EL spectra of forward and reversed blue ACPEL devices with and without a layer of F8BT operated at 110 V and 400 Hz. Insets in (b): dark field images of forward and reversed ACPEL devices with a layer of F8BT operated at 110 V and 400 Hz. The scale bars each correspond to 100μm.

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Fig. 3 Emission mechanism in ACPEL devices and downshifting mechanism of the hybrid devices. G-Cu: copper in ZnS contributing to green emission. B-Cu: copper in ZnS contributing to blue emission.

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In Fig. 2(b), clear yellow peaks at about 527 nm and 536 nm are observed respectively for the forward (reversed) hybrid ACPEL devices with a drop of F8BT. These devices are referred hereafter as forward (reversed) F8BT. The yellow emissions can also be seen from the dark field optical images of both devices as shown in the insets of Fig. 2(b) demonstrating the downshifting of ACPEL emission by the added F8BT. As illustrated in Fig. 3, some photons from the blue and green emission from the emitting layer are absorbed by the F8BT layer with an optical band gap of 2.25 eV [19]. After relaxation, singlet and triplet excitons are formed in the F8BT. When the singlet excitons decay radiatively, yellow light is emitted and gives rise to the EL spectrum for forward and reversed F8BT devices in Fig. 2(b). Note that this downshifting mechanism is independent of the details of the ACPEL emission mechanism outlined above. In addition, the downshifting is more effective for blue photons, as can be inferred from the absorbance spectra in Fig. 2(a). As a result, the blue peaks of the hybrid devices shift to 447 nm with the blue-green lights being absorbed in the F8BT layer. The yellow peak intensity of forward F8BT is higher than that of reversed F8BT while the blue peak intensities are similar. This shows that the downshifting process is more effective in forward F8BT devices. The drop-casted F8BT in reversed F8BT is visually noticeable from the PET substrate side, which means the F8BT-CHCl₃ solution diffuses into the emitting and dielectric layers upon drying. This leads to less F8BT participating in the downshifting process and thus lowers the intensity of yellow emissions in reversed hybrid ACPEL devices.

The CIE color coordinates of forward and reversed ACPEL devices are dependent on the operating frequency as shown in Fig. 4(a). At 2000 Hz, the colors for the forward and reversed blue are located in the blue region with CIE of (0.153, 0.150) and (0.151, 0.137) respectively. When the frequency decreases, the device colors shift to blue-green region. At 100 Hz, their CIE become (0.169, 0.340) and (0.164, 0.320) respectively. (Note the CIE y coordinate has a greater frequency dependence than the x coordinate.) This observation agrees well with the frequency dependent performances of ACPEL devices [20] and is explained by the dipolar field emission theory proposed by Fischer, who assumes that copper-decorated lines are formed near the “needles” and the ZnS adjacent to these lines is richer in copper than the bulk [21]. At high frequencies, when the copper-decorated lines become sharp and concentrated, the blue emission of the copper-rich ZnS predominates. At low frequencies, where the lines are spread out, the green emission of the bulk ZnS predominates [21]. As such, the change of operating frequency provides another channel for color tuning the ACPEL devices.

 

Fig. 4 (a) CIE operated at 110 V and 100, 400, 800, 1200, 1600, 2000 Hz and (b) luminance as function of frequency operated at 110 V for forward and reversed blue ACPEL devices with and without a layer of F8BT.

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In Fig. 4(a), the CIE coordinates for forward and reversed F8BT follow the same trend with the increase in frequency. At the same operating frequency, the hybrid F8BT devices show a green shift in color due to the downshifting by F8BT. The forward F8BT color becomes green with CIE of (0.228, 0.490) when operated at 110 V and 100 Hz. As shown in Fig. 4(b), the forward blue device is brighter than reversed blue. With a layer of F8BT, both the forward and reversed F8BT exhibit a decrease in luminance. When operated at 110 V and 400 Hz, the luminance for forward and reversed blue are 41.11 and 33.5 cd/m² respectively while that for forward and reversed F8BT are reduced to 35.41 and 14.79 cd/m². This is due to the luminance quenching by the F8BT layer during the downshifting process.

3.2 Hybrid ACPEL devices using DCJTB

The PL emission and absorbance spectra for the DCJTB-CHCl₃ solution and DCJTB-PET film are shown in Fig. 5(a). The DCJTB-CHCl₃ solution exhibits a red emission with the peak wavelength of 615 nm and FWHM of 54 nm. When DCJTB is deposited on a PET film, there is a red shift of the PL emission spectrum to the peak wavelength of 630 nm. From the absorbance spectra in Fig. 5(a), both the DCJTB-CHCl₃ solution and DCJTB-PET film are able to absorb a broad spectrum of blue to green light. However, since the absorbance peak wavelength is at 510 nm for both samples, the DCJTB is better at absorbing the emissions from F8BT shown in Fig. 2(a). By contrast, the blue light from ZnS phosphor is not absorbed as strongly, as illustrated by Fig. 2(b) and Fig. 3. This implies that downshifting by DCJTB to longer wavelength photons is less effective than F8BT.

 

Fig. 5 (a) PL emission and absorbance spectra of DCJTB-CHCl₃ solution and DCJTB-PET film; (b) EL spectra of forward and reversed hybrid ACPEL devices with a layer of DCJTB operated at 110 V and 400 Hz. Insets in (b): dark field images of forward and reversed ACPEL devices with a layer of DCJTB operated at 110 V and 400 Hz. The scale bars each correspond to 200μm.

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The forward hybrid ACPEL devices with a layer of DCJTB (forward DCJTB) have only one EL emission peak with the peak wavelength of 452 nm as shown in Fig. 5(b). Compared with the EL spectra of forward blue, there is subdued red emission in forward DCJTB. This may be due to the non-uniformity of the drop-casted DCJTB film on PET. The dark field optical image of forward DCJTB in the inset of Fig. 5(b) shows the existence of circular features with size ranging from hundreds of micrometers to submicrometer. These patterns are probably caused by the Marangoni effect during the evaporation of the chloroform [1]. In addition, the low wettability of the untreated PET surface may be a factor [22]. Further experiments will be conducted to understand the reduced red EL emission. On the other hand, the reversed hybrid devices incorporating DCJTB (reversed DCJTB), show two EL emission peaks with the peak wavelengths of 445 nm and 624 nm. The blue shifting of the low wavelength peak indicates the PL absorption of blue-green lights by DCJTB, which is further proven by the red emissions observed from the dark field image in the inset of Fig. 5(b).

Figure 6(a) shows the CIE coordinates of forward and reversed DCJTB as the operating frequency is varied from 100 Hz to 2 kHz. The forward DCJTB has only a slight red shift in color output compared to the forward and reversed blue devices while a large red shift is observed for the reversed DCJTB device. This demonstrates that the incorporation of DCJTB into forward DCJTB devices fails to yield the desired red component and is therefore not suitable for color tunable ACPEL devices. Both the forward and reversed DCJTB show a frequency dependence of the color coordinates. At 100 Hz, the CIE coordinate for the reversed DCJTB is (0.311, 0.288) showing near white color. As the operating frequency increases to 2 kHz, the downshifting process by DCJTB is enhanced due to the more intense emission from ZnS and thus a purple color with the CIE of (0.253, 0.153) is produced. In Fig. 6(b), the energy loss during the downshifting process produces weaker luminance for the reversed DCJTB compared to reversed blue. The decreased luminance in forward DCJTB further demonstrates the hypothesis of low light transmittance of the DCJTB layer.

 

Fig. 6 (a) CIE operated at 110 V and 100, 400, 800, 1200, 1600, 2000 Hz and (b) luminance as function of frequency operated at 110V for forward and reversed ACPEL devices with and without a layer of DCJTB.

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3.3 Hybrid ACPEL devices with the F8BT and DCJTB mixture

3.3.1 Effects of mass ratio

Figure 7(a) illustrates the EL spectra of reversed hybrid ACPEL devices with a layer of F8BT and DCJTB mixture of different mass ratios operated at 110 V and 400 Hz. Two clear peaks located at the blue and red regions are observed with the peak wavelengths of 448 nm and 627 nm respectively and there is another small hump near the wavelength of 526 nm. As illustrated in Fig. 3, when F8BT and DCJTB are mixed together, the F8BT acts as a sensitizer for the DCJTB. Short wavelength photons from the phosphor are first absorbed and downshifted by the sensitizer. This is followed by energy transfer from F8BT to DCJTB in the form of a further downshifting of yellow light by DCJTB which has a narrower energy gap of 2.0 eV as shown in Fig. 3. As a result, no clear yellow peaks are observed. As the F8BT:DCJTB mass ratio increases, the 627 nm peak intensity decreases, showing less red emission by the DCJTB. Meanwhile, the intensities of the 448 nm peak and 526 nm hump increase with the mass ratio. As the final concentration of F8BT and DCJTB mixture vary with the mass ratio as shown in Table 1, the dependency of peak intensities is mainly due to the change of the proportion of F8BT and DCJTB in the hybrid devices. As a result, the increase of mass ratio generally shifts CIE towards blue region except for the mass ratio of 9:1 as shown in Fig. 7(b). This may be due to the fact that the total dye concentration for the mass ratio of 9:1 is the highest as shown in Table 1. A reddish purple color is achieved with the mass ratio of 4:1 when operated at 110 V and 2000 Hz and the corresponding CIE is (0.336, 0.219). For purplish pink color, the mass ratio is 3:1 and the CIE is (0.386, 0.266) with the operating voltage of 110 V and frequency of 400 Hz.

 

Fig. 7 (a) EL spectra at 110 V and 400 Hz, (b) CIE at 110V and 100, 400, 800, 1200, 1600, 2000 Hz and (c) luminance as a function of frequency for the reversed hybrid ACPEL devices with a layer of F8BT and DCJTB mixture with the mass ratios of 9:1, 7:1, 5:1, 4:1, and 3:1.

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The luminance as a function of frequency for hybrid ACPEL devices with different F8BT:DCJTB mass ratios are shown in Fig. 7(c). It can be seen that the increase of mass ratio leads to slight increase of luminance from 100 Hz to 1600 Hz. On the other hand, for 2000 Hz, increasing F8BT:DCJTB ratio decreases the luminance. As discussed in section 3.1, the phosphor emits an increased portion of blue lights at high frequency and green lights at low frequency. Since DCJTB is more effective in absorbing green light as suggested in Fig. 5(a), an increase in F8BT:DCJTB ratio will reduce the absorption of green light at low to intermediate frequencies and thus lead to higher luminance. At higher frequencies near 200 Hz, the emission spectrum of the ZnS phosphor exhibits a blue shift. The increased blue emission intensity will result in more absorption by F8BT in mixtures with higher F8BT:DCJTB mass ratio. The increased downshifting of blue light thus results in a decrease of the total luminance.

3.3.2 Effects of final mixture concentration

In order to investigate the impact of final mixture concentration on color tuning, different amounts of CHCl₃ are added into a F8BT and DCJTB mixture with the mass ratio of 3:1 as mentioned in section 3.3.1. Final concentrations of 2.40, 1.92, 1.60, 1.37 and 1.20 mg/mL were achieved and the corresponding EL spectra of the hybrid ACPEL devices are shown in Fig. 8(a). It is observed that decreasing the final concentration increases the blue emission intensity but reduces the red peak intensity. No obvious yellow hump is observed. This is understandable because as the dye mixture becomes more dilute, the solution has less organic dye per unit volume and thus produces less red and yellow emission. Less down shifting thus causes more blue light to penetrate the organic layer and the resulting colors are more blueish as the concentration decreases as illustrated in Fig. 8(b). At 110 V and 400 Hz, the CIE coordinates of the hybrid ACPEL devices with the final concentration of 2.40 mg/mL is (0.313, 0.230) while that for the final concentration of 1.20 mg/mL is (0.267, 0.232). White color with the CIE coordinates of (0.313, 0.312) is achieved for the hybrid devices with the final concentration of 1.60 mg/mL and operated at 110 V and 100 Hz as shown in Fig. 8(b). The luminance of all these devices increases with frequency as illustrated in Fig. 8(c). The decrease of the final concentration increases the luminance at all frequencies. Note that there is no crossover of the luminance characteristics as in Fig. 7(c) because all samples in this plot have the same F8BT:DCJTB mass ratio. In Fig. 7(b), the red shift for the mass ratio of 9:1 is due to the increased concentration of DCJTB causing increased energy transfer from the F8BT sensitizer.

 

Fig. 8 (a) EL spectra operated at 110 V and 400 Hz, (b) CIE operated at 110 V and 100, 400, 800, 1200, 1600, 2000 Hz and (c) luminance as a function of frequency at 110 V for reversed hybrid ACPEL devices with a layer of F8BT and DCJTB mixture with final concentrations of 2.40, 1.92, 1.60 and 1.20 mg/mL. The F8BT:DCJTB mass ratio is 3:1 for all samples.

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3.3.3 White light and EL aging

During white light aging, the hybrid ACPEL device was removed from the light box from time to time and was turned on briefly to measure the EL spectrum and luminance. In Fig. 9(a), a red peak contributed by DCJTB and a yellow shoulder generated by the downshifting in F8BT are initially observed for reversed hybrid ACPEL devices with F8BT:DCJTB mass ratio of 9:1. As white light aging progresses, the red peak intensity decreases continuously until it becomes negligible. This suggests that the DCJTB degrades readily under white light illumination. However, the yellow shoulder becomes a yellow peak with aging time and the intensity of this peak increases thereafter. This is because with less DCJTB, energy transfer from F8BT is reduced leading to higher yellow emission. This change in color also indicates that F8BT is more stable under white light aging. In Fig. 9(b), as DCJTB degrades, the device luminance decreases by about 5 cd/m² and becomes stable after 40 hours.

 

Fig. 9 (a) EL spectra and (b) luminance change of the hybrid ACPEL devices with time under white light aging. The F8BT:DCJTB mass ratio is 9:1 for all samples.

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The hybrid ACPEL device behaves differently under EL aging compared with white light aging. As shown in Fig. 10(a), the red peak intensity reduces with time continuously under EL aging. With less DCJTB and less sensitizing effect, the yellow peak intensity increases during the first 22.75 hours but decreases thereafter. This change of the yellow peak intensity can be due to two reasons. First, the F8BT also degrades under EL aging but the degradation rate is much less than that of DCJTB. On the other hand, since little degradation was observed for F8BT under white light aging, reduced incident blue EL may have resulted in decreased yellow emission by F8BT under EL aging. Further studies, however, will have to be conducted to understand the EL aging mechanism. Figure 10(b) shows that unlike white light aging, there is a monotonic decrease of device luminance. After ~114 hours of EL aging, the luminance decreased by 39% of the initial value.

 

Fig. 10 (a) EL spectra and (b) luminance change with time under EL aging for the hybrid ACPEL devices with the operating voltage of 110 V and 400 Hz. The F8BT:DCJTB mass ratio is 9:1 for all samples.

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4. Conclusions

In this study, F8BT and DCJTB organic dyes have been investigated by PL and EL methods for the down shifting capabilities of blue emission from ZnS phosphors. EL excited yellow and red emissions in F8BT and DCJTB are obtained respectively through down shifting and sensitizer-down shift mechanisms. Forward ACPEL devices are unsuitable for color tuning due to poor wetting of the drop-casted DCJTB. However, for the reversed device structure, emission from the hybrid ACPEL device can be tuned through F8BT:DCJTB mass ratio, total dye solution concentration and the excitation frequency. Increasing the mass ratio between F8BT and DCJTB enhances the blue and yellow emission peaks but the red peak intensity is reduced. Similarly, diluting the F8BT and DCJTB mixture diminishes the yellow and red emission peaks and enhancement of the blue peak. Colors ranging from green to white can be obtained from the reversed hybrid ACPEL structure. EL and white light aging results show that DCJTB degrades under both conditions while F8BT does not degrade under white light aging.

Appendix

 

Fig. 11 Chemical structure of (a) F8BT and (b) DCJTB.

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Fig. 12 PL emission spectra of uncured and cured F8BT and DCJTB in resin.

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Fig. 13 Transmittance of ITO-coated PET and PEDOT:PSS.

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Acknowledgments

This work was supported by the project U14-P-040SU funded by Singapore Institute of Manufacturing Technology (SIMTech).

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