1. Introduction

Printed flexible and stretchable lightings have garnered interest in recent years due to the possibility of changing the lighting landscape with large-area thin film lighting sources. Organic light-emitting diodes (OLEDs) and printed EL films such as ACPELDs are two such potential technologies that can replace existing lighting applications such as signage, advertising and solid-state lighting. However, these light-emitting materials are typically photo-sensitive and may change their opto-electrical characteristics under practical circumstances.

Roberts found that ZnS_0.8Se_0.2:Cu phosphors were sensitive to high energy photons (365 nm to 436 nm) and the dielectric constant of their EL films increased with illumination intensity [1]. The illumination intensity caused a shift of the dielectric constant in the same fashion as the change in the excitation voltage. On the other hand, Soudek observed that the resistance of ZnCdS:Ag phosphors reduced under short-wavelength illumination and attributed it to the quenching of luminescence and conductivity [2]. Kronenberg and Accardo studied the impedance changes of ZnS/CdS phosphor particles under different illumination intensities and found that the resistance decreased while the capacitance increased with intensity [3]. ACPELDs, comprising of stacked layers of polymer composites containing both micro- and nano-fillers, are susceptible to dielectric dispersion due to the dielectric relaxation of polymer matrix, fillers and filler/matrix interfaces [4]. The impedance changes on these photo-sensitive phosphor particles can cause dielectric dispersion within the ACPELD structure and requires a different excitation voltage/frequency condition to compensate for the required luminescence output. Existing simplified equivalent circuit models find it difficult to derive the equivalent circuit according to the device architecture due to its high impedance at room temperature. Furthermore, complexity is added if the photo-sensitivity of phosphors is taken into account [5,6].

In this paper, the effects of illumination wavelengths and intensities on the dielectric dispersion of EL film (or ACPELDs) were studied, which is also known as opto-impedance spectroscopy. To identify the material sensitive to high-energy photons and study the opto-impedance response in ACPELDs, the dielectric properties of phosphor (ZnS microparticles) and dielectric (BaTiO₃ nanoparticles) films were also analyzed. Equivalent circuit models for the EL and phosphor films were then constructed and compared.

2. Experimental details

2.1 Device structure and fabrication process

Forward EL films were fabricated using screen printing process and the detailed description can be found in our previous publication [7]. Green ZnS:Cu,Al phosphor microparticles (~20 µm, Gwent C2070209P5) premixed with resin were printed onto the indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrate, followed by a layer of BaTiO₃ nanoparticles in resin (~100 nm, Gwent D2070209P6). Ag nanoparticles in resin (Gwent C2131014D3) were subsequently printed as the back electrode. The resin used in this experiment (Gwent R2070613P2) is a proprietary mixture of an epoxy resin and a polyurethane film former. For external electrical connections, copper stripes were attached over the front and back electrodes. The schematic structure of the EL film is shown in Fig. 1(a) with the device area of 100 mm by 100 mm. The thicknesses of Ag, BaTiO₃ and ZnS layers are 10 µm, 25.5 µm and 31 µm, respectively. The roughnesses of ZnS and BaTiO₃ layers when printed on the PET substrate separately are 5.08 μm and 0.71 μm, respectively, as measured by Nanovea JR25 profilometer. To understand the implications of external illumination on each layer of ACPELDs, a phosphor film (ITO-coated PET/ZnS:Cu,Al/Ag) and a dielectric film (ITO-coated PET/BaTiO₃/Ag) were also screen-printed according to the structures shown in Figs. 1(b) and 1(c). Both have the same area as the EL film. The thickness of Ag layer in both phosphor and dielectric films is 10 µm while the BaTiO₃ layer in the dielectric film and ZnS layer in the phosphor film are 31 µm and 52.4 µm thick respectively. To prevent electrical short, two layers of ZnS phosphor are printed in the phosphor film instead. The resin film was fabricated by drop casting a layer of resin, which was then sandwiched by two ITO-coated PET films. Following a heat treatment at 130 °C for 30 min, the structure according to Fig. 1(d) is formed, with a device area of 40 mm by 40 mm.

 

Fig. 1 Device structures of screen-printed (a) EL, (b) phosphor, (c) dielectric and (d) resin films.

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2.2 Opto-impedance measurement and materials characterization

The opto-impedance spectroscopy study of the fabricated samples was conducted using a customized dielectric measurement setup. The device under test (DUT) was placed in a temperature enclosure with discrete illumination sources. The full width at half maximum (FWHM) of these illumination sources are ~12 nm (399 nm), ~30 nm (520 nm) and ~11 nm (625 nm) accordingly. The illumination intensity, which was controlled by a sourcemeter and measured using an integrating sphere, has a spatial illumination resolution on the DUT of less than 5%. The impedance data was obtained using Autolab PGSTAT302N equipped with a frequency response analyser FRA2 module, which has a current range from 10 nA to 10 mA with resolution of 0.0003% and a potential resolution of 3 µV. The dielectric data was measured by supplying an unbiased voltage signal with a peak voltage of 0.5 V and a frequency range of 10⁻² to 10⁵ Hz with a resolution of 0.0003%. The DUT was maintained at 90 °C by a Peltier-based piezoelectric heater controlled by a Accuthermo ATEC302 temperature controller. The spatial temperature resolution over DUT was +/− 0.2 °C. Due to extremely high impedance, information about the dielectric process of ACPELDs cannot be obtained at room temperature and thus impedance spectroscopy study at temperatures from 70 °C to 120 °C was conducted. Similar results were obtained at different temperatures and therefore only the 90 °C results are presented in this paper. The photoluminescence (PL) emission and excitation spectra and the phosphorescence decay curve were obtained by Fluorolog-3 PL system.

From the opto-impedance data, the real (ε’) and imaginary (ε”) permittivity, loss tangent (tan δ) and imaginary electric modulus (M”) were calculated according to the following equations [8]:

3. Results and discussion

3.1 Opto-impedance at different illumination wavelengths

Figure 2 shows the dielectric properties of the EL film subjected to no illumination and 399 nm, 520 nm and 625 nm illuminations. It is observed that only the 399 nm illumination alters the intermediate to high frequency dielectric response of the EL film. At frequencies below 0.1 Hz, log (ε”) is inversely proportional to log (f) regardless of the illumination conditions as shown in Fig. 2(a). This linearity is also observed in the phosphor, dielectric and resin films as shown in Fig. 3(a). This is a DC conduction characteristic which is attributed to ionic carriers in the resin matrix [10]. These ionic carriers are defects mostly introduced during the resin production process. The real conductivity of these films are given in Fig. 8 in the Appendices. At high frequency, there is AC conductivity while the low frequency plateau corresponds to the DC conductivity. The pure resin has very low DC conductivity. If ZnS or BaTiO₃ particles are added, alternative pathways are provided for the ionic carriers and thus the DC conductivities of the dielectric, phosphor and EL films are increased dramatically. If these mobile ions exchange all the charges at the electrodes, electrical neutrality should be maintained in the device and ε’ should stay constant at low frequencies. However, the proportional increase of ε’ with the decrease of frequency at low frequencies in Fig. 2(b) indicates that hetero space charge layers are formed at the vicinity of the electrodes. This is termed as electrode polarization (EP) [11], which is also present in the dielectric, phosphor and resin films as shown in Fig. 3(b). The linear portion in Figs. 2(a) and 2(b) are not changed by the different incident wavelengths, indicating that the EP and DC conductivity are not affected by illumination.

 

Fig. 2 (a) Imaginary permittivity ε”, (b) real permittivity ε’, (c) loss tangent tan δ and (d) imaginary electric modulus M” of the EL film as a function of frequency without illumination and with 399 nm, 520 nm and 625 nm illuminations.

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Fig. 3 (a) Imaginary permittivity ε”, (b) real permittivity ε’, (c) loss tangent tan δ and (d) imaginary electric modulus M” of the dielectric (BTO), phosphor (ZnS) and EL (ZnS/BTO) films without and with the 399nm illumination and resin film. The data of resin film in (a), (b) and (d) is scaled by 50, 50 and 1/1000 respectively.

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To study the dielectric polarization occurring in the device, both tan δ and M” dispersions are plotted. The humps at 0.1 to 10 Hz in Fig. 2(c) are also seen in the dielectric and phosphor films but not in the resin film in Fig. 3(c). These humps are due to the interfacial polarization or a Maxwell-Wagner-Sillars (MWS) effect at the particle/resin interfaces [12–14]. The MWS effect is typically caused by charge accumulation which leads to the formation of large dipoles at the particle/resin interfaces. With high-energy photon excitation, the MWS hump is enhanced suggesting that more charges are accumulated at the particle/resin interfaces. The complex electric modulus (M*) describes the relaxation of an electrical field under the constraint of a constant displacement vector. The advantage of using M* is that the DC conduction and EP can be adequately suppressed [15]. As a result, no EP peak is recorded in Fig. 2(d) but there are peaks (M”_max) at high frequency which are also observed in the dielectric, phosphor and resin films in Fig. 3(d). These peaks are thus attributed to the dipolar reorientation of the resin matrix [16,17]. The reduction of M”_max under the 399 nm illumination indicates higher polarization in the resin matrix.

To identify the material or layer that is sensitive to high-energy photon illumination, the dielectric response of the three different films were compared before and after the 399 nm illumination as shown in Fig. 3. It is observed that the external illumination does not change the dielectric properties of the dielectric film, implying that the high-energy photons do not directly affect the dielectric properties of the dielectric and electrode layers in ACPELDs, including the resin within these layers. This is due to the large bandgap of BaTiO₃ used in this study, which does not absorb photons from 300 nm to 900 nm through the absorbance measurement by UV-Vis. On the other hand, the permittivity increases for both EL and phosphor films under the external illumination as shown in Figs. 3(a) and 3(b). This suggests that the impedance changes are directly induced by the ZnS microparticles in the phosphor layer. The photo-excitation of ZnS phosphors is further confirmed by the PL emission and excitation measurements. As shown in Fig. 4, the EL film has an EL peak wavelength of around 500 nm when excited at 110 V and 400 Hz. During PL, the ZnS:Cu,Al phosphors can be excited by incident photons with wavelengths below 420 nm. The wavelength spectra of the 520 nm and 625 nm illuminating sources do not overlap with the excitation spectrum of ZnS phosphors and only the 399 nm illumination has sufficient energy to excite ZnS phosphors in the EL and phosphor films. Based on the PL decay curve measurement (see the inset of Fig. 4), these ZnS phosphors exhibit phosphorescence with the lifetime of tens of microseconds, which matches the results shown by Chen et al. [18]. This long afterglow is believed to be due to the detrapping of trapped electrons in sulphur vacancies [19]. The emission centres of ZnS phosphor particles were found to be located both in the bulk [20] as well as on its surface [21]. Thus, when ZnS phosphors are excited by high-energy photons, photo-induced charges may be trapped not only in the sulphur vacancies but also in interfaces or other defect sites, leading to the change of dielectric dispersion.

 

Fig. 4 Normalized PL emission when excited by 400 nm photons and PL excitation spectra with the emission wavelength fixed at 500 nm for green ZnS:Cu,Al phosphors. EL emission spectrum of the EL film operated at 110 V and 400 Hz. Spectra of the 399 nm, 520 nm and 625 nm LEDs. Inset: PL intensity as a function of time after the excitation source is switched off.

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3.2 Opto-impedance at different illumination intensities

To study the interaction of excited charges from the ZnS phosphor particles with its surrounding, the impedances of the EL and phosphor films were measured at different illumination intensity levels. Both the EL and phosphor films exhibit similar dielectric response. Figure 5 shows the dielectric properties of the phosphor film at different illumination intensities. The dispersions of ε”, ε’, tan δ and M” show a significant change even at a low illumination of 0.5 mW/cm² and saturate with higher illumination intensity. In Figs. 5(a) and (b), the permittivity does not change at low frequency, indicating that the increased charges do not affect the DC conductivity of ACPELDs. However, the MWS effect becomes more pronounced with more photo-excited charges as shown in Fig. 5(c). A higher illumination intensity also causes the electric modulus peak to reduce and shift to higher frequency in Fig. 5(d). This indicates that more charges are trapped in the polymer chains of the resin matrix, leading to an increased polarization and decreased M”.

 

Fig. 5 (a) Imaginary permittivity ε”, (b) real permittivity ε’, (c) loss tangent tan δ and (d) imaginary electric modulus M” of the phosphor film as a function of frequency with the 399 nm illumination of different intensities.

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To identify the relaxation processes of illuminated ACPELDs, the imaginary and real part of the relative permittivity Δε* is utilized to omit ionic conduction in ε” and ε’. Δε’ and Δε” are obtained by subtracting the extrapolated low-frequency linear contributions from the ε’ and ε” in Figs. 5(a) and 5(b). Figure 6 shows that both the EL and phosphor films exhibit one small arc followed by a big semicircle regardless of the illumination conditions. This means that no additional relaxation process is generated in the ACPELDs when subjected to high-energy photon excitation and the corresponding equivalent circuits of photo-excited ACPELDs are the same as the unexcited ones [22]. The small arc corresponds to the permittivity contributed by the dipolar reorientation of polymer chains in the resin matrix while the big semicircle represents the MWS effects at the particle/resin interfaces [23]. The MWS effect for the phosphor film is due to the charge accumulation at the ZnS/resin interface. For the EL film, both BaTiO₃/resin and ZnS/resin interfaces contribute to the MWS effect, which is implied by the larger Δε’ shown in Fig. 6(a). The relative permittivity was also observed to increase with intensity, indicating an increase of dipolar polarization in the resin matrix and enhanced MWS effects at the particle/resin interfaces.

 

Fig. 6 Imaginary part (Δε”) vs real part (Δε’) of relative permittivity for (a) EL and (b) phosphor films with the 399 nm illumination of different intensities. Experimental and simulated Nyquist plots of (c) EL and (d) phosphor films without illumination and with 4.9 mW/cm² illumination together with the corresponding equivalent circuit models. High-energy photons excite the ZnS phosphor particles. R_s: series resistance of the electrodes; R_m and C_m: resistance and capacitance of the resin matrix; R_ZnS and C_ZnS: resistance and capacitance of the ZnS/resin interface; R_BTO and C_BTO: resistance and capacitance of the BaTiO₃/resin interface; R_e and CPE_e: resistance and constant phase element representing EP; Z_w: Warburg element.

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These relaxation processes are then represented in the form of equivalent circuits as shown in Figs. 6(c) and 6(d). With each relaxation process corresponding to one RC component, various combinations of these RC components were fitted and the equivalent circuits shown give the best goodness of fit (χ²). The chi square χ² values for the fittings of the EL and phosphor films with and without illuminations are 0.001, 0.0041 and 0.006, 0.0049, respectively. This shows that the fittings are good enough with comparable χ² results by Nurk et al. [24]. R_s is the series resistance of the external connections of ACPELDs. R_m and C_m are the resistance and capacitance of the resin matrix representing the dipolar polarization of the resin polymer chains. The MWS effects at the ZnS/resin and BaTiO₃/resin interfaces are modelled by two pairs of parallel resistance and capacitance: R_ZnS-C_ZnS and R_BTO-C_BTO. EP is represented by a resistance (R_e), a Warburg element (Z_w) [25] and a constant phase element (CPE). The need of a Warburg element in EL and phosphor films are further demonstrated from the Nyquist plots of the phosphor film at higher temperatures as shown in Fig. 9 in the Appendices. From Cha et al.’s physics-based model of polymer-metal composites [26], R_e is associated with the resistance due to charge transport across the resin matrix close to the Ag composite. The impedance of CPE is generally used to describe the imperfect nature of the space charge layer [27] and is depicted as:

In Tables 1 and 2, R_ZnS in the phosphor film is about two times higher than that in the EL film while the C_ZnS values are close. This is due to the fact that the phosphor film is printed with two layers of ZnS particles while the EL film consists of one BaTiO₃ layer and one ZnS layer. The doubled thickness of the ZnS layer in the phosphor film causes the R_ZnS value to double compare to that of the EL film. However, the surface area between ZnS particles and resin is also doubled, leading to similar C_ZnS values in EL and phosphor films with C = εε₀A/d. As shown in Table 1 and 2, the changes of R_ZnS and C_ZnS with illumination intensity exhibit similar responses to earlier studies [1–3]. When high energy photons are applied over the EL and phosphor films, the excited ZnS phosphor particles lower R_ZnS and increase C_ZnS. This is due to the increased photo-current at the ZnS/resin interfaces. These photo-excited ZnS phosphor particles may, in turn, affect the adjacent material represented by circuit components.

Table 1. Simulated values of the equivalent circuit components of the EL film under the 399 nm illumination of different intensities

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Table 2. Simulated values of the equivalent circuit components of the phosphor film under the 399 nm illumination of different intensities

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Without any illumination, R_m in the EL film is 3.08 kΩ while that in the phosphor film is 11 kΩ. With less interface ratio and higher resin content, the ZnS layer has a higher R_m than the BaTiO₃ layer. Thus, R_m in the phosphor film is more than two times higher than that in the EL film. C_m is affected by both resin volume and interaction between fillers and resin. These two factors may compensate each other, leading to a consistent C_m value for both EL and phosphor films. It is also observed that the resistance (R_m) of the resin matrix decreases and the capacitance (C_m) increases gradually with higher illumination intensity for both the EL and phosphor films. This is attributed to the trapped charges at the ZnS phosphor surfaces accumulating at the ZnS/resin interfaces. It is also possible that these charges have sufficient mobility to travel to the resin matrix and form dipoles. A higher illumination intensity will enable more trapped charges in the polymer chains and enhances the dipolar polarization.

Although it is shown that the dielectric and electrode layers are not directly affected by external illumination, both C_BTO in Table 1 and C_e in Table 2 are observed to increase with intensity. As the dielectric layer is in direct contact with the ZnS phosphor layer in the EL film, the accumulation of charges injected from the excited ZnS phosphors can enhance the MWS effect at the BaTiO₃/resin interface. This causes the C_BTO to increase with illumination intensity. The C_e in the EL film is not affected by the photo-excited charges due to the dielectric layer sandwiched between the phosphor layer and the Ag electrode. In the phosphor film, the ZnS phosphor layer is in close contact with the Ag electrode, charges generated during PL are able to reach the Ag electrode and cause more charges accumulated at the Ag/resin interfaces, leading to an increase of C_e. The saturated C_e values at high intensities suggests that the number of trapped charges reaches its maximum, possibly due to the limited trapping sites or mean free path of excited charges. The admittance Y_0w representing the Warburg element is shown in Tables 1 and 2 for the EL and phosphor films and does not change with illumination intensity, which indicates that the diffusional properties of ionic carriers are not affected by the photo-excited ZnS phosphors. It is also noted that Y_0w of the EL film is larger than that of the phosphor film. In the EL film, the Ag electrode is in contact with BaTiO₃ layer while it is directly stacked over the ZnS phosphor layer in the phosphor film. Due to the larger surface-to-volume ratio in the dielectric layer, more ionic pathways lead to larger Y_0w.

3.3 Opto-impedance mechanism in ACPELDs

From the above analyses of the illuminated EL and phosphor films, a mechanism for the opto-impedance behaviors in ACPELDs is proposed as illustrated in Fig. 7. When green ZnS:Cu,Al ACPELDs are illuminated by photons with energy higher than 2.95 eV, electrons in the deep trap level close to the valence band (VB) gain enough energy and are excited to the conduction band (CB) to generate free charges. Some of the free electrons trapped in the donor sites have the possibility to combine with the free holes trapped in the acceptor sites to emit green phosphorescent lights. Some of the free charges generated at the surfaces of ZnS phosphor particles are accumulated at the ZnS/resin interfaces leading to the enhancement of the MWS effect in the phosphor layer. For the ZnS particles which are in close contact or at the vicinity of BaTiO₃ nanoparticles, some of the free charges at the ZnS surface have the possibility of hopping over and accumulated at the BaTiO₃/resin interfaces and thus enhance the corresponding MWS effect. Some other free charges that propagate to the resin matrix are trapped in the potential wells of the polymer chains and generate dipoles. This increases polarization due to the dipolar reorientation in the resin matrix. Due to the larger distance to reach resin matrix, the charging effects over the resin dipoles are small compared to the MWS effects at the BaTiO₃/resin and ZnS/resin interfaces.

 

Fig. 7 Proposed mechanism for the opto-impedance behaviors of ACPELDs. (1) Incident photons with energy of higher than 2.95 eV penetrate through ITO/PET and are absorbed by ZnS phosphor particles; (2) electrons are excited from deep trap level just above valence band (VB) to conduction band (CB); (a) some electrons trapped in donor sites combine with holes trapped in acceptor sites and emit green phosphorescent lights; (b) some trapped charges drift and are accumulated at the BaTiO₃/resin interfaces; (c) minority charges move to the resin matrix to form dipoles; (d) other charges are trapped at the ZnS/resin interfaces.

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4. Conclusions

Opto-impedance spectroscopy was adopted to study the dielectric dispersion of the EL, phosphor and dielectric films at different illumination wavelengths and intensities. High-energy photons affect the dielectric properties of the EL and phosphor films by enhancing the WMS effect at the particle/resin interfaces and increasing the dipolar polarization of the resin matrix. This opto-impedance behavior is caused by the PL excitation of green ZnS phosphors during which high-energy photons (hf > 2.95 eV) have sufficient energy to excite the emission centres and generate charges. When the illumination intensity increases, the MWS effect and dipolar polarization are found to increase. Equivalent circuit models of the EL and phosphor films are constructed based on the three relaxation processes, namely EP, MWS effects at the particle/resin interfaces and dipolar polarization in the resin matrix. Through the analyses of dielectric properties and simulated equivalent circuits, a mechanism is proposed to explain the opto-impedance phenomenon in ACPELDs. Under high energy photon illumination, electrons are first excited to the conduction bands of the ZnS phosphor particles, followed by being trapped in defect sites. Thereafter, these electrons have two paths: radiative and non-radiative. The radiative path enables the emission of the green phosphorescent lights. Non-radiative path consists of three possible outcomes: charge accumulation at the ZnS/resin interfaces and at the BaTiO₃/resin interfaces and dipole generation in the resin matrix.

Appendix

In Fig. 8, the real conductivity of different films is represented by a DC plateau at low frequency and non-linear portion at intermediate to high frequencies. DC conductivity is low in resin but is significantly increased with added fillers. This demonstrates the presence of charge carriers within resin.

 

Fig. 8 Real conductivity as a function of frequency from 10⁻² to 10⁵ Hz at 90 °C for dielectric, phosphor, EL and resin films.

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Figure 9 shows the Nyquist plots of the phosphor film at elevated temperatures. The presence of long tail suggests the need for a Warburg element in the equivalent circuit model.

 

Fig. 9 Nyquist plots of the phosphor film at 110, 120, 130, and 140 °C.

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Funding

Singapore Institute of Manufacturing Technology (SIMTech) (U14-P-040SU).

Acknowledgments

The authors want to thank SIMTech for the fabrication of the device specimens and funding support for this project.

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