1 Introduction

Blending of two or more polymers is considered one of the most promising and advantageous green chemistry processes for creating new compositions with a wide variety of distinct properties [1, 2]. Solid polymer electrolytes (SPE) have advantages over their liquid counterparts including ease of preparation, leakage-free, a wide range of operating temperatures, good shelf life, higher energy density, lower flammability, stability during the charge/discharge processes, thermal stability, and good mechanical behavior. Therefore, developing SPE based on polymeric blends with improved physicochemical properties, enhanced ionic conductivities, low cost and lightweight can be used for advanced flexible, stretchable, and all-solid-state organo-microelectronic energy storage, supercapacitors, harvesting devices, and for solar cells and sensors [3,4,5].

On the other hand, rare-earth (RE) doping was found to create considerable effects on the polymer's physical and chemical properties owing to their distinctive nature of 4f–4f electronic transitions shielded by the filled 5s and 5p shells. Therefore these RE/polymer complexes have been used in a broad range of uses, such as LEDs, laser materials, fiber optics, signal amplification, broadband telecommunications, and some NIR photonic applications [6,7,8,9,10,11]. Zhang et al. [12] reported improvement in the thermal and fluorescence emission properties of the poly(styrene-co-butyl acrylate) by incorporating Eu3+/Yb3+ into the polymer matrix.

Poly(ethylene oxide), PEO is a normal chain polymer that attracts particular attention from researchers. Because of its hydrophilicity, nontoxicity, and biocompatibility, PEO has been widely used for biomedical and biomimetics applications [1]. In addition, it is still the prime choice for manufacturing ion-conducting flexible-type SPE films [4]. Due to its good conductivity, polar character, strong chemical strength, stability and reactivity, low glass-transition temperature, and chemical stability, PEO was recently used in electrical and industrial equipment [13]. Moreover, PEO has a high degree of crystallinity (\({X}_{\mathrm{C}}\)) ~ 73.6% [14], and the fast chain segmental motion facilitates the hopping of ion transportation. However, the poor thermo-mechanical properties of PEO represents a major drawbacks for work at higher temperatures. For SPE of improved efficiency, with enhanced thermal and physical properties, it is essential to blend PEO with another polymer. Dhatarwal and Sengwa studied the electrochemical parameters of PEO (0.75)/poly(vinylidene fluoride) PVDF (0.25) loaded with LiClO4 and TiO2 [4]. Loading Tb3+ into PEO/poly(vinyl pyrrolidone), PVP blend also gave a green emission at 546 nm with an excitation at 370 nm which means the blend act as a visible color luminescent material [15]. Chigome et al. [16] reported that embedding the Eu3+-doped yttrium orthovanadate (YVO4) in PEO nanofibers yield a red light emission upon excitation at 254 nm. This suggested the using of this phosphor material for color display panels.

Poly(vinyl alcohol), PVA is a water-soluble, chemical resistant, high mechanical resistant, high transparent polymer, and has a hydroxyl –OH grouped carbon backbone, that acts as a hydrogen bonding source [10]. Dash et al. [7] synthesized PVA-coated Eu3+-doped (BiOCl, BiOBr, and BiOI) nanoflakes with a strong photoluminescence and photocatalytic properties towards RhB degradation under sunlight irradiation. Doping PVA with Ce3+, Tb3+, and GO at certain concentrations yielded dazzling green emissions that can be employed in photonic devices [8]. Kumar et al. [17] used PVA as a host material to design sensitive biosensor (Ag and Eu:Y2O3) fluorescent film for the detection of hydrogen peroxide and glucose. Ragab [18] studied the role of (0–15 wt%) CsCl on the physical properties of PEO (60%)/PVA (40%). Mahmoud et al. [19] reported the influence of EuCl3, up to 5 wt%, on the thermal properties, optical parameters, the color changes of the PVA, and the photocatalytic degradation of p-nitrophenol. Elsaeedy et al. [20] reported the possible use of Er3+/PVA for varistor device fabrication and UV shielding. Salma and Rudramadevi [21] reported that introducing Sm3+ ions up to 0.4 wt% inside PVA/PVP made the matrix able to improve the reddish-orange photonic emission for luminescent applications. Moreover, Ding et al. [22] reported that Eu0.3Tb0.7 ions/PVA organic complex can effectively shield the UV light and convert it to red light at 612 nm.

The need to blend PVA with PEO is arising due to the low strength of the etheric linkages (C–O–C) of PEO, which act as a functional group, compared to C=C=C in PVA. Moreover, this will enables the blend chains to form coordinative structures with the metal salts [15, 18, 23]. Recently, Eu3+ has attracted increased research interest because of its sharp red and green emissions in the visible region, where a small amount of external light can excite its 4f electrons. Eu3+/PVDF and Eu3+/PEO nanofibers could be used in the textile industries and for designing photoluminescent fabric [24]. The above-mentioned literature survey illustrates that the fabrication of PEO/PVA blends and the effect of RE ions on their physical properties have received little attention. There is no complete report available in the literature, to the authors' knowledge, on the effect of Eu3+ on the physical properties of the PEO–PVA blend. Therefore, this work focuses on the preparation of PEO/PVA blend and Eu3+/blend by solution casting method which is a facile and less-expensive route. The structural, optical, and electrical properties, as well as the possible conduction mechanism in these SPE were investigated by various characterization techniques and the obtained results were discussed in the light of the published data.

2 Experimental section

2.1 Materials and preparation

Europium chloride (EuCl3·6H2O) of molecular weight MW = 366.41 g/mol, purity 99.9, an average particle size of 40–60 µm from Nanoshel LLC, was used as the source of Eu3+ rare earth ions. To confirm this size, an FE-SEM image for EuCl3 was taken and shown in Fig. S1 (see the Supplementary Materials file). PEO of MW = 3 × 105 g/mol, from Alfa Aesar, Germany, and PVA powder, 87–90% hydrolyzed, and MW = (3–7) × 104 g/mol, from Sigma-Aldrich, Germany, were used for the blend formation, where the double distilled water (DDW) was used as a common solvent. Solution-cast preparation method was applied for making the PEO/PVA/rare earth films. 0.8 g of PEO was dissolved in 20 ml DDW at room temperature (RT) using magnetic stirring for about 20 min. 0.2 g of PVA was dissolved in 30 ml DDW using magnetic stirring at 80–85 °C, for 1 h, cooled to RT, and then mixed with PEO solution under stirring for 30 min. For SPE films the ratio × (wt%) of EuCl3 (x = 0, 2, 4, and 6 wt%) was dissolved in 15 ml DDW and mixed with the blend solution. Finally, the solutions were cast into glass Petri dishes and dried at 50° in a furnace till complete removal of the solvent. The films were peeled off the dishes easily and their thickness was evaluated using a digital micrometer and was in the range of 0.15–0.19 mm. A decrease in the film's transparency was seen with the increase in Eu3+ concentration. Moreover, the films were of good flexibility, and their surfaces appeared to be smooth and non-sticky.

2.2 Characterization techniques

The film's micro-structural properties and crystallinity were investigated using X-ray diffraction (XRD) analysis utilizing a Rigaku mini flex diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 5°–75°. The Fourier transform infrared (FTIR) transmittance spectra were recorded in the wavenumber range of 400–4000 cm−1 by a Bruker vertex 70 Spectrophotometer coupled with a diamond attenuated total reflection unit. The surface morphology of the PEO/PVA SPE and the chemical composition analysis were studied by a high-resolution field-emission scanning electron microscope (FE-SEM), Quanta EFI 250—Philips Company, coupled with an EDS unit. Optical transmittance and absorption spectra of the samples were recorded at RT utilizing a UV-3600 UV–Vis–NIR spectrophotometer (Shimadzu, accuracy of ± 0.2 nm) in the range of 200–1350 nm. The current–voltage (IV) curves of PEO–PVA and 6 wt% doped blend were recorded using a computerized Keithley 2635A in the temperature range of 30–100 °C and DC voltage 0–10 V. To make good Ohmic contact, the samples were coated with conductive silver paste.

3 Results and discussion

3.1 XRD and FTIR spectral analysis

Figure 1 displays the XRD patterns of PEO/PVA blend and Eu3+/blend SPE films. The un-doped PEO/PVA blend exhibits two main intense/sharp peaks at 2θ = 19.24° and 23.28° which could be attributed to the (120) and (112) reflection planes of PEO [15, 16]. It is known that PVA alone exhibits a broad peak around 19.7° associated with the (101) plane [17]. This PVA reflection plane may be merged into the (120) plane of PEO. These results confirm the semicrystalline nature of the host blend matrix as also found in the literature [18]. Insertion of Eu3+ created a significant alteration in the blend structure, where the peak intensity is significantly reduced with increasing Eu3+ content to 4 and 6 wt%, showing a considerable decrease in the blend crystallinity. Doping Y3+ at 8.88 wt% in PVA shifted the main peak from 19.7° to 19.98° [10].

Fig. 1
figure 1

XRD patterns for the blend and 2–6 wt% Eu3+/blend SPE films

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In addition, no peaks related to Eu3+ or Eu2O3 are seen in the XRD patterns of Fig. 1, indicating that the inserted ions were introduced into the amorphous regions of the blend. To account for this point quantitatively, the percent crystallinity \({X}_{\mathrm{C}} (\%)\) of the PEO/PVA blend in these SPE films was calculated using the relation based on the integrated area of blend diffraction peaks and the hump area in the range of 2θ = 16°–28° in XRD patterns, according to [10]: \({X}_{\mathrm{C}}=\frac{{A}_{\mathrm{crystalline}}}{{A}_{\mathrm{crystalline}}+ {A}_{\mathrm{amorphous}}}\). The obtained \({X}_{\mathrm{C}} (\mathrm{\%})\) values are listed in Table 1. As noted, the \({X}_{\mathrm{C}}\) of the blend is 46.23% which is smaller than the value reported for PEO [14]. This value initially increased to 48.75% at 2 wt% Eu3+ content, then decreased to 27.89 with increasing Eu3+ content up to 6 wt%. This means that low Eu3+ content makes some ordering character inside the blend matrix, while the electrostatic interaction between Eu3+ and the polymer chains is dominant at high Eu3+ contents that deactivate the admission of the polymeric chains [5, 18]. The decrease of SPE film' crystallinity and reduction of the peaks' intensity were also noticed in the XRD patterns of Er3+/Yb3+ codoped PEO/PVP blend systems [9].

Table 1 Crystallinity degree (\({X}_{\mathrm{C}}(\%)\), direct (Egd) and indirect (Egid) optical band gap at high (I) and low (Ii) energy sides and refractive index (n)
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FTIR spectral analysis is a technique that gives complete information about the functional groups and their interactions and/or complexation with the additives. Figure 2 displays the FTIR spectra of PEO/PVA SPE films. The spectrum of the un-doped blend has a broad band around 3350 cm−1. The intensity of this band is small but greatly improved after EuCl3·6H2O addition. Similarly, the band at 1735 cm−1 is assigned to the C=O stretching vibrations. The first band (of O–H) confirms the existence of PVA and the second one confirms the existence of PEO in the film. C–H symmetric vibration is seen at 2880 cm−1 and a tiny peak at 2945 cm−1 is also observed and arising due to C–H asymmetric vibration. Moreover, the bending vibrations of C–H of the CH2 group is seen at 1445 cm−1 and the asymmetric bending of CH2 is at 1347 cm−1. No obvious peak shifts could be seen, but the intensity of most of these peaks depends on the salt content. Farea et al. [25] reported a shift from 1345 to ~ 1337 cm−1 for CH2 bending after loading the PVA/PEO with CsBr salt. The two neighboring bands at 1242 and 1278 cm−1 are owing to C–O, and C–O–C bending modes [26]. The symmetric/asymmetric stretching of C–O–C appears at 1100 cm−1 [27]. The two sharp peaks at 957 and 847 cm−1 are assigned to CH2 rocking motion and C–O stretching in PEO, respectively [28]. The Assignments of the FTIR characteristic bands of the samples are listed in Table S1 (see the Supplementary Materials file). No changes in the position of these bands mean that the added salt did not change the chemical structure of PEO/PVA.

Fig. 2
figure 2

FTIR transmission spectra of the blend and 2–6 wt% Eu3+/blend SPE films

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3.2 Surface morphology and compositional analysis

At very low magnification, the film appears through SEM as shown in Fig. S2 (see the Supplementary Materials file). Figure 3 shows FE-SEM images for PEO/PVA blend and the blend loaded with 2 and 6 wt% Eu3+. The blend films, Fig. 3a, exhibits rough, cracks-free, and homogenous surface confirming the complete miscibility of the mixed polymers. Loading 2 and 6 wt% Eu3+ made the surface smooth and ruffles like the fingerprint, Fig. 3b, is found on some areas of the film. Under higher magnification (9.77 kx), no important variation is seen in the surfaces of the blend and 2 wt% Eu3+-doped films, the insets of Fig. 3a, b. But the 6 wt% Eu3+-doped PEO/PVA SPE film appears porous with a texture of net-like structure (Fig. 3c, c′). This change in the film surface morphology is certainly due to the added salt.

Fig. 3
figure 3

FE-SEM for the PEO (0.8)/PVA (0.2) blend (a) and the blend loaded with 2 and 6 wt% Eu3+ (b, c) and 6 wt% Eu3+/blend at higher magnification (c′)

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Figure 4 shows the energy-dispersive X-ray (EDX) analysis of the PEO/PVA/EuCl3 SPE films. To confirm the chemical purity of the supplied EuCl3, EDS analysis was performed and shown in Fig. S3 (see the Supplementary Materials file). It is noted that the powder is composed mainly of Eu and Cl. The O signal is coming from the water molecules attached to this compound. In addition, the C signal is coming from the grid used to carry out the material during the investigation. The pure film is composed of carbon and oxygen, and their \({\mathrm{K}}_{\mathrm{\alpha }1}\) signals are at 0.278 and 0.525 keV, respectively, where [C]/[O] = 60.86:39.14 in atomic ratio. The spectrum of 2 wt% EuCl3 loaded films contains [C] and [O] signals at the same positions as in the pure blend but with reduced intensities. Also, [C] > [O] for all of the films. The new peaks at 5.81 (\({\mathrm{L}}_{\mathrm{\alpha }2} \mathrm{line})\) and 1.13 keV (\({\mathrm{M}}_{\mathrm{\alpha }}\) \(\mathrm{line}\)) indicate the presence of the Eu element. The signal at the new peaks at 2.62 keV (\({\mathrm{K}}_{\mathrm{\alpha }1}\) line) confirms the existence of Cl. The same peaks appear in the spectrum of 6 wt% doped films with an improved intensity of Eu and Cl signals at the expense of the intensity of [C] and [O] signals. This result confirms the existence of EuCl3 in the SPE films.

Fig. 4
figure 4

EDS spectra of PEO (0.8)/PVA (0.2) blend and the blend loaded with 2 and 6 wt% Eu3+

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3.3 UV–Vis study

Figure 5a, b shows the transmittance spectra (T%) and the absorption index k [\(k= \frac{\alpha \lambda }{4\pi }\), where \(\alpha \left(\mathrm{the\, absorption \,coefficient }\right)=2.303\frac{\mathrm{Absorbance}}{\mathrm{film \,thickness}}\)]. PEO (0.8)/PVA (0.2) shows 70–75% transmittance. The blend composed of PEO (0.1)/PVA (0.9) showed transmittance of ~ 82% in the visible and IR regions [3]. 2 wt% Eu3+ loading slightly increased T to ~ 76.5% in the visible and IR regions, due to the slight improvement of film's crystallinity as reported in the XRD results. However, increasing Eu3+ content reduced T to about 60% at λ = 700 nm. The transmittance in the range of 60–75% of these SPE films makes them suitable to be involved in many optoelectronic applications. Moreover, Fig. 5b confirms that the films are transparent throughout the visible region and the absorption is in the UV region. Adding Eu3+ ions to the blend matrix increases its ability to absorb UV rays at 275 nm. This band could be assigned to π–π* electronic transitions due to the presence of C=O and C=C unsaturated bonds in the blend. PVA sole polymer was found to exhibit a characteristic peak at 278 nm [17]. The absorption band at 216 nm (see the inset of Fig. 5b) is assigned to the n–π* transition and it shifted to 210 nm with increasing Eu3+ ions content. This change is an indication of the chelate formation of Eu3+ coordinated with the hydroxyl group of PVA [29]. Similar observations were noticed for PEO/PVA/CsCl [18] and PVA/Eu3+ SPE systems [19].

Fig. 5
figure 5

Transmittance (a) and absorption index spectra (b) of PEO (0.8)/PVA (0.2) blend and the blend loaded with 2–6 wt% Eu3+

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It is possible to determine the electron transition nature of Eu3+/PEO/PVA SPE films using the α values, where the optical band gap (Eg) of the films is connected to α by Tauc’s formula: (αhυ)1/m = B(hυ − Eg), where hυ is the incident photon energy, hυ (eV) = 1242/λ (nm)), B is a constant, and the m value determines the nature of the possible optical transition. For example, m = 1/2 or 2 for the direct or indirect allowed transition, and m = 3/2 or 3 for the direct or indirect forbidden transitions. The values of Eg can be obtained by extrapolating the linear part of the curves to a point of zero absorption. Figure 6a, b shows the plots of (αhυ)2 vs. hυ and (αhυ)1/2 vs. hυ for the direct (Egd) and indirect (Egi) optical band gap, respectively. The Egd and Egi values were obtained by extending the linear parts of curves of (αhυ)2 vs. hυ and (αhυ)1/2 vs. hυ, respectively, to zero absorption. It is very interesting to note that the films have a band edge at the lower energies. The values of Egd and Egi in the lower energy side are also listed in Table 1. For the high energy region both Egd and Egi were decreased from 5.3 and 4.38 eV to 4.4 and 4.0 eV, respectively, with increasing Eu3+ ions content from 0.0 to 6 wt%. In the low energy region, the values also decreased from 3.9 (the inset of Fig. 6a) and 2.8 eV to 3.6 and 2.0 eV. This dual-band gap was also reported for NiO-doped PVA films, where Egi were increased from 5.4 to 5.8 eV, in the high energy region and decreased from 3.8 to 2.8 eV in the low energy region, with increasing NiO nanoparticles content from 0.0 to 5 wt% [30]. Two Egi were also reported for Er3+/PVA [20] and the organic alpha-sexithiophene/n-Si at 1.9 eV (onset) and the fundamental one at 2.09 eV [31]. Similar to the Eg decrement after Eu3+ incorporation was also reported for La3+/PVA/PVP SPE, and Sm3+/PVA/PVP where the Eg was decreased from 5.16 to 4.96 eV after doping with 15 wt% La3+[11], and from 5.0 to 4.6 eV after loading 0.5% Sm3+. Elsaeedy et al. [20] reported the shrinking of Egd and Egi of PVA from 5.6 and 4.98 eV to 5.08 and 4.47 eV by adding 37 wt% Er3+. This also indicates that Eu3+ is more effective than La3+, Sm3+, and Er3+ for narrowing the Eg of the SPE films.

Fig. 6
figure 6

Tauc's plots for direct (a) and indirect (b) band gap determination of 0–6 wt% Eu3+/PEO (0.8)/PVA (0.2) blend

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The refractive index (n) is one of the most important parameter for optical device fabrication, coatings, and ant-reflection applications. The n value is related to the material's optical band gap through the following equation [11]:

$$\frac{{n}^{2}-1}{{n}^{2}+2}=1-\sqrt{\frac{{E}_{\mathrm{gd}}}{20}}.$$
(1)

The obtained values of n are listed in Table 2. As seen, n increased from 1.956 to 2.096 with increasing Eu3+ content from 0.0 to 6 wt%. Another relation that relates n with Eg can be written as [21]:

Table 2 Non-linear coefficient parameter (y), and β value for pure and 6 wt% Eu3+ doped films
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$$n= \sqrt{\frac{12.417}{\sqrt{{E}_{\mathrm{g}}-0.365}}}$$
(2)

and the obtained values of n are also given in Table 2. As noticed, the derived n values from Eq. 2 are higher by a constant value (~ 0.4) compared with those derived from Eq. 1. However, in both cases, the n values are inversely proportional with Eg. Increasing n values indicates the improvement of the films' reflectivity by the formation of scattering centers inside the blend matrix.

Moreover, the optical conductivity of the SPE films is related to n and α by the following equation [3]: \(\sigma = \frac{\alpha nC}{4\pi }\), where c is the light velocity (3 × 108 m/s). Figure 7 shows the dependence of \(\sigma\) on the incident photon energy. The \(\sigma\) vs. hυ curves can be divided into four regions, see Fig. 7, where in the region (i) the low energy region (hυ < 2.5 eV) and (iii) the plateau region (2.5 < hυ < 4.4 eV), \(\sigma\) values appear constant. In the regions (ii) 2.5 < hυ < 4.4 eV and (iv) hυ > 5.0 eV, The values of \(\sigma\) increase at region (ii) and sharply increase in region (iv). This sharp increase indicates that the incident photon (UV region) has enough energy to excite the electrons/charge carriers to higher states and thus increase the conductivity of the material [32]. At higher energies, hυ > 5.8 eV the transitions reach the saturation state. Moreover, the values of \(\sigma\) are significantly enhanced with increasing Eu3+ ions content. This finding is consistent with Eg decrement in doping.

Fig. 7
figure 7

Optical conductivity of 0–6 wt% Eu3+/PEO (0.8)/PVA (0.2) blend

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3.4 I–V characteristics, and conduction mechanism

Figure S4 shows the I–V curves of PEO/PVA and 6 wt% Eu3+/blend in the applied voltage range of 0.0–10 V and temperatures in the range of 30–100 °C. At low temperatures, I marginally increases with V. As the applied temperature increase, I increases significantly. This means that the temperature has a decisive effect on I. The linearity or non-linearity of these curves could be determined using the equation: \(I=z {V}^{\mathrm{y}}\) [33], where z is a constant, and y is the non-linear coefficient parameter. The value of y could be used as an indication of the conduction mechanism in the polymers [34]. The Ohmic behavior is preeminent if y = 1, and at y = 2 the trap-space–charge-limited is the leading mechanism. But if x > 2, the conduction mechanism is governed by the space–charge-limited [35]. Figure 8a, b shows the plots of ln I vs. ln V, where two regions (i and ii) with different slopes can be distinguished. The obtained values of slopes (y) are listed in Table 2, for PEO/PVA blend and 6 wt% Eu3+/blend. In region (i), y is in the range of 1.62–1.91 and 1.35–1.62. And in the region (ii) the values are in the range of 1.07–1.47 and 0.96–1.52, for the two samples respectively, and y values for the doped blend are relatively smaller than those of the pure blend. Thus the materials exhibit non-Ohmic behavior.

Fig. 8
figure 8

ln I–ln V curves for PEO/PVA blend (a) and 6 wt% Eu3+/blend (b)

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The conductivity (\({\sigma }_{\mathrm{dc}})\) of the two films was calculated by using the relation; \({\sigma }_{\mathrm{dc}}=\frac{I.d}{V.A}\), where A and d are the cross-sectional area and sample thickness, respectively. As seen from Fig. 9a, b, at RT (300 K) the \({\sigma }_{\mathrm{dc}}\) value is very low but increases with temperature. The Eu3+/blend exhibits higher conductivity compared with the pure blend. For example, at 373 K, the \({\sigma }_{\mathrm{dc}}\) of the pure blend is in the range of 1.16–2.05 × 10–6 S/cm and for the doped SPE film \({\sigma }_{\mathrm{dc}}\) = 1.73–3.36 × 10–6 S/cm. This result is consistent with the UV/Vis data, where Eg of the blend was more shrinkable after loading Eu3+ compared to the pure blend. The obtained conductivity values are consistent with the published data, where Sundaramahalingam et al. [27, 36] reported that \({\sigma }_{\mathrm{ac}}\) of PEO/PVP raised from 1.34 × 10–8 to ~ 1.59 × 10–6 S/cm with increasing LiBr to 4.0 wt% but then decreased to 2.45 × 10–7 S/cm at 5.0 wt% loading ratio. Also, they found a maximum ionic conductivity of 6.157 × 10−7 S/cm at 30 °C for PEO (0.67) /PVP (0.27) doped with 6 wt% NaNO3.

Fig. 9
figure 9

DC conductivity of PEO/PVA (blend) (a) and the blend loaded with 6 wt% Eu3+ (b)

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Regarding the conduction mechanism, the current density J, where \(J= \frac{I}{A}\), and the field strength E, where \(E= \frac{V}{d}\) are related by the following equations [37,38,39]: \(J= {J}_{\mathrm{o}}\mathrm{exp}(\frac{\beta {E}^{0.5}}{{k}_{\mathrm{B}}T})\), where T is the temperature (K), kB is Boltzmann constant, \({J}_{\mathrm{o}}\) and \({\sigma }_{\mathrm{o}}\) are constants, and \(\beta (\mathrm{s})\) and \(\beta (\mathrm{PF})\) are related to Schottky emission and Poole–Frenkel mechanisms, respectively. The Schottky emission is related to the barrier at the surface of a metal or insulator, whereas the Poole–Frenkel emission is related to the barrier in the bulk of the material. \(\beta (\mathrm{s})\) and \(\beta (\mathrm{PF})\) can be calculated theoretically using the following equations: \(\beta \left(\mathrm{s}\right)=\sqrt{\frac{e}{4\pi {\varepsilon }_{\mathrm{o}}{\varepsilon }^{^{\prime}}}}\) and \(\beta (\mathrm{PF}) =2\beta (s)\), where \({\varepsilon }_{\mathrm{o}}\) is the permittivity of free space and \({\varepsilon }^{^{\prime}}\) is the permittivity of the PEO/PVA which was found to be \({\varepsilon }^{^{\prime}}\) =  ~ 6 [23]. Substituting these values in the equation gives us: \(\beta (\mathrm{s})\)= 1.548 × 10−5 and \(\beta (\mathrm{PF})\)= 3.096 × 10–5. Figures 10 and S5 (see the Supplementary Materials file) display the curves of log J vs. E0.5 and log (J/E) vs. E0.5, respectively, for the pure and 6 wt% Eu3+/blend. The \(\beta\) values were determined from the slope of these curves as \(\beta\) = slope × kBT, and the values are listed in Table S2. As noticed in Fig. 10a, b and Table 2, the values of \(\beta\) are in the range of (1.3–1.53) × 10−5 that are very close to the theoretical value of \(\beta (\mathrm{s})\). Thus, the Schottky emission is the most suitable conduction mechanism in PEO/PVA SPE composites.

Fig. 10
figure 10

Log J vs. E0.5 curves for evaluating the conduction mechanism

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4 Conclusions

Eu3+/PEO–PVA SPE with Eu3+ content up to 6 wt% were fabricated by solution casting. XRD analysis showed that the SPE films are semicrystalline with \({X}_{C}\) = 46.23% increased to 48.75% at 2 wt% Eu3+ and then decreased to 27.89%. FT-IR spectral analysis illustrated the presence of all PEO and PVA functional groups and their interaction and complexation with the added ions. FE-SEM images showed that the surface of the blend was rough but cracks-free and homogenous and turned to be smooth and ruffles like the fingerprint after loading 2 and 6 wt% Eu3+. EDX analysis confirmed the presence of both Eu and Cl ions in the blend. UV–Vis study showed that the transmittance and absorption index depends on the Eu3+ content. Both direct and indirect electronic transitions are allowed. The films have dual Egd on the low energy side (3.6–3.9 eV) and on the higher energy side (4.4–5.3 eV) and dual Egid in the ranges of 2.0–2.8 eV and 4.0–4.38 eV, respectively, and shrink of the bandgap with increasing Eu3+ content. The refractive index increased from ~ 1.96 to ~ 2.1 and the optical conductivity sharply increased in the UV region. I–V characteristic curves showed non-Ohmic behavior and the \({\sigma }_{\mathrm{dc}}\) increased to ~ 3.5 × 10–6 S/cm. The blend and SPE films exhibited Schottky emission behavior in the temperature range of 30–100 °C. The improvements in the optical and electrical properties of these SPE films make them a candidate for optoelectronic applications and ion-conducting devices.