1. Introduction

Dye-sensitized solar cells (DSSCs), which were first reported by O’Regan and Grätzel in 1991, are thought to be a promising candidate to replace commercial silicon solar cells because of their low cost of fabrication, eco-friendly feature and simple structure [1–3]. Typically, DSSCs are made up of a dye-sensitized TiO₂ porous membrane as the photoanode, an electrolyte solution containing the I^-/I₃^- redox couples and the platinum-coated fluorine tin oxide (FTO) glass as the counter electrode. As is known, the photovoltaic properties of DSSCs can be greatly influenced by the light harvesting ability of the dye [4,5]. However, the commonly used dyes, such as N3, N-719 and N-749, can only absorb the light in the visible range (400-800 nm) of the total solar irradiation. This means that nearly 50% of the sun light energy in the ultraviolet (~7%) and infrared (~43%) regions has not been harvested by these dyes, resulting in a relatively low efficiency of DSSCs. Nowadays, the introduction of luminescent materials (especially, upconverting nanoparticles) into the TiO₂ porous film is considered to be an alternative method to improve the light harvesting ability of DSSCs as well as the cell efficiency [6–8]. The upconversion (UC) emission is a non-linear optical process in which low-energy photons (infrared light) are absorbed, high-energy photons (visible light) that can be reabsorbed by the dyes are generated, and more electrons will be produced, leading to the increased photovoltaic performances of DSSCs. Diao et al. pointed out that the power conversion efficiency (PCE) of DSSCs was increased from 5.791 to 6.661% using the CeO₂:Er³⁺/Yb³⁺ nanofiber as the upconverting layer [6]. Furthermore, it was also revealed that the Y₂O₃:Er³⁺ nanorods can be used to improve the photovoltaic performances of DSSCs [9]. Although some inspiring achievements have been obtained, they still cannot meet the requirement of practical applications and more efforts should be paid.

Over the last decades, the upconverting materials based on the rare-earth (RE) doped materials, such as fluorides, inorganic oxides and ceramics, have been intensively studied as a result of their superior luminescent properties [10–12]. Among these RE ions, holmium (Ho³⁺) ion is thought to be an excellent activator candidate for upconverting materials because of its unique luminescent properties. Generally, the Ho³⁺ ion exhibits two emissions in the green and red regions corresponding to the (⁵F₄,⁵S₂) → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions, respectively [13,14]. Furthermore, ytterbium (Yb³⁺) ion is usually codoped with other RE ions, such as Er³⁺, Tm³⁺, Pr³⁺ and Ho³⁺, as the sensitizer to improve the luminescent properties due to its large absorption in the infrared region and efficient energy transfer (ET) from Yb³⁺ to these RE ions [6,15–17]. Meanwhile, gadolinium oxide (Gd₂O₃), which has a cubic phase as an important part of the inorganic oxides, is regarded as a good luminescent host lattice because of its high physical durability, thermal stability and low phonon energy (as low as ~600 cm⁻¹) [18,19]. It was reported that strong UC emissions were observed in RE ions (Er³⁺/Yb³⁺) doped Gd₂O₃ nanoparticles which have potential applications in optical temperature sensors and bioimaging [18,20]. However, to the best of our knowledge, the investigation on the UC emission properties of Ho³⁺/Yb³⁺-codoped Gd₂O₃ nanoparticles and especially its application in DSSCs is still insufficient. In this work, a series of sphere-like Ho³⁺/Yb³⁺-codoped Gd₂O₃ nanoparticles were prepared by a simple urea-based homogeneous precipitation method and their phase structure, morphology and luminescent properties were investigated in detail. Additionally, to explore their effect in DSSCs, the Ho³⁺/Yb³⁺-codoped Gd₂O₃ nanoparticles were introduced into the TiO₂ porous membrane to form a hybrid photoelectrode.

2. Experimental details

The conventional urea-based homogeneous precipitation method was employed to fabricate the sphere-like Ho³⁺/Yb³⁺-codoped Gd₂O₃ nanoparticles (abbreviated as Gd₂O₃:0.01Ho³⁺/xYb³⁺, where the Ho³⁺ ion concentration was 1 mol% and x = 0.01, 0.02, 0.03 and 0.04). High-purity powders of Gd(NO₃)₃·6H₂O (99.9%), Ho(NO₃)₃·5H₂O (99.9%) and Yb(NO₃)₃·5H₂O (99.9%) were used as the starting materials. On the basis of the stoichiometric proportion, the Gd(NO₃)₃·6H₂O, Ho(NO₃)₃·5H₂O and Yb(NO₃)₃·5H₂O were weighted and dissolved in the de-ionized (DI) water (200 mL) to form a transparent solution under vigorously mechanical stirring. Subsequently, moderate urea (i.e., molar ratio of urea to RE ions: 30) was added. Then, the mixed solution was sealed in a beaker and heated at 80 °C for 4 h under magnetic stirring. After that, the white precipitate was separated by centrifugation and washed with ethanol and DI water for several times to remove the remained ions. Ultimately, the precipitate was transferred into the aluminum crucible and calcined at 800 °C for 2 h in a furnace to create Ho³⁺/Yb³⁺-codoped Gd₂O₃ nanoparticles.

The DSSCs were assembled via a conventional fabrication process. Briefly, the TiO₂ colloids (PST-18NR) were firstly deposited on the FTO glass to form a transparent photoelectrode with the thickness of about 5 μm by the doctor-blade method and sintered at 500 °C for 2 h. Subsequently, the TiO₂ colloides (PST-400C) containing 2 wt% Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles were screen-printed on the TiO₂ film, forming a hybrid TiO₂ porous film with a thickness of around 5 μm. Thus, the thickness of the total TiO₂ film was approximately 10 μm. After that, the TiO₂-coated FTO glass was soaked in the N-719 dye (3 × 10⁻⁴ M ethanol) for 24 h. For comparison, a pure TiO₂ porous film without upconverting nanoparticles was also prepared. Finally, the sandwich-type DSSCs were fabricated with the platinum-coated FTO glass and an electrolyte (Dyesol, electrolyte HPD).

The X-ray diffractometer (Bruker D8 Advance) with Cu Kα radiation (λ = 0.15406 nm) was applied to check the phase structure of the samples and the MDI JADE 5.0 software was employed to deal with the data. The morphological properties of the obtained materials were characterized by using a field-emission scanning electron microscope (FE-SEM) (LEO SUPPA 55, Carl Zeiss) and a transmission electron microscope (TEM) (JEM-2100F, JEOL). The elemental mappings of the as-synthesized particles were measured by the TEM (JEM-2100F, JEOL). As for the UC measurement, the excitation source was a 980 nm light laser diode and a fluorescence spectrophotometer (Ocean Optics USB 4000) was used to obtain UC emission spectra. The photocurrent density-voltage (J-V) curves of the as-prepared DSSCs were recorded by using a photocurrent system which consists of a solar simulator with 1000 W xenon short arc lamp and a source meter (Keithley 2400). The incident photon to current conversion efficiency (IPCE) characteristics of the DSSCs were measured by utilizing a 300 W xenon arc lamp as the light source coupled to a monochromator (TLS-300x xenon light source, Newport) with an optical power meter (Newport 2935-C).

3. Results and discussion

In order to analyze the crystal structure and phase purity of the obtained phosphors, the X-ray diffraction (XRD) patterns of the Gd₂O₃:0.01Ho³⁺/xYb³⁺ nanoparticles were measured, as shown in Fig. 1. From Fig. 1(a), it is evident that all the diffraction peaks of the as-prepared samples can be indexed to the Ia-3 (206) space group of cubic Gd₂O₃ (JCPDS#12-0797) and no any other additional peaks for impurity phases were detected within the resolution of the device. This means that all the samples had pure cubic phase and the Ho³⁺/Yb³⁺ ions were diffused into the Gd₂O₃ host lattices. Furthermore, as shown in Fig. 1(b), the positions of the diffraction peaks slightly shifted to the larger angle with the addition of Ho³⁺ and Yb³⁺ ions. This is because the large sized Gd³⁺ (1.05 Å, when coordinate number (CN) is 8) ions were substituted by the small sized Yb³⁺ (0.99 Å, when CN is 8) and Ho³⁺ (1.02 Å, when CN is 8) ions, leading to a shrinkage of the lattices. To better understand the lattice shrinkage phenomenon, the lattice parameters of the as-synthesized phosphors were calculated using MDI JADE 5.0 software and the corresponding results are illustrated in Table. 1. Notably, with raising the Yb³⁺ ion concentration from 1 to 4 mol%, the lattice parameters, such as a, b and c, were decreased from 10.7939 to 10.7784 Å, which further confirmed the shrinkage of the lattices.

 

Fig. 1 (a) XRD patterns of Gd₂O₃:0.01Ho³⁺/xYb³⁺ (x = 0.01, 0.02, 0.03 and 0.04) nanoparticles sintered at 800 °C. (b) Magnified XRD patterns in the 2θ range between 44 and 52°.

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Table 1. Structural parameters of Gd₂O₃ host lattice and Gd₂O₃:0.01Ho³⁺/xYb³⁺ (x = 0.01, 0.02, 0.03 and 0.04) nanoparticles

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The structural and morphological properties of the Ho³⁺/Yb³⁺-codoped Gd₂O₃ phosphors were characterized by FE-SEM and TEM images. Figure 2 shows the FE-SEM images of the products. It is clear that the as-prepared samples were made up of uniformly spherical particles with the diameter ranging from about 200 to 300 nm. Note that, all the samples exhibited the same morphology and the particle size did not change with the increase of Yb³⁺ ion concentration, as presented in Fig. 2, suggesting that the doping of the Yb³⁺ ions had little effect on the shape and size of the resultant phosphors. Furthermore, from Fig. 3, one knows that the particle size of sintered samples was slightly decreased along with the relatively rougher surface compared to the as-prepared precursor and the dehydration process can be responsible for the shrinkage of the particle [21]. The TEM image further confirmed that the obtained particles were in the nanometer range with sphere-like morphologies, as described in Fig. 4(a). Meanwhile, the high-resolution TEM (HR-TEM) image showed the clear lattice fringes and the distance between the adjacent lattice fringes was found to be about 0.33 nm which is well consistent with the d-space of the (222) plane of cubic Gd₂O₃ (JCPDS# 12-0797). In addition, the elemental mappings, which were taken from the scanning transmission electron microscope (STEM) image of inset of Fig. 4(b), indicated that the Gd, Ho, Yb and O were uniformly distributed over the entire particles, as illustrated in Fig. 4(c).

 

Fig. 2 FE-SEM images of Gd₂O₃:0.01Ho³⁺/xYb³⁺ nanoparticles: (a) x = 0.01, (b) x = 0.02, (c) x = 0.03 and (d) x = 0.04.

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Fig. 3 FE-SEM images of (a) as-prepared precursor (Gd(OH)CO₃:0.01Ho³⁺/0.03Yb³⁺) and (b) Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles sintered at 800 °C.

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Fig. 4 (a) TEM image and (b) HR-TEM image of Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles. (c) Elemental mappings of Gd, Ho, Yb and O. Inset of (b) shows the STEM image of Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles.

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Figure 5(a) depicts the representative room-temperature UC emission spectrum of Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles under 980 nm light excitation. As shown in Fig. 5(a), the UC emission spectrum consisted of an intensive green UC emission located at about 550 nm and a weak red UC emission centered at approximately 665 nm which are ascribed to the (⁵F₄,⁵S₂) → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺ ions, respectively [17,22]. It is worth noting that the intensity of the green UC emission was nearly six times as high as that of the red UC emission. Hence, under 980 nm light excitation, the emission color of the samples was green, as illustrated in the inset of Fig. 5(a). It is known that the N-719 dye has a strong light absorption in the visible wavelength (300-700 nm) with a maximum absorption band located at about 530 nm [23], which matched well with the UC emissions of Ho³⁺/Yb³⁺-codoped Gd₂O₃ nanoparticles. Therefore, with the introduction of Ho³⁺/Yb³⁺-codoped Gd₂O₃ upconverting nanoparticles into the TiO₂ porous membrane, the absorption range of the N-719 dye can be extended, and thus the enhanced photovoltaic performance of DSSCs is expected to be achieved.

 

Fig. 5 (a) UC emission spectrum of Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles. (b) UC emission intensity as a function of Yb³⁺ ion concentration. (c) Simplified energy level diagram of Ho³⁺ and Yb³⁺ ions. Inset of (a) shows the luminescence image excited by 980 nm light.

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The dependence of Yb³⁺ ion concentration on the UC emission intensity is displayed in Fig. 5(b). It can be seen that both green and red UC emission intensities showed an upward trend with the increment of Yb³⁺ ion concentration and its optimum value was achieved when x = 0.03 due to efficient ET from Yb³⁺ to Ho³⁺ ions. Nevertheless, the UC emission intensity began to decease when the Yb³⁺ ion concentration was over 3 mol%. The main reason for the deceased UC emission properties should be attributed to the energy back transfer (EBT) from Yb³⁺ to Ho³⁺ ions, i.e., (⁵F₄,⁵S₂) (Ho³⁺) + ²F_7/2 (Yb³⁺) → ⁵I₆ (Ho³⁺) + ²F_5/2 (Yb³⁺), as shown in Fig. 5(c) [17]. Generally, the higher the Yb³⁺ ion concentration becomes, the higher the probability of EBT from Ho³⁺ to Yb³⁺ ions is. To better comprehend the UC mechanism in the Ho³⁺/Yb³⁺-codoped Gd₂O₃ nanoparticles, the simplified energy level diagram of Ho³⁺ and Yb³⁺ ions along with possible UC processes is demonstrated in Fig. 5(c). Similar as previous reports on Ho³⁺/Yb³⁺-codoped materials, the ET process from Yb³⁺ to Ho³⁺ ions is dominant [14,17,24]. Briefly, under the excitation at 980 nm, the Yb³⁺ ions absorb the infrared photon energy and the electrons are excited from the ground state to the excited level, i.e., (²F_7/2 (Yb³⁺) + infrared photon → ²F_5/2 (Yb³⁺)). Meanwhile, the energy can be transferred from Yb³⁺ to the adjacent Ho³⁺ ions (²F_5/2 (Yb³⁺) + ⁵I₈ (Ho³⁺) → ²F_7/2 (Yb³⁺) + ⁵I₆ (Ho³⁺)), leading to the population of the ⁵I₆ level. After that, the electrons located at the ⁵I₆ intermediary level are excited to the (⁵F₄,⁵S₂) level either by the absorption of an infrared photon or a second ET from Yb³⁺ to Ho³⁺ ions (²F_5/2 (Yb³⁺) + ⁵I₆ (Ho³⁺) → ²F_7/2 (Yb³⁺) + (⁵F₄,⁵S₂) (Ho³⁺)). Then, the bright green UC emission is observed due to the radiative transition of Ho³⁺ ions to the ground state ((⁵F₄,⁵S₂) → ⁵I₈). In comparison, two different paths are involved to generate the red UC emission. The first one is the non-radiative (NR) transition from the (⁵F₄,⁵S₂) level to the ⁵F₅ level, as described in Fig. 5(c). Subsequently, the red UC emission corresponding to the ⁵F₅ → ⁵I₈ transition is observed. The second one is the NR transition of ⁵I₆ → ⁵I₇ followed by an ET process from Yb³⁺ to Ho³⁺ ions to populate the ⁵F₅ level (²F_5/2 (Yb³⁺) + ⁵I₇ (Ho³⁺) → ²F_7/2 (Yb³⁺) + ⁵F₅ (Ho³⁺)). Finally, the red UC emission centered at about 665 nm is generated through the ⁵F₅ → ⁵I₈ transition.

To identify the effect of as-prepared upconverting nanospheres on the photovoltaic performance of DSSCs, the Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles were dispersed in the TiO₂ porous film to fabricate a novel DSSC device. The surface morphologies of pure and hybrid TiO₂ porous films are shown in Fig. 6. From the FE-SEM image of the hybrid TiO₂ film, it is evident that there were some Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles attached to the surface of the TiO₂ film, which can improve the light scattering properties to achieve a higher photocurrent [25]. Nevertheless, compared with that of the pure TiO₂ porous film, the surface morphology of the hybrid TiO₂ porous film changed a little, suggesting that the UC emission of the Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles was the main factor to affect the cell performance.

 

Fig. 6 FE-SEM images of the surface of (a) pure TiO₂ porous film and (b) hybrid TiO₂ porous film with Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles.

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Figure 7(a) shows the J-V curves of the DSSCs with and without addition of Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles and the corresponding photovoltaic parameters, such as short-circuit current density (J_SC), open-circuit voltage (V_OC), fill factor (FF) and PCE, are summarized in Table 2. As presented in Fig. 7(a) and Table 1, the DSSCs with pure TiO₂ porous film exhibited the photovoltaic characteristics of J_SC = 15.662 mA/cm², V_OC = 0.715 V, FF = 59.77% and PCE = 6.701%. In comparison, with the introduction of Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles into the TiO₂ porous film, the photovoltaic properties of the DSSCs were enhanced, that is, J_SC = 17.091 mA/cm², V_OC = 0.715 V, FF = 60.51% and PCE = 7.403%. It is obvious that the PCE of the DSSCs was greatly improved (by an increment percentage of approximately 10.47%) with the addition of Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles and the increased J_SC should be responsible for the modified PCE. As we know, the increase of the J_SC can be significantly affected by the incidence light wavelength range [3,9]. Here, the Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ upconverting nanoparticles had a strong light absorption in the near-infrared region and the energy can be transferred to the visible region by the UC process, leading to the extension of the absorption range of the DSSCs. As a result, the J_SC and PCE of the DSSCs were increased. On the other hand, the IPCE, which directly shows how efficiently the incident photons are converted to the electrons, is thought to be a key parameter for solar cells. As shown in Fig. 7(b), the IPCE of the DSSCs with the addition of Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles was much higher than that of the referenced DSSCs which coincided well with the result of the J-V curves, demonstrating that photocurrent was increased with the introduction of Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles into the TiO₂ porous film.

 

Fig. 7 (a) J-V curves and (b) IPCE spectra of the DSSCs with and without Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles.

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Table 2. Photovoltaic parameters of DSSCs without and with addition of Gd₂O₃:0.01Ho³⁺/0.03Yb³⁺ nanoparticles.

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4. Conclusion

In summary, high-efficiency upconverting Ho³⁺/Yb³⁺-codoped Gd₂O₃ nanoparticles were synthesized and incorporated into the TiO₂ porous film of DSSCs. Under 980 nm light excitation, strong UC emissions were observed and the UC emission intensity increased gradually with the Yb³⁺ ion concentration, achieving its optimum value when x = 0.03. Furthermore, with the introduction of the Ho³⁺/Yb³⁺-codoped Gd₂O₃ nanoparticles into the TiO₂ porous film, the DSSCs exhibited an efficiency of 7.403%, which indicates a percentage improvement of about 10.47% compared to that of the DSSCs with pure TiO₂ porous film (6.701%). From these results, the Ho³⁺/Yb³⁺-codoped Gd₂O₃ nanoparticles provided excellent UC emission properties and were capable of enhancing the light harvest ability of N-719 dye as well as the photovoltaic performances of DSSCs.