A down-shifting Eu³⁺-doped Y₂WO₆/TiO₂ photoelectrode for improved light harvesting in dye-sensitized solar cells

Abstract

Y_1.86Eu_0.14WO₆ phosphors were prepared using a solid-state reaction method. Their optical properties were analysed, and they was mixed with TiO₂, sintered, and used as a photoelectrode (PE) in dye-sensitized solar cells (DSSCs). The as-prepared photoelectrode was characterized by photoluminescence spectroscopy, diffuse reflectance, electrochemical impedance spectroscopy (EIS) and X-ray diffraction. The photoelectric conversion efficiency of the DSSC with TiO₂:Y_1.86Eu_0.14WO₆ (100:2.5) was 25.8% higher than that of a DSCC using pure TiO₂ as PE. This high efficiency is due to the ability of the luminescent material to convert ultraviolet radiation from the sun to visible radiation, thus improving the solar light harvesting of the DSSC.

1. Introduction

Dye-sensitized solar cells (DSSCs) are part of this third generation, and they have attracted considerable attention because of their low cost, easy fabrication and relatively high conversion efficiencies [1,2]. DSSCs were proposed by O'Regan and Grätzel in 1991 [3]. However, the poor response of DSSCs to red and near-infrared (NIR) light has been a significant impediment for achieving higher photocurrents and efficiencies. Therefore, to develop competitive DSSC technology, further increasing their efficiency by widening their operational spectral range is necessary.

The spectral response of conventional DSSCs is very narrow compared to the whole wavelength range of sunlight. The band between 400 and 700 nm only accounts for approximately 43% of the sun's total radiant energy [4]. To harvest energy in the red and NIR regions of the solar spectrum, new types of dyes, quantum dots and co-sensitizers are currently under development [5–8]. All of these routes still suffer from some disadvantages, however, such as: (i) the new dyes do not absorb at wavelengths above 750 nm, i.e. in the red region; (ii) organic dyes decompose relatively easily upon interacting with the ${{\text{I}}_3}^{-}$ electrolyte; and (iii) quantum dots are sensitive to the presence of moisture or oxygen and have restrictions for their use in sensitized solar cells [9,10]. A novel way to enhance the conversion efficiency in DSSCs is the use of down-shifting (DS), down-conversion (DC) and up-conversion (UC) luminescent materials as components of the solar cells. Generally, DS, DC and UC materials consist of rare earth (RE)-doped inorganic host materials. The intra-4f transitions in RE-doped luminescent materials can be efficiently used as either NIR-to-visible up-convertors or ultraviolet (UV)-to-visible down-convertors for applications in DSSCs.

On the other hand, the literature reports that exposure to UV light produces serious damage to the DSSC since the iodine present in the electrolyte is irreversibility consume under UV light [11]. Down-conversion or down-shifting phosphors are suitable materials for reducing this disadvantage in dye-sensitized solar cells. Recently, Zhang et al. described how the use of the inorganic phosphors LAVO4:Dy3+ introduced into a DSSC, not only improves the electrical conversion efficiency (ca 23%), but also expands considerably the lifetime of the solar cell [12].

This paper is part of our continuing research on the synthesis, applications, characterization and luminescent properties of inorganic phosphors containing rare-earth cations [13–16]. In this report, we focused on the preparation of Eu³⁺-doped Y₂W0₆ and its application in photoelectrodes (PE) in DSSCs in order to investigate the possibility of increasing the spectral response of DSSCS using down-shifting luminescent materials.

2. Experimental

2.1. Materials

All chemical reagents including Eu₂O₃, Y₂O₃, WO₃, TiO₂ nanopowder (21 nm), I₂, LiI, tetrabutylammonium iodide, and t-octylphenoxypolyethoxyethanol as emulsification agent (Triton X-100) were analytical grade and supplied by Sigma-Aldrich. Conducting glass plates of indium tin oxide (ITO) with a surface resistivity of 8–12 Ω sq⁻¹ and the sensitizing dye N-719 (RuL₂(NCS)₂:2TBA (L = 2,2′-bipyridyl-4,4′-dicarboxylic acid)) were also purchased from Sigma-Aldrich and used without further treatment.

2.2. Preparation of Eu³⁺-doped Y₂W0₆ nanoparticles

The Y₂WO₆:Eu³⁺ phase was prepared via a solid-state reaction at high temperature [17,18]. All phosphors were synthesized from a thoroughly ground mixture of the corresponding oxides, Y₂O₃, Eu₂O₃ and WO₃, in stoichiometric proportions. The mixtures were placed in an alumina boat and heated to 973 K for 10 h before cooling below 400 K. The sample was removed from the crucible, ground into a powder, and reheated for 10 h at 1273 K. This procedure was repeated, and the sample was finally heated to 1373 K for another 10 h. All of these synthetic processes were performed under an air atmosphere. Optical inspection of the products showed homogeneous white powders.

Powder X-ray diffraction (PXRD) data were collected using a Bruker D8 Advance diffractometer fitted with a graphite monochromator using Cu Kα radiation (λ = 1.54057 Å) across the range of 10° ≤ 2θ ≤ 60° to determine the phase purity. The experimental PXRD patterns for the doped and un-doped samples were almost in perfect agreement with those reported in the inorganic crystal structure database (ICSD) database [19].

2.3. Preparation of dye-sensitized solar cell

Different amounts of Y_1.86Eu_0.14WO₆ (1%, 2%, 2.5% and 3%) were dispersed in a TiO₂ sol, ultrasonicated, stirred for about 30 min and finally used as slurry. The PE was prepared by the doctor-blade technique [20,21]. After air-drying, the electrodes were sintered at 450°C for 30 min and soaked in an N-719 dye solution (3 × 10⁻⁴ M in ethanol) for 24 h. For comparison, a PE without Y_1.86Eu_0.14WO₆ (i.e. pure TiO₂) was also prepared. Finally, the DSSC was assembled by filling an electrolyte solution (0.60 M tetrabutylammonium iodide, 0.05 M I₂, and 0.10 M LiI in acetonitrile) between the dye-sensitized Y_1.86Eu_0.14WO₆/TiO₂ electrode and a platinized conducting glass electrode. Both electrodes were clipped together, and a cyanoacrylate adhesive was used as a sealant to prevent the electrolyte solution from leaking. For comparison, a DSSC was prepared with the pure TiO₂ PE using the same method.

2.4. Characterization

The phase purities were determined via PXRD analysis (vide supra). The nanocrystalline Y_1.86Eu_0.14WO₆ phosphor surface morphologies were determined via scanning electron microscopy (SEM, JEOL, JSM-6360LV). The photoluminescence (PL) spectra were measured using a JASCO FP-6500 spectrofluorometer. All of the spectra were collected at room temperature. The sample quantities were identical across all experiments so that the photoluminescence intensities could be compared. A Perkin-Elmer Lambda 20 UV-Vis spectrophotometer equipped with a Labsphere RSA-PE-20 diffuse reflectance accessory was used to measure the diffuse reflectance of the samples over the range of 200--600 nm (6.2--2.1 eV). The photovoltaic test of the DSSCs was performed using a PET cell tester (model CT80AAA, Photo Emission Tech., Inc. USA), under simulated AM 1.5 solar illumination of one sun (100 mW cm^–2) from a 300 W Xe lamp. The intensity was adjusted using a calibrated c-Si solar cell. The I–V measurements were attained using a Keithley Sourcemeter model 2400. The electrochemical impedance spectroscopy (EIS) analysis were carried out with Solartron 1280 impedance analyser. Nyquist plots were measured at open circuit voltage (V_oc) over a frequency range of 1–10⁴ under one-sun illumination. To determine the amount of dye desorbed from the photoelectrode a 1 M solution of NaOH was used and the UV-Vis spectra were recorded in a Perkin-Elmer Lambda 25 spectrophotometer.

3. Results and discussion

The crystallite morphology of Y_1.86Eu_0.14WO₆ was inspected via SEM. The results revealed that the samples are spherical and exhibit some tendency to aggregate, as shown in figure 1 [18]. The morphology of the layers was also inspected by SEM. Figure 1 shows the typical SEM images of bare TiO₂ and the doped samples. No remarkable differences were observed. The phosphor was embedded within the structure of TiO₂, which is an important characteristic to obtain a good dye distribution [22].

Figure 1.

SEM images of (a) TiO₂, (b) TiO₂/Y_1.86Eu_0.14WO₆ (100:2.5) and (c) Y_1.86Eu_0.14WO₆.

Additionally, the Scherrer equation (D = 0.90λ/βcosθ) can estimate the crystallite size of the luminescent material from the PXRD results if the crystals are smaller than 1000 Å. In this equation, D is the average grain size, λ is the wavelength of the radiation used in the diffraction experiments (vide supra), θ is the diffraction angle and β is the full width at half maximum (FWHM) of the observed peak [23,24]. Since small angular differences are associated with large spatial distances (inverse space), the broadening of a diffraction peak is expected to reflect some scaling feature in the crystal. The strongest diffraction peak located at 2θ = 29.5° was used to calculate the grain size of Y_1.86Eu_0.14WO₆. Our results indicate that the crystallite size for the sample is approximately 62 nm. The PXRD patterns of pure TiO₂ and Y_1.86Eu_0.14WO₆ (1%, 2%, 2.5% and 3% by weight) dispersed in TiO₂ and pure Y_1.86Eu_0.14WO₆ are displayed in figure 2.

Figure 2.

(a) XRD patterns of TiO₂ mixed with different content of Y_1.86Eu_0.14WO₆. (b) XRD patterns in the 2θ region of 27--30°.

The corresponding UV-Vis diffuse reflectance spectra of the pure TiO₂ and the corresponding Y_1.86Eu_0.14WO₆ (1%, 2%, 2.5% and 3% in weight) dispersed in TiO₂ are superimposed in the figure 3. The band edges are analogous and indicate a band gap energy between 3.13 and 3.15 eV. The commonly reported value for the energy band gap of TiO₂ is 3.2 eV [25]. Ground state diffuse reflectance absorption studies for the as-prepared photoelectrodes show that semiconductor properties of the TiO₂ were not obstructed by the inorganic phosphor.

Figure 3.

UV-visible diffuse reflectance curves of TiO₂ mixed with different content of Y_1.86Eu_0.14WO₆. The band gaps of pristine TiO₂ and TiO₂ with different contents of phosphors were calculated by the Tauc's plot method.

The synthesized phosphor shows the typical excitation and emission bands. The photoluminescence excitation spectrum (PLE) exhibits a broad charge transfer band centred at about 300 nm along with very weak f-f transitions at about 395 nm (⁷F₀ → ⁵L₆), 465 nm (⁷F₀ → ⁵D₂) and 540 nm (⁷F₀ → ⁵D₁). The broad, intense band consists of overlapping O²⁻-Eu³⁺ and O²⁻-W⁶⁺ charge transfer bands. Upon excitation at 300 nm, the resulting emission spectrum originates from the transition between the ⁵D₀ excited state to the ⁷F_j ground states (j = 0, 1, 2, 3, 4) for the 4f⁶ configuration of Eu³⁺, with the most prominent electric dipole transition, ⁷D₀ → ⁵F₂, centred at about 610 nm. Figure 4 shows the excitation and emission spectra of Y_1.86Eu_0.14WO₆ compared with the absorbance of N-719 dye.

Figure 4.

Excitation and emission spectra of Y_1.86Eu_0.14WO₆ compared with the absorbance of N-719 dye.

Using ITO conductive glass as a substrate, the TiO₂:Y_1.86Eu_0.14WO₆ photoelectrodes were prepared using the doctor-blade technique [20,21]. The emission spectra of the film electrodes at a wavelength of 300 nm exhibit an intense peak at 610 nm. The effect of the phosphor phase on the emission intensity is shown in figure 5. The emission intensity reaches a maximum with the TiO₂:Y_1.86Eu_0.14WO₆ (100:2.5) electrode (see inset figure 4). The luminescent material can convert the UV radiation from the sun to visible radiation, thus improving the solar light harvesting of the DSSC.

Figure 5.

Emission spectra of the film electrodes at wavelength of 300 nm.

Figure 6 shows the photovoltaic performance of DSSC using the mixed TiO₂:Y_1.86Eu_0.14WO₆ (100:2.5) and pure TiO₂ PEs. To obtain significant data, five DSSCs were prepared with and without 2.5 wt% of the luminescent material. When the PE consists of pure TiO₂, the DSSC has a short-circuit current density (J_sc) of 8.6 mA cm^–2, an open circuit voltage (V_oc) of 0.76 V, a field-factor (FF) of 0.48 and a photoelectric conversion efficiency (PCE) of 3.1%. The DSSCs with TiO₂:Y_1.86Eu_0.14WO₆ (100:2.5) photoelectrode show a J_sc of 12.3 mA cm^–2, a V_oc of 0.76 V, a FF of 0.43 and a PCE of 3.9%. The DSSC with the down-shifting material exhibits a higher J_sc due to the increase in photo-generated photons in the visible region converted from the UV region of the solar spectrum. By the other hand, the Rs and Rsh values for the DSSC using the mixed TiO₂:Y_1.86Eu_0.14WO₆ (100:2.5) photoelectrode were 9.8 Ω and 1657 Ω, respectively. Whereas for the DSSC using bare TiO₂ the values were and 10.8 Ω cm² and 2036 Ω cm², respectively.

Figure 6.

Current density--voltage characteristics for bare and phosphor doped TiO₂ based DSSCs under illumination of simulated solar light (AM 1.5, 100 mW cm⁻²).

To determine the amount of dye loading in both photoanodes (TiO₂ and TiO₂:Y_1.86Eu_0.14WO₆), the photoelectrodes were soaked in a 1 M NAOH solution for 12 h. The absorption spectra of the desorbed dye represented the amount of dye absorbed for the photoanodes. Figure 7 shows the UV-Vis absorbance spectra of the dye molecules detached from both photoelectrodes. The amount of absorbed N-719 was 4.61 × 10⁻⁹ and 2.94 × 10⁻⁹ mol cm⁻² for TiO₂ and TiO₂:Y_1.86Eu_0.14WO₆, respectively. Remarkable is the fact that the TiO₂:Y_1.86Eu_0.14WO₆ (100:2.5) show less absorption than the pristine TiO₂, their photovoltaic properties are better. The DSSC parameters obtained in our study are shown in table 1.

Figure 7.

UV-Vis absorbance spectra of the N-719 molecules detached from the photoanodes.

Table 1.

Photovoltaic (PV) properties of bare and phosphor doped TiO₂ based DSSCs.

DSSC	Jsc (mA cm−2)	Voc (mV)	FF	PCE (%)	dye (mol cm−2)
TiO2	8.6	757	0.48	3.1	4.61 × 10−9
TiO2:Y1.86Eu0.14WO6	12.3	757	0.43	3.9	2.94 × 10−9

The incident photon-to-current efficiency (IPCE) for the dye-sensitized solar cell of DSSC with down-shifting material and with pure TiO₂ were measured and are shown in figure 8. The IPCE of the dye-sensitized solar cell depends on the incident light harvesting and light scattering [26,27]. The enhancement of the IPCE in the range 300–400 nm can be associated with ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ transitions of Eu³⁺ under excitation at 300 nm (CTB O²⁻-Eu³⁺). The higher IPCE is attributed to the down-shifting from the ultraviolet light to visible light due to the presence of Y_1.86Eu_0.14WO₆ in the photoelectrode [28]. The ultraviolet light converted to visible was reabsorbed by the N-719 dye causing higher IPCE [29]. Our results are in agreement with those reported in the literature where the DS or DC phosphor layers were directly integrated into the DSSC surface to increase the spectral response of the cell [30].

Figure 8.

Incident photon to current conversion efficiency (ICPE) curves for bare and phosphor doped TiO₂ based DSSCs.

On the other hand, the electrochemical impedance spectroscopy (EIS) is an important tool to understand the transport properties within an electrochemical system [31–34]. The EIS spectra of the DSSCs with different photoelectrodes were analysed over a frequency range of 1–10 kHz under one-sun illumination at V_oc. The recorded spectra show a circle at low frequency corresponds to the impedance of TiO₂/electrolyte interface. Figure 9 shows Nyquist plots of DSSC made with and without luminescent material under one-sun illumination. As shown in figure 9, the smallest circle corresponds to phosphor doped DSSC, which means a decrease in the interfacial resistance and an increase in the efficiency of the cell. This also indicates that the charge transport resistance is lower for the DSSC using the mixed TiO₂:Y_1.86Eu_0.14WO₆ (100:2.5) (3.6 Ω) than that using the bare TiO₂ (4.8 Ω). This also indicates less impedance suggesting a faster electron transport, and consequently an improvement in the conversion efficiency. If the concentration of DS phosphor exceeds 2.5%, the charge carrier transportation is hindered. According to Yin et al., the TiO₂:Y_1.86Eu_0.14WO₆ (100:2.5) must have the smaller Rct value. This implies that the electron transfer from the TiO₂:Y_1.86Eu_0.14WO₆ to the electrolyte is more efficient than that of the pristine TiO₂ electrode [35].

Figure 9.

Nyquist plots of the DSSCs based on TiO₂/Y_1.86Eu_0.14WO₆ (100:0; 100:1; 100:2.5 and 100:3) photoelectrodes measured at V_oc under one-sun illumination.

More work in order to prepare Y_2−xEu_xWO₆/TiO₂ core-shell nanoparticles as photoelectrode (PE) for use in dye sensitized solar cells (DSSCs) is in progress.

4. Conclusion

The optimized TiO₂:Y_1.86Eu_0.14WO₆ (100:2.5) mixture was used as a photoelectrode to fabricate DSSCs that were more efficient than DSSCs based on pure TiO₂ photoactive electrodes. The solar conversion efficiency for a DSSC with the mixed PE reached 3.9%, which is 25.8% higher than that of a DSCC using pure TiO₂ as the PE.

On the other hand, ground state diffuse reflectance absorption measurements indicated that the addition of the down-shifting phosphor does not affect the semiconductor character of TiO₂. Finally, in this work we demonstrated that the down-shifted luminescence could be an effective way to improve the sunlight harvesting of solar cells. The last result is supported by electrochemical impedance spectroscopy and optical measurements.

Data accessibility

General method, materials used and data sheets figures of this article are available at Dryad (http://dx.doi.org/10.5061/dryad.t04k1) [36].

Authors' contributions

J.L. designed the study and wrote the manuscript; D.E. and R.S. performed the experiments; I.B. and P.M. analysed the results and revised the manuscript. All authors gave final approval for publication

Competing interests

The authors declare no competing interest.

Funding

This work was financially supported by the Comision Nacional de Investigacion Cientifica y Tecnologica of Chile (CONICYT) Grant 1130248 (FONDECYT), Postdoctoral program (no. 3150213), Grant 1130802 (FONDECYT).