Preparation of long persistent phosphor SrAl₂O₄:Eu²⁺, Dy³⁺ and its application in dye-sensitized solar cells

Abstract

SrAl₂O₄:Eu²⁺, Dy³⁺ powder has been successfully prepared via a combustion method and then introduced into the titanium dioxide (TiO₂) film photoanode of the dye-sensitized solar cell (DSSC). The influence of the amount of doping on the light-to-electric energy conversion efficiency was discussed. At the concentration 7 wt% of SrAl₂O₄:Eu²⁺, Dy³⁺ (weight ratio of phosphor powder to TiO₂), an overall 20 % improvement was achieved compared to that of the DSSC without SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor doping. The enhancement is mainly due to the presence of the long persistent phosphor doping. It can convert ultraviolet to visible luminescence via down-conversion luminescence. Owing to the emission of long persistent light, the DSSC with SrAl₂O₄:Eu²⁺, Dy³⁺ showed an efficiency of 0.0076 % in the dark after illuminated under a simulated solar light for 1 min. This method represents a novel approach to increase the efficiency of DSSC.

1 Introduction

The supply of non-renewable resources is becoming more serious. Global energy consumption is increasing and is expected to double by the end of 2050. So, the search for alternative renewable energy sources is undoubtedly important [1, 2]. Sunlight is a freely available abundant source of solar energy which can irradiate the Earth’s surface with 120,000 TW of solar power [3].

Dye-sensitized solar cell (DSSC) is a photoelectric power generation device, which can convert light photons into electricity effectively. It was firstly reported by Dr. Gratzel’s group in 1991 [4]. Since then, the research of the DSSC has become a hot topic owing to its environmentally friendly and low-cost character. A conventional DSSC is assembled by a photoelectrode of dye adsorbed on porous nanocrystalline titanium dioxide (TiO₂), a counter electrode of platinum [5].

The dye-sensitized TiO₂ photoanode plays an important part in the absorbing sunlight, it can load sensitizers and collect and transport electrons. However, it only absorbs visible light in the wavelength range 300–800 nm. The absorption in the ultraviolet spectrum and red region of the visible spectrum is restricted. Thus, in order to improve the harvest of incident light, many new-type dyes have been under observation [6]. An effective way to improve the conversion efficiency is the utilization of a luminescence layer composed of up-conversion (UC) or down-conversion (DC) phosphor materials [7–9]. It can convert ultraviolet or red irradiation into visible light and more solar irradiation will be utilized, the DSSC photocurrent may increase. Recently, Yao et al. [10] applied a DC phosphor layer composed of ZnO:Eu³⁺, Dy³⁺ to DSSC. It can broaden absorption region of solar spectrum in DSSC and enhanced carrier transportation is achieved. Meanwhile, a down convertor Lu₃Al₅O₁₂:Ce³⁺ (LuAG) phosphor was used as a visible light amplifier for DSSC [11]. LuAG particles emitting green light, which are capable of stimulating Ru-dye molecules, enhanced the light-harvesting properties of a DSSC.

So far, the rare earth luminescent material used in DSSC mainly are fluorescent materials, however, the introduction of long persistent phosphor in the DSSC is rarely reported. Green emitting SrAl₂O₄ co-doped with Eu²⁺, Dy³⁺ ions has been considered one of the best and long lasting phosphorescent materials [12]. In this paper, SrAl₂O₄:Eu²⁺, Dy³⁺ powder has been synthesized by combustion method and applied in the DSSC. The prepared phosphor powder has the characteristic of DC luminescence and light-to-electric energy conversion efficiency is improved. The research result reveals that the long persistent phosphor SrAl₂O₄:Eu²⁺, Dy³⁺ can be a candidate material to improve the efficiency of DSSC.

2 Experimental

2.1 Materials

Tetrabutyl titanate, absolute ethyl alcohol, glacial acetic acid, polyethylene glycol 20,000, 4-tert-butylpyridine (TBP), nitric acid, OP emulsification agent (Triton X-100), iodine, lithium iodide, urea, boric acid, aluminium nitrate and strontium nitrate were analytic purity supplied by Shanghai Chemical Agent Limited, Shanghai, China. The sensitized dye N-719 [RuL₂(NCS)₂, L = 4,4′-dicarboxylate-2,2′-bipyridine] was from SOLARO-NIX SA (Aubonne, Switzerland). Dysprosium oxide and europium oxide were supplied by Shanghai Yuelong Non-Ferrous Metals Limited, Shanghai, China.

2.2 Preparation of SrAl₂O₄:Eu²⁺, Dy³⁺ powder

The SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor powder was synthesized by combustion technique. Amounts of urea were added as reducer and fuel, small quantities of boric acid were used as a flux, metal nitrates were used as oxidizer. Powders were weighted according to the stoichiometry.

The procedure used to prepare the precursor had the following stages. Firstly, Eu₂O₃ (0.088 g) and Dy₂O₃ (0.093 g) were dissolved into concentrated nitric acid (1.4 g/ml) to form nitrate solution. Al(NO₃)₃·9H₂O (7.5 g), Sr(NO₃)₂ (2.116 g), H₃BO₃ (0.18 g) and CO(NH₂)₂ (15 g) were dissolved enough into deionised water to obtain transparent solution, then mixed the two solutions together and stirred at 70 °C for 4 h to make some water evaporate.

After that, the precursor solution was introduced into a muffle furnace maintained at 600 °C. Initially, the solution boiled and underwent dehydration, followed by decomposition with escaping large amounts of gases (oxides of carbon, nitrogen and ammonia) and then spontaneous ignition occurred and underwent smouldering combustion with enormous swelling, producing white foamy and voluminous ash, the whole process was over within less than 5 min, after the product was cooled to room temperature, milled the ash slightly. The SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor was obtained.

2.3 Preparation of SrAl₂O₄:Eu²⁺, Dy³⁺/TiO₂ photoanode

First, the TiO₂ colloid was prepared as reported in our previous work [13–15]. Then a different amount of SrAl₂O₄:Eu²⁺, Dy³⁺ powders (0, 3, …, 9 wt%) were dispersed in the TiO₂ colloid by stirring and ultrasonic treatment for 30 min respectively. After that, keep the colloid stirring under 105 °C until it becomes thicken and homogeneous. The resulting colloid was deposited on the FTO electrode by a doctor blade coating. The first layer with about 10 μm thickness was coated with pure TiO₂. After drying under an infrared lamp, the TiO₂ film was sintered at 450 °C for 30 min and cooled down to 80 °C. The second layer with about 4 μm thickness was coated with SrAl₂O₄:Eu²⁺, Dy³⁺/TiO₂ colloid by the same method. Finally, SrAl₂O₄:Eu²⁺, Dy³⁺/TiO₂ photoanode was obtained. Figure 1 shows a sketch of the structure of the DSSC. For comparison, the DSSC without SrAl₂O₄:Eu²⁺, Dy³⁺ containing layer was assembled.

Fig. 1

Schematic representation of the DSSC

2.4 Fabrication of the DSSC

The SrAl₂O₄:Eu²⁺, Dy³⁺/TiO₂ photoelectrode was immersed into the N719 dye-ethanol solution at room temperature in the dark for 24 h. Then a DSSC was assembled by injecting the redox electrolyte [0.1 M I₂, 0.1 M LiI, 0.6 M N(n-C₄H₉)₄I and 0.5 M TBP in acetonitrile] into interspace between the SrAl₂O₄:Eu²⁺, Dy³⁺/TiO₂ photoelectrode and a platinum counter electrode. The DSSC based on a pure TiO₂ photoelectrode was prepared using the same method.

2.5 Characterization

Field emission scanning electron microscopy (FESEM, SU8000, Hitachi, Japan) was used to study the phosphor morphology. Energy Dispersive Spectrometer (EDS, INCA Energy, OXFORD, UK) was performed to analyze the chemical elements. The crystal structure of the powder was identified by an X-ray diffractometer (XRD, Bruker D8 Advance, Germany) using Cu Kα radiation (λ = 1.5405 Å). The 2 θ angle of the XRD spectra was recorded at a scanning rate of 5° min⁻¹. The photovoltaic testing of the DSSCs was performed by measuring the photocurrent density-cell potential curves under a simulated solar light coming from a 3A solar simulator (94043A, Newport Corporation, America) equipped with a Xe lamp (450 W) and an AM1.5G filter. The impedance measurements were carried out in dark atmosphere and studied on CHI660E (Shanghai Chenhua Device Company, China). The excitation and emission spectra were obtained from a fluorescence spectrometer (Lumina, Thermo Scientific, America). The decay curve was recorded using a fluorescence spectrophotometer (FLS980, Edinburgh, UK) fitted with a xenon lamp after the sample was sufficiently excited for about 15 min. UV–Vis diffuse reflection tests were performed with an UV/Vis–NIR spectrometer (Lambda 950, PerkinElmer, America).

3 Results and discussions

3.1 Morphology and structure characterizations

Field emission scanning electron microscopy (FESEM) measurements were carried out to observe the morphology and size of the sample. Figure 2a shows the FESEM image of the prepared strontium aluminate crystal. Figure 2b shows the surface morphology of the SrAl₂O₄:Eu²⁺, Dy³⁺/TiO₂ photoelectrode.

Fig. 2

FESEM pattern of a SrAl₂O₄:Eu²⁺, Dy³⁺ powder and b SrAl₂O₄:Eu²⁺, Dy³⁺/TiO₂ photoelectrode. c EDS pattern of SrAl₂O₄:Eu²⁺, Dy³/TiO₂ photoelectrode

From Fig. 2a, we can see there are many cracks, voids and pores formed on the surface of the foams due to the escaping gases during combustion reaction. The large amount of escaping gases can dissipate heat so as to prevent the material from sintering and contribute to the formation of nanocrystalline phase. The phosphor particles are irregular. The obvious phenomenon of agglomeration suggests the crystal activity of the generated phosphor powder is very high. Figure 2b indicates the successful doping of SrAl₂O₄:Eu²⁺, Dy³. Energy-dispersive X-ray Spectroscopy (EDS) was carried to analyse the composition of the SrAl₂O₄:Eu²⁺, Dy³/TiO₂ photoelectrode, as exhibited in Fig. 2c, which shows that as-synthesized sample composed of Sr, Al, O, Ti, Dy and Eu Elements and no other spurious impurity elements were detected. The results further confirm the existence of TiO₂ and SrAl₂O₄:Eu²⁺, Dy³.

Figure 3 displays a typical X-ray diffraction pattern of the resultant TiO₂, SrAl₂O₄:Eu²⁺, Dy³⁺ and SrAl₂O₄:Eu²⁺, Dy³/TiO₂ respectively. The pure monoclinic phase diffraction peaks of SrAl₂O₄ are predominant in the XRD pattern. Each diffraction peak in the pattern is similar with the standard pattern of SrAl₂O₄ JCPDS data (No. 34-0379). It means SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor belongs to a P21 space group and a monoclinic system (a = 8.442 Å, b = 8.822 Å, c = 5.160 Å, β = 93.415°). The result indicates the crystalline phase of SrAl₂O₄ when doped with Eu²⁺ and Dy³⁺ rare earth ions. The small amount of Eu²⁺ and Dy³⁺ doped ions enter the host lattice and replace Sr²⁺ ion partly, they have almost no effect on the SrAl₂O₄ phase composition [16]. Besides, other obvious diffraction peaks can’t be found in the XRD pattern, proving the resultant material doesn’t contains other crystalline phases. The phase diffraction peaks of pure TiO₂ are also presented in the XRD pattern. It is clear to see that the diffraction peaks of TiO₂ located at 25.3°, 37.2°, 48.0°, 53.7°, 55.0°, 63.0°, 69.0°, 70.2° and 75.0° can be respectively indexed to the (101), (004), (112), (200), (105), (211), (204), (116), (220), and (215) crystal planes of anatase TiO₂ (JCPDS card 21-1272). Compared with the pure TiO₂ and single phosphor powder, all the major diffraction peaks of TiO₂ and SrAl₂O₄ can be found and no extra peak was detected in the XRD pattern of SrAl₂O₄:Eu²⁺, Dy³/TiO₂, which suggests that the SrAl₂O₄:Eu²⁺, Dy³⁺ particles have combined with the TiO₂ colloid.

Fig. 3

XRD patterns of TiO₂, SrAl₂O₄:Eu²⁺, Dy³⁺ and SrAl₂O₄:Eu²⁺, Dy³/TiO₂

3.2 The properties of SrAl₂O₄:Eu²⁺, Dy³⁺ powder

Figure 4a shows the excitation and emission spectra spectrums of SrAl₂O₄:Eu²⁺, Dy³⁺. It shows a broad band and at maximum excitation peak the corresponding excitation wavelength of is 370 nm which is assigned to the characteristic 4f⁷ → 4f⁶5d¹ transition of the Eu²⁺ ion. The emission spectra at room temperature of SrAl₂O₄:Eu²⁺, Dy³⁺ was shown in the Fig. 4b at excitation wavelength of 370 nm, it yields high bright green luminescence (λ_max = 520 nm) with only one broad band which is due to the typical 4f⁶5d¹ → 4f⁷ transition of luminescent center Eu²⁺ ion [17]. Dy³⁺ ion is an auxiliary active ion and work as a hole trap which generates the trap energy level in the original matrix. It is the hole trapped–transported–detrapped process that results in the properties of long afterglow of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor [18]. The luminescence 520 nm is just within the absorption wavelength range of commonly used dye N719. In addition, the ultraviolet irradiation from the sun can be reabsorbed by the dye N719 via the DC luminescence of SrAl₂O₄:Eu²⁺, Dy³⁺ and contributes to the solar light harvest of DSSC. Besides, from the UV–Vis diffuse reflection spectroscopy shown in Fig. 4c, an obvious absorption was exhibited in the 200–400 nm region which improved the light scattering capabilities of the dye, resulting in an improved light harvesting ability.

Fig. 4

Excitation spectra spectrum (a) and emission spectra spectrum of SrAl₂O₄:Eu²⁺, Dy³⁺ (b) UV–Vis-DRS spectra of TiO₂ nanoparticles and SrAl₂O₄:Eu²⁺, Dy³⁺ powder (c)

The decay characteristics of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor was tested by continuous exciting with 370 nm UV light for about 15 min. The measurements started in the darkness after the excited source was cut off. As we can see in the Fig. 5, the decay processes of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor contain the rapid-decaying process and the slow-decaying one. The initial intensity of phosphor is very high and then the intensity of the afterglow decreases rapidly and finally formed a stable long persistent emission. As we know, the roles of Eu²⁺ ions are the luminescent centers and Dy³⁺ ions serve as the luminescent centers in aluminate phosphors. The first decay process could be attributed to the short survival time of the electron in Eu²⁺ while the slow decay process results from the deep trap energy center of Dy³⁺. It can be seen from the afterglow decay curve that the SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor we have prepared exhibited a good afterglow property.

Fig. 5

Afterglow decay curve of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor after exciting for 15 min

3.3 The photoelectric performance of the DSSC

The photovoltaic parameters including the open-circuit voltage (V_OC), short-circuit density (J_SC), fill factor (FF) and overall light-to-electrical energy conversion efficiency (η) of these DSSCs under a simulated solar light irradiation of 100 mW cm⁻² are summarized in Table 1 and the photocurrent-voltage curves of the DSSC with and without SrAl₂O₄:Eu²⁺, Dy³⁺ dopant are showed in Fig. 6. Table 1 shows that J_SC and increased with the SrAl₂O₄:Eu²⁺, Dy³⁺ dopant level up to 7 wt%, beyond which J_SC decreased. The increase in J_SC comes mainly from the introduction of long persistent phosphors, which caused more sunlight to be harvested, thus increasing J_SC. After that, the large amounts of phosphors influence the TiO₂ crystal gap, which cause the block of electronic transmission and the decrease of J_SC [19, 20]. While FF increased gradually, from 0.662 to 0.729 V.

Table 1 Influence of the amount of SrAl₂O₄:Eu²⁺, Dy³⁺ on the photoelectric properties of the DSSC

Fig. 6

J–V curves of the DSSC with and without SrAl₂O₄:Eu²⁺, Dy³⁺ dopant (7 wt%)

As we can see in Fig. 6, an obvious increasement of the sunlight conversion efficiency for DSSC with SrAl₂O₄:Eu²⁺, Dy³⁺ dopant. The DSSC without dopant displays the photovoltaic parameters: V_OC = 0.797 V, J_SC = 12.54 mA cm⁻², FF = 0.662 η = 6.62 %. And DSSC with SrAl₂O₄:Eu²⁺, Dy³⁺ dopant demonstrates enhanced photovoltaic parameters: V_OC = 0.828 V, J_SC = 13.41 mA cm⁻², FF = 0.715, η = 7.94 %. The enhancement mainly due to the DC luminescence response of SrAl₂O₄:Eu²⁺, Dy³⁺ which transfers the ultraviolet light to visible light and broadens the absorption spectrum of the DSSC. As a result, more photoinduced electrons can be produced to enhance the photocurrent, increasing the harvested sunlight and efficiency of DSSC was improved.

The DSSC with SrAl₂O₄:Eu²⁺, Dy³⁺ dopant has a unique property compared with the pure TiO₂ electrode for its emission of long persistent light. After the DSSCs were illuminated under a simulated solar light for 1 min, the light source was turned off and the photovoltaic performance of the cells was measured in the dark. Table 2 lists the different data between the cells containing the TiO₂ and SrAl₂O₄:Eu²⁺/TiO₂ electrodes. The efficiency of the DSSC with SrAl₂O₄:Eu²⁺, Dy³⁺ dopant is 0.0076 % in the dark, while the efficiency of the DSSC containing the pure TiO₂ electrode is almost zero. The emission of long persistent light from the long afterglow phosphor makes it possible for a cell works in the dark. This method represents a novel approach to improve the DSSC.

Table 2 Photovoltaic parameters of DSSC without and with SrAl₂O₄:Eu²⁺, Dy³⁺ dopant (7 wt%) in the dark after 100 mW cm⁻² illumination for 1 min

As shown in Fig. 7, the photocurrent action (IPCE) spectra show that the absolute IPCE of the cell containing the SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor is higher than the cell containing the pure TiO₂ electrode in both the UV and visible regions, which is in good agreement with the observed higher J_SC. The improvement is mainly due to the introduction of SrAl₂O₄:Eu²⁺, Dy³⁺ which could absorb near UV-photons and causes a visible green emission. The excited dye N719 emits green light and excess electrons generate. As a result light harvesting ability within the electrode is enhanced and the quantum efficiency increases.

Fig. 7

IPCE spectra of the DSSC without and with SrAl₂O₄:Eu²⁺, Dy³⁺ dopant (7 wt%)

3.4 The impedance spectra of the DSSC

The charge transfer and recombination behavior in the DSSC were studied by analyzing the EIS spectra in the dark. The EIS spectra in Fig. 8 are characterized by the presence of two semicircles in a Nyquist plot [21, 22]. The smaller semicircle in high frequency region represents the electron ejection at the platinum counter electrode/electrolyte interface (R₁), and the larger one in medial frequency region is related to chemical capacitance of nanostructured TiO₂ and the charge recombination resistance (R₂) which reveals the charger transfer at the TiO₂/Dye/electrolyte interface and transport in the TiO₂ films.

Fig. 8

Nyquist plots of the impedance data of the DSSC with different amount of SrAl₂O₄:Eu²⁺, Dy³⁺ tested in the dark. The inset is the equivalent circuit model of the DSSC

The detailed results are showed in Table 3. R₁ values are similarly obtained for the DSSC with different amount of SrAl₂O₄:Eu²⁺, Dy³⁺ dopant. While R₂ value increased as the doping amount of SrAl₂O₄:Eu²⁺, Dy³⁺ varies from 0 to 7 wt%. The raised value of R₂ is corresponding to the circumstances for charge recombination from the conduction band of the anode materials to the redox I⁻/I₃ ⁻ couple. The increasing SrAl₂O₄:Eu²⁺, Dy³⁺ in TiO₂ produced some crystal defects, causing electron transport resistance. It means that the charge recombination at the interface of TiO₂/Dye/electrolyte within the DSSC is hindered because of the difficulties in charge transferring in dark condition [23, 24], which is favorable for the improvement of photovoltaic performance.

Table 3 Fitted data for the EIS measurements of the DSSCs

4 Conclusion

In summary, long persistent phosphor SrAl₂O₄:Eu²⁺, Dy³⁺ was successfully synthesized by combustion method and introduced into a DSSC. As a luminescence medium, SrAl₂O₄:Eu²⁺, Dy³⁺ improved light harvesting via a conversion luminescence process and increased photocurrent. SrAl₂O₄:Eu²⁺, Dy³⁺ elevated the energy level of the oxide film and increased the photovoltage. The solar conversion efficiency for a DSSC with SrAl₂O₄:Eu²⁺, Dy³⁺ doping (7:100) reached 7.938 % under a simulated solar light irradiation of 100 mW cm⁻², increased by a factor of 1.32 compared to that of the DSSC without SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor doping. This work demonstrates the feasibility of the long persistent phosphor doping in a DSSC and provides a novel way to enhance the sunlight conversion efficiency for solar cells.