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Morphology luminescence and photovoltaic performance of lanthanide-doped CaWO4 nanocrystals
Journal of Colloid and Interface Science 559 (2020) 162–168
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Morphology luminescence and photovoltaic performance of lanthanidedoped CaWO4 nanocrystals Mingqi Yu a,1, Hongyi Xu a,1, Yanzhen Li a,b, Qilin Dai b,⇑, Guofeng Wang a,⇑, Weiping Qin c a
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China Department of Physics, Jackson State University, Jackson 17660, United states c College of Electronic Science and Engineering, Jilin University, 120012, China b
g r a p h i c a l a b s t r a c t CaWO4:Ln3+ nanocrystals were synthesize by a facile solvothermal method, and study the effects of Ln3+ and the ratios of Ca2+ to WO42 on the morphology and size of CaWO4 nanocrystals. Upconversion and downconversion luminescence behavior show that CaWO4 is an excellent host for rare earth doping. CaWO4:Er nanowires are also used in dye sensitized solar cells (DSSCs) to improve device performance.
a r t i c l e
i n f o
Article history: Received 22 June 2019 Revised 5 September 2019 Accepted 3 October 2019 Available online 8 October 2019 Keywords: CaWO4 Nanocrystals Luminescence Photovoltaic
a b s t r a c t Multifunctional materials have attracted recent attention due to their various applications in many fields. In this work, CaWO4 nanocrystals were prepared by a hydrothermal method. Nanoparticles, nanowires, and micro-sized structures were obtained by controlling the reaction temperature, Ca2+-to-WO2 4 ratio and type of dopant. The influence of rare earth ions on the morphology and luminescence properties was investigated. Upconversion and downconversion luminescence behaviors showed CaWO4 to be an excellent host for rare earth doping. CaWO4:Er3+ nanowires were also used in dye-sensitized solar cells (DSSCs) to improve device performance. Increased light harvesting caused by improved dye loading capacity and enhanced light scattering led to improved cell efficiency. Moreover, reduced charge recombination due to the additional energy levels of CaWO4:Er3+ was another reason for the improved cell efficiency. Therefore, in this work, we demonstrated the synthesis of CaWO4 nanocrystals and the control of their morphology and luminescence by rare earth doping, which has significant future applications in lighting and display. The applications in DSSCs provide a new strategy to achieve high-performance solar cells by using nanocrystals via increased light harvesting and reduced charge recombination. Ó 2019 Elsevier Inc. All rights reserved.
⇑ Corresponding authors. 1
E-mail addresses: [email protected] (Q. Dai), [email protected] (G. Wang). Mingqi Yu and Hongyi Xu contributed equally to this work.
https://doi.org/10.1016/j.jcis.2019.10.011 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
M. Yu et al. / Journal of Colloid and Interface Science 559 (2020) 162–168
1. Introduction In recent years, rare earth-based luminescent nanomaterials have been widely studied due to their low toxicity, high chemical stability, excellent light stability, narrow emission spectra and long luminescence lifetime, leading to important applications in illumination, flat panel display, cell imaging and medical radiology [1-5]. Tungstates are attracting much attention due to their high luminescence quantum yield under the excitation of X-rays, electron beams and ultraviolet radiation [6]. CaWO4 has the advantages of low phonon energy, high radiation damage resistance, good stability and a simple synthesis process, and is the dominate phosphor in the tungstate family. Moreover, these rare earth ions can easily replace Ca2+ ions while retaining the scheelite structure of CaWO4 due to the similar radii of rare earth ions and Ca2+ ions. Rare earth ions have a regulatory effect on nanocrystal structures such as NaYF4, vanadate and tungstate due to the influence of the rare earth ion radius and its interaction with other ions [7-9]. The W6+ ion in CaWO4 exhibits a large charge and a small radius, which can cause enhanced polarization of the cation and induce deformation of the anion, resulting in reduced symmetry and an enhanced energy level splitting of rare earth ions [10,11]. Therefore, CaWO4 is an excellent matrix for rare earth doping, and the luminescent properties of CaWO4 phosphor can be controlled by doped rare earth ions [12-18]. Substantial effort has been made to synthesize rare earth-doped CaWO4 nanocrystals and microcrystals including nanospheres [19,20], nanorods [21], nanowires [22], microspheres [20], and microwires [23]. Nanowires and nanotubes were synthesized by a template-based method [22], which requires an electrospinning facility. Recently, we developed a low-temperature synthesis method to prepare nanowires, nanospheres and microspheres [24]. However, the synthesized particles are aggregated nanoparticles or micro-sized particles. Controlling the synthesis of nanomaterials is critical not only for the corresponding applications but also for understanding the fundamental properties of the nanocrystals. However, the synthesis of rare earth-doped CaWO4, with various morphologies, sizes and monodispersed nanocrystals, is remains challenging using current techniques. Fossil fuels are the main energy source to meet our current energy needs. However, they are non-renewable and have serious environmental pollution issues. Solar energy is one of the most promising technologies to replace fossil fuels because it is clean and renewable energy resource. Solar cells can convert solar energy to electrical energy, which is believed to be one of the most efficient ways to utilize solar energy. Dye-sensitized solar cells (DSSCs) have received widespread attention since 1991 due to their low cost, easy fabrication technique and low toxicity. However, DSSCs show a record efficiency of only 12% by the current techniques [25], which is not sufficiently high for practical applications. Therefore, the further improvement of cell efficiency is a major concern to be addressed for DSSCs. It is well known that the efficiency of a cell can be influenced by the charge injection, transport, recombination, and regeneration of dyes [26,27]. Photoelectric anodes play an important role in electron hole separation, dye adsorption and charge transport. In 1991, O‘Regan et al. applied TiO2 nanoparticles as a photoelectric anode in DSSCs for the first time [28]. The amount of dye adsorption was greatly improved, and the efficiency of DSSCs was considerably enhanced due to the large surface area of the nanostructures. In addition, TiO2 received extensive attention for use as a photoelectric anode in DSSCs due to its chemical stability and its excellent optical, photochemical, and optoelectronic properties [29,30]. It is reported that cell efficiency can be improved by optimizing the photoelectric anode [31]. Common methods for modifying TiO2 photoanodes
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include surface modification, surface treatment, semiconductor recombination, ion doping, and structural optimization. The main purpose of these techniques is to improve the light harvesting efficiency, the rate of photoelectron transmission, and the adsorption rate of dyes. Moreover, charge loss occurs in the charge injection process from dye to TiO2 and can be reduced by inserting an energy level between the dye lowest unoccupied molecular orbital (LOMO) state and the conduction band of TiO2. In this work, we synthesized CaWO4:Ln3+ nanocrystals by a facile solvothermal method and studied the effects of Ln3+ and the ratios of Ca2+ to WO2 4 on the morphology and size of CaWO4 nanocrystals. Various morphologies were obtained, including nanorods, nanowires, microspheres, and self-assembly nanospheres. The Ln3+ doping concentration impacts the CaWO4 nanocrystal growth significantly. The application of CaWO4:Ln3+ nanocrystals in DSSCs was also investigated. For the first time, a TiO2-CaWO4:Ln3+ composite was used as a photoanode for DSSCs. CaWO4:Ln3+ can adsorb N719 dye to increase the light harvesting capability for high-performance devices. The combination of TiO2 and CaWO4:Ln3+ reduces the energy level difference between the dye molecules and TiO2, leading to a reduced charge loss during charge injection. The light scattering ability is enhanced, and the effective light path is extended, leading to increased light harvesting and improved DSSC device performance. 2. Results and discussion We explored the optimal reaction conditions for the preparation of CaWO4 by controlling the reaction temperature and the molar ratio of Ca2+:WO2 4 in the raw materials. Fig. 1(a)–(c) shows the SEM images of the CaWO4 nanocrystals prepared at 140 °C, with different ratios of Ca2+:WO2 4 . Nanowires with a diameter of ~45 nm and a length of 220 nm were obtained at a Ca2+:WO2 4 ratio of 1:0.5 (Fig. 1a). The length increased to 450 nm as the ratio of Ca2+:WO2 increased to 1:1, while the diameter did not show 4 appreciable change (Fig. 1b). Micro-sized spheres were formed as the ratio of Ca2+:WO2 4 increased to 1:2 (Fig. 1c). Therefore, more WO2 groups benefited the production of larger particles. Fig. 1 4 (d)–(f) shows the SEM images of the CaWO4 nanocrystals prepared with different ratios of Ca2+:WO2 4 at 160 °C. It can be observed that the size and morphology of CaWO4 prepared at 160 °C was very similar to those prepared at 140 °C. Fig. 1 (g) and (h) shows the SEM images of CaWO4 nanocrystals prepared at 180 °C with ratios of Ca2+:WO2 4 1:1 and 1:2, respectively. The reaction temperature did not strongly affect the sizes of the nanowires. Liu proposed that decreased sizes could be obtained by decreasing the growth rate via surface charge modifications, leading to a reduced size of NaYF4 [32]. We attribute the reduced diameter of the CaWO4 nanowires to the similar effects caused by Er3+ doping. Fig. S1 shows the XRD data of CaWO4 prepared with different ratios and reaction temperatures. All of the samples are indexed to the scheelite-type tetragonal phase with space group I41/a and JCPDS card No. 41-1431. Fig. 2 (a)–(c) shows the TEM images of CaWO4 with Er3+ doping concentrations of 0%, 1% and 5%. The length of the nanowires decreased from 250 nm to 150 nm, while the diameter remained quite stable (~70 nm) as the doping concentration increased from 0% to 1%. The diameter decreased to 13 nm as the Er doping concentration increased to 5%. The HRTEM image in Fig. 2(d) of the sample with 5% Er3+ doping confirms the interplanar spacing of 0.949 nm, corresponding to the (1 0 1) plane of CaWO4. Irregular CaWO4 nanoparticles are shown in Fig. 2(f), where the Er3+ doping concentration was 50%. An excess of Er ions in the CaWO4 precursor disturbs the growth of the nanowires, resulting in irregular nanoparticles. The inset of Fig. 2(f) shows a high-magnification
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Fig. 1. The SEM images of CaWO4 prepared with different Ca2+:WO2 4 ratios and 2 2+ reaction temperatures: (a) Ca2+:WO2 4 = 1:0.5 (140 °C), (b) Ca :WO4 = 1:1 2 2+ 2+ (140 °C), (c) Ca2+:WO2 4 = 1:2 (140 °C), (d) Ca :WO4 = 1:0.5 (160 °C), (e) Ca : 2+ 2+ 2 2 WO2 4 = 1:1 (160 °C), (f) Ca :WO4 = 1:2 (160 °C), (g) Ca :WO4 = 1:1 (180 °C), and (h) Ca2+:WO2 4 = 1:2 (180 °C).
Fig. 2. TEM images of CaWO4:Er3+ with different Er3+ concentrations: (a) 0%, (b) 1%, (c-d) 5%, (e) 20%, (f) 50%. (g–l): TEM image of CaWO4:5%Er3+ and the corresponding HRTEM-EDX elemental mapping of the selected area.
TEM image of the irregular nanoparticles, which are 100–200 nm in length. Thus, the Er3+ doping concentration has a significant influence on the morphology of CaWO4. The diameters of the CaWO4 nanowires decreased with the increasing Er3+ doping concentration (when the doping concentration is less than 20%). The growth dynamics of the nanowires was disturbed when 50% Er ions were doped into the CaWO4 precursor, leading to irregular nanoparticles 100–200 nm in size. It is reported that the pH of the precursor solution has a significant influence on the morphology of metal tungstate nanomaterials [24,33,34]. Metal tungstate one-dimensional structures can be obtained when the pH of the precursor is greater than 7 [34]. In addition, the sites of the Ca2+ ions are occupied by Ln3+ ions, resulting in electron cloud distortion. With an increased Ln3+ ion radius, the dipole polarizability of CaWO4:Ln3+ increases, showing a stronger electron cloud distortion trend [35]. This can further affect the crystal growth rate and the morphology of CaWO4:Ln3+. The lanthanide ions and the dipolar polarizability also affect the CaWO4:Ln3+ nanocrystal
morphology [32]. With the increase of Ln3+ concentration, the average radius of nanocrystals decreases. As the size of the nanocrystals decreases, the specific surface area increases, and aggregation of nanocrystals is apt to occur [36]. Oriented attachment is another possible reason for the formation of non-spherical or only quasi-spherical morphologies [37]. Further, crystallographic interfaces among the attached crystallites could be slightly misoriented, which may result in dislocations and other crystal defects in the final products [38,39]. This may be the possible reason for the formation of irregular spheres. The replacement of Ca2+ by rare earth ions will produce more electron cloud distortion as the radius of rare earth ions increase. This has more influence on the size of CaWO4:Ln3+ [32]. In addition, the substitution of Ca2+ by trivalent rare earth ions leads to charge compensation. The EDX elemental mapping images of CaWO4:5% Er3+ in Fig. 2 (g–l) confirm the elemental composition of the nanocrystals. The results confirm the homogeneous distribution of Ca, W, O and Er in the nanocrystals, and the homogeneous doping of Er in the CaWO4 host. Fig. S2 shows the XRD results of CaWO4 with different Er3+ doping concentrations. All of the samples show pure phase CaWO4. The peaks at 28.8° shift towards the large-angle side in Fig. S3, which confirms the successful doping of Er3+ in CaWO4 crystal lattice. The values of Ln3+ mentioned in this paper are the initial contents of Ln(NO3)3. The results of the element mapping test (Fig. S4) indicated that the actual content of Er3+ reached 38% when the initial content of Er(NO3)3 was 50%. The higher initial Er3+ content can be attributed to the closer radii of Er3+ and Ca2+. In addition, the actual Er3+ concentration increased with the increase of the initial Er(NO3)3 content. Fig. 3 shows the TEM images of CaWO4 with different Eu3+ doping concentrations. It can be seen that all of the nanowires are connected to each other, and the diameters are very similar for nanowires with different doping concentrations. Fig. S5 shows the XRD patterns of the prepared CaWO4:Eu3+ nanocrystals with different Eu3+ concentrations. The characteristic diffraction peaks of CaWO4:Eu3+ nanocrystals exhibited pure CaWO4 phase. Therefore, Eu3+ doping did not change the morphology as much as doping with Er3+. To analyze the influences of the doping content on the luminescence of the doped particles, CaWO4:Yb3+ and CaWO4:Re3+/Yb3+ [Re = Er, Tm] were prepared with various doping concentration. As observed in Fig. S6, doping with Yb3+ also had a regulatory effect on the morphology of CaWO4. Increased Yb doping led to morphology changes from nanowires to micro-sized structures composed of smaller nanocrystals. Furthermore, we also studied the influence of doping with two rare earth ions on the CaWO4 nanocrystal morphology. SEM images of CaWO4:Er3+/Yb3+ and CaWO4:Tm3+/Yb3+ are shown in Fig. S7. All of the samples were doped with 20% Yb and different amounts of Tm3+ or Er3+. Since the radius of Yb3+ is close to those of Tm3+ and Er3+, on the basis of 20% Yb3+, the addition of Er3+ and Yb3+ led to the morphology changing from nanowires to aggregated microstructures. The XRD patterns of CaWO4:Er3+/Yb3+ in Fig. S8 confirm the pure phase of CaWO4. To investigate the effect of Er3+ substitution on the electronic band structure of CaWO4, DFT theoretical calculations were carried out. Fig. 4 and Fig. S9 show the electronic band structures and state densities of CaWO4 and CaWO4:Er3+. The band gap values of CaWO4 and CaWO4:Er3+ calculated by DFT theory are 0.119 Ha (3.238 eV) and 0.048 Ha (1.306 eV), respectively. The theoretical calculated value of the band gap of CaWO4 is close to the experimental value (3.25 eV for CaWO4 and 1.78 eV for CaWO4:Er3+), which will be discussed below. Er3+ doping led to a narrowing of the band gap. Notably, the density of the state varies greatly from CaWO4 to CaWO4:Er3+. We suggest that the difference in the density of state between
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Fig. 3. TEM images of CaWO4:Eu3+ with different Eu3+ concentration: (a) 3%, (b) 5%, (c) 7%, (d) 10%, (e) 20%, (f) 30%, (g) 40%, and (h) 50%.
CaWO4 to CaWO4:Er3+ could be attributed to the interaction between the d-orbital (Ca2+ ions) and the f-orbital (Er3+ ions). In addition, the Er3+ substitution sites in CaWO4:Er3+ had no distinct effect on the values of the band gap or on the state density. We studied the effects of rare earth doping on the downconversion luminescence properties of the CaWO4 nanocrystals (Fig. 5). The excitation spectra of CaWO4 doped with different Eu3+ concentrations are exhibited in Fig. 5(a). The broad band at 200–350 nm is attributed to the charge transfer bands of O-Eu3+ and O-W [24], 3+ which indicate the energy transfer from WO2 4 to Eu . The sharp peaks at ~362 nm, ~396 nm, ~418 nm, ~466 nm, and ~536 nm are assigned to the Eu3+ transitions of 7F0 ? 5D4, 7F0 ? 5L6, 7F0 ? 5D3, 7 F0 ? 5D2 and 7F0 ? 5D1, respectively. Fig. 5(b) shows the emission spectra (kex = 396 nm). The sharp peaks at ~595 nm, ~618 nm, ~658 nm and ~706 nm are assigned to the Eu3+ transitions of 5 D0 ? 7F1, 5D0 ? 7F2, 5D0 ? 7F3 and 5D0 ? 7F4, respectively. Fig. 5 (c) shows the Eu concentration-dependent luminescence intensity peak at 619 nm. The optimized Eu3+ concentration was 20%, which is consistent with a previous report [40]. The effect of the type and concentration of rare earth ion on the luminescence properties of CaWO4 were investigated by using optical spectra. When the 980-nm diode laser was focused on the CaWO4:Er3+/Yb3+ nanocrystals, a remarkably green emission could be observed by the naked eye, as shown in Fig. 6(a). The UC luminescence spectra of the CaWO4:Er3+/Yb3+ nanocrystals consisted of three components: a strong green emission (515–540 nm) caused by 2H11/2 ? 4I15/2 transition, a green emission (540–570 nm) due to 4S3/2 ? 4I15/2 transition, and a relatively weak red emission (650–680 nm) originating from the 4F9/2 ? 4I15/2 transition. In the magnified UC luminescence spectra of CaWO4:Er3+/Yb3+ (in the upper right corner of Fig. 6(a)), two peaks at ~379 nm and ~410 nm are observed, which can be attributed to the transitions
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G11/2 ? 4I15/2 and 2H9/2 ? 4I15/2, respectively. When the Er3+ doping concentration was 7%, the emission peak intensity of the sample was the highest. When the Er3+ doping concentration was less than 7%, the luminescence intensity was weak, which may be attributed to fewer luminescence centers. Therefore, an increase in the Er3+ doping concentration leads to an increase in the intensity of the emission peak. When the Er3+ doping concentration was greater than 7%, the emission intensity gradually decreased due to concentration quenching. Fig. 6(b) shows the UC luminescence spectra of CaWO4:7%Er3+/20%Yb3+ excited with different power values. The emission peak intensities of the sample are the strongest at a power of 0.64 W. When the power was less than 0.64 W, the luminescence intensity gradually increased as the excitation energy increased. As the excitation power increased, part of the light energy was converted into heat energy, and the emission peak intensity decreased. The UC luminescence properties of CaWO4:Er3+ without Yb3+ were also investigated, as shown in Fig. S10. Yb3+ was more easily excited as a sensitizer to transfer energy to Er3+; therefore, the undoped Yb3+ sample had a lower emission peak than the Yb3+-doped sample. The peak position of the sample did not change. The sample without Yb3+ showed almost no red-light emission. The UC luminescence spectra of CaWO4:Tm3+/Yb3+ with different Tm3+ concentrations under 980 nm laser excitation is shown in Fig. 6(c). Intense UC fluorescence was visible to the naked eye when the laser was focused on the nanocrystals. The blue emission (~478 nm) is ascribed to 1 G4 ? 3H6 transition. The red emission originates from 1G4 ? 3F4 (~652 nm), 3F2 ? 3H6 (~693 nm) and 3F3 ? 3H6 (~709 nm) transitions. The peak emission showed the highest intensity when the Tm3+ doping concentration reached 1%. Fig. 6(d) shows the UC luminescence spectra of CaWO4:1%Tm3+/20%Yb3+ excited with different power levels. The emission peak intensity of the sample was the strongest at 0.64 W. As shown in Fig. 6(f), the CIE coordinates of the UC luminescence were (0.201, 0.761) and (0.111, 0.137) for CaWO4:7%Er3+/20%Yb3+ and CaWO4:1%Tm3+/20%Yb3+, respectively. The prepared CaWO4:Eu3+ nanocrystals were used as a photoanode to improve the cell performance of TiO2-based cells. The photocurrent density-voltage (J-V) curves of pure TiO2 cells and TiO2-CaWO4:Er3+-based cells are shown in Fig. 7(a). The corresponding values of the open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and overall conversion efficiency (g), are shown in Fig. 7(b). The performance of the TiO2CaWO4:20%Er3+-based cell was higher than that of the pure TiO2based cell. The preparation of the photoanodes was consistent with previously published synthetic processes. The overall thickness of the electrode was approximately 10 mm [9,41]. The thickness of the TiO2-CaWO4:Er3+ composite photoanode remained unchanged because the amount of CaWO4:Er3+ in the TiO2-CaWO4:Er3+ com-
Fig 4. Electronic band structures and crystal structure: CaWO4 and CaWO4:Er.
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Fig. 5. (a) Excitation spectra of CaWO4:Eu3+ with different Eu3+ concentrations monitored at 618 nm. (b) Emission spectra of CaWO4:Eu3+ with different Eu3+ concentrations excited at 396 nm. (c) Eu concentration-dependent luminescence intensity of CaWO4:Eu3+.
Fig. 6. UC luminescence spectra of (a) CaWO4:Er3+/20%Yb3+ with different Er3+ concentrations under 980 nm excitation. (b) CaWO4:7%Er3+/20%Yb3+ at different powers. (c) CaWO4:Tm3+/20%Yb3+ with different Tm3+ concentrations under 980 nm excitation. (d) CaWO4:1%Tm3+/20%Yb3+ at different powers.(e) Plots (log–log) of emission intensity versus pump power density of CaWO4:Er3+ (f) CIE diagram of CaWO4:Er3+/Yb3+ and CaWO4:Tm3+/Yb3+.
posite photoanode was infinitesimal (mass ratio: 0.1%, 0.5%, 1%, 3%). The optophotoelectric properties of the DSSCs were also influenced by the Er3+ concentration. The device based on TiO2-0.5% CaWO4:20%Er3+ showed the highest efficiency, as shown in Fig. S11. As mentioned above, the size of the CaWO4 particles first decreases and then increases with increasing Er3+ concentration. When the concentration of Er3+ ions is 20%, the size of the nanocrystal is almost the smallest. It is well known that doping nanoparticles that are too large in size affects the quality of thin films, resulting in a decrease of photoelectric conversion efficiency. Thus, the photoelectric conversion efficiency was found to be optimal when the content of Er was 20%. Several mechanisms are responsible for the enhanced efficiency of the TiO2-CaWO4:Er3+-based cell. First, the larger dye loading
Fig. 7. (a) J-V curves of pure TiO2 and TiO2-CaWO4:Er3+ composite cells. (b) Solar cell parameters of pure TiO2 cells and TiO2-CaWO4:20%Er3+ composite cells with different amounts of CaWO4:20%Er3+ under simulated solar light radiation. (c) UV– Vis absorption spectra of desorption dye from N719-sensitized TiO2 and TiO2CaWO4:Er3+ photoanodes. (d) UV-Vis diffuse reflectance spectra of TiO2 and TiO2CaWO4:Er3+. (e) Band gap of TiO2. (f) Band gap of CaWO4:Er3+. (g) XPS valence-band spectra of samples. (h) Working principle of TiO2-CaWO4:Er3+ DSSCs.
capacity of TiO2-CaWO4:Er3+ leads to more light harvesting and photoelectrons, resulting in improved efficiency (Fig. 7c). Second, CaWO4:Er3+ plays a role in enhancing the light scattering in the device (Fig. 7d), leading to increased effective optical path length
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and light harvesting, which helps improve cell efficiency. Third, as shown in Fig. 7(e) and (f), CaWO4:Er3+ has a similar band gap as TiO2, and the conduction band and valence band positions of CaWO4:Er3+ are different from those of TiO2. The valence band positions of CaWO4 and CaWO4:20%Er3+ are calculated to be ~2.98 eV, as shown in Fig. 7(g). Finally, Fig. 7(h) shows the working principle of TiO2-CaWO4:Er3+ DSSCs. It can be seen that one additional energy level is inserted between the dye and TiO2, leading to reduced electron loss. Therefore, the reduced charge recombination is caused by the introduction of CaWO4 energy levels. CaWO4:Er3+ is complexed with TiO2 to effectively separate photogenerated carriers, which inhibits electron-hole recombination. EIS is utilized to investigate the internal resistance of the DSSC charge-transfer process. Usually, the impedance at low frequency (0.05–1 Hz) is associated with the Nernst diffusion of I3/I within the electrolyte. The impedance in the high-frequency region (1–100 kHz) is related to the capacitance and charge-transfer resistance at the interface of the Pt/I3/I electrolyte. The medium frequency in the range of 1–100 Hz is correlated with the photoelectrode–dye|I3/I electrolyte interface, where the accumulation of electrons and redox shuttles typically occurs. Fig. 8(a) shows the EIS data of the pure TiO2-based cell and the TiO2-CaWO4: Er3+-based cell. The interfacial resistance of the TiO2-dye|I3/I electrolyte interface of the pure TiO2 cell is much smaller than that of the TiO2-CaWO4:Er3+ cell. This indicates that the electron recombination resistance at the photoelectrode-dye|I3/I electrolyte interface increased, which suppressed the electrons in the semiconductor and I3 composite, reducing the generation of dark current and improving the photoelectric conversion efficiency of the battery. The inset in Fig. 8(a) shows the equivalent circuit for impedance spectra fitting. Rs is the series resistance (including FTO sheet resistance), the interface contact resistance and the wire resistance. R2 represents the charge transfer resistance at the photoelectrode-dye|I3/I electrolyte interface. ZDif is the finitelength Warburg impedance. The impedance of the finite-length Warburg diffusion is exhibited as
ZDif ¼ RDif
tanhðjxsÞ1=2 ðjxsÞ1=2
ð1Þ
where RDif = B/Y0 and s = B2. B is a constant phase element. The fitting parameters by the equivalent circuit are listed in Table S3. R2 is 19.89 X for a pure TiO2 cell and 24.35 X for a TiO2-CaWO4:Er3+-
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based cell. In addition, the series resistance (RS) values for pure TiO2 cells and TiO2-CaWO4:Er3+-based cells are 35.82 X and 35.66 X, respectively, indicating that the incorporation of CaWO4: Er3+ does not help the interfacial electron transfer of FTO|TiO2. In DSSCs, the electron recombination time (sn), the electron transport time (sd), and the charge collection efficiency (gcc) are critical parameters for the performance of the DSSCs. IMPS and IMVS are typical methods used to study the electron transfer and recombination process. The IMPS and IMVS response plots of pure TiO2-based cells and TiO2-CaWO4:Er3+-based cells are shown in Fig. 8(b,c). The TiO2-CaWO4:Er3+ composite cell exhibits a longer electron recombination time and a longer electron transport time compared with those of the pure TiO2 cell. Because Er3+ replaces Ti4+ to form a heterojunction, the Fermi level increases and energy transfer occurs, which accelerates charge separation. Additionally, the charge recombination time increases. Further, because the addition of CaWO4:Er3+ causes the film quality of the photoanode to decrease and the conductivity to decrease, the charge transfer time increases. The cell efficiency is increased under the influence of both the charge recombination time and the charge transfer time. The charge collection efficiency can be determined by the charge transfer time and charge recombination time. The charge collection efficiencies (gcc) of DSSCs are determined by the relation: gcc = 1 sd/sn, where sd is the charge transport time and sn is the charge recombination lifetime. Fig. 8(d) shows the charge collection efficiencies of pure TiO2-based cells and TiO2-CaWO4:Er3+-based cells. The charge collection efficiency of TiO2-CaWO4:Er3+-based cells is slightly higher than that of the pure TiO2 cell. The charge collection efficiency of the TiO2-CaWO4:Er3+ cell is slightly higher than that of the pure TiO2 cell. This indicates that the addition of CaWO4:Er3+ contributes to the improvement of cell efficiency. 3. Conclusion In this work, CaWO4 nanocrystals were synthesized by a hydrothermal method. Experimental parameters including the starting material ratio, reaction temperature, and type of rare earth dopant were used to control the nanocrystal growth. Nanowires with different lengths and diameters were thus obtained. Microsized structures and irregular nanostructures can be also achieved by adjusting the doping concentration of rare earth ions. Theoretical calculations show that doping with Er stabilizes CaWO4 more than the undoped nanocrystals. Luminescence properties show the efficient energy transfer from CaWO4 to Eu, producing a red emission, which has potential applications in lighting and display. The upconversion luminescence realized by Yb, Er and Tm doping exhibited three phonon process, indicating that CaWO4 is a promising candidate for upconversion luminescence. The application of CaWO4: Er3+ in DSSCs demonstrates three factors that influence the device performance. 1) the increased light harvesting caused by CaWO4: Er3+ leads to increased device performance. 2) reduced charge recombination due to the additional energy level of CaWO4:Er3+, leading to increased photocurrent density. 3) the optimized band alignment between the dye and CaWO4:Er3+ leads to efficient charge transport, resulting in decreased charge recombination. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21871079).
Fig. 8. (a) Nyquist plots of DSSCs comprised of pure TiO2 cells and TiO2-CaWO4:Er3+ cells. (b)Transport time constant and (c) recombination time constant for DSSCs as a function of applied voltage (or quasi-Fermi energy). (d) Comparison of charge collection efficiency for DSSCs as a function of applied voltage (or quasi-Fermi energy).
Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.10.011.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Morphology luminescence and photovoltaic performance of lanthanidedoped CaWO4 nanocrystals Mingqi Yu a,1, Hongyi Xu a,1, Yanzhen Li a,b, Qilin Dai b,⇑, Guofeng Wang a,⇑, Weiping Qin c a
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China Department of Physics, Jackson State University, Jackson 17660, United states c College of Electronic Science and Engineering, Jilin University, 120012, China b
g r a p h i c a l a b s t r a c t CaWO4:Ln3+ nanocrystals were synthesize by a facile solvothermal method, and study the effects of Ln3+ and the ratios of Ca2+ to WO42 on the morphology and size of CaWO4 nanocrystals. Upconversion and downconversion luminescence behavior show that CaWO4 is an excellent host for rare earth doping. CaWO4:Er nanowires are also used in dye sensitized solar cells (DSSCs) to improve device performance.
a r t i c l e
i n f o
Article history: Received 22 June 2019 Revised 5 September 2019 Accepted 3 October 2019 Available online 8 October 2019 Keywords: CaWO4 Nanocrystals Luminescence Photovoltaic
a b s t r a c t Multifunctional materials have attracted recent attention due to their various applications in many fields. In this work, CaWO4 nanocrystals were prepared by a hydrothermal method. Nanoparticles, nanowires, and micro-sized structures were obtained by controlling the reaction temperature, Ca2+-to-WO2 4 ratio and type of dopant. The influence of rare earth ions on the morphology and luminescence properties was investigated. Upconversion and downconversion luminescence behaviors showed CaWO4 to be an excellent host for rare earth doping. CaWO4:Er3+ nanowires were also used in dye-sensitized solar cells (DSSCs) to improve device performance. Increased light harvesting caused by improved dye loading capacity and enhanced light scattering led to improved cell efficiency. Moreover, reduced charge recombination due to the additional energy levels of CaWO4:Er3+ was another reason for the improved cell efficiency. Therefore, in this work, we demonstrated the synthesis of CaWO4 nanocrystals and the control of their morphology and luminescence by rare earth doping, which has significant future applications in lighting and display. The applications in DSSCs provide a new strategy to achieve high-performance solar cells by using nanocrystals via increased light harvesting and reduced charge recombination. Ó 2019 Elsevier Inc. All rights reserved.
⇑ Corresponding authors. 1
E-mail addresses: [email protected] (Q. Dai), [email protected] (G. Wang). Mingqi Yu and Hongyi Xu contributed equally to this work.
https://doi.org/10.1016/j.jcis.2019.10.011 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
M. Yu et al. / Journal of Colloid and Interface Science 559 (2020) 162–168
1. Introduction In recent years, rare earth-based luminescent nanomaterials have been widely studied due to their low toxicity, high chemical stability, excellent light stability, narrow emission spectra and long luminescence lifetime, leading to important applications in illumination, flat panel display, cell imaging and medical radiology [1-5]. Tungstates are attracting much attention due to their high luminescence quantum yield under the excitation of X-rays, electron beams and ultraviolet radiation [6]. CaWO4 has the advantages of low phonon energy, high radiation damage resistance, good stability and a simple synthesis process, and is the dominate phosphor in the tungstate family. Moreover, these rare earth ions can easily replace Ca2+ ions while retaining the scheelite structure of CaWO4 due to the similar radii of rare earth ions and Ca2+ ions. Rare earth ions have a regulatory effect on nanocrystal structures such as NaYF4, vanadate and tungstate due to the influence of the rare earth ion radius and its interaction with other ions [7-9]. The W6+ ion in CaWO4 exhibits a large charge and a small radius, which can cause enhanced polarization of the cation and induce deformation of the anion, resulting in reduced symmetry and an enhanced energy level splitting of rare earth ions [10,11]. Therefore, CaWO4 is an excellent matrix for rare earth doping, and the luminescent properties of CaWO4 phosphor can be controlled by doped rare earth ions [12-18]. Substantial effort has been made to synthesize rare earth-doped CaWO4 nanocrystals and microcrystals including nanospheres [19,20], nanorods [21], nanowires [22], microspheres [20], and microwires [23]. Nanowires and nanotubes were synthesized by a template-based method [22], which requires an electrospinning facility. Recently, we developed a low-temperature synthesis method to prepare nanowires, nanospheres and microspheres [24]. However, the synthesized particles are aggregated nanoparticles or micro-sized particles. Controlling the synthesis of nanomaterials is critical not only for the corresponding applications but also for understanding the fundamental properties of the nanocrystals. However, the synthesis of rare earth-doped CaWO4, with various morphologies, sizes and monodispersed nanocrystals, is remains challenging using current techniques. Fossil fuels are the main energy source to meet our current energy needs. However, they are non-renewable and have serious environmental pollution issues. Solar energy is one of the most promising technologies to replace fossil fuels because it is clean and renewable energy resource. Solar cells can convert solar energy to electrical energy, which is believed to be one of the most efficient ways to utilize solar energy. Dye-sensitized solar cells (DSSCs) have received widespread attention since 1991 due to their low cost, easy fabrication technique and low toxicity. However, DSSCs show a record efficiency of only 12% by the current techniques [25], which is not sufficiently high for practical applications. Therefore, the further improvement of cell efficiency is a major concern to be addressed for DSSCs. It is well known that the efficiency of a cell can be influenced by the charge injection, transport, recombination, and regeneration of dyes [26,27]. Photoelectric anodes play an important role in electron hole separation, dye adsorption and charge transport. In 1991, O‘Regan et al. applied TiO2 nanoparticles as a photoelectric anode in DSSCs for the first time [28]. The amount of dye adsorption was greatly improved, and the efficiency of DSSCs was considerably enhanced due to the large surface area of the nanostructures. In addition, TiO2 received extensive attention for use as a photoelectric anode in DSSCs due to its chemical stability and its excellent optical, photochemical, and optoelectronic properties [29,30]. It is reported that cell efficiency can be improved by optimizing the photoelectric anode [31]. Common methods for modifying TiO2 photoanodes
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include surface modification, surface treatment, semiconductor recombination, ion doping, and structural optimization. The main purpose of these techniques is to improve the light harvesting efficiency, the rate of photoelectron transmission, and the adsorption rate of dyes. Moreover, charge loss occurs in the charge injection process from dye to TiO2 and can be reduced by inserting an energy level between the dye lowest unoccupied molecular orbital (LOMO) state and the conduction band of TiO2. In this work, we synthesized CaWO4:Ln3+ nanocrystals by a facile solvothermal method and studied the effects of Ln3+ and the ratios of Ca2+ to WO2 4 on the morphology and size of CaWO4 nanocrystals. Various morphologies were obtained, including nanorods, nanowires, microspheres, and self-assembly nanospheres. The Ln3+ doping concentration impacts the CaWO4 nanocrystal growth significantly. The application of CaWO4:Ln3+ nanocrystals in DSSCs was also investigated. For the first time, a TiO2-CaWO4:Ln3+ composite was used as a photoanode for DSSCs. CaWO4:Ln3+ can adsorb N719 dye to increase the light harvesting capability for high-performance devices. The combination of TiO2 and CaWO4:Ln3+ reduces the energy level difference between the dye molecules and TiO2, leading to a reduced charge loss during charge injection. The light scattering ability is enhanced, and the effective light path is extended, leading to increased light harvesting and improved DSSC device performance. 2. Results and discussion We explored the optimal reaction conditions for the preparation of CaWO4 by controlling the reaction temperature and the molar ratio of Ca2+:WO2 4 in the raw materials. Fig. 1(a)–(c) shows the SEM images of the CaWO4 nanocrystals prepared at 140 °C, with different ratios of Ca2+:WO2 4 . Nanowires with a diameter of ~45 nm and a length of 220 nm were obtained at a Ca2+:WO2 4 ratio of 1:0.5 (Fig. 1a). The length increased to 450 nm as the ratio of Ca2+:WO2 increased to 1:1, while the diameter did not show 4 appreciable change (Fig. 1b). Micro-sized spheres were formed as the ratio of Ca2+:WO2 4 increased to 1:2 (Fig. 1c). Therefore, more WO2 groups benefited the production of larger particles. Fig. 1 4 (d)–(f) shows the SEM images of the CaWO4 nanocrystals prepared with different ratios of Ca2+:WO2 4 at 160 °C. It can be observed that the size and morphology of CaWO4 prepared at 160 °C was very similar to those prepared at 140 °C. Fig. 1 (g) and (h) shows the SEM images of CaWO4 nanocrystals prepared at 180 °C with ratios of Ca2+:WO2 4 1:1 and 1:2, respectively. The reaction temperature did not strongly affect the sizes of the nanowires. Liu proposed that decreased sizes could be obtained by decreasing the growth rate via surface charge modifications, leading to a reduced size of NaYF4 [32]. We attribute the reduced diameter of the CaWO4 nanowires to the similar effects caused by Er3+ doping. Fig. S1 shows the XRD data of CaWO4 prepared with different ratios and reaction temperatures. All of the samples are indexed to the scheelite-type tetragonal phase with space group I41/a and JCPDS card No. 41-1431. Fig. 2 (a)–(c) shows the TEM images of CaWO4 with Er3+ doping concentrations of 0%, 1% and 5%. The length of the nanowires decreased from 250 nm to 150 nm, while the diameter remained quite stable (~70 nm) as the doping concentration increased from 0% to 1%. The diameter decreased to 13 nm as the Er doping concentration increased to 5%. The HRTEM image in Fig. 2(d) of the sample with 5% Er3+ doping confirms the interplanar spacing of 0.949 nm, corresponding to the (1 0 1) plane of CaWO4. Irregular CaWO4 nanoparticles are shown in Fig. 2(f), where the Er3+ doping concentration was 50%. An excess of Er ions in the CaWO4 precursor disturbs the growth of the nanowires, resulting in irregular nanoparticles. The inset of Fig. 2(f) shows a high-magnification
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Fig. 1. The SEM images of CaWO4 prepared with different Ca2+:WO2 4 ratios and 2 2+ reaction temperatures: (a) Ca2+:WO2 4 = 1:0.5 (140 °C), (b) Ca :WO4 = 1:1 2 2+ 2+ (140 °C), (c) Ca2+:WO2 4 = 1:2 (140 °C), (d) Ca :WO4 = 1:0.5 (160 °C), (e) Ca : 2+ 2+ 2 2 WO2 4 = 1:1 (160 °C), (f) Ca :WO4 = 1:2 (160 °C), (g) Ca :WO4 = 1:1 (180 °C), and (h) Ca2+:WO2 4 = 1:2 (180 °C).
Fig. 2. TEM images of CaWO4:Er3+ with different Er3+ concentrations: (a) 0%, (b) 1%, (c-d) 5%, (e) 20%, (f) 50%. (g–l): TEM image of CaWO4:5%Er3+ and the corresponding HRTEM-EDX elemental mapping of the selected area.
TEM image of the irregular nanoparticles, which are 100–200 nm in length. Thus, the Er3+ doping concentration has a significant influence on the morphology of CaWO4. The diameters of the CaWO4 nanowires decreased with the increasing Er3+ doping concentration (when the doping concentration is less than 20%). The growth dynamics of the nanowires was disturbed when 50% Er ions were doped into the CaWO4 precursor, leading to irregular nanoparticles 100–200 nm in size. It is reported that the pH of the precursor solution has a significant influence on the morphology of metal tungstate nanomaterials [24,33,34]. Metal tungstate one-dimensional structures can be obtained when the pH of the precursor is greater than 7 [34]. In addition, the sites of the Ca2+ ions are occupied by Ln3+ ions, resulting in electron cloud distortion. With an increased Ln3+ ion radius, the dipole polarizability of CaWO4:Ln3+ increases, showing a stronger electron cloud distortion trend [35]. This can further affect the crystal growth rate and the morphology of CaWO4:Ln3+. The lanthanide ions and the dipolar polarizability also affect the CaWO4:Ln3+ nanocrystal
morphology [32]. With the increase of Ln3+ concentration, the average radius of nanocrystals decreases. As the size of the nanocrystals decreases, the specific surface area increases, and aggregation of nanocrystals is apt to occur [36]. Oriented attachment is another possible reason for the formation of non-spherical or only quasi-spherical morphologies [37]. Further, crystallographic interfaces among the attached crystallites could be slightly misoriented, which may result in dislocations and other crystal defects in the final products [38,39]. This may be the possible reason for the formation of irregular spheres. The replacement of Ca2+ by rare earth ions will produce more electron cloud distortion as the radius of rare earth ions increase. This has more influence on the size of CaWO4:Ln3+ [32]. In addition, the substitution of Ca2+ by trivalent rare earth ions leads to charge compensation. The EDX elemental mapping images of CaWO4:5% Er3+ in Fig. 2 (g–l) confirm the elemental composition of the nanocrystals. The results confirm the homogeneous distribution of Ca, W, O and Er in the nanocrystals, and the homogeneous doping of Er in the CaWO4 host. Fig. S2 shows the XRD results of CaWO4 with different Er3+ doping concentrations. All of the samples show pure phase CaWO4. The peaks at 28.8° shift towards the large-angle side in Fig. S3, which confirms the successful doping of Er3+ in CaWO4 crystal lattice. The values of Ln3+ mentioned in this paper are the initial contents of Ln(NO3)3. The results of the element mapping test (Fig. S4) indicated that the actual content of Er3+ reached 38% when the initial content of Er(NO3)3 was 50%. The higher initial Er3+ content can be attributed to the closer radii of Er3+ and Ca2+. In addition, the actual Er3+ concentration increased with the increase of the initial Er(NO3)3 content. Fig. 3 shows the TEM images of CaWO4 with different Eu3+ doping concentrations. It can be seen that all of the nanowires are connected to each other, and the diameters are very similar for nanowires with different doping concentrations. Fig. S5 shows the XRD patterns of the prepared CaWO4:Eu3+ nanocrystals with different Eu3+ concentrations. The characteristic diffraction peaks of CaWO4:Eu3+ nanocrystals exhibited pure CaWO4 phase. Therefore, Eu3+ doping did not change the morphology as much as doping with Er3+. To analyze the influences of the doping content on the luminescence of the doped particles, CaWO4:Yb3+ and CaWO4:Re3+/Yb3+ [Re = Er, Tm] were prepared with various doping concentration. As observed in Fig. S6, doping with Yb3+ also had a regulatory effect on the morphology of CaWO4. Increased Yb doping led to morphology changes from nanowires to micro-sized structures composed of smaller nanocrystals. Furthermore, we also studied the influence of doping with two rare earth ions on the CaWO4 nanocrystal morphology. SEM images of CaWO4:Er3+/Yb3+ and CaWO4:Tm3+/Yb3+ are shown in Fig. S7. All of the samples were doped with 20% Yb and different amounts of Tm3+ or Er3+. Since the radius of Yb3+ is close to those of Tm3+ and Er3+, on the basis of 20% Yb3+, the addition of Er3+ and Yb3+ led to the morphology changing from nanowires to aggregated microstructures. The XRD patterns of CaWO4:Er3+/Yb3+ in Fig. S8 confirm the pure phase of CaWO4. To investigate the effect of Er3+ substitution on the electronic band structure of CaWO4, DFT theoretical calculations were carried out. Fig. 4 and Fig. S9 show the electronic band structures and state densities of CaWO4 and CaWO4:Er3+. The band gap values of CaWO4 and CaWO4:Er3+ calculated by DFT theory are 0.119 Ha (3.238 eV) and 0.048 Ha (1.306 eV), respectively. The theoretical calculated value of the band gap of CaWO4 is close to the experimental value (3.25 eV for CaWO4 and 1.78 eV for CaWO4:Er3+), which will be discussed below. Er3+ doping led to a narrowing of the band gap. Notably, the density of the state varies greatly from CaWO4 to CaWO4:Er3+. We suggest that the difference in the density of state between
M. Yu et al. / Journal of Colloid and Interface Science 559 (2020) 162–168 4
Fig. 3. TEM images of CaWO4:Eu3+ with different Eu3+ concentration: (a) 3%, (b) 5%, (c) 7%, (d) 10%, (e) 20%, (f) 30%, (g) 40%, and (h) 50%.
CaWO4 to CaWO4:Er3+ could be attributed to the interaction between the d-orbital (Ca2+ ions) and the f-orbital (Er3+ ions). In addition, the Er3+ substitution sites in CaWO4:Er3+ had no distinct effect on the values of the band gap or on the state density. We studied the effects of rare earth doping on the downconversion luminescence properties of the CaWO4 nanocrystals (Fig. 5). The excitation spectra of CaWO4 doped with different Eu3+ concentrations are exhibited in Fig. 5(a). The broad band at 200–350 nm is attributed to the charge transfer bands of O-Eu3+ and O-W [24], 3+ which indicate the energy transfer from WO2 4 to Eu . The sharp peaks at ~362 nm, ~396 nm, ~418 nm, ~466 nm, and ~536 nm are assigned to the Eu3+ transitions of 7F0 ? 5D4, 7F0 ? 5L6, 7F0 ? 5D3, 7 F0 ? 5D2 and 7F0 ? 5D1, respectively. Fig. 5(b) shows the emission spectra (kex = 396 nm). The sharp peaks at ~595 nm, ~618 nm, ~658 nm and ~706 nm are assigned to the Eu3+ transitions of 5 D0 ? 7F1, 5D0 ? 7F2, 5D0 ? 7F3 and 5D0 ? 7F4, respectively. Fig. 5 (c) shows the Eu concentration-dependent luminescence intensity peak at 619 nm. The optimized Eu3+ concentration was 20%, which is consistent with a previous report [40]. The effect of the type and concentration of rare earth ion on the luminescence properties of CaWO4 were investigated by using optical spectra. When the 980-nm diode laser was focused on the CaWO4:Er3+/Yb3+ nanocrystals, a remarkably green emission could be observed by the naked eye, as shown in Fig. 6(a). The UC luminescence spectra of the CaWO4:Er3+/Yb3+ nanocrystals consisted of three components: a strong green emission (515–540 nm) caused by 2H11/2 ? 4I15/2 transition, a green emission (540–570 nm) due to 4S3/2 ? 4I15/2 transition, and a relatively weak red emission (650–680 nm) originating from the 4F9/2 ? 4I15/2 transition. In the magnified UC luminescence spectra of CaWO4:Er3+/Yb3+ (in the upper right corner of Fig. 6(a)), two peaks at ~379 nm and ~410 nm are observed, which can be attributed to the transitions
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G11/2 ? 4I15/2 and 2H9/2 ? 4I15/2, respectively. When the Er3+ doping concentration was 7%, the emission peak intensity of the sample was the highest. When the Er3+ doping concentration was less than 7%, the luminescence intensity was weak, which may be attributed to fewer luminescence centers. Therefore, an increase in the Er3+ doping concentration leads to an increase in the intensity of the emission peak. When the Er3+ doping concentration was greater than 7%, the emission intensity gradually decreased due to concentration quenching. Fig. 6(b) shows the UC luminescence spectra of CaWO4:7%Er3+/20%Yb3+ excited with different power values. The emission peak intensities of the sample are the strongest at a power of 0.64 W. When the power was less than 0.64 W, the luminescence intensity gradually increased as the excitation energy increased. As the excitation power increased, part of the light energy was converted into heat energy, and the emission peak intensity decreased. The UC luminescence properties of CaWO4:Er3+ without Yb3+ were also investigated, as shown in Fig. S10. Yb3+ was more easily excited as a sensitizer to transfer energy to Er3+; therefore, the undoped Yb3+ sample had a lower emission peak than the Yb3+-doped sample. The peak position of the sample did not change. The sample without Yb3+ showed almost no red-light emission. The UC luminescence spectra of CaWO4:Tm3+/Yb3+ with different Tm3+ concentrations under 980 nm laser excitation is shown in Fig. 6(c). Intense UC fluorescence was visible to the naked eye when the laser was focused on the nanocrystals. The blue emission (~478 nm) is ascribed to 1 G4 ? 3H6 transition. The red emission originates from 1G4 ? 3F4 (~652 nm), 3F2 ? 3H6 (~693 nm) and 3F3 ? 3H6 (~709 nm) transitions. The peak emission showed the highest intensity when the Tm3+ doping concentration reached 1%. Fig. 6(d) shows the UC luminescence spectra of CaWO4:1%Tm3+/20%Yb3+ excited with different power levels. The emission peak intensity of the sample was the strongest at 0.64 W. As shown in Fig. 6(f), the CIE coordinates of the UC luminescence were (0.201, 0.761) and (0.111, 0.137) for CaWO4:7%Er3+/20%Yb3+ and CaWO4:1%Tm3+/20%Yb3+, respectively. The prepared CaWO4:Eu3+ nanocrystals were used as a photoanode to improve the cell performance of TiO2-based cells. The photocurrent density-voltage (J-V) curves of pure TiO2 cells and TiO2-CaWO4:Er3+-based cells are shown in Fig. 7(a). The corresponding values of the open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and overall conversion efficiency (g), are shown in Fig. 7(b). The performance of the TiO2CaWO4:20%Er3+-based cell was higher than that of the pure TiO2based cell. The preparation of the photoanodes was consistent with previously published synthetic processes. The overall thickness of the electrode was approximately 10 mm [9,41]. The thickness of the TiO2-CaWO4:Er3+ composite photoanode remained unchanged because the amount of CaWO4:Er3+ in the TiO2-CaWO4:Er3+ com-
Fig 4. Electronic band structures and crystal structure: CaWO4 and CaWO4:Er.
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Fig. 5. (a) Excitation spectra of CaWO4:Eu3+ with different Eu3+ concentrations monitored at 618 nm. (b) Emission spectra of CaWO4:Eu3+ with different Eu3+ concentrations excited at 396 nm. (c) Eu concentration-dependent luminescence intensity of CaWO4:Eu3+.
Fig. 6. UC luminescence spectra of (a) CaWO4:Er3+/20%Yb3+ with different Er3+ concentrations under 980 nm excitation. (b) CaWO4:7%Er3+/20%Yb3+ at different powers. (c) CaWO4:Tm3+/20%Yb3+ with different Tm3+ concentrations under 980 nm excitation. (d) CaWO4:1%Tm3+/20%Yb3+ at different powers.(e) Plots (log–log) of emission intensity versus pump power density of CaWO4:Er3+ (f) CIE diagram of CaWO4:Er3+/Yb3+ and CaWO4:Tm3+/Yb3+.
posite photoanode was infinitesimal (mass ratio: 0.1%, 0.5%, 1%, 3%). The optophotoelectric properties of the DSSCs were also influenced by the Er3+ concentration. The device based on TiO2-0.5% CaWO4:20%Er3+ showed the highest efficiency, as shown in Fig. S11. As mentioned above, the size of the CaWO4 particles first decreases and then increases with increasing Er3+ concentration. When the concentration of Er3+ ions is 20%, the size of the nanocrystal is almost the smallest. It is well known that doping nanoparticles that are too large in size affects the quality of thin films, resulting in a decrease of photoelectric conversion efficiency. Thus, the photoelectric conversion efficiency was found to be optimal when the content of Er was 20%. Several mechanisms are responsible for the enhanced efficiency of the TiO2-CaWO4:Er3+-based cell. First, the larger dye loading
Fig. 7. (a) J-V curves of pure TiO2 and TiO2-CaWO4:Er3+ composite cells. (b) Solar cell parameters of pure TiO2 cells and TiO2-CaWO4:20%Er3+ composite cells with different amounts of CaWO4:20%Er3+ under simulated solar light radiation. (c) UV– Vis absorption spectra of desorption dye from N719-sensitized TiO2 and TiO2CaWO4:Er3+ photoanodes. (d) UV-Vis diffuse reflectance spectra of TiO2 and TiO2CaWO4:Er3+. (e) Band gap of TiO2. (f) Band gap of CaWO4:Er3+. (g) XPS valence-band spectra of samples. (h) Working principle of TiO2-CaWO4:Er3+ DSSCs.
capacity of TiO2-CaWO4:Er3+ leads to more light harvesting and photoelectrons, resulting in improved efficiency (Fig. 7c). Second, CaWO4:Er3+ plays a role in enhancing the light scattering in the device (Fig. 7d), leading to increased effective optical path length
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and light harvesting, which helps improve cell efficiency. Third, as shown in Fig. 7(e) and (f), CaWO4:Er3+ has a similar band gap as TiO2, and the conduction band and valence band positions of CaWO4:Er3+ are different from those of TiO2. The valence band positions of CaWO4 and CaWO4:20%Er3+ are calculated to be ~2.98 eV, as shown in Fig. 7(g). Finally, Fig. 7(h) shows the working principle of TiO2-CaWO4:Er3+ DSSCs. It can be seen that one additional energy level is inserted between the dye and TiO2, leading to reduced electron loss. Therefore, the reduced charge recombination is caused by the introduction of CaWO4 energy levels. CaWO4:Er3+ is complexed with TiO2 to effectively separate photogenerated carriers, which inhibits electron-hole recombination. EIS is utilized to investigate the internal resistance of the DSSC charge-transfer process. Usually, the impedance at low frequency (0.05–1 Hz) is associated with the Nernst diffusion of I3/I within the electrolyte. The impedance in the high-frequency region (1–100 kHz) is related to the capacitance and charge-transfer resistance at the interface of the Pt/I3/I electrolyte. The medium frequency in the range of 1–100 Hz is correlated with the photoelectrode–dye|I3/I electrolyte interface, where the accumulation of electrons and redox shuttles typically occurs. Fig. 8(a) shows the EIS data of the pure TiO2-based cell and the TiO2-CaWO4: Er3+-based cell. The interfacial resistance of the TiO2-dye|I3/I electrolyte interface of the pure TiO2 cell is much smaller than that of the TiO2-CaWO4:Er3+ cell. This indicates that the electron recombination resistance at the photoelectrode-dye|I3/I electrolyte interface increased, which suppressed the electrons in the semiconductor and I3 composite, reducing the generation of dark current and improving the photoelectric conversion efficiency of the battery. The inset in Fig. 8(a) shows the equivalent circuit for impedance spectra fitting. Rs is the series resistance (including FTO sheet resistance), the interface contact resistance and the wire resistance. R2 represents the charge transfer resistance at the photoelectrode-dye|I3/I electrolyte interface. ZDif is the finitelength Warburg impedance. The impedance of the finite-length Warburg diffusion is exhibited as
ZDif ¼ RDif
tanhðjxsÞ1=2 ðjxsÞ1=2
ð1Þ
where RDif = B/Y0 and s = B2. B is a constant phase element. The fitting parameters by the equivalent circuit are listed in Table S3. R2 is 19.89 X for a pure TiO2 cell and 24.35 X for a TiO2-CaWO4:Er3+-
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based cell. In addition, the series resistance (RS) values for pure TiO2 cells and TiO2-CaWO4:Er3+-based cells are 35.82 X and 35.66 X, respectively, indicating that the incorporation of CaWO4: Er3+ does not help the interfacial electron transfer of FTO|TiO2. In DSSCs, the electron recombination time (sn), the electron transport time (sd), and the charge collection efficiency (gcc) are critical parameters for the performance of the DSSCs. IMPS and IMVS are typical methods used to study the electron transfer and recombination process. The IMPS and IMVS response plots of pure TiO2-based cells and TiO2-CaWO4:Er3+-based cells are shown in Fig. 8(b,c). The TiO2-CaWO4:Er3+ composite cell exhibits a longer electron recombination time and a longer electron transport time compared with those of the pure TiO2 cell. Because Er3+ replaces Ti4+ to form a heterojunction, the Fermi level increases and energy transfer occurs, which accelerates charge separation. Additionally, the charge recombination time increases. Further, because the addition of CaWO4:Er3+ causes the film quality of the photoanode to decrease and the conductivity to decrease, the charge transfer time increases. The cell efficiency is increased under the influence of both the charge recombination time and the charge transfer time. The charge collection efficiency can be determined by the charge transfer time and charge recombination time. The charge collection efficiencies (gcc) of DSSCs are determined by the relation: gcc = 1 sd/sn, where sd is the charge transport time and sn is the charge recombination lifetime. Fig. 8(d) shows the charge collection efficiencies of pure TiO2-based cells and TiO2-CaWO4:Er3+-based cells. The charge collection efficiency of TiO2-CaWO4:Er3+-based cells is slightly higher than that of the pure TiO2 cell. The charge collection efficiency of the TiO2-CaWO4:Er3+ cell is slightly higher than that of the pure TiO2 cell. This indicates that the addition of CaWO4:Er3+ contributes to the improvement of cell efficiency. 3. Conclusion In this work, CaWO4 nanocrystals were synthesized by a hydrothermal method. Experimental parameters including the starting material ratio, reaction temperature, and type of rare earth dopant were used to control the nanocrystal growth. Nanowires with different lengths and diameters were thus obtained. Microsized structures and irregular nanostructures can be also achieved by adjusting the doping concentration of rare earth ions. Theoretical calculations show that doping with Er stabilizes CaWO4 more than the undoped nanocrystals. Luminescence properties show the efficient energy transfer from CaWO4 to Eu, producing a red emission, which has potential applications in lighting and display. The upconversion luminescence realized by Yb, Er and Tm doping exhibited three phonon process, indicating that CaWO4 is a promising candidate for upconversion luminescence. The application of CaWO4: Er3+ in DSSCs demonstrates three factors that influence the device performance. 1) the increased light harvesting caused by CaWO4: Er3+ leads to increased device performance. 2) reduced charge recombination due to the additional energy level of CaWO4:Er3+, leading to increased photocurrent density. 3) the optimized band alignment between the dye and CaWO4:Er3+ leads to efficient charge transport, resulting in decreased charge recombination. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21871079).
Fig. 8. (a) Nyquist plots of DSSCs comprised of pure TiO2 cells and TiO2-CaWO4:Er3+ cells. (b)Transport time constant and (c) recombination time constant for DSSCs as a function of applied voltage (or quasi-Fermi energy). (d) Comparison of charge collection efficiency for DSSCs as a function of applied voltage (or quasi-Fermi energy).
Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.10.011.
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