Upconversion luminescence enhancement by Fe3+ doping in CeO2:Yb/Er nanomaterials and their application in dye-sensitized solar cells

To make use of broad spectrum solar energy remains a main target in the photoelectrochemical area. Novel promising photoelectrode CeO2:Fe/Yb/Er nanomaterials supported on upconversion nanomaterials doped with transition-metal ions are reported to improve broad spectrum absorption and scattering properties in dye-sensitized solar cells (DSSCs) for the first time. The results demonstrate that the materials have stronger upconversion luminescence than CeO2:Yb/Er samples when the Fe3+ ion doping concentration is 2 mol% and 33.5% higher photoelectric conversion efficiency than a pure P25 electrode, which are attributed to the special light scattering properties and excellent dye adsorption capacity of the CeO2:Fe/Yb/Er nanomaterials. Accordingly, doping Fe3+ transition metal ions in the upconversion material CeO2:Yb/Er provides a new research idea for improving the photoelectric conversion efficiency of DSSCs.


Introduction
In today's deepening energy crisis, the inexhaustible solar energy has received more and more attention because of its safety, sustainability, and carbon-free nature. 1 In the photovoltaic industries using solar energy, dye-sensitized solar cells are photoelectrochemical systems with dye-adsorbed porousstructured oxides as photoanodes, which have the advantages of low cost, high efficiency and good stability. 2 However, the problem of spectral mismatch between the incident solar spectrum and the band gap of the semiconductor photoanode greatly reduces the utilization of sunlight by the battery. In particular, the near-infrared (NIR) light which accounts for nearly 50% of sunlight is wasted. Hence, enlarging the range of absorbable wavelengths in sunlight is one of the effective measures to reduce energy loss. 3 In order to obtain higher photocurrent values in the NIR region, researchers turned their attention to up-conversion materials that can convert low-energy excitations into highenergy emissions. Up-conversion nanophosphors (UCNPs) with nonlinear optical effects can convert NIR light into visible light by means of energy transfer or multiple absorptions. 4 Then the dye molecule absorbs visible light and produces more electrons, which promotes the conversion of sub-band gap photons into above-band gap photons to decrease transmission loss. Consequently, it is feasible to improve battery performance by introducing UCNPs into solar cells to enhance their utilization of sunlight. 5 In order to realize the above potential applications, it is particularly important to nd a suitable matrix material. The semiconductor oxide material CeO 2 has been widely in many elds such as photocatalysis, water oxidation, phosphors and thermoelectric materials, which is due to its environmental friendliness, thermal stability and chemical stability. 6,7 Especially in this work, the similar ionic radius of Ce 4+ and Yb 3+ , Er 3+ are conducive to the doping of rare earth ions and the lower phonon energy of CeO 2 is favorable for the emission of UC by reducing the multiphonon relaxation, which all lay the foundation for CeO 2 as the host material. 8,9 On the other hand, the remarkable features of CeO 2 such as high refractive index and high optical transparency provide support for its use as a light scattering layer to increase light collection efficiency in DSSCs with double-layer photoanode. 10 Herein, we have chosen CeO 2 as the host material for UCNPs is reasonable, and it is also the unique innovation of our team based on the above two considerations. 11 But the current exploration of CeO 2 -based upconversion materials is mostly achieved by the doping between Er 3+ /Yb 3+ and Er 3+ rare earth ions, which makes it difficult to further improve the luminous intensity. Thereupon, how to overcome this problem becomes a top priority.
As we all know, the probability of electronic transition has a decisive inuence on the intensity of upconversion luminescence in the UCNPs, but it is also easily affected by the local crystal eld around the RE ions. 12 In the sense, in addition to utilizing the plasma effect, photonic crystal effect and constructing a core-shell structure, it is a great signicance to increase the UC efficiency of UCNPs by changing the symmetry of local crystal elds. 13 In published literature reports, researchers have used this approach to achieve their goals. For example, the green light emission enhanced by 34 times and red light emission enhanced by 101 times when the Li + -doping concentration was 0.5 mol%, which were found in Wang's studies. 14 The three-doping system with Mg 2+ ions can signicantly increase the red, green and purple UC emissions in UCNPs by Zhao's research. 15 Compared with alkali ion doping, transition metal (TM) ions have potential to adjust the excited state properties to match the acceptor ions because the exposure of d electrons renders them very sensitive to the environment. Especially, the TM ions Fe 3+ have many energy levels to enable more energy exchange with RE ions. 16 The use of Fe 3+ ions alone in batteries has a number of drawbacks, so we have introduced Fe 3+ into DSSCs by codoping. This paper is the rst time to explore the effect of Fe 3+ on the strength of UC in CeO 2 matrix materials, which has important reference signicance. The incorporation of non-rare earth ion Fe 3+ will adjust the symmetry of the local crystal eld in the materials to enhance UC luminescence intensity. The prepared materials were used as photoanodes for dye-sensitized solar cells, and the good photovoltaic performance was obtained. At the same time, a new approach has been opened for the application of Fe 3+ ions in DSSCs.

Experiments
2.1 Preparation of CeO 2 :Fe/Yb/Er 0.8 g polyvinylpyrrolidone (PVP, K-30) was ultrasonically dispersed into 30 mL ultrapure water, and magnetically stirred with 60 minutes to form a clear solution. Subsequently, 2.4 mmol of metal ions [(96.7 À x) mol% Ce 3+ , 0.3 mol% Yb 3+ , 3 mol% Er 3+ , x mol% Fe 3+ (x ¼ 0.5, 1, 2, 3, 5)] was mixed with PVP solution and stirred vigorously for 60 minutes to make the mixture uniform. The homogenized liquid was poured into a Teon-lined with 50 mL and hydrothermally treated at 200 C for 12 hours. The product was centrifuged at 8000 rpm min À1 and washed several times with deionized (DI) water and ethanol, then dried overnight at 80 C. Moreover, the precipitate was placed in a muffle furnace, heated at 400 C for 1 hour, and then the same heating rate was controlled to continue to rise to 1100 C and held for 3 hours. In addition, the SiO 2 layer was wrapped on the surface of the materials to protect its morphology at high temperature; it was eliminated with NaOH solution (1 M) aer calcination. For comparison, the CeO 2 :Yb/ Er and CeO 2 :Fe nanomaterials were prepared by the same method.

Fabrication of solar cells
For the double layer photoanode, P25 paste was screen-printed on the FTO glasses, then the CeO 2 :Fe/Yb/Er nanoparticles were coated as an upper lm on the basis of the TiO 2 lm. The double layer lms with area of 0.16 cm 2 were sintered at 450 C for 30 minutes to form the photoanodes, which will be immersed in N719 dye solution (4.5 Â 10 À4 M in absolute acetonitrile) for 24 hours. Subsequently, rinsed photoanodes with absolute ethanol and allowed to air dry. The Pt-coated counter electrode and working electrode were assembled to form a sandwich-type battery whose structure is shown in Fig. 1. Finally, 0.5 M tertbutylpyridine, 0.1 M LiI, 0.60 M BMII and 0.05 M I 2 were dissolved in an acetonitrile solution to obtain the I À /I 3 À electrolyte, which was dropped into the battery. 17 For comparison, pure P25 electrode, CeO 2 :Yb/Er electrode and CeO 2 :Fe electrode were also assembled.

Characterization
The X-ray diffraction which used the Cu-Ka radiation (l ¼ 1.5406) has characterized the phase purity of powder. And the overall structure was studied by scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Highresolution transmission electron microscopy (HR-TEM), Energy dispersive X-ray (EDX) and X-ray photoelectron spectra (XPS) have analyzed the atomic composition of the sample. The Fluorescence spectrophotometer (Shimadzu 200) measured all spectra of upconversion photoluminescence emission by using a 980 nm semiconductor laser as the excitation source, because it has steady state uorescence. The diffuse reectance spectra of sample were observed by a UV-vis-NIR spectrophotometer (Cary 5000). Dye desorption measurements were carried out by detaching the N719 dye from photoanode lms in NaOH solution (0.1 M) and the concentration has been evaluated by UV-vis absorption spectroscopy. The I-V and the IPCE curves were valued by using a solar light simulator (Newport 94063) with the air mass 1.5 global (AM 1.5G) light. Electrochemical impedance spectroscopy (EIS) was measured in range of 100 kHz to 0.1 Hz by a computer controlled potentiostat (Autolab 320, Metrohm). This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 18868-18874 | 18869 and the phase structure of CeO 2 did not change. We can learn from the already reported literature that the doping of Fe 3+ in the host lattice is achieved by replacing other ions or occupying the gap sites. What's more important, these two occupancies of Fe 3+ ions in the main matrix both can change the environment around Er 3+ , thereby breaking the forbidden band transition and facilitating the f-f intra-conguration transition of rare earth ions. 18 SEM and TEM images characterize the morphologies of the prepared samples. As shown in Fig. 3a and d, the CeO 2 :Yb/Er sample with a particle diameter of 800 nm is composed of octahedral porous structures. Fig. 3b inset displays the HRTEM image of the sample and clearly shows that the nanoparticles are formed by agglomeration of small nanoparticles with a particle size of 3-5 nm. When the Fe 3+ ions are introduced, the samples of CeO 2 :Fe/Yb/Er (Fig. 3b and e) and CeO 2 :Fe ( Fig. 3c and f) are not only well dispersed but also uniform in size, their particle size was shrunk to about 120 nm. From the comparison of the three samples, it is observed that the doping of Er 3+ and Yb 3+ ions have no effect on the morphology of materials, but the Fe 3+ doping has obviously inuenced the particle size of samples.

Results and discussions
It can be deduced that the ionic radius of Er 3+ (0.89Å) and Yb 3+ (0.868Å) have little difference with the Ce 4+ (0.87Å), whereas Fe 3+ (0.64Å) with much smaller ionic radius than Ce 4+ (0.87Å), which induce to hinder the growth of the crystal. 9 Fig. 4 specically shows the structural details of the CeO 2 :Fe/ Yb/Er crystal. The EDX spectrum (Fig. 4a) conrms the successful doping of Er 3+ , Yb 3+ and Fe 3+ ions in samples.
The elemental mappings in Fig. 4d-g further prove the uniform distribution of Ce, Fe, Yb and Er in the whole nanoparticles. The SAED pattern (Fig. 4b) and HRTEM image (Fig. 4c) display the intercrystalline diffraction rings of CeO 2 :Fe/Yb/Er UCNPs and a typical 0.271 nm lattice spacing attributed to the (111) crystal plane of CeO 2 .
The surface elemental composition and chemical states of CeO 2 :Fe/Yb/Er is analyzed by XPS. It can be know from the XPS survey spectrum (Fig. 5a) that the CeO 2 :Fe/Yb/Er UCNPs only contains several elements of Ce, O, Fe, Er and Yb. The core level XPS spectrum of Ce 3d is attributed to the signal binding energies of Ce 4+ state while some of Ce 3+ state, the binding energy peak at 902.4 eV is attributed to high concentration of Ce 3+ and the binding energy peaks at 919.3, 900.3 eV are attributed to Ce 4+ oxidation state.
As shown in Fig. 5b, the Yb 4d and Er 4d XPS peaks were observed at a lower binding energy position at 192.3 eV and 168.4 eV, respectively. 19-21 XPS signals of Fe 2p (Fig. 5c) at 711 eV can be assigned to Fe 2p 3/2 and Fe 2p 1/2 . The major peak at 530.0 eV corresponds to O 2 and another two shoulder peaks around the main peak at 530 eV may correspond to be oxygen adsorbed water and forming hydroxyl groups. XPS proved the presence of Fe 3+ , Yb 3+ , Er 3+ , Ce 4+ and Ce 3+ ions in the samples.
When the amount of Yb 3+ and Er 3+ is constant, Fig. 6a shows the UC luminescence spectra of CeO 2 :Fe/Yb/Er UCNPs with Fe 3+ -doping concentration from 0.5% to 5% at room temperature under the commercial continuous wave diode 980 nm laser. The upconversion emission intensities were changed with the Fe 3+ doping. Two green emissions ranging from 517 to 532 nm and from 532 to 551 nm were deduced from the 2 H 11/2 / 4 I 15/2 and 4 S 3/2 / 4 I 15/2 transitions respectively. The red emission is observed at 659 nm and 679 nm, which due to the 4 F 9/2 / 4 I 15/2 transition of Er 3+ . 22 It can be known that the intensity of the upconversion photoluminescence changes signicantly with the increase of the Fe 3+ ion concentration, and the luminescence intensity is the strongest at the Fe 3+doping concentration of 2 mol%. While the enhancement rate of green light is obviously higher than the red region (Fig. 6b). It can also be observed from the uorescence spectra of the three materials in Fig. 6c that doping Fe 3+ does not affect the peak position. Furthermore, the green and red emission of CeO 2 :Fe/ Yb/Er (C Fe ¼ 2 mol%) samples are nearly 7 and 3 times stronger   than those of CeO 2 :Yb/Er materials, respectively. But the CeO 2 :Fe samples without doped rare earth ions have no luminescent effect. This phenomenon is probably due to the fact that the Fe 3+ ions with a smaller ion radius in the lattice shorten the average bond lengths of O-Ln bond, which destroys the symmetry of local crystal eld around the RE ions and increases the probability of electric dipole transition, therefore the upconversion luminous intensity is obviously enhanced. But the higher concentration of Fe 3+ may induces severe distortion of the crystal lattice, which will affects the spatial distribution of Er 3+ ions and cause concentration quenching, which eventually induced the uorescence radiation decreased. 12,23 Possible reason is there may be Yb 3+ -Fe 3+ dimer complex formation in the Yb 3+ -Er 3+ -Fe 3+ tridoped system. The mixed electron wave functions of 3d 5 electron-exposed Fe 3+ ions and Yb 3+ ions formed some new energy levels, thus triggering new energy transfer and enhancing UC emission. 18,21 The UV diffuses reectance spectra of four samples (Fig. 7) display that the changes aer doping with Fe 3+ ions are clearly related to the characteristics of unsensitized photoanodes. The reectance characteristics of the CeO 2 :Fe/Yb/Er increase distinctly compared with other materials. Furthermore, the CeO 2 :Fe/Yb/Er electrode shows the highest reectance within the long wavelength region (550-800 nm). When sunlight is irradiated onto DSSCs, the NIR photons can be converted into visible photon by the CeO 2 :Fe/Yb/Er nanomaterial layers. It is indicated that the upconversion luminescent materials can be used as an excellent light scattering medium for expanding the optical path length in the electrode, which will improve its light capturing ability and photoelectric conversion efficiency. It is worth mentioning that the electrode coated with CeO 2 :Fe materials has no advantage in light scattering because of its no upconversion characteristics. 24 As we all know, the dye loading capacity of the photoanode lms will greatly affect the efficiency of the DSSCs. Fig. 8 is an   Obviously, the photoanodes aer printed materials have a signicant improvement in dye adsorption compared to the pure P25 electrode. Among them, the lms of CeO 2 :Fe/Yb/Er and CeO 2 :Fe have the same dye adsorption capacity substantially, and both are larger than CeO 2 :Yb/Er lm. This may be attributed to the smaller particle diameter of CeO 2 :Fe/Yb/Er and CeO 2 :Fe nanomaterials than the CeO 2 :Yb/Er and the enhancement of the ability in dyes adsorbing.
The top and cross-section of the photoanode were characterized by SEM images. Fig. 9a shows that the photoanode of DSSCs is a typical two-layer structure, and its light scattering materials is tightly covered on the host material TiO 2 . The two layers fused to form a large assembly and the boundary is clearly visible. As can be seen in Fig. 9b, the upconversion nanomaterials are adhered to each other and are uniformly arranged on the P25 layer with the help of adhesives which is consisted of ethylcellulose and Triton X-100 terpineol solution. 25 The DSSCs coated with different materials have been tested for I-V (Fig. 10) and the photovoltaic parameters are shown in Table 1. Aer coating CeO 2 :Fe material on the P25 lm, the short-circuit current is improved from 11.85 mA cm À2 which only had the TiO 2 lm up to the 12.32 mA cm À2 and the photoelectric conversion efficiency increased form 5.47% to 5.89%. When a layer of CeO 2 :Yb/Er up-conversion material was added to the P25 layer, J es and h were further raised to 14.17 mA cm À2 and 6.74%, respectively. Here, the enhancement of efficiency may rely on the broadened range of absorbable spectrum, and making fuller use of near-infrared light in sunlight. What is more important, a high conversion efficiency of 7.30% and a short-circuit current of 16.70 mA cm À2 were achieved because the introduction of the composite material CeO 2 :Fe/Yb/ Er on the TiO 2 lm as a light scattering layer. The enhancement of 33.5% than the pure P25 electrode in efficiency was directly ascribed to the dye loading capacity, light capture ability and upconversion effect of CeO 2 :Fe/Yb/Er UCNPs. Especially its stronger upconversion luminescence intensity makes the battery efficiency enhanced by 8.3% compared with the CeO 2 :Yb/Er electrode. Note that its battery efficiency is 23.9% higher than the CeO 2 :Fe battery can be further seen that the upconversion effect plays a vital role in DSSCs.
The IPCE spectrum of batteries is detailed in Fig. 11. In a certain wavelength range, the light capture ability of all photoanodes improved compared to the P25 electrode with the lowest IPCE value. It is attributed that the stronger dye absorption capacity on the particles surface will generate more photoelectrons to remarkably improve the photoelectric conversion efficiency. In addition, the UC luminescence effect is believed to enhance the light-trapping ability of the electrodes by enhancing the optical density. Herein, the CeO 2 :Fe/Yb/Er layer obtains the highest IPCE value because of its stronger   up-conversion luminescence intensity. The measurement result of IPCE is consistent with I-V and UV tests.
In order to investigate the transmission and recombination of photo-generated electrons in DSSCs, the electrochemical impedance measurements of solar cells with four different photoanodes were performed under the condition that the light intensity is 100 mW cm À2 and the scanning frequency ranges from 100 kHz to 0.1 Hz, the result is shown in Fig. 12. The middle large semicircle is ascribed to the recombination resistance and chemical capacitance across the photoanode/dye/ electrolyte interface. The CeO 2 :Fe/Yb/Er electrode doped with Fe 3+ ions obtains the lowest electronic recombination resistance with a value of 12.5 U. This incident conrmed that the increase number of dye molecules and the improvement of light utilization can reduce the impedance at the intermediate frequency, thereby increasing the amount of photo-generated electrons. Thus the purpose of heighten the photoelectric conversion efficiency of DSSCs can be achieved.

Conclusions
In conclusion, the CeO 2 :Fe/Yb/Er nanomaterials with uniform size were perfectly prepared by a hydrothermal method. To guarantee the upconversion luminescent intensity, the Fe 3+ ions were introduced in materials to enhance the intensity by changing the symmetry of the local crystal eld around the Er 3+ ions. At the meanwhile, the behavior of incorporation Fe 3+ ions into the host material improves the light scattering ability of this material. And it also enables materials to gain better dye loading capacity because the smaller ionic radius of Fe 3+ shrank the lattice ruler will obtain a larger specic surface area. Additionally, the photoelectric conversion efficiency of solar cells using CeO 2 :Fe/Yb/Er UCNPs as the light scattering layer can reach 7.30%, which is 33.5% higher than the pure P25 electrodes. And its process of improving the photovoltaic performance of dye-sensitized cells is mainly accomplished by expanding the spectral absorption range and making fuller utilization of sunlight. This illustrates the great possibility and feasibility of the abundant Fe 3+ ions on the earth applied to DSSCs. Furthermore, some explorations in this work that related to the use of transition metal ion Fe 3+ to enhance upconversion luminescence provide a novel idea for the improvement of upconversion materials widely used in scien-tic research.

Conflicts of interest
There are no conicts to declare.  This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 18868-18874 | 18873