A New Up-conversion Material of Ho3+-Yb3+-Mg2+ Tri-doped TiO2 and Its Applications to Perovskite Solar Cells

A new up-conversion nanomaterial of Ho3+-Yb3+-Mg2+ tri-doped TiO2 (UC-Mg-TiO2) was designed and synthesized with a sol-gel method. The UC-Mg-TiO2 presented enhanced up-conversion fluorescence by an addition of Mg2+. The UC-Mg-TiO2 was utilized to fabricate perovskite solar cells by forming a thin layer on the electron transfer layer. The results display that the power conversion efficiency of the solar cells based on the electron transfer layer with UC-Mg-TiO2 is improved to 16.3 from 15.2% for those without UC-Mg-TiO2. It is demonstrated that the synthesized UC-Mg-TiO2 can convert the near-infrared light to visible light that perovskite film can absorb to improve the power conversion efficiency of the devices. Electronic supplementary material The online version of this article (10.1186/s11671-018-2681-4) contains supplementary material, which is available to authorized users.


Background
More attentions have been paid to the perovskite solar cells (PSCs) in the field of solar cells [1][2][3][4][5]. The power conversion efficiency (PCE) of the PSCs has been exceeding 22% within a few years [6]. However, the perovskite materials usually absorb the visible light whose wavelength is less than 800 nm, and more than half of the solar energy is not be utilized, especially in the region of near-infrared (NIR). To solve the issues, one of the effective methods is to apply the up-conversion nanomaterial to perovskite solar cells by converting the NIR light to visible light that the perovskite can utilize [7][8][9]. The beta-phase sodium yttrium fluoride (β-NaYF 4 ) is commonly used as the host lattice for rare earth ions to prepare the up-conversion materials. While the β-NaYF 4 -based up-conversion materials are insulator, which is not beneficial for the electron transfer [ETL] [10].
Titanium dioxide (TiO 2 ) nanocrystal with anatase phase is commonly used as the electron transfer material in the perovskite solar cells due to its suitable energy band structure, low cost, and long stability [11][12][13]. However, the energy band gap of TiO 2 is large (3.2 eV), which hampers its applications. To improve the applications of TiO 2 in visible light and near-infrared region, some methods were explored. One of the effective methods is doping TiO 2 with metal or non-metal [14][15][16]. Yu et al. [17] demonstrated that Ho 3+ -Yb 3+ -F − doped TiO 2 could convert NIR light to visible light that can be absorbed by the dye-sensitized solar cells (DSSCs). Zhang and co-authors [18] proved that Mg-doped TiO 2 can change the Fermi energy level of TiO 2 to enhance the performance of perovskite solar cells.
In this work, we are preferred to combine the rear earth ions (Ho 3+ and Yb 3+ ) and the metal ion (Mg 2+ ) doped TiO 2 together to synthesize a new material with enhanced up-conversion fluorescence. Our purpose is to explore how the addition of Mg 2+ affect the up-conversion fluorescence of TiO 2 and to apply the up-conversion nanomaterial of Ho 3+ -Yb 3+ -Mg 2+ tri-doped TiO 2 to perovskite solar cells. The results display that the addition of Mg 2+ enhanced the up-conversion emission of TiO 2 , and the application of Ho 3+ -Yb 3+ -Mg 2+ tri-doped TiO 2 improved the PCE of PSCs to 16.3% from 15.2%.

Preparation of PSCs
The FTO was washed in detergent, acetone, and isopropanol, and then treated for 15 min with UV-O 3 . A blocking layer was prepared by a spin-coating method using a solution of titanium diisopropoxide bis (acetylacetonate) in 1-butanol with the concentration of 1 M and then heated for 30 min at 500°C. An electron transfer layer (ETL) prepared by a spin-coating method using TiO 2 solution which is obtained by diluting TiO 2 (30NR-D) using ethanol (1:6, mass ratio), and then heated for 10 min at 100°C and 30 min at 450°C. The UC-Mg-TiO 2 was used to fabricate the solar cells by spin-coating a mixed solution of UC-Mg-TiO 2 sol and TiO 2 sol (UC-Mg-TiO 2 :TiO 2 = x:(100 − x), v/v, x = 0, 20, 40, 60, 80, and 100) on the ETL and heating for 30 min at 500°C. A perovskite film was fabricated according to the reported method [20]. In brief, the precursor solution of perovskite was prepared by dissolving FAI (1 M), Au anode was made on the hole transfer layer by thermal evaporation.

Characterization
Photoluminescence (PL) spectra were acquired using a fluorometer of FLS 980 E. A diffractometer of DX-2700 was used to obtain the X-ray diffraction (XRD) patterns. X-ray photoelectron spectra were measured with a spectrometer of XPS THS-103. Absorption spectra were obtained with a spectrophotometer of Varian Cary 5000.

Results and Discussion
The up-conversion fluorescence of the materials was optimized by varying the molar ratio of Ho 3+ and Yb 3+ . The up-conversion emission of Ho 3+ -Yb 3+ co-doped TiO 2 with varying molar ratio of Ho 3+ and Yb 3+ (Ho:Yb:Ti = 1:x:100) was shown in Fig. 1a, which were excited with an 980 nm NIR light. Two strong up-conversion emission peaks were observed at 547 nm and 663 nm. Additional file 1: Figure S1 shows the up-conversion mechanisms of the Ho 3+ -Yb 3+ co-doped TiO 2 . The fluorescence peaks at 663 nm and 547 nm could correspond to the 5 F 5 → 5 I 8 and ( 5 S 2 , 5 F 4 ) → 5 I 8 transitions of Ho 3+ , respectively [21]. It can be seen that the intensity of the up-conversion fluorescence is the largest when the molar ratio of Ho 3+ and Yb 3+ is 1:4. Figure 1b presents   To demonstrate the doping of Ho, Yb, and Mg into TiO 2 , the X-ray photoelectron spectra of UC-Mg-TiO 2 were obtained. The XPS survey spectrum of UC-Mg-TiO 2 was presented in Additional file 1: Figure  S2. Figure 3a shows the high-resolution photoelectron peaks of Ti 2p, which had two peaks of Ti 2p 1/2 and Ti 2p 3/2 located at 463.7 eV and 458.2 eV, respectively. peaks of Ho 4d and Yb 4d, which appear at 163.6 eV and 192.3 eV, respectively. These agree with the reported peak positions [22]. Figure 3d presents the photoelectron peak of Mg 2p located at 49.8 eV [23]. These data displays that Ho, Yb, and Mg atoms were incorporated into TiO 2 . Figure 4a shows the absorption spectra of TiO 2 (30NR-D) and UC-Mg-TiO 2 . There are five absorption peaks appear in the absorption spectrum of UC-Mg-TiO 2 , which are corresponding to characteristic absorption of Ho 3+ and Yb 3+ . It can be seen that the doping of Ho, Yb, and Mg improves the absorption of TiO 2 in visible light region and expands its absorption to NIR range. The Tauc plot can be used to estimate the energy band gap of material [24]. The Tauc plots from the absorption spectra were presented in Fig. 4b. The energy band gap values can be calculated to be 3.09 eV and 3.18 eV for UC-Mg-TiO 2 and TiO 2 (30NR-D), respectively. The UC-Mg-TiO 2 presents a smaller band gap than TiO 2 . Figure 5 shows the SEM photograph of TiO 2 (30NR-D) and UC-Mg-TiO 2 films. The size of the nanoparticle is about 25 nm for 30 NR-D, and particle size is about 28 nm for UC-Mg-TiO 2 . The two films are uniform. Thus, the UC-Mg-TiO 2 displays a similar morphology and particle size to TiO 2 (30NR-D).
The PSCs were fabricated based on the electron transfer layers with and without UC-Mg-TiO 2 . The electron transfer layer with UC-Mg-TiO 2 was prepared by spin-coating the mixed solution of UC-Mg-TiO 2 sol and TiO 2 sol (UC-Mg-TiO 2 :TiO 2 = x:(100 − x), x = 0, 20, 40, 60, 80, and 100, v/v). I-V measurements of the solar cells were performed, and from which the photovoltaic parameters were abstracted. The I sc , V oc , FF, and PCE of the solar cells in this work were obtained by an average of the values of 20 samples. The relation of PCE with the contents of UC-Mg-TiO 2 was displayed in Fig. 6a. Firstly, the PCE of the solar cells becomes large, and after that becomes small with the increase of the UC-Mg-TiO 2 contents, which reaches the maximum value at the content of 60% (UC-Mg-TiO 2 :TiO 2 = 60:40, v/v). Table 1 presents the photovoltaic parameters of solar cells based on the electron transfer layers with and without UC-Mg-TiO 2 . The open-circuit voltage (V oc ) and short-circuit current (I sc ) of the solar cells with UC-Mg-TiO 2 were increased to 1.05 V and 22.6 mA/ cm 2 from 1.03 V and 21.2 mA/cm 2 for the solar cells without UC-Mg-TiO 2 , respectively. Thus, the PCE of the devices based on the electron transfer layer with UC-Mg-TiO 2 was improved to 16.3% from 15.2% for those without UC-Mg-TiO 2 . The typical I-V curves of the devices are shown in Fig. 6b. The PCE histograms of the solar cell performance of 20 samples with and without UC-Mg-TiO 2 are presented in Additional file 1: Figure S3.
Some experiments were carried out to explain the improvement. Figure 7 displays the energy band structures of the materials contained in the solar cells based on some reports [25,26], and the energy band gap from the Tauc plots is shown in Fig. 4b. The conduction band difference between perovskite and TiO 2 becomes larger for UC-Mg-TiO 2 compared with that of TiO 2 (30NR-D), since the UC-Mg-TiO 2 has a smaller band gap than TiO 2 (30NR-D). This may be one of the reasons to give a larger V oc for the devices based on the electron transfer layer with UC-Mg-TiO 2 [27,28]. Figure 8a shows the steady-state photoluminescence (PL) of the perovskite films on the electron transfer layers with and without UC-Mg-TiO 2 . The PL peak located at 760 nm is originated from the perovskite film [29]. The PL intensity of the perovskite film on electron transfer layer with UC-Mg-TiO 2 decreased compared with that of perovskite film on electron transfer layer without UC-Mg-TiO 2 . This implies that the electron transport and extraction of UC-Mg-TiO 2 from the perovskite film is more efficient than that of TiO 2 (30NR-D). This can be further demonstrated by the time-resolved photoluminescence (TRPL) of the samples shown in Fig. 8b. It can be seen that the decay time of TRPL for the perovskite film on electron transfer layer with UC-Mg-TiO 2 is faster than that of perovskite film on electron transfer layer without UC-Mg-TiO 2 . This indicates that the charge transfer for the former is faster than the latter [30,31].   Figure 9a shows the Nyquist plots obtained from the electrochemical impedance spectroscopy (EIS) of the solar cells based on the electron transfer layer with and without UC-Mg-TiO 2 . The Nyquist plots can be fitted by an equivalent circuit which is schematically shown in Fig. 9b. The R s , R rec , and C μ are the series resistance, recombination resistance, and the capacitance of the device [32,33]. The detailed fitting values are presented in Table 2. The R s value of the devices based on the electron transfer layers with UC-Mg-TiO 2 is nearly the same with that of those without UC-Mg-TiO 2 . While the R rec value of the devices based on electron transfer layer with UC-Mg-TiO 2 is larger than that of those without UC-Mg-TiO 2 . This implies that UC-Mg-TiO 2 could effectively decrease the change recombination.
To confirm the contributions of the up-conversion material UC-Mg-TiO 2 to the photocurrent of the solar cells, the I-V measurements were carried out under the simulated solar radiation filtered with a band-pass NIR filter (980 ± 10 nm). Figure 10a displays the I-V curves of the solar cells based on the electron transfer layers with and without UC-Mg-TiO 2 . The short-circuit current (I sc ) of the solar cells with UC-Mg-TiO 2 is obviously larger than that of those without UC-Mg-TiO 2 . This demonstrates the effect of UC-Mg-TiO 2 on the photocurrent of the solar cells, because UC-Mg-TiO 2 converts the near-infrared photons into visible photons, which the solar cells can absorb to produce additional photocurrent [7,17]. Figure 10b shows the IPCE spectra of the solar cells with and without UC-Mg-TiO 2 . The IPCE of the solar cells with UC-Mg-TiO 2 is increased, especially at the range of 400~650 nm, compared with that of those without UC-Mg-TiO 2 . This could be caused by the up-conversion effect of UC-Mg-TiO 2 [7,17].

Conclusions
The up-conversion nanomaterial of Ho 3+ -Yb 3+ -Mg 2+ tri-doped TiO 2 (UC-Mg-TiO 2 ) was synthesized successfully. The up-conversion emissions of the UC-Mg-TiO 2 were enhanced with an addition of Mg 2+ . We applied the UC-Mg-TiO 2 to the PSCs, in which the UC-Mg-TiO 2 was used to modify the electron transfer   Fig. 10 a I-V curves of the solar cells under the simulated solar radiation filtered with a band-pass NIR filter (980 ± 10 nm). b IPCE spectra of the solar cells with and without UC-Mg-TiO 2