Yttrium-doped TiO2 compact layers for efficient perovskite solar cells

Yttrium-doped TiO2 compact layers for efficient perovskite solar cells

Journal of Solid State Chemistry 275 (2019) 206–209 Contents lists available at ScienceDirect Journal of Solid State Chemistry journal homepage: www...

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Journal of Solid State Chemistry 275 (2019) 206–209

Contents lists available at ScienceDirect

Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc

Yttrium-doped TiO2 compact layers for efficient perovskite solar cells Xinlian Deng, Yanqing Wang *, Yan Chen, Zhendong Cui, Chengwu Shi School of Chemistry and Chemical Engineering, Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, Hefei University of Technology, Hefei, 230009, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Y-doped TiO2 compact layers Y-doped TiO2 nanorod arrays Electron transporting Charge recombination Perovskite solar cells

To improve the electron transporting and suppress the charge recombination at the interface of TiO2/perovskite, the smooth and compact yttrium(Y)-doped TiO2 compact layers were successfully prepared via a hydrolysispyrolysis method. The influences of Y-doped on the microstructure, crystal phase, chemical composition, optical absorption of the TiO2 compact layers were investigated, and the electron transporting and charge recombination at the TiO2/perovskite interface of solar cells were systemically analyzed. The planar perovskite solar cells based on the Y-doped TiO2 compact layers with a Y/Ti molar ratio of 5% obtained a best photoelectric conversion efficiency (PCE) of 15.52%, comparing with a PCE of 14.05% from the undoped devices. When the 200-nm length 3% Y-doped TiO2 nanorod arrays were introduced between the 5% Y-doped TiO2 compact layers and the perovskite thin film, the champion cell achieved a PCE of 18.32% under illumination of simulated AM 1.5 sunlight (100 mA cm2).

1. Introduction In the past several years, organic-inorganic hybrid perovskite solar cells (PSCs) have developed rapidly owing to their excellent photovoltaic performance, low cost, simple manufacturing process [1–3], and the power conversion efficiency (PCE) has reached to a current record efficiency of 23.7% [4]. The electron transporting layer (ETL) is proved to play a crucial role in improving the performance of PSCs. Currently, due to the suitable conduction band and favorable photoelectric property, the titanium dioxide (TiO2) has been commonly used as ETL in PSCs [5–7]. However, some limiting factors, such as low electrical conductivity, small electron mobility, and high trap states, hinder to the further improvements of the device efficiency and stability [8]. What's more, the charge recombination existed at the TiO2/perovskite interface should be suppressed in order to improve the electron-extraction behavior [9]. Doping TiO2 with other elements, such as yttrium (Y), lithium (Li), indium (In), cobalt (Co), magnesium (Mg), europium (Eu), samarium (Sm), lanthanum (La), etc., has been widely utilized in PSCs to address these issues [10–14]. For examples, Gr€atzel et al. [11] assembled CH3NH3PbI3 PSCs based on Y-doped mesoscopic TiO2 thin films with undoped TiO2 compact layers and a PCE of 11.2% was obtained. PSCs with Eu-doped mesoscopic TiO2 thin films and pure TiO2 compact layers achieved a PCE of 17.90% [15]. As to the doped TiO2 compact layers, Zhuang et al. [16] prepared Sm-doped TiO2 compact layer to assemble planar

CH3NH3PbI3 PSCs and gave a PCE of 14.10%. Liu et al. [17] prepared the La-doped TiO2 compact layers by spray pyrolysis method and the corresponding CH3NH3PbI3 PSCs with undoped mesoscopic TiO2 thin films achieved a PCE of 17.2%. In our previous work, the planar CH3NH3PbI3-xBrx PSCs with undoped TiO2 compact layers were assembled and exhibited a PCE of 12.82% [18], when introducing 200-nm length Y-doped TiO2 nanorod arrays on top of the undoped TiO2 compact layers, the PCE was further improved to 18.11% [19]. Very recently, Wang et al. [20] demonstrated that Pt doping results in a tailed band level of TiO2, then it could suppress the charge accumulation at the interface of Pt-doped TiO2/perovskite. Wei and Li et al. [21] introduced graphene quantum dots on the surface of mesoporous TiO2 film with both enhanced electron extraction and charge transportation at the interface of perovskite/ETL, and resulting in a best PCE of 20.45%. Zhou et al. [22] reported a facile one-step hydrothermal method to synthesis Er and Mg co-doped TiO2 nanorod arrays with enhanced charge injection efficiency and higher recombination resistance. Though a few undoped and doped TiO2 compact layers based PSCs have been reported, to the best of our knowledge, the strategy of combining both the Y-doped TiO2 compact layers and the Y-doped TiO2 nanorod arrays in fabricating PSCs has not been reported. Herein, the Y-doped TiO2 compact layers were successfully prepared by hydrolysis-pyrolysis method using an acidic isopropanol solution of 0.23 M titanium isopropoxide and 0.013 M HCl with the Y/Ti molar ratio

* Corresponding author. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.jssc.2019.04.022 Received 18 March 2019; Received in revised form 15 April 2019; Accepted 17 April 2019 Available online 21 April 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.

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Fig. 1. Surface and cross-sectional SEM images of Y-doped TiO2 compact layers using the acidic isopropanol solution with the Y/Ti molar ratios of (a, d) 0%, (b, e) 5% and (c, f) 10%.

2. Experimental The Y-doped TiO2 compact layers were prepared by hydrolysispyrolysis method using an acidic isopropanol solution consisted of 0.23 M titanium isopropoxide, 0.013 M HCl, and 0.0115 M YCl3 with the Y/Ti molar ratio of 0%, 5%, or 10% was introduced in the acidic precursor solution, with yttrium chloride hexahydrate (YCl3⋅6H2O, 99.99%) as the Y source [18,19]. The preparation of Y-doped TiO2 nanorod arrays, CH3NH3PbI3-xBrx perovskite thin film, spiro-OMeTAD layer, and gold electrode, as well as the characterization with SEM, XRD, EDS, UV–Vis, PL, EIS, photovoltaic performance measure of PSCs were the same as our previous reports [18, 19]. 3. Results and discussions 3.1. Microstructure and chemical composition of Y-doped TiO2 compact layers Fig. 1 shows the surface and cross-sectional SEM images of pristine and Y-doped TiO2 compact layers. It can be clearly seen that the grain size of TiO2 nanoparticles decreased with the increase of the Y/Ti molar ratio in the acidic precursor solution from 0% to 5%, and which means that Y doping can suppress the growth of TiO2 nanoparticles. While increasing the doping ratio to 10%, the grain size of TiO2 nanoparticles became uneven and the surface of TiO2 compact layers became rough. The XRD pattern and UV–Vis spectrum of the corresponding undoped and Y-doped TiO2 compact layers were shown in Fig. S1, Supporting Information. The SEM image, XRD pattern and UV–Vis–NIR absorption spectrum of the perovskite thin film on the 5% Y-doped TiO2 compact layers were illustrated in Fig. S2, Supporting Information. Fig. 2 presents the XPS spectra of the Y 3d and Ti 2p in the TiO2 compact layers. The Y 3d5/2 and Y 3d3/2 peaks located at 157.88 eV and 159.79 eV, respectively, with a peak splitting of 1.91 eV, demonstrating the presence of Y3þ in the TiO2 compact layers, which was in accordance with the previous reports [23]. The Ti 2p1/2 and Ti 2p3/2 peaks of the undoped TiO2 located at 464.14 eV and 458.39 eV, respectively, with a peak splitting of 5.75 eV, and the Ti 2p1/2 and Ti 2p3/2 peaks of the Y-doped TiO2 located at 464.02 eV and 458.28 eV with a peak splitting of 5.74 eV. The Ti 2p1/2 and Ti 2p3/2 binding energy of Y-doped TiO2 was lower than that of pure TiO2, and this means that due to the stronger metallicity and lower valence state of Y3þ than Ti4þ, may be Y3þ can donate electrons easier than Ti4þ. In addition, according to the XPS analysis, the Y/Ti molar ratio in the TiO2 compact layers was 5%, which was the same as the Y/Ti molar ratio in the acidic isopropanol solution.

Fig. 2. XPS spectra of undoped and Y-doped TiO2 compact layers. (a) Y 3d peaks, (b) Ti 2p peaks.

of 0%, 5%, and 10%, respectively. The influences of Y/Ti molar ratio on the microstructure, crystal phase, chemical composition, optical absorption of the corresponding TiO2 compact layers were investigated by field emission scanning electron microscopy (SEM), X-ray diffraction (XRD) and ultraviolet–visible spectroscopy (UV–Vis). Photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) were applied to analyze the charge separation and recombination at the TiO2/perovskite interface. The photovoltaic performances of Y-doped and pure TiO2 compact layers PSCs were compared. 207

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Fig. 3. Photocurrent-photovoltage characteristics of the perovskite solar cells using the acidic isopropanol solution with the Y/Ti molar ratios of 0%, 5% and 10% (a), and the IPCE spectrum of 5% (b). Fig. 4. Photoluminescence spectra (a) and Nyquist plots (b) of compact layers using the acidic isopropanol solution with the Y/Ti molar ratios of 0%, 5% and 10%.

Table 1 Photovoltaic performance parameters of Y-doped TiO2 compact layers based perovskite solar cells. Y/Ti molar ratios

Voc (V)

Jsc (mA⋅cm2)

FF (%)

PCE (%)

0%

0.99 0.99  0.02 1.00 1.00  0.01 0.98 0.98  0.02

20.42 19.89  1.72 21.20 20.97  1.19 19.85 19.83  0.19

69.49 68.06  1.43 73.22 70.12  7.09 69.48 64.80  5.91

14.05 13.41  1.09 15.52 14.77  2.18 13.52 12.53  0.99

5% 10%

Best Average Best Average Best Average

Table 2 Parameters obtained by fitting the experimental spectra with the equivalent circuit Rs (Rcr CPE). Y/Ti molar ratios

Rs (Ω)

Rcr (Ω)

Y0 (107 F⋅sn1)

n

0% 5% 10%

12.80 13.63 13.07

621.7 684.7 241.9

0.6 0.4 2.0

0.97 0.97 0.88

Average: 6 solar cells.

doped TiO2 compact layers was 20.04 mA cm2, which was in close agreement with the Jsc from the J-V curves.

3.2. Photovoltaic performance of planar PSCs with Y-doped TiO2 compact layers Fig. 3(a) is the photocurrent-photovoltage (J-V) characteristics of the corresponding solar cells, the PSCs based on pure TiO2 compact layers possess the best PCE of 14.50% with the short-circuit photocurrent density (Jsc) of 20.42 mA cm2, open-circuit voltage (Voc) of 0.99 V, and fill factor (FF) of 69.49%. As to the device with 5% Y-doped TiO2 compact layers, the champion cell presented a PCE of 15.52% with Jsc of 21.20 mA cm2, Voc of 1.00 V, and FF of 73.22%, and the average PCE of 14.77  2.18% with Jsc of 20.97  1.19 mA cm2, Voc of 1.00  0.01 V, and FF of 70.12  7.09%. These results should be related to the smooth and compact Y-doped TiO2 compact layers. However, the PEC, Jsc, Voc and FF of solar cell based on 10% Y-doped TiO2 compact layers decreased to 13.52%, 19.85 mA cm2, 0.98 V, and 69.48% respectively, which may be caused by the uneven TiO2 nanoparticles and the rough surface of TiO2 compact layers. The main cell parameters are also summarized in Table 1. From the incident photon-to-electron conversion efficiency (IPCE) spectrum (Fig. 3(b)), the integrated Jsc of the solar cell with 5% Y-

3.3. Charge separation and recombination at the TiO2/perovskite interface The charge separation and recombination at the interface of TiO2/ perovskite can be investigated by analyzing the corresponding PL and EIS data [18,19]. Fig. 4(a) illustrates the photoluminescence spectra of the perovskite thin films on the TiO2 compact layers, and it shows that the intensities of luminescence peaks at 762 nm for 5% Y-doped TiO2 compact layers were weaker than the undoped one, which means that the electron transporting was faster and the accumulation of conduction electron was less in the former one. However, the undoped TiO2 compact layers were more effective than the 10% Y-doped TiO2 ones in electron transporting, which should be related to the uneven TiO2 nanoparticles and rough surface of the later one, and it may hinder the charge separation at the interface of TiO2/perovskite. The results above imply that moderate Y-doped TiO2 compact layers can improve the charge separation at the TiO2/perovskite interface. Fig. 4(b) shows the Nyquist plots of 208

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4. Conclusions The smooth and compact Y-doped TiO2 compact layers were successfully synthesized by a hydrolysis-pyrolysis method with YCl3⋅6H2O as the Y source. The planar PSCs with 5% Y-doped TiO2 compact layers achieved a best PCE of 15.52%, and the device with both 5% Y-doped TiO2 compact layers and 3% Y-doped TiO2 nanorod arrays exhibited a best PCE of 18.32%. The improved cell performance is attributed to the enhance electron transporting and suppress charge recombination at the TiO2/perovskite interface with the Y-doped TiO2. Acknowledgements This work was financially supported by the Fundamental Research Funds for the Central Universities (JZ2017HGTB0230), the National Natural Science Foundation of China (51602089, 51472071, and 51272061), and the Talent Project of Hefei University of Technology (75010-037004, 75010-037003). Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.jssc.2019.04.022. References [1] S. Lv, L. Han, J. Xiao, L. Zhu, J. Shi, H. Wei, Y. Xu, J. Dong, X. Xu, D. Li, S. Wang, Y. Luo, Q. Meng, X. Li, Chem. Commun. 50 (2014) 6931–6934. [2] Q. Dong, Y. Fang, Y. Shao, P. Mulligan, J. Qiu, L. Cao, J. Huang, Science 347 (2015) 967–970. [3] Q. Chen, H. Zhou, Z. Hong, S. Luo, H.-S. Duan, H.-H. Wang, Y. Liu, G. Li, Y. Yang, J. Am. Chem. Soc. 136 (2014) 622–625. [4] National Renewable Energy Laboratory, Best Research-Cell Efficiencies, https:// www.nrel.gov/pv/assets/pdfs/pv-efficiencies.pdf. accessed: March 2019. [5] G.E. Eperon, V.M. Burlakov, P. Docampo, A. Goriely, H.J. Snaith, Adv. Funct. Mater. 24 (2014) 151–157. [6] M.-C. Wu, S.-H. Chan, M.-H. Jao, W.-F. Su, Sol. Energy Mater. Sol. Cells 157 (2016) 447–453. [7] K. Wojciechowski, S.D. Stranks, A. Abate, G. Sadoughi, A. Sadhanala, N. Kopidakis, G. Rumbles, C.-Z. Li, R.H. Friend, A.K.Y. Jen, H.J. Snaith, ACS Nano 8 (2014) 12701–12709. [8] E.L. Unger, E.T. Hoke, C.D. Bailie, W.H. Nguyen, A.R. Bowring, T. Heumüller, M.G. Christoforo, M.D. McGehee, Energy Environ. Sci. 7 (2014) 3690–3698. [9] B. Conings, L. Baeten, C. De Dobbelaere, J. D'Haen, J. Manca, H.-G. Boyen, Adv. Mater. 26 (2014) 2041–2046. [10] H. Zhou, Q. Chen, G. Li, S. Luo, T.-b. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Science 345 (2014) 542–546. [11] P. Qin, A.L. Domanski, A.K. Chandiran, R. Berger, H.J. Butt, M.I. Dar, T. Moehl, N. Tetreault, P. Gao, S. Ahmad, M.K. Nazeeruddin, M. Gratzel, Nanoscale 6 (2014) 1508–1514. [12] J.K. Kim, S.U. Chai, Y. Ji, B. Levy-Wendt, S.H. Kim, Y. Yi, T.F. Heinz, J.K. Nørskov, J.H. Park, X. Zheng, Adv. Energy Mater. (2018) 1801717. [13] F. Giordano, A. Abate, J.P. Correa Baena, M. Saliba, T. Matsui, S.H. Im, S.M. Zakeeruddin, M.K. Nazeeruddin, A. Hagfeldt, M. Graetzel, Nat. Commun. 7 (2016) 10379. [14] J. Peng, T. Duong, X. Zhou, H. Shen, Y. Wu, H.K. Mulmudi, Y. Wan, D. Zhong, J. Li, T. Tsuzuki, K.J. Weber, K.R. Catchpole, T.P. White, Adv. Energy Mater. 7 (2017) 1601768. [15] Z. Xu, J. Wu, T. Wu, Q. Bao, X. He, Z. Lan, J. Lin, M. Huang, Y. Huang, L. Fan, Energy Technol. 5 (2017) 1820–1826. [16] Y. Xiang, Z. Ma, J. Zhuang, H. Lu, C. Jia, J. Luo, H. Li, X. Cheng, J. Phys. Chem. C 121 (2017) 20150–20157. [17] H. Li, B. Zheng, Y. Xue, S. Liu, C. Gao, X. Liu, Sol. Energy Mater. Sol. Cells 168 (2017) 85–90. [18] G. Xiao, C. Shi, Z. Zhang, N. Li, L. Li, J. Solid State Chem. 249 (2017) 169–173. [19] X. Deng, Y. Wang, Z. Cui, L. Li, C. Shi, Superlattices Microstruct. 117 (2018) 283–287. [20] L.-L. Jiang, Z.-K. Wang, M. Li, C.-H. Li, P.-F. Fang, L.-S. Liao, Solar RRL, 2018, p. 1800149. [21] D. Shen, W. Zhang, F. Xie, Y. Li, A. Abate, M. Wei, J. Power Sources 402 (2018) 320–326. [22] H. Chen, W. Zhu, Z. Zhang, W. Cai, X. Zhou, J. Alloys Compd. 771 (2019) 649–657. [23] M. Li, Y. Huan, X. Yan, Z. Kang, Y. Guo, Y. Li, X. Liao, R. Zhang, Y. Zhang, ChemSusChem 11 (2018) 171–177.

Fig. 5. Photocurrent-photovoltage characteristics (a) and the IPCE spectrum (b) of the perovskite thin film on the TiO2 nanorod arrays using the hydrothermal grown solution with the Y/Ti molar ratios of 3%.

the PSCs and the corresponding fitted data by an equivalent circuit Rs (Rcr CPE) are listed in Table 2. When the Y/Ti molar ratio increased from 0% to 5%, the charge recombination resistance (Rcr) increased from 621.7 Ω to 684.7 Ω. This indicates that Y doping can improve the electron transporting in the TiO2 compact layers and suppress the charge recombination at the interface of TiO2/perovskite, which was in accordance with the PL spectra. The Rcr decreased from 684.7 Ω to 241.9 Ω while the Y/Ti molar ratio increased from 5% to 10%, and this should be related to the uneven TiO2 nanoparticles and rough surface of TiO2 compact layers caused by the heavy Y doping. 3.4. Photovoltaic performance of the PSCs combine both Y-doped TiO2 compact layers and Y-doped TiO2 nanorod arrays Fig. 5(a) is the J-V characteristics of the champion solar cells based on the Y-doped TiO2 nanorod arrays using the Y/Ti molar ratio of 3% in the precursor solution [19]. By introducing the 200 nm length 3% Y-doped TiO2 nanorod arrays on the 5% Y-doped TiO2 compact layers, an oriented direction electron transporting path was established and the charge separation at the TiO2/perovskite interface was improved. The PCE of the corresponding PSCs was improved to 18.32% with Jsc of 23.55 mA cm2, Voc of 1.04 V, and FF of 74.83%, and the average PCE was 17.55  0.86% with Jsc of 22.83  0.78 mA cm2, Voc of 1.04  0.02 V, and FF of 73.19  1.79%. In addition, the integrated Jsc from IPCE spectrum is 22.54 mA cm2 (Fig. 5(b)), which was in accordance with the measured result. Fig. S3 and Fig. S4 (Supporting Information) show the SEM image, XRD pattern, UV–Vis–NIR absorption spectrum of the 3% Y-doped TiO2 nanorod arrays and those characterization data of the perovskite thin films on the TiO2 nanorod arrays, respectively.

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