Adjustment of Conduction Band Edge of Compact TiO2 Layer in

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 36708-36714

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Adjustment of Conduction Band Edge of Compact TiO2 Layer in Perovskite Solar Cells Through TiCl4 Treatment Takurou N. Murakami,*,† Tetsuhiko Miyadera,† Takashi Funaki,† Ludmila Cojocaru,‡ Said Kazaoui,† Masayuki Chikamatsu,† and Hiroshi Segawa‡,§ †

Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba City, 305-8565, Japan ‡ Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan § Department of General Systems Studies, Graduate School of Arts and Science, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan S Supporting Information *

ABSTRACT: Perovskite solar cells (PSCs) without a mesoporous TiO2 layer, that is, planar-type PSCs exhibit poorer cell performance as compared to PSCs with a porous TiO2 layer, owing to inefficient electron transfer from the perovskite layer to the compact TiO2 layer in the former case. The matching of the conduction band levels of perovskite and the compact TiO2 layer is thus essential for enhancing PSC performance. In this study, we demonstrate the shifting of the conduction band edge (CBE) of the compact TiO2 layer through a TiCl4 treatment, with the aim of improving PSC performance. The CBE of the compact TiO2 layer was shifted to a higher level through the TiCl4 treatment and then shifted in the opposite direction, that is, to a lower level, through a subsequent heat treatment. These shifts in the CBE were reflected in the PSC performance. The TiCl4treated PSC showed an increase in the open-circuit voltage of more than 150 mV, as well as a decrease of 100 mV after being heated at 450 °C. On the other hand, the short-circuit current decreased after the treatment but increased after heating at temperatures higher than 300 °C. The treated PSC subjected to subsequent heating at 300 °C exhibited the best performance, with the power conversion efficiency of the PSC being 17% under optimized conditions. KEYWORDS: perovskite solar cells, compact TiO2 layer, TiCl4 treatment, postheating treatment, conduction band edge shift

1. INTRODUCTION

Among the various inorganic-materials-based techniques explored for modifying the TiO2 surface, the chemical bath deposition method using an aqueous TiCl4 solution (i.e., a TiCl4 treatment) is also used conventionally with dye-sensitized solar cells to improve the photocurrent, voltage, and PCE.24−29 During this treatment, a thin TiO2 layer can grow on the TiO2 crystals. Previous reports have suggested that the performance of planar-type PSCs can be improved through a TiCl 4 treatment that results in fewer voids in the interface between the TiO2 and perovskite layers when it is accompanied by a wettability-improving treatment in which the perovskite precursor solution is dropped on the TiO2 surface.21 On the other hand, the TiO2 produced after the TiCl4 treatment lowers the charge-transfer resistance at the TiO2/perovskite interface.20 However, the cause of this lowering of the chargetransfer resistance because of the TiCl4 treatment remains unclear. It is likely that the TiO2 thin layer formed by the TiCl4

The power conversion efficiency (PCE) of organic−inorganic metal halide PSCs has improved dramatically in the past five years.1−7 The band gap of perovskite materials, such as methylammonium iodide and lead iodide, is approximately 1.6 eV, and it has been suggested that the theoretical performance of solar cells based on these materials should be the following: short-circuit photocurrent (Jsc) of 23.5 mA cm−2, open-circuit voltage (Voc) of 1.18 V, fill factor (FF) of 0.78, and PCE of approximately 21.6%.7−10 To achieve the ideal PCE, it is necessary to reduce the energy loss at the material interfaces in the cells. For example, cells without a porous TiO2 layer, that is, planar-type PSCs, show poorer cell performances and a high degree of hysteresis in the I−V curve, depending on the sweep direction of the bias voltage.11−17 A likely reason could be the impedance components, such as the large capacitance at the TiO2/perovskite interface. For this reason, there have been several studies on how to modify the TiO2 surface for efficient charge transfer using fullerene derivatives, amino acids, and inorganic materials.18−23 © 2017 American Chemical Society

Received: May 26, 2017 Accepted: October 5, 2017 Published: October 5, 2017 36708

DOI: 10.1021/acsami.7b07496 ACS Appl. Mater. Interfaces 2017, 9, 36708−36714

Research Article

ACS Applied Materials & Interfaces treatment changes the level of the CBE of the compact TiO2 layer, leading to changes in the electrical properties of the TiO2 layer. In fact, O’Regan et al. showed a shift in the CBE of a mesoporous nanocrystalline TiO2 layer with a TiCl4 treatment.25−27 Additionally, Liu et al. reported a higher Ti3+/Ti4+ ratio in the newly formed TiO2 in a thin porous TiO2 layer produced through a TiCl4 treatment, which changed the work function.23 In this study, we fabricated electrochemical cells and evaluated the shift in the CBE of the compact TiO2 layer because of the TiCl4 treatment to clarify the effect of the treatment. Furthermore, the effect of the postheating treatment temperature on the thin TiO2 layer formed by the TiCl4 treatment was successfully evaluated separately from the effects of the TiCl4 treatment on the entire compact TiO2 layer. We also elucidated the relationship between the CBE potential and the performance of the resultant PSCs to determine the optimum level of the TiO2 CBE for electron transfer.

formed on the FTO glass substrate at room temperature, and the substrate was heated at 120 °C for 50 min. On the heating of the substrate, its color changed to dark brown, which is the color of perovskite. The spiro-OMeTAD solution was then spin-coated on the perovskite layer, once the substrate had cooled to room temperature. After drying the substrate overnight, Au was deposited on it using a vacuum heat evaporation system (RD-1230, Sanvac Co.) to form the cathode. The photocurrent−voltage (I−V) characteristics of the cells were measured using a Keithley 2400 SourceMeter and a solar simulator unit (CEP-25FT, Bunkokeiki Co.) with AM 1.5G filters. The intensity of the light (100 mW cm−2) was calibrated using a standard singlecrystal silicon photovoltaic device with a KG5 optical filter. The aperture sizes of the cells used for evaluating the effects of the TiCl4 treatment and those of the optimized conditions were set by using a photomask on top of the cells and were fixed at 0.03 and 0.12 cm2, respectively. The incident-photon-to-electron conversion efficiency (IPCE) spectra of the cells were measured using the same unit as that employed for the I−V measurements; a light-intensity-controllable monochromator was also used for the purpose.

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION The CBE potential of the sprayed TiO2 layer was evaluated by the charge-extraction method (see Supporting Information).32,33 To evaluate the CBE level of each TiO2 layer, the potential energy of the electrons in the TiO2 layer, which corresponds to the Fermi level, should be compared at the same electron density. In other words, a higher CBE is required to obtain a higher Fermi level with the same amount of accumulated electrons in TiO2. Here, the applied voltage in the steady state (Vapp) is the potential difference between the Fermi level of TiO2 and the redox potential of the electrolyte used. Therefore, the absolute value of Vapp is not the same as the Voc of the PSCs. However, it is possible to evaluate the relationship between the Fermi level and the electron density in TiO2. This relationship is reflected in the CBE level of TiO2. To calculate the electron density, the thickness and area of the TiO2 layer were taken to be 40 nm (based on a scanning electron microscopy image; see Figure S1) and 0.36 cm2 (based on the cell structure), respectively; the surface roughness of the electrodes was not considered because it was assumed that every electrode had the same roughness. Figure 1 shows the relationship between the electron density in the TiO2 layer and Vapp of the electrochemical cells. For the same electron density

Fluorine-doped SnO2-coated glass (FTO glass), which is transparent and conductive, was purchased from Nippon Sheet Glass Co., Ltd. (sheet resistance ≤10 Ω sq−1). A compact layer of TiO2 was coated on the FTO glass substrate by the spray pyrolysis method as described below.30 A TiO2 precursor solution of titanium tetraisopropoxide was mixed with acetyl acetone to obtain a 2 M stock solution of the titanium acetylacetonate (Ti-acac) complex. This Ti-acac solution was diluted at a 3:1 ratio with 2-propanol to 75 vol %. Next, this solution was further diluted with ethanol at a 1:9 ratio (10 vol %). Then, 8 mL of the diluted solution was sprayed onto the FTO substrate at 350 °C over an area of 144 cm2, and the substrate was subsequently heated at 450 °C for 30 min.30 Electrochemical cells with a sandwich-like geometry were prepared for characterizing the deposited TiO2 layer. The cells were fabricated using the TiO2-coated FTO glass substrate, a platinized FTO glass substrate (as the counter electrode), and iodide triiodide (as the redox electrolyte).31 The electron densities of the compact TiO2 layer at different applied voltages (Vapp) were measured using the chargeextraction method.32,33 The underlying theory of this method has been described by Duffy et al. Further, the method is described in detail in Supporting Information. The relationship between the capacitance of the TiCl4-treated FTO glass sample and the applied voltage was determined through electrochemical impedance spectroscopy (EIS), which was performed using a computer-controlled potentiostat equipped with a frequency response analyzer (Bio-Logic, SP-300). The frequency range for the EIS measurements was 200 kHz to 0.1 Hz. The “Randomize and Simplex” method of the Z-fit tool in the software EC-Lab (v10.33) was used for curve fitting the EIS data. To observe the changes induced in the crystals of the TiO2 layer by the TiCl4 treatment, grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were performed at beamline BL46XU of the SPring-8 synchrotron radiation facility, Japan. To fabricate the PSCs, a perovskite precursor solution was used. The solution was prepared by mixing 153 mg of PbCl2 and 257 mg of methylammonium hydro iodide (CH3NH3I, MAI) in 611 μL of dimethylformamide (DMF).21 To produce the cell with the optimal performance, 290 mg of PbI2 and 95.4 mg of MAI were dissolved in 300 μL of DMF and 200 μL of dimethyl sulfoxide, and this solution was used as the perovskite precursor solution.4,34 To form the holetransport layer, (2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl-amine]9,9′-spirobifluorene (spiro-OMeTAD) was used. To prepare the spin-coating solution, 56 mg of spiro-OMeTAD (SHT-263, Merck) was dissolved in 650 μL of chlorobenzene. To this was added 20 μL of 4-tert-butylpyridine and 14 μL of a lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) acetonitrile solution, which was prepared by adding 50 mg LiTFSI to 80 μL of acetonitrile. The perovskite precursor solution was coated on the TiO2 layer

Figure 1. Relationship between electron density and applied bias voltage (Vapp) in electrochemical cells with TiO2 layer deposited on FTO glass. Labels “Spray”, “Spray + TiCl4”, and “Spray + TiCl4 + Heat” represent the compact TiO2 layer, the TiCl4-treated compact TiO2 layer, and the TiCl4-treated compact TiO2 layer subjected to subsequent heating at 450 °C, respectively. 36709


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ACS Applied Materials & Interfaces of approximately 1 × 1020 cm−3, the Vapp value of the TiCl4treated compact TiO2 layer was higher than that of the untreated TiO2 layer. Further, the Vapp value of the layer heated after the TiCl4 treatment was lower than that of the layer not heated after the treatment. The CBE of the compact TiO2 layer deposited by spraying was raised by 200 mV after the TiCl4 treatment. However, after heating at 450 °C for 30 min, it was lowered by 110 mV. These results suggest that the CBE of the compact TiO2 layer can be readily raised and lowered by the TiCl4 treatment and by post-treatment heating, respectively. The upward shift in the Fermi level with the TiCl4 treatment is in keeping with the results of a Liu’s study, wherein a TiCl4 treatment was used to form thin porous TiO2 layers with a thickness of 40 nm on indium tin oxide substrates.23 On the other hand, O’Regan et al. showed a downward shift in the CBE of a mesoporous nanocrystalline TiO2 layer (3 μm thick) due to a TiCl4 treatment.27 If the CBE level in the entire TiO2 compact layer after TiCl4 treatment was dominated by the property of the newly formed TiO2 on the TiCl4-treated surface, the difference between the tendencies in the CBE shifts can probably be attributed to the difference in the surface properties of the TiO2 layers before treatment. The particles of the TiO2 layer in O’Regan’s study, which were synthesized via hydrothermal methods, were single crystals with a diameter of approximately 20 nm exposed crystalline facets,25 while the TiO2 compact layer prepared via spray pyrolysis did not exhibit single-crystal particles and crystalline facets. For these reasons, the CBE of the thin TiO2 layer formed with the TiCl4 treatment should be evaluated. To observe the characteristics of the TiO2 layer formed owing to the TiCl4 treatment, the EIS spectra of the electrochemical cells with the TiCl4-treated FTO glass substrate were measured under dark conditions. The EIS spectra and the fitted curves for various bias voltages are shown in Figure S3. The equivalent circuit used for fitting the EIS spectra of the electrochemical cells as well as for determining the quasicapacitance from the constant-phase element is shown in Figure S4 and Scheme S5.35−37 The relationship between the quasi-capacitance and Vapp is shown in Figure 2. The capacitance of the bare FTO substrate does not change with Vapp. This capacitance can be considered the geometric

electrostatic capacitance, which is the capacitance attributable to the electrical double layer between the electrode and the electrolyte.38 On the other hand, the capacitance of the TiCl4treated FTO substrate increases with an increase in Vapp, which was applied as the bias voltage. This capacitance is probably a chemical capacitance attributable to the density of states (DOS) of the electrons in the thin TiO2 layer formed after the TiCl4 treatment.38 For the same Vapp, the capacitance increases with an increase in the heating temperature. Therefore, the CBE of TiO2 formed by the TiCl4 treatment shifts to a deeper level with an increase in the heating temperature. Thus, it can be concluded that the TiO2 formed by the TiCl4 treatment changes the CBE potential of the entire TiO2 layer on the FTO substrate. For this reason, the direction of the shift in the CBE with the TiCl4 treatment depends on the level of the CBE before the treatment. To observe the changes in the crystallinity of the TiO2 layer with the TiCl4 treatment, GIWAXS measurements were performed on the thin TiO2 layer formed on the FTO glass substrate. The diffraction spectrum is shown in Figure 3. On

Figure 3. GIWAXS patterns of FTO glass (FTO), compact TiO2 layer on FTO glass (TiO2), TiCl4-treated compact TiO2 layer on FTO glass (TiO2 + TiCl4), and TiCl4-treated compact TiO2 layer on FTO glass subjected to subsequent heating at 450 °C (TiO2 + TiCl4 + heat). Inset shows magnified images of patterns at approximately 18 nm−1.

the basis of the JCPDS database (card 00-001-0562), the compact TiO2 layer on the FTO glass was identified as consisting of anatase crystals. Further, no other peaks related to crystalline TiO2 were observed; this was the case even after the TiCl4 treatment. The peak related to crystalline anatase at 18.1 nm−1 shifted slightly, to 17.9 nm−1, after the TiCl4 treatment. However, after heating at 450 °C for 30 min, the peak shifted back to 18.1 nm−1; this shift was accompanied by an increase in the intensity. It is likely that the thin TiO2 layer formed after the TiCl4 treatment contains anatase crystals with a slightly higher lattice constant as well as a smaller crystal phase and a shallower CBE. Further, after the post-treatment heating process, the crystallinity of the TiO2 layer is improved, with the CBE becoming deeper than is the case for a lower crystallinity. This change in crystallinity could also be related to the defect concentration in TiO2, which decreases with the increasing heating temperature. Meanwhile, the DOS for electrons in the conduction band in TiO2 increases with the crystallinity, which in turn, lowers the CBE.39,40

Figure 2. Relationship between quasi-capacitance and applied bias voltage (Vapp) for TiCl4-treated FTO glass sample. The equivalent circuit shown was used to fit the EIS spectra. Labels “Bare FTO”, “TiCl4”, “TiCl4 + 150”, “TiCl4 + 300”, and “TiCl4 + 450” represent the untreated FTO glass substrate, TiCl4-treated FTO glass sample, and TiCl4-treated FTO glass samples subjected to subsequent heating at 150, 300 °C, and 450 °C, respectively. 36710


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Figure 4. I−V parameters of PSCs as a function of the postheating treatment temperature. Open and filled circles indicate parameters corresponding to forward sweep from Jsc to Voc and backward sweep from Voc to Jsc, respectively. Dashed and solid horizontal lines indicate parameters corresponding to forward and backward sweeps, respectively, for a cell with a compact TiO2 layer without TiCl4 treatment but heated at 450 °C for 30 min. Twelve replicate cells were tested for each postheating treatment temperature.

The effects of this adjustment of the CBE through the TiCl4 treatment on PSC performance were also observed. Figure 4 shows that the PSC performance depended on the temperature of the post-treatment after the TiCl4 treatment. The solid and dashed horizontal lines in each figure represent the I−V characteristics for backward and forward sweeps, respectively, for a cell not subjected to the TiCl4 treatment. The cells heated at temperatures below 150 °C exhibit lower Jsc and FF values but higher Voc values. These effects can probably be attributed to the raised CBE with the TiCl4 treatment. On the other hand, Jsc and FF increase but Voc decreases with an increase in the heating temperature. This can be explained by the fact that the CBE potential shifts to a deeper level with an increase in the heating temperature. Figure 5 shows the typical I−V curves of the high-PCE cells subjected to various treatments; the curves were plotted based on the data in Figure 4. In the case of the cell subjected to the TiCl4 treatment but not heating, the I−V curve exhibits a high degree of hysteresis. On the other hand, for the cell heated after the TiCl4 treatment, the hysteresis is lower and of the same degree as that for the cell not subjected to the TiCl4 treatment. The mechanism responsible for the hysteresis observed in the I−V curves of PSCs remains a matter of debate.16,41−44 However, we believe that the following mechanism is responsible for the phenomenon. In the case of the compact TiO2 layer with a shallow CBE, during the I−V measurements performed using a forward bias sweep, the generated electrons must pass through a region with a low DOS to reach the FTO layer; these electrons fill the states at a slow rate. On the other hand, in the case of the TiO2 layer with a deep CBE, the region with the low DOS is located at a deeper level and is filled more readily by the generated electrons; thus,

Figure 5. I−V curves for cell with highest PCE, as determined from data in Figure 4. Labels “No treatment”, “TiCl4”, “TiCl4 + 150”, “TiCl4 + 300”, and “TiCl4 + 450” indicate cells with untreated compact TiO2 layer, TiCl4-treated compact TiO2 layer, and TiCl4-treated compact TiO2 layer subjected to subsequent heating at 150, 300, and 450 °C, respectively.

most of the electrons pass through the region with a high DOS. In both cases, when the I−V measurements are performed using a backward bias sweep, the region with the low DOS is already filled by the electrons from the backward current, owing to which the generated electrons can pass through to the high DOS region (Scheme S6). This assumption is also supported by the results shown in Figure S5. The TiO2 layer not subjected to heating showed the highest charge-transfer resistance, with its resistance decreasing with the heating temperature. The FF value also decreased with an increase in the charge-transfer resistance. Further, PSCs that use Nb2O5 for the electron36711


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method was slightly lower (∼50 mV) than the Voc of the cell prepared without the antisolvent step. The higher Voc without the antisolvent method is possibly due to the effects of Cl from the PbCl2 in the perovskite precursor solution because Cl on TiO2 may reduce recombination.46

transport layer have been found to exhibit a higher degree of hysteresis and a lower FF than PSCs with a TiO2 layer.45 Nb2O5 has a higher CBE than TiO2, which may be one of the reasons accounting for these differences. The cell heated at 300 °C after the TiCl4 treatment showed the highest PCE (13.9%) during screening, with the other parameters of the cell being the following: Jsc of 18.6 mA cm−2, Voc of 1.107 V, and FF of 0.67. Moreover, in this study, in order to ensure that the PSCs exhibited higher PCE values, the perovskite layer was prepared using the antisolvent method.34 In this method, which involves the use of MAI and PbI2 to form the perovskite precursor solution, chlorobenzene, which is a poor solvent, is dropped into the perovskite precursor solution during the spin-coating of the perovskite layer to ensure a flat surface and large perovskite grains.34 The I−V curve and IPCE spectrum of the cell subjected to the TiCl4 treatment and subsequent heating at 300 °C are shown in Figures 6 and 7,

4. CONCLUSIONS In this study, we found that the CBE of the compact TiO2 layer is raised after a TiCl4 treatment and lowered by a subsequent heat treatment. The TiO2 layer formed because of the TiCl4 treatment affects the CBE of the entire TiO2 electrode. In addition, the lattice constant of the anatase TiO2 crystals formed because of the TiCl4 treatment may be slightly higher but reverts to the normal value with heating. The performances of the PSCs based on the treated samples were in keeping with the changes observed in the CBE potential with the TiCl4 treatment and the subsequent heat treatment. Voc increased with the TiCl4 treatment while Jsc and FF decreased in the absence of a subsequent heat treatment. Further, it was observed that the highest PCE, which was 17.2%, was achieved after post-TiCl4-treatment heating at 300 °C.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07496. SEM images of TiCl4-treated compact TiO2 on FTO glass, charge-extraction method, model of electron accumulation and extraction, time chart of applied voltage, current, and electron number during chargeextraction measurements, evaluation of CBE, model of charge accumulation in DOS for TiO2 with different CBEs, model for relationship between electron density and Vapp of TiO2 layer, presumed energy diagram of perovskite solar cells, Nyquist plots obtained from EIS measurements of electrochemical cells, equivalent circuit used for fitting Nyquist plots, formula for estimating quasi-capacitance, relationship between applied bias voltage and charge-transfer resistance, relationship between Vapp and series resistance, impedance parameters of TiCl4-treated FTO glass sample, performance of cells, presumed electron states in photoanode of PSCs, and cross-sectional SEM image of a PSC (PDF)

Figure 6. I−V curve of cell with highest PCE (cell with TiCl4-treated compact TiO2 layer subjected to subsequent heating at 300 °C).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Takurou N. Murakami: 0000-0002-2066-3982 Tetsuhiko Miyadera: 0000-0002-3661-8297 Figure 7. IPCE spectrum of cell corresponding to Figure 6.

Present Address

L.C.: Laboratory for Photovoltaic Energy Conversion, AlbertLudwigs University of Freiburg, Georges-Kohler-Allee 105, 79110, Freiburg im Breisgau, Germany.

respectively. The cell exhibited a PCE of 17.2%, Jsc of 22.5 mA cm−2, Voc of 1.062 V, and FF of 0.72 (the aperture area was 0.12 cm2); this was the highest efficiency observed during backward sweeping in this study. Further, the highest IPCE of this cell was 90%. However, a large hysteresis was still observed in the I−V curve with respect to the forward and backward sweeps. Hence, in future studies, we intend to explore methods of fabricating cells that do not exhibit this hysteresis. On the other hand, the Voc of the cells prepared using the antisolvent

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Prof. S. Uchida for his valuable discussions and also thank Dr. T. Koganezawa, Ms. M. Chiken, Ms. H. Kodama, and Ms. S. Makiyama for their technical 36712


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assistance. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The GIWAXS measurements were performed at SPring8 BL46XU with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal no. 2016A1514).

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Adjustment of Conduction Band Edge of Compact TiO2 Layer in

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