Adjustment of Conduction Band Edge of Compact TiO2 Layer in

Subscriber access provided by AUSTRALIAN NATIONAL UNIV

Article

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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07496 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

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

§

Graduate School of Arts and Science, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan

* Corresponding Author: Takurou N. Murakami E-mail: [email protected]

Keywords: Perovskite solar cells, compact TiO2 layer, TiCl4 treatment, post-heating treatment, conduction band edge shift

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 TiCl4-treated 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.


Page 2 of 30

Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction 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 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 TiCl4 treatment that results in fewer voids in the interface between the TiO2 and perovskite layers when it is



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 charge-transfer resistance because of the TiCl4 treatment remains unclear. It is likely that the TiO2 thin layer formed by the TiCl4 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 due to the TiCl4 treatment to clarify the effect of the treatment. Furthermore, the effect of the post-heating 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.

2. Experimental Section Fluorine-doped SnO2-coated glass (FTO glass), which is transparent and conductive, was


Page 4 of 30

Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 charge-extraction 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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 hole-transport 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-butyl pyridine 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 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-1213, Sanvac Co.), in order 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.


Page 6 of 30

Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The intensity of the light (100 mW cm-2) was calibrated using a standard single-crystal 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 cm2 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.

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 of approximately 1 × 1020 cm-3, the Vapp value of the TiCl4-treated 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


Page 8 of 30

Page 9 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 quasi-capacitance 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 TiCl4-treated 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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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. Based on 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 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


Page 10 of 30

Page 11 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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, 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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 electron-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 in order 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, 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


Page 12 of 30

Page 13 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hysteresis. On the other hand, the Voc of the cells prepared using the antisolvent method was slightly lower (ca. 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

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.

Acknowledgements The authors would like to thank Dr. T. Koganezawa, Ms. M. Chiken, Ms. H. Kodama, and Ms. S. Makiyama for their technical assistance. This work was supported by the New Energy and Industrial



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Technology Development Organization (NEDO) of Japan. The GIWAXS measurements were performed at SPring-8 BL46XU with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal no. 2016A1514). Supporting Information Available: < Adjustment of Conduction Band Edge of Compact TiO2 Layer in PSCs Through TiCl4 Treatment > This material is available free of charge on the Internet at http://pubs.acs.org L. Cojocaru’s present affiliation: Laboratory for Photovoltaic Energy Conversion, Albert-Ludwigs University of Freiburg, Georges-Kohler-Allee 105, 79110, Freiburg im Breisgau, Germany

References 1.

Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T.; Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells, J. Am. Chem. Soc., 2009, 131, 6050–6051.

2.

Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; and Snaith, H. J.; Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites Science, 2012, 338, 643–647.

3.

Kim, H.-S.; Lee, C.-R.; Im, J.-H; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser J. E.; Grätzel, M.; Park, N-G.; Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%, Sci. Rep., 2012, 2, 591.


Page 14 of 30

Page 15 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.

Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I.; Solvent Engineering for High-Performance Inorganic–Organic Hybrid Perovskite Solar Cells, Nat. Mater., 2014, 13, 897–903,

5.

Wu, Y.; Yang, X.; Chen, W.; Yue, Y.; Cai, M.; Xie, F.; Bi, E.; Islam, A.; Han, L.; Perovskite Solar Cells with 18.21% Efficiency and Area Over 1 cm2 Fabricated by Heterojunction Engineering Nat. Energy, 2016, 1, 16148.

6.

Saliba, M.; Matsui, T.; Seo, J.-Y.; Domanski, K.; Correa-Baena, J.-P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Tress, W.; Abate, A.; Hagfeldt, A.; Grätzel, M.; Cesium-Containing Triple Cation Perovskite Solar Cells: Improved Stability, Reproducibility and High Efficiency, Energy Environ. Sci., 2016, 9, 1989–1997.

7.

Saliba, M.; Matsui, T.; Domanski, K.; Seo, J.-Y.; Ummadisingu, A.; Zakeeruddin, S. M.; Correa-Baena, J.-P.; Tress, W. R.; Abate, A.; Hagfeldt, A.; Grätzel, M.; Incorporation of Rubidium Cations Into Perovskite Solar Cells Improves Photovoltaic Performance, Science, 2016, 354, 206–209

8.

Shirayama, M.; Kadowaki, H.; Miyadera, T.; Sugita, T.; Tamakoshi, M.; Kato, M.; Fujiseki, T.; Murata, D.; Hara, S.; Murakami, T. N.; Fujimoto, S.; Chikamatsu, M.; Fujiwara, H.; Optical Transitions in Hybrid Perovskite Solar Cells: Ellipsometry, Density Functional Theory and Quantum Efficiency Analyses for CH3NH3PbI3, Phys. Rev. Appl., 2016, 5, 014012.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

9.

N.-G. Park, M. Grätzel, T. Miyasaka, K. Zhu and K. Emery, Towards Stable and Commercially Available Perovskite Solar Cells, Nature Energy, 2016, 1, 16152.

10. Tress, W.; Marinova, N.; Inganäs, O.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Graetzel, M.; Predicting the Open-Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: The Role of Radiative and Non-Radiative Recombination, M; Adv. Energy Mater., 2015, 5, 1400812. 11. Wojciechowski, K.; Stranks, S. D.; Abate, A.; Sadoughi, G.; Sadhanala, A.; Kopidakis, N.; Rumbles, G.; Li, C.-Z.; Friend, R. H.; Jen, A. K.-Y.; Snaith, H. J.; Heterojunction Modification for Highly Efficient Organic-inorganic Perovskite Solar Cells, ACS Nano, 2014, 8, 12701– 12709. 12. Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S. D.; Wang, J. T.-W.; Wojciechowski, K.; Zhang, W.; Anomalous Hysteresis in Perovskite Solar Cells, J. Phys. Chem. Lett., 2014, 5, 1511−1515. 13. Kim, H.-S.; Park, N.-G.; Parameters Affecting I–V Hysteresis of CH3NH3PbI3 Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer, J. Phys. Chem. Lett., 2014, 5, 2927−2934. 14. Almora, O.; Zarazua, I.; Mas-Marza, E.; Mora-Sero, I.; Bisquert, J.; Garcia-Belmonte, G.; Hysteresis, and Electrode Polarization in Lead Halide Perovskite Solar Cells, J. Phys. Chem. Lett., 2015, 6, 1645−1652.


Page 16 of 30

Page 17 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15. Cojocaru, L.; Uchida, S.; Jayaweera, P. V. V.; Kaneko, S.; Nakazaki, J.; Kubo, T.; Segawa, H.; Origin of the Hysteresis in I-V Curves for Planar Structure Perovskite Solar Cells Rationalized with a Surface Boundary Induced Capacitance Model, Chem. Lett., 2015, 44, 1750–1752. 16. Meloni, S.; Moehl, T.; Tress, W.; Franckevicˇius, M.; Saliba, M.; Lee, Y. H.; Gao, P.; Nazeeruddin, M.

K.; Zakeeruddin, S. M.; Rothlisberger, U; Graetzel, M.; Ionic

Polarization-induced Current–Voltage Hysteresis in CH3NH3PbX3 Perovskite Solar Cells, Nat. Comm., 2016, 7, 10334. 17. Yu, H.; Lu, H.; Xie, F.; Zhou, S.; Zhao, N.; Native Defect-induced Hysteresis Behavior in Organolead, Iodide Perovskite Solar Cells, Adv. Funct. Mater., 2016, 26, 1411–1419. 18. Ogomi, Y.; Morita, A.; Tsukamoto, S.; Saitho, T.; Shen, Q.; Toyoda, T.; Yoshino, K.; Pandey, S. S.; Ma, T.; Hayase, S.; All-Solid Perovskite Solar Cells with HOCO-R-NH3+I− Anchor-Group Inserted between Porous Titania and Perovskite, J. Phys. Chem. C, 2014, 118, 16651−16659. 19. Zhou, Z.; Pang, S.; Liu, Z.; Xu, H.; Cui, G.; Interface Engineering for High-performance Perovskite Hybrid Solar Cells, J. Mater. Chem. A, 2015, 3, 19205–19217. 20. Zhao, J.; Wang, P.; Wei, L.; Liu, Z.; Fang, X.; Liu, X.; Rena, D.; Mai, Y.; Efficient Charge-transport in Hybrid Lead Iodide Perovskite Solar Cells, Dalton Trans., 2015, 44, 16914. 21. Cojocaru, L.; Uchida, S.; Sanehira, Y.; Nakazaki, J.; Kubo, T.; Segawa, H.; Surface Treatment of the Compact TiO2 Layer for Efficient Planar Heterojunction Perovskite Solar Cells, Chem. Lett., 2015, 44, 674–676.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

22. Hwang, I.; Yong, K.; Novel CdS Hole-Blocking Layer for Photostable Perovskite Solar Cells, ACS Appl. Mater. Interfaces, 2016, 8, 4226−4232. 23. Liu, Z.; Chen, Q.; Hong, Z.; Zhou, H.; Xu, X.; Marco, N-D.; Sun, P.; Zhao, Z.; Cheng, Y-B.; Yang, Y.; Low-Temperature TiOx Compact Layer for Planar Heterojunction Perovskite Solar Cells, ACS Appl. Mater. Interfaces, 2016, 8, 11076−11083 24. Nazeeruddin, M. K.; Péchy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Grätzel, M.; Engineering of Efficient Panchromatic Sensitizers for Nanocrystalline, TiO2-Based Solar Cells, J. Am. Chem. Soc., 2001, 123, 1613–1624. 25. Sommeling, P. M.; O’Regan, B. C.; Haswell, R. R.; Smit, H. J. P.; Bakker, N. J.; Smits, J. J. T.; Kroon, J. M.; van Roosmalen, J. A. M.; Influence of a TiCl4 Post-Treatment on Nanocrystalline TiO2 Films in Dye-Sensitized Solar Cells, J. Phys. Chem. B, 2006, 110, 19191–19197 26. O’Regan, B. C.; Durrant, J. R.; Calculation of Activation Energies for Transport and Recombination in Mesoporous TiO2/Dye/Electrolyte Films–Taking into Account Surface Charge Shifts with Temperature, J. Phys. Chem. B, 2006, 110, 8854–8547 27. O’Regan, B. C.; Durrant, J. R.; Sommeling, P. M.; Bakker, N. J.; Influence of the TiCl4 Treatment on Nanocrystalline TiO2 Films in Dye-Sensitized Solar Cells. 2. Charge Density, Band Edge Shifts, and Quantification of Recombination Losses at Short Circuit, J. Phys. Chem. C, 2007, 111, 14001–14010


Page 18 of 30

Page 19 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28. Lee, S-W.; Ahn, K-S.; Zhu, K.; Neale, N. R.; Frank, A. J.; Effects of TiCl4 Treatment of Nanoporous TiO2 Films on Morphology, Light Harvesting, and Charge-Carrier Dynamics in Dye-Sensitized Solar Cells, J. Phys. Chem. C, 2012, 116, 21285−21290 29. Sakai, N.; Miyasaka, T.; Murakami, T. N.; Efficiency Enhancement of ZnO-Based Dye-Sensitized Solar Cells by Low-Temperature TiCl4 Treatment and Dye Optimization, J. Phys. Chem. C, 2013, 117, 10949–10956. 30. Kavan, L.; Grätzel, M.; Highly Efficient Semiconducting TiO2 Photoelectrodes Prepared by Aerosol Pyrolysis, Electrochim. Acta, 1995, 40, 643–652. 31. Wang, Z.-S.; Koumura, N.; Cui, Y.; Takahashi, M.; Sekiguchi, H.; Mori, A.; Kubo, T.; Furube, A.; Hara, K.; Hexylthiophene-Functionalized Carbazole Dyes for Efficient Molecular Photovoltaics: Tuning of Solar-Cell Performance by Structural Modification, Chem. Mater., 2008, 20, 3993–4003. 32. Duffy, N. W.; Peter, L. M.; Rajapakse R. M. G.; Wijayantha, K. G. U.; A Novel Charge Extraction Method for the Study of Electron Transport and Interfacial Transfer in Dye Sensitised Nanocrystalline Solar Cells, Electrochem. Commun., 2000, 2, 658–662. 33. Sunahara, K.; Ogawa, J.; Mori, S.; A Method to Measure Electron Lifetime in Dye-sensitized Solar Cells: Stepped Current Induced Measurement of Cell Voltage in the Dark, Electrochem. Commun., 2011, 13, 1420–1422.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

34. Wakamiya, A.; Endo, M.; Sasamori, T.; Tokitoh, N.; Ogomi, Y.; Hayase, S.; Murata, Y.; Reproducible Fabrication of Efficient Perovskite-Based Solar Cells: X-ray Crystallographic Studies on the Formation of CH3NH3PbI3 Layers, Chem. Lett., 2014, 43, 711–713. 35. Huang, V. M.-W.; Vivier, V.; Orazem, M. E.; Pébère, N.; Tribollet, B.; The Apparent Constant-Phase-Element Behavior of a Disk Electrode with Faradaic Reactions A Global and Local Impedance Analysis, J. Electrochem. Soc., 2007, 154, C99–C107. 36. Orazem, M. E.; Tribollet, B.; The Electrochemical Society Series: Electrochemical Impedance Spectroscopy, Wiley, 2008. 37. Burg, G. J.; Eeden, A. V. D.; Sluyters-Rehbach, M.; Sluyters, J. H.; The Analysis of Electrode Impedances Complicated by the Presence of a Constant Phase Element, J. Electroanal. Chem., 1984, 176, 275–295. 38. Bisquert, J.; Chemical Capacitance of Nanostructured Semiconductors: Its Origin and Significance for Nanocomposite Solar Cells, Phys. Chem. Chem. Phys., 2003, 5, 5360–5364. 39. Zhang, Q.; Gao, L.; Guo, J.; Effects of Calcination on The Photocatalytic Properties of Nanosized TiO2 Powders Prepared by TiCl4 Hydrolysis, Appl. Catal. B-Environ., 2000, 26, 207– 215. 40. Benko, G.; Skårman, B.; Wallenberg, R.; Hagfeldt, A.; Sundstro1m, V.; Yartsev, A. P.; Particle Size and Crystallinity Dependent Electron Injection in Fluorescein 27-Sensitized TiO2 Films, J. Phys. Chem. B, 2003, 107, 1370–1375.


Page 20 of 30

Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


41. Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S. D.; Wang J. T-W.; Wojciechowski, K.; Zhang, W.; Anomalous Hysteresis in Perovskite Solar Cells, J. Phys. Chem. Lett., 2014, 5, 1511–1515 42. Kim, H-S.; Park, N-G.; Parameters Affecting I–V Hysteresis of CH3NH3PbI3 Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer, J. Phys. Chem. Lett., 2014, 5, 2927–2934 43. Ravishankar, S.; Almora, O.; Echeverría-Arrondo, C.; Ghahremanirad, E.; Aranda, C.; Guerrero, A.; Fabregat-Santiago, F.; Zaban, A.; Garcia-Belmonte, G.; Bisquert, J.; Surface Polarization Model for the Dynamic Hysteresis of Perovskite Solar Cells, J. Phys. Chem. Lett., 2017, 8, 915– 921 44. Cojocaru, L.; Uchida, S.; Jayaweera, P. V. V.; Kaneko, S.; Nakazaki, J.; Kubo, T.; Segawa, H.; Origin of the Hysteresis in I–V Curves for Planar Structure Perovskite Solar Cells Rationalized with a Surface Boundary-Induced Capacitance Model, Chem. Lett., 2015, 44, 1750–1752 45. Kogo, A.; Numata, Y.; Ikegami, M.; Miyasaka, T.; Nb2O5 Blocking Layer for High Open-Circuit Voltage Perovskite Solar Cells, Chem. Lett., 2015, 44, 829–830 46. Tan, H.; Jain, A.; Voznyy, O.; Lan, X.; García de Arquer, F. P.; Fan, J. Z. Quintero-Bermudez, R.; Yuan, M.; Zhang, B.; Zhao, Y.; Fan, F.; Li, P.; Quan, L. N.; Zhao, Y.; Lu, Z.-H.; Yang, Z.; Hoogland, S.; Sargent, E. H.; Efficient and Stable Solution-Processed Planar Perovskite Solar Cells via Contact Passivation, Science, 2017, 355, 722–726



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

Page 22 of 30

Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1.

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.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2.

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 °C, 300 °C, and 450 °C, respectively.


Page 24 of 30

Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3.

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.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4.

Figure 4. I-V parameters of PSCs as a function of the post-heating 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 post-heating treatment temperature.


Page 26 of 30

Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5.

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 °C, 300 °C, and 450 °C, respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6.

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


Page 28 of 30

Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7.

Figure 7. IPCE spectrum of cell corresponding to Figure 6



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents (TOC) Graphic


Page 30 of 30

Adjustment of Conduction Band Edge of Compact TiO2 Layer in

Recommend Documents