Spontaneous Synthesis of Highly Crystalline TiO2 Compact

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Spontaneous Synthesis of High Crystalline TiO Compact/ Mesoporous Stacked Film by Low-Temperature SteamAnnealing Method for Efficient Perovskite Solar Cells Yoshitaka Sanehira, Youhei Numata, Masashi Ikegami, and Tsutomu Miyasaka ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03532 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Materials & Interfaces

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Spontaneous Synthesis of High Crystalline TiO2

2

Compact/Mesoporous Stacked Film by Low-

3

Temperature Steam-Annealing Method for Efficient

4

Perovskite Solar Cells

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Yoshitaka Sanehira†*, Youhei Numata‡, Masashi Ikegami†, and Tsutomu Miyasaka†*

6 7

†

Graduate School of Engineering, Toin University of Yokohama, 1614 Kurogane-cho, Aoba,

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Yokohama, Kanagawa, 225-8503, Japan.

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‡Research Center for Advanced Science and Technology (RCAST), The university of Tokyo,

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4-6-1 Komaba, Meguro, Tokyo, 153-8904, Japan.

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KEYWORDS

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Perovskite solar cell, electron transport layer, TiO2, low-temperature process, steam-annealing

15

method, solvent-free hydrothermal synthesis

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 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

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ABSTRACT

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Highly crystalline TiO2 nanostructured films were synthesized by simple steam treatment

3

of a TiCl4 precursor film under saturated water vapor atmosphere at 125 °C, here referred to as

4

the steam-annealing method. In a single TiO2 film preparation step, a bilayer structure

5

comprising a compact bottom layer and a mesoporous surface layer was formed. The

6

mesoporous layer was occupied by bipyramidal nanoparticles, with a composite phase of anatase

7

and brookite crystals. Despite the low-temperature treatment process, the crystallinity of the TiO2

8

film was high, comparable with that of the TiO2 film sintered at 500 °C. The compact double-

9

layered TiO2 film was applied to perovskite solar cells (PSCs) as an electron-collecting layer.

10

The PSC exhibited a maximum power conversion efficiency (PCE) of 18.9% with an open-

11

circuit voltage (VOC) of 1.15 V. The PCE and VOC were higher than those of PSCs using a TiO2

12

film formed by 500 °C sintering.

13 14

Introduction

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The photovoltaic power conversion efficiency (PCE) of organic-inorganic hybrid perovskite

16

solar cells (PSCs) has improved in the decade since their invention, 1 reaching above 22%. 2-11 As

17

the PCE is now approaching the highest value predicted in the early stages of PSC development,

18

12,13

19

suitable properties for industrialization, particularly in terms of the stability of devices and

20

design of a low-cost production process. Fabrication of PSCs by low-temperature processes is

21

one area that has attracted attention. A photoactive layer of organic-inorganic hybrid lead halide

22

perovskites can be prepared by a simple solution process followed by annealing at temperatures

research trends are shifting away from improving the efficiency of cells and toward ensuring


2

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lower than 150 °C, regardless of the type of A-site cations in an ABX3 perovskite structure,

2

usually with Pb2+ as the B-site and halogen ions as the X-site. 5,6,8,9,10 Such low-temperature

3

process enables fabrication of devices using plastic substrates, thereby potentially facilitating

4

large-scale and low-cost printing processes for the manufacturing of lightweight flexible devices.

5

14-18

6

by using organic or carbonaceous materials as electron transporting materials (ETMs), rather

7

than metal oxides such as stable TiO2. As improving thermal stability of plastic-based PSCs is

8

desirable, metal oxides would be promising ETMs if the coating process for these materials

9

could be simplified to reduce cost and process time.

However, low-temperature manufacturing of perovskite cells can usually be achieved only

10

The most widely used ETM in PSCs is n-type semiconductor TiO2, which is generally

11

prepared by a high-temperature sintering processes at 450-550 °C to ensure high performance of

12

the cell. The high-temperature synthesis effectively reduces the lattice defects (recombination

13

centers) in ETMs by sufficient oxidation of the metal oxide, which removes discontinuity in

14

grain boundaries inhibiting electron diffusion between nanoparticles in a metal oxide layer.

15

However, there have been intense investigations into the sintering-free preparation of metal

16

oxide ETMs such as TiO2, 14 ZnO, 15,16 and SnO2 17,18 in attempts to develop a cost-effective and

17

thermally stable ETM. High PCE has also been reported by using a low-temperature ETM layer

18

formed by spin-coating of TiO2 nanoparticles that had been previously crystalized, and by

19

heating at a temperature close to the glass transition temperature of plastic substrate, typically

20

less than 150 °C. 22 We have also prepared mesoporous ETM layers from highly crystalline TiO2

21

nanocrystals of brookite phase by spin coating and 150 °C annealing treatments.

22

methods are based on the “necking” of nanocrystalline particles by surface chemical reactions;

23

however, this process requires commercial nanoparticles as a source of coating material. If a

17,19

19-21

These


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Page 4 of 29

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TiO2 ETM layer is directly prepared on substrates from Ti precursors, a reduction in the process

2

cost can be achieved by low-temperature preparation methods. Sputtering 14,18,20 and atomic layer

3

deposition 23 of metal oxides have been also reported as methods of forming a thin ETM layer on

4

substrates at relatively low temperatures. These methods, however, require special materials,

5

equipment, and skills to form an optimized ETM layer. Notable, another facile way to synthesize

6

a crystalline mesoporous film directly on substrates is a hydrothermal reaction, which has been

7

also used for ETM layer preparation a low-temperature solution process. 24 For example, a TiO2

8

nanowire array can be synthesized by hydrothermal reaction, and such a nanostructure has

9

reportedly been used for ETMs of PSCs.

25, 26

Although hydrothermal reactions require

10

significant amounts of precursor materials relative to the surface area of the substrates, and most

11

of the source materials become waste after the synthesis, the advantages for ETM preparation

12

include ability to control the morphology and crystallinity. High PCE is also expected if

13

hydrothermally synthesis directly on substrate from a small amount of precursor source materials

14

is possible.

15

Here, to achieve a solvent-free hydrothermal-like synthesis method, which is cost-effective and

16

ecologically friendly, we developed a steam-annealing method using only a small amount of

17

precursor and vapor source. A substrate coated with precursor was treated under a saturated

18

water vapor atmosphere. This steam treatment promoted the hydrolysis of a precursor by

19

supplying a large excess of water for detachment of the ligand, and enhanced crystal growth by

20

dehydrating condensation of hydroxyl-rich intermediates. W. J. Zeng et al. previously reported a

21

similar heating treatment. 27 In that work, a precursor film of titanium tetrabutoxide was exposed

22

to saturated water vapor to form the ETM layer of PSCs. A high annealing temperature of

23

180 °C was used, and the aim was to introduce hydroxyl groups on the Ti oxide surface which


4

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1

would lead to the PSCs having an increased open-circuit voltage (VOC). By contrast, we used

2

TiCl4 aqueous solution as a precursor and obtained a highly crystalline TiO2 layer by applying

3

steam-annealing at only 125 °C under saturated water vapor. Additionally, we demonstrated that

4

such steam-annealing could simultaneously synthesize a TiO2 monolithic structure by stacking of

5

a compact layer and a mesoporous layer on a substrate by a simple one-step heating process.

6

In this paper, we report the synthesis of a compact TiO2 film at a low temperature by the

7

steam-annealing method, and its application for fabricating highly efficient PSCs. By changing

8

the thickness of the precursor films and the steam-annealing conditions, a bilayer structure of

9

compact/mesoporous TiO2 layers with high crystallinity and a high effective surface area was

10

obtained. As a result, PSCs using the steam-annealed TiO2 film showed high photovoltaic

11

performance compared with the corresponding PSCs using TiO2 film prepared by sintering at

12

500 °C.

13 14

Experimental

15

Materials

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TiCl4 aqueous solution (16.0-17.0 wt.%, Kanto Chemical Co., Inc.), PbI2 (99.99%, Tokyo

17

Chemical Industry Co., Ltd.), PbBr2 (for perovskite precursor, Tokyo Chemical Industry Co.,

18

Ltd.), methylamine hydrobromide (MABr, >98.0%, Tokyo Chemical Industry), formamidine

19

hydroiodide (FAI, >98.0%, Tokyo Chemical Industry Co., Ltd.), CsI (>99.0%, Tokyo Chemical

20

Industry Co., Ltd.), 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-

21

OMeTAD, 99%, Merck & Co., Inc.), 4-tert-butylpyridine (TBP, Wako Pure Chemical Industries

22

Ltd.), lithium bis(trifluoromethylsulfonyl)imide salt (Li-TFSI, >98.0%, Tokyo Chemical

23

Industry Co., Ltd.), methyl methacrylate polymer (PMMA, Tokyo Chemical Industry Co., Ltd.),


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Page 6 of 29

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N,N-dimethylformamide (DMF, >99.5%, Wako Pure Chemical Industries Ltd.), dimethyl

2

sulfoxide (DMSO, >99.0%, Wako Pure Chemical Industries Ltd.), acetonitrile (>99.8%, Wako

3

Pure Chemical Industries Ltd.), and chlorobenzene (CB, >99.8%, Sigma-Aldrich Co. LLC) were

4

used as received without purification.

5

TiO2 layer preparation

6

A Fluorine-doped tin oxide (FTO) conductive glass substrate (size: 25 mm x 25 mm, sheet

7

resistance: 10 Ω/cm2, Nippon Sheet Glass Co. Ltd.) was washed using a sonicator in detergent

8

(2% Hellmanex solution), water, acetone, and 2-propanol, each for 15 min, followed by UV-O3

9

treatment of the surface under irradiation of a low-pressure Hg lamp for 15 min. In typical

10

preparation conditions, the TiCl4 aqueous solution was diluted with deionized water to a

11

concentration of 0.5 M. A Ti-precursor layer was formed on the FTO substrate by spin-coating

12

of the diluted TiCl4 solution at 2000 rpm for 60 s, and dried at 125 °C for 30 min. For the steam-

13

annealing treatment, the substrate was placed on a stand and sealed into a Teflon© container

14

(inner volume: 100 mL) with 5 mL of water on the bottom, positioned so that the substrate and

15

water could not directly contact each other, and kept in a dry oven at 125 °C for 1-4 h. After

16

steam-annealing, the container was allowed to cool down to room temperature before being

17

opened, and the substrate was subjected to the following PCS fabrication without any additional

18

heat treatment. To comparison of photovoltaic properties, mesoporous TiO2 film which spin-

19

coated a TiO2 nanoparticle dispersion solution on the compact TiO2 film and sintered at 500 °C

20

for 30 min by electric furnace under the dry condition were also prepared. 19

21

Solar cell fabrication

22

A triple-cation mixed-halide type perovskite was prepared based on the method reported by

23

Saliva et al.

8

PbI2, PbBr2, MABr, and FAI were dissolved in a mixture solvent of DMF and


6

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DMSO (4:1 volume ratio) at concentrations of 1.1 M, 0.2 M, 0.2 M, and 1.0 M, respectively. To

2

the solution, 1.5 M CsI/DMSO was added, so that the ratios of cations and halogens were

3

approximately FA:MA:Cs = 80:15:5 and Br:I = 85:15, respectively. The precursor solution thus

4

prepared was spread on the TiO2-coated substrate by a two-step spin-coating method (step 1:

5

1000 rpm, 10 s; step 2: 5000 rpm, 30 s) under a dry atmosphere (20-30%RH), and 300 µL of CB

6

was dropped at 25 s after the start of the spin-coating. The as-coated layer was then annealed in a

7

dry oven at 120 °C for 1 h and cooled down to room temperature to form a perovskite layer, with

8

thickness of around 450-500 nm. A CB solution containing 70 mM spiro-OMeTAD, 70 mM

9

TBP and 20 mM Li-TFSI was spin-coated at 3000 rpm for 30 s on the perovskite layer as a hole

10

transport layer. An Au layer (active area: 0.09 cm2) as a counter electrode was thermally

11

deposited on these layers.

12

Analysis

13

Photovoltaic characteristics of PSCs were evaluated by measuring J-V curves under irradiation

14

of simulated sunlight (A.M. 1.5G, 100 mW/cm2) using a solar simulator (PEC-L01, Peccell

15

Technologies, Inc.). Incident photon-to-electron conversion efficiency (IPCE) was measured

16

with scanning IPCE equipment (PEC-S20, Peccell Technologies, Inc.). Nanostructures of the

17

TiO2 layer and solar cells were observed by field-emission scanning electron microscope (SEM,

18

SU8000, Hitachi High-Technologies Co.). For cross-sectional SEM observation, the sample

19

surface was coated with Pt nanoparticles by using the ion sputter coater (MC1000, Hitachi High-

20

Technologies Co.). To identify the crystal structure of the steam-annealed TiO2 particle prepared

21

on the substrate, the lattice image and selected area electron diffraction (SAED) pattern of the

22

TiO2 particle were also observed using high-resolution transmission electron microscope (TEM,

23

HD-2700, Hitachi High-Technologies Co.). The crystallinity of the TiO2 layer was also


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Page 8 of 29

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evaluated by grazing incidence X-ray diffraction (GI-XRD) pattern (D8 Discover, Bruker-AXS

2

K.K.). Absorbance spectra of TiO2/FTO substrates were measured by ultraviolet-visible

3

spectrophotometer equipped with an integration sphere (UV-3600, Shimadzu Co.). Time decay

4

analysis of photoluminescence (PL) was measured by a C11367 Quantaurus-Tau compact

5

fluorescence lifetime spectrometer (Hamamatsu Photonics K.K.). Prior to measurement, a thin

6

protective layer was formed on the surface of the perovskite layer by spin-coating of CB solution

7

containing 10 mg/mL PMMA. Samples were photoexcited by a 365 nm LED light source, and

8

PL intensity was collected at around 760 nm.

9 10

Result and discussion

11

The steam-annealing treatment reported here was generally performed by heating a precursor

12

coating film obtained by spin-coating a 0.5 M TiCl4 aqueous solution on a FTO substrate in a

13

sealed vessel filled with saturated steam. SEM images of steam-annealed films on an FTO

14

substrate at 125 °C with a different reaction times (1, 2, and 4 h, described as "SA1h", "SA2h",

15

and "SA4h", respectively) are shown in Figure 1. SEM images of dry-annealed films (125 °C, 1

16

h, described as "DA125") and a high-temperature-sintered film (500 °C, 30 min, described as

17

"DA500") prepared from the same precursor films in an electric furnace without the use of water

18

vapor are also show in Figure 1.

19

In the case of the DA125 film without water vapor, a compact and smooth layer comprising

20

fine particles with size smaller than sputtered Pt nanoparticles (for SEM sample treatment,

21

approximately less than 5 nm) was formed (Figure 1a, 1b), whereas for the sintered DA500 film,

22

there was a compact layer packed with relatively large particles of ca. 20 nm in diameter, and

23

small gaps between particles could be seen in the cross-section image (Figure 1c, 1d). By


8

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1

contrast, the steam-annealing treatment produced a compact layer covering the FTO crystal

2

surface, with large particles of 20-50 nm in diameter protruding from the surface (Figure 1e, 1g,

3

1i). The large particles had a bipyramidal shape, and their numbers were looked like increased

4

with increasing reaction time. Bottom layer of the SA1h (Figure 1f) looks dark color which small

5

particles with unclear shapes were densely packed, but dark color layer in SA2h becomes thin

6

(Figure 1h) and then completely converted to compact layer filled by particles less than 10 nm in

7

diameter in SA4h (Figure 1j). At that time, the total thickness from the bottom layer to the

8

protruding particles was not changed at about 60 nm for all steam-annealed samples. As a result

9

of the steam-annealing treatment, the morphology of TiO2 layer is quite changed, and the

10

roughness and surface area both increased compared to the dry-annealing treatment.

11

Figure 2 shows GI-XRD patterns of the dry-annealed and steam-annealed films. While the

12

DA125 film remained in an amorphous phase, showing only the peaks due to FTO, DA500 film

13

showed a peak at 25.3° derived from a (101) facet of the anatase phase of TiO2 (JCPDS No. 00-

14

021-1272). By contrast, the steam-annealed films showed broad peaks near 25.3°, which

15

intensity gradually increased with reaction time. In the case of the SA4h, the intensity of the

16

25.3° peak finally reached a value equal to that of the DA500, which underwent 500 °C sintering.

17

When the XRD patterns of DA500 and SA4h are superimposed (Figure S1), new peak at around

18

30.8°, 36.3° and 37.3° appeared in the SA4h film, which was attributed to (211), (102) and (021)

19

facet of the brookite phase of TiO2 (JCPDS No. 01-076-1934), respectively. Additionally, it can

20

be seen that the peak at around 25.3° of the SA4h film broadened to higher angle side than that

21

of the DA500 film like the shoulder peak, and that consisted of (210) and (111) facets of the

22

brookite phase and a (101) facet of the anatase phase. Further, sintering of the SA4h film at

23

500 °C did not substantially change the morphology of the nanostructure or the crystallinity


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Page 10 of 29

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compared with before sintering (Figure S2). These results corroborate that a 4 h steam-annealing

2

treatment is capable of complete conversion of the precursor layer into TiO2 nanoparticles even

3

at the low temperature of 125 °C.

4

To determine the crystal phase of the steam-annealed TiO2 film, the lattice structure of

5

nanoparticles grown on the FTO substrate was observed by high-resolution TEM (Figure 3a).

6

The surfaces of FTO crystals were covered with a thin compact layer consisting of small

7

particles of TiO2 (less than 10 nm in diameter) and large bipyramidal particles that grew up from

8

the surface. A clear lattice structure was observed in the TEM images of each single nanoparticle,

9

indicating that the bipyramidal particle is a single crystal with high crystallinity. However, the

10

SAED patterns indicated that the crystal structure differed between the particles, with each of

11

them being a single crystal of either anatase (Figure 3b) or brookite phase (Figure 3c). From

12

these results, we conclude that the steam-annealed film prepared from a TiCl4 aqueous solution

13

is composed of a mixture of anatase and brookite nanoparticles with high crystallinity.

14

The pressure inside the reaction container that is sealed so as not to escape steam during the

15

heating is equal to the saturated vapor pressure of water at 125 °C, and that inside of the

16

container is considered to be in a state like hydrothermal reaction. Hydrothermal synthesis of

17

mixtures of anatase and brookite TiO2 nanoparticles in an acidic TiCl4 solution has been reported

18

elsewhere.

19

treatment, it is considered that a similar hydrothermal reaction occurs in a thin solution layer on

20

the substrate surface as shown in Scheme 1. TiCl4 partly hydrolyzed in an aqueous solution

21

before spin-coating is formed a metastable intermediate estimated as TiOXClY(OH)Z (2X + Y + Z

22

= 4) by partly dehydration condensation in the dried precursor film (Scheme 1a). This

23

intermediate is considered nonvolatile and hydrophilic and contained partly soluble Ti-species

28-31

Regarding the mechanism of TiO2 crystal growth during steam-annealing


10

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1

those are convenient state for the steam-annealing. Owing to the deliquescence of the

2

intermediate under a saturated steam atmosphere, the precursor layer absorbs water and forms an

3

acidic reaction solution layer by re-dissolving of the intermediate which remains on the substrate

4

surface without re-evaporation (Scheme 1b). Although the presence of enough water molecules

5

would enhance hydrolysis and dehydration condensation of the intermediate, leading to the

6

formation of a dense compact TiO2 layer at the bottom of the precursor film, simultaneously

7

hydrothermal crystal growth occurs on the re-dissolution layer (Scheme 1c). At this time, the

8

crystallization of TiO2 proceeds from the top to the bottom of the precursor layer along with

9

reaction time passed as shown in cross-sectional SEM images (Figure 1) and GI-XRD pattern

10

(Figure 2). Finally, as all the products are deposited directly on the substrate, a TiO2 layer with a

11

sufficient thickness can be formed from a very small amount of source material (Scheme 1d).

12

Since euhedral nanoparticles with high crystallinity are formed on the surface, there is no doubt

13

that a hydrothermal reaction occurs in the liquid phase.

14

Steam-annealed films with different treatment times were investigated with respect to the

15

photovoltaic properties of PSCs, and compared with PSCs using other dry-annealed films after

16

PSCs’ fabrication by methods reported in the Experimental section. The best J-V curve and

17

incident photon-to-electron conversion efficiency (IPCE) spectrum obtained from the PSC are

18

shown in Figure 4a and Figure 4b, respectively. Corresponding photovoltaic characteristics of

19

short-circuit current density (JSC), VOC, fill factor (FF), PCE, and series resistance (RS) are

20

summarized in Table on the Figure 4a. More details on the photovoltaic properties of the best

21

cells and average values collected for more than eight cells are provided in Table S1 and S2,

22

respectively, but the trend remains unchanged between these two cases.


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Page 12 of 29

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In the SEM images of the cross-section, all the PSCs showed approximately the same film

2

thickness in each layer, with no difference in structure compared with a so-called planar-type cell

3

(Figure S3). The DA125 film gave the lowest PCE in all cells, with lower JSC and FF and higher

4

RS values compared with other films. It also gave low IPCEs across the entire wavelength range,

5

although it had photoresponses up to around 800 nm, similar to the other PSCs. In a previous

6

study, it was shown that electron collection from a perovskite layer using a low-temperature-

7

prepared Ti-oxide layer was suppressed because of high sheet resistance and charge

8

recombination probability of amorphous TiO2 layer.

9

necessary to increase the surface area and/or the crystallinity of TiOX layer. By sintering at a

10

high temperature, the photovoltaic performance of the DA500 film was improved compared with

11

that of the DA125 film. However, despite the low-temperature process, the PSCs of steam-

12

annealed films achieved higher PCEs than that of the DA500 film, owing to increasing FF and

13

decreasing RS as the steam-annealing time increased. Hysteresis of J-V curves between the

14

forward scan and the backward scan, also decreased with longer steam-annealing time, possibly

15

owing to a crystallization of bottom compact layer and an increase in specific surface area

16

caused by the formation of large bipyramidal particles on the surface. IPCE spectra (Figure 4b)

17

of the PSCs with steam-annealing films showed higher efficiency on the long-wavelength side

18

with increasing reaction time. Since the absorbance spectra of SA films in the visible light region

19

did not indicate significant change by the steam-annealing time (Figure S4), it is irrelevant to the

20

improvement of the IPCE spectra. As can be seen from the GI-XRD patterns in Figure 2,

21

crystallization of the amorphous composition that inhibited the charge transfer progressed as the

22

steam-annealing increased, leading to improved photovoltaic properties. The best photovoltaic

23

characteristics of the SA4h film, which were higher than those of the DA500 film, included a JSC

20,21,32

To enhance electron collection, it is


12

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1

of 22.3 mA/cm2, VOC of 1.10 V, FF of 0.75, and PCE of 18.4%. Thus, the results show that the

2

steam-annealed TiO2 layer worked better as ETM of PSCs than those annealed at higher

3

temperatures, even with the low-temperature treatment.

4

To further improve the photovoltaic performance of PSCs using the steam-annealed TiO2 layer,

5

a thicker film with a larger surface area was prepared by changing the TiCl4 concentration. The

6

resulting films obtained by steam-annealing for 4 h (described as "tSA4h") are shown in Figure 5.

7

The surface of the tSA4h film (Figure 5a) was covered by nanoparticles of 20-50 nm in diameter,

8

containing bipyramidal particles similar to those seen on the SA4h film; however, the cross-

9

section (Figure 5b) showed a bilayer structure, with stacking of a compact layer with the same

10

thickness as that of the SA4h film on the bottom and a mesoporous layer on top. The total

11

thickness of the bilayer structure showed correlation with the precursor concentration and was

12

about 110 nm for a 1.0 M TiCl4 solution used to the tSA4h film. The GI-XRD patterns of steam-

13

annealed TiO2 films with different thicknesses are compared in Figure 5c. The intensities of all

14

peaks related to TiO2 increased with increasing TiO2 film thickness. In particular, the peaks near

15

30.8°, 36.3° and 37.3° which were derived from (211), (102) and (021) facets of brookite phase,

16

respectively, could be clearly distinguished. The thick bilayer TiO2 film obtained by steam-

17

annealing of TiCl4 was composed of a mixture of anatase and brookite phase nanoparticles with

18

high crystallinity, similar to the 60 nm SA4h film. Since no further increment in peak intensity,

19

nor any crystal phase change, were observed in the GI-XRD patterns with additional sintering at

20

500 °C (Figure S5), we could conclude that the tSA4h film prepared from a higher-concentration

21

precursor represented TiO2 film fully transformed to crystallized TiO2 without amorphous

22

residuals.


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Page 14 of 29

1

The bilayer TiO2 film with stacked compact/mesoporous layers was tested as the ETM layer of

2

PSCs; the J-V curves of the best-performing PSCs are shown in Figure 6a, together with that for

3

the SA4h film (thickness: ca. 60 nm). Moreover, the compact/mesoporous TiO2 film prepared by

4

spin-coating of nanoparticle dispersion solution on the DA500 and sintering at high-temperature

5

(500 °C, 30 min) is also examined as PSCs (described as "mDA500"). The photovoltaic

6

characteristics of those PSCs are summarized in table and inserted in the same Figure. In the case

7

of mDA500, JSC increased more than that of DA500 due to introduction of mesoporous layer

8

which enhances electron collection from the perovskite layer, FF and VOC still remain lower than

9

that of the steam-annealed films. It is considered that the thick mesoporous/perovskite mixed

10

layer of the mDA500 has higher resistance to charge transfer than that of the tSA4h (Figure S6).

11

The highest PCE in this work was obtained with the tSA4h film; the best cell yielded a JSC of

12

22.5 mA/cm2, VOC of 1.15 V, FF of 073, and PCE of 18.9%. The shape of the IPCE spectra for

13

the tSA4h film was almost consistent with that of the SA4h film (Figure 6b) which measured in

14

open circuit state, in agreement with the matching of the JSC values in the J-V curves. The

15

distribution of photovoltaic characteristics is shown in Figure S7. Apparently, the tSA4h film

16

showed higher VOC than that of the SA4h film in J-V curves, but the distributions of VOC are

17

within a similar range and have equivalent average value (Figure S7b). The improvement of

18

photovoltaic performance in the PSC (Figure S7d) of the tSA4h film was due to the

19

improvement in FF (Figure S7c) and the elimination of hysteresis in the J-V curves. Generally,

20

introduction of a mesoporous ETM layer, which increases the contact area for electron collection,

21

enables rapid capture of photoinduced electrons from the perovskite layer and efficient charge

22

transfer to the FTO and external circuit. The same principal explains why the tSA4h film with a


14

Page 15 of 29 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

stacked structure of compact and mesoporous layers showed better photovoltaic performance

2

than the thin SA4h film.

3

PL measurement was applied to simplified cells without the spiro-OMeTAD layer and the

4

counter electrode to focus on observation of a behavior of electron injection in the

5

perovskite/TiO2 interfaces. Figure 7 shows the time decay and fitting curves of PL obtained from

6

the perovskite layer on DA500 and tSA4h film. The PL spectra and detailed fitting parameters of

7

each sample are summarized in Figure S8 and Table S3, respectively. The PL emission intensity

8

decreased earlier for tSA4h than for DA500, indicating that photogenerated electrons in the

9

perovskite layer are promptly injected into the TiO2 layer. Similar to the disappearance of

10

hysteresis in the J-V curves, these results indicate that the tSA4h film excels at charge collection

11

from the perovskite layer, as the area of the perovskite/ETM interface increased owing to the

12

formation of highly crystalline bipyramidal nanoparticles on the surface of the compact TiO2

13

layer. This is consistent with the FF and PCE for the steam-annealed TiO2 film being superior to

14

those of the high-temperature-sintered TiO2 film.

15 16

Conclusion

17

In summary, the steam-annealing method produced a monolithic structure of dense and

18

compact and a high crystalline TiO2 film by heating a TiCl4 precursor film spin-coated on an

19

FTO substrate at 125 °C under a saturated water vapor atmosphere. As the result of a

20

hydrothermal reaction in the thin solution layer that formed on the substrate surface during

21

treatment, the obtained film densely covered the substrate surface and contained TiO2

22

nanoparticles with bipyramidal shape, having anatase and brookite phases. The configuration of

23

the nanostructure of film could be controlled by changing the concentration of the precursor


15


Page 16 of 29

1

solution, and a bilayer structure of composed by stacking a compact layer of uniform thickness

2

and a mesoporous layer of variable thickness could be obtained by a simple one-step heating

3

process. PSCs fabricated with steam-annealed TiO2 ETM layers achieved photovoltaic

4

characteristics superior to those of PSCs using TiO2 film sintered at 500 °C; a maximum PCE of

5

18.9% was obtained. All preparation processes in the present study were performed at 125 °C,

6

which is lower than the heat resistance limit of commercial plastic substrates. The method is thus

7

expected to have applications in the fabrication of flexible photovoltaic devices.

8


16

Page 17 of 29 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


17


1

Figure 1. Top view and cross-sectional SEM image of TiO2 films coated on FTO substrate and

2

dry-annealed at (a, b) 125 °C for 1 h, and (c, d) 500 °C for 30 min without water vapor, and

3

steam-annealed at 125 °C for (e, f) 1 h, (g, h) 2 h and (i, j) 4 h, respectively.

FTO

B(102) B(021)

FTO

B(211)

DA125

A(101) B(210) B(111) FTO

4

Intensity (arb. units)

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

Page 18 of 29

DA500 SA1h SA2h SA4h

20

5

25

30 2θ (˚)

35

40

6

Figure 2. GI-XRD patterns of TiO2 films annealed at different conditions. Positions of peaks

7

were referenced from JCPDS No. 00-021-1271 (TiO2, anatase phase, as A), No. 01-076-1934

8

(TiO2, brookite phase, as B), and No. 01-070-6995 (SnO2, as FTO).

9

10


18

Page 19 of 29 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

Figure 3. (a) Cross-sectional TEM image of TiO2 film (SA4h) steam-annealed at 125 °C, 4 h,

2

and high-magnification lattice image of (b) anatase phase and (c) brookite phase particles. SAED

3

patterns (inset) were obtained for each individual particle.

4 a H2 O

b H2O

c

H2O

H2O

Precursor layer FTO substrate

5

d

( Ti : Soluble Ti-species)

TiOXClY(OH)Z (2X+Y+Z=4)

H2O

HCl H2O

Compact TiO2 layer TiOXClY(OH)Z + H2O →TiOXClY-1(OH)Z+1 + HCl

TiOXClY(OH)Z → TiO2 +HCl +H2O

TiO2

6

Scheme 1. Schematic image of TiO2 film formation by the steam-annealing treatment. A

7

hydrothermal-like reaction occurs on the substrate surface by the following processes: (a) Step 1,

8

flocculation of water vapor from surrounding saturated steam atmosphere caused by

9

deliquescence of TiCl4 hydrolysis intermediates; (b) Step 2, fixing in solution state by vapor

10

pressure depression due to partial dissolving of intermediates; (c) Step 3, crystal growth of TiO2

11

nanoparticles by a dissolution-reprecipitation reaction in the thin solution layer; (d) Step 4, re-

12

evaporation of volatile components. “Ti” in circle indicates a soluble Ti-species.

13


19


100 SA1h SA2h SA4h b

25 20

DA500

TiO2 JSC VOC FF PCE RS layer mA/cm2 V % Ω DA125 2.4 0.98 0.38 0.9 1323 DA500 21.3 1.11 0.68 16.1 52 SA1h 22.6 1.14 0.60 15.4 78 SA2h 22.4 1.13 0.68 17.1 69 SA4h 22.3 1.10 0.75 18.4 43

15 10 5

SA1h SA2h SA4h

80

DA500 IPCE (%)

a Photocurrent density (mA/cm2)

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

Page 20 of 29

60 40 20

DA125

DA125 0

0 0

0.2

0.4

0.6 0.8 Voltage (V)

1

1.2

400

500

600 700 Wavelength (nm)

800

1 2

Figure 4. Best data of (a) J-V curves and (b) IPCE spectra obtained from PSCs on DA125

3

(purple), DA500 (blue), SA1h (green), SA2h (yellow), and SA4h (red). The dotted and solid

4

lines in Figure 4a indicate the forward scan (-0.05 V → 1.20 V) and reverse scan (1.20 V → -

5

0.05 V), respectively.

6

7 8

Figure 5. (a) Top and (b) cross-section SEM views and (c) GI-XRD pattern of the bilayer TiO2

9

film steam-annealed at 125 °C for 4 h.


20

Page 21 of 29

1 25

tSA4h 20

b

15

JSC TiO2 layer mA/cm2 mDA500 22.8 SA4h 22.3 tSA4h 22.5

10 5

100

tSA4h mDA500

80

mDA500 SA4h

IPCE (%)

Photocurrent density (mA/cm2)

a

VOC

FF PCE V % 1.08 0.73 17.9 1.10 0.75 18.4 1.15 0.73 18.9

SA4h

60 40 20

0

0 0

0.2

2

0.4

0.6 0.8 Voltage (V)

1

1.2

400

500

600 700 Wavelength (nm)

800

3

Figure 6. (a) J-V curves and (b) IPCE spectra obtained from the best-performing mDA500

4

(black), SA4h (blue) and tSA4h (red); the dotted line and solid line in the J-V plot indicate the

5

forward scan (-0.05 V → 1.20 V) and reverse scan (1.20 V → -0.05 V), respectively.

6 1 Photoluminescence (norm.)

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


FTO/DA500/Perovskite/PMMA 0.1

FTO/tSA4h/Perovskite/PMMA

0.01 0

7

100

200 300 Time (ns)

400

8

Figure 7. Normalized decay curves of PL intensity measured for the perovskite layer prepared

9

on DA500 (blue) and tSA4h (red). Black solid lines in the plot area indicate fitting curves of

10

each sample.


21


Page 22 of 29

1 2

ASSOCIATED CONTENT

3

Supporting Information

4

Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

5

Overapplying image of GI-XRD patterns of DA500 and SA4h (Figure S1), SEM images

6

and GI-XRD patterns of additional-sintered SA4h and tSA4h films (Figure S2), tables of

7

photovoltaic characteristics and cross-sectional SEM images for PSCs (Table S1, S2 and

8

Figure S3), absorbance spectra of the steam-annealed films on the FTO substrate (Figure

9

S4), GI-XRD patterns of additional-sintered tSA4h films (Figure S5), cross-sectional

10

SEM images and distribution of photovoltaic characteristics of tSA4h and mDA500

11

(Figure S6 and S7), and PL spectra and lifetime of perovskite layer on the tSA4h film

12

(Figure S8 and Table S3).

13 14

AUTHOR INFORMATION

15

Corresponding Author

16

* E-mail: [email protected]

17

[email protected]

18

ORCID

19

Yoshitaka Sanehira: 0000-0003-2030-2690

20

Youhei Numata: 000-0001-8350-7226


22

Page 23 of 29 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


Tsutomu Miyasaka: 0000-0001-8535-7911

2 3

Notes

4

The authors declare no competing financial interest.

5 6

ACKNOWLEDGMENT

7

This research was supported by the Japan Science and Technology Agency under the

8

Advanced Low Carbon Technology R&D program (JST-ALCA) and Grant-in-Aid for Young

9

Scientists B (17K14533) of Japanese Society for Promotion of Science (JSPS). We appreciate

10

Prof. Hiroshi Segawa for allowing access to SEM, XRD and other research facilities at Research

11

Center for Advanced Science and Technology (RCAST), The university of Tokyo. Cordial

12

thanks are due to Hitachi High-Technologies Co. for the use of the HD-2700 for obtaining the

13

TEM images.

14 15

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15 16 17 18 19 20


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1 2 3 4

5

TOC GRAPHICS

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Spontaneous Synthesis of Highly Crystalline TiO2 Compact

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