High Consistency Perovskite Solar Cell with a Consecutive Compact

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High Consistency Perovskite Solar Cell with a Consecutive Compact and Mesoporous TiO Film by One-Step Spin-Coating 2

Xu-Hui Zhang, Jia-Jiu Ye, Liang-Zheng Zhu, Hai-Ying Zheng, Xue-Peng Liu, Pan Xu, and Songyuan Dai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11860 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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ACS Applied Materials & Interfaces

High Consistency Perovskite Solar Cell with a Consecutive Compact and Mesoporous TiO2 Film by One-Step Spin-Coating Xu-Hui Zhanga,b, Jia-Jiu Yea,b, Liang-Zheng Zhua,b, Hai-Ying Zhenga,b, Xue-Peng Liua,b, Xu Pan*a and Song-Yuan Dai*c

a

Key Laboratory of Novel Thin Film Solar Cells, Institute of Applied Technology, Hefei Institutes

of Physical Science, Chinese Academy of Sciences, Hefei, 230031, P. R. China b

University of Science and Technology of China, Hefei, 230026, P. R. China

c

Beijing Key Laboratory of Novel Thin Film Solar Cells, North China Electric Power University,

Beijing, 102206, P. R. China

ABSTRACT: Generally, in classic mesoscopic perovskite solar cells (PSCs), the compact blocking layer and mesoporous scaffold layer prepared by two steps or more, will inevitably form an interface between them. It is undoubted that the interface contact is not conducive to electron transport and would increase the recombination in the device, resulting in the inferior performance of PSCs. In this work, we constructed a consecutive compact and mesoporous (CCM) TiO2 film to substitute the compact blocking layer and scaffold layer for mesoscopic PSCs. The bottom of CCM TiO2 film was dense and the top was mesoporous with large uniform macropores. The two parts of the film were consecutive, which could promote the electron transport rate and decrease the charge recombination effectively. Moreover, due to the existence of macropores in the CCM TiO2 film, it was propitious to the deposition of perovskite and charge separation for the perovskite layer. Over 15.0% of average power conversion efficiency (PCE) with high consistency photovoltaic performances was 1

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achieved for the CCM TiO2 film based mesoscopic PSCs, which is higher than that with classic mesoporous structure.

KEYWORDS: mesoscopic perovskite solar cells; consecutive; macropore; charge recombination; high consistency

1. INTRODUCTION Recently, perovskite solar cells (PSCs) have attracted intense attention because of the perovskite’s superior properties of tunable optical properties, long electron-hole diffusion length, high absorption coefficient and solution process ability.1,2 With a surprising increase of the power conversion efficiency (PCE) from 3.8% to 22.1% during last 7 years,3-9 today, it has become the fastest growing solar cell technology. The structures of PSCs can be divided into two branches: mesoscopic nanostructure PSCs and planar heterojunction structure PSCs.10-11 The compact layer and the mesoporous layer are two basic components in the mesoscopic PSCs,12 the compact layer (CL) can collect electrons and block holes from the perovskite layer effectively,13-15 while the mesoporous layer acts as the electron transporting layer.16-19 Both the compact layer and the mesoporous layer play important roles in enhancing the performance of mesoscopic PSC. Methods for the compact layer preparation include

spray

pyrolysis,20-22

hydrolysis,23-25

atomic

layer

deposition,26

spin-coating,27-28 and physical vapor deposition.29 The mesoporous layer also has various

preparation

methods

such

as

spin-coating,30

2


in-situ

hydrothermal

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synthesis,31-32

electrochemical

deposition,33

screen-printing,34

liquid

phase

deposition,35 and sprayed method.36 For the classic mesoscopic PSC (Meso-PSC, the device structure is shown in Figure 1b) fabrication, there are two limitations that may hinder the improvement of device efficiency. Firstly, two or more steps should be spent on the preparation of compact layer and mesoporous layer. There will be an interface between the compact layer and mesoporous layer because the two layer are prepared separately. Since the interface contact is not conducive to the electron transport, the charge recombination will be increased, leading to the decrease of PSCs performance.37 The general approach of reducing this recombination is treating the compact blocking layer and mesoporous scaffold layer with diluted TiCl4 aqueous solution.5,38-40 Secondly, the commonly used mesoporous TiO2 nanocrystal film is too dense for the perovskite permeating and deposition. It will result in incomplete filling with the perovskite and hinder charge separation of the perovskite layer.25 Accordingly, a rough surface such as a TiO2 layer with both mesopores and macropores has been considered to solve this problem. For this purpose, Nam-Gyu Park and co-workers added polymer vehicle ethyl cellulose into the TiO2 paste, a mixed porous nanostructure of TiO2 film with small mesopores and large macropores was obtained.41 Herein, a new and simple consecutive compact and mesoporous (CCM) TiO2 film was prepared by one-step spin-coating to substitute the compact blocking layer and scaffold layer for mesoscopic PSCs. The consecutive structure could reduce the interface resistance and help to the transport of electrons for mesoscopic PSCs, and 3



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therefore the charge recombination of the device was decreased effectively. Furthermore, due to the existence of macropores in the CCM TiO2 film, it is propitious to the deposition of perovskite and charge separation for the perovskite layer. Compared with Meso-PSC, the CCM TiO2 film based mesoscopic PSCs (CCM PSC, the device structure is shown in Figure 1a) had achieved a higher PCE and better photovoltaic performances consistency.

Figure 1. (a) Device structure of CCM PSC, (b) device structure of Meso-PSC.

2. EXPERIMENTAL Synthesis of CH3NH3I Hydroiodic acid (57 wt% in water, Aldrich, 30 mL) and methylamine (40% in methanol, Aldrich, 28.6 mL) were mixed in a round-bottom flask (250 mL) and stirred at 0 °C for 2 h. The product was precipitated by rotary evaporation and removing the solvent at 50 °C for 60 min. Then the generated yellowish powder was washed by diethyl ether for three times. After being dried in a vacuum oven at 60 °C for 24 h, the pure CH3NH3I was successfully prepared. Synthesis of TiOx Precursor Solution Titanium(IV) isopropoxide (99%, Aldrich, 5 mL), ethanolamine (99+%, Aldrich, 4


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2.5 mL) and 2-methoxyethanol (99+%, Aldrich, 25 mL) were mixed in a three-necked flask. A thermometer and a condenser were equipped with it. By a silicon oil bath, the mixed solution was heated to 80 °C for about 2h during stirring, then increased the heating temperature to 120 °C and kept for 1 h. Then this two-step heating process (80 and 120 °C) was repeated. Argon gas protection was used during the synthesis.42 Device Fabrication Substrate preparation: FTO glass (Pilkington TEC 15) 15 Ω/square was cut into pieces with dimension of 15 mm × 20 mm and etched with HCl aqueous (2 M)and Zn powder. The etched FTO substrates were immersed in alkali solution for 20 min, and then cleaned with detergent. After being ultrasonicated with deionized water and absolute ethanol, the FTO substrates were dried with a hair dryer. Finally, the FTO substrates were taken into a muffle furnace for annealing at 510 °C for 30 min. CCM TiO2 layers preparation: The CCM TiO2 layers were spin-coated on the FTO substrate at 5,000 rpm for 30 s by using different CCM TiO2 pastes, then heated at 510 °C for 30 min. The CCM TiO2 pastes were prepared by mixing a commercial TiO2 paste (Dyesol18NRT, Dyesol) with isopropanol and TiOx precursor solution with different weight ratio. There are five different CCM TiO2 layers we have prepared: CCM-TiO2-a, CCM-TiO2-b, CCM-TiO2-c, CCM-TiO2-d and CCM-TiO2-e, which represents the CCM TiO2 layers prepared by different CCM TiO2 pastes with the weight ratio of m(TiO2 paste):m(isopropanol):m(TiOx precursor solution) are 1:2.5:1, 1:3.5:1, 0.8:3.5:1, 0.5:3.5:1 and 0.3:3.5:1, respectively. Mesoporous TiO2 layer preparation: A compact layer of TiO2 (c-TiO2) was 5



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deposited on the FTO substrate by a reported method.43 By spin-coating a diluted TiO2 paste onto the FTO/c-TiO2 substrate at 3,500 rpm for 30 s and then annealing at 510 °C for 30 min, the mesoporous TiO2 layer was prepared. The diluted TiO2 paste was made by diluting commercial TiO2 paste (Dyesol18NRT, Dyesol) in absolute ethanol (weight ratio, 1:3.5). We used two-step sequential deposition to fabricate the CH3NH3PbI3 layer. 1 M of PbI2 solution was prepared by adding PbI2 (462 mg, 99%, Aldrich) powder in N,N-dimethylformamide (DMF, 99.8%, Sigma-Aldrich, 1 mL) with stirring at 70 °C. 40 µL of PbI2 DMF solution was spin-coated on the porous TiO2 film at 3,000 rpm for 30 s. After drying at 25 °C for 5 min, the PbI2 film was heated at 70 °C for 30 min. When it cooled to room temperature, 100 µL of CH3NH3I solution (0.063 M) in isopropanol was dropped on the PbI2 for 20 s, then spin-coated at 4,000 rpm for 30 s and

annealed

at

70

°C

for

30

min.

The

spiro-OMeTAD

(2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene,

45

µL)

solution was dropped on the perovskite layer and spin-coated at 3,000 rpm for 20 s. The spiro-OMeTAD solution was made by a reported method.22 Finally, a 60 nm of Au contact was deposited on top of the device by thermally evaporating. Characterization The morphologies of all samples were characterized by a field-emission scanning electron microscope (FE-SEM, sirion200, FEI Corp., Holland) and atomic force microscopy (AFM, using a MultiMode V (Veeco) viewer and analyzer). Absorption spectra were measured on an ultraviolet-vis (Uv-vis) spectrophotometer (U-3900H, HITACHI, Japan). PL spectra of the perovskite films which was deposited onto different substrates were recorded by a spectrofluorometer (photon technology 6


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international) and analyzed by the software Fluorescence. The exciting wavelength was 473 nm and excited by a standard 450 W Xenon CW lamp. Transient absorption (LKS80, England) measurements were performed to characterize the charge recombination dynamics. The pump light wavelength and probe light wavelength for the samples were 500nm and 1000 nm respectively. The repetition rate was 5Hz and laser energy was150 µJ cm-2 for the transient absorption measurements. The current-voltage characteristics (J-V curves) were measured by a solar simulator (solar AAA simulator, Oriel USA) with a source meter (Keithley Instruments, Inc., OH) at 100 mW cm-2, AM 1.5 G illumination. The active area for each device was 0.09 cm2 and ensured by masking a black mask on the device. With a 50 mV s-1 of scan rate, the applied bias voltage for the reverse scan and forward scan were from 1.2 V to -0.1 V and from -0.1 V to 1.2 V. The incident monochromatic photon-to-current conversion efficiency (IPCE) of the perovskite solar cells were measured using an IPCE measurement system (Newport Corporation, CA). A Xe lamp was used as the light source. By using an Autolab analyzer (Metrohm, PGSTAT 302N, Switzerland), the electrochemical impedance spectroscopy (EIS) were measured at -1.0 V under dark, in the frequency range of 1 Hz to1MHz.

3. RESULTS AND DISCUSSION

7



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Figure 2. Top-view SEM images of (a, b) the Meso-TiO2 film and (c-h) different CCM TiO2 films. (c) CCM-TiO2-a, (d) CCM-TiO2-b, (e, f) CCM-TiO2-c, (g) CCM-TiO2-d, (h) CCM-TiO2-e.

Figure 3. Cross-sectional SEM images of (a) the Meso-TiO2 film and (b-d) different CCM TiO2 films. (b) CCM-TiO2-a, (c) CCM-TiO2-b, (d) CCM-TiO2-c, (e) CCM-TiO2-d, (f) CCM-TiO2-e.

In this work, we used a mixed TiO2 paste that contains TiOx precursor solution and diluted commercial TiO2 paste to prepare CCM TiO2 films. Due to the existence of TiOx precursor solution, it would form a thin compact TiO2 layer at FTO by hydrolysis.42 Moreover, during the hydrolysis, a number of the TiO2 nanoparticles were agglomerated. This agglomeration made the spacing between partial TiO2 nanoparticles become larger, therefore the large macropores in the CCM TiO2 film were formed. As the CCM TiO2 films prepared by one-step spin-coating, the dense 8


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bottom part and the porous up part were prepared at the same time, it were consecutive. Top-view SEM images of the Meso-TiO2 film (including compact TiO2 film and mesoporous TiO2 film) and the CCM TiO2 films were shown in Figure 2, and the cross-sectional SEM images were shown Figure 3. Notably, both the concentration of TiOx precursor solution and the content of TiO2 paste are necessary for a successful preparation of the CCM TiO2 films. At a low content of TiO2 paste, the CCM TiO2 films (CCM-TiO2-d and CCM-TiO2-e) became too thin or not continuous for the macroporous/mesoporous pores; while the high concentration of TiOx precursor solution and content of TiO2 paste would make the CCM TiO2 films (CCM-TiO2-a and CCM-TiO2-b) too thick or dense. Obviously, these films were not propitious to the deposition of perovskite and detrimental to the enhancement of devices efficiency. The J-V curves and photovoltaic performances summary of PSCs based on different CCM TiO2 films were shown in Figure S1 and Table S1 respectively, which were correlated with the prediction conclusion above. When the CCM TiO2 paste weight ratio of m(TiO2 paste):m(isopropanol):m(TiOx precursor solution) was 0.8:3.5:1, a perfect morphology was obtained by the CCM-TiO2-c film (Figure 2e, 2f and 3d). Compared with the Meso-TiO2 film (the top-view and cross-sectional SEM images were shown in Figure 2a, 2b and 3a) which only contains mesopores, the CCM-TiO2-c film contains mesopores and macropores at the same time. The diameter of macropores was about 200~250 nm, and the macroporous/mesoporous pores were continuous and uniformly disperse. In Figure S2, panels a-b showed the AFM images of the Meso-TiO2 film and CCM-TiO2-c film, respectively. Both the thickness of 9



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Meso-TiO2 film and CCM-TiO2-c film were about 250 nm. The square average roughnesses of the CCM-TiO2-c film and the Meso-TiO2 film were 28.7 and 9.97 nm, suggesting that the CCM-TiO2-c film has a coarser surface than the Meso-TiO2 film due to the existence of a plenty of macropores. The rough surface of TiO2 layer was propitious to the deposition of perovskite.40 Therefore, we used the CCM-TiO2-c film as both compact blocking layer and mesoporous scaffold layer to fabricate the mesoscopic PSC. For clarity of presentation, in the following sections, we designated the CCM-TiO2-c film as CCM TiO2 film, and designated the PSC based on CCM-TiO2-c film as CCM PSC.

Figure 4. Top-view SEM images of perovskite CH3NH3PbI3 film deposited on (a) the Meso-TiO2 film and (b) the CCM TiO2 film. Cross-sectional SEM image of 10


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perovskite CH3NH3PbI3 film deposited on (c) the Meso-TiO2 film and (d) the CCM TiO2 film.

The CH3NH3PbI3 perovskite layers were deposited on the porous TiO2 films by two-step sequential deposition. Figure 4 showed SEM images of perovskite-coated Meso-TiO2 film (Figure 4a, 4c) and CCM TiO2 film (Figure 4b, 4d). We found that the perovskite film deposition could be governed by the TiO2 films’ porosity. From Figure 4a, the perovskite film with a thickness of 300 nm and crystal size of 100~400 nm was formed on the top of the Meso-TiO2 film. Because of the Meso-TiO2 film is dense and the mesopores size is small, the unfilled mesopores in the TiO2 film still existed. As for the CCM TiO2 film, a dense and full covered perovskite layer was deposited on the entire TiO2 architecture. The perovskite layer deposited on the CCM TiO2 film was in a uniform crystal size about 200 nm and with few voids. Hence, for the perovskite deposition, the CCM TiO2 architecture can facilitate both the pore-filling and crystal quality of perovskite deposited on it. The optical absorption spectra of perovskite CH3NH3PbI3 films deposited on Meso-TiO2 film and CCM TiO2 film were shown in Figure S3. There was a notable increase in the absorbance of perovskite CH3NH3PbI3 films deposited on the CCM TiO2 film, probably due to the CCM TiO2 architecture which could result in a full-filling and high quality perovskite layer. This result was in accordance with the SEM data.

11



glass/CH3NH3PbI3

PL Intensity (a.u.)

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glass/c-TiO2/m-TiO2/CH3NH3PbI3

glass/CCM TiO2/CH3NH3PbI3

700 720 740 760 780 800 820 840 Wavelength (nm) Figure

5.

Steady-state

PL

spectra

for

glass/CH3NH3PbI3,

glass/c-TiO2/m-TiO2/CH3NH3PbI3, and glass/CCM TiO2/CH3NH3PbI3.

Figure 5 showed the steady-state fluorescence (PL) spectra of glass/CH3NH3PbI3, glass/c-TiO2/meso-TiO2/CH3NH3PbI3 and glass/CCM TiO2/CH3NH3PbI3 samples. Both the thickness of glass/c-TiO2/meso-TiO2/CH3NH3PbI3 and glass/CCM TiO2/CH3NH3PbI3 were about 480 nm. For exploring the charge transfer efficiency of light-excited charge in the semiconductor devices, the PL spectrum is an effective characterization method. From Figure 5, all the emission peaks of the samples located at 770 nm. Both the Meso-TiO2 layer and the CCM TiO2 layer had a significant quenching effect for the fluorescence of perovskite layer. Moreover, the glass/CCM TiO2/CH3NH3PbI3 showed a lower peak intensity, indicating a lower recombination and prospectively lead to a better photovoltaic performance. This result verified that the continuous film can help to reduce the recombination and it is conducive to the electron diffusion for the perovskite layer. 12


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a

3.0x10-4

b

1.5x10-4

FTO/c-TiO2/m-TiO2/CH3NH3PbI3/spiro

FTO/CCM TiO2/CH3NH3PbI3/spiro

Fitting

Fitting

1.0x10-4

∆A

2.0x10-4

∆A

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


1.0x10-4

0.0

0.0

-1.0x10-4

5.0x10-5

0

20

40

60

-5.0x10-5

80

0

20

Time (µs)

40

60

80

Time (µs)

Figure 6. TA response of (a) FTO/c-TiO2/m-TiO2/CH3NH3PbI3/spiro and (b) FTO/CCM TiO2/CH3NH3PbI3/spiro.

To further understand the electron transfer efficiency of CCM TiO2 film and Meso-TiO2 film in PSCs, we employed transient absorption (TA) response to detect the recombination dynamics between TiO2 layer and spiro-OMeTAD layer. Electrons and holes in perovskite layer were generated while the device under the solar radiation and then respectively injected into the TiO2 layer (electron transport layer) and spiro-OMeTAD layer (hole transport layer). However, charge recombination of electrons in TiO2 with holes in spiro-OMeTAD were also occurred during the charge generation. If electrons in the TiO2 transferred effectively, the recombination between electrons and holes in the device would occurs difficultly, and the longer recombination time would be obtained.44-45 The TA responses in Figure 6 were measured from the spiro-OMeTAD side of the devices. By fitting the absorption signals with a monoexponential decay function, the time constant of 9.85 (FTO/c-TiO2/m-TiO2/CH3NH3PbI3/spiro-OMeTAD) (Figure 6a) and 12.44 µs 13



(FTO/CCM TiO2/ CH3NH3PbI3/spiro-OMeTAD) (Figure 6b) were obtained. The recombination

time

for

CH3NH3PbI3/spiro-OMeTAD

the was

structure much

of

longer

than

FTO/CCM the

TiO2/

structure

of

FTO/c-TiO2/m-TiO2/CH3NH3PbI3/spiro-OMeTAD, indicating that CCM TiO2 film was more conducive for the electron transport. In comparison to the Meso-TiO2 film (c-TiO2/m-TiO2), the CCM TiO2 film was consecutive and no interface contact for the TiO2 compact layer and TiO2 mesoporous layer, so the electrons transferred more

a 20

20

80

16

60

12

40

8

-2

b 100 IPCE (%)

16 12 CCM PSC CCM PSC Meso-PSC Meso-PSC

8 4 0 0.0

0.2

Reverse Forward Reverse Forward

20

0.4 0.6 0.8 Voltage (V)

1.0

0 300

1.2

4

Meso-PSC CCM PSC

400 500 600 700 Wavelength (nm)

0 800

Integrated Jsc (mA cm-2)

effective and recombination in the device could be suppressed greatly.

Current density (mA cm )

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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Figure 7. (a) J-V curves of reverse and forward for the Meso-PSC and CCM PSC, (b) the corresponding IPCE spectra.

Table 1. Photovoltaic performances summary of the devices of CCM PSC and Meso-PSC. Samples

Voc (V)

Jsc (mA cm-2)

FF

PCE (%)

CCM PSC

1.10

19.00

0.75

15.76

Meso-PSC

0.99

17.79

0.67

11.90

14


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The photovoltaic performances of the champion cells of CCM PSC and Meso-PSC were shown in Figure 7a and Table 1. The CCM PSC showed a PCE of 15.76%, Voc of 1.10 V, Jsc of 19.00 mA cm-2, and FF of 0.75, while Meso-PSC showed the PCE of 11.90%, Voc of 0.99 V, Jsc of 17.79 mA cm-2, and FF of 0.67. Compared to the Meso-PSC, the improvement of CCM PSC performance mainly reflected in the great rise of Voc and FF. This can be explained as charge carrier recombination reduced contributed by the consecutive CCM TiO2 film. Meanwhile, owing to the CCM TiO2 architecture could facilitate both the pore-filling and crystal quality of perovskite layer, a higher light-harvest capability would be obtained, the Jsc of CCM PSC was increased. From the IPCE spectra in Figure 7b, the integrated Jsc of CCM PSC and Meso-PSC were 19.01 mA cm-2 and 15.58 mA cm-2, which agreed with the measured values. Generally, the photocurrent hysteresis of PSCs may result from the defects between the interphase in the devices, the charge traps from the poor quality perovskite layer or an unbalanced charge transport rate.46-47 The J-V curves (Figure 7a) of the CCM PSC showed no hysteresis, but a great photocurrent hysteresis was observed for the Meso-PSC. To further understand why the CCM PSC could suppress the J-V hysteresis effectively, electrochemical impedance spectroscopy (EIS) measurement was employed. The EIS plots of Meso- PSC and CCM PSC showed in Figure S4 were measured at a bias voltage (-1.0 V) under dark. There were a semicircle and an incomplete semicircle in the plots. For the PSCs, the value intercepted by the high-frequency on the real axis was equivalent to the total series 15



resistance (Rs). The low frequency (the incomplete semicircle in the right) featured resistance should be assigned to the recombination resistance (Rrec), and the high frequency (the semicircle in the left) featured resistance was corresponded with the charge transport resistance (Rct).37,48 From Figure S4, both the Rct and Rs of CCM PSC were lower than that in the Meso-PSC. It demonstrated that the CCM PSC has a lower interface resistance. Compared to the compact TiO2 layer and the mesoporous layer in Meso-PSC, the CCM TiO2 film had a higher conductivity to the electrons transport and less resistant to the charges. The defects between the interphase or unbalanced charge transport rate of the CCM PSC had been improved, so the J-V hysteresis was suppressed greatly. Meanwhile, the high quality and good pore-filling perovskite film also made a positive contribution to the greatly suppressed J-V hysteresis of CCM PSC.

b

Current density (mA cm-2)

a 1.2 1.1 Voltage (V)

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1.0 Meso-PSC CCM PSC

0.9 0.8

Meso- PSC

CCM PSC

20

18

16

14

Meso- PSC CCM PSC Meso- PSC

16


CCM PSC

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d

c 0.76

16

0.72

14 PCE (%)

Fill Factor

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


0.68 0.64

Meso- PSC CCM PSC

0.60 Meso- PSC

12 10

Meso-PSC CCM PSC

8

CCM PSC

Meso-PSC

CCM PSC

Figure 8. The average values (with s.d.) of (a) Voc, (b) Jsc, (c) FF and (d) PCE for 10 samples of CCM PSCs and Meso-PSCs in once experiment. The error bars represented maximum and minimum values while the middle line in each box represented the median value. Unfilled squares indicate mean values.

Besides, the CCM PSC showed a very high uniformity of the performances for the devices. The average values (with s.d.) of Voc, Jsc, FF and PCE for 10 samples of CCM PSCs and Meso-PSCs in once experiment were shown in Figure 8. Detailed statistics were shown in Table S2 and Table S3. From the Figure 8, we can see all the parameters of CCM PSCs were more uniform than Meso-PSCs. Two factors may lead to this high consistency performances. Firstly, the addition of TiOx precursor solution makes the TiO2 paste slurry very homogeneous and conducive to the film formation of TiO2 layer, so the CCM films are extremely uniform. A high quality perovskite film would be deposited on the uniform CCM film. Meanwhile, the macropores in the CCM films facilitated both the pore-filling and crystal quality of perovskite layer. High quality perovskite film with full pore-filling could improve the consistency of the devices effectively. Secondly, in classic mesoscopic perovskite solar cells (PSCs), 17



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the compact blocking layer and mesoporous scaffold layer were usually prepared by two steps or more, while the CCM TiO2 film only needs one step. The simpler preparation also can reduce uncertainty factors in the preparation of devices. Therefore, the consistency of device performances for CCM PSC was significantly increased.

4. CONCLUSION In summary, we constructed an efficient CCM TiO2 film to substitute the classic compact blocking layer and scaffold layer for mesoscopic PSCs. A mixed TiO2 paste that contains TiOx precursor solution and diluted commercial TiO2 paste was used to prepare the CCM TiO2 film. Owing to the hydrolysis of TiOx precursor solution, a thin compact TiO2 layer was formed on the bottom of CCM TiO2 film, while the top shown a consecutive mixed porous structure. Results of PL, TA response and EIS analysis demonstrated that this consecutive film could promote the electron transport rate and decrease the charge recombination effectively. Moreover, the top of the CCM TiO2 film had a high surface roughness because of its special mesoporous structure with continuous lager uniform macropores and small mesopores. It was propitious to the deposition of perovskite and charge separation for the perovskite layer. A maximum PCE of 15.76% was achieved by the PSCs based on the CCM TiO2 film, which was higher than that based on the classic mesoporous structure (11.9 %). Furthermore, high consistency performances of devices based on the CCM TiO2 film were fabricated by two-step sequential deposition. This work represents a practical 18


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step toward the realization of simple and high consistency performances mesoporous perovskite solar cell.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: J-V curves and photovoltaic performance summary of PSCs based on different CCM TiO2 films, AFM images of Meso-TiO2 film and CCM-TiO2-c film, Absorption spectrum of perovskite CH3NH3PbI3 films deposited on the Meso-TiO2 film and the CCM TiO2 film, Photovoltaic performance summary of the devices of CCM PSC and Meso-PSC, EIS plots of the Meso- PSC and CCM PSC measured under dark.

AUTHOR INFORMATION *

Corresponding Authors

[email protected] (X Pan) (+86 55165593222); [email protected] (SY Dai) (+86 1061772268). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China under Grant No. 2015CB932200, the National Natural Science Foundation of 19



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China under Grant No. 21273242, and State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (Grant No.LAPS14012).

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High Consistency Perovskite Solar Cell with a Consecutive Compact

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