SnO2 Double Electron Transport Layer Guides Improved Open

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ZnO/SnO2 Double Electron Transport Layer Guides Improved Open Circuit Voltage for Highly Efficient CH3NH3PbI3-based Planar Perovskite Solar Cells Duo Wang, Cuncun Wu, Wei Luo, Xuan Guo, Bo Qu, Lixin Xiao, and Zhijian Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00293 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 23, 2018

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ACS Applied Energy Materials

ZnO/SnO2 Double Electron Transport Layer Guides Improved Open Circuit Voltage for Highly Efficient CH3NH3PbI3-based Planar Perovskite Solar Cells Duo Wang, † Cuncun Wu, † Wei Luo, Xuan Guo, Bo Qu, Lixin Xiao,* and Zhijian Chen * State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, People’s Republic of China.

KEYWORDS: Double electron transport layer, Perovskite solar cell, Planar, ZnO/SnO2, Open circuit voltage

ABSTRACT The electron transport layer (ETL), as an important component of planar perovskite solar cells (P-PSCs), can effectively extract photon-generated electrons from perovskites and convey them to the cathode, by this token, its properties directly determine the photovoltaic performances of P-PSCs. Herein, we introduce ZnO/SnO2 double electron transport layer for CH3NH3PbI3-based P-PSCs, achieving a high open circuit voltage (VOC) of 1.15 V with the power conversion efficiencies (PCE) of 19.1% when the SnO2 based devices with a Voc of 1.07 V and a PCE of 18.0%, to the best of our knowledge, which is the highest Voc by using

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inorganic electron transport layer for pure CH3NH3PbI3-based P-PSCs so far. This result demonstrates that a higher Fermi energy (EF) and conduction band minimum (ECBM) of ETL could drive a higher Voc and a better PCE.

INTRODUCTION Since hybrid organic−inorganic perovskite solar cells (PSCs) was first reported by Miyasaka in 20091, which have led to significant impact on solar energy research due to their low cost, simple fabrication, and outstanding photovoltaic properties including outstanding light harvesting, high charge carrier mobility, and long diffusion length2-11. Today, the highest efficiency of PSCs have achieved certified power conversion efficiencies (PCE) exceeding 22%12. Generally, typical PSCs comprising an n−i−p structure are made up of a working electrode (cathode)/electron transport layer (ETL)/perovskite layer/hole transport layer (HTL)/counter electrode (anode)8. The ETL and HTL can effectively extract photon-generated electrons and holes from perovskites and conveying them to the anode and cathode, by this token, they play important roles in the photovoltaic performances13-14. The PSCs with high PCE are usually fabricated with TiO2 as the electron transport layer (ETL) and 2,20,7,70-tetrakis (N,N-di-pmethoxyphenylamine)-9,90-spirobifluorene (Spiro-OMeTAD) as the hole transport layer (HTL)15-16. In order to improve the electron extraction ability, a mesoporous TiO2 layer is often added on planar TiO2 to enhance the surface area17. The mesoporous structure has more complicated production process and higher cost, therefore, planar perovskite solar cells based on the double electron transport layer have arisen as the requirement for application18-20. Moreover, TiO2, especially nanoparticles in the mesoporous layer as a photo catalyst would decompose the


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perovskite under UV radiation, which is the dominant factor responsible for the photo instability of PSCs. In this case, SnO2 ETL have become one of the major ETL for P-PSCs since Miyasaka et al. and Yan et al. independently applied it to P-PSCs21-22. SnO2 is an excellent electron transport material with a higher high electron mobility (µe)21 compared with TiO223 as well as its well-matched energy levels with CH3NH3PbI3. Combining the superiority of SnO2 and TiO2, Taiho Park et al. developed a fully solution processed SnO2/a-TiO2 bilayer ETL for P-PSCs, and have obtained high Voc and PCE8. Besides, ZnO is also known to have an electron mobility that is substantially higher than that of TiO224, which makes it an ideal choice for an electronselective contact4. Although SnO2 ETL or ZnO ETL has excellent and unique properties in the PPSCs, there are still some faultiness such as the loss of open circuit voltage (VOC) and shortcircuit current density (Jsc), and ZnO has a strong ability to decompose perovskite (Table S2 and Figure S2 (b)). Due to both SnO2 and ZnO have simpler process and better properties than TiO2, taking the advantages of SnO2 and ZnO into consideration, in this work, we propose ZnO/SnO2 double layer as the ETL, which leads to perovskite solar cells with high Voc and a maximum power efficiency of 19.1%, which is the highest Voc so far by using entire inorganic electron transport layer for pure CH3NH3PbI3-based Planar Perovskite Solar Cells. Furthermore, to gain a deeper understanding

for

the

higher

Voc,

we

demonstrated

the

devices

structure

of

ITO/ZnO/SnO2/MAPbI3/Spiro-OMeTAD/Ag and ITO/SnO2/MAPbI3/Spiro-OMeTAD/Ag (as the referenced device), and a series of characterization analysis has been carried out.


3


RESULTS AND DISCUSSION The X-ray photoelectron spectroscopy (XPS) characterization was carried out to investigate the chemical composition of the ETLs. The XPS measurements demonstrate the existence of ZnO, SnO2 and ZnO/SnO2 ETLs as shown in Fig. 1. The energy band alignment was studied by using UPS to reveal the built-in fields formed at the interfaces in ITO/ZnO, ITO/SnO2 and ITO/ZnO/SnO2, and the results are shown in Fig. 2. The work functions (WF) calculated by subtracting the spectrum width from the photon energy of exciting radiation (21.2 eV) are determined to be -3.94 eV for ZnO25. The energy level of valence band maximum (VBM), EVBMEF, is derived by extrapolating the linear portion of the low binding energy edge of the peak to the energy axis. As shown in the inset of Fig. 2(a), the values of EVBM-EF for ZnO are determined to be -3.29 eV, so further we can obtain that the values of EVBM comes to -7.23 eV. Similarly, From Fig. 2(b) and (c), The WF are determined to be -3.98 eV and -3.95 eV, and the

1500000

Zn 2p

Sn 3p

O 1s

Sn 4d

2000000

O 1s

Sn 3d Sn 3d-

Sn 4d

2500000

Sn 3p

Sn 3d

ITO/ZnO ITO/SnO2 ITO/ZnO/SnO2

3000000

Zn 3d; Sn 3d-

values of EVBM are determined to be -7.72eV and -7.46eV for SnO2 and ZnO/SnO2, respectively.

Intensity (a.u.)

Zn 2p

Sn 3p

500000

O 1s

1000000 In 3d

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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0

0

200

400

600 Binding energy (eV)

800

1000

Figure 1. XPS spectra of different ETLs on ITO substrates.


4

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(a)

(b)

400000

400000

ZnO

SnO2

350000

Intensity (CPS)

350000 300000

Intensity (CPS)

300000

EVB=-7.23eV

250000

200000

150000

150000

100000

100000

50000

EVB-EF=3.29eV

0

0

2

4

6

8

EVB=-7.72eV

250000

200000

10

50000

EF=3.94eV

12

16

18

20

EF=3.98eV

EVB-EF=3.74eV

0

14

Binding energy (eV)

0

2

4

6

8

10

12

14

16

18

20

Binding energy (eV)

(c)

500000

ZnO/SnO2 400000

EVB=-7.46eV

Intensity (CPS)

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


300000

200000

100000

EF=3.95eV

EVB-EF=3.51eV

0

0

2

4

6

8

10

12

14

16

18

20

Binding energy (eV)

Figure 2. UPS spectra of different thin films. (a) ITO/ZnO. (b) ITO/SnO2. (c) ITO/ZnO/SnO2.

The Voc depends on the energy difference between the values of the quasi Fermi energy level of ETL and HTL. Fig. 2 shows that, ITO/ZnO/SnO2 has a higher EF than ITO/SnO2, which utter a prediction that the device based on ZnO/SnO2 double electronic layer will have a higher VOC than the devices based on SnO2 single electronic layer.


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(a)

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(b)

(c)

(d)

Figure 3. Schematic illustration of electron extraction from the perovskite to the ITO via different ETLs, device architecture and energy level diagram. (a) Electron extraction via SnO2 ETL.

(b) Electron extraction via ZnO/SnO2 ETL.

(c) Device architecture of the

ITO/ZnO/SnO2/MAPbI3/Spiro-OMeTAD/Ag cells in this study. (d) Energy levels (relative to vacuum) of the various device components.

The electron extraction process in P-PSCs could be divided into three steps: (i). Free electrons photoproduct perovskites and drift to the ETL, (ii). Electrons transport through the ETL, (iii). Electrons are collected by cathode of ITO. Andre M. et al. reported the band gap of


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ZnO is ~ 3.3 eV26, and Chung-Hsin Lu et al. reported the band gap of SnO2 is ~ 3.6 eV27. Combining the values of EVBM in Fig. 2, the energy level of conduction band minimum (CBM), ECBM, are driven to be -3.93 eV and -4.12 eV for ZnO and SnO2, respectively. Fig. 3 (a) and (b) provides an illustration to understand the processes of electron extraction from the perovskite to the ITO when electronics passed through SnO2 or ZnO/SnO2 ETL. Fig. 3 (a) represents the electron extraction processes in the device of SnO2 as an electronic layer, it has the same processes as regular n−i−p structure. As shown in Fig. 3 (b), there are extra processes electron injection (iv) and transportation (v) processes in the ZnO/SnO2 double ETL based device, which effectively increase Voc, however, as the short-circuit current density (Jsc) decreased slightly, the structure of double electronic layer need to be adjusted to achieve high PCE of device. (a)

(b)

Figure 4. SEM images of (a) ITO/ SnO2/ MAPbI3 film. (b) ITO/ZnO/SnO2/MAPbI3 film.

We investigated the morphology of ITO/SnO2/MAPbI3 and ITO/ZnO/SnO2/MAPbI3 films using the scanning electron microscope (SEM) as shown in Fig. 4 (a) and (b), respectively. Both of the perovskite film have smooth and uniform surface with no pinholes. Moreover, the


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morphology of ITO/SnO2, ITO/ZnO/SnO2 and ITO/ZnO/SnO2/MAPbI3 films using AFM as shown in Fig. S1. The root mean square roughness of the ITO/ SnO2 film (Fig. S1 (a)) is 1.86 nm, while that of ITO/ZnO/SnO2 film (Fig. S1 (b)) is 2.14 nm, this can be explained that ZnO nanoparticles

lead

to

a

greater

roughness

of

the

film.

The

AFM

image

of

ITO/ZnO/SnO2/MAPbI3 film (Fig. S1 (c)) exhibits a similar grain size and morphology to SEM image of it (Fig. 4 (b)). Fig. S2 shows the XRD patterns of different films on ITO/glass substrates:

(a)

ITO,

ITO/ZnO,

ITO/SnO2,

and

ITO/ZnO/SnO2

films.

(b)

ITO,

ITO/SnO2/MAPbI3, ITO/ZnO/SnO2/MAPbI3 and ITO/ZnO/MAPbI3 films. Fig. S2 (a) can demonstrate the existence of ZnO and SnO2, and Fig. S2 (b) shows that ITO/SnO2/MAPbI3 and ITO/ZnO/SnO2/MAPbI3 films exhibit the main peak at 14.12° and 28.48°, corresponding to the (110) and (220) planes of the MAPbI3 crystal, corresponding to the (110) and (220) planes of the MAPbI3 crystal28, while ITO/ZnO/MAPbI3 films, corresponding to the (001) planes which showed the presence of PbI2, which gives an evidence of ZnO has a strong ability to decompose perovskite . In order to further understand the optical properties of the films of various components, the ultraviolet-visible (UV-Vis) spectra have been conducted to examine the light harvesting changes

among

the

different

films

(ITO,

ITO/ZnO,

ITO/SnO2,

ITO/ZnO/SnO2,

ITO/SnO2/MAPbI3, and ITO/ZnO/SnO2/MAPbI3 ) (Fig. S3 (a) and (b)). Fig. S3 (a) shows a similar absorption between ITO/SnO2/MAPbI3 and ITO/ZnO/SnO2/MAPbI3 films, while Fig. S3 (b) shows the absorption of different substrates films. To confirm the effect of the ZnO/SnO2 double electron transport layer in P-PSCs, the devices with structure of ITO/ZnO/SnO2/MAPbI3/Spiro-OMeTAD/Ag were fabricated, as shown in Fig. 3 (c), and the cross-section SEM were showed in Figure S4. As a contrast, the devices with of


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ITO/SnO2/MAPbI3/Spiro-OMeTAD/Ag were also fabricated. The energy-level diagram of perovskite solar cells are given in Fig. 3(d), and the energy levels of ZnO and SnO2 were obtained by Fig. 2 and Fig. 3 (a) (b). Based on the relative energy levels of the various device components (Fig. 3 (d)), free charge carriers generated in the MAPbI3 layer can be extracted by either transferring an electron to the underlying ZnO/SnO2 double layer. However, because ZnO layer has a higher ECBM than SnO2 layer, we predict the devices with ZnO/SnO2 double ETL will drive higher Voc and smaller JSC. To explore optimal structure of ZnO/SnO2, we fabricated a series of devices with different thickness of ZnO of about 0 nm, 45 nm, 65 nm and 80 nm, respectively. The thickness of ZnO were measured by step profiler. The current density voltage (J-V) curves of the best performance devices based on different thickness of ZnO were measured under simulated 100 mW/cm2 AM 1.5 irradiation, and the corresponding device parameters are summarized in Table 1, and the mean photovoltaic parameters of these devices are showed in Table S1. It’s noticed that the device with ZnO film of 65 nm thickness exhibits the best photovoltaic performance, generates a PCE of 19.1%, along with a Voc of 1.15 V, a Jsc of 21.74 mA/cm2 and an FF of 0.764. To check the reproducibility of the performance of Device S1 and S2, we fabricated and measured 40 separate devices, respectively. The PCE distribution histogram of Device S1 and S2 were shown in Figure S5. The average efficiency of Device S1 for the 40 cells was 17.54%, while the average efficiency of Device S2 for the 40 cells was 17.01%. Meanwhile, the comparison of photovoltaic parameters of the device with the structure of ITO/ZnO/MAPbI3/Spiro-OMeTAD/Ag compared with the Device S1 and S2 were showed in Table S2.


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Table 1. Comparison of photovoltaic parameters of the perovskite solar cell based on ZnO/SnO2 ETL with different thickness of ZnO. Thickness of ZnO (nm)

Jsc (mA·cm-2)

Voc (V)

FF

PCE (%)

0

22.75

1.07

0.739

18.0

45

21.23

1.13

0.721

17.3

65

21.74

1.15

0.764

19.1

80

20.82

1.14

0.716

17.0

The current–voltage (J–V) characteristics and incident photon-to-current efficiency (IPCE) spectra of the best devices are shown in Fig. 5. The devices with the structure of ITO/ZnO (65 nm)/SnO2 (30 nm)/MAPbI3/Spiro-OMeTAD/Ag is denoted as Device S1, while the referenced device with the structure of ITO/SnO2 (30 nm)/MAPbI3/Spiro-OMeTAD/Ag is denoted as Device S2. The J-V curve was measured at a rate of 0.1V/s. The measured Jsc, Voc, fill factor and PCE of Device S1 were 21.74 mA/cm2, 1.15 V, 0.764 and 19.1%, respectively. While Device S2 gives a Voc of 1.07 V, a Jsc of 22.75 mA/cm2, an FF of 0.739, and yielding a PCE of 18.0%. It is obviously observed that the device with ZnO/SnO2 double electron transport layer (Device S1) held a significant improvement in Voc, PCE and FF, while Jsc displayed a slight decrease compared to the referenced device (Device S2). Moreover, this result coincides with the previous prediction that the higher Voc was forced by the higher EF of ITO/ZnO/SnO2 film, and the lower Jsc was caused by the higher ECBM of ZnO compared with SnO2. The incident photonto-current efficiency (IPCE) (Fig. 5 (b)) were measured to further demonstrate the accuracy of


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the output Jsc and afford an integrated photocurrent of 20.1 mA/cm2 and 20.6 mA/cm2 for Device S1 and Device S2, respectively, which approximate to the measured Jsc. (a)

(b) 25

1.0

20

0.8

15

IPCE

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


Device S1 Device S2

0.6

10

0.4

5

0.2

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 300

400

Device S1

(Jsc=20.1 mA/cm2))

Device S2

(Jsc=20.6 mA/cm2))

500

600

700

800

Wavelength(nm)

Voltage(V)

Figure 5. J–V curves and IPCE spectrum of the highest-performing device in this study. Device S1: ITO/ZnO/SnO2/MAPbI3/Spiro-OMeTAD/Ag; Device S2: ITO/SnO2/MAPbI3/SpiroOMeTAD/Ag. (a) J–V characteristics measured under 100 mWcm22 AM1.5G illumination for device S1 and S2. (b) IPCE spectrum and the value of corresponding integrated Jsc.

In order to thoroughly investigate the effects of ZnO/SnO2 ETL on the device performance, the recombination process of PSCs was analyzed by steady-state Photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. Fig. 6 (a) shows steady PL spectra of SnO2/MAPbI3 and

ZnO/SnO2/MAPbI3 on

glass, respectively.

The PL intensity of

ZnO/SnO2/MAPbI3 was enhanced compared with that of SnO2/MAPbI3. This indicates that ZnO/SnO2 double layer ETL has decreased electron transportation that reduces Jsc of device. TRPL was measured by monitoring the peak emission at 770 nm, as shown in Fig. 6 (b). The data can be fitted with a triple exponential (Table S3), yielding three PL lifetime constants (τ1,


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τ2, and τ3). For SnO2/MAPbI3, τ1 is 3.84 ns with a ratio of 56.0%, τ2 is 24.07 ns with a ratio of 39.0% and τ3 is 132.56 ns with a ratio of 5.0%, yielding an average PL lifetime of 17.70 ns. While for ZnO/SnO2/MAPbI3, τ1 is 5.12 ns with a ratio of 56.0%, τ2 is 32.59 ns with a ratio of 36.0% and τ3 is 151.49 ns with a ratio of 8.0%, yielding an average PL lifetime of 26.26 ns. These data indicated that there was a stronger combination of electrons in the transmission process when they through ZnO/SnO2 ETL, resulting in the decline of Jsc, and this is well in line with the fact that the Jsc decreased in the previous result. (a)

(b)

1E+04

8.0E+05

SnO2/MAPbI3 SnO2/MAPbI3 (Fitting) ZnO/SnO2/MAPbI3 ZnO/SnO2/MAPbI3 (Fitting)

SnO2/MAPbI3 ZnO/SnO2/MAPbI3

1E+03

Intensity(norm.)

6.0E+05

Intensity (CPS)

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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4.0E+05

2.0E+05

0.0E+00

1E+02

1E+01

1E+00 680

700

720

740

760

780

800

820

840

0

200

400

600

Time(ns)

Wavelength (nm)

Figure 6. (a) Steady-state photoluminescence (PL) and (b) transient photoluminescence of SnO2/MAPbI3 and ZnO/SnO2/MAPbI3 films on glass.

CONCLUSIONS We demonstrate ZnO/SnO2 double layer as the ETL, which leads to perovskite solar cells with a high Voc of 1.15 V and a maximum power efficiency of 19.1%, which is the highest Voc


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performance so far by using inorganic electron transport layer for pure CH3NH3PbI3-based Planar Perovskite Solar Cells. The Voc of the device with ZnO/SnO2 ETL has increased by 0.08 V compared with the device with only SnO2 ETL (from 1.07 V to 1.15 V), and the PCE raises from 18.0% to 19.1% in despite of the slightly decreased Jsc. Moreover, a higher EF and ECBM of ETL are believed to drive a larger Voc and a better PCE. These indicate that it is a very significant method to achieve a higher Voc for P-PSCs by matching ETLs with a higher EF and ECBM.

EXPERIMENTAL SECTION Materials. 180 nm of indium tin oxide (ITO) coated glass substrates with a sheet resistance of 8 Ω/sq were purchased from Huayulianhe Co., Ltd. Zinc acetate dehydrate (AR) were purchased from Xilong Scientific Co., Ltd. SnO2 colloid precursor (tin(IV) oxide, 15% in H2O colloidal dispersion), anhydrous N,N-dimethylformamide (DMF), anhydrous dimethyl sulfoxide (DMSO), and chlorobenzene were obtained from Alfa Aesar. PbI2 (99.9985%) and spiro-OMeTAD were purchased from Xi’an Polymer Light Technology Co., Ltd. CH3NH3I (99.5%, denoted as MAI) was

acquired

from

Borun

New

Material

Technology

Co.,

Ltd.

Lithium

bis(trifluoromethylsulfonyl) imide (Li-TFSI) and 4-tertbutylpyridine (TBP) were obtained from Aldrich. All these commercially available materials were used as received without any further purification. Device Fabrication. ITO-coated glass was sequentially cleaned in deionized water, acetone, and ethanol under ultrasonic each for 30 min. The cleaned ITO-coated glass substrates were then dried with a nitrogen stream and treated by oxygen plasma cleaning for 15 min immediately. ZnO nanoparticles were prepared according to literature procedures29-30. The ZnO nanoparticle


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solution (6 mg/ml) was spin coated onto glass/ITO substrates at 3000 rpm for 30 s and then baked on a hot plate at 130 °C for 10 min. The SnO2 colloid precursor was ultrasonically diluted by deionized water (1:6 volume ratio) for 60 min31. The glass/ITO/ZnO substrates treated by oxygen plasma cleaning for 20 s, and then the acquired SnO2 solution was spin coated onto glass/ITO/ZnO substrates at 4000 rpm for 30 s and then baked on a hot plate at 150 °C for 30 min. The perovskite film was deposited by a modified one-step spin coating method. 1.25 mmol MAI and PbI2, 89 µL DMSO was mixed in 1 mL DMF at room temperature and to form a precursor solution. The perovskite CH3NH3PbI3 (denoted as MAPbI3) film was fabricated by a homemade low-pressure-assisted method as reported in literature32-33. The perovskite precursor solution was spin coated onto the cooled glass/ITO/ZnO/SnO2 substrate at 4000 rpm for 8 s. The coated film was quickly moved to a small sample chamber which was pumped immediately for 70 s (below 15 Pa). A brown and transparent film was obtained34. After these, the gas-pump dried film was baked on a hot plate at 100 °C for 10 min for the conventional thermal annealing method35. Spiro-OMeTAD solution, which is made by mixing 72.3 mg Spiro-OMeTAD, 39 µL of TBP and 20 µL of Li-TFSI solution (520 mg Li-TFSI in 1 mL acetonitrile) in 1 mL chlorobenzene, was spin-coated on the perovskite layer at 4000 rpm for 30 s17. Above, all processes were performed in ambient atmosphere. Finally, an 100 nm Ag cathode was deposited by thermal evaporation at a pressure of < 2.0 × 10−3 Pa and a evaporation rate of < 0.3 nm/s. The fabricated solar cells with 0.1 cm2 active area. Characterization. The morphology of samples was measured using a scanning electron microscope (SEM) (Hitachi S-4800) and an atomic force microscope (AFM) (Agilent Series 5500). The current density−voltage (J−V) curves were measured with a scan rate of 0.1 V/s under 100 mW/cm2 AM 1.5G simulated illumination of Newport solar simulator using a


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Keithley 2611 Semiconductor Characterization System. The XPS (Kratos, Axis Ultra) analysis was performed at RT, equipped with a monochromatic Al Kα X-ray source (1486.7 eV). The Xray diffraction (XRD) patterns were obtained by using a D/MAX-2000 X-ray diffractometer with monochromatic Cu Kα irradiation (l 1/4 1.5418 Å) at a scan rate of 6°/min. The absorption spectrum was recorded with a UV-visible spectrophotometer (Agilent 8453). Photoluminescence (PL) (excitation at 485 nm) was measured with NaonLog infrared fluorescence spectrometer (Nanolog FL3-2Ihr). Transient PL measurement was measured using UltraFast lifetime Spectrometer (Delta flex). The incident photon-to-current conversion efficiency (IPCE) spectrum was observed using a lock-in amplifier (model SR830 DSP) coupled with a 1/4 m monochromator (Crowntech M24-s) and 150 W tungsten lamp (Crowntech). All of the measurements of the solar cells were performed in an ambient atmosphere at room temperature without encapsulation.

ASSOCIATED CONTENT Supporting Information. Additional AFM images, XRD patterns, UV-Vis spectra and Transient PL spectroscopy results (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected]


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Notes The authors declare no competing financial interest. Author Contributions † These

authors contributed equally.

ACKNOWLEDGMENT This work was supported by the National Key Basic Research and Development Program of China (Grant No.2016YFB041003) and the National Natural Science Foundation of China (11574009, U1605244, 61575005, 11574013, 61775004). ABBREVIATIONS CCR2, CC chemokine receptor 2; CCL2, CC chemokine ligand 2; CCR5, CC chemokine receptor 5; TLC, thin layer chromatography.

REFERENCES (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-light Sensitizers for Photovoltaic Cells. Journal of the American Chemical Society 2009, 131 , 6050-6051. (2) Docampo, P.; Hanusch, F. C.; Stranks, S. D.; Döblinger, M.; Feckl, J. M.; Ehrensperger, M.; Minar, N. K.; Johnston, M. B.; Snaith, H. J.; Bein, T. Solution Deposition-Conversion for Planar Heterojunction Mixed Halide Perovskite Solar Cells. Advanced Energy Materials 2014, 4. (3) 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. (4) Liu, D.; Kelly, T. L. Perovskite Solar Cells with a Planar Heterojunction Structure Prepared


16

Page 17 of 21 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


Using Room-temperature Solution Processing Techniques. Nature photonics 2014, 8, 133138. (5) Mei, A.; Li, X.; Liu, L.; Ku, Z.; Liu, T.; Rong, Y.; Xu, M.; Hu, M.; Chen, J.; Yang, Y. A Holeconductor–free, Fully Printable Mesoscopic Perovskite Solar Cell with High Stability. Science 2014, 345, 295-298. (6) Xiao, M.; Huang, F.; Huang, W.; Dkhissi, Y.; Zhu, Y.; Etheridge, J.; Gray-Weale, A.; Bach, U.; Cheng, Y. B.; Spiccia, L. A Fast Deposition-crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin-film Solar Cells. Angew Chem Int Ed Engl 2014, 53, 9898-903. (7) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542546. (8) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-8. (9) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344. (10) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. (11) Chang, J.; Zhu, H.; Li, B.; Isikgor, F. H.; Hao, Y.; Xu, Q.; Ouyang, J. Boosting the Performance of Planar Heterojunction Perovskite Solar Cell by Controlling the Precursor Purity of Perovskite Materials. Journal of Materials Chemistry A 2016, 4, 887-893. (12) National Center for Photovoltaics (NCPV) at the National Renewable Energy Laboratory


17


Page 18 of 21

(NREL). www.nrel.gov/pv/assets/images/efficiency-chart.png (accessed July 2017) 2017. (13) Kim, G.-W.; Kang, G.; Kim, J.; Lee, G.-Y.; Kim, H. I.; Pyeon, L.; Lee, J.; Park, T. Dopantfree Polymeric Hole Transport Materials for Highly Efficient and Stable Perovskite Solar Cells. Energy & Environmental Science 2016, 9, 2326-2333. (14) Song, S.; Moon, B. J.; Hörantner, M. T.; Lim, J.; Kang, G.; Park, M.; Kim, J. Y.; Snaith, H. J.; Park, T. Interfacial Electron Accumulation for Efficient Homo-junction Perovskite Solar Cells. Nano Energy 2016, 28, 269-276. (15) Bi, D.; Tress, W.; Dar, M. I.; Gao, P.; Luo, J.; Renevier, C.; Schenk, K.; Abate, A.; Giordano, F.; Baena, J.-P. C. Efficient Luminescent Solar Cells Based on Tailored Mixed-cation Perovskites. Science advances 2016, 2, e1501170. (16) 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.; Gratzel, M. Cesium-containing Triple Cation Perovskite Solar Cells: Improved Stability, Reproducibility and High Efficiency. Energy Environ Sci 2016, 9, 1989-1997. (17) Wu, C.; Huang, Z.; He, Y.; Luo, W.; Ting, H.; Li, T.; Sun, W.; Zhang, Q.; Chen, Z.; Xiao, L. TiO 2 /SnO x Cl y Double Layer for Highly Efficient Planar Perovskite Solar Cells. Organic Electronics 2017, 50, 485-490. (18) Eperon, G. E.; Burlakov, V. M.; Docampo, P.; Goriely, A.; Snaith, H. J. Morphological Control for High Performance, Solution-Processed Planar Heterojunction Perovskite Solar Cells. Advanced Functional Materials 2014, 24, 151-157. (19) Leijtens, T.; Eperon, G. E.; Pathak, S.; Abate, A.; Lee, M. M.; Snaith, H. J. Overcoming Ultraviolet Light Instability of Sensitized TiO(2) with Meso-superstructured Organometal tri-halide Perovskite Solar Cells. Nat Commun 2013, 4, 2885.


18

Page 19 of 21 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


(20) Zhang, W.; Saliba, M.; Moore, D. T.; Pathak, S. K.; Horantner, M. T.; Stergiopoulos, T.; Stranks, S. D.; Eperon, G. E.; Alexander-Webber, J. A.; Abate, A.; Sadhanala, A.; Yao, S.; Chen, Y.; Friend, R. H.; Estroff, L. A.; Wiesner, U.; Snaith, H. J. Ultrasmooth Organicinorganic Perovskite Thin-film Formation and Crystallization for Efficient Planar Heterojunction Solar Cells. Nat Commun 2015, 6, 6142. (21) Ke, W.; Fang, G.; Liu, Q.; Xiong, L.; Qin, P.; Tao, H.; Wang, J.; Lei, H.; Li, B.; Wan, J.; Yang, G.; Yan, Y. Low-temperature Solution-processed Tin Oxide as an Alternative Electron Transporting Layer for Efficient Perovskite Solar Cells. J Am Chem Soc 2015, 137, 6730-3. (22) Song, J.; Zheng, E.; Bian, J.; Wang, X.-F.; Tian, W.; Sanehira, Y.; Miyasaka, T. LowTemperature SnO2-based Electron Selective Contact for Efficient and Stable Perovskite Solar Cells. Journal of Materials Chemistry A 2015, 3, 10837-10844. (23) Choi, J.; Song, S.; Horantner, M. T.; Snaith, H. J.; Park, T. Well-Defined Nanostructured, Single-Crystalline TiO2 Electron Transport Layer for Efficient Planar Perovskite Solar Cells. ACS Nano 2016, 10, 6029-36. (24) Zhang, Q.; Dandeneau, C. S.; Zhou, X.; Cao, G. ZnO Nanostructures for Dye-Sensitized Solar Cells. Advanced Materials 2009, 21, 4087-4108. (25) Fan, Z.; Yao, K.; Wang, J. Photovoltaic Effect in an Indium-tin-oxide/ZnO/BiFeO3/Pt Heterostructure. Applied Physics Letters 2014, 105. (26) Ghosh, B.; Ray, S. C.; Pontsho, M.; Sarma, S.; Mishra, D. K.; Wang, Y. F.; Pong, W. F.; Strydom, A. M. Defect Induced Room Temperature Ferromagnetism in Single Crystal, Poly-crystal, and Nanorod ZnO: A Comparative Study. Journal of Applied Physics 2018, 123. (27) Das, S.; Som, S.; Yang, C.-Y.; Lu, C.-H. Optical Temperature Sensing Properties of SnO 2 :


19


Page 20 of 21

Eu 3+ Microspheres Prepared via the Microwave Assisted Solvothermal Process. Materials Research Bulletin 2018, 97, 101-108. (28) Sun, W.; Li, Y.; Xiao, Y.; Zhao, Z.; Ye, S.; Rao, H.; Ting, H.; Bian, Z.; Xiao, L.; Huang, C.; Chen, Z. An Ammonia Modified PEDOT: PSS for Interfacial Engineering in Inverted Planar Perovskite Solar Cells. Organic Electronics 2017, 46, 22-27. (29) Beek, W. J.; Wienk, M. M.; Kemerink, M.; Yang, X.; Janssen, R. A. Hybrid Zinc Oxide Conjugated Polymer Bulk Heterojunction Solar Cells. The Journal of Physical Chemistry B 2005, 109, 9505-9516. (30) Pacholski, C.; Kornowski, A.; Weller, H. Self ‐ assembly of ZnO: From Nanodots to Nanorods. Angewandte Chemie International Edition 2002, 41, 1188-1191. (31) Jiang, Q.; Zhang, L. Q.; Wang, H. L.; Yang, X. L.; Meng, J. H.; Liu, H.; Yin, Z. G.; Wu, J. L.; Zhang, X. W.; You, J. B. Enhanced Electron Extraction Using SnO2 for High-efficiency Planar-structure HC(NH2)(2)PbI3-based Perovskite Solar Cells. Nat. Energy 2017, 2, 1-7. (32) Ding, B.; Gao, L.; Liang, L.; Chu, Q.; Song, X.; Li, Y.; Yang, G.; Fan, B.; Wang, M.; Li, C.; Li, C. Facile and Scalable Fabrication of Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells in Air Using Gas Pump Method. ACS Appl Mater Interfaces 2016, 8, 20067-73. (33) Li, X.; Bi, D.; Yi, C.; Décoppet, J.-D.; Luo, J.; Zakeeruddin, S. M.; Hagfeldt, A.; Grätzel, M. A Vacuum Flash–assisted Solution Process for High-efficiency Large-area Perovskite Solar Cells. Science 2016, 353, 58-62. (34) Gao, L.-L.; Liang, L.-S.; Song, X.-X.; Ding, B.; Yang, G.-J.; Fan, B.; Li, C.-X.; Li, C.-J. Preparation of Flexible Perovskite Solar Cells by a Gas Pump Drying Method on a Plastic Substrate. Journal of Materials Chemistry A 2016, 4, 3704-3710. (35) Luo, W.; Wu, C.; Sun, W.; Guo, X.; Xiao, L.; Chen, Z. High Crystallization of Perovskite


20

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Film by a Fast Electric Current Annealing Process. ACS Appl Mater Interfaces 2017, 9, 26915-26920.

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SnO2 Double Electron Transport Layer Guides Improved Open

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