Fullerene Derivative Modified SnO2 Electron Transport Layer for

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Fullerene Derivative Modified SnO2 Electron Transport Layer for Highly Efficient Perovskite Solar Cells with Efficiency over 21% Tiantian Cao, Kang Chen, Qiaoyun Chen, Yi Zhou, Ning Chen, and Yongfang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09238 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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ACS Applied Materials & Interfaces

Fullerene

Derivative

Modified

SnO2

Electron

Transport Layer for Highly Efficient Perovskite Solar Cells with Efficiency over 21% Tiantian Cao,a‡ Kang Chen,a‡ Qiaoyun Chen,a Yi Zhou,a Ning Chen*a and Yongfang Lia,b

a

Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical

Engineering and Materials Science, Soochow University, Suzhou 215123, P.R. China.

b

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese

Academy of Sciences, Beijing, 100190, P.R. China.

KEYWORDS Perovskite solar cells, fullerene derivative, SnO2 electron transport layer, trap states, surface passivation

ABSTRACT

ACS Paragon Plus Environment

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Tin dioxide (SnO2) has been widely applied as an electron transport layer (ETL) for the

n-i-p type perovskite solar cells (Pero-SCs). However, the existence of defects at the surface of SnO2 and the hysteresis behavior of the devices with SnO2 ETL limit its application in the Pero-SCs. In this study, a fullerene derivative pyrrolidinofullerene C60substituted phenol (NPC60-OH) was synthesized and applied to modify the SnO2 ETL in Pero-SCs for the first time. The systematic and comparative characterizations demonstrated that, after the introduction of NPC60-OH modification layer on the SnO2 ETL, the perovskite film in the corresponding device have enlarged grain size and these devices present enhanced electron transport and decreased charge recombination velocity. Besides, the NPC60-OH layer could significantly reduce the trap-state density in the perovskite film. As a result, a champion power conversion efficiency (PCE) of 21.39% was achieved for the SnO2/NPC60-OH based Pero-SCs, with suppressed hysteresis and improved stability, while the control devices with pristine SnO2 ETL showed a lower PCE of 19.04%.

INTRODUCTION


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Organic-inorganic hybrid perovskite solar cells (Pero-SCs) are promising photovoltaic device and have attracted intense attention worldwide recently.1 This type of solar cell devices showed excellent electronic and optical properties, such as high carrier mobility, large absorption coefficient, low trap density and tunable bandgaps. 2-5 Continuous efforts have been devoted to further improve the power conversion efficiencies (PCEs) of PeroSCs, including the fine control of perovskite compositions, a high quality perovskite film, as well as the efficient hole transport layer (HTL) and electron transport layer (ETL).6-14 The PCE of the Pero-SCs has skyrocketed from 9.7% to the certified 24.2% in 7 years.15, 16

Among the abovementioned factors determining the performance of the Pero-SCs, the electron transporting material (ETM) plays an essential role in improving device performance. So far, n-i-p type Pero-SCs mainly employed mesoporous or compact titanium oxide (TiO2) layer as ETL, which requires high-temperature (> 450 oC) and longduration sintering treatment to obtain the conductive TiO2 phase.17, 18 This treatment limits the application of Pero-SCs based on TiO2 ETL with flexible substrates. In addition, the


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relatively low conductivity of TiO2 may lead to decreased PCE and severe hysteresis of the Pero-SCs.19, 20 Thus, alternatively, several low temperature and solution-processable ETLs such as zinc oxide (ZnO),21 tin oxide (SnO2) have been applied to replace TiO2 recently. Among them, SnO2 possesses favorable energy level alignment and higher electron mobility, is regarded as a promising ETL for planar heterojunction n-i-p PeroSCs.22

However, even though SnO2 offers significant advantages as ETL compared to TiO2 in Pero-SCs, most of the SnO2 based devices can’t eliminate hysteresis behavior completely. Besides, intrinsic defects still exist at the surface of SnO2, which can capture electron and may cause sever charge recombination and poor electron transport.23 Nevertheless, recent reports suggested that interfacial modification can efficiently passivate the trap state and suppress the charge recombination between ETL and the perovskite active layer.24, 25

Fullerene and its derivatives have been proven to be an excellent interfacial material due to their ability to passivate the charge traps and suppress device hysteresis.26-29 Xu


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and co-workers reported a fullerene derivative C60 pyrrolidine tris-acid (CPTA). After modification of CPTA atop of SnO2, the flexible MAPbI3 solar cells showed an enhanced PCE of 18.36% and long-term stability over 46 days in air.22 Furthermore, in a recent study, Zhan et al. reported a long chain fullerene derivative with 2 hydroxyl anchoring groups (C9) to modify SnO2 ETL, C9 modifying layer can promote charge extraction and passivate the defects. Consequently, the device based on C9/SnO2 exhibited a PCE of 21.3 % with negligible hysteresis and good stability.27 These results suggest that fullerene derivative could be efficient interfacial layer to improve the performance of Pero-SCs based on SnO2 ETL.

In this work, we synthesized a fullerene derivative pyrrolidinofullerene C60-substituted phenol (NPC60-OH) using a simple one-step method, and then employed it in the PeroSCs for the first time. In the previous study, NPC60-OH had been applied as a selfassembled monolayer on top of ZnO layer via a soaking method to improve the PCE of the inverted organic solar cell.30 Herein, NPC60-OH was successfully introduced into the Pero-SCs via a spin-coating method to modify SnO2 ETL. As a fullerene derivative, the


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application of NPC60-OH fullerene has several advantages.31-33 It could be synthesized under low-temperature of 125 oC by a facile one-step process, involving only the nonhalogen solvent of toluene. Moreover, the NPC60-OH/SnO2 ETLs possessed a high electron mobility, which promoted the electron transportation and further enhanced the PCE of device. After the insertion of NPC60-OH, the grain size of the perovskite film was enlarged and the trap-states inside perovskite film was decreased. As a result, the device based on SnO2/NPC60-OH demonstrated an enhanced PCE of 21.39 %, while the control device with pristine SnO2 ETL only obtained a PCE of 19.04%.


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RESULT AND DISCUSSION

(a)

CH3 N

+

CH3-NH-CH2-COOH

+

(b)

OH

CHO

Toluene

OH

Reflux

NPC60-OH

868.229

720.152

500

1000

1500

m/z

2000

2500

Figure 1. (a) The synthetic route for fullerene derivative of NPC60-OH, (b) The MALDITOF mass spectrum of NPC60-OH. NPC60-OH was synthesized according to the previous report using one-step method, fullerene C60, 4-hydroxybenzaldehyde and N-methylglycine were dissolved in toluene and


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then the mixture was refluxed under N2 atmosphere.31-33 The detailed synthetic route and purification steps were illustrated in Figure 1a. Mass spectrum analysis was employed to confirm the successful synthesis of NPC60-OH (Figure 1b). The MALDI-TOF spectrum of NPC60-OH (C69H11ON) presented a major peak at 868.229 m/z, which agrees well with the theoretical molecular weight of NPC60-OH (869.852). The minor peak was assigned to fragment peak of C60 (720.152). XPS measurement was performed for the pristine SnO2 and SnO2/NPC60-OH films, to investigate the interaction between the two layers. As shown in Figure 2a, the XPS survey spectrum of SnO2/NPC60-OH illustrated a much higher C1s peak at 285 eV compared to that of the SnO2 substrate, which proved the existence of NPC60-OH layer atop of the SnO2. In addition, a minor peak at 399 eV was observed on the XPS curve of SnO2/NPC60-OH substrates (Figure S1c), which agrees well with N1s peak observed on the spectrum of the NPC60-OH power (Figure S1b), further confirming the existence of NPC60-OH film on top of the SnO2 layer.


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(b)

(a) Sn3d

SnO2

O1s

SnO2/NPC60-OH

Sn3d O1s C1s

1000

800

600

400

200

Binding energy (eV)

SnO2

Intensity (a.u.)

Intensity (a.u.)

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

SnO2/NPC60-OH

500

495

490

485

480

Binding energy (eV)

Figure 2. (a) XPS survey spectra of SnO2 and SnO2/NPC60-OH, (b) XPS curves for pristine SnO2 and NPC60-OH modified SnO2 at Sn 3d level.

Figure 2b presented the binding energy for the Sn3d level of pristine SnO2 and SnO2/NPC60-OH films. It displayed the characteristic peaks at 495.0 eV, 486.7 eV of the NPC60-OH modified SnO2 layer, respectively, which is slightly higher than that of pristine SnO2 (494.7 eV and 486.4 eV). This negative shift of binding energy at Sn3d level suggested that the negative charges around Sn atoms were increased after the modification of NPC60-OH, which is likely caused by the interaction between SnO2 and NPC60-OH. The hydroxyl group of phenol attached on the NPC60-OH could be regarded as Lewis base, thus, this weak interaction could be formed by the electron donation from


9


NPC60-OH to the under-coordinated Sn caused by oxygen vacancy in SnO2.27 Therefore, the XPS studies confirmed the interaction between SnO2 and NPC60-OH layer.

Ag Spiro-OMeTAD Perovskite SnO2/NPC60-OH ITO

500 nm -2

-2.30

-3

-4.43 -4.75

-5

ITO

-6

-3.89

Perovskite

-4

-4.14

NPC60-OH

OH

SnO2

N

Spiro-OMeTAD

CH3

E (eV)

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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-4.60 Ag

-5.22

-5.44

Figure 3. (a) Cross-sectional SEM image of the Pero-SC with the NPC60-OH layer, (b) chemical structure of NPC60-OH, (c) energy level diagrams of the corresponding materials of the device.


10

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To further examine the effect of NPC60-OH layer on the device performance, the planar

n-i-p

type

Pero-SCs

with

the

configuration

of

ITO/SnO2

(or

SnO2/NPC60-

OH)/Perovskite/Spiro-OMeTAD/Ag were fabricated. Figure 3a presents the crosssectional scanning electron microscopy (SEM) image of the Pero-SC with the NPC60-OH layer, and all the functional layers of device were distinguished using different color. The original cross-sectional SEM image was depicted in Figure S2 as well. The thicknesses of SnO2 ETL and NPC60-OH layer were ~ 25 nm and ~ 4 nm, respectively (measured by spectroscopic ellipsometer, as discussed below in Figure 7a). Figure 3a demonstrates that the perovskite active layer with a thickness of ~360 nm was deposited on top of the ETL and was then covered with a Spiro-OMeTAD HTL with a thickness of ~190 nm. Additionally, the UV-Vis spectra of SnO2, SnO2/NPC60-OH before and after washing with DMF: DMSO (95:5, v:v) were performed to prove the existence of NPC60-OH during the two-step fabrication method of perovskite. As illustrated in Figure S3, the absorbance of SnO2/NPC60-OH only showed very slight decrease after washing by mixed DMF/DMSO solvent. This result suggests that NPC60-OH fullerene layer could survive upon the coating of the PbI2 DMF/DMSO solution.


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Moreover, the chemical structure of NPC60-OH and energy level diagrams of the PeroSC were displayed in Figure 3b and Figure 3c, respectively. The work functions (WFs) of SnO2 and SnO2/NPC60-OH were determined using ultraviolet photoelectron spectroscopy (UPS), and the detailed UPS results of SnO2 and SnO2/NPC60-OH in the secondary electron cut-off region were presented in Figure S4 in supporting information. The energy levels of other materials referred to the previous literatures.27, 34-36 The WFs of SnO2 and SnO2/NPC60-OH were calculated to be 4.43 eV and 4.14 eV, respectively, suggesting that the introduction of NPC60-OH layer efficiently lowered the WF of SnO2 layer. The reduction in the WF of SnO2 after the modification of NPC60-OH layer may be attributed to the formation of interface dipoles.37-40 The shift of Sn3d peak in XPS after the insertion of NPC60-OH (Figure 2b) indicated the interaction between SnO2 and NPC60-OH fullerene layer. The electron-donating property and the functional phenolic hydroxyl group of NPC60-OH might cause the formation of the dipoles on the surface of SnO2, which likely led to the reduced WF of SnO2.37 This reduced energy band gap between ETL and perovskite active layer could facilitate the charge transport and decrease the charge recombination at the interface.41


12

(a

SnO2

Reserve scan Forward scan

-5

)

-15 -20

-10 -15

SnO2

-20

-25 -0.2 0.0

0.2

(d) 100

0.4

0.6

V (V)

0.8

1.0

-25 -0.2 0.0

1.2

(e

0.2

60

15

40

10

20

5

SnO2 SnO2/NPC60-OH 400

500

600

700

Wavelength (nm)

800

0 900

)

0.6

0.8

1.0

1.2

25

-15

10 SnO2

5

SnO2/NPC60-OH

0 0

20

40

60

80 100 120 140

Time (s)

SnO2/NPC60-OH

-20 -25 -0.2 0.0

(f)

PCE ~ 18.50 %

15

Reserve scan Forward scan

-10

PCE ~ 20.91 %

20

PCE (%)

20

0.4

V (V)

2

Integrated J sc (mA cm- )

25

80

0 300

2

-10

0 -5

J (mA/cm )

2

2

(c

0

SnO2/NPC60-OH

J (mA/cm )

-5

J (mA/cm )

)

(b)

0

EQE (%)

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


Count

Page 13 of 52

0.2

0.4

0.6

0.8

1.0

1.2

20

21

22

V (V) 20 18 16 14 12 10 8 6 4 2 0 15

SnO2 SnO2/NPC60-OH

16

17

18

19

PCE (%)

Figure 4. (a) J-V curves of the champion Pero-SCs with SnO2 and SnO2/NPC60-OH ETL, J–V characteristics with forward and reverse scans of the Pero-SCs with (b) SnO2 and (c) SnO2/NPC60-OH ETL under the illumination of AM1.5G, 100 mW cm-2. (d) EQE spectra and integrated photocurrent densities of the corresponding devices without and with NPC60-OH layer, (e) Steady-state photocurrent output of the devices based on different ETL measured at the maximum power point (0.92 V for SnO2, and 0.96 V for SnO2/NPC60-OH), (f) PCE histograms of 50 devices with SnO2 and SnO2/NPC60-OH ETL. Figure 4a depicted the current density-voltage (J-V) characteristics of the champion devices based on SnO2 and SnO2/NPC60-OH ETLs (the optimized concentration of


13


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NPC60-OH was 0.75 mg/mL). And the J-V curves of devices with the other concentration of NPC60-OH modified SnO2 ETL were depicted in Figure S5 in supporting information. The detailed photovoltaic parameters of control devices based on pristine SnO2 and PeroSCs with different concentration of NPC60-OH were summarized in Table 1. As shown in Figure 4a and Table 1, the device based on SnO2/NPC60-OH ETL achieved a summit PCE of 21.39% with an open circuit voltage (Voc) of 1.13 V, a short circuit current density (Jsc) of 23.37 mA cm-2 and a fill factor (FF) of 80.73%, while the control device with pristine SnO2 only obtained a Voc of 1.11V, a Jsc of 22.67 mA cm-2, a FF of 75.86% and a corresponding PCE of 19.04%, respectively. For comparison, the widely used C60 fullerene and fullerene derivative PC60BM were also applied to modify SnO2 ETL, the photovoltaic parameters of the champion device based on these ETL were displayed in Table 1, and all the average values with errors were listed in Table S1. As presented in Table 1, the PCEs of devices with SnO2/C60 and SnO2/PC60BM were 19.07% and 20.25%, respectively. Thus, compared with C60 or PC60BM, NPC60-OH demonstrates better performance as modification layer in the corresponding devices. Intrinsic hysteresis is a long-existing problem for the n-i-p type Pero-SCs,

42

thus the J-V characteristics of


14

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devices based on SnO2 and SnO2/NPC60-OH ETLs under reverse and forward scans were measured to examine the hysteresis behavior of Pero-SCs( Figure 4b and 4c). The hysteresis index (H-index) can be determined by H-index = [PCE(reverse) PCE(forward)]/PCE(reverse),27, 43,44 Table S2 presented a decreased H-index of 5.3 % for the device based on the SnO2/NPC60-OH ETL compared to that of Per-SC with pristine SnO2 ETL (21.4%), suggesting the introduction of NPC60-OH layer efficiently suppressed the hysteresis behavior of device.

(a)

(b)

1.18

24.5

1.16

J sc (mA/cm )

1.14

Voc (V)

2

1.12 1.10 1.08 1.06 1.04 1.00

(c)

24.0 23.5 23.0 22.5

1.02

22.0

SnO2

SnO2/NPC60-OH

85

(d)

SnO2

SnO2/NPC60-OH

23 22

80

21

PCE (%)

FF (%)

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


75 70

20 19 18 17 16

65

15 14

60

SnO2

SnO2/NPC60-OH

SnO2

SnO2/NPC60-OH


15


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Figure 5. Statistical distributions of the photovoltaic parameters of (a) Voc, (b) Jsc, (c) FF and (d) PCE for devices based on SnO2 and SnO2/NPC60-OH ETLs.

The external quantum efficiency (EQE) spectra and the integrated Jscs of Pero-SCs without and with NPC60-OH layer were displayed in Figure 4d. The maximum EQE of devices based on SnO2 and SnO2/NPC60-OH ETL was 93.8% and 97.4%, respectively. In addition, for the SnO2/NPC60-OH based device, the integrated photocurrent densities calculated from EQE spectrum exhibited a higher value of 23.25 mA cm-2, compared to that of the control device (22.43 mA cm-2). The results of integrated photocurrent agree well with the corresponding Jscs from the J-V curves (Figure 3a and Table 1). Figure 4e showed the steady-state photocurrent output of the Pero-SCs without and with NPC60OH layer measured under the maximum power point (0.92 V for SnO2, and 0.96 V for SnO2/NPC60-OH). The stabilized efficiency of 18.50 % and 20.91% were obtained based on the device with SnO2 and SnO2/NPC60-OH ETL, respectively, which are consistent with results from J-V measurement. Besides, a high PCE of the Pero-SC with SnO2/NPC60-OH ETL was kept stable at 20.9% over 140 s. In contrast, a decrease of the


16

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PCE for the pristine SnO2 based device was observed over 100 s (Figure 4e). Therefore, the SnO2/NPC60-OH based Pero-SCs showed better stability than the control device.

Furthermore, in order to evaluate the reproducibility of the Pero-SCs, 50 individual devices based on different ETLs were fabricated, and the PCE distribution histogram was presented in Figure 4f. The SnO2/NPC60-OH based devices demonstrated a narrower PCE distribution of 19.83(±0.71)% compared to the control device (17.51(±0.92)%), and the average PCE was enhanced by ≈13.2% with the introduction of the NPC60-OH layer into the Pero-SC. The detailed comparisons of the photovoltaic parameters of different devices were illustrated in Figure 5. It revealed that, after the introduction of NPC60-OH layer, the Voc and FF were specifically enhanced while only a slight improvement was observed for the Jsc . The improvement of Voc may be attributed to the reduced work function of SnO2/NPC60-OH (as discussed above in Figure 3c), which will further decrease the energy band gap between ETL and perovskite layer, and shows the reduced energy loss when electron transfers from perovskite to SnO2 ETL. Therefore, it leads to the higher


17


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Voc of the devices based on SnO2/NPC60-OH.45, 46 The enhancement of Jsc and FF will be discussed in the following section.

Table 1. The detailed photovoltaic parameters of Pero-SCs with different ETLs under the illumination of AM1.5G, 100 mW cm-2.

ETL SnO2

SnO2/NPC60-OH

SnO2/C60a SnO2/PC60BM a

Concentration (mg/mL) /

1.11

Jsc (mA/cm2) 22.67

0.25

1.11

23.31

77.97

20.21

0.5

1.14

22.87

79.20

20.59

0.75

1.13

23.37

80.73

21.39

1

1.11

23.42

79.79

20.78

1.25

1.12

23.62

76.76

20.35

1.5

1.12

23.45

74.97

19.74

/

1.08

22.67

78.00

19.07

0.75

1.14

23.22

76.95

20.25

1

1.14

23.38

74.85

19.96

Voc (V)

FF (%)

PCE (%)

75.86

19.04

the thickness of C60 is ~4nm


18

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(b)

(a)

(c)

500 nm SnO2

310 nm

Percentage (%)

20% 15% 10% 5% 0

35% SnO2/NPC60-OH

30%

25%

0%

500 nm

(d)

35% 30%

Percentage (%)

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


100 200 300 400 500 600 700 800

Size (nm)

360 nm

25% 20% 15% 10% 5% 0%

0

100 200 300 400 500 600 700 800 900

Size (nm)

Figure 6. Top view SEM images of the perovskite films spin-coated on (a) SnO2, (b) SnO2/NPC60-OH layer. Histograms of grain size distributions of perovskite films based on (c) SnO2, (d) SnO2/NPC60-OH layer.

The previous reports suggested that the quality of the perovskite film has a great influence on the device performance.47,

48

Thus, in order to validate the effect of the

additional NPC60-OH layer on the surface morphology of perovskite films, top-view SEM measurement was performed. Figure 6a and 6b showed the top view SEM images of the perovskite films spin-coated on the pristine SnO2 and SnO2/NPC60-OH layer, in which both perovskite films showed a continuous and uniform film without obvious pinholes.


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Nevertheless, the perovskite deposited on SnO2/NPC60-OH illustrated a much bigger grain size than that of the devices based on the SnO2 ETL. As observed in Figure 6c and 6d, the average grain size of perovskite film grown on pristine SnO2 and SnO2/NPC60-OH layer was 310 nm and 360 nm, respectively. These results clearly shows that the grain size of the perovskite film was enlarged with the insertion of NPC60-OH layer, which indicated an enhanced quality of perovskite film and agreed with the enhanced device performance. Moreover, to further investigate why the insertion of the NPC60-OH layer could increase the grain size of the perovskite film, the contact angles of water based on SnO2 and SnO2/NPC60-OH substrates were performed. As shown in Figure S6, while the SnO2/NPC60-OH presented a bigger contact angle of 64o, the contact angles of SnO2 without and with UVO treatment were 52o and 10o, respectively. This moderate nonwetting surface could suppress the heterogeneous nucleation and enhance the grain boundary mobility, which likely led to the enlarged grain size.27, 49


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(a)

(b)

150 SnO2

100

SnO2/NPC60

50 0 -50

SnO2 SnO2/NPC60-OH VTFL=0.68 V

1E-3 1E-4

VTFL=0.87 V

1E-5

-100 -150

0.1

0.01

Current (A)

Current (mA)

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


-5

-4

-3

-2

-1

0

1

2

3

4

5

1E-6

Voltage (V)

0.1

Voltage (V)

1

Figure 7. (a) I-V characteristics of the devices based on ITO/SnO2/Au and ITO/SnO2/NPC60-OH/Au, (b) dark current-voltage curves of the electron-only device based

on

ITO/SnO2/Perovskite/PCBM/Ag,

and

ITO/SnO2/NPC60-

OH/Perovskite/PCBM/Ag

The current-voltage (I-V) measurements were conducted in the dark condition to further evaluate the charge transport properties of the SnO2 ETL without and with NPC60-OH layer. The devices based on ITO/SnO2/Au and ITO/SnO2/NPC60-OH/Au were fabricated, and the corresponding I-V curves were illustrated in Figure 7a. The direct current (DC) conductivities (σo) of SnO2 and SnO2/NPC60-OH can be determined by the slope of the

I-V curves based on the following equation, I = σoAd-1V, in which A is the detected area,


21


Page 22 of 52

and d represents the thickness of the samples.45 The average thickness of SnO2 and SnO2/NPC60-OH were 25.2 nm and 29.3 nm (measured by spectroscopic ellipsometer). Therefore, according to the above equation, the corresponding σos could be calculated as 2.64 ×10-4 mS cm-1 and 1.05×10-3 mS cm-1, respectively. This indicates that the conductivity of the corresponding device has been notably enhanced after the insertion of NPC60-OH layer atop of SnO2 layer, and the electron can transport more efficiently from the perovskite active layer to the ETL. This result also provides a rational explanation for the enhanced Jsc and FF of the Pero-SCs with the insertion of NPC60-OH layer.

The space-charge-limited-currents (SCLC) method based on the electron only devices with the structure of ITO/SnO2 (or SnO2/NPC60-OH)/Perovskite/PCBM/Ag were used to measure the trap-state density within the perovskite film.50 The corresponding dark I-V curves of the devices with different ETLs were shown in Figure 7b, in which the I-V curves include three parts, the linear ohmic response region at the low bias voltage, the trapfilling region and the trap-free SCLC region at the high voltage.51 And the applied voltage at the kink-point voltage is defined as the trap-filled limit voltage (VTFL), which is linearly


22

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proportional to the density of trap states (Nt) of perovskite layer according to the following equation.52 53

𝑁𝑡 =

2𝜀𝜀0 𝑒𝐿2

𝑉𝑇𝐹𝐿

(1)

Where ε is the relative dielectric constant of the perovskite, ε0 is the vacuum permittivity, e is the elementary charge of the electron, L is the thickness of perovskite film. As shown in Figure 7b, the VTFL of the devices based on SnO2 and SnO2/NPC60-OH were 0.87 V (the orange line) and 0.68 V (the blue line). Based on the above equation, the calculated

Nt were 3.48×1016 cm-3 and 2.72×1016 cm-3, respectively. Therefore, after introducing NPC60-OH layer to modify SnO2 ETL, the trap-state density of perovskite film decreased by ~22% compared to the perovskite based on pristine SnO2 layer, which suggested that NPC60-OH can dramatically reduce and passivate the trap-state density in perovskite film. A plausible explanation for the decreased trap-state density was the increased grain size of perovskite film with the modification of NPC60-OH (as shown in Figure 6), which supplied further evidence for the higher FF of the devices.


23


1.4

(b)

1.2

Perovskite SnO2/Perovskite

1.0

SnO2/NPC60-OH/Perovskite

3

10

PL intensity

(a) Normalized intensity

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 24 of 52

0.8 0.6 0.4

2

10

0.2

Perovskite SnO2/Perovskite SnO2/NPC60-OH/Perovskite

0.0 700 720 740 760 780 800 820 840

Wavelength (nm)

20

40

60

80

100

Time (ns)

Figure 8. (a) Steady-state PL and (b) time-resolved PL spectra of the perovskite films deposited on different ETLs.

The steady-state photoluminescence (PL) and time-resolved PL measurements were carried out to investigate the charge transfer and recombination dynamics that occurred at the ETL/Perovskite interface.54 Figure 8a presented the steady-state PL spectra of pristine perovskite film, and the perovskite film deposited on SnO2 ETL, SnO2/NPC60-OH ETL, respectively. It showed that an emission peak of perovskite film was located at 776 nm, the PL intensities of SnO2/Perovskite and SnO2/NPC60-OH/Perovskite were obviously lower than that of the pristine perovskite layer. In particular, a significant quenching (90 %) was observed in the SnO2/NPC60-OH/Perovskite film while the devices with SnO2/Perovskite film showed a PL quenching of 40%. This remarkable fluorescence


24

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quenching after introducing NPC60-OH layer suggests that, NPC60-OH film could facilitate the charge transport between perovskite and ETL, which is consistent with the improved

Jsc and FF of the corresponding devices.

In order to further study the effect of NPC60-OH fullerene layer on the dissociation and recombination of the charge carriers, time-resolved PL was conducted. Figure 8b exhibited the time-resolved PL spectra of the perovskite films deposited on different ETLs. The PL decay curves were fitted by a biexponential decay function, including a fast decay and a slow decay component,55 which corresponding to the PL quenching originated from the ETL/perovskite interfacial charge transfer and radiative recombination of free charges, respectively.43, 56 The detailed fitting parameters were listed in Table S3, according to the fitting data, the average decay lifetime (τave) were calculated based on the following equation.57, 58

𝐴1𝜏21 + 𝐴2𝜏22

𝜏𝑎𝑣𝑒 = 𝐴1𝜏1 + 𝐴2𝜏2 (2)

As shown in Table S3, the pristine perovskite film exhibited a τave of 178.81 ns, while the perovskite film deposited on SnO2 and SnO2/NPC60-OH displayed average decay


25


lifetimes of 39.95 ns and 24.98 ns, respectively. The decay lifetime of SnO2/NPC60OH/Perovskite decreased dramatically compared to that of the SnO2/Perovskite device. These results imply that, after using NPC60-OH layer to modify SnO2, electron transferred more efficiently from the perovskite active layer to the ETL and less recombination occurred inside the perovskite layer. The facilitated charge transfer and suppressed recombination led to the enhanced Jsc and FF.

(b)

(a) SnO2

100

Fitted SnO2 SnO2/NPC60-OH

80

Z'' )

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 26 of 52

Rs

Fitted SnO2/NPC60-OH

60

Rtr

Rrec

CPE1

CPE2

40 20 0

0

20

40

60

80

100 120 140 160

Z' )

Figure 9. (a) Nyquist plots of the devices based on pristine SnO2 and SnO2/NPC60-OH ETL, (b) the equivalent circuit model for fitting the impedance spectra

The electrical impedance spectroscopy (EIS) measurement was conducted to further study the effect of NPC60-OH layer on the interfacial charge transport and recombination


26

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behavior of devices. Figure 9a showed the Nyquist plots of the Pero-SCs with SnO2 and SnO2/NPC60-OH ETL, and the devices based on different ETL were measured in the dark condition near the Voc value. The corresponding equivalent circuit model was presented in Figure 9b, in which the Rs is the series resistance, the Rtr represents charge transfer resistance and the Rrec is assigned to the charge recombination resistance.43,

59

Furthermore, the Rtr and Rrec reflect the electron transport and the hole-electron recombination, respectively.60

The detailed parameters used for fitting were tabulated in Table S4. As shown in Table S4, the device based on SnO2/NPC60-OH ETL showed a much smaller Rtr value (5.90 Ω) compared to the Pero-SC with pristine SnO2 ETL (106.00 Ω), indicating that the modification of NPC60-OH layer could significantly enhance the electron transport of the device. In addition, the Pero-SCs without and with NPC60-OH layer presented the Rrecs value of 20.65 Ω and 86.45 Ω, respectively. The lager Rrec of SnO2/NPC60-OH based device suggested that the hole-electron recombination was effectively inhibited at the interface of SnO2/NPC60-OH and perovskite layer, which may be attributed to the


27


increased grain size of perovskite as discussed above (Figure 7). Therefore, these results also revealed that , after introducing the NPC60-OH layer, the electron transported more efficiently and hole-electron recombination was suppressed, which was beneficial for the enhanced FF value.61

N2 Condition

0

(e) 1.14

10

20

30

40

50

Time (d)

(f) 24.0 )

1.12

J sc (mA/cm

2

1.10 1.08 1.06

SnO2

1.04

SnO2/NPC60-OH

1.02 1.00

Air Condition

0

30

60

90

120 150 180

Time (h)

23.5 23.0 22.5 22.0 21.5 21.0 20.5 20.0

70

SnO2 SnO2/NPC60-OH N2 Condition

0

10

20

30

40

Time (d)

50

65 60

SnO2

55

SnO2/NPC60-OH

50

(g)75

N2 Condition

0

10

20

30

40

50

Time (d)

70

SnO2 SnO2/NPC60-OH

30

60

90

SnO2 SnO2/NPC60-OH

55 Air Condition

0

65 60

120 150 180

Time (h)

50

Air Condition

0

30

60

90

120 150 180

Time (h)

Normalized PCE

SnO2/NPC60-OH

(d)1.0

(c)75 FF (%)

SnO2

23.5 23.0 22.5 22.0 21.5 21.0 20.5 20.0

FF (%)

) 2

J sc (mA/cm

Voc (V)

1.14 1.12 1.10 1.08 1.06 1.04 1.02 1.00

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2

(h)1.0 Normalized PCE

(b)24.0

(a) 1.16

Voc (V)

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 28 of 52

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2

SnO2 SnO2/NPC60-OH N2 Condition

0

10

20

30

Time (d)

40

50

SnO2 SnO2/NPC60-OH

0

30

60

90

Air Condition

120 150 180

Time (h)

Figure 10. Stabilities of the Pero-SCs with pristine SnO2 and SnO2/NPC60-OH ETL stored in nitrogen atmosphere. (a) Voc, (b) Jsc (c) FF, and (d) normalized PCE. Ambient stabilities of the devices by storing without any encapsulation in ambient condition (temperature: 25±5 ºC, relative humidity: 20-30%) for SnO2 and SnO2/NPC60-OH based sample. (e) Voc, (f) Jsc (g) FF, and (h) normalized PCE.


28

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Finally, the stabilities of the devices based on pristine SnO2 and SnO2/NPC60-OH ETL were also evaluated. As depicted in Figure 10a-10d, it showed the stabilities of the PeroSCs with pristine SnO2 and SnO2/NPC60-OH ETL stored under nitrogen atmosphere in the glovebox. After 45 days storage ，the PCE of the control device based on pristine SnO2 ETL dropped to 80% of the initial value, while the NPC60-OH modified Pero-SCs still kept ≈90% of the initial PCE. Furthermore, Figure 10e-10h presented the ambient stabilities of the devices with different ETLs by storing in ambient air without any encapsulation (temperature: 25±5 ºC, relative humidity: 20-30%). It revealed that, for over 180 h storage in air condition, the unencapsulated SnO2 and SnO2/NPC60-OH based devices retained 81% and 88% of their original PCE, respectively.

CONCLUSIONS

In summary, a fullerene derivative NPC60-OH with a phenolic hydroxyl group on the side chain was prepared using a simple one-step process, and then utilized as modification layer in planar Pero-SCs for the first time. With the insertion of NPC60-OH layer, the Pero-


29


Page 30 of 52

SC demonstrated superior properties than the control device based on bare SnO2 ETL. The NPC60-OH based devices showed a reduced energy band gap between ETL and perovskite film, due to the suitable WF of SnO2/NPC60-OH, which contributed to the high

Voc. The average grain size of perovskite film increased from 310 nm to 360 nm when deposited on SnO2/NPC60-OH substrate, beneficial for the enhanced FF and PCE. Moreover, the comprehensive studies of DC conductivities, PL and EIS measurements and the SCLC revealed that, NPC60-OH layer could simultaneously promote the charge transport efficiency, suppress charge recombination and passivate the trap-state density of the perovskite film. Therefore, the abovementioned factors collectively led to the enhanced PCE of 21.39% and the improved stability for the SnO2/NPC60-OH based device. This work indicated that NPC60-OH was a promising modification material on ETL for the n-i-p type Pero-SCs.

EXPERIMENTAL SECTION

Materials


30

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C60 (99.9%) was purchased from Puyang Yongxin Fullerene Technology Co., Ltd. 4hydroxybenzaldehyde and N-methylglycine were obtained from Adamas Reagent Co., Ltd. and used without further purification.

Formamidinium iodide (FAI), methylammonium bromide (MABr), methylammonium chloride (MACl) were purchased from Xi’an Polymer Light Technology Crop. Lead iodide (PbI2, 99.999%) was bought from Alfa Aesar. Spiro-OMeTAD was obtained from Xi’an Polymer Light Technology Crop.

Device fabrication

The ITO glass substrates (10 Ω sq−1) were bought from South China Science & Technology Company Limited. ITO substrates were cleaned by sequential ultrasonic bath in detergent, water, deionized water, acetone, ethanol and isopropanol for 15 min each. And then the ITO glass dried in a nitrogen flow before treating with ultraviolet-ozone (UVO) for 20 min. The SnO2 precursor was obtained by dissolving 23 mg tin chloride dihydrate into 1mL anhydrous ethanol and then stirring for 2 hours. A thin SnO2 layer was prepared by spin-coating onto the pre-cleaned ITO substrate using the SnO2 precursor


31


Page 32 of 52

at 2000 rpm for 45s, then the substrate was annealed at 150 °C for 30 min and the following 180 °C for 60 min in an ambient atmosphere. The fullerene derivative of NPC60OH was dissolved in chlorobenzene (CB) with different concentration (0.25, 0.5, 0.75, 1.0, 1.25 and 1.5 mg mL-1). Then the NPC60-OH solutions were spin-coated on the SnO2based substrates at a speed of 1500 rpm for 45s. The C60 layer was deposited by vacuum evaporation and the thickness was controlled to 4 nm. The PC60BM was dissolved in CB with the concentration of 0.75 and 1.0 mg mL-1.The following fabrication of perovskite steps were performed in a glovebox under nitrogen atmosphere.

For the perovskite precursor, PbI2 was dissolved in the mixed solvents of N, Ndimethylformamide (DMF, 99.8%, J&K) and dimethyl sulfoxide (DMSO, 99.8%, J&K) (DMF/DMSO, 95:5, v/v). And then the FAMA precursor solution was prepared with a mass ratio of FAI: MACl: MABr = 60: 6: 6 (FAI, 60 mg/mL in anhydrous isopropanol), the PbI2 and FAMA solutions were both stirred at 70 °C for 10 h. The fabrication of perovskite film was prepared by two-step method according to the literature.35, 62 The PbI2 solution was spin-coated on the ITO/SnO2/ NPC60-OH substrate at 4500 rpm for 45s, and then the


32

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mixed FAMA solution was dropped onto the PbI2 film at 20s during this step. The halfcrystallization perovskite film was annealed at 125 °C for 5 min and the yellowish sample turned dark brown immediately after annealing. After the substrate cooling down to room temperature, the Spiro-OMeTAD hole transport layer was deposited by spin coating on top of perovskite layer at 3500 rpm for 40s. For the Spiro-OMeTAD solution, 72.3 mg Spiro-OMeTAD dissolved in 1 mL chlorobenzene (Sigma-Aldrich), then 28.8 μL of 4tertbutylpyridine (tBP, Sigma-Aldrich) and 17.5 μL of bis(trifluormethane)sulfonamide lithium salt (Li-TFSI, 520 mg/mL in acetonitrile, Sigma-Aldrich) were mixed into the SpiroOMeTAD solution. Finally, Ag electrode (≈100 nm thick) was deposited by thermal evaporation under 10-6 Torr. And a shadow mask was used to define the effective active area of the device (0.0757 cm2).

Measurements and characterization


33


Page 34 of 52

XPS and UPS were recorded on an ESCALAB 250Xi instrument (Thermo Fisher, USA). The UV-vis absorption spectra of SnO2/perovskite and SnO2/NPC60-OH/perovskite were measured using an Agilent Cary 6000. The mass spectrum of NPC60-OH was performed on a Bruker MALDI-TOF-MS (Ultraflextreme, Germany). UV-Vis spectra were recorded with Agilent Carry 6000. The J-V curves and steady-state efficiencies of devices were conducted on a Keithley 2400 source meter and the measurement was under AM1.5G illumination in the glovebox. The EQE spectra were acquired using a QE-R3011 system (Enli Technology Co., Ltd.). The S-4700 (Japan) instrument was used to collect the SEM images, and the distribution of perovskite grain size was analyzed using a Nano measurer 1.2 software. Contact angles were collected on an instrument of DAS100 (Germany). The thicknesses of SnO2 and SnO2/NPC60-OH were obtained using M-2000V spectroscopic ellipsometer (J.A. Woollam Co., USA). A FLS980 instrument (UK) with excitation wavelength of 470 nm was used to record the steady-state PL spectra. And the timeresolved PL spectra were carried out using a Lifespec II produced by Edinburgh Instrument (UK). The electrical impedance spectroscopy (EIS) was acquired on an IM6


34

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electrochemical workstation (Zahner Zennium, Germany) under dark condition, and the impedance spectra were analyzed using Z-view software.

ASSOCIATED CONTENT

Supporting Information

The detailed synthetic process, the mass spectra of NPC60-OH, and the J-V characteristics of the perovskite solar cells with different electron transport layer.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Author Contributions ‡ T.C. and K.C. contributed equally to this work.

Notes The authors declare no competing financial interest.


35


Page 36 of 52

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (NO. 51302178),

the

Natural

Science

Foundation

(NSF)

of

Jiangsu

Province

(SBK2017021655). Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the project of scientific and technologic infrastructure of Suzhou (SZS201708).

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The table of contents entry

CH3 N OH

0 SnO2

-5

SnO2/NPC60-OH

2

J (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


-10

Voc = 1.13 V Jsc = 23.37 mA/cm2 FF = 80.73% PCE = 21.39%

-15 -20 -25 -0.2

0.0

0.2

0.4

0.6

V (V)

0.8

1.0

1.2


51


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52

Fullerene Derivative Modified SnO2 Electron Transport Layer for

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