In Situ Cesium Modification at Interface Enhances the Stability of

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In-situ Cesium Modification at Interface Enhances Stability of Perovskite Solar Cells Yao Zhao, Yicheng Zhao, Wenke Zhou, Qi Li, Rui Fu, Dapeng Yu, and Qing Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10616 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 4, 2018

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ACS Applied Materials & Interfaces

2

In-situ Cesium Modification at Interface Enhances Stability of Perovskite Solar Cells

3

Yao Zhao, Yicheng Zhao, Wenke Zhou, Qi Li, Rui Fu, Dapeng Yu, Qing Zhao*

4

State Key Laboratory for Mesoscopic Physics and Electron Microscopy Laboratory,

5

School of Physics, Peking University, Beijing 100871, China.

6

Collaborative Innovation Center of Quantum Matter, Beijing 100084, China

7

Keyword: Perovskite solar cells, interface modification, stability, ion migration,

8

cesium acetate

9

ABSTRACT:

1

10

A consensus has been reached that organic transport layer (e.g. Spiro-OMeTAD) in

11

perovskite solar cell (PSC) is prone to be impacted by mobile ions in perovskite film

12

during long-term operation. Here we incorporate cesium acetate, as a buffer layer into

13

perovskite solar cells to mitigate this detrimental behavior, in which cesium acetate is

14

sandwiched between perovskite and organic transport layer. The mobile ions that

15

migrate towards organic transport layer (e.g. MA+) are gradually consumed by cesium

16

acetate, resulting in cesium-rich perovskite at the interface. This in-situ reaction and

17

the subsequent Cs incorporation greatly enhance the operational stability of PSC

18

without efficiency loss. The optimized PSC presents power conversion efficiency of 1 ACS Paragon Plus Environment

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20.9% with open-circuit voltage of 1.18 V, maintaining ~80% of its initial efficiency

20

after 4500 min continuous operation at maximum power point (MPP). This new

21

strategy opens up a new opportunity for fabricating stable perovskite solar cells.

22

23

1. INTRODUCTION

24

CH3NH3PbX3 (X=Cl, Br, or I) perovskite possesses many distinct properties, such

25

as tunable bandgap, long carrier diffusion length and long carrier lifetime, making it

26

an ideal material for photovoltaic applications.1-10 Although the power conversion

27

efficiency (PCE) of perovskite solar cells have reached 22.7%,11 the instability issue

28

of PSC still hinders the development of large-area module and further

29

commercialization. Many efforts have been made to protect perovskite itself from

30

moisture, which was thought to be a dominant factor affecting the performance of

31

PSC.12-18 However, moisture-induced perovskite decomposition would be trivial after

32

strict encapsulation, considering commercialized silicon solar cells are also fully

33

encapsulated.

34

Recently, we reported that the organic hole transport layer (HTL) in PSC is

35

physically soft and prone to be destroyed by ion migration in perovskite.19

36

Quantitatively, we observed that the degradation of HTLs by ionic penetration covers

37

about half of the efficiency loss in PSC after few hours’ continuous working under

38

illumination. To overcome this issue, some modification methods based on 2 ACS Paragon Plus Environment

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39

perovskite/HTL interface were reported. Our group used CuSCN as a buffer layer to

40

protect Spiro-OMeTAD from ion migration effect.20 Mei et al. adopted HTL-free

41

device structure and carbon-based electrode to make it immune to ion penetration.21

42

In addition, many efforts have been reported to enhance the stability of PSCs based on

43

invert structure using ion migration-inert compact inorganic HTL.22-27 However,

44

stability improvement based on these methods results in a severe PCE sacrifice

45

because of the weakened hole transporting property.

46

Herein, we demonstrate an effective modification strategy by spin-coating cesium

47

acetate solution on perovskite film to improve the stability of PSC without sacrificing

48

device efficiency. Cesium acetate film would react with perovskite in the first stage to

49

form a Cs-rich perovskite at the surface. Cs incorporation is expected to suppress

50

ionic (e.g. MA+) migration during device operation and therefore protect the organic

51

transporting layer.28 Furthermore, the residue cesium acetate atop the perovskite film

52

can further protect the organic HTL from ionic penetration due to an in-situ reaction.

53

The device with proper modification of cesium acetate shows a PCE exceeding 20%.

54

With MPP tracking under 1-sun illumination, the long-term stability test shows that

55

our optimized device remains no loss of its initial PCE while pristine device lost

56

~20% of its initial PCE after 450 minutes continuous working. We speculate the

57

enhanced MPP stability stems from dual effects of Cs-rich interface and residue

58

cesium acetate at the interface of perovskite and HTL.

59

2. RESULTS AND DISCUSSION 3 ACS Paragon Plus Environment


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Cs0.05FA0.8MA0.15PbI2.85Br0.15 precursor solution was first spin-coated on

61

Glass/ITO/TiO2 substrate by one-step method. After annealing under 100oC for 10

62

min, cesium acetate solution was spin-coated on the as-prepared perovskite film

63

without post annealing process, and then Spiro-OMeTAD was spin-coated on the

64

modified film (Figure 1a). Scan electron microscopy (SEM) is used to study the

65

change of perovskite film morphology before and after cesium acetate modification.

66

Figure 1b shows a uniform and condense perovskite film with well-defined grains.

67

After spin-coating cesium acetate on as-prepared perovskite film, a quite different

68

morphology was observed (Figure 1c), implying a cesium acetate film surface. After

69

exposing the film to ambient air for 2 hours, some bright spots appeared on the

70

surface of perovskite film (Figure 1d). We attribute the bright spots to the cesium

71

acetate since no PbI2 signal is found in the film from XRD analysis (Figure S1). In

72

addition, energy dispersive spectroscopy (EDS) also strongly suggests the bright spot

73

as cesium acetate (Figure S2 and Table S1,S2). From cross-section SEM image

74

(Figure 1e), a thin layer with amorphous structure beneath Spiro-OMeTAD was

75

observed, which might be corresponding to the residue cesium acetate. Cross-section

76

SEM image reveals an uniform perovskite film for pristine solar cells (Figure S3).

77

The result suggests that cesium acetate still remains after spin coating

78

Spiro-OMeTAD in modified sample.

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79

80 81

82

Figure 1. (a) Schematic diagram of experimental procedures of cesium acetate

83

modification. (b) Top view SEM image of pristine perovskite film. (c) Top view SEM

84

image of fresh modified perovskite film. (d) Top view SEM image of modified

85

perovskite film post for 2 hours in ambient air. (e) Cross section SEM image of

86

modified perovskite film after spin-coating Spiro-OMeTAD.



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87

X-ray photoelectron spectroscopy (XPS) was used to analyze surface composition

88

of pristine and modified perovskite film, which is Cs0.07FAxMA0.93-xPbI2.55Br0.45 and

89

Cs0.1FAxMA0.9-xPbI2.55Br0.45 (Figure S4,S5 and Table S3). Cs content is increased at

90

perovskite

91

FAPbI3+xCsCH3COO=CsxFA1-xPbI3+xFACH3COO, which could explain the change

92

of perovskite surface composition.30 This reaction also leads to a blue shift of

93

steady-state photoluminescence (PL) spectra (Figure 2a). Steady-state PL signal peak

94

emitted from perovskite film deposited on glass shifts from 763 nm to 760 nm after

95

modification, which also indicates Cs ratio increased after modification for Cs-rich

96

perovskite has a higher bandgap. However, the blue shift is not observed when the PL

97

signal is collected from the glass side of perovskite film (Figure S6). In addition, we

98

measured reflectance spectra to further illustrate that this modification strategy has no

99

impact on perovskite/glass side (Figure S7,S8), suggesting that the reaction only

100

surface

after

modification.

Jiang

et

al.

proposed

a

reaction

occurs on the surface of perovskite film.

101

To examine the effect of Cs incorporation on band structure, ultraviolet

102

photoemission spectroscopy (UPS) was used to obtain energy band information of

103

perovskite films deposited on ITO/TiO2 substrates. Figure 2b shows that the value of

104

EF-EV decreases from 1.64 eV to 1.45 eV after modification, indicating valence band

105

maximum (VBM) shifts upward to EF. Figure 2c indicates work function

106

(Wf=Evaccum-EF) changes from 4.13 eV to 3.95 eV after modification. One can obtain

107

from UPS that EV position of pristine, modified perovskite and Sprio-OMeTAD is 6 ACS Paragon Plus Environment

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108

-5.77 eV, -5.40 eV and -5.35 eV, respectively (Figure S9). Moreover, we need to

109

clarify that the variation of EV position is originated from the conduction and valence

110

band together shift considering the bandgap of pristine and modified perovskite film

111

is 1.617 eV and 1.627 eV, respectively (Figure S10). We sketched an energy level

112

diagram distribution at perovskite/Spiro-OMeTAD interface for the pristine and

113

modified device (Figure 2e). Energy barrier between perovskite and Spiro-OMeTAD

114

was calculated in this way: the energy band structure would bent after perovskite

115

contacting with Spiro-OMeTAD, and they share the same EF, ∆E = − −

116

− , where EFP, EVP, EFS and EVS are perovskite EF, perovskite EV,

117

Spiro-OMeTAD EF and Spiro-OMeTAD EV, respectively. One can see energy barrier

118

∆E reduced from 0.84 eV to 0.65 eV along with energy band shift, and thus the hole

119

injection efficiency will be enhanced. We further measured time-resolved

120

photoluminescence decay (TRPL) spectra (Figure 2d) to determine carrier lifetime.

121

The carrier life of pristine and modified sample is 408 ns and 306 ns, respectively. We

122

think that the difference stems from the new formed high Cs ratio perovskite

123

containing many deep energy level defects due to lack of annealing process. However,

124

the defects could be screened after we spinning coating Spiro-OMeTAD on it because

125

the (Cs-doped perovskite & cesium acetate)/Spiro-OMeTAD interface defects density

126

is much lower than that of pristine one as we will discuss later.



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127 128

Figure 2. (a) Steady-state PL spectra of pristine and modified perovskite film

129

deposited on glass. (b) UPS spectra of pristine and modified perovskite film. (c)

130

Cut-off edge of pristine and modified perovskite film, Wf refers to work function

131

(Wf=Evaccum-EF). (d) TRPL measurement for pristine and modified perovskite film

132

deposited on glass. (e) Brief energy level diagram of perovskite/Spiro-OMeTAD

133

interface energy band arrangement of pristine and modified device.

134 8 ACS Paragon Plus Environment

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135

To study the effect of varying cesium acetate modification condition on the property

136

of PSC, we used 6 mg/mL cesium acetate isopropyl solution to spin-coat on the

137

as-prepared perovskite film with 5000 rpm, 3000 rpm and 1000 rpm speed,

138

respectively. Photocurrent density (J)-voltage (V) curves (Figure 3a and Figure S11)

139

show that 5000 rpm results in the best photovoltaic performance. Therefore, we used

140

this condition as modification sample. The best-performing modified PSC shows a

141

reverse-scan PCE of 20.9% (VOC =1.18 V, FF= 78%, JSC= 22.7 mA/cm2), while the

142

pristine PSC shows a PCE of 19.7% (VOC=1.17 V, FF= 77%, JSC=21.5 mA/cm2)

143

(Figure 3a). To check the reproducibility of the performance of the pristine and

144

modified PSC, we fabricated 30 devices. The statistics of the PCE measured under

145

reverse scan with scan rate of 0.1 V/s are shown in Figure 3b. The average efficiency

146

for the 30 modified devices is 19.9%, with a VOC =1.17 V, a Jsc =22.36 mA/cm−2, and

147

a FF = 0.76. In addition, pristine PSC has achieved a 19.1% PCE, a Voc = 1.16 V, a

148

Jsc = 21.42 mA/cm−2, and a FF = 0.77 (Table 1). Forward and reverse scan of pristine

149

and modified device (Figure S12-S13) indicated a reduced hysteresis due to the

150

supressed ion migration effect.

151

The external quantum efficiency (EQE) measurements show spectra range from 300

152

nm to 800 nm (Figure S14), with a maximum point at 520 nm. Compared with

153

pristine PSC, modified PSC possesses higher EQE response in the range of 300 to 760

154

nm with the highest response values up to 95%. We ascribe the enhanced EQE to the

155

enhanced hole inject efficiency (Figure S15) and also the analysis above. The 9 ACS Paragon Plus Environment


156

integrated current density based on the EQE measurement is 21.3 mA/cm2 (pristine)

157

and 22.5 mA/cm2 (modified), which is in good agreement with the JSC values from

158

J-V measurement. Figure 3c is the cell performance under MPP tracking with bias

159

voltage 0.92 V in the initial 100 s, and the initial PCE is 19.57% and 18.72% for

160

modified and pristine PSC, respectively.

161 25

(b)

20

24

(c)

Pristine

20

Modified

PCE (%)

10

20 15

16

Pristine Modified

15

C ount

(a)

Current Density (mA/cm2)

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Pristine Modified

10 5

5 0 0.0

162

4

0.2

0.4

0.6

0.8

1.0

1.2

0 18

0

19

20

21

22

Voltage (V)

0

50

100

Time (s)

PCE (%)

163

Figure 3. (a) J-V curve of champion pristine and modified PSC. (b) Statistics PCE

164

distribution of 30 pristine and modified devices. (c) Steady-state stability test of the

165

pristine and modified PSC under bias voltage 0.92 V in an N2-filled glove box at

166

room temperature.

167 168

Table l. Photovoltaic parameters based on the average of 30 pristine and modified

169

devices.

PSC

PCE (%)

FF (%)

VOC (V)

JSC (mA/cm2)

Pristine

19.13

76.9

1.16

21.42

Modified

19.93

76.3

1.17

22.36 10

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170 171

172

Long-term operational stability tests were performed on pristine and modified PSCs

173

under MPP tracking with bias voltage 0.92 V and continuous 1-sun illumination in

174

glove box at room temperature (Figure 4a). Specifically, 80% PCE from staring point

175

is maintained for modified PSC after 4500 min, which is much better than pristine

176

PSC. We also prepare a “modified+annealing” sample, which is the modified PSC

177

followed by an annealing process to removed residue cesium acetate, to further make

178

clear the mechanism behind the enhanced stability. As is shown in Figure 4a,

179

“modified+annealing” PSC only shows a little improvement compared to pristine

180

PSC, but still worse than the modified PSC.

181

As our previous studies suggested, when PSC is illuminated, organic cation is prone

182

to penetrate into Spiro-OMeTAD under electric field (Figure 4b) and PSCs will

183

degrade soon.19 We also demonstrate that Cs+ could be pinning points that suppress

184

ion migration in perovskite,20,33 which is beneficial to improving the device stability.

185

Stability of “modified+annealing” PSC improved compared with pristine one because

186

the high Cs ratio at interface mitigates the organic cation migration (Figure 4c, 1).

187

However, since the “modified+annealing” PSC experiences an annealing process,

188

there

189

perovskite/Spiro-OMeTAD interface is not good enough to fully inhibit ion

190

migration. In contrast, as for the modified PSC, on one hand, high Cs ratio at interface

is

not

much

CsCH3COO

left

at

the

interface,

therefore,

the



Page 12 of 31

191

suppresses ion migration (Figure 4c, 1). On the other hand, residue CsCH3COO tends

192

to consume these unblocked mobile ions at the interface via a reaction route of

193

FA+/MA++CH3COO-=(FA/MA)CH3COO (Figure 4c, 2). The reaction is further

194

proved by a real-time video carried out in the lab (supporting video 1). This process

195

plays a vital role in stabilizing mobile ions in PSC during continuous operation. Since

196

FACH3COO and MACH3COO are not mobile molecule, together with the Cs pining

197

effect, ion migration was further supressed, resulting in an excellent MPP stability of

198

modified PSC.

199

We designed another experiment to illustrate Cs-doped layer and cesium acetate

200

can be also used as a block layer to suppress organic cation migration into HTL. XPS

201

measurement is used to study the elemental distribution in HTL of pristine and

202

modified device after 20 hours’ operation (denoted as “operated pristine device” and

203

“operated modified device”). The results and detailed analyses can be found in Figure

204

S16.

205


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206 207

Figure 4. Operational stability under MPP tracking of (a) Pristine, modified and

208

“modified+annealing” PSC under 0.92 V and continuous light illumination in glove

209

box. (b) Schematic diagram of pristine PSC before and after stability test. When

210

exposed PSC under illumination, organic cation would go into HTL easily since the

211

ion migration barrier energy is small. (c) Schematic diagram of modified PSC before

212

and after stability test. After modification, 1: Cs would suppress ion migration in the 13 ACS Paragon Plus Environment


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213

first place and 2: residue cesium acetate will react with unblocked organic cation

214

driving by election field at interface. These combined effects enhanced the stability of

215

modified PSC.

216

To verify that the defects could be screened after we spinning coating

217

Spiro-OMeTAD onto modified perovskite film, impedance spectrum (IS) was used.

218

IS is a well-established and widely used technique in many type solar cells.34-37

219

Capacitances and resistances parameters derived from IS in photovoltaic devices, can

220

be decoupled by analyzing the frequency-dependent alternating-current response with

221

appropriate equivalent circuits. The equivalent circuit (Figure S17) is used for curve

222

fitting, where the subscripts ext, rec, i, geo, and inter stand for external,

223

recombination, internal, geometry, and interface, respectively. The radius of the

224

semicircle increases along with the negative bias voltage absolute value increases for

225

both fresh pristine and modified solar cells (Figure S18a,c), corresponding to a

226

widened space charge region. However, after 20 hours’ operation in nitrogen filled

227

glovebox, pristine solar cells presented the opposite tendency (Figure S18b) while

228

modified one retained its initial tendency (Figure S18d). We also observed negative

229

capacitance at -0.4 V in pristine cells after 20 hours’ operation at around 1 kHz

230

(Figure 5a). But in modified cells at same condition, the negative capacitance did not

231

appear (Figure 5b). As previous study suggested, negative capacitance is related to

232

sub-bandgap defects induced by ion migration induced degraded solar cells.19,38-40 We


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233

also analyzed density of defect states based on angular-frequency-dependent

234

capacitance by using the equation below :

235

= −

(1)

236

where Vbi, W, and Vapp represent the built-in voltage, the width of the space charge

237

region, and the applied voltage, respectively.41,42 The frequency ω can be converted

238

into the energy level of defects (Ea) using the expression below43:

239

= exp −" ⁄#

240

Finally, one can obtain the defects state energy distribution43-47 under different

241

(2)

applied bias voltages as

242

" = −

243

From Figure 5c,d, we can conclude that defects density of modified device is

244

lower compared to that of pristine one, which illustrates the (Cs-doped & cesium

245

acetate)/Spiro-OMeTAD owns a better interface than perovskite/Spiro-OMeTAD

246

to screen defects in perovskite. Moreover, after 20 hours’ operation, deep level

247

defects density of pristine device increases obviously as degraded solar cell.19

248

However, for modified device, only shallow level defects have increased after 20

249

hours’ operation, which is considered to have little effect on the device.19

250

Therefore, the modification strategy could indeed reduce interface defects density

251

and prevent the HTL change of doping level from MA+ migration.

% &

(3)



(b)

Capacitance (µs)

100

Pristine before operation Pristine after operation

10-1 10-2

100

Capacitance (µs)

(a)

10-3

(c)

101

102

103

104

Frequency (Hz）

105

10-1 10-2

10-4 100

106

Pristine before operation Pristine after operation

0.80 0.60 0.40 0.20

102

103

104

Frequency (Hz）

105

106

0.06

0.08

0.10

0.12

0.14

0.16

Modified before operation Modified after operation

0.80 0.60 0.40 0.20 0.00 0.04

Ea (eV)

252

101

(d) 1.00

1.00

0.00 0.04

Modified befor operation Modified after operation

10-3

Nt (1017cm-3eV-1)

10-4 100

Nt (1017cm-3eV-1)

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

Page 16 of 31

0.06

0.08

0.10

0.12

0.14

0.16

Ea (eV)

253

Figure 5. The capacitance-frequency relation at -0.4 V bias in (a) fresh pristine solar

254

cells and pristine solar cells after 20 hours’ operation (b) fresh modified solar cells

255

and modified solar cells after 20 hours’ operation. The arrow in panel a indicates the

256

transition point where the capacitance value becomes negative at −0.4 bias. Note that

257

we use absolute values in panel a and that the transition point indicates a value change

258

from negative to positive. Defect distributions in (c) fresh pristine solar cells and

259

pristine solar cells after 20 hours’ operation (d) fresh modified solar cells and

260

modified solar cells after 20 hours’ operation in nitrogen filled glove box.

261

262

3. CONCLUSION


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263

We developed an in-situ modification method to modify the interface between

264

perovskite active layer and Spiro-OMeTAD. This modification increases Cs ratio at

265

the interface and leaves residue cesium acetate that reduces defects density and

266

stabilizes the unblocked mobile ions of perovskite. The stability of modified PSC is

267

greatly enhanced, retaining 80% of its initial PCE after 4500 min’ MPP tracking. This

268

work offers an effective approach to modifying the interface between perovskite and

269

HTL toward a greatly improved operational stability while maintaining a high PCE of

270

mixed cation perovskite solar cells.

271 272

4. EXPERIMENTAL SECTION

273

4.1. TiO2 Nanocrystal Synthesis

274

The following experiments are all carried out in ambient environment. Firstly, 2 mL

275

TiCl4 (99% Alfa-Aesar) was injected very slowly into 8 mL ethanol with 3000 rpm

276

stirring speed to avoid local overheating of ethanol. The reaction vial was put into

277

ice-water mixture. After 30 min, 40 mL of anhydrous benzyl alcohol was added to the

278

previous solution and stirred for 10 min. The result solution was sealed in a vial and

279

heated in 80oC water bath for about 12 hours. The as-prepared TiO2 nanocrystals were

280

then precipitated from the as-obtained solution by the addition of 200 ml diethyl ether

281

and isolated by centrifugation at 3000 rpm for 2 min. The solid was subsequently

282

washed by adding anhydrous ethanol and diethyl ether (volume ratio 1:5), followed



Page 18 of 31

283

by a same centrifugation process. This washing procedure was repeated twice. The

284

washed TiO2 nanocrystals were dispersed into anhydrous chloroform and anhydrous

285

methanol (1:1 volume ratio).

286

4.2. Solar Cell Fabrication

287

ITO substrates were cleaned with acetone and isopropanol in an ultrasonic bath in

288

sequence. These substrates were then spin-coated with as-obtained TiO2 nanocrystal

289

solution at 3000 rpm for 30 seconds and annealed at 150oC for 30 minutes on a

290

hotplate. For the perovskite layer, the Cs0.05FA0.81MA0.14PbI2.55Br0.45 precursor

291

solution (1.4 M) was prepared with molar ratios of PbI2/PbBr2 and FAI/MABr both

292

fixed at 0.85:0.15, molar ratio of CsI/(FAI+MABr)=0.05:0.95, and the molar ratio of

293

(FAI+MABr+CsI)/(PbI2+PbBr2) was fixed at 1:1. The perovskite films were

294

deposited onto the TiO2 substrates with two-step spin-coating procedures. The first

295

step was 2200 rpm for 10 s with an acceleration of 200 rpm/s. The second step was

296

5000 rpm for 40 s with a ramp-up of 1000 rpm/s. Chlorobenzene (100 µL) was

297

dropped on the spinning substrate during the second spin-coating step at 10 s before

298

the end of the procedure. To form a thick but still smooth perovskite film,

299

chlorobenzene was slowly dropped on the precursor film within 3 seconds to allow

300

sufficient extraction of extra DMSO through the entire precursor film. The substrate

301

was then immediately transferred on a hotplate and heated at 100°C for 10 min. After

302

cooling down to room temperature, PSC modification is accomplished by

303

spin-coating 6 mg/mL cesium acetate isopropyl solution at 5000 rpm speed on the 18 ACS Paragon Plus Environment

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304

as-prepared perovskite film and the hole transport layer (HTL) was then coated on the

305

sample.

306

Spiro-OMeTAD/chlorobenzene (90 mg/mL) solution with addition of 15 mL

307

Li-TFSI/acetonitrile (210 mg/mL), and 40 mL TBP. Finally, 100 nm gold was

308

deposited as an electrode using a thermal evaporator under a pressure of 8*10-6 Pa.

309

4.3. Photovoltaic characterization

The

precursor

of

the

HTL

was

prepared

by

using

1

mL

310

J-V curves were obtained by an Agilent B2912 Series precision source/measure unit

311

and a solar simulator (Solar IV-150A, Zolix). Light intensity was calibrated with a

312

Newport calibrated KG5-filtered Si reference cell. A black mask was used to define

313

the cells’ area. The J-V curves were tested from 1.8 V to -0.1 V with a scan velocity

314

of 100 mV/s (voltage step of 10 mV and delay time of 200 ms). For the MPP test,

315

PCE (t) was measured by setting the bias voltage to the MPP voltage and then tracing

316

the current density in a nitrogen-filled glovebox without encapsulation. The long term

317

stability test at continuous MPP conditions and 1 sun, AM 1.5G illumination was

318

carried out in a nitrogen-filled glovebox at a constant device temperature of 25°C by

319

setting the bias voltage to MPP and tracking the current output. The MPP was updated

320

every 1 s by measuring the current response to a small perturbation in the potential. A

321

420 nm cut off UV filter was applied in front of the solar cells during the MPP

322

tracking tests. The active area of our PSC is 0.049 cm2.

323

4.4. Perovskite film and solar cell characterization.



Page 20 of 31

324

The surface morphology of the perovskite thin film and cross section of the solar

325

cell devices were characterized by scanning electron microscopy (SEM) (Helios600i,

326

FEI) under an electron beam accelerated at 5 kV. PL spectra were measured using a

327

blue laser (470 nm in wavelength) on FLS 920 spectrograph. XPS measurements were

328

conducted on a Thermo Fisher Scientific ESCALAB 250X system. Al Kα (1486.6

329

eV) was used as X-ray source and the analyzer was put at 54.7° relative to the source.

330

UPS measurement is carried out on ESCALAB 250xi. The Valence band (VB) spectra

331

were measured with a monochromatic He I light source (21.2 eV) and a Thermo

332

Scientific ESCALAB 250 XI analyzer. A sample bias of -5 eV was applied to observe

333

the secondary electron cutoff (SEC).

334

EQE measurement process. Firstly, the standard silicon solar cell was required to

335

scan under laser 300-850 nm in Zolix Solar Cell Scan 100 IPCE Measurement

336

System, and the instrument would automatically record data every 5 nm to set a

337

standard. The to be tested device was putted into a self-made box filled with nitrogen

338

gas, and then we put the box into Zolix Solar Cell Scan 100 IPCE Measurement

339

System. We need to adjust laser position to make sure a selected pixel was fully

340

spotted under laser. The scan range was 300-850 nm, and the instrument

341

automatically records data every 5 nanometers. Finally, the software would compare

342

perovskite solar cell with standard silicon solar cell light response data to calculate the

343

EQE response automatically.

344 20 ACS Paragon Plus Environment

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345

■ ASSOCIATED CONTENT

346

Supporting Information

347

XRD, EDS, SEM, XPS, backside PL spectra, reflectance spectra, absorption spectra,

348

PL decay spectra of pristine and modified perovskite film, UPS spectra of Spiro, J-V

349

curve of 1000 rpm and 3000 rpm PSC, J-V curves from forward and reverse scan,

350

EQE of pristine and modified device, XPS analyses for four kinds of designed sample,

351

Impedance spectra of fresh/operated pristine and modified devices, Cs ratio calculated

352

from XPS result.

353

■ AUTHOR INFORMATION

354

Corresponding author: [email protected]

355

■ ACKNOWLEDGMENTS

356

This work was supported by National Natural Science Foundation of China (NSFC

357

51622201, 91733301, 61571015, 51872007).

358

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TOC image

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Norm. PCE (a.u.)

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0.8 Au

0.6

Spiro-OMeTAD Cesium acetate

0.4 Perovskite ITO+TiO2

0.2 0.0 0

547

200

400

600

800

1000

Time (min)


In Situ Cesium Modification at Interface Enhances the Stability of

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