Dopant-free benzothiadiazole bridged hole transport materials for highly stable and efficient perovskite solar cells

Journal Pre-proof Dopant-free benzothiadiazole bridged hole transport materials for highly stable and efficient perovskite solar cells Xiang Zhou, Fantai Kong, Yuan Sun, Yin Huang, Xianxi Zhang, Rahim Ghadari PII:

S0143-7208(19)31232-X

DOI:

https://doi.org/10.1016/j.dyepig.2019.107954

Reference:

DYPI 107954

To appear in:

Dyes and Pigments

Received Date: 28 May 2019 Revised Date:

28 August 2019

Accepted Date: 3 October 2019

Please cite this article as: Zhou X, Kong F, Sun Y, Huang Y, Zhang X, Ghadari R, Dopant-free benzothiadiazole bridged hole transport materials for highly stable and efficient perovskite solar cells, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.107954. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Side groups were introduced on core acceptor to develop efficient dopant-free D-A-D HTMs for PSCs.

1

Dopant-free Benzothiadiazole Bridged Hole Transport Materials for

2

Highly Stable and Efficient Perovskite Solar Cells

3 4

Xiang Zhou Ghadari

a,b

, Fantai Kong a,*, Yuan Sun a, Yin Huang a,b, Xianxi Zhang c, Rahim

d

5

a

6

Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, 230088, P.

7

R. China. E-mail address: [email protected] (F. T. Kong)

8

b

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

9

c

Shandong Provincial Key Laboratory / Collaborative Innovation Center of Chemical Energy

Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Applied

10

Storage & Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng

11

University, Liaocheng, 252000, P. R. China

12

d

13

Chemistry, University of Tabriz, 5166616471 Tabriz, Iran

14

Abstract

15

D-A-D typed hole transport materials have been designed and synthesized with

16

benzothiadiazole acceptor unit by introducing electron-withdrawing group fluorine

17

atoms and electron-donating group alkoxy as side group. It is found that the three hole

18

transport materials have a low-lying HOMO energy level and higher hole mobility.

19

Furthermore, when these materials are applied in perovskite solar cell, the higher hole

20

mobility (1.02×10-3 cm2·V−1·s−1) make the B3, which the fluorine atoms on core

21

acceptor without dopant, get a power conversion efficiency of 12.1%, while

Computational Chemistry Laboratory, Department of Organic and Biochemistry, Faculty of

1

22

spiro-OMeTAD based dopant-free perovskite solar cell only shows a 7.96% efficiency.

23

In addition, the long-term stability of the perovskite solar cells based all the three hole

24

transport materials improved greatly, especially B1-based cells show a favorable

25

long-term stability, which keeps a 90% initial efficiency after 10 days at a relative

26

humidity of 30%. Therefore, the D-A-D typed hole transport materials have a good

27

application prospects in the field of perovskite solar cells with the incorporation of

28

appropriate side group.

29

Keyword: perovskite solar cells, hole transport materials, D-A-D, acceptor, side

30

group

31

1. Introduction

32

Perovskite solar cells (PSCs) have attracted more and more attention, owing to its

33

long hole and electron diffusion lengths, low-cost manufacturing process, and high

34

power conversion efficiencies (PCEs), which has shown an exciting enhancement

35

from 3.8% to 25.2% [1-6]. In PSCs, the hole transport materials (HTM) plays a key

36

role in extracting and transferring holes, suppressing undesired charge recombination

37

losses

38

Donor-Acceptor-Donor (D-A-D) typed hole transport material with alkoxy or fluorine

39

atoms were introduced to increase the stability and photovoltaic performance of PSCs

40

instead of traditional spiro-OMeTAD.

41

In the previous study, Donor-π bridge-Donor (D-π-D) typed molecules with a

and

enhancing

the

stability

[7-9].

In

this

manuscript,

three

2

42

conjugated electron rich unit such as thiophene [10], carbazole [11], truxene [12],

43

indolocarbazole [13], azulene [14], triphenylamine [15], etc., have been extensively

44

developed and applied in PSCs. And the results have confirmed that the conjugated

45

electron rich core unit in D-π-D structure will upraise the highest occupied molecular

46

orbital (HOMO) energy level of the materials [16] and cause stability issues due to the

47

decreased oxidation potential [17]. In contrast, as the electron rich core changes to an

48

electron deficient core, the Donor-Acceptor-Donor (D-A-D) structure can not only

49

obtain lower HOMO energy level and higher hole mobility in organic solar cells

50

[18-20], but also enhance the intrinsic stability of the organic sensitizer for

51

dye-sensitized solar cells [21]. Although there have been some reports [22-25]

52

focused on the effect of different D-A-D HTMS on PSCs performance, the influence

53

of such acceptor core in D-A-D typed HTMs on the stability and photovoltaic

54

performance of PSCs needs further investigation.

55

With this in idea, we designed and synthesized three dopant-free D-A-D typed HTMs

56

B1, B2 and B3 with triphenylamine as the donor unit and benzothiadiazole as the

57

acceptor unit (The structure shown in Fig 1) in this work. In order to improve the

58

device

59

lithium-bis(trifluoromethanesulfonyl)-imide (Li-TFSI) or tert-butylpyridine (t-BP)

60

[26-28], electron-withdrawing group fluorine atoms (B3) and electron-donating group

61

alkoxy (B1) was introduced on the core acceptor in comparison with conversional

62

benzothiadiazole D-A-D HTMs. The device results indicate that the PSCs with the

stability

towards

the

instability

caused

by

the

additives

like

3

63

D-A-D typed HTMs we designed have an improvement in stability and photovoltaic

64

performance.

65

Fig 1. Molecular structures of B1, B2 and B3.

66

2. Results and Discussion

67 68

Fig 2. (a) Absorption and emission spectra in dichloromethane (DCM) solution. (b) Cyclic voltammogram (CV).

69

Table 1. Photophysical and electrochemical properties of B1, B2 and B3.

70 71 72 73 74 75

HTM

λmax[a]/nm

λPL[a]/nm

Eg[b]/eV

HOMO[c]/eV

LUMO[d]/eV

µ/cm2·V−1·s−1

B1

445

635

2.38

-5.23

-2.85

5.25×10-4

B2

485

665

2.23

-5.25

-3.02

7.51×10-4

B3

465

653

2.34

-5.27

-2.93

1.02×10-3

[a]

Absorption and emission spectra were measured in DCM solution. Optical band gap is calculated from the intersection of absorption and emission spectra. [c] HOMO level is obtained from CV with the calibrate of ferrocene, EHOMO = E1/2ox vs. Fc/Fc+ + 0.67 vs. NHE + 4.44 vs. Vacuum. [d] ELUMO = Eg + EHOMO. [b]

76

The normalized UV-Vis absorption and fluorescence emission spectra of B1, B2,

77

and B3 in dichloromethane solution are shown in Fig 2.a. The corresponding data are

78

listed in Table 1. It can be noted that the three materials exhibit two characteristic 4

79

absorption bands. In the lower wavelength region, three materials having an

80

absorption peak at almost the same position of 320 nm, which can be attributed to the

81

π–π* electron transition of the molecular conjugated backbone [29]. In the visible

82

region, the maximum absorption peaks of B1, B2, and B3 is 445, 485, and 465 nm,

83

respectively. This series of peaks can be attributed to the intra-molecular charge

84

transfer (ICT) from the electron rich donor groups to the electron deficient acceptor

85

groups [30]. The peak of B1 at 445 nm shows a blue shift, which can be explained

86

that introducing electron-donating group alkoxy reduces the electron-withdrawing

87

ability of the benzothiadiazole core unit, and eventually weakens the ICT effect.

88

However, when the two fluorine atoms are introduced to the core acceptor, the peak of

89

B3 at 465 nm also shows a blue shift relative to the peak of B2 at 485 nm. Because

90

the atomic radius of fluorine (0.50 Å) is larger than that of hydrogen (0.25 Å), and

91

expected to increase the steric hindrance between the fluorine atoms on the

92

benzothiadiazole acceptor and the adjacent triphenylamine [31]. We attribute the blue

93

shift to the reduced conjugation length of B2 [32]. The optical band gap (Eg) of B1,

94

B2, and B3 calculated from the intersection of the corresponding normalized

95

absorption and fluorescence emission spectra are 2.38, 2.20, and 2.34 eV, respectively.

96

In addition, all of the materials show large stokes shifts (B1: 190 nm, B2: 180 nm, and

97

B3: 188 nm), it represents there is a large structural change between the ground- and

98

the excited-state, which is beneficial for the pore-filling of HTMs [33].

5

99 100 101

Fig 3. (a) PSCs architecture. (b) Scanning Electron Microscopy (SEM) cross-section image of the corresponding PSCs. (c) energy level diagram of the PSCs with the B1, B2 and B3.

102

The electrochemical properties of the materials were investigated by cyclic

103

voltammogram (CV) measurements, as shown in Fig 2.b. We can see that all three

104

materials are displayed a couple of highly reversible redox peaks within the scan

105

range, which can be assigned to the oxidation of triphenylamine unit. The HOMO

106

levels of the B1, B2, and B3 calculated from the CV data are -5.23, -5.25, and -5.27

107

eV, respectively. The lowest HOMO level of B3 indicates that the incorporation of the

108

strong electron-withdrawing group can reduce the HOMO level of the molecule.

109

While introducing the electron-donating group alkoxy lead the HOMO level of B1

110

increase slightly. The corresponding LUMO levels of the B1, B2, and B3 are -2.85,

111

-3.02, and -2.93 eV, which are calculated from ELUMO = Eg + EHOMO. We find that the

112

LUMO levels of three HTMs are higher than the conduction band (CB) of

113

CH3NH3PbI3 (-3.93 eV), which could block the electron transfer from perovskite to

114

Au and hence prevent the interfacial charge recombination effectively.

6

115 116 117

Fig 4. Square root of current density - Voltage curves obtained from the hole-only devices (FTO/PEDOT: PSS/HTM/Au).

118 119 120

Fig 5. (a) Optimized ground state geometry and (b) HOMO and LUMO orbitals of B1, B2 and B3.

121

To clear the effect of the different electron-withdrawing ability of the core

122

acceptor units on the hole mobility of these D-A-D HTMs, we developed the 7

123

hole-only devices and measured by the space-charge-limited current (SCLC)

124

according to previous reports [34]. Fig 4 shows the measured J-V curves, the recorded

125

curves were fitted in space charge limited current (SCLC) region [35] and calculated

126

by Mott-Gurney law [36, 37]. The hole mobility value for B3 was 1.02×10-3

127

cm2·V−1·s−1, which is higher than that of B1 （5.25×10-4 cm2·V−1·s−1） and B2

128

（7.51×10-4 cm2·V−1·s−1）. All three D-A-D typed materials show a higher hole

129

mobility, indicating that there is a strong ICT effect between the triphenylamine arms

130

and the core acceptor. This can be confirmed by further studies of electronic structures

131

and distribution, which were calculated by the density functional theory (DFT) with

132

the Gaussian 16 program package. The corresponding HOMO and LUMO of B1, B2

133

and B3 are shown in the Fig 5. It can be found that the electron distribution is similar

134

for three materials due to their similar D-A-D structure. As we can see the HOMO

135

spread over the triphenylamine arms and the core acceptor, while the LUMO

136

primarily located on the core acceptor. Obviously, the existence of core acceptor

137

induces the ICT effect and that enhances the hole mobility. Furthermore, the lowest

138

hole mobility value of B1 could be attributed to the introduction of electron-donating

139

group alkoxy, thereby reduces the electron-withdrawing ability of the acceptor unit in

140

the D-A-D structure, which results a weaken of ICT effect. In addition, the dihedral

141

angles between two triphenylamine arms and core acceptor of B1 is -44.7° and 47.1°,

142

which is too much higher than that of B2 and B3. While the increase in dihedral angle

143

caused by the introduction of the alkoxy will lead to a worse π-π stacking [38]. So the 8

144

lower hole mobility of B1 could be a combination of worse π-π stacking and weak

145

ICT effect. Furthermore, even though the dihedral angle between triphenylamine and

146

core acceptor of B3(-38.7° on both sides) is a little bigger than that of B2 (-35.6° on

147

both sides), the stronger electron-withdrawing ability of core acceptor, which impact

148

the hole transport stronger than π-π stacking, make a better hole mobility for B3.

149

Therefore, we can effectively regulate the hole mobility of D-A-D typed HTMs by

150

introducing side groups to change electron-withdrawing ability of core acceptor and

151

π-π stacking.

152 153

Fig 6. The steady-state PL spectra of perovskite and with the B1, B2 and B3.

154

To verify the superior hole transport properties of the HTMs and identify the

155

influence of different side groups on the hole extraction and transportation properties

156

at the perovskite/HTM interface, we investigated the perovskite/HTM interface by 9

157

steady-state and time-resolved photoluminescence (PL) measurements. The

158

steady-state PL of the perovskite with/without HTMs is shown in Fig 6. Upon being

159

excited at 473 nm, a strong emission spectrum of the pristine perovskite was observed

160

at 778 nm, while all the perovskite/HTM bilayers show a dramatic quenching.

161

Furthermore, we can find that all bilayers show a blue shift, which can be explained

162

by the chemical interaction between MAPbI3 and HTM [12]. In fact, to some extent,

163

the PL quenching efficiency represents the separation efficiency of different HTMs in

164

MAPbI3/HTMs interface. As depicted in Fig 6, B1, B2 and B3 have a quenching

165

efficiency of 80, 90 and 93%, respectively. The result demonstrates that the

166

hole-electron separation efficiency in MAPbI3/B3 interface is the highest, which

167

results from the core acceptor of B3 with a highest electron-withdrawing ability.

168

Meanwhile, There has a lowest hole-electron separation efficiency in MAPbI3/B1

169

interface, which can be assigned to the introduce of the alkoxy reduce the hole

170

mobility of the HTM. By the way, we can see the steady-state fluorescence intensity

171

of B1 is too high before the characteristic fluorescence peak of perovskite, that's

172

because the B1 material has a strong fluorescence performance, and a strong

173

fluorescence peak appear at 620 nm when excitation wavelength is 473 nm.

174

Furthermore, the time-resolved photoluminescence (Fig S3) measurement shows that

175

the PL decay time (te) is 12 ns, 9.9 ns and 7.6 ns for perovskite/B1, B2 and B3

176

interface, respectively. Which is consistent with the steady-state photoluminescence.

177 178

Table 2. The photovoltaic performance of PSCs with pristine B1, B2, B3 and spiro-OMeTAD. 10

HTM B1 B2 B3 spiro-OMeTAD

Jsc (mA cm2) 16.96 18.58 20.90 20.76

Voc (V) 0.90 0.92 0.91 0.88

FF (%) 44 60 64 43

PCE (%) 6.95 10.3 12.1 7.96

179

180 181

Fig 7. (a) J-V curves of the based-B1, B2, B3 and spiro-OMeTAD dopant-free PSCs. (b) Corresponding IPCE spectrum of the PSC devices.

182

To compare the performance of the B1, B2 and B3-based PSC devices under AM

183

1.5G illumination (100 mW·m-2), we further applied the three materials to PSCs.

184

Generally, while the hole mobility of HTM is up to 10-4 -10-3 cm2·V−1·s−1 [39], there

185

is no need to add the dopants like Li-TFSI or t-BP which can results moisture into the

186

PSCs and decompose perovskite materials [26-28]. Therefore, the HTMs used in the

187

following PSCs are all with no dopant. The corresponding J-V curves are depicted in

188

Fig 7.a, and these key parameters are shown in Table 2. B1-based device achieved a

189

PCE of 6.95%, with a Voc of 0.90 V, a Jsc of 16.96 mA cm−2, and a FF of 0.44. While

190

the B2-based device yield a PCE of 10.3% (with a Voc of 0.92 V, a Jsc of 18.58 mA

191

cm−2, and a FF of 0.60), which both of B1 and B2-based devices’ PCE are lower than

192

that of B3-based device (a PCE of 12.1%, with a Voc of 0.91 V, a Jsc of 20.9 mA cm−2,

193

and a FF of 0.64). Therefore, we can see that the photovoltaic performance of PSCs is

194

closely related to the core acceptor of D-A-D typed hole transport materials.

195

Compared with B2-based device, the lower Jsc (was confirmed by the incident

196

photon-to-current efficiency (IPCE), as shown in Fig 7.b) and the FF of B1-based

197

PSCs can be attributed to the lower hole mobility of B2 [14, 40], and is also related to

198

the resistance of the corresponding devices. It has been confirmed that the FF is 11

199

highly dependent on the photoactive layer/electrode interface, which can be reflected

200

by the series resistance (Rs) and shunt resistance (Rsh). In general, a lower Rs and

201

higher Rsh may lead to a higher FF.[41]. In this work, Rs and Rsh were directly

202

extracted from the J-V curves [42]. The Rs of B1-based device is 228 Ω cm2, which is

203

much higher than that of B2-based device (113 Ω cm2). And the corresponding Rsh of

204

B1-based device (2.4 KΩ cm2) is lower than that of B2 (8.2 KΩ cm2), which indicate

205

that less charge recombination and reduced leakage current in the devices with B2

206

[43]. These results well explain the higher FF of B2-based devices compared with B1.

207

In addition, the increase of Rs not only affects FF, but also reduces the Jsc of the

208

devices [44]. On the other hand, the highest PCE of B3-based PSCs can be ascribed to

209

the higher fill factor and photocurrent. The results indicate that the introduction of

210

appropriate side groups can effectively improve the PSCs performance, which

211

through conscious regulate the electron-withdrawing ability of core acceptor in

212

D-A-D typed HTMs. By the way, under the same conditions, spiro-OMeTAD-based

213

PSCs device without dopant only got a PCE of 7.96%. Therefore, the D-A-D typed

214

HTMs we designed and synthesized have a good application prospect in the field of

215

perovskite in the future.

12

216 217 218

Fig 8. Nyquist spectra of the PSCs with B1, B2 and B3 at 0.8 V under dark condition and the corresponding equivalent circuit.

219

To investigate the charge transfer and recombination process in the PSC devices

220

with different HTMs, we further performed the electrochemical impedance

221

spectroscopy (EIS) under dark condition. Fig 8 shows the EIS Nyquist plots of the

222

PSCs with different HTMs at 0.8 V. In the EIS spectra, the arc in the middle

223

frequency region (about 10-100 kHz) is related to the recombination resistance (Rrec)

224

[45, 46]. Hence, the B3-based devices show a higher Rrec than B1 and B2-based,

225

which indicates that the charge recombination rate in the B3-based devices is slower

226

than that in the other two devices.

13

227 228 229

Fig 9. Stability of the PSCs based on the B1, B2 and B3 (25℃, 30% RH in air environment ).

14

230 231

Fig 10. The water contact angle of B1 (a), B2 (b), B3 (c) and spiro-OMeTAD (d) surfaces.

232

Finally, we tested the long-term stability of the corresponding PSCs under dark

233

condition (with a relative humidity of 30% and temperature of 25 ℃ in air

234

environment). After 10 days, B2 and B3-based PSCs maintains 85% and 79% of its

235

initial efficiency, respectively, which is less than that of B1-based device (90%) owing

236

to the hydrophobic properties of alkoxy. It is worth to mention that the PSCs based

237

three materials we designed all have a better stability than spiro-OMeTAD, which

238

reduce efficiency by 34% under the same conditions. As the HTMs is on top of the

239

PSC device, the hydrophobic nature of HTMs is closely related the stability of PSCs

240

and plays an important role in preventing water invade [26, 47]. In general, a larger

241

water contact angle means a better hydrophobic property, thus increasing device

242

stability. So we measured the contact angle of water on the HTMs and the results are

243

shown in Fig 10. The results are consistent with the long-term stability test. The water

244

contact angle of B1 (98.1°), B2 (89.4°) and B3 (87.2°) are all higher than that of

245

spiro-OMeTAD (76.2°). In particular, the maximum contact angle of B1 (98.1°)

246

which introduced a hydrophobic alkoxy group, indicating that we can further 15

247

strengthen the hydrophobic property of D-A-D typed HTMs by introduce appropriate

248

groups. In addition, we have test the device stability under the thermal tress, under

249

85 ℃ thermal stress, although the efficiency of the corresponding devices decreases

250

rapidly (as shown in Fig S2), B1-based perovskite solar cells still shows better

251

stability than that of B2 and B3.

252

3. Conclusions

253

In summary, three D-A-D typed HTMs (B1, B2 and B3) has been designed and

254

synthesized with different side groups on benzothiadiazole acceptor unit. It’s found

255

that the long-term stability of the PSCs based three materials we designed all

256

improved greatly which ascribed the hydrophobic nature of HTMs. And the changed

257

electron-withdrawing ability of acceptor unit and π-π stacking by the introduction of

258

electron-withdrawing group fluorine atoms or electron-donating group alkoxy, can

259

effectively regulate the hole mobility of the HTMs. The dopant-free B3-based PSCs

260

get a PCE of 12.1% and when introduce electron-donating group alkoxy, B1-based

261

PSCs shows an impressive long-term stability which contain 90% of its initial

262

efficiency after 10 days at a 30% relative humidity. The results demonstrate the

263

D-A-D typed HTMs have a very good prospect substitute spiro-OMeTAD and

264

indicate that the design direction of D-A-D typed HTMs in the future, not only the

265

electron-withdrawing ability of the core acceptor should be improved, but also the

266

dihedral angle between the core acceptor and the triphenylamine arms should be

16

267

considered to reduce as much as possible which is advantageous to the π-π stacking.

268

In addition, introducing hydrophobic groups like alkoxy into the D-A-D typed

269

molecules can get a device with excellent long-term stability.

270

Acknowledgments

271

This work was financially supported by the National Key R&D Program of China

272

(2018YFB1500101),

273

2015CB932200), CAS-Iranian Vice Presidency for Science and Technology Joint

274

Research Project (No. 116134KYSB20160130).

National

Basic

Research

Program

of

China

(No.

275 276

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277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296

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Highlights 1) Three dopant-free D-A-D typed HTMs with benzothiadiazole core were developed. 2) Introducing fluorine or alkoxy in benzothiadiazole will hinder the π-π stacking. 3) Long-term stability of PSCs with D-A-D typed HTMs is enhanced greatly. 4) Increasing electron-withdrawing ability of core may improve hole mobility.