1

An Electron-Rich 2-Alkylthieno[3,4-b]thiophene Building Block with Excellent

Electronic and Morphological Tunability for High-Performance Small-Molecule Solar

Cells

Shengjie Xu, ‡a, c Zichun Zhou, ‡a,c Haijun Fan,a Longbin Ren,a,c Feng Liu, *,b,d Xiaozhang Zhu,*,a,c and

Thomas P. Russelld,e

a Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of

Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: xzzhu@iccas.ac.cn

b Department of Physics and Astronomy, Shanghai Jiaotong University, Shanghai 200240, China. E-mail:

iamfengliu@gmail.com

c University of Chinese Academy of Sciences, Beijing 100049, China

d Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts

01003, United States

e Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United

States

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2016

2

5.10 eV

3.04 eV

: 664.9 nmf : 1.41

5.34 eV

3.03 eV

: 601.3 nmf : 1.24

Fig. S1 DFT-calculated orbital energy diagram and pictorial representation of frontier orbitals for STB-n (STB-1: R1 = n-octyl, R2 = ethyl; STB-2: R1 = EH, R2 = ethyl; STB-3: R1 = EH, R2 = methyl) and the reference compound STB-r (EH = 2-ethylhexyl, HD = 2-hexyldecyl). Calculations were conducted at the DFT//B3LYP/6-31G** level. Alkyl substituents were replaced by methyl groups to simplify the calculations.

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

-2.0x10-5

-1.0x10-5

0.0

1.0x10-5

2.0x10-5

I/A

Potential (V vs Fc/Fc+)

STB-3

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-2.0x10-5

-1.5x10-5

-1.0x10-5

-5.0x10-6

0.0

5.0x10-6

1.0x10-5

1.5x10-5

I/A

Potential (V vs Fc/Fc+)

STB-1

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

-2.5x10-5

-2.0x10-5

-1.5x10-5

-1.0x10-5

-5.0x10-6

0.05.0x10-6

1.0x10-5

1.5x10-5

I/A

Potential (V vs Fc/Fc+)

STB-r

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-2.5x10-5

-2.0x10-5

-1.5x10-5

-1.0x10-5

-5.0x10-6

0.05.0x10-6

1.0x10-5

1.5x10-5

2.0x10-5

I/A

Potential (V vs Fc/Fc+)

STB-2(a) (b)

(c) (d)

Fig. S2 Cyclic voltammogram of (a) STB-1, (b) STB-2, (c) STB-3, and (d) STB-r in diluted CH2Cl2 solution with a scan rate of 100 mV s-1.

3

(a) (b)

(c)

100 200 300 400 50040

50

60

70

80

90

100

Wei

ght(%

)

Temperature(oC)

STR-1

T5%= 364oC

100 200 300 400 50040

50

60

70

80

90

100

T5%= 368oC

Wei

ght(%

)

Temperature(oC)

STB-2

100 200 300 400 50040

50

60

70

80

90

100

T5%= 365oC

Wei

ght (

%)

Temperature(oC)

STB-3

Fig. S3 Thermal gravimetric analysis (TGA) curves of compounds (a) STB-1, (b) STB-2, and (c) STB-3.

Table S1. Photovoltaic performance of STB-n-based solar cells with different thicknesses with device structure of ITO/PEDOT:PSS/active layer/Ca/Ala

Cpd. Thickness (nm)

Voc

(V)Jsc

(mA cm–2)Fill factor

(FF)PCE(%)

STB-1 80 0.935 10.55 0.64 6.30 100 0.938 11.20 0.63 6.61 120 0.938 10.87 0.63 6.42

STB-2 80 0.900 13.82 0.66 7.34100 0.901 13.12 0.66 7.84120 0.906 12.84 0.67 7.79

STB-3 80 0.917 12.85 0.70 8.24100 0.921 13.03 0.71 8.47120 0.923 12.79 0.71 8.38

a With CHCl3 vapor annealing.

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Table S2. Photovoltaic performance of STB-3-based solar cells with different thicknesses with device structure of ITO/PEDOT:PSS/active layer/PFN/Ala

Thickness (nm)

Voc

(V)Jsc

(mA cm–2)Fill factor

(FF)PCE[%]

80 0.925 13.66 0.72 9.09 120 0.928 14.24 0.70 9.26 140 0.933 14.18 0.68 9.01 160 0.931 13.82 0.69 8.87

aWith CHCl3 vapor annealing.

Fig. S4 Tapping-mode AFM height images of optimal blend films cast from chloroform solution: (a) STB-1:PC71BM, the RMS roughness is 0.75 nm. (b) STB-2:PC71BM, the RMS roughness is 0.47 nm. (c) STB-3:PC71BM, the RMS roughness is 0.41 nm. (d) STB-1:PC71BM, with SVA treatment, the RMS roughness is 0.77 nm. (e) STB-2:PC71BM, with SVA treatment, the RMS roughness is 0.49 nm. (f) STB-3:PC71BM, with SVA treatment, the RMS roughness is 0.56 nm. (Atomic force microscope investigation was performed using Bruker MultiMode 8 AFM in “tapping” mode.)

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1.0 1.5 2.0 2.5 3.0 3.530

40

50

60

70

80

90

100

110

120

130

J0.5 / A

0.5 m

-1

Vappl-Vbr-Vbi / V

after SVA before SVA

1.5 2.0 2.5 3.0 3.5

40

60

80

100

120

140

160

180

after SVA before SVA

J0.5 / A

0.5 m

-1

Vappl-Vbr-Vbi / V

1.0 1.5 2.0 2.5 3.0 3.52030405060708090

100110120130

J0.5 / A

0.5 m

-1

Vappl-Vbr-Vbi / V

after SVA before SVA

1.5 2.0 2.5 3.0 3.5

60

80

100

120

140

160

180

after SVA before SVA

J0.5 / A

0.5 m

-1Vappl-Vbr-Vbi / V

1.5 2.0 2.5 3.0 3.540

60

80

100

120

140

160

after SVA before SVA

J0.5 / A

0.5 m

-1

Vappl-Vbr-Vbi / V

1.0 1.5 2.0 2.5 3.0 3.520

30

40

50

60

70

80

90

100

110

120

after SVA before SVA

J0.5 / A

0.5 m

-1

Vappl-Vbr-Vbi / V

(a) (b)

(c) (d)

(e) (f)

Fig. S5 J0.5 vs V plots: STB-1/PC71BM (1:1.5, w/w) hole-only diode (a) and electron-only diode (b), STB-2/PC71BM (1:1.5, w/w) hole-only diode (c) and electron-only diode (d), STB-3/PC71BM (1:1.5, w/w) hole-only diode (e) and electron-only diode (f).

0.01 0.1 1

10

100

STB-1 STB-2 STB-3

J ph (A

m-2)

Veff (V)

Fig. S6 Photocurrent density versus effective voltage (Jph–Veff) characteristics for three devices under constant incident light intensity (AM 1.5G, 100 mW cm−2).

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20 40 60 80 1001

10

J ph

(mA

cm-2

)

Light Intensity (mW cm-2)

=0.973(a) (b)

20 40 60 80 1001

10

J ph (m

A cm

-2)

Light Intensity (mW cm-2)

=0.977

20 40 60 80 1001

10

J ph (m

A cm

-2)

Light Intensity (mW cm-2)

=0.981(c)

20 40 60 80 1001

10

J ph (m

A cm

-2)

Light Intensity (mW cm-2)

=0.992(d)

Fig. S7 Measured Jph of STB-1-based (a), STB-2-based (b), STB-3-based (c) and STB-3-based with PFN cathode interlayer (d) solar cells plotted against light intensity on the logarithmic scale.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-4

-3

-2

-1

0

1

2

Voltage (V)

Curr

ent d

ensi

ty (m

A cm

-2)

STB-r as-cast STB-r SVA

400 500 600 7000

5

10

15

20

25

30

EQE

(%)

Wavelength (nm)

STB-r as-cast STB-r SVA

(a) (b)

Fig. S8 (a) Characteristic density vs voltage (J-V) curves and (b) external quantum efficiency (EQE) curves of STB-r.

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Table S3. Photovoltaic performance of STB-r-based solar cells with device structure of ITO/PEDOT:PSS/active layer/Ca/Al

Cpd. Treatment Voc

(V)Jsc

(mA cm–2)Fill factor

(FF)PCE(%)

STB-r none 1.12 2.51 0.49 1.39 SVAa 1.11 3.25 0.64 2.24

aWith CHCl3 vapor annealing.

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NMR Charts6,6-(4,8-Bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b]dithiophene-2,6-diyl)bis(2-octylthieno[3,4-b]thiophene-4-carbaldehyde) (6)

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6,6-(4,8-Bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b]dithiophene-2,6-diyl)bis(2-(2-ethylhexyl)thieno[3,4-b]thiophene-4-carbaldehyde) (7)

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(5Z,5Z)-5,5-((6,6`-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b]dithiophene-2,6-diyl)bis(2-octylthieno[3,4-b]thiophene-6,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxo-thiazolidin-4-one) (STB-1)

11

(5Z,5Z)-5,5-((6,6-(4,8-Bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b]dithiophene-2,6-diyl)bis(2-(2-ethylhexyl)thieno[3,4-b]thiophene-6,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (STB-2)

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(5Z,5Z)-5,5-((6,6-(4,8-Bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b]dithiophene-2,6-diyl)bis(2-(2-ethylhexyl)thieno[3,4-b]thiophene-6,4-diyl))bis(methanylylidene))bis(3-methyl-2-thioxothiazolidin-4-one) (STB-3)

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(5Z,5Z)-5,5-((5,5-(4,8-Bis(5-(2-hexyldecyl)thiophen-2-yl)benzo[1,2-b:4,5-b]dithiophene-2,6-diyl)bis(thiophene-5,2-diyl))bis(methanylylidene))bis(3-octyl-2-thioxothiazolidin-4-one) (STB-r)

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MALDI-TOF Mass Spectrum of STB-1

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MALDI-TOF Mass Spectrum of STB-2

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MALDI-TOF Mass Spectrum of STB-3