Synthesis and Photovoltaic Properties of a Series of Narrow Bandgap

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Synthesis and Photovoltaic Properties of a Series of Narrow Bandgap Organic Semiconductor Acceptors with Absorption Edge Reaching 900 nm Xiaojun Li, He Huang, Haijun Bin, Zhengxing Peng, Chenhui Zhu, Lingwei Xue, Zhi-Guo Zhang, Zhanjun Zhang, Harald Ade, and Yongfang Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03928 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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Chemistry of Materials

Synthesis and Photovoltaic Properties of a Series of Narrow Bandgap Organic Semiconductor Acceptors with Absorption Edge Reaching 900 nm

Xiaojun Li,a b He Huang, b Haijun Bin, a b Zhengxing Peng,d Chenhui Zhu,e Lingwei Xue,a Zhi-Guo Zhang,a Zhanjun Zhang, b *Harald Ade,d *Yongfang Li a b c *

a

CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key

Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China; b

School of Chemistry and Chemical Engineering, University of Chinese Academy of

Sciences, Beijing 100049, China; c

Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical

Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China; d

Department of Physics and ORaCEL, North Carolina State University, Raleigh, North

Carolina 27695, USA; e

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley,

California 94720, USA. E-mail: [email protected] (Y. F Li), [email protected] (H. Ade), [email protected] (Z. J. Zhang)

ABSTRACT Three n-OS acceptors with Eg < 1.4 eV was synthesized by introducing double bond π-bridges in ITIC (ITVIC) and with monofluorine (ITVfIC) or bifluorine (ITVffIC) substituents on its end groups, and the structure-properties relationship of the acceptors were systematically studied. The three n-OS films show broad absorption covering the wavelength range of 550~900 nm with narrow Eg values of 1.40 eV for 1

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ITVIC, 1.37 eV for ITVfIC and 1.35 eV for ITVffIC. Additionally, the fluorine substitution down-shifted the HOMO and LUMO energy levels of the compounds. The photovoltaic properties of the n-OS acceptors were investigated by using a medium bandgap conjugated polymer J71 as donor. The optimized PSCs based on J71:ITVffIC demonstrated a power conversion efficiency (PCE) of 10.54% with a high Jsc of 20.60 mA cm-2 and a Voc of 0.81 V, and the highest Jsc reached 22.83 mA cm-2. The high Jsc values of the devices could be attributed to the broad absorption and lower-lying HOMO energy levels of the acceptor. In considering the Voc of 0.81 V and the narrow bandgap of 1.35 eV for the acceptor ITVffIC, the energy loss (Eloss) of the ITVffIC-based PSCs was reduced to 0.54 eV which is the lowest in the PSCs with PCE higher than 10%. The results indicate that ITVffIC is a promising narrow Eg acceptor for the application in the tandem and semitransparent PSCs.

INTRODUCTION Polymer solar cells (PSCs) with p-type conjugated polymer as donor and fullerene derivative (such as PCBM) or n-type organic semiconductor (n-OS) as acceptor, have drawn considerable attention as an emerging energy conversion technology in recent years1-5, due to their advantages of simple device structure, light weight and capability to be fabricated into flexible and semitransparent devices. In the development of the PSCs, the design and synthesis of high-performance donor and acceptor photovoltaic materials play crucial role6-14. Especially, in the last two years, the development of narrow bandgap (Eg) A-D-A structured n-OS small molecule acceptors have promoted a rapid increase of the power conversion efficiency (PCE) of the PSCs to over 12%15-17. In comparison with traditional fullerene derivative acceptors, the advantages of the A-D-A structured n-OS acceptors include good morphology stability and the easy tuning of their electronic energy levels and absorption spectra by using different electron-donating (D) and electron-accepting (A) units in the molecules. For example, the n-OS acceptor ITIC, with a fused-ring central donating unit and two INCN (2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene) propanedinitrile) accepting end groups, 2


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possesses strong and broad film absorption in the wavelength region from 560 to 780 nm with an optical Eg of 1.58 eV and suitable LUMO and HOMO energy levels for the application as acceptors.18 By selecting medium or broad bandgap conjugated polymers as donor, PCE of the PSCs with ITIC as acceptor reached 10~12%6,19-21. Nevertheless, there is still some room for the acceptors to further extend its absorption to even longer wavelength (such as 900 nm) in the near-infrared (NIR) region for matching with the solar spectrum and enhancing harvest of solar light. In trying to reducing energy bandgap (Eg) of the photovoltaic materials for increasing short circuit current density (Jsc) of the PSCs, the energy level matching of donor and acceptor materials should be carefully considered. The open circuit voltage (Voc) of the PSCs is proportional to the energy difference between the LUMO (the lowest unoccupied molecular orbital) of the acceptor and the HOMO (the highest occupied molecular orbital) of the donor. Obviously, up-shifting LUMO level of acceptor and down-shifting HOMO level of donor could increase Voc of the PSCs. However, the shift of the energy levels is limited by the request of enough LUMO and HOMO energy offsets (ΔELUMO and ΔEHOMO) between the donor and acceptor materials which should be larger than exciton binding energy of the organic semiconductor for efficient charge separation, and the Voc value of the PSCs is limited by the lowest Eg among the donor and acceptor materials and the devices energy loss (Eloss) which is usually larger than 0.6 eV. Therefore, it is very important to tune the LUMO and HOMO energy levels of the photovoltaic materials when reducing its Eg values for simultaneously obtaining high Jsc and high Voc. Very interestingly, the recently reported high performance PSCs with narrow bandgap n-OS ITIC acceptor displayed high Jsc and high Voc simultaneously, benefitted from the high efficiency charge separation even with a ΔEHOMO as small as less than 0.1 eV and low Eloss of ca. 0.6 eV6. However, detailed studies related to the effect of the energy levels of the narrow bandgap acceptors on the photovoltaic performance of the PSCs are still seldom. Here, we modified the molecular structure of ITIC and synthesized three low bandgap A-D-A structured n-OS acceptors by introducing double bond π-bridges 3



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between its donating and accepting units for further reducing its Eg22-24 and attaching fluorine substituents on its accepting end groups to tune (down-shift) its energy levels7, 20

, for enhancing the NIR light harvest and studying the effect of the energy levels of

the acceptor on the photovoltaic performance of the PSCs.

RESULTS AND DISCUSSION Synthesis and Thermal Stability Figure 1 shows the molecular structures and synthetic routes of the three acceptors ITVIC, ITVfIC and ITVffIC. The introduction of the double bond into IT-CHO to get ITV-CHO was accomplished by using tributyl (1,3-dioxolan-2-ylmethyl) phosphonium bromide and NaH at room temperature, and quenched by 10% HCl, with a high yield of 87%, then Knoevenagel condensation of ITV-CHO with compound 1, 2, 3 in chloroform afforded the acceptors ITVIC, ITVfIC and ITVffIC respectively in high yield. The synthesis details were described in the Experimental section. These three compounds exhibit good solubility in organic solvents. The

thermal

stability

of

these

compounds

was

investigated

using

thermogravimetric analysis (TGA), as shown in Figure S1 in SI. The TGA plots of ITVIC, ITVfIC and ITVffIC show decomposition temperature at 5% weight loss (Td) of 335 °C, 324°C and 324°C respectively, which indicates that the thermal stability of these three compounds are good enough for the application in PSCs.

4


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Figure 1. a) Chemical structures of the n-OS acceptors and the polymer donor J71. b)

Synthetic routes for ITVIC, ITVfIC and ITVffIC.

Absorption Spectra, Electronic Energy Levels and Electron Mobilities Figure 2a and Figure 2b show absorption spectra of ITVIC, ITVfIC and ITVffIC in chloroform solutions and in thin films respectively. The solution absorption spectra of the three acceptors (see Figure 2a) show similar absorption bands in the wavelength range from 550 to 800 nm with a little red-shift from ITVIC to ITVfIC with monofluorine substitution and to ITVffIC with bifluorine substitution on its end groups. In the films, the three n-OS acceptors all show greatly red-shifted and broadened 5



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absorption spectra than those of their solutions, with a broad absorption band covering the wavelength range of 550~900 nm (see Figure 2b). The absorption spectrum of ITVIC film is significantly red-shifted with an absorption edge at 885 nm corresponding to a Eg of 1.40 eV in comparison with the absorption edge at 780 nm and Eg of 1.58 eV for ITIC film, which should be attributed to the introduction of double band π-bridge in the molecule. The fluorination further red-shifted the film absorption edges to 900 nm (Eg = 1.37 eV) for ITVfIC and to 915 nm (Eg = 1.35 eV) for ITVffIC. The optical properties of the n-OS acceptors were listed in Table 1 for a clear comparison. Table 1. Physicochemical Properties of J71 Polymer Donor and the n-OS Accepters.

a

λmaxa

λedgea

Egb

EHOMOc

ELUMOc

EHOMOd

(nm)

(nm)

(eV)

(eV)

(eV)

(eV)

J71

580

632

1.96

-5.40

-3.24

ITVIC

755

885

1.40

-5.46

-3.97

-5.56

ITVfIC

780

900

1.37

-5.56

-4.01

-5.60

ITVffIC

780

915

1.35

-5.58

-4.04

-5.65

Absorption of the films. b Calculated from the absorption edge of the polymer films: Eg = 1240/λ

edge.

c

Calculated according to the equation ELUMO/HOMO = −e (Ered/ox + 4.36) (eV). d Measured from

the ultraviolet photoelectron spectroscopy (UPS)

The LUMO and HOMO energy levels of ITVIC, ITVfIC and ITVffIC were measured by electrochemical cyclic voltammetry with Ag/AgCl as reference electrode and Fc/Fc+ couple used as internal standard, and their cyclic voltammograms were shown in Figure 2c. The HOMO and LUMO energy levels were estimated from the onsets of oxidation and reduction potentials (Eox/red), respectively, according to the equations: EHOMO/LUMO = -e (Eox/red + 4.36) (eV). (Redox potential of Fc/Fc+ is 0.44 V vs Ag/AgCl in our measurement system, and we take the energy level of Fc/Fc+ as 4.8 eV below vacuum.) The EHOMO values of ITVIC, ITVfIC and ITVffIC were calculated to be -5.46, -5.56 and -5.58 eV, ELUMO values are -3.97, -4.01 and -4.04 eV, respectively. Obviously, the ELUMO and EHOMO values of the compounds are 6


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downshifted by the fluorine substitution. In comparison with the EHOMO of -5.48 eV and ELUMO of -3.83 eV for ITIC15, the EHOMO of ITVIC is upshifted a little and its ELUMO is downshifted by 0.14 eV due to the insertion of the double bond π-bridges. Figure 2d displays the LUMO and HOMO energy levels of the three acceptors together with those values of the polymer donor J71 and electrode buffer layer materials. We also used ultraviolet photoelectron spectroscopy (UPS) to verify the EHOMO values obtained from CV measurements, as shown in Figure S2 in SI. The EHOMO values from the UPS measurements are -5.56, -5.60 and -5.65 eV for ITVIC, ITVfIC and ITVffIC respectively (see Table S1 in SI), which are close to the values measured by cyclic voltammetry and show the same down-shifting tendency from ITVIC to ITVfIC and to ITVffIC. (b)

1.0

ITVIC ITVfIC ITVffIC

0.8

Absorbance(a. u.)

Absorbance(a. u.)

(a)

0.6 0.4 0.2 0.0 300

400

500

600

700

800

1.0


0.8 0.6 0.4 0.2 0.0 300

900

400

500

600

700

800

900

1000

Wavelength (nm)

Wavelength (nm)

(d)

(c)

-3.24

Current (a.u.) 0.0

0.2

0.4

0.6

-5.40

0.8

Potential (V vs. Ag/AgCl)

-0.5

0.0

0.5

1.0

-5.46

-5.56

PDINO


-5.1 PSS: PEDOT

ITVffIC

ITO

-4.04 -4.3

ITVIC

-4.7

Fc/Fc+

-4.01

ITVfIC

Current (a.u.)

-3.63 -3.97

J71

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


Al

-5.58 -6.21

1.5

Potential (V vs. Ag/AgCl)

Figure 2. UV−vis absorption spectra of ITVIC, ITVfIC and ITVffIC in (a) chloroform solution and (b) film state, (c) cyclic voltammograms of ITVIC, ITVfIC and ITVffIC, the inset shows the cyclic voltammogram of ferrocene/ferrocenium (Fc/Fc+) couple used as an internal reference, (d) energy level diagram of the materials used in the PSCs. 7



Electron mobility is another important property for the acceptors used in PSCs. We measured the electron mobilities of the acceptors by the space charge-limited current (SCLC) method with the electron only device structure of ITO/ZnO/active layer/PDINO/Al. The measurement results are shown in Figure S3 in SI, and the calculated electron mobilities of ITVIC, ITVfIC and ITVffIC are 2.02×10−4, 1.80×10−4 and 1.13×10−4 cm2 V−1 s−1 respectively, which are close (a little lower) to the electron mobility of ITIC15, and the similar phenomena of slightly decrease of electron mobilities with fluorization can be observed in other system25,28. Photovoltaic Performance In order to investigate photovoltaic performance of the acceptors, we fabricated the PSCs with the medium Eg polymer J71 as donor, the three narrow Eg n-OS compounds as acceptor, ITO/PEDOT:PSS as positive electrode and PDINO/Al as negative electrode. The donor/acceptor weight ratio was optimized to be 1:1.2 and thermal annealing at 160 °C for 2 min was performed for improving the photovoltaic performance of the PSCs. Figure 3(a) shows the current density-voltage (J-V) curves of the PSCs based on J71:n-OS (1:1.2, w/w) with thermal annealing at 160 °C for 2 min under the illumination of AM1.5G, 100 mW/cm2, and the input photon to converted current efficiency (IPCE) spectra of the corresponding devices. The photovoltaic

90

5

80

-2

)

performance data of the devices are listed in Table 2 for a clear comparison.

0

70

J71:ITVIC J71:ITVfIC J71:ITVffIC

-5 -10

60

IPCE (%)

Current Density (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

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-15

50 40

J71:ITVIC J71:ITVfIC J71:ITVffIC

30 20

-20

10 -25 -0.2

0.0

0.2

0.4

0.6

0 300

0.8

400

500

600

700

800

900

Wavelength (nm)

Voltage (V)

Figure 3. (a) J−V curves of the PSCs based on J71:acceptors (1:1.2, w/w) with thermal annealing at 160 °C for 2 min, under the illumination of AM 1.5G, 100 mW cm−2; (b) 8


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IPCE spectra of the corresponding PSCs

The J71:ITVIC-based PSC demonstrated a high Voc of 0.88 V and a moderate PCE of 7.34% with a relatively low short circuit current density (Jsc=14.47 mA/cm2), which may be due to too low ΔEHOMO of 0.06 eV (see Table 2) for the donor and acceptor materials in the active layer. For the PSCs based on J71:ITVfIC, Voc was decreased to 0.84 V due to the down-shifted LUMO level of ITVfIC acceptor, while Jsc and PCE were increased significantly to 19.73 mA/cm2 and 9.72% respectively, which could be benefitted from the increased ΔEHOMO to a reasonable value of 0.16 eV. With the further down-shift of the LUMO and HOMO energy levels of the acceptor ITVffIC, the Voc was further decreased to 0.80 V and Jsc was further increased to 22.83 mA/cm2 for the PSCs based on J71:ITVffIC with ΔEHOMO = 0.18 eV. Interestingly, the Jsc values of these devices go up with the increase of ΔEHOMO of their donor and acceptor materials, indicating that larger ΔEHOMO in the range of 0.16~0.18 eV (see Table 2) should be beneficial to the efficient charge separation of the excitons in the acceptors. The IPCE spectra in Figure 3(b) clearly show the contribution of the acceptors absorption to the photocurrent between 600 and 900 nm. The IPCE values of the PSCs with different acceptors in the wavelength range of 600~900 nm increase from ITVIC to ITVfIC to ITVffIC, which is in the same order as the change of ΔEHOMO values in the active layer of the devices. The results reveal that the exciton charge separation efficiency of the n-OS acceptors decreases with the decrease of the ΔEHOMO. The integrated Jsc values from IPCE spectra agree well with the Jsc values measured from J-V curves within 4% mismatch. The morphology of active layers of the PSCs may also influence their photocurrent Jsc. Figure S4 in SI shows the atomic force microscopy (AFM) and transmission electron microscopy (TEM) images of the devices. It can be seen that these devices based on different acceptors display similar morphology with nanostructure network and average surface roughness (Rq) of ca. 0.90 ~ 1.16 nm, which indicate that the large Jsc difference in the PSCs is not from the active layer morphology. 9



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We also measured the charge carrier mobilities of the PSCs with different acceptors by using the SCLC method under the annealed film condition. The hole and electron mobilities were measured with the hole only device of ITO/PEDOT: PSS/active layer/Au and electron only device of ITO/ZnO/active layer/PDINO/Al, respectively, and the results were shown in Figure S5 in SI. The three blend films with J71 as donor and the three n-OS as acceptor exhibited similar hole and electron mobilities. The hole mobilities of the blend films with ITVIC, ITVfIC and ITVffIC as acceptors are 3.66×10-4, 3.62×10-4 and 2.06×10-4 cm2 V–1 s–1, and the electron mobilities are 2.82×10-4, 4.31×10-4 and 2.15×10-4 cm2 V–1 s–1 respectively (see Table S2 in SI), which indicates that the charge carrier mobility is also not the reason for the difference of the Jsc values of the PSCs. These results confirm that the larger ΔEHOMO of 0.18 eV is beneficial to the increased Jsc of the ITVffIC-based PSCs. Table 2. Photovoltaic Performance Parameters of the PSCs Based on J71/Accepters (1:1.2, w/w) with different treatment conditions, under the Illumination of AM1.5G, 100 mW/cm2 FF

PCE

calculated Jsc

ΔEHOMOd

Elosse

(mA cm-2)

(%)

(%)

(mA cm−2)

(eV)

(eV)

0.89

14.47

57.64

7.34

14.36

0.06

0.52

(0.892±0.006)f

(14.13±0.23)

(55.36±0.86)

(7.12±0.16)

ITVfIC

0.84

19.73

58.67

9.72

19.12

0.16

0.53

(TAa)

(0.834±0.003)

(19.66±0.54)

(56.98±1.12)

(9.48±0.21)

ITVffIC

0.80

22.83

52.66

9.61

22.00

0.18

0.55

(TAa)

(0.791±0.002)

(22.33±0.37)

(53.84±1.02)

(9.57±0.21)

ITVffIC

0.78

19.81

49.48

7.64

19.15

0.18

0.57

(as castb)

(0.782±0.001)

(19.04±0.30)

(50.06±0.58)

(7.45±0.12)

ITVffIC

0.81

20.60

63.18

10.54

19.77

0.18

0.54

(19.71±0.60)

(62.28±1.01)

(10.24±0.24)

Acceptor

Voc

(treatment)

(V)

ITVIC (TAa)

(TA+additivec) (0.798±0.004)

Jsc

a) TA: thermal annealing at 160 °C for 2 min; b) as-cast: without post-treatment; c) TA+additive: with 0.5% CN solvent additive treatment and thermal annealing at 160 °C for 2 min; d) HOMO=EHOMOdonor - EHOMOacceptor. e）Eloss=Eg–eVoc, Eg is the lowest energy bandgap of the donor and acceptor components. 10


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It should be mentioned that the device energy loss (Eloss) is quite small for the PSCs with the three narrow bandgap n-OS as acceptors. Eloss is defined as Eloss=Eg– eVoc, where Eg is the lowest energy bandgap of the donor and acceptor components. In the PSCs studied in this work, the n-OS acceptors possess the lowest Eg. According to the Eg values of the n-OS acceptors (Table 1) and the Voc values of the corresponding PSCs (Table 2), the Eloss values were calculated to be 0.52 eV for the J71:ITVIC-based device, 0.53 eV for the J71:ITVfIC-based device, and 0.55 eV for J71:ITVffIC-based PSC, which were also listed in Table 2. The low Eloss values of the PSCs indicate that the three narrow bandgap n-OS molecules are promising NIR acceptor materials for tandem PSCs and semitransparent PSCs. 90 80

0

70

J71:ITVffIC As cast TA SA+TA

-5 -10

60

IPCE (%)

Current Density (mA cm

-2

)

5

-15

50 40

J71:ITVffIC As cast TA SA+TA

30 20

-20

10

-25 -0.2

0.0

0.2

0.4

0.6

0 300

0.8

400

500

Voltage (V)

20 -2

PCE

16

11

11

10

10

9

9

8

8

7

7

PCE

22

18

600

700

800

900

Wavelength (nm)

24

JSC(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


6

6

5

5

4

4

14 12 10 0.3

3

3 0.4

0.5

0.6

VOC(eV)

0.7

0.8

0.9

2

1.10

1.15

1.20

1.25

1.30

1.35

1.40

Eg(eV)

2 0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

Eloss(eV)

Figure 4. (a) J−V curves of the devices with D/A weight ratio of 1:1.2 without extra treatment (As cast), with thermal annealing (TA) and with solvent additive and TA (SA+TA) under the illumination of AM 1.5G, 100 mW cm−2; (b) IPCE spectra of the devices with D/A weight ratio of 1:1.2; Plots of (c) Jsc vs. Voc , (d) PCE vs. Eg. and (e) PCE vs. Eloss. of the PSCs based on the NIR photovoltaic material with Eg < 1.4 eV reported in literatures (the red points in the plots indicate the results from this work). 11



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In considering the high Jsc of 22.83 mA/cm2, relatively high Voc of 0.80 V, but lower FF of 52.66% for the ITVffIC-based PSCs, we carried out further optimization on the device by using 0.5% chloronaphthalene (CN) as solvent additive. Figure 4(a) shows the J-V curves of the PSCs based on J71:ITVffIC (1:1.2, w/w) with different treatment conditions, and the photovoltaic performance data of the devices were also listed in Table 2. The PSC without thermal annealing and solvent additive treatment delivered a PCE of 7.64% with a Voc of 0.78 V, Jsc of 19.81mA/cm2, and FF of 49.48%. After thermal annealing treatment at 160 °C for 2 min, the PCE, Voc, Jsc and FF of the device were improved to 9.61%, 0.80 V, 22.81 mA/cm2 and 52.66% respectively. With the treatment of solvent additive and thermal annealing, the FF value was significantly improved to 63.18% and the Voc was slightly increased to 0.81 V, leading to an improved PCE of 10.54% even though the Jsc was slightly decreased to 20.60 mA/cm2. The Jsc values of the PSCs were confirmed by the IPCE spectra of the corresponding devices, as shown in Figure 4(b). Figures 4(c-e) display the relationship of Jsc vs. Voc, PCE vs. the lowest Eg of the photovoltaic materials and the PCE vs. Eloss of the PSCs based on the NIR photovoltaic materials (donors or acceptors) with Eg < 1.4 eV reported in literatures9,25-37, respectively. Table S3 in SI compared the Voc, Jsc, PCE, Eg and Eloss of the related PSCs with the results of this work. Obviously, the ITVffIC-based PSC demonstrated the highest Jsc of 22.83 mA/cm2 for the device with Voc >0.8 V, and the highest PCE of 10.54% among the PSCs based on the NIR photovoltaic material with Eg < 1.4 eV and with Eloss < 0.6 eV reported in literatures. GIWAXS and Morphology Analysis In order to elucidate some of the effect of thermal annealing and solvent additive on the photovoltaic performance of the acceptors, Grazing Incidence Wide Angle X-ray Scattering (GIWAXS)40 was conducted to determine the molecular packing of ITVffIC-based films with different treatment conditions. 2D patterns and line-cut profiles for neat films are shown in Fig S6 in SI. Molecular stacking distances for neat films and blend films are summarized in Table S4 in SI. For neat ITVffIC film, its lamellar (100) peak is located at 0.44 Å-1 and π-π stacking (010) peak is at 1.65 Å-1, 12


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corresponding to a lamellar distance of 14.27 Å and a π-π stacking distance of 3.81 Å. Figure 5 shows the 2D GIWAXS patterns of the J71:ITVffIC blend films with different treatment conditions (as cast, with thermal annealing, or with solvent additive and thermal annealing). For all the three blend films, (100) peaks are observed in the in-plane direction and (010) peaks are found in the out-of-plane direction (Figure. 5d,e), which suggest the molecules in the blend adopt face-on polymer backbone orientations to the substrates. We note that overall, the diffraction pattern is very weak and neither the polymer nor the n-OS acceptor is well ordered. After thermal annealing with or without solvent additive, π-π stacking distance of J71 is decreased to 3.65 Å (q=1.72 1/Å) from 3.72 Å (q=1.69 1/Å) and coherence length is increased to around 21 Å from 17.6 Å compared to as-cast J71:ITVffIC film. For details of the analysis and fitting, please see Figure. S7 in SI. Tight π-π stacking of polymer chains is known to assist intermolecular charge transport to the electrodes in many systems,38, 39 which can help improve short-circuit current density (Jsc) and fill factor (FF). The molecular packing is comparable after thermal annealing no matter whether the solvent additive is added, which implies the solvent additive doesn’t have much influence on molecular packing and a similar result was also reported.41 The improved device performance after adding the solvent additive may be attributed to the mesoscale morphology associated with phase separation rather than molecular packing.

13



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Figure 5. 2D GIWAXS patterns for J71:ITVffIC (a) without extra treatment (As cast), (b) with thermal annealing (TA), (c) with solvent additive and TA (TA+SA). Line cut of GIWAXS patterns (d) in the In-Plane direction and (e) in the Out-of-Plane direction.

The effect of thermal annealing and solvent additive treatment on the active layer morphologies of the PSCs was studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM), as shown in Figure 6, the AFM images show that the blend films with different conditions are smooth and incremental phase separations were observed for films. The root-mean-square surface roughness values for as-cast, TA and TA+SA films are 1.27 1.16 and 1.43 nm respectively. From the TEM images (Figure 6a-c), compared to those of the as-cast film, a more defined phase separation of donor and acceptor with a bicontinuous interpenetrating network was observed in the film with thermal annealing (TA). As phase segregation increased, geminate recombination may be reduced and the overall yield of charge photogeneration thus improves, which results in higher Jsc. After adding the additive, the phase separation in the J71:ITVffIC blend film was obviously enhanced, which may mean the film has a greater pure phase region. The enhanced phase separation may be beneficial for charge extraction and for effectively suppressing the bimolecular 14


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recombination and achieving higher FF, which result in the improvement of the device performance.

Figure 6. a-c) TEM patterns of the J71:ITVffIC blend films in different condition. d-f) AFM height images of the J71:ITVffIC blend films in different condition.

Conclusions In summary, by introducing double bond π-bridge in ITIC and fluorine substitution on its electron-accepting end groups, we synthesized three new NIR n-OS compounds ITVIC, ITVfIC and ITVffIC, with Eg of 1.40, 1.37 and 1.35 eV, and EHOMO of -5.46, -5.56 and -5.58 eV respectively for the application as acceptor in PSCs. Insertion of vinylene π-bridge broadened the absorption and reduced the bandgap of the n-OS molecules, and the fluorine substitution down-shifted the EHOMO and ELUMO of the compounds. Photovoltaic properties of the three acceptors were studied by using conjugated polymer J71 with EHOMO= -5.40 eV as donor. It was found that the Jsc values of the PSCs with different acceptors were tightly related to theΔEHOMO between the J71 donor and the acceptor. The Jsc value increases from 14.47 mA/cm2 for the ITVIC-based PSC with EHOMO = 0.06 eV to 19.73 mA/cm2 for the ITVfIC-based PSC with EHOMO = 0.16 eV and 22.83 mA/cm2 for the ITVffIC-based PSC with EHOMO = 15



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0.18 eV. The optimized PSC based on J71:ITVffIC demonstrated a high PCE of 10.54 % with Voc = 0.81 V, Jsc = 20.60 mA cm−2, and FF = 63.18%, and a low Eloss of 0.54 eV. The PCE of 10.54% is the highest efficiency reported for the PSCs based on the NIR photovoltaic material with Eg < 1.4 eV. The results indicate that ITVffIC is a promising narrow band gap NIR acceptor for the application in tandem/multi-junction, semitransparent, and ternary PSCs.

Experimental section Measurements: 1H NMR spectra were measured on a Bruker DMX-400 spectrometer with d–chloroform as the solvent and trimethylsilane as the internal reference. UV−visible absorption spectra were measured on a Hitachi U-3010 UV-vis spectrophotometer. Mass spectra were recorded on a Shimadzu spectrometer. Thermogravimetric analysis (TGA) was conducted on a Perkin-Elmer TGA-7 thermogravimetric analyzer at a heating rate of 20 °C/min and under a nitrogen flow rate of 100 mL/min. UV-vis absorption spectra of active layers of the all PSCs were measured on a Hitachi U-3010 UV-vis spectrophotometer. The electrochemical cyclic voltammetry was performed on a Zahner IM6e Electrochemical Workstation, in an acetonitrile solution of 0.1 mol/L n-Bu4NPF6 at a potential scan rate of 100 mV/s with an Ag/AgCl reference electrode and a platinum wire counter electrode. The film morphology was measured using an atomic force microscope (AFM, SPA-400) using the tapping mode. Device fabrication and characterization: The PSCs were fabricated with a structure of ITO/PEDOT: PSS (40 nm)/active layer/cathode. A thin layer of PEDOT: PSS was deposited through spin-coating on precleaned ITO-coated glass from a PEDOT: PSS aqueous solution (Baytron P VP AI 4083 from H. C. Starck) at 4000 rpm and dried subsequently at 150 °C for 15 min in air. Then the device was transferred to a nitrogen glove box, where the active blend layer of J71 polymers and acceptors was spin-coated from its chloroform solution onto the PEDOT: PSS layer under a spin-coating rate of 3000 rpm. After spin-coating, the active layers were annealed at 160 oC for 2 min for 16


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the devices with thermal annealing treatment. The thickness of the active layers is ca. 100 nm. Then methanol solution of PDINO at a concentration of 1.0 mg mL−1 was deposited atop the active layer at 3000 rpm for 30 s to afford a PDINO cathode buffer layer with thickness of ca. 10 nm. Finally, top Al electrode was deposited in vacuum onto the cathode buffer layer at a pressure of ca. 5.0 × 10 −5 Pa. The active area of the device was 4.7 mm2. The current density-voltage (J-V) characteristics of the PSCs were measured in glovebox on a computer-controlled Keithley 2450 Source-Measure Unit. Oriel Sol3A Class AAA Solar Simulator (model, Newport 94023A) with a 450 W xenon lamp and an air mass (AM) 1.5 filter was used as the light source. The light intensity was calibrated to 100 mW cm−2 by a Newport Oriel 91150V reference cell. The input photon to converted current efficiency (IPCE) was measured by Solar Cell Spectral Response Measurement System QE-R3-011 (Enli Technology Co., Ltd., Taiwan). The light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell. Mobility Measurements: Hole and electron mobilities were measured using the the space

charge

limited

ITO/PEDOT:PSS/J71:

current

(SCLC)

acceptor(1:1.2,

w/w)

method.

Device

/Au

hole-only

for

structures

are

devices

and

ITO/ZnO/J71: acceptor (1: 1.2, w/w) /PDINO/Al for electron-only devices. The SCLC mobilities were calculated by MOTT-Gurney equation:

9 r  0 V 2 J 8L3

(1)

Where J is the current density, εr is the relative dieletiric constant of active layer material usually 2-4 for organic semiconductor, herein we use a relative dielectric constant of 4, ε0 is the permittivity of empty space, μ is the mobility of hole or electron and L is the thickness of the active layer, V is the internal voltage in the device, and V = Vapp-Vbi, where Vapp is the voltage applied to the device, and Vbi is the built-in voltage resulting from the relative work function difference between the two electrodes (in the hole-only and the electron-only devices, the Vbi values are 0.2 V and 0V respectively). Materails: All chemicals and solvents were purchased from J&K, Alfa Aesar and TCI 17



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Chemical Co. respectively. IT-CHO was bought from Solarmer Company and the J71 was synthesized according to the procedure reported in the literatures6. Synthesis of ITV-CHO :To a solution of the mixture of IT-CHO (1.0 equiv.) and tributyl(1,3-dioxolan-2-ylmethyl)phosphonium bromide (1.1 equiv.) in anhydrous tetrahydrofuran was added sodium hydride (60% dispersed in mineral oil, 3.0 equiv.) under argon gas atmosphere, and the resulting turbid solution was stirred at room temperature for 16 h. After completion of the reaction, the excess NaH was quenched using 10% HCl solution under cooling and the reaction mixture was brought to acidic pH and stirred at room temperature for 4-5 h. The contents in the reaction flask were concentrated and the organic contents were extracted into ethyl acetate. The organic layer was washed with water followed by brine, dried by anhydrous MgSO4, filtered and evaporated to dryness to afford the crude aldehyde which after purification by silica-gel column chromatography using dichloromethane gave product as yellow solid in high yield of 87%. 1H NMR (400 MHz, CDCl3) δ 9.60 (d, J = 7.6 Hz, 2H), 7.54 (dd, J = 15.1, 5.6 Hz, 6H), 7.16 (d, J = 8.0 Hz, 8H), 7.11 (d, J = 8.1 Hz, 8H), 6.43 (dd, J = 15.5, 7.6 Hz, 2H), 2.61-2.52 (m, 8H), 1.64-1.54 (m, 8H), 1.31-1.24 (m, 24H), 0.86 (t, J = 6.5 Hz, 12H).

13

C NMR (101 MHz, CDCl3) δ 192.35, 154.26, 147.08, 146.58,

144.68, 142.40, 142.24, 140.44, 139.48, 136.96, 136.32, 128.67, 127.89, 126.37, 125.58, 117.56, 77.34, 77.02, 76.70, 63.01, 35.58, 31.69, 31.23, 29.71, 29.15, 22.58, 14.07. HRMS (TOF) m/z calcd for [M ]+ C74H78O2S4 1126.4885, found 1126.4870. Synthesis of ITVIC: To a three-necked round-bottomed flask were added ITV-CHO (225 mg, 0.2 mmol), 1,1-dicyanomethylene-3-indanone (279 mg, 1.4 mmol), chloroform (50 mL), and pyridine (1 mL). The mixture was deoxygenated with nitrogen for 30 min and then refluxed for 12 h. After cooling to room temperature, the mixture was poured into methanol (200 mL) and filtered. The residue was purified by column chromatography on silica gel using petroleum ether/dichloromethane (1:1) as eluent, yielding a dark green solid (245 mg, 83%).1H NMR (400 MHz, CDCl3) δ 8.65 (dd, J = 5.6, 2.8 Hz, 2H), 8.51 (dd, J = 14.5, 11.8 Hz, 2H), 8.40 (d, J = 11.7 Hz, 2H), 7.89 (dd, J = 5.0, 3.4 Hz, 2H), 7.77-7.69 (m, 4H), 7.58 (s, 2H), 7.51-7.47 (m, 4H), 7.17 (q, J = 8.4 Hz, 16H), 2.62-2.52 (m, 8H), 1.64-1.57 (m, J = 15.4, 7.6 Hz, 8H), 1.35-1.24 18


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(m, 24H), 0.86 (t, J = 6.7 Hz, 12H).

13

C NMR (101 MHz, CDCl3) δ 192.35, 154.26,

147.08, 146.58, 144.68, 142.40, 142.24, 140.44, 139.48, 136.96, 136.32, 128.67, 127.89, 126.37, 125.58, 117.56, 77.34, 77.02, 76.70, 63.01, 35.58, 31.69, 31.23, 29.71, 29.15, 22.58, 14.07. HRMS (TOF) m/z calcd for [M ]+ C98H86N4O2S4 1478.5634, found 1478.5621. Synthesis of ITVfIC: To a three-necked round-bottomed flask were added ITV-CHO (225 mg, 0.2 mmol), 2-(5or 6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (300 mg, 1.4 mmol), chloroform (50 mL), and pyridine (1 mL). The mixture was deoxygenated with nitrogen for 30 min and then refluxed for 12 h. After cooling to room temperature, the mixture was poured into methanol (200 mL) and filtered. The residue was purified by column chromatography on silica gel using dichloromethane (1:1) as eluent, yielding a dark green solid (266 mg, 88%).1H NMR (300 MHz, CDCl3) δ 8.69 (dd, J = 8.6, 4.3 Hz, 1H), 8.57-8.31 (m, 6H), 7.89 (dd, J = 8.3, 5.2 Hz, 1H), 7.62-7.48 (m, 7H), 7.41 (t, J = 8.3 Hz, 2H), 7.22-7.09 (m, 16H), 2.64-2.52 (m, 8H), 1.68-1.53 (m, 8H), 1.30 (s, 24H), 0.86 (t, J = 6.5 Hz, 12H).

13

C

NMR (75 MHz, CDCl3) δ 188.02, 187.92, 168.40, 164.99, 158.34, 158.09, 154.96, 149.41, 147.16, 146.66, 146.28, 146.08, 143.82, 143.35, 142.42, 142.32, 142.18, 140.42, 140.31, 139.20, 139.15, 136.69, 135.87, 135.85, 133.54, 133.52, 128.81, 127.89, 127.57, 125.93, 125.81, 124.87, 124.75, 123.10, 122.03, 121.72, 117.93, 114.64, 114.40, 114.22, 114.17, 112.90, 112.56, 110.71, 110.50, 70.47, 63.03, 35.61, 31.70, 31.26, 29.19, 22.58, 14.09. HRMS (TOF) m/z calcd for [M ]+ C98H84F2N4O2S4 1514.5439, found 1514.5436. Synthesis of ITVffIC: To a three-necked round-bottomed flask were added IT-CHO (225

mg,

0.2

mmol),

2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)

malononitrile (320 mg, 1.4 mmol), chloroform (50 mL), and pyridine (1 mL). The mixture was deoxygenated with nitrogen for 30 min and then refluxed for 12 h. After cooling to room temperature, the mixture was poured into methanol (200 mL) and filtered. The residue was purified by column chromatography on silica gel using dichloromethane (1:1) as eluent, yielding a dark green solid (270 mg, 87%).1H NMR (400 MHz, CDCl3) δ 8.54-8.39 (m, 6H), 7.70-7.51 (m, 8H), 7.16 (q, J = 8.6 Hz, 16H), 19



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2.64-2.50 (m, 8H), 1.67-1.55 (m, 8H), 1.40-1.21 (m, 24H), 0.86 (t, J = 6.8 Hz, 12H). 13

C NMR (101 MHz, CDCl3) δ 187.02, 157.34, 155.04, 153.28, 153.13, 149.71,

147.21, 146.83, 146.68, 143.92, 143.27, 142.45, 139.45, 139.09, 136.72, 134.96, 128.81, 127.87, 124.06, 123.95, 122.94, 117.98, 115.05, 114.20, 114.08, 112.64, 112.46, 70.19, 63.05, 35.60, 31.69, 31.24, 29.18, 22.57, 14.07. HRMS (TOF) m/z calcd for [M ]+ C98H82F4N4O2S4 1550.5251, found 1550.5253.

Supporting Information: Supporting Information is available free of charge on the ACS Publications website. TGA plots, UPS measurements, TEM patterns and AFM images of the J71:acceptors blend films, GIWAXS data of neat films and analysis and J1/2~(Vappl-Vbi-Vs) characteristics for hole and electron mobilities measurements by SCLC method.

Acknowledgements This work was supported by the Ministry of Science and Technology of China (973 project, No. 2014CB643501) and NSFC (Nos. 91633301, 91433117, 91333204, 21374124, and 51673200) and the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB12030200. GIWAXS analysis by NCSU was supported by ONR grant N00141512322. X-ray data were acquired at beamlines 7.3.3 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. E. Schaible of the ALS (DOE) assisted with the measurements and/or provided instrument maintenance.

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Table of contents Synthesis and Photovoltaic Properties of a Series of Narrow Bandgap Organic Semiconductor Acceptors with Absorption Edge Reaching 900 nm Xiaojun Li, He Huang, Haijun Bin, Zhengxing Peng, Chenhui Zhu, Lingwei Xue, Zhi-Guo Zhang, Zhanjun Zhang, Harald Ade, Yongfang Li. For polymer solar cells (PSCs), high-performance narrow bandgap n-OS acceptors are needed for matching with solar spectrum and for use in tandem and semitransparent PSCs. Three new NIR n-OS acceptors ITVIC, ITVfIC and ITVffIC with Eg < 1.4 eV were synthesized, by introducing double bond π-bridge in ITIC and fluorine substitution on its electron-accepting end groups. It was found that the Jsc values of the PSCs with different acceptors were tightly related to theΔEHOMO between the J71 donor and the acceptor. The optimized PSC based on J71:ITVffIC demonstrated a high PCE of 10.54 % with a low Eloss of 0.54 eV. This PCE is the highest efficiency reported for the PSCs based on the NIR photovoltaic material with Eg < 1.4 eV.

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Synthesis and Photovoltaic Properties of a Series of Narrow Bandgap

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