Alkyl Chain End Group Engineering of Small Molecule Acceptors for

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Alkyl Chain End Group Engineering of Small Molecule Acceptors for Non-Fullerene Organic Solar Cells Jianfei Qu, Zhao Mu, Hanjian Lai, Hui Chen, Tao Liu, Shuai Zhang, Wei Chen, and Feng He ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00851 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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ACS Applied Energy Materials

Alkyl Chain End Group Engineering of Small Molecule Acceptors for Non-Fullerene Organic Solar Cells Jianfei Qu†, Zhao Mu†, Hanjian Lai†, Hui Chen†, Tao Liu†, Shuai Zhang†, Wei Chen∗,‡,§ and Feng He∗,† †

Department of Chemistry, Southern University of Science and Technology, Shenzhen

518055, P. R. China. ‡

Materials Science Division, Argonne National Laboratory, 9700 Cass Avenue, Lemont,

Illinois 60439, United States. §

Institute for Molecular Engineering, The University of Chicago, 5640 South Ellis

Avenue, Chicago, Illinois 60637, United States. *E-mail: [email protected] (F.H.) *E-mail: [email protected] (W.C.)

Keywords: alkyl chains, end groups, molecular packing, organic solar cells, non-fullerene acceptors

1

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Abstract: Alkyl chain engineering is widely used to prepare high-performance donor materials. However, relatively few studies have been focused on the alkyl chain optimization of acceptor materials. Herein, a series of new A-D-A (acceptor-donor-acceptor) type small molecule

acceptors

(ITBTR-C2,

ITBTR-C4,

ITBTR-C6

and

ITBTR-C8)

with

indacenodithieno[3,2-b]thiophene (IDTT) as the core, benzothiadiazole (BT) as the π bridge, and ethyl-, butyl-, hexyl-, and octyl-substituted 2-(1,1-dicyanomethylene) rhodanine as the end groups, respectively, are successfully synthesized to systematically investigate the alkyl substituent effects on the physical, chemical, and electronic properties of A-D-A type small molecule acceptors. All molecules exhibit a strong and broad absorption from 600 to 800 nm as well as similar HOMO and LUMO energy levels. ITBTR-C6 with hexyl substitution shows the highest electron mobility and better phase separation morphology after blending with a donor polymer (PBDB-T). Therefore, inverted bulk heterojunction organic solar cells based on ITBTR-C6:PBDB-T blends exhibits the highest power conversion efficiency (PCE) of 8.26% with an open-circuit voltage (VOC) of 0.89 V, a high short-circuit current density (JSC) of 15.80 mA/cm2, and a fill factor (FF) of 58.21%, while the PCEs of ITBTR-C2-, ITBTR-C4- and ITBTR-C8-based devices are 7.04%, 7.43% and 7.93%, respectively. After solvent vapor and thermal annealing, both the JSC and FF values of the ITBTR-C6-based device are further increased, leading to a PCE of 9.29%. The results demonstrate that the alkyl chain substitution of A-D-A type small molecule acceptors is critical, and an appropriate adjustment of the alkyl chains can effectively enhance device performance.

INTRODUCTION 2


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Solution-processed organic solar cells (OSCs) based on a bulk heterojunction (BHJ) structure have attracted much attention from academic and industrial researchers due to their many advantages such as low cost, light weight, mechanical flexibility, and large area.1-6 For many years, fullerene derivatives, such as PC61BM and PC71BM, have been the most commonly used acceptor materials owing to their high electron mobility, suitable energy level, isotropic charge transport, and ability to form appropriate phase separation.7-9 Fullerene-based single-junction OSCs can achieve power conversion efficiencies (PCEs) that exceed 11%.10 However, the intrinsic drawbacks of the fullerene derivatives, such as weak absorption in the visible and near-infrared regions, high cost of preparation, high difficulty of purification, and limited tunability of their LUMO energy levels, have hindered their further development.11-15 Over the past three years, non-fullerene acceptor-based OSCs have been making a dramatic breakthrough. In 2015, Zhan et al. reported an A-D-A type small molecule acceptor (ITIC), based on a bulky seven-ring fused core indacenodithieno[3,2-b]thiophene (IDTT), and end-capped with 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile groups. ITIC exhibits strong absorption between 550~800 nm, a high electron mobility, excellent solubility in common solvents, and good phase separation after blending with donor materials. OSCs based on PTB7-Th and ITIC blends shows a PCE up to 6.8%, which is the record of non-fullerene OSCs at that time.16 Compared to the traditional fullerene derivative accepters, the A-D-A type fused ring acceptors can be easily synthesized and purified, and their absorption spectra and energy levels can be readily tuned.17-29 With these advantages, non-fullerene acceptor-based single-junction OSCs have shown a PCE over 13%.30-37

3



Basically, there are three design strategies for donor polymers, which are also feasible for A-D-A type small molecule non-fullerene materials, including molecular backbone modulation,19,38-43 functional substitutions,17,25,44-47 and alkyl chain optimization.21,48-50 Several investigations have been focused on modifying the molecular backbones and functional substitutions. However, it is very important to select an appropriate flexible alkyl chain for the small molecule with a definite conjugated backbone. This alkyl chain can determine the intermolecular interactions between the small molecule and the donor polymer-small molecule and further affect the solubility, film-forming property, electron mobility, and miscibility of the small molecule with the donor polymers. Therefore, systematic studies of the influence of alkyl chains on the properties of small molecule non-fullerene acceptors are significant and can provide insight into the structure-property relationships. Herein, we designed a series of novel A-D-A type small molecules, named ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8, and their chemical structures are shown in Chart 1. The four small molecules exhibited identical molecular backbones with IDTT as the electron-rich unit, benzothiadiazole (BT) as the π bridge, and different alkyl-substituted 2-(1,1-dicyanomethylene) rhodanine molecules as electron-deficient end groups. The effect of the alkyl chains on the electronic structures, molecular packing, film morphology, charge transport, photovoltaic properties of the four small molecule acceptors were systematically investigated by optical absorption spectroscopy, cyclic voltammetry (CV), atomic force microscopy (AFM), space-charge limited current (SCLC) mobility measurements, grazing

4


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incidence wide-angle X-ray scattering (GIWAXS), and photovoltaic response analysis. Non-fullerene OSCs were fabricated using the polymer PBDB-T as the donor and four small molecules as the acceptors respectively. ITBTR-C6 with hexyl end groups showed the best photovoltaic performance with a PCE of 8.26% due to its increased molecular packing order and high charge transport property. After solvent vapor annealing and thermal annealing, the PCE of the PBDB-T:ITBTR-C6-based device further increased to 9.29%. These results imply that the alkyl chain end group engineering of A-D-A type non-fullerene small molecule acceptors is a very promising strategy to further tune the optoelectronic properties.

RESULTS AND DISCUSSION Chart 1. Chemical Structures of ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8. C6H13

NC

C6H13

S N O R

N

NC

S

N

S

S S

S

CN N O

N

S

N

R

ITBTR-C2, R = -C2H5 ITBTR-C4, R = -C4H9 ITBTR-C6, R = -C6H13

S CN

C6H13

ITBTR-C8, R = -C8H17 C6H13

Scheme 1. Synthetic Routes of ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8.

The synthetic routes of ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8 are shown in Scheme 1. Compounds 1, 2, 4a, 4b, 4c and 4d were synthesized according to previously reported approaches.12,51,52 Key intermediate 3 was synthesized with relatively high yields by 5



Stille coupling of 1 and 2. ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8 were prepared by Knoevenagel condensation reactions between 3 with 4a, 4b, 4c and 4d, respectively, and the compounds were further purified by recycling preparative high-performance liquid chromatography with chloroform as the eluent after column chromatography purification on silica gel. All of the compounds were soluble in common solvents, such as chloroform and chlorobenzene. The donor polymer PDBD-T was synthesized according to the literature,53 and its number/weight average molecular weights were 50.3/91.9 kDa with a polydispersity index of 1.82, which was tested by high-temperature gel permeation chromatography (150 °C) with 1,2,4-trichlorobenzene as the eluent. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed to investigate the thermal properties of ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8, which are shown in Figure S5 in the Supporting Information. According to the thermogravimetric analysis (TGA), four small molecules exhibits good thermal stability with a decomposition temperature (Td, 5% weight loss) greater than 380 °C under nitrogen. Four small molecules show similar glass transition temperature (Tg) at 110 °C. No melting peak can be observed for ITBTR-C2 and ITBTR-C4 below 280 °C, while ITBTR-C6 shows a melting peak at 227 °C. As the increasement of the alkyl chains, ITBTR-C8 shows a lower melting point of 184 °C, indicating that the introduction of long alkyl chains can obviously lower the crystallinity.

6


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-1

-1

ITBTR-C2 ITBTR-C4 ITBTR-C6 ITBTR-C8

1.2 1.0 0.8 0.6 0.4 0.2 0.0 300

400

500 600 700 Wavelength (nm)

800

900

Normalized Absorbance (a.u.)

1.4 (a)

5

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


Absorption (10 L mol cm )

Page 7 of 26

1.0 (b)


0.8 0.6 0.4 0.2 0.0 300

400


800

900

Figure 1. (a) UV-vis absorption spectra of ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8 in a chloroform solution with a concentration of 1×10-5 mol/L. (b) Normalized UV-vis absorption spectra of the ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8 films. The ultraviolet visible (UV-vis) absorption spectra of ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8 measured in chloroform solution with a concentration of 10-5 mol/L are shown in Figure 1a, and the corresponding data are listed in Table 1. In the dilute solution, the four molecules show broad absorption spectra in the range of 600~800 nm due to their strong intramolecular charge transfer. Meanwhile, almost identical absorption spectra with high absorption coefficients exceeding 1.2×105 L mol-1 cm-1 for these molecules are observed, indicating that the alkyl chain end groups have no noticeable effect on their electronic structures in the ground state. The UV-vis absorption spectra of four small molecular films spin-coated from chlorobenzene solution are shown in Figure 1b. From solution to films, the maximum absorption peaks of the molecules redshift from 651~654 nm to 697~711 nm. Notably, ITBTR-C6 and ITBTR-C8 show relatively redshifted absorption spectra, which may indicate stronger intermolecular interaction due to their longer alkyl chains. The optical bandgaps calculated from the thin-film absorption onsets for ITBTR-C2, ITBTR-C4 and 7



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ITBTR-C6 are identical at 1.52 eV, while that of ITBTR-C8 is slightly lower at 1.49 eV. To evaluate their electrochemical properties, cyclic voltammetry was performed to determine the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, and the corresponding redox curves are shown in Figure S6 in the Supporting Information. The corresponding HOMO/LUMO energy levels of ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8 are -5.30/-3.70 eV, -5.30/-3.71 eV, -5.30/-3.71 eV and -5.28/-3.70 eV, respectively. The results indicate that the HOMO and LUMO energy levels mainly depend on the molecular backbones, while the length of the alkyl chain end groups has a negligible effect on the energy levels of the four A-D-A type small molecules. Table 1. Optical and Electrochemical Properties of ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8.

materials

λ sol max (nm)

λ film max (nm)

λ film onset (nm)

Ega (eV)

ELUMOb (eV)

EHOMOb (eV)

Egc (eV)

ITBTR-C2

651

697

814

1.52

-3.70

-5.30

1.60

ITBTR-C4

654

697

812

1.52

-3.71

-5.30

1.59

ITBTR-C6

654

711

813

1.52

-3.71

-5.30

1.59

ITBTR-C8

651

711

830

1.49

-3.70

-5.28

1.58

a

The band gaps were obtained from the onset of UV-vis absorption in each film. b The

LUMO and HOMO energy levels were measured by CV. c The band gaps were calculated using the following equation: Eg = ELUMO - EHOMO.

The photovoltaic properties of ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8 were evaluated with an inverted device structure of ITO/ZnO/active layer/MoO3/Ag, and the most 8


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commonly used wide band gap polymer PBDB-T was chosen as the donor material. The characteristic current density-voltage (J-V) curves and the EQE spectra for these blended film-based optimized devices are shown in Figure 2, and the corresponding photovoltaic parameters of these devices are listed in Table 2. The four small molecule-based OCSs showed the best device performance with a D/A ratio of 1:1 (total concentration of 20 mg/mL) and 0.3% 1,8-diiodooctane (DIO) as the additive in the chlorobenzene solution. Under this condition, the four non-fullerene acceptor-based solar cells showed almost the same open-circuit voltage (VOC) of 0.89-0.90 eV, resulting from their similar LUMO energy levels. The shortest alkyl chain substituted molecule ITBTR-C2-based device showed the lowest short-circuit current (JSC) of 13.47 mA/cm2, leading to a low PCE of 7.04%. As the length of the alkyl chain end group of the small molecules increased, the device performance gradually improved. The PBDB-T:ITBTR-C6-based solar cell showed the best device performance with a PCE of 8.26%, a JSC of 15.80 mA/cm2, and a fill factor (FF) of 58.21%. When the alkyl chains further increased, the PCE of PBDB-T:ITBTR-C8 based device dropped to 7.93% due to its reduced JSC of 14.88 mA/cm2. The corresponding external quantum efficiencies (EQEs) of ITBTR-C2-, ITBTR-C4-, ITBTR-C6- and ITBTR-C8-based OSCs were also characterized to evaluate the photoresponse of the blends, which are shown in Figure 2b. The EQE curves exhibited a broad photoresponse in the range of 300-800 nm for all solar cells. Especially for the PBDB-T:ITBTR-C6-based device, the EQE exceeded 60% from 550 to 720 nm with a peak of 68% at 620 nm. Therefore, the ITBTR-C6-based device yielded the highest JSC.

9



10

70 (b)

(a)

5

60

0

50


-5 -10

EQE (%)

-2

Current Denstiy (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

-15 -20 -0.2

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40 30 ITBTR-C2 ITBTR-C4 ITBTR-C6 ITBTR-C8

20 10

0.0

0.2 0.4 0.6 Voltage (V)

0.8

1.0

0 300

400


800

900

Figure 2. (a) J-V characteristics and (b) EQE spectra of the PBDT-T:ITBTR-C2, PBDT-T:ITBTR-C4, PBDB-T:ITBTR-C6 and PBDT-T:ITBTR-C8 devices. Table 2. Photovoltaic Performance of the PBDB-T: Acceptor-based Devices. Acceptor

VOC (V)

JCS (mA cm-2)

FF (%)

PCE (%)

ITBTR-C2

0.89 ± 0.01 (0.89)

13.38 ± 0.13 (13.47)

58.42 ± 0.83 (59.03)

6.96 ± 0.12 (7.04)

ITBTR-C4

0.90 ± 0.01 (0.90)

14.75 ± 0.22 (14.94)

54.15 ± 1.02 (55.09)

7.33 ± 0.15 (7.43)

ITBTR-C6

0.89 ± 0.01 (0.89)

15.72 ± 0.24 (14.94)

57.83 ± 0.64 (58.21)

8.18 ± 0.10 (8.26)

ITBTR-C8

0.90 ± 0.01 (0.90)

14.74 ± 0.18 (14.88)

58.47 ± 0.98 (59.35)

7.85 ± 0.13 (7.93)

The electron mobilities and hole mobilities of four blends were measured through space-charge-limited current (SCLC) method, and the current density-voltage (J-V) curves and the results are shown in Figure S7 and Table S1. The structure of electron-only device is ITO/ZnO/PBDB-T:Acceptor/Ca/Al,

and

the

structure

of

hole-only

device

is

ITO/PEDOT:PSS/ PBDB-T:Acceptor/ MoO3/Ag. The hole mobilities (µh) of ITBTR-C2-, ITBTR-C4-, ITBTR-C6- and ITBTR-C8-based blend films are 3.21×10-6, 1.92×10-5, 5.97×10-5 and 2.75×10-5 cm2·V-1·s-1, while the electron mobilities (µe) are 8.62×10-6, 2.36×10-5, 6.55×10-5 and 3.43×10-5, respectively. The µh/µe ratios of four blend films are 0.37, 0.81, 0.91 and 0.80, respectively. The highest and balanced hole and electron mobilities could 10


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help explain the high JSC and FF achieved in the PBDB-T:ITBTR-C6-based device. The device performance of the PBDB-T:ITBTR-C6-based solar cell was further optimized by solvent vapor annealing using THF for 1 min, then thermal annealing 10 min at 120 °C after deposition of the electrode (Figure 3 and Table 3). As shown in Figure 3b, the EQE of the optimized device showed an overall enhancement, causing the JSC to increase from 15.80 to 16.30 mA/cm2. Additionally, the FF also increased from 58.21% to 64.18%, and then the PCE eventually increased to 9.29% (Table 3). 10

(a)

70 (b)

5

60

0

50

ITBTR-C6 without annealing ITBTR-C6 with annealing

-5

EQE (%)

-2

Current Denstiy (mA cm )

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


-10

30 20

-15 -20 -0.2

40

ITBTR-C6 without annealing ITBTR-C6 with annealing

10 0.0

0.2 0.4 0.6 Voltage (V)

0.8

1.0

0 300

400


800

900

Figure 3. (a) J-V characteristics and (b) EQE spectra of the PBDT-T:ITBTR-C6 without and with annealing film-based devices. Table 3. Photovoltaic Performance of the PBDB-T:ITBTR-C6-based Devices Without and With Annealing. JCS (mA cm-2)

ITBTR-C6

VOC (V)

without annealing

0.89 ± 0.01 (0.89)

15.62 ± 0.21 (15.80) 58.02 ± 0.26 (58.21) 8.18± 0.11 (8.26)

with annealing

0.88 ± 0.01 (0.88)

16.13 ± 0.18 (16.30) 63.56 ± 0.46 (64.18) 9.17 ± 0.14 (9.29)

FF (%)

PCE (%)

The effect of different alkyl chains on the morphologies of the blend films was investigated by atomic force microcopy (AFM) and transmission electron microscopy (TEM). As shown in 11



Figure S8 in the Supporting Information, relatively smooth surfaces with no significant difference were observed for all four small molecule-based blend films. No obviously a large domain can be observed, which means all small molecules form good phase separation after blending with the donor material. Relatively obvious fibrous structures can be seen in the TEM images of the PBDB-T:ITBTR-C2 and PBDB-T:ITBTR-C6 films, demonstrating that lamellar phase separation morphologies are formed. Compared to the PBDB-T:ITBTR-C6 film, the domain size of PBDB-T:ITBTR-C2 is much small, which may hinder charge transport, leading to a low electron mobility, finally resulting in the lowest JSC. Although the domain sizes of PBDB-T:ITBTR-C4 and PBDT-T:ITBTR-C8 are similar to the ITBTR-C6-based blends, the film orders are relatively low, therefore leading to the lower JSC. The PBDB-T:ITBTR-C6 film can form nanostructures with the highest order and a suitable domain size, and thus shows the best photovoltaic performance.

12


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Figure 4. TEM images of the (a) PBDT-T:ITBTR-C2, (b) PBDT-T:ITBTR-C4, (c) PBDB-T:ITBTR-C6, and (d) PBDT-T:ITBTR-C8 blend films. The influence of the alkyl chain end groups on the molecular packing and orientation of the neat films was investigated by grazing incidence wide angle X-ray scattering (GIWAXS). The two-dimensional GIWAXS patterns and the corresponding out-of-plane and in-plane line-cuts of the ITBTR-C2, ITBTR-C4, ITBTR-C6 and ITBTR-C8 neat films are shown in Figure 5. It is evident that ITBTR-C2 possesses a higher propensity to crystallize in the neat films casted from chlorobenzene (CB) than the other three small molecules. ITBTR-C4, ITBTR-C6 and ITBTR-C8 show similar patterns with (100) scatter peaks at qxy = 0.30 Å-1 that correspond to the formation of lamellar structures with a d-spacing of 20.9 Å. The π-π stacking peaks, which are located at 1.76 Å-1 (d = 3.6 Å), can be observed in the out-of-plane 13



direction, indicating a face-on orientation, which is beneficial for charge transport. In addition, compared to ITBTR-C4 and ITBTR-C8, ITBTR-C6 shows a relatively sharp scattering peak at 0.30 Å-1 in the in-plane direction, implying more ordered packing. The 2D GIWAXS patterns and line profiles of the blend films and the PBDB-T neat film are compiled in Figure S9 in the Supporting Information. The pristine PBDB-T film shows a scatter peak at qxy = 0.29 Å-1, which means the formation of lamellar structures with a d-spacing of 21.7 Å. The stacking peak located at 1.69 Å-1 corresponds to a π-π distance of 3.7 Å, which can be observed in the out-of-plane direction, implying that PBDB-T adopts a face-on orientation. All blend films show almost the same patterns with a pronounced lamellar packing peak located at qxy = 0.30 Å-1 and a corresponding d spacing of 20.9 Å. Meanwhile, the obvious π-π stacking peak located at qz = 1.76 Å-1 with a distance of 3.6 Å can be observed for all four blend films. The scattering peaks of the blending films at 0.34 Å-1 in the out-of-plane direction and at qz = 0.65 Å-1 can be ascribed to PBDB-T, demonstrating that the polymer in the blended films retains the same molecular orientation as its pristine film. All blend films show more sharply scattered peaks at 0.30 Å-1 in the in-plane direction than in the PBDB-T film, indicating that these acceptors still retain their crystallinity after blending with the donor polymer. Notably, the strong crystallization peaks of ITBTR-C2 are disappeared in the blend film, indicating that its aggregation ability is effectively suppressed and a better phase separation morphology is formed. All the small molecule acceptors maintain the face-on configuration in the blend films, which facilitate charge transport, and therefore result in higher device performance. Moreover, the full width at half-maximum values of the scattering peaks at 0.30 Å-1 of the ITBTR-C2-, ITBTR-C4-, ITBTR-C6- and ITBTR-C8-based blend 14


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films are 0.211, 0.186, 0.161 and 0.174 Å-1, respectively, implying that the crystal size in the four

blend

films

exhibit

the

following

sequence:

PBDB-T:ITBTR-C2

12%. Nat. Photonics 2016, 11, 85. (10) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027. (11) Hummelen, J. C.; Knight, B. W.; LePeq, F.; Wudl, F.; Yao, J.; Wilkins, C. L. Preparation and Characterization of Fulleroid and Methanofullerene Derivatives. J. Org. Chem. 1995, 60, 532-538. (12) Holliday, S.; Ashraf, R. S.; Nielsen, C. B.; Kirkus, M.; Röhr, J. A.; Tan, C.-H.; 18


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25



Table of Contents (TOC) 8.5

16 2

JSC (mA/cm )

8.0

15

7.5

14

ITBTR-C2, R = -C2 H5 ITBTR-C4, R = -C4 H9 13 2 ITBTR-C6, R = -C6 H13 ITBTR-C8, R = -C8 H17

7.0 4

6

ITBTR-CX

26


8

6.5

PCE (%)

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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Alkyl Chain End Group Engineering of Small Molecule Acceptors for

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