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Propeller-Shaped Acceptors for High-Performance NonFullerene Solar Cells: Importance of Molecular Geometry Rigidity Qinghe Wu, Donglin Zhao, Jinghui Yang, Valerii Sharapov, Zhengxu Cai, Lianwei Li, Na Zhang, Andriy Neshchadin, Wei Chen, and Luping Yu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04287 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017

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Chemistry of Materials

Propeller-Shaped Acceptors for High-Performance Non-Fullerene Solar Cells: Importance of Molecular Geometry Rigidity †∇

† ∇

†

†

†

†

†

†

Qinghe Wu,†,∇ Donglin Zhao,† ,∇ Jinghui Yang,† Valerii Sharapov,† Zhengxu Cai, † Lianwei Li,† Na Zhang,† Andriy Neshchadin† Wei ‡§

Chen

and Luping Yu*

†

†

Department of Chemistry and The James Franck Institute, The University of Chicago, 929 E 57th Street, Chicago, IL 60637 Materials Science Division, Argonne National Laboratory, 9700 Cass Avenue, Lemont, Illinois 60439, USA § Institute for Molecular Engineering, the University of Chicago, 5747 South Ellis Avenue, Chicago, Illinois 60637, USA

‡

ABSTRACT: This paper describes the synthesis and application of βTPB6 and βTPB6-C as the electron acceptors for organic solar cells. Compound βTPB6 contains four covalently bonded PDIs with BDT-Th core at β-position. The free rotation of PDIs renders βTPB6 with varying molecular geometry. The cyclization of βTPB6 yields βTPB6-C with high rigidity of molecular geometry and enlarged conjugated skeleton. The inverted solar cells based on βTPB6-C and PTB7-Th as the donor polymer exhibited the highest efficiency of 7.69 % with Voc of 0.92 V, Jsc of 14.9 mAcm-2 and FF of 0.56, which is 31% higher than that for βTPB6 based devices. The larger fraction of βTPB6-C and PTB7-Th than that of βTPB6:PTB7-Th in blend film take a face-on orientation packing pattern for π-systems that benefits the charge transport and hence higher PCE value than that for βTPB6:PTB7-Th. It was also found proper DIO:DPE additive further enhance this trend, which results in increase of PCE value for βTPB6-C:PTB7-Th while decrease of PCE value for βTPB6:PTB7-Th.

Introduction As the development of low band gap donor polymers reaches a plateau in their organic photovoltaic performance, research effort has been directed towards development of replacement of fullerene derivatives as electron acceptor. The fullerene derivatives such as PC61BM, PC71BM have been used as the electron acceptor in the champion bulk-heterojunction (BHJ) solar cells due to their unique structure and exceptional high performances.1-4 However, the intrinsic drawbacks of fullerene are also clear, including poor visible light harvesting, high cost and poor morphology stability in the blend film.5-7 Thus, nonfullerene electron acceptors that can overcome the issues associated with fullerene and exhibit high performances become desirable. The non-fullerene acceptor can potentially show superior advantages, including readily tunable chemical structure, ease in synthesis and purification and low cost.8-16 Although many electron deficient moieties such as naphthalene diimide (NDI), perylene diimide (PDI), benzo[c]1,2,3thiadiazole (BT), 2-(2,3-dihydro-3-oxo-1H-inden-1ylidene)propane-dinitrile (INCN) etc. can be used to synthesize the non-fullerene acceptors,7, 10, 17-18 there is much science left to be done in acceptor design and development.10, 16, 19-20 Recently, several examples of high performance non-fullerene OPV have been disclosed.21-27 The most widely studied strategy is to design the electron acceptors with a π-conjugated backbone of twisted 3D geometry that could improve the morphological compatibility with the donor polymer and lead to enhanced photovoltaic performance.28-33 However, introducing steric hindrance and/or different orientation of chemical bonding to create the twisted 3D molecular geometry of acceptors would inevitably result in the conformational isomers in the solid packing state. It was shown that molecular conformation could impact on OPV performance.9, 20, 34-35

In this paper, we designed and synthesized two compounds βTPB6 and βTPB6-C. In compound βTPB6, four PDIs are covalently bonded with BDT-Th core at β-position and free rotation of PDIs can result in βTPB6 with varying molecular geometry. The cyclization between PDIs and BDT-Th core generates βTPB6-C that not only has enlarged conjugated skeleton, but also possess a more rigid molecular geometry than βTPB6. The detailed study revealed that the rigidity of molecular geometry has great impact on packing patterns of the molecules and the blend films in the solid state, thus the OPV properties. The results demonstrate that careful design and control in the rigidity and geometry of acceptor molecules is an effective strategy to create high performance acceptors for OPV. Result and discussion Synthesis. The βTPB6 was synthesized by Suzuki coupling of BDT-Th-4Bpin with 4 equivalent β-monobrominated PDIs. As revealed in our previous paper,36 the functionalization at bay-position of PDI could leads to the distortion in the conjugated backbone of PDI that is detrimental to OPV properties. This problem can be addressed by cyclization between BDT and PDIs, and thiophenes and PDIs, resulting in a large PDIBDT-PDI and two bridged PDI-Th cores. The cyclization is accomplished by treating substrate with iron chloride and βTPB6-C is obtained in moderate yield, which exhibits high solubility in common organic solvent like chloroform, chlorobenzene. The structure of two compounds was characterized and confirmed by mass spectrum, 1H NMR and elemental analysis.

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Scheme 1. Synthetic Route of βTPB6 and βTPB6-C. Bpin

Bpin

S S

Bpin O

R1 N

N R1

R1 O N O

O O

Bpin O

N R1

O

R1 N O

O

R1 N O

O

O

S

R1 N O

S

S Pd2(dba)3/P(MeOPh)3

O

R1 N

S

S

R1 N O

O

O

O

S

FeCl3

S S

K2CO 3 THF/H2 O

S O

Br

O N O O R1

N O R1 O R1 N

N R1

O O

N O R1

β TPB6

O

N R1

R1 N

O R1 =

S O O

C 6H13 C 6H13

O β TPB6-C R1 =

O

N OR1

O

N R1 O C 6H 13 C 6H 13

Figure 1: (a) the top view of optimized geometries βTPB6 using DFT calculation; (b) the side view of optimized geometries βTPB6-C using DFT calculation; (c) cyclic voltammograms of βTPB6 and βTPB6-C films with Fc/Fc+ as the reference; (d) Schematic of energy level of βTPB6, βTPB6-C and PTB7-Th; (e) the solution and film absorption of βTPB6 and βTPB6-C; (f) film emission spectra of PTB7-Th and the photoluminescence quenching of PTB7-Th in βTPB6:PTB7-Th and βTPB6-C:PTB7-Th blend film, excited at 640 nm. DFT calculation. To investigate the structural and electronic properties of the two compounds, density functional theory calculations using the Gaussian package b3lyp/6-31g(d) were performed to evaluate the frontier molecular orbitals and geometry of βTPB6 and βTPB6-C. In order to facilitate the calculation, the long alkyl chains were replaced with methyl group. The LUMO and HOMO orbitals are shown in Figure s1. In βTPB6, the LUMO orbitals localize in PDI unit while HOMO electron density localize in BDT-Th core, implying the charge polarization in the excited state. However, in βTPB6-C, the LUMO orbitals localize in the ladder type of core PDI-BDT-PDI while HOMO orbitals concentrated in the bridged PDI-Th. The optimized molecular geometries of βTPB6 and βTPB6-C are presented in Figure 1 a, b. In βTPB6, dihedral angle between PDIs and BDT, PDIs and thiophene, and thiophene and BDT are 53.0°, 53.0°, 55.2°, 55.2°, 52.9°, 52.9°, respectively, resulting in a twisted geometry of four PDIs. In βTPB6-C, PDI-BDT-PDI and two bridged PDITh cores show good planarity. Due to the steric hindrance, the

torsion with a dihedral angle of 60° between two bridged PDIThs and PDI-BDT-PDI core is introduced and propellershaped molecular geometry is formed. The major difference between these two compounds is their conjugated skeleton size and molecular geometry rigidity. The PDIs in βTPB6 have more freedom to rotate and result in varying molecular configurations of βTPB6. The βTPB6-C has more planar and rigid building units and the rotation of PDI-Ths at bay position of PDI-BDT-PDI core is more confined than that in βTPB6 due to steric hindrance which leads to a rigid molecular geometry of βTPB6-C. Electronic and optical properties. The energy levels of the two compounds were deduced from cyclic voltammetric measurements (CV), and the results are shown in Figure 1c. Compared with LUMO energy levels of -3.79 eV for βTPB6, βTPB6-C exhibits a \higher value of -3.75 eV. By increasing the difference between LUMO of acceptor and HOMO of donor, βTPB6-C gains the advantage of improving the Voc value of solar cell devices, thus help to enhance the overall


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solar cell performance. Due to HOMO localized on PDI-Th, much deeper HOMO energy level of -6.21 eV for βTPB6-C is observed while that for βTPB6 is -5.92 eV, which is similar with the DFT calculation result, -5.79 eV for βTPB6 and -6.01 eV for βTPB6-C. Thus, it results in a large bandgap of 2.46 eV for βTPB6-C, which is 0.33 eV larger than that for βTPB6. The trend is consistent with that in solution absorption spectrum. The solution and film absorption spectrum is shown in figure 1e. The solution spectrum of βTPB6 shows the vibronic peaks between 450 and 600 nm with stronger 0-0 (I00) absorption peaks than 0-1 (I01) transition, which is very similar to that of PDI. The maximum extinction coefficient of βTPB6 at 533 nm is 4.5×10-4 M-1cm-1. The film absorption of βTPB6 made by spin-casting chloroform solution exhibit stronger 0-1 (I01) absorption peaks than 0-0 (I00) transition. The blue shift of highest absorption spectrum indicates strong intermolecular interaction of βTPB6 in solid state, leading to the formation of excimer in very dilute solution (10-9 mol/L). The concentration-dependent emission spectrum offers evidence for this phenomenon (See Figure S2). The strong intermolecular interaction of βTPB6 in solid state can be explained by its low rigidity of molecular geometry, adjustable to facilitate the close packing of PDIs. The UV-vis absorption of βTPB6-C in chlorobenzene (10-8 mol/L) exhibits five vibronic peaks between 350 nm and 550 nm with maximum extinction coefficient of 1.15×10-5 M-1cm-1 at 502 nm. To help the peak assignment, the absorption spectra of model compounds PDI-Th and PDI-BDT-PDI in solution were measured (See Figure S3). It can be clearly observed that the peaks 0-1 (I01) and 0-2 (I02) are from the absorption of PDI-Ths while the absorption peaks of 0-0 (I00), 0-3 (I03) and 0-4 (I04) are from the absorption of PDI-BDT-PDI. However, the 0-0 (I00) absorption peak of βTPB6-C is apparently blue shifted compared with that of PDI-BDT-PDI, which is caused by the interruption of electron density distribution in PDI-BDT-PDI after introducing two bridged PDI-Ths at bay positions. Due to the introduction of bridged PDI-Ths, the βTPB6-C shows weak intermolecular interaction in solid state, as indicated by the similarity of film absorption in shape and intensity with solution absorption spectrum (Fig.1e). OPV Properties. To evaluate the photovoltaic properties of these two compounds, the inverted solar cell devices were fabricated with the configuration of ITO/ZnO/βTPB or βTPBC:PTB7-Th/MoO3/Ag. The device performance was measured under a simulated solar illumination of 100 mW/cm2 Am 1.5G under nitrogen atmosphere. The J-V characteristics and EQE spectrum are presented in Figure 2. The photovoltaic properties were summarized in table 1. The active layer with a thickness of around 100 nm was spin-casted from hot chlorobenzene solution. The acceptor/donor mass ratios ranging from 2:1 to 0.8:1 were first investigated and the best acceptor/donor ratio is 1.5:1 for both βTPB-C/βTPB:PTB7-Th devices. The βTPB6:PTB7-Th devices with 1.5:1 blend ratio show the optimized average PCE of 5.58 % with Voc of 0.82 eV, Jsc of 11.9 mAcm-2 and FF of 0.57, while βTPB-C:PTB7-Th devices give the higher average PCE of 6.58 % with Voc of 0.92 eV, Jsc

of 12.8 mAcm-2 and FF of 0.56. The Voc value of βTPB-C based devices is indeed higher than that with βTPB due to its higher LUMO energy level after cyclization. Although the LUMO energy level difference of 0.09 eV between βTPB-C and PTB7-Th is much lower than the empirical 0.3 eV driving force for efficient charge separation, the βTPB-C:PTB7-Th devices still demonstrated promising photovoltaic performance. A small amount of 1,8-diiodooctane (DIO) and diphenylether (DPE) can further increase the Jsc values from 12.8 to 14.7 mAcm-2 and result in the enhanced average PCE of 7.56 % and the highest PCE is 7.69 % with Voc of 0.92 V, Jsc of 14.9 mAcm-2 and FF of 0.56. There is no performance enhancement for the βTPB:PTB7-Th based devices after adding a small amount of additives of DIO and DPE. The 2.5% DIO and 2.5% DPE deteriorate the Jsc from 11.9 mAcm-2 to 10.4 mAcm-2, but slightly enhanced the Voc and FF, which are typical characteristics of larger domain size caused by additives, and the overall result is a lower PCE of 5.16 %. The external quantum efficiency (EQE) of these devices was measured to estimate the Jsc. The Jsc values calculated from EQE are all in less than 10% deviation from Jsc deduced from OPV devices. The curves of EQE spectrum is very similar with their corresponding blend film absorption, indicating both the donor and acceptors make contribution to the Jsc. It should be noted that even the HOMO energy level difference between βTPB6-C and PTB7-Th is around 1 eV, the holes generated in acceptor βTPB6-C still can be efficiently transferred to donor polymer PTB7-Th, as evidenced by the high EQE value in the spectral range between 400-550 nm. The charge separation and recombination dynamics were also investigated by using the method of charge dissociation probability P (E, T) and light intensity dependence of Jsc, as shown in figure 2e,f. By plotting photocurrent density Jph (defined by JL-JD; JL and JD are light and dark current density) against the effective voltage Veff (defined by Vo-V, Vo is the voltage where Jph = 0 ) in logarithmic scale, the P (E, T) can be calculated according to the equation of Jph /Jsat, where Jsat is the Jph reaches its saturation at high reverse voltage which means all the photogenerated exitons were dissociated to free charges and swept out. The P (E, T) under Jsc condition for βTPB6/PTB7-Th without /with additives are 90 % and 87 %, respectively, which is in accordance with the slightly decrease of Jsc after add DIO and DPE additive. The P (E, T) under Jsc condition for βTPB6-C/PTB7-Th without /with additives are both 90 %, indicating the relatively efficient exciton dissociation at interfaces. The measurement of the Jsc as a function of illumination intensity in logarithmic scale was performed to evaluate the bimolecular recombination kinetics. The higher value of slope implies the weaker free carrier recombination. If the slope reaches 1, all free carrier can be swept out and collected by electrode. The linear scaling of photocurrent to light intensity is demonstrated for all four devices with the exponential factors of 0.96 and 0.94 for βTPB6:PTB7-Th devices without/with additive, 0.96 and 0.95 for βTPB6C:PTB7-Th devices without/with additive. The relative high and similar values imply that the bimolecular recombination in all the four devices is similar and weak.

Table 1: J-V characteristics of solar cell devices based on βTPB6:PTB7-Th and βTPB6-C:PTB7-Th blend film; the hole and electron mobility of blend films using SCLC method.



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Device

Additives

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Jsc (mAcm-2)

Voc (V)

FF

Eff(%) (best device)

µe (cm-2V-1s-1)

µh (cm-2V-1s-1)

βTPB6

N

11.9 ±0.6

0.82±0

0.57±0.01

5.58±0.27 (5.85)

5.23x10-5

2.72x10-4

βTPB6

DIO+DPE 2.5+2.5%

10.4 ±0.3

0.85±0

0.59±0.00

5.16±0.17 (5.33)

1.88x10-5

1.62x10-4

βTPB6-c

N

12.8 ±0.2

0.92±0

0.56±0.00

6.58±0.13 (6.72)

4.45x10-5

2.01x10-4

βTPB6-c

DIO+DPE 2.5+2.5%

14.7 ±0.4

0.92±0

0.56±0.01

7.56±0.13 (7.69)

4.67x10-5

2.67x10-4

Figure 2. (a),(b) J-V characteristics of βTPB6:PTB7-Th and βTPB6-C:PTB7-Th based solar cells without/with 2.5%DIO and 2.5%DPE additives; (c),(d) External quantum efficiency spectra of βTPB6:PTB7-Th and βTPB6-C:PTB7-Th based solar cells without/with 2.5%DIO and 2.5%DPE additives; (e) the photocuttent density (Jsc) versus effective voltage (Veff) characteristics of the four solar cell devices; (f) the short current density (Jsc) agaist the light density of the four solar cell devices. Active layer characterization. The blend films UV-vis absorption was measured and presented in Figure 4 e,f. It was found that the shape and intensity of absorption spectrum (except the absorption peak around 470 nm) of βTPB6-C in blend film is very similar with that in neat film. This phenomenon indicates that the βTPB6-C in blend films majorly maintains the same packing order as in neat film, due to its enlarged conjugation skeleton and rigid molecular geometry. However, the absorption of βTPB6 in the blend film shows a sharp 0-0 vibrational peak at 533 nm and a lower 0-1 vibrational peak at 499 nm, different from its neat film absorption, but very similar with its solvent absorption spectrum. Thus, the configuration of βTPB6 in blend film takes a different packing pattern, indicating that the donor polymer chains act as solvent and the βTPB6 has a good miscibility in blend film. This phenomenon was also evidenced in 2D grazing-incidence wide-angle X-ray scattering (GIWAXS) data.

Figure 3. 2D GIWAXS patterns of films on ZnO-modified Si substrates. a) neat βTPB6 film; b), blend film of βTPB6:PTB7-Th without DIO:DPE additive; c), blend film of βTPB6:PTB7-Th with 2.5%DIO:2.5%DPE additive; d) neat βTPB6-C film; e), blend film of βTPB6-C:PTB7-Th without DIO:DPE additive; f), blend film of βTPB6-C:PTB7-Th with 2.5%DIO:2.5%DPE additive.


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Figure 4. (a) in-plane and (b) out-plane line cuts of neat βTPB6 film and blend films of βTPB6:PTB7-Th without/with 2.5%DIO:2.5%DPE additive. (c) in-plane and (d) out-plane line cuts of neat βTPB6-C film and blend films of βTPB6-C:PTB7-Th without/with 2.5%DIO:2.5%DPE additive. (e) solution and film absorption spectra of βTPB6, blend film absorption of βTPB6:PTB7-Th; (f) solution and film absorption of βTPB6-C, blend film absorption of βTPB6-C:PTB7-Th. The GIWAXS analysis was used to investigate the crystallinity and molecular orientation of the pure and blend films. The diffraction patterns are presented in Figure 3 and the out-ofplane/in-plane line cuts from GIWAXS patterns in Figure 4. The GIWAXS pattern of βTPB6 and βTPB6-C neat films both show two strong arc-like scattering, suggesting their crystalline nature and molecular orientation isotropy, which behave like PC61BM/PC71BM. The Bragg reflections at qy ≈ 0.27 Å-1, 1.40 Å-1 and 0.29 Å-1, 1.36 Å-1 was observed for βTPB6 and βTPB6-C, which corresponds to the d-spacing of 23.3, 4.5 and 22.7, 4.6 Å, respectively. The polymer PTB7-Th has two Bragg reflections at qy ≈ 0.27 Å-1 (lamellar d-spacing) and 1.65 Å-1 (π−π stacking). Its Bragg reflections at 0.27 Å-1 is overlapped with acceptors while π−π stacking reflections at 1.65 Å-1 can be used to study the molecular orientation of PTB7-Th. In the blend films, the ratios of the Bragg reflection (0.27 Å-1, 1.65 Å-1) intensity in out-of-plane (qz) to in-plane (qy) are summarized in Table S1 and used to evaluate the molecular orientation of PTB7-Th and acceptors. In both blend films, the π−π stacking reflections of PTB7-Th can be observed both in out-of-plane (qz) and in-plane (qy) directions. For blend films of βTPB6:PTB7-Th and βTPB6-C:PTB7-Th, the ratios of the π−π stacking reflection (1.65 Å-1) intensity in qz direction to qy direction are 1.22 and 1.25, while the ratios for lamellar packing reflection are 2.91 and 1.64. These data indicate a larger fraction of PTB7-Th and βTPB6-C in βTPB6C:PTB7-Th blend film take a face-on orientation than that in βTPB6:PTB7-Th blend film. Using DPE:DIO as additive, the ratios of π−π stacking and lamellar packing reflections are changed from 1.25 and 1.64 to 1.29 and 1.44 in βTPB6C:PTB7-Th blend film, which mean additives promote both PTB7-Th and βTPB6-C taking face-on orientation. However, these values are changed from 1.22 and 2.91 to 1.10 and 5.83 in blend film of βTPB6:PTB7-Th, which implies that additives make more PTB7-Th and βTPB6 take an edge-on orientation. The face-on orientation is beneficial for vertical charge transport and corresponding OPV performance. These results

are reflected in the OPV performances as shown above. Furthermore, the reflections of βTPB6 at qy ≈ 1.40 Å-1 in neat film down-shifts to 1.31 Å-1 in blend film, further reflecting the βTPB6 takes a different packing pattern in the blend film. Therefore, the ease of changing molecular geometry makes the packing pattern of βTPB6 sensitive to the varying surroundings. This characteristic feature promotes larger fraction of βTPB6 and PTB7-Th than βTPB6-C and PTB7-Th takes an edge-on orientation that is detrimental for OPV performance.

Figure 5. AFM of films of (a) βTPB6:PTB7-Th film as deposited, (b) βTPB6:PTB7-Th film with 2.5%DIO:2.5%DPE, (c) βTPB6-C:PTB7-Th film as deposited, (d) βTPB6-C:PTB7Th film with 2.5%DIO:2.5%DPE. The electron and hole mobility of the devices are measured by using space-charge-limited current (SCLC) method with device configuration of ITO/PEDOT:PSS/A:PTB7-Th/MoO3/Ag for hole and ITO/ZnO/A:PTB7-Th/Al for electron. The device based on βTPB6 exhibited electron and hole mobility of



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5.23x10-5 cm-2V-1s-1 and 2.72x10-4 cm-2V-1s-1, respectively. The DPE and DIO co-additives significantly reduce its value to 1.88x10-5 cm-2V-1s-1 and 1.62x10-4 cm-2V-1s-1, which is consistent with observation in GIWAXS data that additives promote βTPB6 and PTB7-Th taking an edge-on orientation. The device based on βTPB6-C exhibied the electron and hole mobility of 4.45x10-5 cm-2V-1s-1 and 2.01x10-4 cm-2V-1s-1, and slightly higher hole and electron mobility of 4.67 x10-5 cm-2V1 -1 s and 2.67x10-4 cm-2V-1s-1 is observed with DIO and DPE additives. All of the mobility data are consistent with the trend in OPV performances. The atomic force microscopy (AFM) was employed to investigate the films morphology of the blend films. Both βTPB6:PTB7-Th and βTPB6-C:PTB7-Th blend films exhibit fibrous morphology with fine and similar domain size, suggesting their 3D molecular geometry lead to the formation of favorable morphology for solar cells. The good blend film morphology could be further evidenced by the efficient photoluminescence quenching of PTB7-Th when it is excited at 640 nm (Figure 1f), indicating the efficient charge separation following the excitation of the donor. The obvious larger domain size is observed for both films after adding DIO and DPE additives. The additives also increase the root-mean-square (RMS) roughness of the blend films from 0.72 nm to 3.24 nm for βTPB6:PTB7-Th, from 0.67 nm to 1.13 nm for βTPB6C:PTB7-Th. The large roughness increase of βTPB6:PTB7-Th indicates the large domains are formed after adding additives, which is also responsible for its reduced Jsc and PCE values. Conclusion Two new compounds βTPB6 and βTPB6-C were synthesized. The cyclized βTPB6-C lacks strong intramolecular charge transfer, resulting in a large blue-shift in optical absorption and larger band gap. DFT calculation revealed that both βTPB6 and βTPB6-C show a 3D molecular geometry and can form favorable blend film morphology with PTB7-Th. Efficient photoluminescence quenching of PTB7-Th was observed in blend films, indicating the efficient charge separation following the excitation of the donor. The more flexible rotation of PDIs renders βTPB6 with varying molecular geometry, thus, its packing pattern in solid state can be easily changed when the surrounding varies. A larger fraction of βTPB6 and PTB7-Th in blend film of are found taking an edge-on orientation than corresponding βTPB6-C and PTB7-Th in blend film. Due to enhanced rigidity of molecular geometry and conjugated skeleton, larger fraction of βTPB6-C and PTB7-Th take a face-on orientation in solid state. The βTPB6-C based inverted solar cells shows the highest efficiency of 7.69 % with Voc of 0.92 eV, Jsc of 14.9 mAcm-2 and FF of 0.56, which is 31% higher than that for βTPB6 based devices. The results indicated that control the interaction between donor and acceptor via tuning molecular geometry can help to enhance the OPV performance for non-fullerene solar cell, a possible new pathway to design high efficient non-fullerene acceptors. ASSOCIATED CONTENT The supporting information is available. It includes synthesis, absorption spectra, photoluminescence spectra, theoretical calculation and device fabrication and characterization.

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AUTHOR INFORMATION Corresponding author: [email protected] Author contribution: ∇ Authors made equal contribution. ACKNOWLEDGEMENTS This work was supported by U. S. National Science Foundation grant (NSF DMR-1263006) and NSF MRSEC program at the University of Chicago (DMR-0213745), DOE via the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award number DESC0001059 and NIST via CHIMAD program. W.C. gratefully acknowledges financial support from the US Department of Energy, Office of Science, Materials Sciences and Engineering Division. We also thank Dr. Joseph Strzalka and Dr. Zhang Jiang for the assistance with GIWAXS measurements. Use of the Advanced Photon Source (APS) at the Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. REFERENCES

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