Nonfullerene Polymer Solar Cells Based on a Main-Chain Twisted

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Letter

Non-Fullerene Polymer Solar Cells Based on A Main-Chain Twisted Low Bandgap Acceptor with Power Conversion Efficiency of 13.2% Weiping Wang, Baofeng Zhao, Zhiyuan Cong, Yuan Xie, Haimei Wu, Quanbin Liang, Sha Liu, Feng Liu, Chao Gao, Hongbin Wu, and Yong Cao ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00627 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Energy Letters

Non-Fullerene Polymer Solar Cells Based on A Main-Chain Twisted Low Bandgap Acceptor with Power Conversion Efficiency of 13.2% Weiping Wang,

a, †

Baofeng Zhao,a, † Zhiyuan Cong, a, † Yuan Xie,b Haimei Wu,a Quanbin Liang,b

Sha Liu,b Feng Liu, c,* Chao Gao,a,* Hongbin Wu, b,* Yong Cao b a

State Key Laboratory of Fluorine & Nitrogen Chemicals, Xi’an Modern Chemistry Research

Institute, Xi’an, Shaanxi, 710065, P. R. China. E-Mail: [email protected] (C. Gao). b

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, P. R. China. E-Mail: [email protected] (H. B. Wu) c

Department of Physics and Astronomy, and Collaborative Innovation Center of IFSA (CICIFSA), Shanghai Jiaotong University, Shanghai, 200240, P.R. China. E-Mail: [email protected] (F. Liu).

ACS Paragon Plus Environment

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ABSTRACT. A new acceptor–donor–acceptor-structured non-fullerene acceptor, 2,2'-((2Z,2'Z)(((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7diyl)bis(4-((2-ethylhexyl)oxy)thiophene-4,3-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile

(i-IEICO-4F),

is

designed

and

synthesized via main-chain substituting position modification of 2-(5,6-difluoro-3-oxo-2,3dihydro-1H-indene-2,1-diylidene)dimalononitrile. Unlike its planar analogs IEICO-4F with strong absorption in near-infrared region, i-IEICO-4F exhibits twisted main-chain configuration, resulting in 164 nm blue-shifts and leading to complementary absorption with the wide bandgap polymer (J52). A high solution molar extinction coefficient of 2.41×105 M−1 cm−1, and sufficiently high energy of charge-transfer excitons of 1.15 eV in J52:i-IEICO-4F blend were observed, in comparison with that of 2.26×105 M-1cm-1 and 1.10 eV for IEICO-4F. An power conversion efficiency of 13.18% with an open-circuit voltage (0.849 V), a short-circuit current density of 22.86 mA cm-2, and a fill factor of 67.9% were recorded in J52:i-IEICO-4F-based polymer solar cells (PSCs), demonstrating that this main-chain twisted strategy can be a guideline that facilitate the development of new acceptors to maximize the efficiency in PSCs.


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ACS Energy Letters

Recently, polymer solar cells (PSCs) based on non-fullerene low bandgap small molecules (NFAs) as electron acceptors have attracted intense research attention, owing their tunable absorptive region, electronic structure and aggregation behavior as compared to fullerene-based acceptors (PC61BM, PC71BM etc.).

1-4

To date, the most efficient PSCs are now exceeding 13%

power conversion efficiency (PCE) under laboratory conditions, realized through the use of finely selected conjugated polymer donors and a variety of aromatic fused rings-based acceptordonor-acceptor (A-D-A) n-type small molecular acceptors, either in binary or in ternary/tandem PSCs.5,6 Hence, one can arrive at a comprehensive view of picture for the development of electron acceptors, that the main-chain planar molecules with bulkily electronic-donating fused aromatic core (D) and electronic-withdrawing segment (A) is of crucial importance in achieving efficient charge separation and charge generation. These new molecules not only possess suitable energy level, but also exhibit complementary absorption with a variety of wide bandgap polymer donors, thus can efficiently harvest solar photons in the visible and even the near infrared (NIR) region. 7-9 The main-chain planar configuration 10 is one of the most important strategies to form intramolecular charge transfer (ICT) in n-type organic semiconductors to tune their photophysical behaviors, molecular energy levels and improve charge transport properties.11,12 However, typical electronic-donating fused aromatic rings usually possess high planarity configuration, thus suffer from strong aggregation and give rise to the formation of large crystalline domains in bulk heterojunction films. As a result, larger phase-separated domain and much reduced exciton diffusion/separation efficiencies, and ultimately low PCEs were observed in these PSCs.13 Later, to minimize the negative impacts of the fused aromatic planar structures on the A-D-A NFAs, a variety of approaches including the introduction of suitable rigid flexible

15

14

or

out-of-plane side chains onto the fused aromatic rings, adoption of proper π-bridged


3


segement

16

and appropriate end groups

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had been demonstrated to target favourable molecular

planarity and aggregation properties. It is important to note that although seveal NFAs based on new structures have been developed recently, 18-27 promising NFAs that deliver high effiency, like IEIC,28 ITIC,29 IDIC,30 O-IDTBR,31 ITIC-Th,32 m-ITIC,33 IT-1M,34 ITCC,35 ITIC-Th136 mainly follow the above-mentioned "planar main-chain/twisted side-chain" design strategy. On the other side, there have been several studies using main-chain non-planar molecules as NFAs for high-performance PSCs. For instance, Chen and coworkers synthesized a spirobifluorene (D) and diketopyrrolopyrrole (A) moieties based NFA (SF(DPPB)4) with crossshaped molecular configuration, the devices based on the acceptor exhibiting a PCE of 5.16% and high open-circuit voltage (VOC) of 1.14 V, when blended with poly(3-hexyl)thiophene (P3HT).37 Recently, Peng and coworkers reported a wide bandgap three-dimensional spirobifluorene-based small molecule

(SFBRCN), the devices from which showed a high

efficiency of 10.26% in binary device and 12.27% in ternary-blend device.38,39 Besides, threedimensional (3D) perylenediimide (PDI)-based NFAs were also successfully developed by several groups. Owing to their high planarity in the PDI core, the resultant devices showed improved device performance,40-44 although their overall efficiency is limited by too large bandgap and up-shifted lowest unoccupied molecular orbital (LUMO) energy level resulting from the twisted main-chain configuration. 45 Recently, a series of ultra-narrow bandgap NFAs (namely IEICO,46 IEICO-4F,47 and IEICO4Cl,48 etc.) with intensively bathochromic shift in absorption spectra were reported by Hou and coworkers, which show promising photovoltaic parameters in both binary/ternary and tandem PSCs. We also noted that a finely twisted structural modulation in these NFAs is highly desired, such that can provide truly complementary acceptor-donor pairs for solar photon harvest. Taking


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ACS Energy Letters

the above considerations into accounts, in this contribution, we herein synthesized a main-chain twisted small molecule organic semiconductor, 2,2'-((2Z,2'Z)-(((4,4,9,9-tetrakis(4-hexylphenyl)4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene4,3-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1diylidene))dimalononitrile (i-IEICO-4F, Figure 1a), which is an isomer of the low bandgap acceptor (IEICO-4F,

47

Figure 1a) by attaching the electron-withdrawing segment 2-(5,6-

difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2F-INCN) in the 4-position instead of 5-position at the neighbouring ((2-ethylhexyl)oxy)thiophene unit. Due to intramolecular steric hindrance caused main-chain twisting, i-IEICO-4F solid film exhibits an absorption edge of 797 nm, which is 164 nm blue-shifted relative to IEICO-4F (961 nm). Moreover, i-IEICO-4F possesses a greater molar extinction coefficient than IEICO-4F. While more complementary absorption between i-IEICO-4F and a fluorobenzotriazole-based wide bandgap polymer (J52)

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will inevitably benefit solar photons harvest, higher lying energy of

charge-transfer excitons (ECT) in this system also benefits higher open-circuit voltage (VOC). As a result, polymer solar cells based on J52:i-IEICO-4F shows a high PCE of 13.18%, with a relatively high VOC of 0.849 V, a short-circuit current density (JSC) of 22.86 mA cm-2, and an enhanced fill factor (FF) of 67.9%. The PCE value is among one of the best results for J52-based binary devices reported so far.47-50 Given the importance of main-chain distorting strategy in determining the photovoltaic properties of the non-fullerene acceptor, we believe it can provide a new design pathway for maximizing light harvesting and photocurrent generation in complementary acceptor-donor systems. The new acceptor i-IEICO-4F was rationally designed by employing quantum chemistry calculations using density functional theory (DFT) with the B3LYP/6–31G** basis set. 51 Figure


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1b and Figure S1, S2 (SI) shows the optimized molecular geometries and frontier molecular orbitals of the two small molecules, from which it can be seen that attaching 2F-INCN at different positions of the bridged thiophene ring show great impacts on the molecular geometries and electron density distributions of the highest occupied molecular orbital (HOMO) level and lowest unoccupied molecular orbital (LUMO) level. The side-view of IEICO-4F (Figure 1b) exhibits a planar geometry with dihedral angle of 0.3° (θ1 and θ4) between the bridged thiophene and planar indacenodithieno[3,2-b]thiophene (IDTT) core as well as 1.8° (θ2 and θ3) between the thiophene ring and acceptor 2F-INCN segment. However, obviously twisted mainchain is observed for i-IEICO-4F, with the enlarged dihedral angles are 25.9° (θ1), 12.2° (θ2), 19.1° (θ3) and 28.8°(θ4), respectively. Very recently, Zhu and coworkers reported a twisted 14π-electron indenoindene core electron acceptor (NITI), showing dihedral angle of 25° between the planar thieno[3,4-b]thiophene combination and indenoindene core but still remaining planar conformation between the thieno[3,4-b]thiophene-bridged segment and the end-group.52 Compare to NITI, i-IEICO-4F possesses more twisted characteristics, which decreases the πconjugating degree of the molecular main-chain and results in elevated LUMO energy level (3.33 eV) relative to IEICO-4F (-3.38 eV) (Figure S1, S2), which might be benefit to the VOC of the PSCs.


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ACS Energy Letters

(a)

C6H13

F O

O CN

NC

S

F

C6H13

F

*

F

NC

C6H13

IEICO-4F

F

S S

S

CN

S

O

O

S

S

S

(b)

F

C6H13

* n

S N N N

S

C6H13

C6H13

C8H17

J52

F

O

C6H13

F

O NC CN S

S

S

S

NC

CN

O

F O

F

C6H13

C6H13

i-IEICO-4F

Figure 1.(a) Chemical structures and (b) Simulated molecular geometries obtained by DFT calculations for IEICO-4F and i-IEICO-4F. F C6H13

Me3Sn

1

SnMe3

C6H13

Br

S

C4H9 S

O S

OHC a

CHO

S

C 4H 9

O

C6H 13

C 2H 5

CN

S

C6H 13

C6H13

3

O O

S

NC

C 4H 9

F

S S

F

F

CN NC

O

CN b

C6H13

C6H13 C 2H 5

F

NC

O

S C6H13

2

F

O

C2H 5

O

+ S

C6H13

C2H5

C4H9 OHC

S

C6H13

C6H13

C6H13

C 4H 9

C2H5

i-IEICO-4F

Scheme 1. Synthetic route of i-IEICO-4F. Reagents and conditions: a) Pd(PPh3)4, toluene, reflux, 6h; b) 2FINCN, pyridine, CHCl3.

IEICO-4F was synthesized in our laboratory according to the literature.47 The synthetic route toward i-IEICO-4F is shown in Scheme 1. The key intermediate 5-bromo-4-((2ethylhexyl)oxy)thiophene-3-carbaldehyde ethylhexyl)oxy)thiophene

46

(2)

was

synthesized

through

2-bromo-3-((2-

with lithium diisopropylamide (LDA) according to a modified

routine of 5-bromo-4-((2-ethylhexyl)oxy)thiophene-2-carbaldehyde with a yield of 62.8%. After Stille coupling reaction of organic tin compound (1)

29

with compound (2), the dialdehyde

compount (3) was obained as a red solid with high yield of 87.1%. Target molecule i-IEICO-4F was then synthesized by Knoevenagel reaction of dialdehyde (3) and 2F-INCN in 67.2% yield as a black solid. All intermiediates and the obtained acceptor i-IEICO-4F were fully characterized by 1H NMR,13C NMR (Figure S3-S6), elemental and mass analyses. i-IEICO-4F is soluble in


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common solvents such as chloroform, chlorobenzene, and o-dichlorobenzene (>20 mg mL−1), which is enough to fabricate solution-processed OPV devices. Thermogravimetric analysis (TGA) indicats that i-IEICO-4F is thermally stable at up to 342 °C (5 wt% loss, Figure S8, SI). The UV-Vis-NIR absorption spectra of IEICO-4F and i-IEICO-4F in diluted chloroform solution are shown in Figure 2a. The IEICO-4F solution has absorption maxima (λmax) at 806 nm, with a high molar extinction coefficient (εmax) of 2.26×105 M-1cm-1. For i-IEICO-4F, the λmax in solution blue-shifts to 708 nm, while εmax is increased by around 6.6% relative to IEICO4F, reaching 2.41×105 M-1cm-1. To evaluate the performance of i-IEICO-4F as electron acceptor, a wide bandgap polymer J52 (Figure 1a, Eg=1.96 eV) was selected as electron donor material due to its overall optical and electronic properties. Films absorption spectra of J52, IEICO-4F and i-IEICO-4F are depicted in Figure 2b. Relative to IEICO-4F, absorption of i-IEICO-4F film exhibits a significant blue-shift of 164 nm. The optical bandgap (Eg) of i-IEICO-4F film estimated from the absorption edge (λedge, ~797 nm) is 1.56 eV, while IEICO-4F film shows an Eg of 1.29 eV (corresponding to λedge of 961 nm). As can be clearly seen in Figure 2b, IEICO4F shows broader absorption covering the visible and NIR solar spectrum in range from 600 to 1000 nm, while i-IEICO-4F exhibits a truly complementary absorption with J52, indicating both electron acceptors can be good candidate for highly efficient PSCs. On the other side, owing to the phase-separated nature of donor and acceptor domains in the photo active layer, the generation of photocurrent involves exciton diffusion to the heterojunction interface and consequent dissociation therein. As exciton dissociation critically depends on the intermediate state of charge-transfer exciton (CTE) formed at the interface between the donor and the acceptor, in order to facilitate efficient exciton dissociation, it is


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ACS Energy Letters

required that the donor/acceptor singlet exciton energy exceeds that of CTE (ECT). Hence the weakly bound CTE is populated and dissociated into free charges.53 Therefore, efficient dissociation of charge transfer excitons is an prerequisite for promising device performance in organic solar cells.54 To investigate the electronic property of CTE and estimate the energy offset for exciton dissociation in these two material systems, we utilize electroluminescence (EL) spectra as a powerful technique, to detect the presence, determine the energy and quantify the emission efficiency of the of CTEs that formed in the blend.55 The normalized EL spectra of each blend with J52 are shown in Figure 2c and Figure 2d, respectively, together with the EL spectra of pristine i-IEICO-4F and IEICO-4F included for comparison. The EL spectrum of each blend contains a mixture of emissive interfacial charge transfer states in longer wavelength region (lower energy) and IEICO-4F/i-IEICO-4F singlet emission in shorter wavelength region (higher energy). As the lifetime of injected charge carriers (~ µs) in the EL process is larger than fluorescence lifetime (~1-10 ns), thus formation of a thermal equilibrium of CT states is readily reached, followed by radiative decay. Hence, the lower energy emission peak corresponds to the energy of the CT states (ECT). The determined ECT is ~1.15 eV for J52:i-IEICO-4F blend, and ~1.08 eV for J52:IEICO-4F, respectively. The higher ECT in the J52:i-IEICO-4F system is also responsible for the higher open-circuit voltage (VOC) as compared to the J52:IEICO-4F system. With the ECT obtained from the EL spectra of emissive interfacial charge transfer states, the energy offset driving charge separation for photo-induced electron transfer (PET) from donor to acceptor and photo-induced hole transfer (PHT) from acceptor to donor can be calculated through ∆EC1=Eg1-ECT and ∆EC2=Eg2-ECT,56 respectively, where Eg is the optical gap of the donor and acceptor and can be determined from the absorption edge of each material. As exhibited in Figure 2b, the optical bandgaps (Egs) of J52, IEICO-4F and i-IEICO-4F are 1.96 eV, 1.29 eV


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ACS Energy Letters

and 1.56 eV, respectively. Thus, the ∆EC1/∆EC2 is calculated as 0.88 eV/0.21 eV for the J52:IEICO-4F blend, and 0.81 eV/0.41 eV for the J52:i-IEICO-4F blend, respectively. Therefore, J52:i-IEICO-4F shows a driving force (the energetic offset between both energy gaps) ≥0.3 eV for PET and PHT, which is a minimal to populate the CT state in many organic photovoltaic systems,57 while the energetic offset for PHT in the J52:IEICO-4F blend is much lower than this threshold, suggesting J52:i-IEICO-4F system can harvest solar photons in dual

2.5

i-IEICO-4F in CHCl3 2.0 1.5 1.0 0.5 0.0 300

400

500

600

1.2

a

IEICO-4F in CHCl3

Normalized Absorbance

Extinction Coefficient (105 M-1 cm-1)

charge generation pathways more efficiently as compared to J52:IEICO-4F.

700

800

900

Wavelength (nm)

1.2

0.6 0.4 0.2 1100

Wavelength (nm)

0.8 0.6 0.4 0.2

1.2

EL intensity (a.u)

0.8

1000

b

400

500

600

700

800

900

1000

1.4

c

IEICO-4F J52:IEICO-4F (Si detector) J52:IEICO-4F (InGaAs detector)

900

J52 IEICO-4F i-IEICO-4F

Wavelength (nm)

1.0

0.0 800

1.0

0.0 300

1000

1.4

EL intensity (a.u)

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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1200

i-IEICO-4F J52:i-IEICO-4F (Si detector) J52:i-IEICO-4F (InGaAs detector)

d

1.0 0.8 0.6 0.4 0.2 0.0 700

800

900

1000

1100

Wavelength (nm)

Figure 2.(a) Absorption coefficients of IEICO-4F, i-IEICO-4F in chloroform solutions; (b) Normalized absorption spectra of J52, IEICO-4F and i-IEICO-4F films; (c) EL spectra of pristine IEICO-4F and


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ACS Energy Letters

J52:IEICO-4F blend (1:1.3), the green solid line is the smoothed curve as a guide to the eye; (d) EL spectra of pristine i-IEICO-4F and J52:i-IEICO-4F (1:1); the EL spectra with wavelength shorter than 1100 nm was acquired using a high-sensitivity spectrometer (QE Pro, Ocean Optics) with a calibrated Si detector, while spectra at longer wavelength (> 1100 nm) was obtained using a NIR spectrometer (NIR 512, Ocean Optics) with a InGaAs detector.

Thin film morphology of neat materials and BHJ thin films were studied by grazing incident wide-angle X-ray diffraction (GIXD). The two-dimensional (2D) GIXD patterns and the corresponding line-cuts of neat J52, IEICO-4F, i-IEICO-4F films and their blends are shown in Figure 3a-b. J52 is a typical face-on material, which showed a (100) diffraction peak in the inplane (IP) direction at 0.29 Å−1 and a (010) diffraction peak in the out-of-plane (OOP) direction at 1.73 Å−1. The corresponding inter-lamellae spacings are 21.6 and 3.63 Å. The neat film of IEICO-4F showed a strong π−π stacking peak at 1.84 Å−1 in OOP direction, corresponding to π−π stacking distance of 3.41 Å. In comparison, the i-IEICO-4F neat film showed a π−π stacking peak at 1.81 Å−1 in OOP direction, corresponding to π−π stacking distance of 3.47 Å. The crystal coherence length for IEICO-4F and i-IEICO-4F was estimated to be 29.5 Å and 25.6 Å, respectively. Besides, the π-π stacking peak intensity and area for IEICO-4F is higher. Thus IEICO-4F is better ordered, which owns to better chain planarity. Such a trend was also seen for the inter-alkyl stacking, in which IEICO-4F showed a sharper and more intensive (100) peak in IP direction. The (100) packing for IEICO-4F was located at 0.30 Å−1, which is slightly smaller than i-IEICO-4F (0.32 Å−1). The J52:IEICO-4F and J52:i-IEICO-4F blend films show combined diffraction features of J52 and the two acceptors. Yet, both in (100) region and π−π stacking region, the donor and acceptor material scattering features were highly mixed, which thus prohibited detailed fitting analysis. However, the sharper (100) and π−π stacking peak in


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J52:IEICO-4F blends indicated that better crystallinity of IEICO-4F donor was translated to BHJ thin film.

Figure 3. (a) 2D GIXD diffraction images of J52, IEICO-4F, i-IEICO-4F pristine films as well as J52:IEICO-4F and J52:i-IEICO-4F blend films. (b) IP (dotted line) and OOP (solid line) X-ray scattering profiles extracted from the 2D GIXD images. (c) RSoXS scattering profiles of J52:IEICO-4F and J52:iIEICO-4F blend films.

Resonant soft x-ray scattering (RSoXS) at carbon edge (284 eV) is used to investigate the phase separation of J52:IEICO-4F and J52:i-IEICO-4F blend films. The circular averaged scattering profiles were shown in Figure 3c. More intensive scattering was seen for J52:IEICO-


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ACS Energy Letters

4F blends, with a broad scattering hump located at lower q regions. Thus stronger and larger phase separation was presented. Debye-Beuche equation, (I(q))−1/2 = K(a3Q)−1 (1+ a 2q 2 ) , in which K is a constant, a is the correlation length, was used to analysis the RSoXS profiles. J52:IEICO4F blend thin film gave a correlation length of 20.3 nm and J52:i-IEICO-4F blend film gave a correlation length of 12.8 nm. Meanwhile, global fitting by Guinier-Porod method yielded a domain size of 25.8 and 18.5 nm for J52:IEICO-4F and J52:i-IEICO-4F blend films respectively. These results revealed that the none-flat molecule i-IEICO-4F in blends led to reduced size of phase separation, which can be due to the reduced crystallinity of the acceptor material. The photovoltaic performance of i-IEICO-4F was established through fabrication of a serie of devices with an inverted device architecture (indium tin oxide (ITO)/ZnO/J52:i-IEICO4F/MoO3/Al), with PSCs from J52:IEIC-4F blends (D/A weight ratio, 1:1.3) included for comparison. By varying the blend ratio, optimum was reached when J52/i-IEICO-4F (D/A) is 1:1. It is noteworthy that some methods that can successfully tune film morphology, such as thermal annealing (TA) at 130 oC for 10 min can leads to an overall increase in device performance (Figure S10 and Table S1). Figure 4a shows the current density−voltage (J−V) curves of the best devices from each acceptor, as obtained under the illumination of AM1.5G, 100 mW/cm2. The photovoltaic parameters deduced from the J-V curves are summarized in Table 1 for a comparison. The best J52: IEICO-4F device delivers an overall efficiency of 8.20%, with a JSC of 20.48 mA cm-2, a FF of 56.9 % and a VOC of 0.704 V. We note that the previously published work showed slightly higher efficiency and VOC,47 probably due to different device configuration. Despite of its relatively narrower range in photoresponse relative to IEICO-4F devices, the i-IEICO-4F devices shows excellent JSC of 22.86 mA cm-2. Besides,


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significant improvements in VOC (0.849 V) and FF (67.9 %) are reached, giving rise to an impressive PCE of 13.18 %. The average PCE over 15 individual PSCs is 12.76% for J52:iIEICO-4F devices. To the best of our knowledge, the efficiency is among one of the best for the J52-based binary PSCs.47-50 Another important device parameter, the external quantum efficiency (EQE), which is often used to elucidate the photoresponse of solar cells, is depicted in Figure 4b. Clearly, the IECIO4F-based device shows a broad photoresponse covering the entire visible spectrum region and part of UV/NIR spectrum region (300-1000 nm), with a maximal EQE peak value of 67.8% at 550 nm. However, the EQE is strongly wavelength-dependent, with a deep valley at range between 600 nm and 750 nm, and noticeably lower photoresponse in longer wavelength region being clearly seen. While the observed undesired valley can be mainly ascribed to insufficient absorption due to low absorption coefficient at that region, the reasons behind the lower EQE at longer wavelength can be on the one hand a lower absorption coefficient relative to J52, and more importantly, on the other hand a decline of exciton dissociation efficiency due to the smaller energy offset (∆EC2=0.21 eV) that drives hole transfer (PHT) from acceptor to donor. In contrast, the i-IEICO-4F device exhibits relatively narrower but stronger photoresponse in the range of 300-830 nm, with maximal value of 75.5% at 590 nm. The averaged photoresponse of iIEICO-4F device in longer wavelength region between 600 nm and 750 nm is remarkably higher than that of IEICO-4F device (70% vs. 50%), indicating the i-IEICO-4F device reveals better exciton dissociation efficiency.


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80

b

0 70

J52:IEICO-4F J52:i-IEICO-4F

-4

60

EQE (%)

2

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

ACS Energy Letters

-8 -12

50 40 30

-16 20

-20

a

-24 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

J52:IEICO-4F J52:i-IEICO-4F

10 0 300

400

500

600

700

800

900

1000

Wavelength (nm)

Voltage (V)

Figure 4. (a) J-V curves of the inverted PSC devices fabricated by J52:IEICO-4F and J52:i-IEICO-4F; (b) EQE curves of the corresponding PSC devices.

Table 1.Detailed photovoltaic parameters for the J52:IEICO-4F and J52:i-IEICO-4F PSC devices.

a)

Device

VOC [V]

JSC [mA/cm2]

FF [%]

PCEmax(PCEavea) [%]

J52:IEICO-4F

0.704

20.48

56.9

8.20 (8.06)

J52:i-IEICO-4F 0.849 22.86 Calculated from 15 individual PSCs.

67.9

13.18 (12.76)

To gain deeper insight into how does the improved device performance in the i-IEICO-4F devices come, the charge carriers mobilities of the two NFAs-based active layers were recorded by using the space charge limited current (SCLC) method,58 and the measured results are shown in Figure S11 in SI. We note that both of hole (µh) and electron mobilities (µe) of the two NFAsbased devices are improved after TA treatment. For the J52: IEICO-4F film, µh and µe are 1.11×10-4 cm2 V−1 s−1 and 1.35×10-4 cm2 V−1 s−1, respectively, whereas for the J52:i-IEICO-4F film, much improved mobilities of 2.59×10-4 cm2 V−1 s−1 and 3.83×10-4 cm2 V−1 s−1 were obtained for µh and µe, which are favorable in achieving higher FF and higher JSC.33


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The phase separation at the surface of the blend films was studied by atomic force microscopy (AFM) with the results depicted in Figure S12. Both blend films exhibit similar morphological features and a smooth surface, with a small root-mean-square (RMS) roughness of 0.9 nm for ascast films and ~1.1 nm after thermal annealing (TA) treatments, indicating both acceptors have good miscibility in morphology with J52. After TA treatment, the RMS of both blends is slightly increased, suggesting an increase in material crystallinity in the active layers.33 Furthermore, nanoscale phase separation morphologies with proper domain sizes are also observed in J52:iIEICO-4F film, as evidenced by TEM images (Figure S13), which accounts for the higher JSC and FF of the corresponding devices. In order to investigate the charge recombination behavior in the devices, we further studied the effect of varying incident light intensity (P) on the short-circuit current density (JSC), and the results are presented in Figure 5a. Both devices show the relationship of JSC∝ Pα,59 with α=0.99 and α=1.00 for the IEICO-4F device and i-IEICO-4F, respectively. The unity of power exponent in the i-IEICO-4F devices suggests that under short-circuit condition, all of the photogenerated charge carries swept out of the bulk and collected by the electrodes prior to recombination, while a slight decrease in the exponent in IEICO-4F devices may originate from bimolecular recombination loss or space charge effects.60 Consistent with these observations, the i-IEICO-4F device shows higher FF relative to the IEICO-4F device (67.6% vs. 56.9%), resulted from less bimolecular recombination loss and less severe space charge build-up.61 To further illuminate the influence of isomer on device characteristics, transient photocurrent (TPC) and transient photovoltage (TPV) measurements

62,63

were undertaken to determine

lifetime, density of charge carriers and their recombination dynamics in the active layer of each device. As shown in Figure 5b, when illuminated at the same light intensity, as compared to the


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the IEICO-4F device, the i-IEICO-4F device shows higher charge carrier density, consistent with the observed higher JSC as shown above, despite of its more narrow absorption spectrum. Besides, the higher charge carrier density in the i-IEICO-4F device also suggests an increased quasi Fermi level splitting upon photoexcitation, which is a measure of how far the active layer away from equilibrium, fully consistent with the observed higher VOC in the i-IEICO-4F device. The measured lifetime of charge carriers in the devices as a function of charge density is depicted in Figure 5c. At any given charge density probed in this study, the i-IEICO-4F devices shows longer carrier lifetime that corresponds to a slower decay dynamics, in good agreement with observed highest charge density at the same light intensity. 17

1.4x10 17 1.2x10

a

-2

JSC (mA cm )

Carrier Density (cm-3)

J52:IEICO-4F (s=0.991) J52:i-IEICO-4F (s=1.000)

10

10

-2

17

10

16

8x10

16

6x10

16

4x10

10

100

-5

10

-6

10

16

2x10

16

4x10

16

6x10

-3

Charge Density (cm )

16

17

8x10 10

Recombination Coefficient (cm3s-1)

c

J52:IEICO-4F-based device J52:i-IEICO-4F-based device

2

100

Light Intensity (mW/cm )

-4

10

b

J52:IEICO-4F-based device J52:i-IEICO-4F-based device

Light intensity (mW cm )

Lifetime (s)

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

ACS Energy Letters

d

J52:IEICO-4F-based device J52:i-IEICO-4F-based device -11

10

-12

10

-13

10

16

16

2x10

4x10

16

6x10

16

8x10

17

10

-3

Charge Density (cm )


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Figure 5. (a) Light intensity dependence of the short-circuit current density of the devices. TPV and TPC analysis of BHJ solar cells fabricated using IEICO-4F or i-IEICO-4F as acceptor: (b) The measured charge densities in the devices as a function of VOC for different bias light intensities. (c) The derived charge carrier lifetime, determined from TPV measured under device open-circuit condition, as a function of charge density. (d) The measured non-geminate recombination rate coefficient, extracted from carrier lifetime and charge densities, as a function of charge densities.

With the data in Figure 5b and 5c, the nongeminate recombination rate coefficient k(n) as a function of carrier density can be determined via k (n) =

1 and shown in Figure 5d. At any τ (n)n

given light intensity (n), k(n) for the IEICO-4F device is invariably larger than that of the iIEICO-4F device, suggesting nongeminate recombination loss in the the i-IEICO-4F device is obviously alleviated as compared to the IEICO-4F device. We also note that the measured recombination rate coefficients for the devices (~10-11-10-13 cm3s-1) is much lower than that of Langevin recombination (~10-10 cm3s-1, defined as k L =

eµ

ε rε 0

), suggesting non-geminate

recombination is the dominating loss pathway for these devices.64 In short, the isomer i-IEICO4F device shows superior charge extraction properties with higher charge density, longer lifetime and lower recombination rate, leading to a better performance than the IEICO-4F device. In conclusion, we have rationally designed and synthesized a new NF acceptor i-IEICO-4F, which is a main-chain isomer of the narrow bandgap NF acceptor (IEICO-4F). Despite finely structural modification is adopted, obviously twisted main-chain is observed for this isomer, giving rise to largely blue-shifting in film absorption, obvious increment in molar extinction coefficient and enlarged energy of charge-transfer excitons. Polymer solar cells based on J52:iIEICO4F exhibit superior charge extraction properties with higher charge density, longer


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ACS Energy Letters

lifetime and lower recombination rate. As a consequence of these combined characteristics, the VOC, JSC and FF of J52:i-IEICO-4F-device are simultaneously improved, leading to an excellent PCE of 13.18%, which is among one of the best results of J52-based binary devices. We believe that this main-chain twisted strategy could provide an useful guideline for molecular design of highly efficient non-fullerene acceptors. ASSOCIATED CONTENT Supporting Information. Experiments details, 1H NMR,

13

C NMR spectra for the acceptor,

optimal photovoltaic properties, and J1/2-V mobility measurement, AFM and TEM are available. AUTHOR INFORMATION Corresponding Author *E-Mail: [email protected] (C. Gao). *E-Mail: [email protected] (H. B. Wu) * E-Mail: [email protected] (F. Liu). †

W. Wang, B. Zhao and Z. Cong contribute equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the Natural Science Foundation of China (No. 51521002 and 91333206), Shaanxi key scientific and technological innovation team project (2016KCT-28) and Shaanxi key project in industrial field (2017ZDXM-GY-046). TPR were supported by the


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U.S. Office of Naval Research under contract N00014-15-1-2244. Parts of this research were conducted at beamline 7.3.3 and 11.0.1.2, and Molecular Foundry at Lawrence Berkeley National Laboratory, which was sustained by the DOE, Office of Science, and Office of Basic Energy Sciences. REFERENCES (1) Lin, Y.; Zhan, X. Non-Fullerene Acceptors for Organic Photovoltaics: An Emerging Horizon. Mater. Horiz., 2014, 1, 470-488 (2) Lin, Y.; Zhan, X. Oligomer Molecules for Efficient Organic Photovoltaics. Acc. Chem. Res., 2016, 49, 175-183. (3) Fu, H.; Wang, Z.; Sun, Y. Advances in Non-Fullerene Acceptor Based Ternary Organic Solar Cells. Sol. RRL, 2018, 2, 1700158. (4) Wu, Y.; Bai, H.; Wang, Z.; Cheng, P.; Zhu, S.; Wang, Y.; Ma, W.; Zhan, X. A Planar Electron Acceptor for Efficient Polymer Solar Cells. Energy Environ. Sci., 2015, 8, 32153221. (5) Qin, Y.; Chen, Y.; Cui, Y.; Zhang, S.; Yao, H.; Huang, J.; Li, W.; Zheng, Z.; Hou, J. Achieving 12.8% Efficiency by Simultaneously Improving Open-Circuit Voltage and Short-Circuit Current Density in Tandem Organic Solar Cells. Adv. Mater., 2017, 29, 1606340. (6) Cui, Y.; Yao, H.; Gao, B.; Qin, Y.; Zhang, S.; Yang. B.; He, C.; Xu, B.; Hou, J. FineTuned Photoactive and Interconnection Layers for Achieving over 13% Efficiency in a Fullerene-Free Tandem Organic Solar Cell. J. Am. Chem. Soc., 2017, 139, 7302-7309.


20

Page 21 of 29 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

ACS Energy Letters

(7) Li, Y; Zhong, L.; Wu, F.-P.; Yuan, Y.; Bin, H.-J.; Jiang, Z.-Q.; Zhang, Z.; Zhang, Z.G.; Li, Y.; Liao, L.-S. Non-Fullerene Polymer Solar Cells Based on SelenopheneContaining Fused-Ring Acceptor with Photovoltaic Performance of 8.6%. Energy Environ. Sci., 2016, 9, 3429-3435. (8) Li, Y.; Qian, D.; Zhong, L.; Lin, J.-D.; Jiang, Z.-Q.; Zhang, Z.-G.; Zhang, Z.; Li, Y.; Liao, L.-S.; Zhang, F. A Fused-Ring Based Electron Acceptor for Efficient NonFullerene Polymer Solar Cells with Small HOMO Offset. Nano Energy, 2016, 27, 430438. (9) Li, S.; Ye, L.; Zhao, W.; Liu, X.; Zhu, J.; Ade, H.; Hou, J. Design of a New SmallMolecule Electron Acceptor Enables Efficient Polymer Solar Cells with High Fill Factor. Adv. Mater., 2017, 29, 1704051. (10)

Zhan, X.; Tan, Z.; Domercq, B.; An, Z.; Zhang, X.; Barlow, S.; Li, Y.; Zhu, D.;

Kippelen, B.; Marder, S. R. A High-Mobility Electron-Transport Polymer with Broad Absorption and Its Use in Field-Effect Transistors and All-Polymer Solar Cells. J. Am. Chem. Soc., 2007, 129, 7246-7247. (11)

Bai, H.; Wang, Y.; Cheng, P.; Wang, J.; Wu, Y.; Hou, J.; Zhan, X. An Electron

Acceptor Based on Indacenodithiophene and 1,1-Dicyanomethylene-3-indanone for Fullerene-Free Organic Solar Cells. J. Mater. Chem. A, 2015, 3, 1910-1914. (12)

Li, R.; Liu, G.; Xiao, M.; Yang, X.; Liu, X.; Wang, Z.; Ying, L.; Huang, F.; Cao,

Y. Non-Fullerene Acceptors Based on Fused-Ring Oligomers for Efficient Polymer Solar Cells via Complementary Light-Absorption. J. Mater. Chem. A, 2017, 5, 239262393.


21


(13)

Page 22 of 29

Li, Z.; Xu, X.; Zhang, W.; Meng, X.; Ma, W.; Yartsev, A.; Inganäs, O.;

Andersson, M. R.; Janssen, R. A. J.; Wang, E. High Performance All-Polymer Solar Cells by Synergistic Effects of Fine-Tuned Crystallinity and Solvent Annealing. J. Am. Chem. Soc., 2016, 138, 10935-10944. (14)

Wang, J.; Wang, W.; Wang, X.; Wu, Y.; Zhang, Q.; Yan, C.; Ma, W.; You, W.;

Zhan, X. Enhancing Performance of Nonfullerene Acceptors via Side-Chain Conjugation Strategy. Adv. Mater., 2017, 29, 1702125. (15)

Lin, Y.; He, Q.; Zhao, F.; Huo, L.; Mai, J.; Lu, X.; Su, C.-J.; Li, T.; Wang, J.;

Zhu, J.; et al. A Facile Planar Fused-Ring Electron Acceptor for As-Cast Polymer Solar Cells with 8.71% Efficiency. J. Am. Chem. Soc., 2016, 138, 2973- 2976. (16)

Zhao, J.; Li, Y.; Zhang, J.; Zhang, L.; Lai, J. Y. L.; Jiang, K.; Mu, C.; Li, Z.;

Chan, C. L. C.; Hunt, A.; et al. The Influence of Spacer Units on Molecular Properties and Solar Cell Performance of Non-Fullerene Acceptors. J. Mater. Chem. A, 2015, 3, 20108-20112. (17)

Baran, D.; Ashraf, R. S.; Hanifi, D. A.; Abdelsamie, M.; Gasparini, N.; Röhr, J.

A.; Holliday, S.; Wadsworth, A.; Lockett, S.; Neophytou, M.; et al. Reducing the Efficiency–Stability–Cost Gap of Organic Photovoltaics with Highly Efficient and Stable Small Molecule Acceptor Ternary Solar Cells. Nat. Mater., 2016, 16, 363-369. (18)

Qiu, N.; Zhang, H.; Wan, X.; Li, C.; Ke, X.; Feng, H.; Kan, B.; Zhang, H.; Zhang,

Q.; Lu, Y.; et al. A New Nonfullerene Electron Acceptor with a Ladder Type Backbone for High-Performance Organic Solar Cells. Adv. Mater., 2017, 29, 1604964.


22

Page 23 of 29 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

ACS Energy Letters

(19)

Li, M.; Liu, Y.; Ni, W.; Liu, F.; Feng, H.; Zhang, Y.; Liu, T.; Zhang, H.; Wan, X.;

Kan, B.; et al. A Simple Small Molecule As An Acceptor for Fullerene-Free Organic Solar Cells with Efficiency Near 8%. J. Mater. Chem. A, 2016, 4, 10409-10413. (20)

Liu, F.; Zhou, Z.; Zhang, C.; Vergote, T.; Fan, H.; Liu, F.; Zhu, X. A Thieno[3,4‑

b]thiophene-Based Non-fullerene Electron Acceptor for High-Performance BulkHeterojunction Organic Solar Cells. J. Am. Chem. Soc., 2016, 138, 15523-15526. (21)

Xu, S.; Zhou, Z.; Fan, H.; Ren, L.; Liu, F.; Zhu, X.; Russellde, T. P. An Electron-

Rich 2-Alkylthieno[3,4-b]thiophene Building Block with Excellent Electronic and Morphological Tunability for High-Performance Small-Molecule Solar Cells. J. Mater. Chem. A, 2016, 4, 17354-17362. (22)

Dai, S.; Zhao, F.; Zhang, Q.; Lau, T.-K.; Li, T.; Liu, K.; Ling, Q.; Wang, C.; Lu,

X.; You, W.; et al. Fused Nonacyclic Electron Acceptors for Efficient Polymer Solar Cells. J. Am. Chem. Soc., 2017, 139, 1336-1343. (23)

Lin, Y.; Li, T.; Zhao, F.; Han, L.; Wang, Z.; Wu, Y.; He, Q.; Wang, J.; Huo, L.;

Sun, Y.; et al. Structure Evolution of Oligomer Fused-Ring Electron Acceptors toward High Efficiency of As-Cast Polymer Solar Cells. Adv. Energy Mater.,2016, 6, 1600854. (24)

Cao, Q.; Xiong, W.; Chen, H.; Cai, G.; Wang, G.; Zheng, L.; Sun, Y. Design,

Synthesis, and Structural Characterization of the First DithienocyclopentacarbazoleBased n-Type Organic Semiconductor and Its Application in Non-Fullerene Polymer Solar Cells. J. Mater. Chem. A, 2017, 5, 7451-7461. (25)

Guo, W.; Zhao, B.; Xin, J.; Liu, H.; Mi, Y.; Zhang, J.; Guo, Z.; Wei, W.; Ma,

W.; Gao, C.; et al. Non-fullerene Small Molecular Acceptors Based on


23


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Dithienocyclopentafluorene and Dithienocyclopentacarbazole Cores for Polymer Solar Cells. Dyes Pigments, 2017, 144, 48-57. (26)

Zhao, B.; Wang, W.; Xin, J.; Wu, H.; Liu, H.; Guo, Z.; Cong, Z.; Ma, W.; Gao,

C. Absorptive Behaviors and Photovoltaic Performances Enhancements of Alkoxyphenyl Modified Indacenodithieno[3,2-b]thiophene-Based Non-fullerene Acceptors. ACS Sustainable Chem. Eng., 2018, 6, 2177-2187. (27)

Zhang, J.; Zhao, B.; Mi, Y.; Liu, H.; Guo, Z.; Bie, G.; Wei, W.; Gao, C.; An, Z. A

New Wide Bandgap Small Molecular Acceptor Based on Indenofluorene Derivatives for Fullerene-Free Organic Solar Cells. Dyes Pigments, 2017, 140, 261-268. (28)

Lin, Y.; Zhang, Z.-G.; Bai, H.; Wang, J.; Yao, Y.; Li, Y.; Zhu, D.; Zhan, X. High-

performance Fullerene-free Polymer Solar Cells with 6.31% Efficiency. Energy Environ. Sci., 2015, 8, 610-616. (29)

Lin, Y.; Wang, J.; Zhang, Z. -G.; Bai, H.; Li, Y.; Zhu, D.; Zhan, X. An Electron

Acceptor Challenging Fullerenes for Efficient Polymer Solar Cells. Adv. Mater., 2015, 27, 1170-1174. (30)

Lin, Y.; Zhao, F.; Wu, Y.; Chen, K.; Xia, Y.; Li, G.; Prasad, S. K. K.; Zhu, J.;

Huo, L.; Bin, H.; et al. Mapping Polymer Donors toward High-Efficiency Fullerene Free Organic Solar Cells. Adv. Mater., 2017, 29, 1604155. (31)

Holliday, S.; Ashraf, R. S.; Wadsworth, A.; Baran, D.; Yousaf, S. A.; Nielsen, C.

B.; Tan, C.-H.; Dimitrov, S. D.; Shang, Z.; Gasparini, N.; et al. High-Efficiency and Air-Stable P3HT-Based Polymer Solar Cells with A New Non-Fullerene Acceptor. Nat. Commun., 2016, 7, 11585.


24

Page 25 of 29 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

ACS Energy Letters

(32)

Lin, Y.; Zhao, F.; He, Q.; Huo, L.; Wu, Y.; Parker, T. C.; Ma, W.; Sun, Y.;

Wang, C.; Zhu, D.; et al. High-Performance Electron Acceptor with Thienyl Side Chains for Organic Photovoltaics. J. Am. Chem. Soc., 2016, 138, 4955-4961. (33)

Yang, Y.; Zhang, Z.-G.; Bin, H.; Chen, S.; Gao, L.; Xue, L.; Yang, C.; Li, Y.

Side-Chain Isomerization on An n‑type Organic Semiconductor ITIC Acceptor Makes 11.77% High Efficiency Polymer Solar Cells. J. Am. Chem. Soc., 2016, 138, 1501115018. (34)

Li, S.; Ye, L.; Zhao, W.; Zhang, S.; Mukherjee, S.; Ade, H.; Hou, J. Energy-Level

Modulation of Small-Molecule Electron Acceptors to Achieve over 12% Efficiency in Polymer Solar Cells. Adv. Mater., 2016, 28, 9423-9429. (35)

Yao, H.; Ye, L.; Hou, J.; Jang, B.; Han, G.; Cui, Y.; Su, G. M.; Wang, C.; Gao, B.;

Yu, R.; et al. Achieving Highly Efficient Nonfullerene Organic Solar Cells with Improved Intermolecular Interaction and Open-Circuit Voltage. Adv. Mater., 2017, 29, 1700254. (36)

Zhao, F.; Dai, S.; Wu, Y.; Zhang, Q.; Wang, J.; Jiang, L.; Ling, Q.; Wei, Z.; Ma,

W.; You, W.; et al. Single-Junction Binary-Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency. Adv. Mater., 2017, 29, 1700144. (37)

Li, S.; Liu, W.; Shi, M.; Mai, J.; Lau, T.-K.; Wan, J.; Lu, X.; Li, C.-Z.; Chen, H.

A Spirobifluorene and Diketopyrrolopyrrole Moieties Based Non-Fullerene Acceptor for Efficient and Thermally Stable Polymer Solar Cells with High Open-Circuit Voltage. Energy Environ. Sci., 2016, 9, 604-610.


25


(38)

Page 26 of 29

Zhang, G.; Yang, G.; Yan, H.; Kim, J.-H.; Ade, H.; Wu, W.; Xu, X.; Duan, Y.;

Peng, Q. Efficient Nonfullerene Polymer Solar Cells Enabled by a Novel Wide Bandgap Small Molecular Acceptor. Adv. Mater., 2017, 29, 1606054. (39)

Xu, X.; Bi, Z.; Ma, W.; Wang, Z.; Choy, W. C. H.; Wu, W.; Zhang, G.; Li, Y.;

Peng, Q. Highly Efficient Ternary-Blend Polymer Solar Cells Enabled by a Nonfullerene Acceptor and Two Polymer Donors with a Broad Composition Tolerance. Adv. Mater., 2017, 1704271. (40)

Li, H.; Hwang, Y.-J.; Courtright, B. A. E.; Eberle, F. N.; Subramaniyan, S.;

Jenekhe, S. A. Fine-Tuning the 3D Structure of Nonfullerene Electron Acceptors Toward High-Performance Polymer Solar Cells. Adv. Mater., 2015, 27, 3266-3272. (41)

Hwang, Y.-J.; Li, H.; Courtright, B. A. E.; Subramaniyan, S.; Jenekhe, S. A.

Nonfullerene Polymer Solar Cells with 8.5% Efficiency Enabled by a New Highly Twisted Electron Acceptor Dimer. Adv. Mater., 2016, 28, 124-131. (42)

Qiu, N.; Yang, X.; Zhang, H.; Wan, X.; Li, C.; Liu, F.; Zhang, H.; Russell, T. P.;

Chen, Y. Nonfullerene Small Molecular Acceptors with a Three-Dimensional (3D) Structure for Organic Solar Cells. Chem. Mater., 2016, 28, 6770-6778. (43)

Liu, Y.; Lai, J. Y. L.; Chen, S.; Li, Y.; Jiang, K.; Zhao, J.; Li, Z.; Hu, H.; Ma, T.;

Lin, H.; et al. Efficient Non-fullerene Polymer Solar Cells Enabled by TetrahedronShaped Core Based 3D-structure Small-Molecular Electron Acceptors. J. Mater. Chem. A, 2015, 3, 13632-13636. (44)

Liu, Y.; Mu, C.; Jiang, K.; Zhao, J.; Li, Y.; Zhang, L.; Li, Z.; Lai, J. Y. L.; Hu,

H.; Ma, T.; et al. A Tetraphenylethylene Core-Based 3D Structure Small Molecular


26

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Acceptor Enabling Efficient Non-Fullerene Organic Solar Cells. Adv. Mater., 2015, 27, 1015-1020. (45)

Lin, H.; Chen, S.; Hu, H.; Zhang, L.; Ma, T.; Lai, J. Y. L.; Li, Z.; Qin, A.; Huang,

X.; Tang, B.; et al. Reduced Intramolecular Twisting Improves the Performance of 3D Molecular Acceptors in Non-Fullerene Organic Solar Cells. Adv. Mater., 2016, 28, 8546-8551. (46)

Yao, H.; Chen, Y.; Qin, Y.; Yu, R.; Cui, Y.; Yang, B.; Li, S.; Zhang, K.; Hou, J.

Design and Synthesis of a Low Bandgap Small Molecule Acceptor for Efficient Polymer Solar Cells. Adv. Mater., 2016, 28, 8283-8287. (47)

Yao, H.; Cui, Y.; Yu, R.; Gao, B.; Zhang, H.; Hou, J. Design, Synthesis, and

Photovoltaic Characterization of a Small Molecular Acceptor with an Ultra-Narrow Band Gap. Angew. Chem. Int. Ed., 2017, 56, 3045-3049. (48)

Cui, Y.; Yang, C.; Yao, H.; Zhu, J.; Wang, Y.; Jia, G.; Gao, F.; Hou, J. Efficient

Semitransparent Organic Solar Cells with Tunable Color Enabled by An UltralowBandgap Nonfullerene Acceptor. Adv. Mater., 2017, 29, 1703080. (49)

Bin, H.; Zhang, Z.-G.; Gao, L.; Chen, S.; Zhong, L.; Xue, L.; Yang, C.; Li, Y.

Non-Fullerene Polymer Solar Cells Based on Alkylthio and Fluorine Substituted 2DConjugated Polymers Reach 9.5% Efficiency. J. Am. Chem. Soc., 2016, 138, 46574664. (50)

Yu, R.; Zhang, S.; Yao, H.; Guo, B.; Li, S.; Zhang, H.; Zhang, M.; Hou, J. Two

Well-Miscible Acceptors Work as One for Efficient Fullerene-Free Organic Solar Cells. Adv. Mater., 2017, 29, 1700437.


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(51)

Page 28 of 29

Sun, C.; Wu, Z.; Hu, Z.; Xiao, J.; Zhao, W.; Li, H.-W.; Li, Q.-Y.; Tsang, S.-W.;

Xu, Y.-X.; Zhang, K.; et al. Interface Design for High-efficiency Non-fullerene Polymer Solar Cells. Energy Environ. Sci., 2017, 10, 1784-1791. (52)

Xu, S.; Zhou, Z.; Liu, W.; Zhang, Z.; Liu, F.; Yan, H.; Zhu, X. A Twisted

Thieno[3,4-b]thiophene-Based

Electron

Acceptor

Featuring

a

14-π-Electron

Indenoindene Core for High-Performance Organic Photovoltaics. Adv. Mater., 2017, 29, 1704510. (53)

Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells.

Chem. Rev., 2010, 110, 6736-6767. (54)

Proctor, C. M.; Albrecht, S.; Kuik, M.; Neher, D.; Nguyen, T. Overcoming

Geminate Recombination and Enhancing Extraction in Solution-Processed Small Molecule Solar Cells. Adv. Energy Mater., 2014, 4, 1400230 (55)

Tvingstedt, K.; Vandewal, K.; Gadisa, A.; Zhang, F.; Manca, J.; Inganäs, O.

Electroluminescence from Charge Transfer States in Polymer Solar Cells. J. Am. Chem. Soc., 2009, 131, 11819-11824. (56)

Stoltzfus, D. M.; Donaghey, J. E.; Armin, A.; Shaw, P. E.; Burn, P. L.; Meredith,

P. Charge Generation Pathways in Organic Solar Cells: Assessing the Contribution from the Electron Acceptor. Chem. Rev., 2016, 116, 12920-12955. (57)

Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. The Energy of Charge-Transfer

States in Electron Donor–Acceptor Blends: Insight into the Energy Losses in Organic Solar Cells. Adv. Func. Mater., 2009, 19, 1939-1948.


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Page 29 of 29 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

ACS Energy Letters

(58)

Zhang, M.; Wang, J.; Zhang, F.; Mi, Y.; An, Q.; Wang, W.; Ma, X.; Zhang, J.;

Liu, X. Ternary Small Molecule Solar Cells Exhibiting Power Conversion Efficiency of 10.3%. Nano Energy, 2017, 39, 571-581. (59)

Kyaw, A. K. K.; Heeger, A. J.; Wang, D. H.; Gupta, V.; Leong, W. L.; Ke, L.;

Bazan, G. C. Intensity Dependence of CurrentVoltage Characteristics and Recombination in High-Efficiency Solution-Processed Small-Molecule Solar Cells. ACS Nano, 2013, 7, 4569-4577. (60)

Cowan, S. R.; Roy, A.; Heeger, A. J. Recombination in Polymer-Fullerene Bulk

Heterojunction Solar Cells. Phys. Rev. B, 2010, 82, 245207. (61)

He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y.

Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells. Adv. Mater., 2011, 23, 4636-4643. (62)

Shuttle, C. G.; O’Regan, B.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.; de

Mello, J.; Durrant, J. R. Experimental Determination of the Rate Law for Charge Carrier Decay In a Polythiophene: Fullerene Solar Cell. Appl. Phys. Lett., 2008, 92, 093311. (63)

Hamilton, R.; Shuttle, C. G.; O’Regan, B.; Hammant, T. C.; Nelson, J.; Durrant, J.

R. Recombination in Annealed and Nonannealed Polythiophene/Fullerene Solar Cells: Transient Photovoltage Studies versus Numerical Modeling. J. Phys. Chem. Lett., 2010, 1, 1432-1436. (64)

Yan, J.; Liang, Q.; Liu, K.; Miao, J.; Chen, H.; Liu, S.; He, Z.; Wu, H.; Wang, J.;

Cao, Y. Optimized Phase Separation and Reduced Geminate Recombination in High Fill Factor Small-Molecule Organic Solar Cells. ACS Energy Lett., 2017, 2, 14-21.


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Nonfullerene Polymer Solar Cells Based on a Main-Chain Twisted

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