Effect of Isomerization on High-Performance Nonfullerene Electron

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Effect of Isomerization on High-Performance Nonfullerene Electron Acceptors Jiayu Wang, Junxiang Zhang, Yiqun Xiao, Tong Xiao, Runyu Zhu, Cenqi Yan, Youquan Fu, Guanghao Lu, Xinhui Lu, Seth R. Marder, and Xiaowei Zhan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04027 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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Journal of the American Chemical Society

Effect of Isomerization on High-Performance Nonfullerene Electron Acceptors Jiayu Wang,† Junxiang Zhang,|| Yiqun Xiao,§ Tong Xiao,‡ Runyu Zhu,† Cenqi Yan,† Youquan Fu,† Guanghao Lu,‡ Xinhui Lu,§ Seth R. Marder,|| and Xiaowei Zhan*,† †

Department of Materials Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China

||

School of Chemistry and Biochemistry, and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States

§

Department of Physics, Chinese University of Hong Kong, New Territories, Hong Kong, China

‡

Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an 710054, China

ABSTRACT: We design and synthesize two isomeric fused-ring electron acceptors, FNIC1 and FNIC2, which have same end-groups and same side-chains, but isomeric fused-nine-ring cores. Subtle changes in the two isomers influence their electronic, optical, charge-transport, and morphological properties. As compared with FNIC1, FNIC2 film exhibits a red-shifted absorption peak at 794 nm (752 nm for FNIC1), larger electron affinity of 4.00 eV (3.92 eV for FNIC1), smaller ionization energy of 5.56 eV (5.61 eV for FNIC1), and higher electron mobility of 1.7 × 10−3 cm2 V−1 s−1 (1.2 × 10−3 cm2 V−1 s−1 for FNIC1). The as-cast organic solar cells based on PTB7-Th:FNIC2 blends exhibit power conversion efficiency (PCE) of 13.0%, which is significantly higher than that of PTB7-Th:FNIC1-based devices (10.3%). Semitransparent devices based on PTB7-Th:FNIC2 blends exhibit PCEs varying from 9.51% to 11.6% at different average visible transmittance (AVT, 20.3% to 13.6%), significantly higher than those of PTB7-Th:FNIC1-based devices (7.58%-9.14% with AVT of 20.2% to 14.7%).

Introduction Organic solar cells (OSCs) possess some merits such as their potential to be light weight, flexible, semitransparent in the visible, and to be manufactured on large-area substrates, at low cost and with short energy payback times.1-3 The most common architecture of the active layer is bulk heterojunction (BHJ), consisting of electron donor materials and electron acceptor materials.4,5 Fullerenes and their derivatives have dominated as acceptor materials during the past two decades, for their excellent electron transport properties.6 Meanwhile, various nonfullerene systems have been developed to overcome the shortcomings of fullerenes such as weak absorption, morphological instability, and difficulties in property tunability,7-9 among which rylene imides/amides10-17 and fused ring electron acceptors (FREAs) exhibit promising performance and are two attractive systems in current research.18,19

The concept of FREA was proposed by Zhan et al., in 2015.20 FREAs consist of a side chain-substituted electron-donating planar fused ring flanked with two strong electron-withdrawing end-groups. The above three components: fused-ring core,21-36 end-groups,22,37-45 and side-chains24,46-50 have been modified to explore their roles in determining photovoltaic performance, and in several cases have results in very high performance of OSCs. With respect to the fused-ring core, the conjugation length, substitution and composition significantly affect the absorption, energy level, morphology, mobility and performance of a molecule. For example, inserting or replacing weakly electron-donating benzene-based moieties with more electron-donating thiophene-based units can red-shift absorption, reduce the ionization energy (IE) , and increase crystallinity and mobility,29,31 introducing electron-rich atoms such as oxygen can red-shift absorption and lower optical gap.33 In addition, another effect that can

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also influence the properties of a molecule is “isomeric effect”, which has been widely investigated in organic field-effect transistors and proven to affect absorption, IE and electron affinity (EA), molecular packing, and charge transport,51-53 but has not been extensively explored for FREAs.54,55

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benzo[1,2-b:4,5-b']dithieno[3,2-b]thiophene (BDTT) fused with diarylcyclopentadienylthiophene, while that of FNIC2 is benzo[1,2-b:4,5-b′]dithiophene fused with diarylcyclopentadienylthieno[3,2-b]thiophene. The end-groups of the two isomers are 3-(1,1dicyanomethylene)-5,6-difluoro-1-indanone. Compared with FNIC1, FNIC2 exhibits red-shifted and broader absorption, larger EA, smaller IE, and higher electron mobility. The as-cast OSCs based on blends of FNIC2 and polymer donor PTB7-Th show a champion PCE of 13.0%, significantly higher than PTB7-Th:FNIC1-based devices (10.3%). The PCE of 13.0% is also the highest value for as-cast binary OSCs (the best value was 12.8% reported in literature).60 The semitransparent OSCs (ST-OSCs) based on PTB7-Th:FNIC2 exhibit PCEs varying from 9.51% to 11.6% with average visible transmittance (AVT) ranging from 20.3% to 13.6%, significantly higher than those based on PTB7-Th:FNIC1 (PCEs of 7.58%-9.14% with AVT of 20.2%-14.7%).

It is well known that different isomers can result in very different properties. For example, in the case of OSC, the classical fullerene acceptor PC71BM is a mixture of three isomers if synthesized via traditional route,56 among which one isomer shows very poor power conversion efficiency (PCE) while the other two show good performance.57 For classical nonfullerene acceptor perylene diimides (PDIs), isomers such as 1,6-/1,7-regioisomers and bay/ortho isomers also exhibit different photovoltaic properties.58,59 Thus, it is beneficial to investigate isomeric effects, especially the isomeric effects of fused-ring cores on FREAs. Herein, we design and synthesize two isomeric FREAs FNIC1 and FNIC2 (Chart 1). The core of FNIC1 is

Chart 1. Chemical Structures of FNIC1, FNIC2, and PTB7-Th.

common organic solvents, such as dichloromethane, chloroform, and o-dichlorobenzene (o-DCB). Thermogravimetric analysis (TGA) (Figure S1, Supporting Information) shows that FNIC1 and FNIC2 possess good thermal stability with decomposition temperatures (5% weight loss) of 347 and 316 °C in nitrogen, respectively (Table 1).

Results and Discussion Synthesis and Characterization The compounds were synthesized as shown in Scheme 1 and fully characterized by mass spectrometry, 1H NMR, 13 C NMR, and elemental analysis (see Supporting Information). FNIC1 and FNIC2 are readily soluble in Table 1. Basic Properties of FNIC1 and FNIC2 compound

a

Td (°C)

λmaxabs (nm) solution

film

Ega

εb

(eV)

(M−1 cm−1)

Eox/Eredc

IE

EA

(V)

(eV)

(eV)

µe (10−3 cm2 V−1 s−1)

5

0.81/−0.88

5.61

3.92

1.2

0.76/−0.80

5.56

4.00

1.7

FNIC1

347

734

752

1.48

2.5 × 10

FNIC2

316

750

794

1.38

2.1 × 105

Estimated from the absorption edge in film. b Molar absorptivity at λmax in solution. c The onset oxidation and reduction


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Journal of the American Chemical Society potentials vs. FeCp2+/0.

Scheme 1. Synthetic Routes for FNIC1 and FNIC2. C6H13

C6H13

S

S

S Sn

S

S

Sn

S

Br

Pd(PPh3)4

+

S

(1) BrMg

S

S

S

S

toluene, 110 °C 80%

C2H5OOC

S

C2H5OOC

C6H13

S

S

S

R R S

THF, reflux

COOC2H5

S

C6H13

C6H13

(2) Amberlyst 15 toluene, 110 °C 30%

S R R

S C6H13

C6H13

BDTT-2Sn

S

S

S

S

FN1

BDTT-T C6H13 S

R R

CN

S

(1) n-BuLi, THF, -78 °C OHC

(2) DMF, -78 °C to r.t. 79%

S

S

O

S

S

R R

F

C6H13

R=

FNIC1 CHCl3, 65 °C 55%

S S

pyridine

CN

CHO + F IC2F

C6H13 FN1-CHO

C6H13

C6H13

C6H13

S

S Br

S Sn

Sn S

S

Pd(PPh3)4

+ S

C2H5OOC

S S C2H5OOC

toluene, 110 °C 76%

S

COOC2H5 S

S S

S

(1) BrMg

S

THF, reflux (2)

S

C6H13

S

C6H13

Amberlyst 15 toluene, 110 °C 30%

S

S

S

R R

S

S

C6H13

C6H13

BDT-TT

BDT-2Sn

R R

S

FN2

C6H13 S S

(1) n-BuLi, THF, -78 °C OHC (2) DMF, -78 °C to r.t. 76%

CN R R

S

O S

S

S

R R S

C6H13

CN

CHO +

pyridine FNIC2

S F

F

R=

C6H13

CHCl3, 65 °C 47%

IC2F

FN2-CHO

Figure 1. (a) UV-vis absorption spectra of FNIC1, FNIC2 and PTB7-Th. (b) Cyclic voltammograms for FNIC1 and FNIC2 in CH3CN / 0.1 M [nBu4N]+[PF6]– at 100 mV s-1, and the horizontal scale refers to an Ag/AgCl electrode.



The normalized optical absorption spectra of FNIC1 and FNIC2 in chloroform solution (10–6 M) and thin films are shown in Figure 1a. Both compounds exhibit strong absorption ranging from 600-900 nm, covering visible and extending into near-infrared (NIR) region. FNIC1 shows an absorption maximum at 734 nm with a high molar absorptivity of 2.5 × 105 M–1 cm–1 in solution, while FNIC2 shows a red-shifted maximum at 750 nm but a slightly lower molar absorptivity of 2.1 × 105 M–1 cm–1. The absorption maxima of FNIC1 and FNIC2 films are at 752 and 794 nm, respectively, red-shift 18 and 44 nm relative to those in solution, respectively. The optical gaps of FNIC1 and FNIC2 estimated from the absorption edge of the thin film are 1.48 and 1.38 eV, respectively (Table 1). The electrochemical properties of FNIC1 and FNIC2 were investigated by cyclic voltammetry (CV) with films on a glassy carbon working electrode in 0.1 M [nBu4N]+[PF6]– CH3CN solution at a potential scan rate of 100 mV s–1 (Figure 1b). With the caveat that the reduction waves of both FNIC1 and FNIC2 are irreversible, IE and EA energy levels (Table 1) are very tentatively estimated from the onset oxidation and reduction potentials, respectively, assuming the absolute energy level of FeCp2+/0 to be 4.8 eV below vacuum (oxidation potential of FeCp2+/0 versus Ag/AgCl was measured to be 0.43 V). The IE and EA of FNIC1 are 5.61 and 3.92 eV, respectively; while FNIC2 shows a smaller IE of 5.56 eV and a larger EA of 4.00 eV. The electron mobilities of FNIC1 and FNIC2 were measured using the space charge limited current (SCLC) method in electron-only devices with a structure of Al/FNIC1 or FNIC2/Al(Figure S2).61 The electron mobilities of FNIC1 and FNIC2 are 1.2 × 10−3 cm2 V−1 s−1 and 1.7 × 10−3 cm2 V−1 s−1, respectively (Table 1). Photovoltaic Properties Low optical gap polymer donor PTB7-Th (Chart 1) possesses strong absorption at 500-750 nm which complements those of the acceptors (Figure 1a). Energy levels of PTB7-Th (IE = 5.20 eV, EA = 3.59 eV) fit those of the acceptors. Thus, we used the blends of PTB7-Th: FNIC1 (or FNIC2) to fabricate BHJ OSCs with a structure of indium tin oxide (ITO)/ZnO/PTB7-Th: FNIC1 (or FNIC2)/MoOx/Ag (active area: 4.0 mm2). Both as-cast devices based on PTB7-Th:FNIC1 and PTB7-Th:FNIC2 blends show the best performance at donor/acceptor ratio of 1:1 (w/w, Table S1). PTB7-Th:FNIC1-based devices exhibit VOC of 0.774 V, JSC of 19.97 mA cm−2, FF of 0.664, and PCE of 10.3%. Consistent with the larger EA, red-shifted and broader absorption, and higher mobility of

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FNIC2, PTB7-Th:FNIC2-based devices exhibit lower VOC of 0.741 V, notably higher JSC of 23.93 mA cm−2, and notably higher FF of 0.734, resulting in a high PCE of 13.0% (Table 2, Figure 2a). Larger devices (active area: 12.0 mm2) were also fabricated and measured with masks in different sizes (Table S2). PTB7-Th:FNIC1-based devices exhibit PCEs of 9.76% and 9.45% under 4.0 mm2 and 11.0 mm2 masks, respectively; PTB7-Th:FNIC2-based devices exhibit PCEs of 12.7% and 12.4% under 4.0 mm2 and 11.0 mm2 masks, respectively. The external quantum efficiency (EQE) spectra of the optimized devices are shown in Figure 2b. The EQE maxima of PTB7-Th:FNIC1 and PTB7-Th:FNIC2 blends are 79.2% at 600 nm and 80.2% at 760 nm, respectively. Owing to the broader and red-shifted absorption, PTB7-Th:FNIC2 shows higher EQE from 700 nm to 900 nm relative to PTB7-Th:FNIC1, contributing to a higher JSC. The JSC of PTB7-Th:FNIC1 and PTB7-Th:FNIC2 blends calculated from integration of EQE spectra with the AM 1.5G reference spectrum are 19.51 and 22.76 mA cm−2, respectively, consistent with JSC values measured from J−V (the error is < 5%, Table 2). Charge recombination in the devices was investigated by measuring VOC and JSC with different incident light intensities (Plight) (Figure 2c and Figure 2d). The relationship between VOC and Plight can be described by the formula of VOC ∝ ln Plight.62 If bimolecular recombination dominates in the device, the slope should be equal to 1 kBT/q, where kB is Boltzmann constant, T is temperature, q is the elementary charge; if monomolecular recombination (trap-assisted recombination) dominates, the slope should be equal to 2 kBT/q;63 if surface recombination dominates, the slope should be equal to 0.5 kBT/q.64,65 The slope for PTB7-Th:FNIC1 and PTB7-Th:FNIC2 blends is 0.91 kBT/q and 1.05 kBT/q, respectively, suggesting surface recombination exists in PTB7-Th:FNIC1-based devices while bimolecular recombination dominates in PTB7-Th:FNIC2-based devices. The relationship between JSC and Plight can be described by the formula of JSC ∝ PS, S = 1 indicates all free carriers are swept out and collected at the electrodes prior to recombination, while S < 1 indicates some extent of bimolecular recombination.66 The values of S of PTB7-Th:FNIC1 and PTB7-Th:FNIC2 blends are 0.92 and 0.94, respectively, indicative of weak bimolecular recombination. The hole and electron mobilities of the blended films were measured by the SCLC method with device structures of ITO/PEDOT:PSS/PTB7-Th:acceptor/Au for holes (Figure S3a) and Al/PTB7-Th:acceptor/Al for electrons (Figure S3b). The hole and electron mobilities of


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PTB7-Th:FNIC1-based devices are 5.6 × 10−4 cm2 V−1 s−1 and 6.0 × 10−4 cm2 V−1 s−1, respectively; while PTB7-Th:FNIC2-based devices show hole mobility of 1.5 × 10−3 cm2 V−1 s−1 and electron mobility of 1.4 × 10−3 cm2 V−1 s−1, which are higher than those of PTB7-Th:FNIC1 (Table 2). High mobilities and balanced hole/electron transport are beneficial to high JSC and FF. Another benefit of blending low optical gap donor and low optical gap acceptor to form active layer is to convert NIR light to electricity while leaving visible region transparent or semitransparent, thus realizing visible semitransparent device without reducing active layer thickness, which is a promising practical application of OSCs.28,67 The ST-OSCs based on PTB7-Th:FNIC1 exhibit PCEs varying from 7.58% to 9.14% (Figure S4, Table 2, Table S2, Table S3) with AVT ranging from 20.2% to

14.7% (Figure S5, Table S3); while those based on PTB7-Th:FNIC2 exhibit PCEs varying from 9.51% to 11.6% (Figure S4, Table 2, Table S2, Table S3) with AVT ranging from 20.3% to 13.6% (Figure S5, Table S3). The Comission Internationale de I’Eclairage (CIE) 1931 color coordinate of the transmitted light under CIE Standard illuminant D65 (Figure S6), correlated color temperature (CCT), and coloring rendering index (CRI) of the ST-OSCs are summarized in Table S3. The optical property in the ST devices, including the portion of absorption (BHJ and parasite), transmission, and reflection, were analyzed and shown in Figure S7. With reducing thickness of Ag electrode, the transmittance of the device increases, the reflection slightly decrease, the BHJ absorption decreases, leading to decreased EQE.

Table 2. Photovoltaic Performance and Mobilities of the Optimized Devices Based on PTB7-Th:Acceptor Devicea PTB7-Th:FNIC1c PTB7-Th:FNIC2c PTB7-Th:FNIC1d PTB7-Th:FNIC2e

VOC (V)b

JSC (mA cm−2)b

FFb

PCE (%)b

0.772 ± 0.003

19.84 ± 0.43

0.651 ± 0.012

9.98 ± 0.23

(0.774)

(19.97)

(0.664)

(10.3)

0.732 ± 0.006

23.81 ± 0.31

0.727 ± 0.007

12.7 ± 0.1

(0.741)

(23.93)

(0.734)

(13.0)

0.769 ± 0.003

18.27 ± 0.23

0.634 ± 0.013

8.91 ± 0.22

(0.772)

(18.33)

(0.646)

(9.14)

0.727 ± 0.002

21.42 ± 0.48

0.716 ± 0.014

11.1 ± 0.2

(0.728)

(21.87)

(0.726)

(11.6)

a

calcd JSC (mA cm−2)

µh (cm2 V−1 s−1)

µe (cm2 V−1 s−1)

19.51

5.6×10−4

6.0×10−4

22.76

15×10−4

14×10−4

17.88

-

-

20.99

-

-

PTB7-Th:acceptor = 1:1 (w/w), the device area is 4.0 mm2. b Average values with standard deviation were obtained from 20 devices, the values in parentheses are the parameters of the best devices. c Opaque devices with 70 nm Ag electrodes. d ST devices with 20 nm Ag electrodes, AVT = 14.7%. e ST devices with 20 nm Ag electrodes, AVT = 13.6%.



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Figure 2. (a) The current density-voltage (J-V) curves, (b) EQE spectra, (c) VOC versus light intensity, and (d) JSC versus light intensity of the optimized devices with the structure of ITO/ZnO/PTB7-Th:acceptor/MoOx/Ag.

Film Morphology Grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were carried out to investigate the molecular packing in films.68 The two -dimensional (2D) GIWAXS patterns and the corresponding intensity profiles in the out-of-plane (qz) and in-plane directions (qr) of FNIC1, FNIC2, PTB7-Th:FNIC1, PTB7-Th:FNIC2 films are presented in Figure S8 and Figure 3(a-c), respectively. FNIC1 exhibits weak scattering features in the neat film, suggesting its low crystallinity, while FNIC2 shows “face-on” oriented scattering features with definite lamellar and π-π peaks at qr = 0.360 Å-1 (d = 1.74 nm) and qz = 1.80 Å-1 (d = 0.349 nm), respectively (Figure S8). This is in accordance with the observed lower electron mobility of FNIC1 than that of FNIC2. The PTB7-Th:FNIC1 blend film exhibits a weak “face-on” ordering with the lamellar peak concentrated in the qr direction (qr = 0.280 Å-1, d = 2.24 nm) and the π-π peak concentrated in the qz direction (qz = 1.65 Å-1, d = 0.381 nm), consistent with the lattice constants of previously reported PTB7-Th crystalline scattering domains.68,69 The PTB7-Th:FNIC2 blend film also presents a preferential “face-on” ordering with the lamellar peak at qr = 0.280 Å-1 (d = 2.24 nm) and the π-π

peak at qz = 1.71 Å-1 (d = 0.367 nm). The tighter π-π stacking distance might originate from the co-crystallization of PTB7-Th and FNIC2 due to the relatively higher crystallinity of FNIC2, contributing to the higher carrier mobility and FF. Grazing incidence small-angle X-ray scattering (GISAXS) measurements were performed to understand the phase separation information. The in-plane intensity profiles and the 2D GISAXS patterns of the blend films are presented in Figure 3d and Figure S9, respectively. The data are fitted with Debye-Anderson-Brumberger (DAB) and fractal-like network models, to account for the scattering contribution from the intermixing amorphous phase and acceptor domains respectively.70 The acceptor domain sizes (2Rg) are estimated to be 22.4 and 21.6 nm for PTB7-Th:FNIC1 and PTB7-Th:FNIC2 films, respectively, within the reasonable range for efficient exciton dissociation. The corresponding correlation lengths of the amorphous intermixing phase are fitted to be 55.7 and 50.8 nm. the relative smaller intermixing regions for PTB7-Th:FNIC2 might reduce the recombination and facilitate the charge collection from these regions, thereby resulting in higher JSC and FF of the device.


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Figure 3. 2D GIWAXS patterns of (a) PTB7-Th:FNIC1 and (b) PTB7-Th:FNIC2 blend films. (c) The corresponding GIWAXS intensity profiles along the in-plane (scattered line) and out-of-plane (solid line) directions. (d) The GISAXS intensity profiles and best fittings along the in-plane direction.

Film-Depth-Dependent Light Absorption 71,72

Film-depth-dependent light absorption spectroscopy was utilized to study the vertical phase segregation of the active layers. Figure 4a and 4b show light absorption spectra of PTB7-Th:FNIC1 and PTB7-Th:FNIC2 films at different film depth, and each spectrum is according to a sub-film is with thickness ca. 12-16 nm. Both PTB7-Th:FNIC1 and PTB7-Th:FNIC2 films show apparent vertical phase variation, due to the phase evolution between air/liquid-layer and liquid-layer/ZnO interfaces during spin coating. Upon fitting the light absorption profiles of sub-films using the absorption spectra of corresponding neat materials, we obtained film-depth dependent composition distribution along film depth directions of the active layers (Figure 4c and 4d, where depth 0 and 100 nm represent the active layer/MoOx and active layer/ZnO interfaces, respectively). For both PTB7-Th:FNIC1 and PTB7-Th:FNIC2 blends, donor (PTB7-Th) contents in the half part of the film at depth 0-50 nm (half film near MoOx) are higher than 50 wt%, while acceptor (FNIC1 and FNIC2) contents in the other half part (half film near ZnO) are overall > 50%. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was also used to analyze the

vertical phase segregation (Figure S10). The ToF-SIMS results suggest both acceptors are more concentrated at the half film near ZnO, consistent with the light absorption results. This vertical segregation, with donor and acceptor dominating in the regions near MoOx and ZnO, is beneficial to hole and electron transport towards MoOx/Ag and ZnO/ITO electrodes, respectively. However, as compared with PTB7-Th:FNIC1 whose vertical composition distribution shows a “S” like profile, PTB7-Th:FNIC2 film is featured with a better-defined bilayer interpenetrated structure, which could reduce both surface and bimolecular recombination and improve the charge mobility in the photovoltaic devices. Moreover, the absorption profile of PTB7-Th:FNIC1 film shows a relatively broad peak with some shoulders in the wavelength range of 500-900 nm (Figure 4e). However, in the same range the absorption spectra of PTB7-Th: FNIC2 film are featured with several well-resolved absorption peaks (Figure 4f), implying that the crystalline ordering of molecules in PTB7-Th:FNIC2 is better than that in PTB7-Th:FNIC1, consistent with GIWAXS results.



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Figure 4. Film-depth-dependent variation of PTB7-Th: acceptor films. (a, b) Light absorption spectra of PTB7-Th:FNIC1 (a) and PTB7-Th:FNIC2 (b) films at different film depth. For clarity, the spectra are vertically shift. Each spectrum is corresponding to a sub-film with thickness ca. 12-16 nm. (c, d) Composition distribution as a function of film depth for PTB7-Th:FNIC1 (c) and PTB7-Th:FNIC2 (d). (e, f) Light absorption spectra of half films. The film is assumed to be divided into two half films, namely half film near MoOx and half film near ZnO.


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Conclusions In summary, we designed and synthesized two isomeric FREAs, FNIC1 and FNIC2, and investigated the isomeric effects on optical, electronic, morphological, charge-transport, and photovoltaic properties. FNIC1 and FNIC2 have same end-groups and same side-chains, but isomeric fused-ring cores. Compared with its counterpart FNIC1 based on thiophene fused benzo[1,2-b:4,5-b']dithieno[3,2-b]thiophene core, FNIC2 based on thieno[3,2-b]thiophene fused benzo[1,2-b:4,5-b′]dithiophene core shows smaller IE and larger EA, thus narrower optical gap and red-shifted absorption, which is beneficial to higher JSC. FNIC2 possesses higher crystallinity in neat film and can co-crystallize with PTB7-Th in blended films, thus exhibits higher mobilities, facilitating enhancement in JSC and FF. Isomeric effects also influence the vertical phase segregation, PTB7-Th:FNIC2 blend shows a better-defined bilayer interpenetrated structure with proper donor/acceptor vertical distribution fitting inverted geometry, reducing both surface and bimolecular recombination. The as-cast OSCs based on PTB7-Th:FNIC2 exhibit PCEs of up to 13.0%, significantly higher than PTB7-Th:FNIC1-based devices (10.3%). The semitransparent OSCs based on PTB7-Th:FNIC2 exhibit PCEs varying from 9.51% to 11.6% at different AVT from 20.3% to 13.6%, significantly higher than those of PTB7-Th:FNIC1-based devices (7.58%-9.14%) at AVT from 20.2% to 14.7%.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed experimental procedures including synthesis, characterization, and device fabrication, semitransparent device data, and additional characterization data, such as TGA, SCLC, GIWAXS, GISAXS, and ToF-SIMS.

Author Information Corresponding Author *[email protected] Notes The authors declare no competing financial interest.

Acknowledgements

X.Z. thanks the National Natural Science Foundation of China (Grant No. 21734001 and 51761165023). Y.X. and X.L. thank the financial support from Research Grant Council of Hong Kong (General Research Fund No.14314216 and Theme-based Research Scheme No. T23-407/13-N), NSFC/RGC Joint Research Scheme No. N_CUHK418/17, and the beam time and technical supports provided by 19U2 beamline at SSRF, Shanghai. G.L. thanks the “National Young 1000 Talents” Program of China. S.R.M. and J.Z. acknowledge the support of the Department of the Navy, Office of Naval Research Award Nos. N00014-14-1-0580 and N00014-16-1-2520.

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Effect of Isomerization on High-Performance Nonfullerene Electron

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