Tuning Acceptor Strength of Ladder-type N-type Materials to

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A New Thieno[3,2-b]thiophene-Based Acceptor: Tuning Acceptor Strength of Ladder-type N-type Materials to Simultaneously Achieve Enhanced Voc and Jsc of Nonfullerene Solar Cells Shao-Ling Chang, Fong-Yi Cao, Wen-Chia Huang, Po-Kai Huang, Kuo-Hsiu Huang, Chain-Shu Hsu, and Yen-Ju Cheng ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00563 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 24, 2018

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ACS Energy Letters

A New Thieno[3,2-b]thiophene-Based Acceptor: Tuning Acceptor Strength of Ladder-type N-type Materials to Simultaneously Achieve Enhanced Voc and Jsc of Nonfullerene Solar Cells Shao-Ling Chang, Fong-Yi Cao, Wen-Chia Huang, Po-Kai Huang, Kuo-Hsiu, Huang, ChainShu Hsu and Yen-Ju Cheng* Department of Applied Chemistry, National Chiao Tung University, 1001 University Road, Hsinchu Taiwan 30010. Corresponding Author

E-mail: [email protected]

Abstract A new thieno[3,2-b]thiophene-incorporated acceptor TTC has been developed. The TTC acceptor was installed in a haptacyclic ladder-type core (BDCPDT) to furnish an n-type BDCPDT-TTC. The standard PBDB-T:BDCPDT-IC device showed a PCE of 9.33% with a Voc of 0.86 V and a Jsc of 16.56 mA/cm2. By molecular engineering of the acceptor unit, the BDCPDT-TTC:PBDB-Tbased device delivered an enhanced efficiency of 10.29% with simultaneously enhanced Voc of

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0.94 V and Jsc of 17.72 mA/cm2. The incorporation of the electron-donating thieno[3,2b]thiophene unit into the acceptor moiety decreases the electron-accepting strength, thereby upshifting the HOMO/LUMO energy levels to decrease the ∆EHOMO and Eloss, achieving a larger Voc. Secondly, the extended conjugated bicyclic thieno[3,2-b]thiophene ring beneficially induces additional optical transition at the short wavelengths, leading to the improvement of Jsc. Alternatively, the BDCPDT-FIC installed with the fluorinated acceptor shows the more red-shifted absorption to achieve a high Jsc of 19.12 mA/cm2.

TOC GRAPHICS

Organic photovoltaic cells (OPVs) have great promise to be integrated with flexible electronic products.1-2 The n-type materials have been exclusively dominated by fullerene derivatives which exhibit strong electron affinity, ultrafast electron transfer and isotropic electron transport. However, mono-adduct fullerene derivatives such as PC61BM or PC71BM have several intrinsic shortcomings including weak light-absorbing ability, untunable energy levels, poor morphological stability and photochemical instability. Although various high-efficiency p-type polymers have been specifically synthesized for the PCBM-based devices,3-6 the deficiencies of fullerene-based


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ACS Energy Letters

materials cause a bottleneck that restricts the further advance of OPVs. Development of nonfullerene acceptor (NFA) materials, i.e. organic-based light-absorbing chromophores, is urgently needed for the next generation of n-type acceptors.7-16 The current state-of-the-art organic-based acceptors consist of a push-pull A-LD-A architecture where a central ladder-type electron-rich donor (LD) structure is coupled with two electron-withdrawing 1,1-dicyanomethylene-3-indanone (IC) acceptors (A).17-19 The success of this new class of NFAs lies in the rigid and coplanar LD units which extends effective conjugated length and promotes intermolecular interactions for improving charge mobility and photon-harvesting capability.20 However, simultaneously obtaining a large Voc and a high Jsc in the PSCs is still a great challenge.21-24 To efficiently dissociate excitons, the LUMO/HOMO energy level difference between donor and acceptor (∆ELUMO and ∆EHOMO) in bulk heterojunction should be greater than 0.3 eV.25,26 However, the recent studies revealed that the ∆ELUMO and ∆EHOMO can be smaller than 0.3 eV. There is a growing body of research that also recognizes the importance of reducing ∆EHOMO between donor and acceptor in the NFA-based devices, which is beneficial for reducing Eloss and thus obtaining high Voc.27-31 The photon energy loss (Eloss = Egopt - eVoc) is expressed as the energy difference between the absorbed photon and released electron.25,32,33 We recently developed a benzodi(cyclopentadithiophene) (BDCPDT) core end-capped with two IC acceptors to form BDCPDT-IC showing a high efficiency of 9.33% with a Voc of 0.86 V and a Jsc of 16.56 mA/cm2.34 In an aim to further improve the Voc and Jsc simultaneously, modulation of the electron-deficient strength of acceptor through molecular engineering is the most promising and effective strategy to optimize the optical and electronic characteristics.35,36 The IC moiety is a benzene-fused 5-member ring containing a strong electron-withdrawing dicyanovinylidenyl group and a weak electron-withdrawing acyl moiety. The electron-accepting


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strength of the IC can be adjusted by incorporating different substituents on the benzene ring or replacing the benzene ring by other aromatic or heteroaromatic units.35-43 In this research, we first develop a new promising acceptor unit TTC (2-(5-oxo-5,6-dihydro-7H-cyclopenta[b]thieno[2,3d]thiophen-7-ylidene)malononitrile) simply replacing the benzene ring in IC with a more electronrich thieno[3,2-b]thiophene unit. The sulfur atoms in thieno[3,2-b]thiophene with more loosely held electrons are more easily polarized which might have a tendency to form S…S interactions that facilitates π-stacking and charge transport relative to carbon atom.44,45 The diformylated BDCPDT unit was condensed with the TTC acceptor to form a new A-LD-A-based BDCPDTTCC n-type material. The more electron-rich thieno[3,2-b]thiophene moiety upshifts the LUMO energy level of BDCPDT-TTC to improve the Voc. Moreover, the BDCPDT-TTC exhibits more pronounced absorption at the shorter wavelengths around 300-500 nm due to the additional optical transition of the TTC unit. Compared to the standard BDCPDT-IC device with a PCE of 9.33%, the BDCPDT-TTC-based device exhibits an impressive efficiency of 10.29% with simultaneously enhanced Voc of 0.94 V and Jsc of 17.72 mA/cm2. On the other hand, the BDCPDT was also coupled with the more electron-deficient fluorinated 1,1-dicyanomethylene-3-indanone (FIC) acceptor to afford a new BDCPDT-FIC for comparison. Higher electron-withdrawing ability of fluorine atom enhances the stronger intermolecular charge transfer (ICT), resulting in better light-harvesting ability and thus better Jsc values.46-48 The BDCPDT-FIC-based device delivers a PCE of 8.12 % with an impressive Jsc of 19.12 mA/cm2.


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ACS Energy Letters

R

R

S

S

O S

C6H13

R

O S

O

S

CN

S

CN

S S

S

CN

n

S

NC

S

O

R= 2-Ethylhexyl

S R

C6H13

C6H13

PBDB-T

C6H13

BDCPDT-IC F

F

S C6H13

C6H13 O S

CN

S

NC

S

S

CN

CN

S

S

NC C6H13

C6H13

O

CN

S

O S

Jsc

S Voc

C6H13

CN

S

CN

S

O C6H13

C6H13

BDCPDT-TTC

C6H13

BDCPDT-FIC F

F

Figure 1. Molecular structure of the materials in this research. The synthetic procedures of the TTC, BDCPDT-TTC, and BDCPDT-FIC are depicted in Scheme 1. Chlorination of compound 1 with thionyl chloride led to the formation of compound 2. Without further purification, the AlCl3-catalyzed intramolecular electrophilic acylation of compound 2 with malonyl dichloride formed compound 3 in 52% yield. Knoevenagel condensation of compound 3 with 1 equivalent of malononitrile furnished the TTC in 55% yield. TTC is a mixture of two inseparable regio-isomers, which were used directly without further purification. The synthesis of the key intermediate BDCPDT-CHO has been reported.34 Condensation of BDCPDT-CHO with TTC and FIC led to the final BDCPDT-TTC and BDCPDTFIC, respectively. All intermediates were fully characterized by 1H-NMR, 13C-NMR, and HR-MS. Thermal properties were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). BDCPDT-TTC and BDCPDT-FIC showed a thermal decomposition temperature (Td) of 374 oC and 341 oC, respectively (Figure S1). However, there is no thermal transition observed in the DSC measurements (Figure S2), suggesting that BDCPDT-TTC and BDCPDT-FIC have more amorphous character.


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S

O

Cl

OH

S

O S

O S

Cl

CHCl3

O Cl

S

1

CN

O

Cl

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O

S NC

S

Cl

AlCl3, CH2Cl2 52 %

S

2

O

CN

O

S

CN

CH3COONa, DMSO 55 %

S

S O

TTC isomers

3

CN

CN

S S O R

TTC isomers Pyridine R

OHC

S

R

S

S

S

R

S

NC

S

S

O

CN

S

R

CN

R F

BDCPDT-TTC

F

S

R

BDCPDT-CHO

CN

Chloroform 51 %

CHO

R

CN

CN

S O

F R F

FIC

R

S

S

CN CN

O CN

Pyridine Chloroform 58 %

S

NC

O

S

R

R

BDCPDT-FIC R=

F

C6H13

F

Scheme 1. Synthesis of TTC, BDCPDT-TTC, and BDCPDT-FIC. The normalized absorption spectra of the three materials in solution and thin film are shown in Figure 2a and 2b. The optical properties are listed in Table 1. BDCPDT-TTC exhibits a maximum peak at 667 nm in CHCl3 solution, which is ca. 20 nm blue-shifted relative to that of BDCPDT-IC. This hypsochromic absorption of BDCPDT-TTC comes from the more electronrich thieno[3,2-b]thiophene (TT) unit which weakens intermolecular charge transfer (ICT) between the ladder-type segment (BDCPDT) and the electron-withdrawing TTC groups. Nevertheless, due to the optical transition of TTC unit, BDCPDT-TTC has an extra absorption band at 350-470 nm which is not observed from BDCPDT-IC and BDCPDT-FIC counterpart. The simulated absorption spectrum of BDCPDT-TTC by DFT calculations also shows an additional band at 409 nm (f = 0.42) (Figure S3). Due to the more electron-deficient FIC unit with two electron-withdrawing fluoro atoms, BDCPDT-FIC exhibits the most red-shifted absorption profile.


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ACS Energy Letters

Figure 2. Absorption spectra of the three materials (a) in chlorobenzene and (b) in thin film; (c) cyclic voltammograms of the materials; (d) energy levels of the materials.

The oxidation and reduction potentials of the three materials were determined by cyclic voltammetry (CV) (Figure 2c). HOMO and LUMO levels of BDCPDT-TTC, BDCPDT-IC, and BDCPDT-FIC were determined to be −5.38/−3.78, −5.41/−3.87, and −5.52/−4.00 eV (Table 1). The energy diagram of the materials is depicted in Figure 2d. BDCPDT-TTC exhibits a highestlying LUMO energy level (–3.78 eV) which is advantageous to obtain high Voc in photovoltaic devices. Notably, the HOMO energy offset (∆EHOMO) between BDCPDT-TTC and the p-type polymer PBDB-T used in this research is only 0.02 eV, which is expected to achieve higher performance as a result of minimizing energy loss (Eloss). Nevertheless, the fluorination of


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BDCPDT-FIC downshifts the HOMO and LUMO levels. The larger ∆EHOMO between PBDB-T and BDCPDT-FIC would lead to larger Eloss and thus lower Voc.

Table 1. The optical and electrochemical properties of the materials. λmax (nm) Compound CB

Film

λonset

Egopt

EHOMO

ELUMO

Egele

(nm)a

(eV)b

(eV)

(eV)

(eV)

BDCPDT-TTC

671

700

782

1.58

-5.38

−3.78

1.60

BDCPDT-IC

696

727

808

1.53

-5.41

−3.87

1.56

BDCPDT-FIC

710

748

833

1.49

-5.52

−4.00

1.52

CB = chlorobenzene, acalculated in the solid state, bEgopt = 1240/λonset.

To investigate the structural and electronic influences of the different acceptor moieties on the energy level and the molecular conformation of the materials, quantum-chemical calculations were performed and shown in Figure 3. Methyl groups are used to replace the hexyl groups for simplicity. Regardless of the acceptor structures, all the three NFAs have highly coplanar frameworks which are important for efficient charge transport. Furthermore, the calculated HOMO/LUMO levels are −5.49 eV/−3.49 eV, −5.57 eV/ −3.58 eV and −5.79 eV/ −3.77 eV for BDCPDT-TTC, BDCPDT-IC, and BDCPDT-FIC, respectively. which is qualitatively consistent with experimental results. As expected, BDCPDT-FIC showed downshifted HOMO/LUMO levels result form the introduction of fluorine. As mentioned previously, the blue-shifted absorption of BDCPDT-TTC is attributable to the weaker ICT. Estimation of dipole moment of the NFA materials can be used to quantify the electron distribution along the backbone. Due to the symmetrical structures, the


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overall dipole moments of the NFAs will be canceled out. Therefore, we also intentionally calculated the local dipole moments of the half molecule of the three NFAs. The dipole moment of BDCPDT-IC is estimated to be 8.65 Debye. BDCPDT-TTC, with the more electron-rich TT unit to weaken the acceptor strength, exhibits a much lower dipole moment of 6.78 D, whereas BDCPDT-FIC with the stronger acceptor shows a larger dipole moment of 9.19 D. The larger diople moment facilitates stronger intramolecular charge transfer. Molecular engineering of the electron-accepting units can effectively modulate the absorption properties and HOMO/LUMO energy levels. (a)

(b)

(c)

LUMO = -3.49

LUMO = -3.58

LUMO = -3.77

HOMO = -5.49

HOMO = -5.57

HOMO = -5.79

C6H13

F

CN

S

S

O

S S

S O

S

C6H13 C6H13

S

C6H13

NC

-9.39 Debye

-7.03 Debye

-6.32 Debye C6H13

S

CN CN

NC

C6H13 O

S

NC

S O

CN

S S

CN

S O

F

C6H13

C6H13

NC

NC

F C6H13

BDCPDT-TTC

C6H13

BDCPDT-IC

F O

S S S

CN NC

C6H13 C6H13

BDCPDT-FIC

Figure 3. HOMO/LUMO frontier molecular orbitals, side view of the optimized geometry, and dipole moments of the half segments. (a) BDCPDT-TTC, (b) BDCPDT-IC and (c) BDCPDTFIC, respectively, calculated at the level of B3LYP/6-31G(d,p).


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We fabricated the inverted devices using the ITO/ZnO/active layer/MoO3/Ag configuration. PBDB-T was chosen as a p-type polymer. The PBDB-T:BDCPDT-IC-based device characteristics described in our previous work are used for comparison.34 Similar processing conditions were employed for BDCPDT-TTC and BDCPDT-FIC-based devices for systematic comparison. Figure 4 depicts the J-V curves and the corresponding external quantum efficiency (EQE) spectra. The device parameters are listed in Table 2. The PBDB-T:BDCPDT-TTC (1:1 in wt %) devices using 0.5 vol % 1,8-diiodooctane delivered a high efficiency of 9.64% with a high Voc of 0.92 V, a Jsc of 17.12 mA/cm2, and an FF of 61.19%. Compared with the BDCPDT-ICbased device (Voc = 0.86 V, Jsc = 16.56 mA/cm2, FF = 65.52%, PCE = 9.33%), BDCPDT-TTCbased device exhibits a higher Voc due to the up-shifted LUMO level of BDCPDT-TTC. The strengthened absorption of BDCPDT-TTC at the shorter wavelengths as a result of thieno[3,2b]thiophene (TT) unit leads to the improved Jsc. Consequently, the BDCPDT-TTC-based device exhibits much higher EQEs in the 350-500 nm region than the BDCPDT-IC based device. After increasing the content of DIO to 1 vol%, the efficiency can be further improved to 10.29%, with a Voc of 0.94 V, an improved Jsc of 17.72 mA/cm2 and an FF of 61.78 %, indicating that efficient exciton separation can take place with a small ∆EHOMO between the donor and acceptor. Meanwhile, the PBDB-T:BDCPDT-FIC device gives a PCE of 8.12%, with a Voc of 0.70 V, a Jsc of 19.12 mA/cm2. Although the Voc is decreased owing to the lower-lying LUMO energy level, the fluorinated BDCPDT-FIC with more red-shifted absorption produces a highest Jsc. As shown in Figure 4b, PBDB-T:BDCPDT-FIC-based device shows larger EQE values in the longer wavelength region from 700–850 nm. Table 2. Parameters of ITO/ZnO/polymer:acceptor/MoO3/Ag devices.


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ACS Energy Letters

Devicea

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Eloss

PBDB-T:BDCPDT-TTC

0.92

17.12

61.19

9.64

0.66

PBDB-T:BDCPDT-TTCb

0.94

17.72

61.78

10.29

0.64

PBDB-T:BDCPDT-ICb

0.86

16.56

65.52

9.33

0.67

PBDB-T:BDCPDT-FIC

0.70

19.12

60.67

8.12

0.79

a

Polymer/acceptor = 1:1 (w/w), 0.5% DIO (v/v).

b

1 vol % DIO as the additive.

Figure 4. (a) J-V curves and (b) EQE spectra of the devices. Grazing incidence wide angle X-ray scattering (GIWAXRD) measurements were used to study the molecular packing and orientation. The 2-dimensional GIWAXS patterns of the neat films and blend films are shown in Figure 5. The corresponding 1-dimensional profiles in the inplane and out-of-plane directions are also provided in Figure S4. The neat BDCPDT-TTC and BDCPDT-FIC films showed a lamellar (100) peak in the out-of-plane direction and a strong diffraction halo at around 1.25 Å-1. BDCPDT-FIC particularly exhibited an obvious π−π stacking peak at qxy =1.68 Å-1 in the in-plane direction, indicating that the BDCPDT-FIC thin film adopts an edge-on orientation. BDCPDT-FIC/PBDB-T blend film exhibited π−π stacking peaks both at


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qxy ≈ 1.75 Å-1 and qz ≈ 1.72 Å-1, implying the coexistence of both face-on and edge-on orientations. However, BDCPDT-TTC:PBDB-T blend film exhibited a much stronger (010) diffraction in the out-of-plane direction at qz ≈ 1.72 Å-1. The face-on π-π stacking is advantageous for vertical charge carrier transport in the device.

Figure 5. GIWAXRD images of (a) BDCPDT-TTC and (b) BDCPDT-FIC neat films, the blend films of (c) PBDB-T:BDCPDT-TTC and (d) PBDB-T:BDCPDT-FIC. Atomic force microscopy (AFM) was used to further investigate the morphologies of these three blend films. From the AFM height images in Figure 6(a-c), the root-mean-square roughness values of PBDB-T:BDCPDT-FIC, PBDB-T:BDCPDT-IC, and PBDB-T:BDCPDT-TTC blend films are 2.8 nm, 4.3 nm, and 12.2 nm, respectively. From the AFM phase images in Figure 6(df), all the three blend films exhibited the homogenous morphologies, indicating good miscibility


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ACS Energy Letters

of PBDB-T polymer and BDCPDT-based NFAs. We have added a paragraph to discuss the AFM measurements.

Figure 6. Height (a-c) and phase (d-f) images of atomic force microscopy: PBDB-T:BDCPDTFIC (a, d), PBDB-T:BDCPDT-IC (b, e), and PBDB-T:BDCPDT-TTC (c, f). In summary, a new thieno[3,2-b]thiophene-incorporated acceptor TTC unit was developed. The TTC acceptor was condensed with a haptacyclic ladder-type donor (BDCPDT) to furnish a new A-LD-A type n-type material, BDCPDT-TTC. Due to the suitable HOMO/LUMO energy levels and complementary absorption, the BDCPDT-TTC was combined with the p-type polymer PBDB-T. The standard BDCPDT-IC-based device showed a PCE of 9.33% with a Voc of 0.86 V and a Jsc of 16.56 mA/cm2. By simply replacing the benzene ring in IC unit with the thieno[3,2b]thiophene to form TTC, the BDCPDT-TTC-based device exhibited a highest efficiency of 10.29% with simultaneously enhanced Voc of 0.94 V and Jsc of 17.72 mA/cm2. Firstly, the


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incorporation of the electron-rich thieno[3,2-b]thiophene unit into the acceptor moiety decreases the electron-accepting strength, thus lifting up the HOMO/LUMO energy levels to decrease the Eloss and achieve a large Voc. Secondly, the extended conjugated bicyclic thieno[3,2-b]thiophene ring beneficially induces additional optical transition at the short wavelengths around 300-500 nm, leading to the improvement of Jsc. Alternatively, the BDCPDT-FIC installed with the fluorinated acceptor shows the strong intramolecular charge transfer and thus more red-shifted absorption to achieve a high Jsc of 19.12 mA/cm2. We envision that the TTC acceptor unit can be used to couple with other well-known ladder-type cores to create new non-fullerene accetpor materials which are promising to exhibite improved both Voc and Jsc. This research demonstrates that molecular engineering of the end-group acceptor is effective to modulate the absorption profile and orbital energy of the A-LD-A type n-type material for further boosting non-fullerne-based OPV efficiency.

ASSOCIATED CONTENT Supporting Information: Synthetic procedures, DSC/TGA measurements, computational details, 1D-GIWAXS profiles and 1H and 13C NMR spectra.

AUTHOR INFORMATION E-mail: [email protected] ACKNOWLEDGMENT This work is supported by Ministry of Science and Technology, Taiwan (grant No. MOST1073017-F009-003) and Ministry of Education, Taiwan (SPROUT Project-Center for Emergent


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Functional Matter Science of National Chiao Tung University). We thank the National Synchrotron Radiation Research Center (NSRRC), and Dr. U-Ser Jeng and Dr. Chun-Jen Su at BL23A1 station.

REFERENCES (1) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868-5923. (2) Li, Y. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Acc. Chem. Res. 2012, 45, 723733. (3) Cheng, S.-W.; Chiou, D.-Y.; Tsai, C.-E.; Liang, W.-W.; Lai, Y.-Y.; Hsu, J.-Y.; Hsu, C.-S.; Osaka, I.; Takimiya, K.; Cheng, Y.-J. Angular-Shaped 4,9-Dialkyl α- and βNaphthodithiophene-Based Donor–Acceptor Copolymers: Investigation of Isomeric Structural Effects on Molecular Properties and Performance of Field-Effect Transistors and Photovoltaics. Adv. Funct. Mater. 2015, 25, 6131-6143. (4) Wu, J.-S.; Cheng, S.-W.; Cheng, Y.-J.; Hsu, C.-S. Donor-Acceptor Conjugated Polymers Based on Multifused Ladder-Type Arenes for Organic Solar Cells. Chem. Soc. Rev. 2015, 44, 1113-1154. (5) Tsai, C.-E.; Yu, R.-H.; Lin, F.-J.; Lai, Y.-Y.; Hsu, J.-Y.; Cheng, S.-W.; Hsu, C.-S.; Cheng, Y.-J. Synthesis of a 4,9-Didodecyl Angular-Shaped Naphthodiselenophene Building Block To Achieve High-Mobility Transistors. Chem. Mat. 2016, 28, 5121-5130.


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