Article pubs.acs.org/Macromolecules
Angular-Shaped 4,10-Dialkylanthradiselenophene and Its Donor− Acceptor Conjugated Polymers: Synthesis, Physical, Transistor, and Photovoltaic Properties Yu-Ying Lai, Huan-Hsuan Chang, Yun-Yu Lai, Wei-Wei Liang, Che-En Tsai, and Yen-Ju Cheng* Department of Applied Chemistry, National Chiao Tung University, 1001 University Road, Hsin-Chu 30010, Taiwan S Supporting Information *
ABSTRACT: An angular-shaped and isomerically pure 4,10di(2-octyl)dodecylanthradiselenophene (aADS) was successfully developed. The expedient synthesis to form the framework of aADS with two lateral side chains regioselectively at its 4,10-positions is via a base-induced propargyl− allenyl isomerization/6π-electrocyclization/aromatization protocol. This pentacyclic distannylated aADS unit was then copolymerized with dithienyldiketopyrrolopyrrole (DPP) and dithienyl-5,6-difluoro-2,1,3-benzothiadiazole (DTFBT) acceptors with different alkyl side chains to afford four donor− acceptor copolymers: PaADSDPP, PaADSDTFBT-C4, PaADSDTFBT-C8, and PaADSDTFBT-C8C12. UV−vis spectroscopy and cyclic voltammetry revealed that PaADSDPP has the narrowest energy band gap, and PaADSDTFBT-C8C12 has larger band gap than PaADSDTFBT-C4 and PaADSDTFBT-C8. Two layer ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) calculations were implemented to investigate the disparity in optical, electrochemical, and device properties between these polymers. Both experimental and theoretical data suggest that the aliphatic side chains play a significant role in determining the physical, transistor, and photovoltaic properties of the polymers. PaADSDTFBT-C4 and PaADSDTFBTC8 exhibited organic-field-effect-transistor hole mobilities of 2.7 × 10−2 and 1.0 × 10−2 cm2 V−1 s−1, greatly outperforming that of PaADSDTFBT-C8C12 with a mobility of 5.4 × 10−6 cm2 V−1 s−1. Polymer solar cells were fabricated on the basis of ITO/ PEDOT:PSS/polymer:PC71BM/Ca/Al configuration. The efficiency decreased as the increase of bulkiness of the aliphatic side chains installed on DTFBT units (4.4% for PaADSDTFBT-C4, 3.5% for PaADSDTFBT-C8, 0.3% for PaADSDTFBT-C8C12). Atomic force microscopy images reveal that the degree of aggregation for the polymer:fullerene blends is influenced significantly by the bulkiness of aliphatic side chain installed on DTFBT. Noticeable aggregation was found for the PaADSDTFBTC8C12:PC71BM blend. These results are in good agreement with the computational results elucidating that the intermolecular interactions between the polymers and PC71BM are sterically hindered by the bulky 2-octyldodecyl groups. This work not only presents a promising selenophene-based aADS building block but also provides insights into the side-chain engineering for donor−acceptor conjugated copolymers.
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INTRODUCTION Over the past few years, much research effort has been devoted to polymer solar cells (PSCs) consisting of organic p-type (donor) and n-type (acceptor) photoactive materials.1 The advancement of PSCs has been driven significantly by the development of ptype conjugated polymers. Construction of new building blocks with multifused conjugated structures is essential to expand the material library for p-type conjugated polymers. 2 The acenedithiophene (AcDT) family, linearly fused benzene rings with two terminal thiophenes, are particularly a superb class of conjugated semiconductors (Scheme 1).3 Tricyclic benzodithiophene (BDT) derivatives,4 the shortest AcDT, have led to numerous successful polymers for highly efficient PSCs and organic field-effect transistors (OFETs). Very recently, tetracyclic naphthodithiophene (NDT)5 derivatives have been also demonstrated as promising building blocks to construct conjugated copolymers.6 Depending on the fused junction between benzene rings and thiophenes, NDT can have different © XXXX American Chemical Society
Scheme 1. Chemical Structures of Typical Acenedithiophenes (AcDTs)
Received: July 12, 2015 Revised: September 21, 2015
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4.24%, outperforming the solar efficiency of the linear ADTbased (lADT) copolymer.10 Structurally similar to thiophene, selenophene with selenium to substitute sulfur has received increasing interest in development of next-generation organic semiconductors due to its many intrinsic advantages over thiophene.11 Computational calculations suggest that polyselenophene has higher quinoidal character and less tendency of twisting than polythiophene, resulting in stronger intermolecular interactions.11a,12 Selenium is more polarizable than sulfur, which might induce an intermolecular Se−Se interaction.13 These properties are advantageous characteristics for improving light-harvesting ability and charge carrier mobility. It is therefore of great interest to develop a variety of new acenediselenophene (AcDS) derivatives by replacing thiophene in AcDT with selenophene and investigate chalcogen effects with the corresponding AcDT family. Nevertheless, in comparison with thiophene-based counterparts, research on selenophene-containing materials is relatively limited due to the fact that the functionalization of the less reactive selenophene is synthetically more challenging.14 Applications of many traditional reactions used for thiophene are still unknown for selenophene. Furthermore, implantation of aliphatic side chains into the AcDS is necessary to improve the solubility of the resulting AcDS-based copolymers, which requires new synthetic methodologies. In this research, for the first time, we designed a new strategy to successfully synthesize an isomerically pure and angular-shaped anthradiselenophene (aADS) framework. The key synthetic step is the base-induced intramolecular 6π electrocyclization of a 2,5-bis(4-octyl-tetradec1-ynyl)-1,4-bis(selenophen-2-yl)benzene intermediate, forming the core structure of aADS with two lateral octyldodecanyl side chains regioselectively at its 4,10-positions. The side-chain engineering has been demonstrated to have significant impact on the bulk properties of D−A copolymers.15 The length, branch, and rigidity of the side chains on the conjugated framework could dramatically influence the solubility, crystallization, and molecular packing. The aADS unit can be selectively stannylated at the α-positions of selenophene and copolymerized with dithienyl-
isomeric structures such as linear-shaped NDT (lNDT) and angular-shaped NDT (aNDT) units. It has been demonstrated by Takimiya and co-workers that the geometrical shape of NDT has substantial influence on the solid-state packing of the resultant polymers.6b The aNDT-based polymer can assemble into more ordered packing structure than the lNDT analogue, leading to much higher PSC efficiencies.6b Further installation of one more benzene ring into a NDT leads to a pentacyclic anthradithiophene (ADT, Scheme 1), which has also attracted considerable attention because of its extended conjugation along with diverse isomeric structures.7 Linear anthradithiophene (lADT) with two trialkylsilyl groups at 5, 11 or 3, 9 positions of central anthracene moiety has been successfully demonstrated as a superb class of materials for solution processed OFET.8 However, the incorporation of lADT units into polymers has only resulted in limited success because of the poor solubility as well as undefined lADT structures with syn/anti isomers.7b,9 We previously reported the synthesis of an isomerically pure and angular-shaped ADT (aADT, Scheme 2) with four lateral alkyl Scheme 2. Chemical Structures of Alkylated Angular Anthradithiophene (aADT) and Angular Anthradiselenophene (aADS)
side chains at the 4, 5, 10, and 11 positions via multiple Suzuki− Miyaura cross-coupling reactions.10 This electron-rich unit was copolymerized with a dithienyldiketopyrrolopyrrole (DPP) acceptor to afford a donor−acceptor (D−A) PaADTDPP copolymer with good solution processability. The aADT-based polymer delivered a power conversion efficiency (PCE) of
Scheme 3. Chemical Structures of aADS-Based Donor−Acceptor Copolymers, PaADSDTFBT-C4, PaADSDTFBT-C8, PaADSDTFBT-C8C12, and PaADSDPP
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Macromolecules Scheme 4. Synthesis of Sn-aADS Monomer
Scheme 5. Proposed Mechanisms of Cyclization
under Stille-coupling reaction conditions to yield compound 4. The key step to construct the aADS core involves base-induced 6π-electrocyclization of compound 4 in the presence of 1,8diazabicyclo[5.4.0]undec-7-ene (DBU) in refluxing N-methylpyrrolidone (NMP), forming the central anthracene core of aADS with the alkyl chains at the 4,10-positions. Proposed mechanisms are shown in Scheme 5. The propargyl moiety is first isomerized to an allene group with the assistance of DBU as a base. Intramolecular 6π-electrocyclization followed by aromatization leads to the formation of the benzene ring. To the best of our knowledge, it is the first example demonstrating that selenophene can undergo dieneyne-based cyclization to form a benzene moiety. We envisage that this synthetic strategy can be utilized to create various benzoselenophene-containing conjugated materials. Subsequently, the aADS was lithiated by n-butyllithium followed by quenching with trimethyltin chloride to generate the distannyl Sn-aADS in a moderate yield of 42%. Sn-aADS was
diketopyrrolopyrrole (DPP) and dithienyl-5,6-difluoro-2,1,3benzothiadiazole (DTFBT) units with various alkyl side chains to afford four donor−acceptor copolymers: PaADSDPP, PaADSDTFBT-C4, PaADSDTFBT-C8, and PaADSDTFBTC8C12 (Scheme 3). Their synthesis, molecular properties, and applications in solution-processable PSCs as well as OFETs will be presented.
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RESULTS AND DISCUSSION Synthesis. The synthetic route for Sn-aADS monomer is depicted in Scheme 4. Trimethylsilylacetylene was deprotonated by n-butyllithium followed by reacting with 2-octyldodecyl bromide in the presence of hexamethylphosphorus triamide (HMPT) at 0 °C to furnish compound 1. The trimethylsilyl group was deprotected by K2CO3 in methanol/THF to give compound 2 in 93% yield. Sonogashira coupling of 1,4-dibromo2,5-diiodobenzene with compound 2 afforded compound 3, which was then coupled with 2-(tributylstannyl)selenophene C
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Macromolecules Scheme 6. Synthesis of PaADSDTFBT-C4, PaADSDTFBT-C8, PaADSDTFBT-C8C12, and PaADSDPP
Table 1. Summary of the Intrinsic Properties of PaADSDTFBT-C4, PaADSDTFBT-C8, PaADSDTFBT-C8C12, and PaADSDPP. λmax (nm) polymer
Mn (kg/mol) d
PDI
Td (°C)
THF
film
HOMO (eV)
LUMO (eV)
Egopt (eV)a
Egoptfilm (eV)b
Egoptcv(eV)c
375 551 380 551 373 554 367 688
383 604 384 605 379 589 358 690
−5.58
−3.60
1.72
1.65
1.98
−5.63
−3.65
1.73
1.71
1.98
−5.85
−3.75
1.92
1.80
2.10
−5.31
−3.50
1.53
1.51
1.81
PaADSDTFBT-C4
N/A
N/A
461
PaADSDTFBT-C8
36.4
2.3
451
PaADSDTFBT-C8C12
19.6
2.0
455
PaADSDPP
81.7
1.7
432
a opt Eg was estimated from the onset of UV spectra in THF. bEgoptfilm was estimated from the onset of UV spectra in thin film. cEgoptcv was estimated from cyclic voltammetry. dNot applicable.
Thermal and Optical Properties. Thermal stability of the polymers was analyzed by thermogravimetric analysis (TGA) (Figure 1). PaADSDTFBT-C 4 , PaADSDTFBT-C 8 , PaADSDTFBT-C8C12, and PaADSDPP exhibited sufficiently high decomposition temperatures (Td) of 461, 451, 455, and 432 °C, respectively. As suggested by differential scanning calorimetry (DSC), no thermal transition was observed for all the polymers between 20 and 330 °C, implying their amorphous character in this temperature scanning range. UV−vis absorption spectra of the four polymers in THF solution and in thin film are depicted in Figure 2, and the corresponding optical parameters are summarized in Table 1. PaADSDTFBT-C4, PaADSDTFBT-C8, PaADSDTFBT-C8C12, and PaADSDPP showed two major absorption bands. In THF, the absorption spectra of PaADSDTFBT-C 4 and PaADSDTFBT-C8 are similar to each other, suggesting that that the main-chain planarity and linearity of PaADSDTFBT do not vary from the butyl group to the octyl group, which can be supported by our computational results (see the Theoretical Calculations section). However, when 2-octyldodecyl was chosen as the substituent, it is evident to see that the optical band gap (Egopt) in THF increased visibly. Based on the
then copolymerized with 4,7-bis(4-butyl-5-bromothiophen-2yl)-5,6-difluoro-2,1,3-benzothiadiazole (DTFBT-C4), 4,7-bis(5bromo-4-octylthiophen-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (DTFBT-C 8 ), 4,7-bis[5-bromo-4-(2-octyldodecyl)thiophen-2-yl]-5,6-difluoro-2,1,3-benzothiadiazole (DTFBTC 8 C 12 ), and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2ethylhexyl)pyrrolo[3,4-c]-pyrrole-1,4(2H,5H)-dione (DPP) monomers to provide four donor−acceptor copolymers: PaADSDTFBT-C4, PaADSDTFBT-C8, PaADSDTFBT-C8C12, and PaADSDPP, respectively (Scheme 6). Subsequent to successive Soxhlet extraction and reprecipitation, the average molecular weight (Mn) and polydispersity index (PDI) values are Mn = 36.4 kg/mol, PDI = 2.3 for PaADSDTFBT-C8, Mn = 19.6 kg/mol, PDI = 2.0 for PaADSDTFBT-C8C12, and Mn = 81.7 kg/ mol, PDI = 1.7 for PaADSDPP (Table 1). Because of the poor solubility of PaADSDTFBT-C4 in THF, the molecular weight of PaADSDTFBT-C4 cannot be determined by the gel permeation chromatography (GPC) instrument. The poor solubility of PaADSDTFBT-C4 in THF suggests that it may possess much higher molecular weight than PaADSDTFBT-C 8 and PaADSDTFBT-C8C12 or the butyl groups are not sufficient enough to solubilize the polymer. D
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by GPC, approximately corresponding to 10 repeating units. Therefore, it is likely that polymerization degree for PaADSDTFBT-C8C12 does not reach the effective conjugation length. The low Mn of PaADSDTFBT-C8C12 may be thus responsible for the larger Egopt. The absorption profiles of the PaADSDTFBT-based polymers all shift toward longer wavelengths from the solution state to the solid state, suggesting that certain degree of polymer-chain aggregation occurred in the solid state. Similar to the solution-phase absorption, PaADSDTFBTC4 and PaADSDTFBT-C8 both exhibited more bathochromic shifts and smaller optical band gaps in thin films than PaADSDTFBT-C8C12 (Table 1 and Figure 2). The λmax shift in the longer wavelengths from THF solution to thin film is 53 nm for PaADSDTFBT-C4, 54 nm for PaADSDTFBT-C8, and 35 nm for PaADSDTFBT-C8C12, indicating that the aggregation is more significant for PaADSDTFBT-C4 and PaADSDTFBT-C8. As for PaADSDPP, the absorptions in THF solution and film are comparable and the optical band gap was estimated to be ca. 1.50 eV. Electrochemical Properties. Cyclic voltammetry (CV) was employed to examine the electrochemical properties and determine the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the polymers (Figure 3 and Table 1). The LUMO energy levels
Figure 1. Thermogravimertic analysis (TGA) of PaADSDTFBT-C4, PaADSDTFBT-C8, PaADSDTFBT-C8C12, and PaADSDPP.
Figure 3. Cyclic voltammograms of PaADSDTFBT-C 4 , PaADSDTFBT-C8, PaADSDTFBT-C8C12, and PaADSDPP in the thin films at a scan rate of 50 mV/s.
of PaADSDTFBT-C4, PaADSDTFBT-C8, PaADSDTFBTC8C12, and PaADSDPP were determined to be −3.60, −3.65, −3.75, and −3.50 eV. The HOMO energies of PaADSDTFBTC4, PaADSDTFBT-C8, and PaADSDTFBT-C8C12 are −5.58, −5.63, and −5.85 eV, respectively, and that of PaADSDPP was estimated to be −5.31 eV. PaADSDPP has the narrowest electrochemical band gap (Egele) and PaADSDTFBT-C8C12 has larger Egele than PaADSDTFBT-C4 and PaADSDTFBT-C8, which are consistent with the results acquired from the optical measurements. Theoretical Calculations. Simplified structures, 2aADSDTFBT-C4, 2aADSDTFBT-C8, 2aADSDTFBT-C8C12, and 2aADSDPP, with two repeating units were chosen as the computational model compounds for simulating PaADSDTFBT-C4, PaADSDTFBT-C8, PaADSDTFBT-C8C12, and PaADSDPP, respectively. The aliphatic side chains are intentionally aligned in an antiparallel manner since this alignment can allow better intermolecular π-to-π stacking
Figure 2. Normalized absorption spectra of PaADSDTFBT-C4, PaADSDTFBT-C8, PaADSDTFBT-C8C12, and PaADSDPP in (a) THF solutions and (b) in thin films.
computation (see the Theoretical Calculations sections), the alternation of dihedral angle between the aADS and DTFBT is not susceptive to the change of side chain substituent from butyl or octyl to 2-octyldodecyl, suggesting that the greater Egopt of PaADSDTFBT-C8C12 in THF may not result from the mainchain twisting caused by the bulkier 2-octyldodecyl moiety. The Mn of PaADSDTFBT-C8C12 was determined to be 19.6 kg/mol E
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Figure 4. (a) Antiparallel arrangement of aliphatic side chains on conjugated systems. (b) Optimized geometry for 2aADSDTFBT-C4 (face view and edge view). (c) Optimized geometry for 2aADSDTFBT-C8 (face view and edge view). (d) Optimized geometry for 2aADSDTFBT-C8C12 (face view and edge view). (e) Optimized geometry for 2aADSDPP (face view and edge view). D = dihedral angle.
(Figure 4a). In order to clarify the steric, electronic, and optical disparity caused by different side chains, two layer ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) calculations were carried out. The ONIOM methods provide very useful approaches to calculating large molecules with high accuracy but less computational time by defining two or three layers within the structure that are treated at various levels of theory. 16 In our computation, for geometry optimization, B3LYP/6-31G(d) was applied to conjugated systems and UFF (a molecular mechanics method) was employed to alkyl side chains. For time-dependent density functional theory (TDDFT) calculations, MPW1PW91/6-
31G(d) was used to replace B3LYP/6-31G(d) on the basis of our common observation that the TDDFT results obtained by MPW1PW91 are often more comparable to experimental values than those calculated by B3LYP. The computational details are described in the Supporting Information. The calculated UV−vis spectra in THF for 2aADSDTFBT-C4, 2aADSDTFBT-C8, 2aADSDTFBT-C8C12, and 2aADSDPP are depicted in Figure 5. Although the calculated absorptions for 2aADSDTFBT-based molecules are more bathochromic shift than the experimental values, the correlations between them are qualitatively good. As suggested by Figure 5a, the calculated optical properties of 2aADSDTFBT-C4, 2aADSDTFBT-C8, and 2aADSDTFBTF
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Figure 5. (a) Calculated UV−vis spectra of 2aADSDTFBT-C4, 2aADSDTFBT-C8, and 2aADSDTFBT-C8C12 in THF. (b) Calculated UV−vis spectrum of 2aADSDPP.
Figure 6. Calculated molecular frontier orbitals (plots, isovalue = 0.02 au) and corresponding energies (eV) for (a) 2aADSDTFBT-C4, (b) 2aADSDTFBT-C8, (c) 2aADSDTFBT-C8C12, and (d) 2aADSDPP.
and electronic properties of the aADSDTFBT-based polymers. Nevertheless, the experimental UV−vis absorption and cyclic voltammetry measurements reveal that the disparity in optical and electronic properties does exist between PaADSDTFBT-C4/ PaADSDTFBT-C8 and PaADSDTFBT-C8C12. The most likely explanation accounting for this disparity is the degree of polymerization. As estimated by GPC, PaADSDTFBT-C8C12 has an average 10 repeating units. The conjugation length of
C8C12 remain essentially unchanged with the variation of side chains. By analyzing the optimized geometries of 2aADSDTFBT-C4, 2aADSDTFBT-C8, and 2aADSDTFBTC8C12, it is found that the dihedral angles between aADS and DTFBT units are comparable to each other (Figure 4). The calculated molecular orbitals and corresponding energies listed in Figure 6 also suggest that the change of side chain from butyl to octyl or 2-octyldodecyl has little effect on the main-chain optical G
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Figure 7. Typical output curves (a, c, e) and transfer plots (b, d, f) of the OFET devices based on PaADSDTFBT-C4, PaADSDTFBT-C8, and PaADSDPP, respectively, with ODTS-SAM layer.
Table 2. OFET Characteristics of the Polymer Thin Films polymer PaADSDTFBT-C4 PaADSDTFBT-C8 PaADSDTFBT-C8C12 PaADSDPP
SAM layer ODTS ODTS ODTS ODTS
annealing temp (°C)
mobility (cm2 V−1 s−1) −2
1.0 × 10 2.7 × 10−2 5.4 × 10−6 1.9 × 10−3
200 200 200 200
on/off ratio
Vt (V)
2.0 × 10 7.0 × 107 2.8 × 104 4.0 × 104
−16 −22 −9 −11
5
4, it is found that the vacant sites in the front of DTFBT unit for 2aADSDTFBT-C4 and 2aADSDTFBT-C8 are occupied by 2octyldodecyl groups for 2aADSDTFBT-C8C12. Therefore, it can be envisioned that the 2-octyldodecyl groups installed on DTFBT unit may hinder the intermolecular stacking among PaADSDTFBT-C8C12 molecules and the intermolecular interactions between PaADSDTFBT-C8C12 and other molecules, such as fullerenes. Organic Field Effect Transistors. To investigate the mobility of the polymers, organic field effect transistors (OFETs) were fabricated in the bottom-gate/top-contact
PaADSDTFBT-C8C12 may not exceed the actual effective conjugation length, thus resulting in the larger optical and electrochemical band gaps. It should be noted that the steric repulsion stems from the 2-octyldodecyl side chains between aADS and DTFBT units (Figure 4d) may hinder the polymerization reaction, thus leading to the relatively low Mn of PaADSDTFBT-C8C12. Even though the change of side chains from butyl to octyl or 2octyldodecyl has negligible impact on the main-chain geometry of the aADSDTFBT-series polymers, the increasing bulkiness would suppress intermolecular interactions. As shown in Figure H
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Figure 8. J−V characteristics of ITO/PEDOT:PSS/polymers:PC71BM/Ca/Al under simulated AM1.5G illumination of 100 mW/cm2 and corresponding external quantum efficiency (EQE) spectra.
Table 3. Summary of Device Parameters polymer
blend ratio polymer:PC71BM
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
SCLC (cm2 V−1 s−1)
PaADSDTFBT-C4 PaADSDTFBT-C8 PaADSDTFBT-C8C12 PaADSDPP
1:2 1:2 1:1 1:2
0.84 0.78 0.84 0.70
8.19 6.37 0.84 2.95
66 67 46 58
4.4 3.5 0.3 1.2
5.75 × 10−5 3.58 × 10−7
Figure 9. Atomic force microscopy height images: (a) PaADSDTFBT-C4:PC71BM, (b) PaADSDTFBT-C8:PC71BM, (c) PaADSDTFBTC8C12:PC71BM, and (d) PaADSDPP:PC71BM.
geometry. The polymer-based OFETs using SiO2 as gate dielectric were treated with octadecyltrichlorosilane (ODTS) to form a self-assembly monolayer (SAM) and thermally annealed at 200 °C. The hole mobility was deduced from the
transfer characteristics of the devices in the saturation regime (Figure 7 and Table 2). PaADSDTFBT-C4 and PaADSDTFBTC8 exhibited higher hole mobilities of 1.0 × 10−2 and 2.7 × 10−2 cm2 V−1 s−1 than PaADSDTFBT-C8C12 with a mobility of 5.4 × I
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Macromolecules 10−6 cm2 V−1 s−1. The trend of FET mobility is in good agreement with the optimized geometries by ONIOM calculations. The presence of the 2-octyldodecyl groups in PaADSDTFBT-C8C12 obstructs the intermolecular interactions between polymers, giving rise to the much lower OFET mobility by 4 orders of magnitude. Another minor reason likely responsible for the low OFET mobility of PaADSDTFBTC8C12 lies in the low Mn of PaADSDTFBT-C8C12, which would result in the shorter intramolecular charge-transport path. Photovoltaic Characteristics. Bulk heterojunction solar cell devices with ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al configuration were fabricated by spin-coating the polymer/ PC71BM blends from o-dichlorobenzene solutions. The device parameters were measured under a simulated AM1.5 G illumination of 100 mW/cm2. The J−V curves of the devices are depicted in Figure 8, and the optimized device characteristics are summarized in Table 3. The device based on the PaADSDTFBT-C8C12:PC71BM (1:2 in wt %) blend exhibited an open-circuit voltage (Voc) of 0.84 V, a short-circuit current (Jsc) of 0.84 mA/cm2, and a fill factor (FF) of 46%, delivering a low PCE of 0.3%. The low Jsc value of this device is ascribed to the bulky 2-octyldodecyl moieties which hinder the electronic communication between the DTFBT acceptor and PC71BM. When the side chain is switched to 1-butyl or 1-octyl, the resultant devices exhibited efficiency of 4.4% for PaADSDTFBTC4 and 3.5% for PaADSDTFBT-C8, respectively. The efficiency improvement was mainly as the result of enhanced Jsc and FF values, suggesting that the PaADSDTFBT-C 4 and PaADSDTFBT-C8 based solar devices have richer donor− acceptor interfacial area for efficient charge separation than the PaADSDTFBT-C8C12-based device. For the PaADSDTFBTbased polymers, the power conversion efficiency (PCE) decreased as the increase of bulkiness of the aliphatic side chains installed on DTFBT units. Hole-only devices (ITO/PEDOT:PSS/polymer/Au) were also fabricated to estimate the hole mobility of PaADSDTFBT-C4 and PaADSDTFBT-C8C12 according to the space-charge-limited current (SCLC) theory. The SCLC mobility (Table 3) reveals that PaADSDTFBT-C8C12 has a much lower mobility value than PaADSDTFBT-C4, suggesting again that the 2-octyldodecyl groups in PaADSDTFBT-C8C12 could obstruct the intermolecular interactions between polymers. The inferior SCLC mobility of PaADSDTFBT-C4 is believed to be one of the major reasons causing the low PCE value of the PaADSDTFBT-C8C12-based device. On the other hand, the device based on the PaADSDPP:PC71BM (1:2 in wt %) blend gave a Voc of 0.76 V, a Jsc of 2.95 mA/cm2, and an FF of 58%, resulting in a low PCE of 1.2%. The inferior PCE may also be linked to the existence of bulky aliphatic substituents on the DPP acceptors. Morphology. Atomic force microscopy (AFM) was employed to investigate the morphology of polymer:PC71BM blends. As revealed in Figure 9, obvious aggregation was found for the PaADSDTFBT-C8C12:PC71BM and PaADSDPP:PC71BM blends. The presence of bulky and branched aliphatic substituents on the DTFBT and DPP acceptors may hinder the intermolecular interactions between PaADSDTFBT-C8C12 or PaADSDPP with PC71BM, resulting in aggregation of PC71BM in the blends. The PaADSDTFBT-C4: PC71BM and PaADSDTFBT-C8:PC71BM have yet much more homogeneous thin-film structure, indicating that the PaADSDTFBT-C4 and PaADSDTFBT-C8 would have more interfacial contact with PC71BM, being beneficial for efficient charge separation in the solar device. The richer polymer−
fullerene interfacial area could be further supported by the greater Jsc and FF values for the PaADSDTFBT-C4 and PaADSDTFBT-C8 based solar devices (Table 3). Overall, the AFM images are in good agreement with the solar-efficiency data listed in Table 3.
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CONCLUSIONS Selenophene-based organic semiconductors have emerged as an important class of materials for solar cell and transistor applications considering selenophene’s distinctive nature to enhance light-harvesting ability and charge mobility. For the first time, we have successfully developed an angular-shaped and isomerically pure 4,10-di(2-octyl)dodecylanthradiselenophene (aADS) synthesized via a base-induced propargyl−allenyl isomerization and intramolecular 6π-electrocyclization strategy. The aADS unit was then copolymerized with dithienyldiketopyrrolopyrrole (DPP) and dithienyl-5,6-difluoro-2,1,3-benzothiadiazole (DTFBT) acceptors with different alkyl side chains to afford four donor−acceptor copolymers: PaADSDPP, PaADSDTFBT-C4, PaADSDTFBT-C8, and PaADSDTFBTC8C12. All the polymers possess sufficiently high decomposition temperatures above 430 °C, supporting their high thermal stability toward further applications. UV−vis spectroscopy and cyclic voltammetry revealed that PaADSDPP has the narrowest energy band gap and PaADSDTFBT-C8C12 has larger band gap than PaADSDTFBT-C4 and PaADSDTFBT-C8. Two layer ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) calculations were carried out to investigate the steric, electronic, and optical properties of the polymers. The computation suggested that the change of side chain from butyl to octyl or 2-octyldodecyl does not affect the main-chain geometry for the aADSDTFBT-based polymers. The larger optical and electrochemical band gaps of PaADSDTFBT-C8C12 might result from the lower degree of polymerization. Nevertheless, the optimized geometries indicated that the 2octyldodecyl groups installed on DTFBT units hinder the intermolecular stacking among PaADSDTFBT-C8C12 or the intermolecular interactions between PaADSDTFBT-C8C12 and PC71BM. As a result, PaADSDTFBT-C4 and PaADSDTFBT-C8 exhibited organic-field-effect-transistor hole mobilities of 2.7 × 10−2 and 1.0 × 10−2 cm2 V−1 s−1, outperforming that of PaADSDTFBT-C 8C 12 (5.4 × 10 −6 cm2 V −1 s−1 ). Bulk heterojunction solar devices were fabricated on the basis of the ITO/PEDOT:PSS/polymer:PC 71BM/Ca/Al configuration. The device efficiencies were 4.4% for PaADSDTFBT-C4, 3.5% for PaADSDTFBT-C8, 0.3% for PaADSDTFBT-C8C12, and 1.2% for PaADSDPP. Again, the efficiency decreased as the increase of bulkiness of the aliphatic side chains installed on DTFBT and DPP units, which is in resonance with the computational results that the electronic communication between the DTFBT units and PC71BM are hindered by the bulky 2-octyldodecyl groups. Atomic force microscopy images disclose that the bulkiness of aliphatic side chain installed on DTFBT or DPP has correlation with the morphological aggregation. Obvious aggregation was found for the PaADSDTFBT-C 8 C 12 :PC 71 BM and PaADSDPP:PC 71 BM blends. This research not only develops a selenophene-based aADS building block which can lead to various new promising materials for solution processable organic electronics but also provides useful insights into the side-chain engineering for the D−A conjugated copolymers. J
DOI: 10.1021/acs.macromol.5b01541 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
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organic layer was dried over MgSO4, and the solvent was removed in vacuo. The residue was purified by column chromatography on silica gel (hexane/ethyl acetate, v/v, 20/1) and then recrystallized from hexane to give a yellow solid as the product (2.32 g, 52%). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.04 (dd, J = 5.6,1.2 Hz, 2H), 7.72 (dd, J = 4.0,1.2 Hz, 2H), 7.69 (s, 2H), 7.32 (dd, J = 5.6, 4.0 Hz, 2H), 2.44 (d, J = 6.0 Hz, 4H), 1.59−1.63 (m, 2 H),1.25−1.37 (m, 64 H), 0.87 (t, J = 6.4 Hz, 12H). 13C NMR (CDCl3, 100 MHz, ppm): δ 146.78, 136.01, 133.57, 131.59, 129.43, 128.66, 120.66, 96.59, 80.92, 37.17, 33.73, 31.91, 29.94, 29.69, 29.65, 29.36, 26.85, 24.32, 22.69, 14.12. MS (FAB, M•+, C58H90Se2•+) calcd: 946.5373. Found: 946.5371. Synthesis of aADS. To a degassed N-methyl-2-pyrrolidone (NMP, 2.5 mL) solution of 2,5-bis(4-octyltetradec-1-ynyl)-1,4-bis(selenophen2-yl)benzene (1g, 1.05 mmol) was added 1,8-diazabicycloundec-7-ene (DBU) (0.33 mL, 2.22 mmol). The resulting mixture was refluxed for 2 days, cooled to room temperature, diluted with water (20 mL), and extracted with ethyl acetate (10 mL × 3). The combined organic layer was washed with brine solution (20 mL) and dried over anhydrous MgSO4. After filtration, the solvent was removed under vacuum, and the residue was purified by column chromatography on silica gel (hexane) to give a white fine powder as aADS (0.2 g, 20%). 1H NMR (CDCl3, 400 MHz): δ 8.45 (s, 2H), 8.11 (d, J = 6.0 Hz, 2H), 7.84 (d, J = 6.0 Hz, 2H), 7.62 (s, 2H), 2.97 (d, J = 7.2 Hz, 4H), 1.86 (br, 2H), 1.23−1.41 (m, 64 H), 0.84−0.88 (m, 12H). 13C NMR (CDCl3, 100 MHz): δ 142.00, 139.58, 136.14, 129.87, 128.96, 127.66, 127.54, 125.19, 123.22, 39.98, 38.48, 33.56, 31.92, 31.90, 31.89, 30.04, 29.70, 29.66, 29.63, 29.61, 29.36, 29.34, 29.32, 26.60, 22.67, 14.11. MS (FAB, M•+, C58H90Se2•+) calcd: 946.5373. Found: 946.5377. Synthesis of Sn-aADS. To a THF (30 mL) solution of aADS (0.5 g, 0.53 mmol) was added a hexane solution of n-BuLi (2.5M, 0.53 mmol) dropwise at 0 °C. The mixture was stirred at this temperature for 1 h. A THF solution of chlorotrimethylstannane (1.0 M, 1.6 mmol) was then introduced dropwise. It was quenched with water (50 mL) and extracted with ether (50 mL × 3). The collected organic layer was dried over MgSO4, and the solvent was removed in vauco. The residue was recrystallized from hexane to give a brown solid as Sn-aADS (0.35 g, 52%). 1H NMR (CDCl3, 400 MHz): δ 8.45 (s, 2H), 7.97 (s, 2H), 7.58 (s, 2H), 2.99 (d, J = 7.2 Hz, 4H), 1.87 (br, 2 H), 1.24−1.38 (m, 64H), 0.84−0.88 (m, 12H), 0.48 (s, 18H). 13C NMR (CDCl3, 100 MHz): δ 146.62, 143.21, 140.84, 135.75, 135.64, 129.61, 128.82, 125.11, 123.75, 39.89, 38.48, 33.67, 31.89, 30.07, 29.69, 29.65, 29.34, 26.73, 22.66, 14.10, −7.90. MS (FAB, M•+, C64H106Se2Sn2•+) calcd: 1274.4663. Found: 1274.4666. Synthesis of PaADSDTFBT-C4. To a 50 mL round-bottom flask was introduced Sn−aADS (200 mg, 0.157 mmol), dibrominated DTFBT-C4 (95.2 mg, 0.157 mmol), Pd2(dba)3 (2.86 mg, 0.003 mmol), tri(o-tolyl)phosphine (3.96 mg, 0.013 mmol), and dry chlorobenzene (4 mL). The mixture was bubbled with nitrogen for 10 min at room temperature. The reaction was then carried out in a microwave reactor under 270 W for 50 min. In order to end-cap the resultant polymer, tributyl(thiophen-2-yl)stannane (33 mg, 0.088 mmol) was added to the mixture, and the microwave reaction was continued for 10 min under 270 W. Subsequent to the addition of tributyl(thiophen-2-yl)stannane, another end-capping reagent, 2-bromothiophene (14 mg, 0.085 mmol) was added, and the reaction was continued for another 10 min under otherwise identical conditions. The mixture was then added into methanol dropwise. The precipitate was collected by filtration and washed by Soxhlet extraction with acetone and hexane sequentially for 3 days. The crude polymer was dissolved in hot THF, and the residual Pd catalyst and Sn metal in the THF solution were removed by Pd-thiol gel and Pd-TAAcOH (Silicycle Inc.). After filtration and removal of the solvent, the polymer was redissolved in THF and reprecipitated by methanol. The resultant polymer was collected by filtration and dried under vacuum for 1 day to afford a give a dark-green fiber-like solid (110 mg, 50%). 1H NMR (CDCl3, 400 MHz, ppm): δ 6.80−8.80 (br, 8H), 2.60−3.80 (br, 4H), 1.75−2.20 (br, 4H), 0.94−1.73 (br, 74H), 0.63− 0.93 (br, 18H). Synthesis of PaADSDTFBT-C8. To a 50 mL round-bottom flask was introduced Sn−aADS (200 mg, 0.157 mmol), dibrominated DTFBT-C8 (112.8 mg, 0.157 mmol), Pd2(dba)3 (2.86 mg, 0.003 mmol),
EXPERIMENTAL SECTION
General Measurement and Characterization. All chemicals are purchased from Aldrich or Acros and used as received unless otherwise specified. 1H and 13C NMR spectra were measured using Varian 300 and 400 MHz instrument spectrometers. Thermogravimetric analysis (TGA) was recorded on a PerkinElmer Pyris under nitrogen atmosphere at a heating rate of 10 °C/min. Absorption spectra were recorded on a HP8453 UV−vis spectrophotometer. Molecular weight of polymers was measured on a Viscotek VE2001GPC, and polystyrene was used as the standard (THF as the eluent). Electrochemical cyclic voltammetry (CV) was conducted on a CH Instruments Model 611D. A carbon glass coated with a thin polymer film was used as the working electrode and Ag/Ag+ electrode as the reference electrode, while 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile was the electrolyte. CV curves were calibrated using ferrocence as the standard, whose oxidation potential is set at −4.8 eV with respect to zero vacuum level. The HOMO energy levels were obtained from the equation HOMO = −(Eoxonset − E(ferrocene)onset + 4.8) eV. The LUMO levels of polymer were obtained from the equation LUMO = −(Eredonset − E(ferrocene)onset + 4.8) eV. Synthesis of (4-Octyl-tetradec-1-ynyl)trimethylsilane. To an anhydrous THF solution of trimethylsilylacetylene (8.15 g, 83 mmol) was added n-butyllithium (2.5 M, 83 mmol) dropwise at 0 °C. The mixture was stirred at this temperature for 1 h. Hexamethylphosphoramide (HMPA, 14.87 g, 83 mmol) and 2-octyldodecyl bromide (15 g, 41.5 mmol) were added sequentially into the mixture, and it was stirred at room temperature overnight. The reaction mixture was then poured into water and extracted with diethyl ether. The residue was purified by column chromatography on silica gel (hexane) to afford a transparent oil as the product (8.86 g, 56%). 1H NMR (CDCl3, 400 MHz): δ 2.18 (d, J = 6.4 Hz, 2 H), 1.48−1.49 (m, 1 H), 1.26−1.32 (m, 32 H), 0.88 (t, J = 6.8 Hz, 6 H), 0.15 (s, 9H). 13C NMR (CDCl3, 100 MHz): δ 106.63, 85.23, 37.02, 33.48, 31.92, 29.90, 29.68, 29.65, 29.61, 29.57, 29.35, 29.33, 26.69, 24.22, 22.68, 14.11, 0.18. Synthesis of 4-Octyl-tetradec-1-yne. To a solution of (4octyltetradec-1-ynyl)trimethylsilane (13.02 g, 34.4 mmol) in methanol (135 mL) and THF (60 mL) was added anhydrous K2CO3 (11.88 g, 85.9 mmol). The mixture was stirred for 12 h at room temperature, poured into water, and extracted with ethyl acetate. The collected organic layer was washed with ammonium chloride aqueous solution and dried over anhydrous Mg2SO4. The solvent was removed in vacuo, and the crude residue was purified by silica gel chromatography (hexane) to afford a transparent oil as the product (9.8 g, 93%). 1H NMR (CDCl3, 400 MHz): δ 2.17 (dd, J = 5.6, 2.8 Hz, 2 H), 1.92 (t, J = 2.6 Hz, 1H), 1.48−1.41 (m, 1 H), 1.26−1.32 (m, 32 H), 0.88 (t, J = 6.8 Hz, 6 H). 13C NMR (CDCl3, 100 MHz): δ 83.38, 68.86, 36.84, 33.34, 31.91, 29.89, 29.66, 29.64, 29.59, 29.35, 29.32, 26.75, 22.68, 22.63, 14.10. Synthesis of 1,4-Dibromo-2,5-bis(4-octyl-tetradec-1-ynyl)benzene. To a degassed toluene (10 mL) solution of diisopropylamine (10 mL) was added 1,4-dibromo-2,5-diiodobenzene (4.77 g, 9.78 mmol), Pd(PPh3)2Cl2 (140 mg, 0.2 mmol), CuI (74 mg, 0.39 mmol), and 4-octyltetradec-1-yne (6 g, 19.56 mmol). The mixture was stirred for 16 h at 100 °C. Water (60 mL) and ammonium chloride (1 M, 60 mL) were then added. The resulting mixture was extracted with diethyl ether (50 mL × 3), and the combined organic layer was dried over anhydrous MgSO4, filtered, and concentrated in vacuo. The residue was purified by column chromatography on silica gel (hexane) to give a yellow oil as the product (4.43 g, 53.6%). 1H NMR (CDCl3, 400 MHz): δ 7.58 (s, 2H), 2.44 (d, J = 5.6 Hz, 4H), 1.59−1.61 (m, 2H), 1.26−1.41 (m, 64 H), 0.88 (t, J = 7.2 Hz, 12H). 13C NMR (CDCl3, 100 MHz, ppm): δ 136.02, 126.54, 123.50, 97.01, 79.14, 37.16, 33.60, 31.92, 29.91, 29.66, 29.62, 29.36, 29.34, 26.82, 23.82, 22.69, 14.11. MS (FAB, M•+, C50H84Br2•+) calcd: 844.4919. Found: 844.4910. Synthesis of 2,5-Bis(4-octyltetradec-1-ynyl)-1,4-bis(selenophen-2-yl)benzene. A mixture of 1,4-dibromo-2,5-bis(4octyltetradec-1-ynyl)benzene (4 g, 4.73 mmol), 2-(tributylstannyl)selenophene (4.2 g, 10.0 mmol), Pd(PPh3)4 (0.22 g, 0.19 mmol), and degassed toluene (60 mL) was heated at 100 °C under a nitrogen atmosphere for 16 h. The reaction mixture was poured into water (150 mL) and extracted with ethyl acetate (250 mL × 3). The combined K
DOI: 10.1021/acs.macromol.5b01541 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
polymers were deposited on octadecyltrichlorosilane (ODTS)-treated SiO2/Si substrates by spin-coating their chlorobenzene solutions (10 mg/mL). Then, the thin films were annealed at 200 °C for 10 min. Gold source and drain contact (40 nm in thickness) were deposited by vacuum evaporation on the organic layer through a shadow mask, affording a bottom-gate, top-contact device configuration. Electrical measurements of OTFT devices were carried out at room temperature in air using a 4156C (Agilent Technologies). The field effect mobility was calculated in the saturation regime by using the equation Ids = (μWCi/2L)(Vg − Vt)2, where Ids is the drain-source current, μ is the field effect mobility, W is the channel width (1 mm), L is the channel length (100 μm), Ci is the capacitance per unit area of the gate dielectric layer, Vg is the gate voltage, and Vt is threshold voltage. PSCs Fabrication. ITO/glass substrates were ultrasonically cleaned sequentially in detergent, water, acetone, and isopropanol (IPA). The cleaned substrates were covered by a 30 nm thick layer of PEDOT:PSS (Clevios P provided by Stark) by spin-coating. After annealing in a glovebox at 150 °C for 30 min, the samples were cooled to room temperature. An active-layer solution was prepared by adding PC71BM (purchased from Nano-C) into an o-dichlorobenzene (ODCB) solution of polymer, and it was then heated at 80 °C and stirred overnight at the same temperature. Prior to deposition, the active-layer solution was filtered (0.45 μm filters) and spin-coated on top of PEDOT:PSS to form the active layer. The cathode made of calcium (350 nm thick) and aluminum (1000 nm thick) was sequentially evaporated through a shadow mask under high vacuum (
