S,N-Heteroacene-Based Copolymers for Highly Efficient Organic Field

Subscriber access provided by Eastern Michigan University | Bruce T. Halle Library

Article

S,N-Heteroacene-Based Copolymers for Highly Efficient Organic Field Effect Transistors and Organic Solar Cells: Critical Impact of Aromatic Subunits in Ladder #-System Chin-Lung Chung, Hsieh-Chih Chen, Yun-Siou Yang, Wei-Yao Tung, JianWei Chen, Wen-Chang Chen, Chun-Guey Wu, and Ken-Tsung Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15584 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38 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 Applied Materials & Interfaces

S,N-Heteroacene-Based Copolymers for Highly Efficient Organic Field Effect Transistors and Organic Solar Cells: Critical Impact of Aromatic Subunits in Ladder π-System Chin-Lung Chung,†,# Hsieh-Chih Chen,*‡,# Yun-Siou Yang,⊥ Wei-Yao Tung,§ Jian-Wei Chen,‡ Wen-Chang Chen,§ Chun-Guey Wu,⊥ and Ken-Tsung Wong*†,∥

† Department

of Chemistry, National Taiwan University, Taipei 106, Taiwan

‡ Department

of Fiber and Composite Materials, Feng Chia University, Taichung 407, Taiwan

⊥

Department of Chemistry, National Central University, Taoyuan 320, Taiwan

§ Department ∥

of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan

Institute of Atomic and Molecular Science Academia Sinica, Taipei 106, Taiwan

KEYWORDS: Pentacyclic heteroacenes, Quinoidal, Organic field effect transistors, Organic solar cells, Semi-ladder type polymers

ABSTRACT: Three novel donor-acceptor (D–A) alternating polymers containing ladder-type pentacyclic heteroacenes (PBo, PBi and PT) are synthesized, characterized and further applied to organic field effect transistors (OFETs) and polymer solar cells (PSCs). Significant aspects of quinoidal characters, electrochemical properties, optical absorption, frontier orbitals, backbone

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 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

Page 2 of 38

coplanarity, molecular orientation, charge carrier mobilities, morphology discrepancies and the corresponding device performances are notably different with various heteroarenes. PT exhibits stronger quinoidal mesomeric structure, linear and coplanar conformation, smooth surface morphology and better bimodal crystalline structures, which is beneficial to extend the π−conjugation and promotes the charge transport via 3-D transport pathways, and in consequence improves overall device performances. OPV based on PT polymer achieves a power conversion efficiency of 6.04% along with a high short-circuit current density (JSC) of 14.68 mA cm-2, and a high hole mobility of 0.1 cm2 V-1 s-1 is fulfilled in an OFET, which is superior to those of its counterparts, PBi and PBo.

INTRODUCTION Over the last decades, polymer-based organic field effect transistors (OFETs) and organic solar cells (OSCs) have attracted extensive interest owing to their great advantages such as flexibility, light weight, low cost, and easy fabrication, in contrast to their inorganic counterparts.1, 2 In order to improve the electronic π-delocalization and harvest more photon flux, the conjugated in-chain donor–acceptor (D–A) alternating copolymers have been recognized as an effective strategy for obtaining low optical energy gap materials. The frontier molecular orbital energies of the resultant polymers can be feasibly modulated with molecular structure engineering on the D and/or A constituents.3-15 To achieve high efficiency polymer solar cells (PSCs), the polymeric electron donors need to have adequate solubility for solution processability and maintain suitable energy gaps to harvest more solar energy as possible to maximize the short-circuit current density (JSC). In addition, the high hole mobility and favorable film morphology are also crucial for efficient exciton dissociation and charge carrier transport as well as suitable highest occupied molecular orbital (HOMO) energy levels to boost high open-circuit voltage (VOC).16 To date, single junction


2

Page 3 of 38 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


PSCs with power conversion efficiencies (PCEs) surpassing 10% have been fulfilled,17-20 giving optimistic prospects for future commercial applications. Among various polymeric donors, only limited D–A alternating copolymers exhibit good characters in both OFETs and OPVs owing to their discrepancies in device architectures and charge transport orientations.21-23 Thus, seeking versatile polymers that could be advantageous in combination with both types of devices is of great interest. Recently, aromatic molecules with conformation locked by covalent bonds have been demonstrated as a promising strategy to design multi-fused ladder-type building blocks for D–A alternating copolymers. The planar backbone structure can boost the π−electron delocalization to give a higher degree of quinoidal mesomeric structure along the molecular main chain for giving reduced bond length alternation, lower energy gap and limited rotational disorder,24-33 and rendering the excitons splitting into free charge carriers more facile. It is generally accepted that the modulation of aromatic and quinoidal characters of building components can effectively tune the solubility, photophysical properties, frontier orbitals, backbone coplanarity, intermolecular packing, film morphology, and charge carrier mobility of the resultant D–A alternating copolymers. For example, the thiophene-based polymers typically show a bathochromic shifted absorption in comparison with the benzene-based analogues due to the lower degree of aromaticity.34,

35

Although the effect of replacing phenylene with thiophene as a π−bridge spacer on optoelectronic properties has been addressed,36-38 further structural modification to systematically investigate both structure-property relationship and device performances of the ladder-type fused aromatic systems has not been fully explored yet.39 In this context, we report herein, for the first time, the design and synthesis of a series of novel D–A alternating copolymers namely, PBo, PBi and PT (Scheme 1), consisting of pentacyclic


3


Page 4 of 38

heteroacenes as donor units and 5,6-difluoro-2-octyl-2H-benzo[d][1,2,3]triazole as an acceptor moiety. These new polymers were investigated to comprehensively explore the structural effects of heteroarenes on the solid-state structures and optoelectronic properties in solution-processed OFET and OPV. The polymer PT exhibits a narrower energy gap and a broader red-shift absorption spectrum than those of PBo and PBi, resulting in a superior PCE of 6.04% along with a high JSC of 14.28 mA cm-2. While applies to OFETs, PT exhibits a higher hole mobility of 0.1 cm2 V-1 s-1. This can be attributable to the fact that PT possesses stronger quinoidal configuration, linear and coplanar conformation, and bimodal texturing, which can expand the π−conjugation and ameliorate the charge transport via 3-D transport routes, and thus improve overall device performances.

EXPERIMENTAL SECTION Synthesis of (3,3'-dibromo-[2,2'-bithieno[3,2-b]thiophene]-5,5'-diyl)bis(triisopropylsilane) (2). To a solution of (5-bromothieno[3,2-b]thiophen-2-yl)(triisopropyl)silane (1) (8.5 g, 22.6 mmol) in anhydrous THF (100 mL) at -78 °C under argon atmosphere was added freshly made lithium diisopropylamide (LDA) (1 M, 24.9 mmol), and then the mixture was stirred for 30 min, followed by the addition of CuCl2 (3.4 g, 24.9 mmol). The reaction mixture was slowly warmed to room temperature and stirred overnight. Afterwards, the solution was poured into H2O and extracted with Et2O, and the organic phase was rinsed by NH4Cl(aq) and brine, dried over anhydrous MgSO4, concentrated and flash chromatographed to give compound 2 as an off-white solid. (6 g, 8.0 mmol, 71%) M.p. 197-199 °C; 1H NMR (CD2Cl2, 400 MHz) δ 7.46 (s, 2H), 1.44-1.40 (m, 6H), 1.17 (d, J = 7.6 Hz, 36H); 13C NMR (CD2Cl2, 100 MHz) δ 146.3, 140.7, 140.5, 131.6, 128.3, 105.3, 31.2, 18.9, 12.4; IR (KBr): ν 2942, 2864, 1463, 1381, 1339, 1308, 1289, 1062, 1000, 954, 880, 816 cm1;

HRMS (m/z, FAB+) calcd for C30H4479Br2S4Si2: 746.0231, found 746.0217; calcd for


4

Page 5 of 38 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


C30H4479Br81BrS4Si2: 748.0211, found 748.0201; calcd for C30H4481Br2S4Si2: 750.0190, found 750.0182. Synthesis of 9-(heptadecan-9-yl)-9H-thieno[2',3':4,5]thieno[3,2-b]thieno[2',3':4,5]thieno [2,3-d]pyrrole (3). A solution of dibromide 2 (3.0 g, 4.0 mmol), sodium tert-butoxide (6.15 g, 64 mmol), Pd(dba)2 (230 mg, 0.4 mmol), and dppf (444 mg, 0.8 mmol) in toluene (100 mL) was stirred at room temperature for 30 min. To the resulting solution was then added heptadecan-9amine (1.54 g, 6.0 mmol), and the mixture was stirred at 110 °C for 12 h. After the resulting mixture was cooled to room temperature, H2O was added to the mixture and extracted with CH2Cl2. The combined organic layer was dried over anhydrous MgSO4 and concentrated under reduced pressure. Tetrabutylammonium fluoride (1M in THF, 10 mL) was added to a solution of the crude product in THF (40 mL) and the solution was stirred for 3h at room temperature. The reaction mixture was extracted with ethyl acetate/H2O and the crude product was further purified by column chromatography with Hexane to afford 3 as a light-yellow solid. (1.8 g, 3.4 mmol, 85%) M.p. 125127 °C; 1H NMR (CD2Cl2, 400 MHz) δ 7.33 (d, J = 4.8 Hz, 2H), 7.32 (d, J = 7.2 Hz, 2H), 4.484.43 (m, 1H), 2.26-2.17 (m, 2H), 2.05-1.97 (s, 2H), 1.26-1.05 (m, 24H), 0.80 (t, J = 6.8 Hz, 6H); 13C

NMR (CD2Cl2, 100 MHz) δ 138.8, 124.3, 121.8, 61.8, 36.8, 32.3, 31.2, 29.8, 29.7, 27.2, 23.1,

14.4; IR (KBr): ν 2944, 2924, 2848, 1633, 1523, 1504, 1465, 1455, 1396, 1364, 1324, 1083, 1014, 898, 863, 784, 723, 706 cm-1; HRMS (m/z, FAB+) calcd for C29H39NS4: 529.1965, found 529.1964. Synthesis of 9-(heptadecan-9-yl)-2,7-bis(trimethylstannyl)-9H-thieno[2',3':4,5]thieno[3,2b]thieno[2',3':4,5]thieno[2,3-d]pyrrole (M1). A solution of 3 (1.43 g, 2.7 mmol) in anhydrous THF (50 mL) was added dropwise t-BuLi (1.55 M in pentane, 5.2 mL, 8.1 mmol) at -78 °C under argon atmosphere and then stirred for 1h. Trimethyltin chloride solution (1 M in hexane, 10.8 mL, 10.8 mmol) was subsequently added in one portion into the reaction mixture. The reaction mixture


5


Page 6 of 38

was stirred overnight and slowly warmed to room temperature, quenched with water and extracted with ether. The combined extracts were washed with brine, dried over anhydrous MgSO4. The solvent was removed by rotary evaporation and the crude product was reprecipitated in methanol to afford M1 as yellow solid and used without further purifications (1.3 g, 1.52 mmol, 56%). M.p. 120-122 °C; 1H NMR (CD2Cl2, 400 MHz) δ 7.32 (s, 2H), 4.53-4.48 (m, 1H), 2.30-2.21 (m, 2H), 2.05-1.97 (m, 2H), 1.31-1.06 (m, 24H), 0.81 (t, J = 6.8 Hz, 6H), 0.43 (s, 18H). 13C NMR (CD2Cl2, 100 MHz) δ 140.9, 137.8, 128.8, 61.5, 36.7, 32.3, 29.8, 29.7, 29.6, 27.2, 23.1, 14.4, -7.9. IR (KBr): ν 2922, 2849, 1559, 1507, 1457, 1396, 1363, 1294, 1270, 1237, 1083, 1016, 899, 770, 761, 736, 706 cm-1; HRMS (m/z, FAB+) calcd for C35H55NS4Sn2: 857.1261, found 857.1253. Synthesis

of

11-(heptadecan-9-yl)-11H-benzo[4,5]thieno[3,2-b]benzo[4,5]thieno[2,3-d]

pyrrole (5). A solution of 3,3'-dibromo-2,2'-bibenzo[b]thiophene (4) (5.0 g, 11.8 mmol), sodium tert-butoxide (18.2 g, 0.18 mol), Pd(dba)2 (679 mg, 1.18 mmol), and dppf (1.31 g, 2.36 mmol) in toluene (240 mL) was stirred at room temperature for 30 min. To the resulting solution was then added heptadecan-9-amine (4.5 g, 17.7 mmol), and the mixture was stirred at 110 °C for 12 h. After the resulting solution was cooled to room temperature, H2O was added to the mixture and extracted with CH2Cl2. The combined organic layer was dried over anhydrous MgSO4 and concentrated under reduced pressure. The crude product was further purified by column chromatography with Hexane as eluent to afford 5 as an off-white solid. (3.4 g, 6.6 mmol, 56%) M.p. 112-115 °C; 1H NMR (CD2Cl2, 400 MHz) δ 8.10 (d, J = 8.4 Hz, 2H), 7.90 (td, J = 8 Hz, J = 0.4 Hz, 2H), 7.48-7.42 (m, 2H), 7.33-7.30 (m, 2H), 5.19-5.14 (m, 1H), 2.46-2.39 (m, 2H), 2.142.07 (m, 2H), 1.37-1.08 (m, 24H), 0.80 (t, J = 7.2 Hz, 6H); 13C NMR (CD2Cl2, 100 MHz) δ 142.7, 142.4, 139.8, 137.1, 128.5, 128.3, 125.1, 124.9, 124.8, 123.7, 123.4, 122.5, 120.2, 117.1, 113.9, 60.9, 35.7, 32.3, 29.8, 29.7, 29.6, 27.8, 23.1, 14.4. IR (KBr): ν 2920, 2851, 1767, 1645, 1587, 1493,


6

Page 7 of 38 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


1475, 1431, 1403, 1295, 1218, 1164, 1128, 1100, 1063, 1030, 749, 728, 709 cm-1; HRMS (m/z, FAB+) calcd for C33H43NS2: 517.2837, found 517.2844. Synthesis

of

3,8-dibromo-11-(heptadecan-9-yl)-11H-benzo[4,5]thieno[3,2-b]benzo[4,5]

thieno[2,3-d]pyrrole (6). Bromine (0.23 mL, 4.4 mmol) was added dropwise to a solution of compound 5 (1.04 g, 2 mmol) in chloroform (50 mL). The reaction mixture was stirred for 12 hours at room temperature. After poured into water, the reaction mixture was extracted with dichloromethane and finally with a sodium thiosulphate solution. The extracts were dried over anhydrous MgSO4 and the crude product was further purified by column chromatography with Hexane as eluent to give the dibromo compound 6 as a white solid (0.6 g, 0.9 mmol, 44%). M.p. 96-98 °C; 1H NMR (CD2Cl2, 400 MHz) δ 7.99 (s, 2H), 7.93 (d, J = 8.8 Hz, 2H), 7.56-7.52 (m, 2H), 5.08-5.00 (m, 1H), 2.39-2.30 (m, 2H), 2.10-2.02 (m, 2H), 1.33-1.10 (m, 24H), 0.81 (t, J = 7.2 Hz, 6H); 13C NMR (CD2Cl2, 100 MHz) δ 144.3, 144.0, 139.4, 136.7, 128.0, 127.5, 127.3, 127.1, 127.0, 123.5, 121.0, 117.4, 117.0, 116.8, 114.3, 61.1, 35.6, 32.2, 29.7, 29.6, 27.8, 23.1, 14.4; IR (KBr): ν 2953, 2924, 2853, 1860, 1715, 1580, 1541, 1493, 1463, 1435, 1399, 1354, 1284, 1254, 1159, 1083, 860, 799, 722 cm-1; HRMS (m/z, FAB+) calcd for C33H4179Br2NS2: 673.1047, found 673.1048; calcd for C33H4179Br81BrNS2: 675.1027, found 675.1017; calcd for C33H4181Br2NS2: 677.1006, found 677.1002. Synthesis

of

11-(heptadecan-9-yl)-3,8-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-

11H-benzo[4,5]thieno[3,2-b]benzo[4,5]thieno[2,3-d]pyrrole (M2). A solution of 6 (4.56 g, 6.7 mmol) in anhydrous THF (60 mL) was added dropwise n-BuLi (1.6 M in THF, 17 mL, 27 mmol) at -78 °C under argon atmosphere and then stirred for 1h. 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2dioxaborolane (7.5 mL, 34 mmol) was subsequently added in one portion into the reaction mixture. The reaction mixture was stirred overnight and slowly warmed to room temperature, quenched


7


Page 8 of 38

with water and extracted with ether. The combined extracts were washed with brine, dried over anhydrous MgSO4. The solvent was removed by rotary evaporation and the crude product was reprecipitated in methanol to afford M2 as yellow solid and recrystallized by pentane (2.73 g, 3.5 mmol, 59%). M.p. 160-162 °C; 1H NMR (CD2Cl2, 400 MHz) δ 8.29 (s, 2H), 8.08 (dd, J = 8.2 Hz, J = 1.2 Hz, 2H), 7.81-7.78 (m, 2H), 5.20-5.15 (m, 1H), 2.47-2.38 (m, 2H), 2.14-2.07 (m, 2H), 1.39 (s, 24H), 1.37-0.94 (m, 24H), 0.78 (t, J = 7.2 Hz, 6H);

13C

NMR (CD2Cl2, 100 MHz) δ 142.4,

142.1, 140.4, 137.8, 131.8, 131.7, 130.7, 130.6, 130.2, 130.0, 122.0, 119.6, 118.7, 115.6, 84.5, 61.0, 35.7, 32.3, 29.8, 29.6, 27.8, 25.3, 23.1, 14.4; IR (KBr): ν 2977, 2927, 2854, 1783, 1662, 1591, 1424, 1348, 1260, 1214, 1144, 1100, 963, 868, 845, 818, 744, 719 cm-1; HRMS (m/z, FAB+) calcd for C45H65B2NO4S2: 769.4541, found 769.4541. Polymerization for PT. Distannyl monomer M1 (0.3 mmol), dibromo-monomer 4,7-dibromo5,6-difluoro-2-octyl-2H-benzo[d][1,2,3]triazole (2FBTz) (0.3 mmol), P(o-tol)3 (16 mol% with respect to the M1), and Pd2dba3 (2 mol% with respect to the M1) were dissolved in degassed chlorobenzene (5 mL). Afterwards, the reaction mixture was heated up to 150 °C for 10 min via microwave. After end-capping with 2-stannylthiophene and 2-bromothiophene, the mixture was cooled and poured into methanol and the precipitate was filtered through a Soxhlet thimble, and then subjected to Soxhlet extraction with methanol, acetone, hexane, CH2Cl2, and chloroform. The polymer extracted from chloroform were reprecipitated in methanol to give PT as a dark solid and dried overnight under vacuum at 60 °C. (209 mg, 88%, Mn = 25.2 kg mol-1, Mw = 28.4 kg mol-1, PDI = 1.12). 1H NMR (CDCl3, 400 MHz) δ 8.61 (brs, 2H), 5.05 (brs, 2H), 4.36 (brs, 1H), 2.43 (brs, 2H), 1.60-0.95 (m, 28H). Polymerization for PBo. M2 (0.3 mmol), dibromo-monomer 2FBTz (0.3 mmol), P(o-tol)3 (16 mol% with respect to the M2), and Pd2dba3 (2 mol% with respect to the M2) and Aliquat 336 (two


8

Page 9 of 38 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


drops) were dissolved in deoxygenated toluene/2M Na2CO3(aq) (20 mL, 5:1, v/v). The reaction mixture was heated at 90 °C for 48 h. The solution was dropwise added into methanol. The precipitate was collected by filtration and washed by Soxhlet extraction with methanol, acetone, hexane, CH2Cl2, and chloroform sequentially. The polymer extracted from chloroform was precipitated in methanol to give PBo as an orange solid and dried overnight under vacuum at 60 °C. (100 mg, 43%, Mn = 21.6 kg mol-1, Mw = 33.2 kg mol-1, PDI = 1.54) 1H NMR (CDCl3, 400 MHz) δ 8.56-8.54 (m, 2H), 8.23 (brs, 2H), 8.10 (brs, 2H), 5.23 (brs, 2H), 4.80 (brs, 3H), 2.53 (brs, 4H), 2.17 (brs, 6H), 1.58-0.80 (m, 75H). Polymerization for PBi. Distannyl monomer M3 (0.3 mmol), dibromo-monomer 2FBTz (0.3 mmol), P(o-tol)3 (16 mol% with respect to the M3), and Pd2dba3 (2 mol% with respect to the M3) were dissolved in degassed chlorobenzene (5 mL). Afterwards, the reaction mixture was heated up to 150 °C for 10 min via microwave. After end-capping with 2-stannylthiophene and 2bromothiophene, the mixture was poured into methanol to afford the precipitate. The polymer was filtered through a Soxhlet thimble, and then subjected to Soxhlet extraction with methanol, acetone, hexane, and CH2Cl2. The polymer extracted from CH2Cl2 was precipitated in methanol to give PBi as a dark red solid and dried overnight under vacuum at 60 °C. (160 mg, 68%, Mn = 26.5 kg mol1,

Mw = 34.9 kg mol-1, PDI = 1.31). 1H NMR (CDCl3, 400 MHz) δ 7.69-7.00 (brs, 6H), 5.63-5.32

(m, 1H), 4.89-4.36 (m, 2H), 3.14 (brs, 1H), 2.44-0.65 (m, 27H). Fabrication and characterization of organic field effect transistors. The OFET devices were manufactured in top-contact configuration on silicon wafer substrates with a thermally grown 300nm-thick SiO2 dielectric layer (capacitance per unit area Co = 10 nF cm−2) on highly doped n-type Si (100) as a gate electrode, which was selected as the device substrate. The silicon substrates utilized were successively pre-cleaned by toluene, acetone, isopropanol, and dried under steam


9


Page 10 of 38

nitrogen. The clean silicon substrates were modified with octadecyltrichlorosilane (OTS) selfassembled monolayer in accordance with the reported literature.40 Polymer thin films were fabricated by spin-coating from o-DCB (10 mg mL-1) at a spin rate of 1600 rpm for 60 s. The polymer films were thermally annealed at 150 °C for 1 h. Then 80-nm-thick gold electrodes were deposited through thermal evaporation with defined masks (channel length and width were defined as 50 and 1000 μm, respectively). The characteristics of the OFETs were measured by using a Keithley 4200 source meter in an inert nitrogen-filled glove box. Fabrication and characterization of organic solar cells. Photovoltaic devices were fabricated with inverted configuration of ITO/ZnO/PEIE/polymer:PC71BM/MoOx/Ag. The active layer solutions were prepared by dissolving polymer (PT, PBo and PBi) and PC71BM in o-DCB in a weight ratio of 1:2 with a polymer concentration of 10 mg mL-1 and were heated to 70 °C and stirred 12 h for complete dissolution. The patterned ITO-glass was pre-cleaned in succession according to the reported literatures.6,23,46 The substrates were further treated in an UV-ozone chamber for 20 min. The cathode interlayers of ZnO and PEIE were prepared according to the literature.41 Afterwards, the active layer was spin-cast in an inert nitrogen-filled glove box, followed by slow drying process and subsequently annealed at 120 °C for 10 min. Thereupon, methanol was dropped onto the active film with a spin-casting rate at 2000 rpm for 60 s. Finally, a 10-nm-thick MoOx and a 100 nm Ag electrode were sequentially deposited on the active layer by thermal evaporation. The effective area of one cell was 0.08 cm2. The J−V characteristics of the PSCs were obtained using a Keithley 2400 source meter under illumination of AM 1.5G solar simulated light. An IPCE measurement was performed with QE10 equipment (PV Measurement, Inc).


10

Page 11 of 38 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


RESULTS AND DISCUSSION Synthesis and characterization The syntheses of monomers and copolymers are outlined in Scheme 1. Detailed experimental procedures are given in the experimental section. The heteroacenes 3, 5 and 7 were employed to explore the structural effects of ladder-type heteroaryl subunits on the nature of D−A copolymers. The heteroacene 3 is derived from a dithienopyrrole (DTP) core fusing with thiophene ring at both end while phenylene was fused to DTP core to give the molecule 5. Switch the configuration of thiophene and phenylene rings in the heteroacene 5 delivered the heteroacene 7. The fluorinated 2-alkyl-benzo[d][1,2,3]triazoles was selected as the acceptor subunit, which can provide a benefit of introducing soluble sidechains to furnish three D–A copolymers PBo, PBi and PT. The starting materials 1 and 4 were synthesized according to the published procedures.42,43 Base-catalyzed halogen dance reaction of 1 was successfully achieved to furnish the rearranged 6bromothieno[3,2-b]thiophen-2-yl)-(trimethyl)silane, which was oxidatively dimerized in situ by treatment with copper chloride to produce 2. The dibromide 2 was readily converted to heteropentacene 3 by tandem Buchwald-Hartwig coupling and subsequent desilylation. Compound 3 was transformed into the distannylated monomer M1 with t-BuLi deprotonation, followed by quenching with SnMe3Cl. Finally, distannylated M1 and 4,7-dibromo-5,6-difluoro-2octyl-2H-benzo[d][1,2,3]triazole (2FBTz)44 were coupled by Stille polycondensation under microwave heating conditions to acquire the copolymer PT (Mn = 25.2 kg mol-1, polydispersity index (PDI) = 1.12). Similar to the synthesis of compound 3, the Pd-catalyzed tandem C-N bond formation of 4 with a heptadecan-9-amine provided a phenylene-fused dithienopyrrole 5. The regioselective bromination occurred at 2,8 positions of 5 to yield 6. Treatment of 6 with n-butyl lithium followed by quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane


11


Page 12 of 38

successfully afforded diboronic ester M2 which was polymerized with 2FBTz acceptor under Suzuki coupling conditions to afford the alternating copolymer PBo (Mn = 21.6 kg mol-1, PDI = 1.54). The monomer 7 which is the regioisomer of compound 5 was converted to organotin reagent M3 according to the literature procedure.45 Monomer M3 was copolymerized with the acceptor, 2FBTz, by Stille coupling to give PBi (Mn = 20.6 kg mol-1, PDI = 1.55). TGA traces of these polymers are shown in Figure S1a (Supporting Information, SI). The thermal decomposition temperatures (5% weight loss) of PBo, PBi and PT copolymers are found to be 341 °C, 388 °C and 379 °C, respectively, indicating that the thermal stability of these polymers is sufficient for application in optoelectronics. No obvious phase transitions (Figure S1b in SI) were identified in the differential scanning calorimetry (DSC) scans upon heating and cooling treatments due to the rigid backbone that restricts the chain motion.46 Electrochemical properties To determine the electrochemical properties of heteropentacenes (3, 5 and 7) and copolymers (PBo, PBi and PT) cyclic voltammetry (CV) was performed (Figure 1a and b). The corresponding data are summarized in Table 1 and Table S1 (SI). Accordingly, the HOMO and the lowest unoccupied molecular orbital (LUMO) energy levels of polymers were calculated along the following equation:47 =−

−

− 5.1

=−

−

− 5.1

The corresponding energy levels of heteropentacenes 3, 5 and 7, and co-polymers PBo, PBi and PT are shown in Figure 1c. As shown in Figure 1a, the oxidation potential of heteropentacene 5 exhibits reversible redox behavior while molecules 3 and 7 show irreversible oxidation waves because of the existence of an α site on the thiophene unit.48 Moreover, the onset oxidation


12

Page 13 of 38 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


potentials (

) of compound 5, 7 and 3 were confirmed to be 0.62, 0.51 and 0.35 V, respectively.

It is interesting to reveal that the quinoidal character of the heteropentacene 7 is superior to that of the regioisomeric 5, and thus uplifts its HOMO energy level. In addition, further replacement of the phenylene with thiophene moiety in the heteropentacene system leads to a higher quinoidal character, giving rise to a higher HOMO energy level of heteropentacene 3 in comparison with 7. The HOMO energy levels of PBo, PBi and PT were corroborated to be -5.69, -5.43 and -5.33 eV, respectively. The trend of the relative HOMO energy levels in these polymers is conformable to those observed in the corresponding heteroaryl components. Although all of these polymers were decorated with the same 2FBTz moiety, PBi features a lower LUMO energy level compared with that of regioisomeric polymer PBo, suggesting that the position of phenylene moiety in the heteropentacene fragment has a significant influence on the electronic interaction between donor and acceptor unit. Additionally, in regard to PBo with outer-fused phenylene, PBi with the embedded phenylene in the heteropentacene possesses a narrower energy gap owing to the better π-electron delocalization along the polymer backbone. Again, it is reasonable to see that the energy gap of PT polymer with thiophene-fused configuration was further shrank by 0.4 eV, resulting in a more bathochromic absorption in comparison with that of PBo polymer (vide infra). The result indicates that the aromatic stabilization resonance energy of flanking-fused arenes in the polycyclic π-system plays a crucial role in ascertaining the polymer energy gap. In general, the quinoidalization has a larger impact on the HOMO energy levels than the LUMO energy levels. In this regard, the HOMO energy levels of linearly fused polycyclic compounds and polymers were also calculated to rationalize the structure-property relationship of the fused conjugated system (Figure S2 and Scheme S1 in SI). The relative trends of HOMO energy levels of the polymers and monomer 5, 7 and 3 were found to be well consistent with those observed


13


Page 14 of 38

from the cyclic voltammetry. As shown in Figure S2, SI, pentacene exhibits a higher HOMO energy level than that of anthracene because of the longer π-conjugation. However, molecule 5 with extended conjugation possesses a similar HOMO energy level as compared to that of parent DTP, suggesting that laterally fused phenylene has a dominant contribution to the enhancement of aromaticity in the heterocycles. Thus, molecule 5 exhibits a similar HOMO energy level in comparison with DTP despite of the extension of π-conjugation. In addition, the replacement of phenylene with thiophene in the pentacene π-system (syn-ADT or anti-ADT) results in a deeper HOMO energy level owing to the introduction of sulfur atom. Conversely, the replacement of phenylene with thiophene in the heterocyclic π-system (compound 3) leads to a higher HOMO energy level as compared to molecule 5 due to the enhancement of qunoidal character, suggesting that quinoidalization is more accessible in the linearly fused heterocyclic π-system than that of the pure hydrocarbon system. Photophysical properties. The absorption spectra of monomers and copolymers in solutions and as thin films are depicted in Figure 2 and Figure S3 (see SI). Table 1 summarizes the relevant data. As shown in Figure S3, all monomers exhibit intense absorption bands with well-resolved vibronic shoulders, indicating highly rigid and coplanar features of the heteropentacenes. Compared to monomer 7 with a maximum absorption peak (λmax) at 303 nm, the λmax of monomer 5 and 3 displays a bathochromic shift at 347 and 360 nm, respectively. Nevertheless, monomer 7 shows an unexpectedly low-energy transition at 392 nm which may stem from the vibronic transition with low oscillator strength. This low-probability electronic transition of N-alkyldithieno[2,3-b:7,6-b]carbazole was also observed by Y. Geng et al.49 The optical energy gaps of molecules 5, 3 and 7 deduced from the absorption onsets were found to be 3.48, 3.32 and 3.11 eV, respectively, which are in good agreement with


14

Page 15 of 38 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


the trends of their corresponding emission spectra (Figure S4 in SI). In contrast to the tendencies observed in the pentacyclic monomers, polymers show the absorption bands with λmax at 456, 520 and 624 nm for PBo, PBi and PT, respectively. Because PBo and regioisomeric PBi have identical acceptor moiety in their backbone, the discrepancy in the λmax reveals that the quinoidal character of pentacyclic donor segment of PBi is higher than that of PBo, and hence PBi shows a much redshifted absorption than PBo. It is clear that the location of phenylene ring in the fused π-conjugated system affects the degree of quinoidal nature. In comparison with PBo and PBi, PT exhibits a bathochromic shift with relatively broad absorption band (Figure 2), suggesting that the replacement of phenylene with thiophene in a ladder π-system is a facile strategy to form a new system with higher degree quinoidal structures. Moreover, the greater electron-rich nature of thiophene in comparison with phenylene promotes the stronger intramolecular charge transfer between electron-donating unit and electron-withdrawing moiety upon photo-excitation. In the solid state, PT polymer displays bathochromically shifted absorption band, implying stronger intermolecular π–π stacking interactions in the thin film. Optical energy gaps (ΔEopt) of PBo, PBi and PT determined from the absorption onset are 2.40, 2.23 and 1.76 eV, respectively, which is fairly consistent with the trends obtained from the cyclic voltammetry. Apparently, the broad absorption spectrum toward longer wavelength endows PT with a better light harvesting ability in the visible region, assuring that an enhancement of photocurrent generation is foreseeable in photovoltaic devices. Bond length alternation The bond length alternation (BLA) calculation is a valuable method to determine the propensity of quinoidalization in the electronic ground state of conjugated molecules.50, 51 In order to probe the effects of introducing different heteroacenes as donor moieties, the ground state geometries of


15


Page 16 of 38

polymers were optimized by density functional theory (DFT) at B3LYP/6-311G(d,p) level for the degree of BLA. Figure 3a shows the resonance structures of the truncated D–A fragment of polymers. The selected computed bond lengths are summarized in Table S2, SI. The degree of BLA of the 2FBTz acceptor, deduced from the difference between the bond length of C4-C5 and the average bond length of C3-C4 and C5-C6 of the 2FBTz acceptor, were estimated to be 0.049 Å, 0.045 Å and 0.043 Å for PBo, PBi and PT, respectively. In addition, the bond distance between heteropentacenes donor and 2FBTz acceptor (C6-C7) units are in the following order: PBo (1.479 Å) > PBi (1.457 Å) > PT (1.453 Å). The BLA data of acceptor steadily decreased from PBo to PT, indicating that PBi and PT moieties with flanking-fused thiophene facilitate the inter-ring πelectron delocalization and imparts a higher degree of quinoidalization to the conjugated backbone while connected to the electron withdrawing moiety 2FBTz. Consequently, the bond length of C6C7 for PBi and PT are shorter toward more double bond character. It is worthy to note that the degree of Mulliken charge variation upon excitation demonstrates that PT has a more delocalized ground state since the lowest energy transition originates from ICT (Figure S5 in SI). We next investigated the impact of central aromatics (carbazole vs. DTP) in pentacyclic conjugated system. The degree of BLA of thiophenes, which were flanking-fused onto carbazole (PBi) or DTP (PT), was also calculated and found to be 0.034 Å and 0.020 Å, respectively. The result indicates that the fused thiophene of PT has a higher tendency to form a quinoidal structure as compared to that of PBi. Molecular simulations To investigate the electronic and optical properties of the pentacyclic arenes and D–A πconjugated systems, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations at B3LYP/6-311G(d,p) level were performed with the methyl substituted side chain


16

Page 17 of 38 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


for simplicity.22 One repeating unit was adopted as simplified model compound for the simulation of polymers (Table S3, SI). The wave functions of frontier molecular orbitals (HOMO/LUMO) and the nearby HOMO-1 are shown in Table S4 (SI). For heteropentacenes, the vertical excitation energy of 5, 7 and 3 with the highest transition probability was estimated to be 333, 296 and 355 nm, respectively, which are consistent with the λmax in the absorption spectra. The most probable transitions of heteropentacene 5 and 3 can be assigned to the HOMO to LUMO, whereas the most probable transition of heteropentacene 7 is mainly attributed to the HOMO-1 to LUMO. It is noteworthy that the orbital spatial distributions of HOMO for heteropentacene 5 and 3 are very similar to that of the HOMO-1 for heteropentacene 7. By contrast, the HOMO-1 orbital distributions for 5 and 3 bear a striking resemblance to that of the HOMO for 7. Accordingly, the weak transition peak settled at 392 nm in the absorption spectrum of heteropentacene 7 can be ascribed to the HOMO to LUMO transition with an extremely low oscillator strength (f = 0.02). Moreover, the electron density distribution of the HOMO for PBo, PT and PBi are mainly delocalized over the pentacyclic donor moieties while the electron density associated with the LUMO for PBo, PT and PBi are primarily localized on the acceptor unit (see Table S4). It reveals that all polymers have a pronounced ICT character upon photo-excitation because the most probable transitions of polymers are ascribed to the transition from HOMO to LUMO.22 On the other hand, in order to analyze the minimum energy conformations of polymers, all polymers were truncated to be D−A trimers for geometry optimization. Compared to PBo, PT and PBi exhibit higher planarity through the entire backbone, which may stem from the intrachain interaction between S atom in the fused thiophenes of donor moieties and the F atom in the acceptor units (Figure 4).52 The dihedral angles between donor and acceptor were found to be 37.6o−38.9o for PBo, 0.2o−5.5o for PBi and 0.2o−1.5o for PT, respectively. The backbone


17


Page 18 of 38

coplanarity of PBi and PT can facilitate D−A electronic coupling, resulting in a longer effective conjugation length along the polymer backbone and the red-shifted absorption. In addition to the coplanar geometry between donor and acceptor, PT has a more linear backbone conformation in comparison with the zigzagged PBi, which may have positive influence on the film crystallinity/order and facilitates charge carrier mobility.53 OFET performance The charge transfer behavior of these three copolymers was explored by using bottom gate/top contact (BGTC) OFETs. The copolymer layers were spin-cast from o-dichlorobenzene (o-DCB) on the octadecyltrichlorosilane (ODTS)-modified SiO2 (300-nm-thick) to form a self-assembled monolayer. Thermal annealing of the polymer layers was also carried out for improving the film order before the deposition of the electrodes. The optimized thermal annealing temperature is 150 °C for these targeted polymers. The mobility was calculated from the saturation region, and the electrical performance are summarized in Table 2. Figure 5 displays the typical transfer and output characteristics of OTFTs. All devices show p-type characteristics and operate in an accumulation mode. The estimated OFET hole mobilities (μ) are (3.04±0.15) ×10-2, (8.31±0.15) × 10-4, (9.09±0.21) × 10-2 cm2V-1s-1 for PBo, PBi and PT, respectively, with corresponding on-off ratios (Ion/Ioff) in the range of 104~105. In the saturation region (Vds > Vg – VT), Id can be depicted in the following equation:54 =

(

−

)

where μ is the field-effect hole mobility, Ids is the drain-to-source current and Vg is the gate voltage. W and L are the width and length of source-drain electrodes, respectively. VT is the threshold voltage. Co is the capacitance of gate insulator per unit area (Co = 10 nF cm−2). The p-type transfer characteristics of OFETs were measured with gate voltage sweeping under Vds of -80 V and the


18

Page 19 of 38 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


studied polymers also showed typical output characteristics. PBo and PT have significantly higher carrier mobilities than PBi. We speculate that the small crystallite structure with the interconnected networks may offer highly efficient pathways for charge carrier transport (Figure S6, SI). In comparison, PT shows a highest hole mobility of 0.1 cm2 V-1 s-1, approximately 100 times larger than the hole mobility of PBi, which will be highly beneficial to achieve high performance solar cells. Photovoltaic properties To evaluate the photovoltaic performances of these copolymers PBo, PBi and PT, BHJ PSCs with a device architecture of ITO/ZnO/PEIE/polymer:PC71BM/MoOx/Ag were fabricated and tested under AM 1.5G illumination (100 mW cm-2). The optimized D/A ratios for the active layers of the PSCs were found to be 1:2 (w/w) for the three targeted PSCs. Numerous solvent additives were used to optimize the morphology of the blended active layer to uplift the solar energy conversion efficiencies, unfortunately, the solar cells based on PBo, PBi and PT copolymers were unresponsive to any solvent additives. We discovered that the optimized thickness of the active layers was around 70-130 nm for all devices. The current density–voltage (J–V) characteristics and incident photon-to-current efficiencies (IPCE) of the photovoltaic devices are exhibited in Figure 6a and b. Representative characteristics of the solar cells are summarized in Table 3. In the optimized 1:2 PT:PC71BM device, the open-circuit voltage (VOC) reaches 0.63 V, with a shortcircuit current (JSC) of 14.28 mA cm-2 and a fill factor (FF) of 67.1%, offering a high power conversion efficiency (PCE) of 6.04%. Conversely, a device based on optimized 1:2 PBo:PC71BM possesses a low PCE of 1.96%, with a VOC of 0.91 V, a JSC of 5.00 mA cm-2 and a FF of 43.1%. Comparatively, device based on 1:2 PBi:PC71BM exhibits a lowest PCE of 1.28% with a VOC of 0.74 V, a JSC of 3.79 mA cm-2, and a FF of 45.5%. It is noteworthy that device based on PT polymer


19


Page 20 of 38

exhibited a lower VOC than those of PBi- and PBo-based devices due to the higher quinoidal feature, which primarily increases the π electron density, and thus raises up the HOMO energy level to some extent. The trends in VOC are also in accord with the electrochemical potentials. Compared to PBi- and PBo-based devices, the high JSC of PT-based device can be attributed to its broad and red-shifted absorption spectrum. Notably, the PCE of the device based on PBi is severely limited by its low JSC, regardless of the fact that the UV–vis absorption spectrum of PBi is significantly red-shifted than that of the PBo one. The reason can be attributable to the inferior morphology and nongeminate recombination loss (vide infra). Nevertheless, PBi-based device has similar JSC and FF in comparison with that of PBo but the overall device performance is lower owning to the smaller VOC. Moreover, the high photovoltaic performance of PT polymer should be benefited from its high hole mobility, broad absorption spectrum, optimal morphology of the active layer, and efficient exciton dissociation and charge carrier collection (vide infra). Furthermore, the IPCE spectra agree well with the absorption spectra of the blends, demonstrating a close correlation with the photocurrents. Comparatively, PT-based device demonstrated very broad panchromatic spectra over the entire spectral range relative to other polymer blends and exceeding 50% in the 350-680-nm range. The integrated JSC values from the IPCE spectra are 4.66, 3.33 and 13.70 mA cm-2 for the devices based on PBo, PBi and PT devices, respectively, which are in accord with the JSC values obtained from the J–V curves within 5% mismatch. The photocurrent (Jph) as a function of effective voltage (Veff) was carried out to explore the detailed information on exciton dissociation probabilities and charge collection efficiencies of the PSCs. Jph is defined as JL − JD, where JL and JD are the current densities under illumination and in the dark, respectively. Veff is defined as V0 − Vbias, where V0 is the voltage at which the photocurrent is zero and Vbias is the applied external voltage bias.55 Consequently, Veff can determine the electric


20

Page 21 of 38 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


field in the devices and thus influences the carrier transport and extraction. As shown in Figure 6c, Jph linearly increases at low Veff range and adequately saturates at a high Veff region (Veff ≥ 2 V). Therefore, exciton dissociation and charge collection efficiency could be acquired from the ratio Jph/Jsat due to the fact that mobile charge carriers rapidly migrate towards the corresponding electrodes with minimal recombination. The calculated Jph/Jsat ratios for the optimized PT:PC71BM, PBo:PC71BM and PBi:PC71BM solar cells are 96.5%, 74.4% and 58.4%, respectively, indicating efficient exciton dissociation and charge collection in PT-based solar cells. Conversely, the relatively low Jph/Jsat ratios can account for the relatively lower FF in PBi and PBo-based devices. Moreover, PT-based device demonstrates higher saturated Jph at lower Veff. This is in accordance with the higher JSC and FF observed in the PT:PC71BM PSC. In contrast, PBi and PBo-based devices exhibit no clear field-independent saturation region, indicating significant electron–hole recombination. In these blends, a stronger electric field is needed to dissociate the excitons and extract the charge carriers.55 Additionally, charge recombination characteristics were also examined by the photocurrent as a function of different light intensity (Figure 6d). The relationship between JSC and light intensity (P) can be described by the formula of JSC ∝ Pα.56 The JSC show a linear dependence on the light intensity in logarithmic coordinates with a slope of 0.93, 0.86 and 0.77 for PT, PBi and PBo-based devices, respectively, suggesting efficient sweep-out of charge carriers and very weak bimolecular recombination in PT:PC71BM PSCs. Consequently, PT-based device demonstrates higher JSC and FF. In order to further understand how different donor moieties impact the phase separation in polymer:PC71BM blend films, we utilized tapping-mode atomic force microscopy (TM-AFM) to observe the surface morphological structure as shown in Figure 7. PBi:PC71BM shows unevenly


21


Page 22 of 38

large aggregated domains with non-bicontinuous feature, which is not beneficial for exciton diffusion, corresponding to the poor solar performance of PBi (Figure 7b,e). On the contrary, PT:PC71BM blend (Figure 7c,f) reveals smooth, uniform and distinct phase-separated structure. This finer interpenetrating structure of both domains with widths about 20~30 nm, which is favorable for efficient exciton dissociation and charge carrier transport, leading to the superior performance of PT-based PSCs. In addition, there is no obvious large-scale accumulation of PBo or PC71BM but nano-fibrillar structures are perceived throughout. That is to say, PBo and PC71BM domains are more homogeneously distributed on the film surface (Figure 7a,d). The root-meansquare roughness values of blend films based on PBo, PBi and PT are 1.8, 4.5 and 2.6 nm, respectively. As regards PBo and PT blends, there is no apparent large aggregates in the morphology, the low JSC of PBo can be attributed to its poor light absorption and low mobility (vide supra). The molecular ordering and crystallinity in both pristine and blend films was investigated using grazing incidence wide angle X-ray scattering (GIWAXS) measurements and the results are shown in Figure 8. The diffraction peak at qz ~0.2 Å-1 (d100 = ~31.4 Å) corresponds to the (100) reflections of polymers (preferential edge-on orientation), which can be assigned to the alkyl chain packing distance. In addition, clear (010) π−π stacking arcs were observed at ~1.56 (~4.03 Å) and ~1.66 Å-1 (~3.79 Å) for PBi and PT films, respectively, representing a higher degree of face-on molecular packing in the out-of-plane direction. For the PT:PC71BM blend film, fullerene domain reveals an amorphous halo at q~ 1.32 Å-1, however, the π−π stacking diffraction peak still can be distinctly observed in the out-of-plane. Conversely, for the PBi:PC71BM blend film, the π−π stacking of PBi along out-of-plane was disappeared. These results indicate that PT polymer possesses stronger intermolecular π−π interactions and higher degree of molecular ordering, not only in the pristine


22

Page 23 of 38 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


film but also in the blend film. It has been reported that the face-on orientation of polymer molecules can facilitate charge transport and results in high PCEs.57-59 In this study, PT-based devices reveal higher PCEs than those of PBi- and PBo-based devices, which may be attributable to the higher carrier mobility and favorable phase separation in PT blend films.

CONCLUSION In summary, a series of soluble semi-ladder type D–A copolymers (PBo, PBi and PT) employing various pentacyclic heteroacenes as electron-donating units has been synthesized, characterized and utilized as the active layer for OFET and OSC applications. We found that heteroaryl structure plays an important role in controlling of quinoidal character, and thus impart an effective means for tuning the frontier orbitals and optical energy gaps of the resulting copolymers. More importantly, the nature of heteroaryl components strongly governs the molecular orientation and morphology discrepancy that leads copolymer to perform various charge carrier mobility as well as device performance. Among them, PT-based device exhibits a high PCE of 6.04% with a VOC of 0.63 V, a JSC of 14.28 mA cm-2 and FF of 67.1% without any additive, which is higher than those of its counterparts, PBi and PBo. In addition, PT also manifested the highest hole mobility of 0.1 cm2 V-1 s-1. The combined result suggested that PT has a potential advantage for dual OFET and OPV capabilities ascribing to its higher quinoidal character, linear and coplanar conformation, smooth surface morphology and better bimodal crystalline structures. These characteristics are highly beneficial to extend the π−conjugation and ameliorate the charge transport via 3-D transport pathways, and thus improve overall device performances. The results obtained from this work provide new insights into how the aromatic subunit in coplanar heteroacene-based system affects the morphology, optoelectronic properties and device performances. Along this line, new


23


Page 24 of 38

copolymers employing tailor-made fully fused coplanar components could be reasonably anticipated to achieve higher charge carrier mobilities and solar energy conversion efficiencies.

Scheme 1. Synthetic routes of the monomers and copolymers. Reagents and conditions: (i) LDA, THF, -78 °C then CuCl2, -78 °C to r.t.; (ii) heptadecan-9-amine, Pd(dba)2, dppf, NaOtBu, toluene, reflux; (iii) TBAF, THF, r.t.; (iv) t-BuLi, THF, Me3SnCl, -78 °C to r.t.; (v) Br2, CHCl3, r.t.; (vi) n-BuLi, isopropyl pinacol borate, -78 °C to r.t.; (vii) n-BuLi, THF, Me3SnCl, -20 °C to r.t.


24

Page 25 of 38 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


Figure 1. Cyclic voltammograms of (a) monomers and (b) polymers. (c) Energy level diagrams of monomers and polymers. Cyclic voltammograms of monomers in solutions and polymers in thin films.


25


Page 26 of 38

Figure 2. UV–vis absorption spectra of polymers in solution and as thin films.

Figure 3. (a) Resonance structures and (b) selected carbon labeling of the truncated polymers. The size of the colored circles between two mesomeric forms represents the relative contribution of the resonance structures in ground state.


26

Page 27 of 38 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


Figure 4. Minimum energy conformations of truncated D-A trimers of PBo, PBi and PT with their calculated dihedral angles.


27


Page 28 of 38

Figure 5. (a) Transfer plots and (b-d) output curves of the FET devices based on PBo, PBi and PT with OTS-SAM layer.


28

Page 29 of 38 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


Figure 6. (a) J–V curves, (b) IPCE spectra, (c) photocurrent versus effective voltage, and (d) light intensity dependence of JSC for the best solar cells with a device configuration of ITO/ZnO/PEIE/polymer:PC71BM (1:2, w/w)/MoOx/Ag.


29


Page 30 of 38

Figure 7. Morphology characterization of TM-AFM topography images (upper row) and phase images (lower row) of 1:2 PBo:PC71BM blend film (panels a and d), 1:2 PBi:PC71BM blend film (panels b and e) and 1:2 PT:PC71BM blend film (panels c and f). The imaging size is 1 μm × 1 μm for each panel.


30

Page 31 of 38 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


Figure 8. 2D GIXRD patterns of the polymer neat films (upper row) and polymers:PC71BM blend films (lower row).


31


Page 32 of 38

Table 1. Photophysical and Electrochemical Parameters for Monomers and Polymers Polymer

λmax (nm)a

λonset (nm)a

λmax (nm)b

ΔEopt (eV)c

HOMO (eV)d

LUMO (eV)e

ΔEcv (eV)f

(V)

(V)

PBo

456

511

456

2.40

0.59

-1.74

-5.69

-3.36

2.33

PBi

520

551

524

2.23

0.33

-1.64

-5.43

-3.46

1.97

PT

624

675

642

1.76

0.23

-1.64

-5.33

-3.46

1.87

a

Measured in solution. bMeasured in thin films. cOptical energy gaps (ΔEopt) calculated from the onset of absorption spectra in thin films. dCalculated from the onset oxidation potentials of CV. eCalculated from the onset reduction potentials of CV. f Electrochemical energy gaps (ΔEcv) calculated from the difference between and .

Table 2. P-Type FET Characteristics of Polymers

a

Polymer

SAM layer

Annealing temperature (°C)

μa (cm2V-1s-1)

Ion/Ioff

PBo

OTS

150

(3.04±0.15) × 10-2

2.93 × 105

2.39

-4

4

-3.77

4.50 × 104

8.53

PBi

OTS

150

(8.31±0.15) × 10

PT

OTS

150

(9.09±0.21) × 10-2

1.20 × 10

VT (V)

Statistical data was obtained from 10 devices.

Table 3. Photovoltaic Parameters of Optimized Solar Cells Polymera

Polymer:PC71BMb

VOC (V)c

JSC (mA cm-2)c

FF (%)c

PCE (%)c

PBo PBi PT

1:2 1:2 1:2

0.91 (0.91±0.01) 0.74 (0.74±0.01) 0.63 (0.63±0.01)

5.00 (4.78±0.25) 3.79 (3.82±0.20) 14.28 (14.12±0.23)

43.1 (42.5±1.2) 45.5 (44.2±1.6) 67.1 (66.3±2.3)

1.96 (1.86±0.13) 1.28 (1.21±0.05) 6.04 (5.90±0.12)

a

The best blend ratios of polymer:PC71BM (w/w) were 1:2 without any additives. bInverted device configuration of ITO/ZnO/PEIE/polymer:PC71BM (1:2, w/w)/MoOx/Ag. cThe best values are given, followed by the averages with standard deviation of 20 optimized devices in parentheses.

ASSOCIATED CONTENT Supporting Information. General measurements and characterization, and NMR spectra. The Supporting Information is available free of charge on the ACS Publications website.


32

Page 33 of 38 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


AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Hsieh-Chih Chen). *E-mail: [email protected] (Ken-Tsung Wong). ORCID Hsieh-Chih Chen: 0000-0003-4060-9489 Ken-Tsung Wong: 0000-0002-1680-6186. Author Contributions # C.-L.C. and H.-C.C. contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support of this work by the Ministry of Science and Technology, Taiwan (MOST 104-2113-M-002 -006-MY3 and MOST 105-2113-M-035-003MY2). We also thank Dr. Pin-Jiun Wu, National Synchrotron Radiation Research Center, for the two-dimensional grazing incidence wide-angle X-ray scattering (GIWAXS) support.


33


Page 34 of 38

REFERENCES 1. Mei, J.; Diao, Y.; Appleton, A. L.; Fang, L.; Bao, Z., Integrated materials design of organic semiconductors for field-effect transistors. J. Am. Chem. Soc. 2013, 135, 6724-6746. 2. Mazzio, K. A.; Luscombe, C. K., The future of organic photovoltaics. Chem. Soc. Rev. 2015, 44, 78-90. 3. Chochos, C. L.; Choulis, S. A., How the structural deviations on the backbone of conjugated polymers influence their optoelectronic properties and photovoltaic performance. Prog. Polym. Sci. 2011, 36, 1326-1414. 4. Li, G.; Zhu, R.; Yang, Y., Polymer solar cells. Nat. Photon. 2012, 6, 153-161. 5. Xin, H.; Subramaniyan, S.; Kwon, T.-W.; Shoaee, S.; Durrant, J. R.; Jenekhe, S. A., Enhanced open circuit voltage and efficiency of donor–acceptor copolymer solar cells by using indene-c60 bisadduct. Chem. Mater. 2012, 24, 1995-2001. 6. Chen, H.-C.; Chen, Y.-H.; Liu, C.-C.; Chien, Y.-C.; Chou, S.-W.; Chou, P.-T., Prominent short-circuit currents of fluorinated quinoxaline-based copolymer solar cells with a power conversion efficiency of 8.0%. Chem. Mater. 2012, 24, 4766-4772. 7. Ye, L.; Zhang, S.; Zhao, W.; Yao, H.; Hou, J., Highly efficient 2d-conjugated benzodithiophene-based photovoltaic polymer with linear alkylthio side chain. Chem. Mater. 2014, 26, 3603-3605. 8. Li, Y.; Yao, K.; Yip, H.-L.; Ding, F.-Z.; Xu, Y.-X.; Li, X.; Chen, Y.; Jen, A. K. Y., Elevenmembered fused-ring low band-gap polymer with enhanced charge carrier mobility and photovoltaic performance. Adv. Funct. Mater. 2014, 24, 3631-3638. 9. Li, W.; Yang, L.; Tumbleston, J. R.; Yan, L.; Ade, H.; You, W., Controlling molecular weight of a high efficiency donor-acceptor conjugated polymer and understanding its significant impact on photovoltaic properties. Adv. Mater. 2014, 26, 4456-4462. 10. Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L., Recent advances in bulk heterojunction polymer solar cells. Chem. Rev. 2015, 115, 12666-12731. 11. Dou, L.; Liu, Y.; Hong, Z.; Li, G.; Yang, Y., Low-bandgap near-ir conjugated polymers/molecules for organic electronics. Chem. Rev. 2015, 115, 12633-12665. 12. Hu, H.; Jiang, K.; Yang, G.; Liu, J.; Li, Z.; Lin, H.; Liu, Y.; Zhao, J.; Zhang, J.; Huang, F.; Qu, Y.; Ma, W.; Yan, H., Terthiophene-based d-a polymer with an asymmetric arrangement of alkyl chains that enables efficient polymer solar cells. J. Am. Chem. Soc. 2015, 137, 14149-14157. 13. Cai, P.; Chen, Z.; Zhang, L.; Chen, J.; Cao, Y., An extended π-conjugated area of electrondonating units in d–a structured polymers towards high-mobility field-effect transistors and highly efficient polymer solar cells. J. Mater. Chem. C 2017, 5, 2786-2793. 14. Li, W.; Hendriks, K. H.; Furlan, A.; Wienk, M. M.; Janssen, R. A., High quantum efficiencies in polymer solar cells at energy losses below 0.6 ev. J. Am. Chem. Soc. 2015, 137, 2231-2234. 15. Chen, Z.; Brown, J.; Drees, M.; Seger, M.; Hu, Y.; Xia, Y.; Boudinet, D.; McCray, M.; Delferro, M.; Marks, T. J.; Liao, C.-Y.; Ko, C.-W.; Chang, Y.-M.; Facchetti, A., Benzo[d][1,2,3]thiadiazole (isobt): Synthesis, structural analysis, and implementation in semiconducting polymers. Chem. Mater. 2016, 28, 6390-6400. 16. Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S., Synthesis of conjugated polymers for organic solar cell applications. Chem. Rev. 2009, 109, 5868-5923. 17. Ouyang, X.; Peng, R.; Ai, L.; Zhang, X.; Ge, Z., Efficient polymer solar cells employing a non-conjugated small-molecule electrolyte. Nat. Photon. 2015, 9, 520-524.


34

Page 35 of 38 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


18. Chen, J. D.; Cui, C.; Li, Y. Q.; Zhou, L.; Ou, Q. D.; Li, C.; Li, Y.; Tang, J. X., Singlejunction polymer solar cells exceeding 10% power conversion efficiency. Adv. Mater. 2015, 27, 1035-1041. 19. Zhang, J.; Zhang, Y.; Fang, J.; Lu, K.; Wang, Z.; Ma, W.; Wei, Z., Conjugated polymersmall molecule alloy leads to high efficient ternary organic solar cells. J. Am. Chem. Soc. 2015, 137, 8176-8183. 20. He, Z.; Xiao, B.; Liu, F.; Wu, H.; Yang, Y.; Xiao, S.; Wang, C.; Russell, T. P.; Cao, Y., Single-junction polymer solar cells with high efficiency and photovoltage. Nat. Photon. 2015, 9, 174-179. 21. Bronstein, H.; Frost, J. M.; Hadipour, A.; Kim, Y.; Nielsen, C. B.; Ashraf, R. S.; Rand, B. P.; Watkins, S.; McCulloch, I., Effect of fluorination on the properties of a donor–acceptor copolymer for use in photovoltaic cells and transistors. Chem. Mater. 2013, 25, 277-285. 22. Wu, J.-S.; Cheng, Y.-J.; Lin, T.-Y.; Chang, C.-Y.; Shih, P.-I.; Hsu, C.-S., Dithienocarbazole-based ladder-type heptacyclic arenes with silicon, carbon, and nitrogen bridges: Synthesis, molecular properties, field-effect transistors, and photovoltaic applications. Adv. Funct. Mater. 2012, 22, 1711-1722. 23. Lu, C.; Chen, H.-C.; Chuang, W.-T.; Hsu, Y.-H.; Chen, W.-C.; Chou, P.-T., Interplay of molecular orientation, film formation, and optoelectronic properties on isoindigo- and thienoisoindigo-based copolymers for organic field effect transistor and organic photovoltaic applications. Chem. Mater. 2015, 27, 6837-6847. 24. Mishra, A.; Popovic, D.; Vogt, A.; Kast, H.; Leitner, T.; Walzer, K.; Pfeiffer, M.; MenaOsteritz, E.; Bauerle, P., A-d-a-type s,n-heteropentacenes: Next-generation molecular donor materials for efficient vacuum-processed organic solar cells. Adv. Mater. 2014, 26, 7217-7223. 25. He, F.; Wang, W.; Chen, W.; Xu, T.; Darling, S. B.; Strzalka, J.; Liu, Y.; Yu, L., Tetrathienoanthracene-based copolymers for efficient solar cells. J. Am. Chem. Soc. 2011, 133, 3284-3287. 26. Osaka, I.; Kakara, T.; Takemura, N.; Koganezawa, T.; Takimiya, K., Naphthodithiophenenaphthobisthiadiazole copolymers for solar cells: Alkylation drives the polymer backbone flat and promotes efficiency. J. Am. Chem. Soc. 2013, 135, 8834-8837. 27. Chung, C.-L.; Chen, C.-Y.; Kang, H.-W.; Lin, H.-W.; Tsai, W.-L.; Hsu, C.-C.; Wong, K.T., A–d–a type organic donors employing coplanar heterocyclic cores for efficient small molecule organic solar cells. Org. Electron. 2016, 28, 229-238. 28. Wu, J.-S.; Lin, C.-T.; Wang, C.-L.; Cheng, Y.-J.; Hsu, C.-S., New angular-shaped and isomerically pure anthradithiophene with lateral aliphatic side chains for conjugated polymers: Synthesis, characterization, and implications for solution-prossessed organic field-effect transistors and photovoltaics. Chem. Mater. 2012, 24, 2391-2399. 29. Zhong, H.; Li, Z.; Deledalle, F.; Fregoso, E. C.; Shahid, M.; Fei, Z.; Nielsen, C. B.; Yaacobi-Gross, N.; Rossbauer, S.; Anthopoulos, T. D.; Durrant, J. R.; Heeney, M., Fused dithienogermolodithiophene low band gap polymers for high-performance organic solar cells without processing additives. J. Am. Chem. Soc. 2013, 135, 2040-2043. 30. Son, H. J.; Lu, L.; Chen, W.; Xu, T.; Zheng, T.; Carsten, B.; Strzalka, J.; Darling, S. B.; Chen, L. X.; Yu, L., Synthesis and photovoltaic effect in dithieno[2,3-d:2′,3′-d′]benzo[1,2b:4,5-b′]dithiophene-based conjugated polymers. Adv. Mater. 2013, 25, 838-843. 31. 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, 11131154.


35


Page 36 of 38

32. Ashraf, R. S.; Chen, Z.; Leem, D. S.; Bronstein, H.; Zhang, W.; Schroeder, B.; Geerts, Y.; Smith, J.; Watkins, S.; Anthopoulos, T. D.; Sirringhaus, H.; de Mello, J. C.; Heeney, M.; McCulloch, I., Silaindacenodithiophene semiconducting polymers for efficient solar cells and high-mobility ambipolar transistors†. Chem. Mater. 2011, 23, 768-770. 33. Anthony, J. E., Functionalized acenes and heteroacenes for organic electronics. Chem. Rev. 2006, 106, 5028-5048. 34. Raimundo, J.-M.; Blanchard, P.; Brisset, H.; Akoudad, S.; Roncali, J., Proquinoid acceptors as building blocks for the design of efficient π-conjugated fluorophores with high electron affinity. Chem. Commun. 2000, 939-940. 35. Demeter, D.; Jeux, V.; Leriche, P.; Blanchard, P.; Olivier, Y.; Cornil, J.; Po, R.; Roncali, J., Tuning of the photovoltaic parameters of molecular donors by covalent bridging. Adv. Funct. Mater. 2013, 23, 4854-4861. 36. Shen, P.; Liu, X.; Jiang, S.; Huang, Y.; Yi, L.; Zhao, B.; Tan, S., Effects of aromatic πconjugated bridges on optical and photovoltaic properties of n,n-diphenylhydrazone-based metalfree organic dyes. Org. Electron. 2011, 12, 1992-2002. 37. Wan, Z.; Jia, C.; Duan, Y.; Zhou, L.; Lin, Y.; Shi, Y., Phenothiazine–triphenylamine based organic dyes containing various conjugated linkers for efficient dye-sensitized solar cells. J. Mater. Chem. 2012, 22, 25140. 38. Ting, H. C.; Chen, Y. H.; Lin, L. Y.; Chou, S. H.; Liu, Y. H.; Lin, H. W.; Wong, K. T., Benzochalcogenodiazole-based donor-acceptor-acceptor molecular donors for organic solar cells. ChemSusChem 2014, 7, 457-465. 39. Deng, Y.; Chen, Y.; Zhang, X.; Tian, H.; Bao, C.; Yan, D.; Geng, Y.; Wang, F., Donor– acceptor conjugated polymers with dithienocarbazoles as donor units: Effect of structure on semiconducting properties. Macromolecules 2012, 45, 8621-8627. 40. Wang, Y.; Lieberman, M., Growth of ultrasmooth octadecyltrichlorosilane self-assembled monolayers on sio2. Langmuir 2003, 19, 1159-1167. 41. Courtright, B. A.; Jenekhe, S. A., Polyethylenimine interfacial layers in inverted organic photovoltaic devices: Effects of ethoxylation and molecular weight on efficiency and temporal stability. ACS Appl. Mater. Interfaces 2015, 7, 26167-26175. 42. Zhang, X.; Côté, A. P.; Matzger, A. J., Synthesis and structure of fused α-oligothiophenes with up to seven rings. J. Am. Chem. Soc. 2005, 127, 10502-10503. 43. Balaji, G.; Valiyaveettil, S., Synthesis and properties of symmetric and unsymmetric dibenzothienopyrroles. Org. Lett. 2009, 11, 3358-3361. 44. Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W., Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer-fullerene solar cells. J. Am. Chem. Soc. 2011, 133, 4625-4631. 45. Deng, Y.; Chen, Y.; Liu, J.; Liu, L.; Tian, H.; Xie, Z.; Geng, Y.; Wang, F., Low-band-gap conjugated polymers of dithieno[2,3-b:7,6-b]carbazole and diketopyrrolopyrrole: Effect of the alkyl side chain on photovoltaic properties. ACS Appl. Mater. Interfaces 2013, 5, 5741-5747. 46. Zhang, Z.; Lin, F.; Chen, H.-C.; Wu, H.-C.; Chung, C.-L.; Lu, C.; Liu, S.-H.; Tung, S.-H.; Chen, W.-C.; Wong, K.-T.; Chou, P.-T., A silole copolymer containing a ladder-type heptacylic arene and naphthobisoxadiazole moieties for highly efficient polymer solar cells. Energy Environ. Sci. 2015, 8, 552-557. 47. Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C., Electrochemical considerations for determining absolute frontier orbital energy levels of conjugated polymers for solar cell applications. Adv. Mater. 2011, 23, 2367-2371.


36

Page 37 of 38 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


48. Heth, C. L.; Tallman, D. E.; Rasmussen, S. C., Electrochemical study of 3-(nalkylamino)thiophenes: Experimental and theoretical insights into a unique mechanism of oxidative polymerization. J. Phys. Chem. B 2010, 114, 5275-5282. 49. Chen, Y.; Tian, H.; Yan, D.; Geng, Y.; Wang, F., Conjugated polymers based on a s- and n-containing heteroarene: Synthesis, characterization, and semiconducting properties. Macromolecules 2011, 44, 5178-5185. 50. Roncali, J., Molecular engineering of the band gap of π-conjugated systems: Facing technological applications. Macromol. Rapid Commun. 2007, 28, 1761-1775. 51. Brédas, J. L., Relationship between band gap and bond length alternation in organic conjugated polymers. J. Chem. Phys. 1985, 82, 3808-3811. 52. Lee, Y.-S.; Lee, J. Y.; Bang, S.-M.; Lim, B.; Lee, J.; Na, S.-I., A feasible random copolymer approach for high-efficiency polymeric photovoltaic cells. J. Mater. Chem. A 2016, 4, 11439-11445. 53. Wu, Y.; Li, Z.; Ma, W.; Huang, Y.; Huo, L.; Guo, X.; Zhang, M.; Ade, H.; Hou, J., Pdt-st: A new polymer with optimized molecular conformation for controlled aggregation and pi-pi stacking and its application in efficient photovoltaic devices. Adv. Mater. 2013, 25, 3449-3455. 54. Zaumseil, J.; Sirringhaus, H., Electron and ambipolar transport in organic field-effect transistors. Chem. Rev. 2007, 107, 1296-1323. 55. Mihailetchi, V. D.; Wildeman, J.; Blom, P. W., Space-charge limited photocurrent. Phys. Rev. Lett. 2005, 94, 126602. 56. Lee, O. P.; Yiu, A. T.; Beaujuge, P. M.; Woo, C. H.; Holcombe, T. W.; Millstone, J. E.; Douglas, J. D.; Chen, M. S.; Frechet, J. M., Efficient small molecule bulk heterojunction solar cells with high fill factors via pyrene-directed molecular self-assembly. Adv. Mater. 2011, 23, 5359-5363. 57. Choi, M.-H.; Ko, E. J.; Han, Y. W.; Lee, E. J.; Moon, D. K., Control of polymer-packing orientation in thin films through chemical structure of d-a type polymers and its application in efficient photovoltaic devices. Polymer 2015, 74, 205-215. 58. Ide, M.; Saeki, A.; Koizumi, Y.; Koganezawa, T.; Seki, S., Molecular engineering of benzothienoisoindigo copolymers allowing highly preferential face-on orientations. J. Mater. Chem. A 2015, 3, 21578-21585. 59. Tumbleston, J. R.; Collins, B. A.; Yang, L.; Stuart, A. C.; Gann, E.; Ma, W.; You, W.; Ade, H., The influence of molecular orientation on organic bulk heterojunction solar cells. Nat. Photon. 2014, 8, 385-391.


37


Page 38 of 38

Table of Contents Graphic


38

S,N-Heteroacene-Based Copolymers for Highly Efficient Organic Field

Recommend Documents