Design of New Electron Acceptor Materials for Organic Photovoltaics

ARTICLE pubs.acs.org/cm

Design of New Electron Acceptor Materials for Organic Photovoltaics: Synthesis, Electron Transport, Photophysics, and Photovoltaic Properties of Oligothiophene-Functionalized Naphthalene Diimides Eilaf Ahmed, Guoqiang Ren, Felix S. Kim, Emily C. Hollenbeck, and Samson A. Jenekhe* Department of Chemical Engineering and Department of Chemistry, University of Washington, Seattle, Washington 98195-1750, United States

bS Supporting Information ABSTRACT: A homologous series of six novel oligothiophene
naphthalene diimide-based oligomer semiconductors with a donor
acceptor architecture, NDI-nTH (n = 1, 2, 3, 4) and NDI-nT (n = 2, 3), was synthesized and used to explore a set of criteria for the design of non-fullerene electron acceptor materials for organic solar cells. Thin films of the oligomer semiconductors had optical band gaps that varied from 2.1 eV in NDI-1TH and 1.6 eV in NDI-3TH to 1.4 eV in NDI-4TH, demonstrating good potential for light harvesting and exciton generation. The LUMO energy levels of the oligomer semiconductors were similar (ca.
4.0 eV), but the HOMO levels varied from
5.5 eV in NDI-3TH and NDI-4TH to
6.1 eV in NDI-1TH, showing that suitable energy band offsets necessary for efficient photoinduced charge transfer could be achieved with current donor polymers. Single-crystal X-ray structures of NDI-3TH and NDI4TH showed a slipped face-to-face π-stacking with short intermolecular distances (0.321
0.326 nm), which enabled facile selfassembly of single-crystalline nanowires from solution. Spin coated thin films of NDI-nTH and NDI-nT were mostly crystalline and had field-effect electron mobilities of up to (2
9)  10
4 cm2/(V s). Bulk heterojunction solar cells incorporating one of the n-type oligomer semiconductors as the electron acceptor and poly(3-hexylthiophene) as the electron donor showed a power conversion efficiency of 1.5% with an open circuit voltage of 0.82 V and a bicontinuous nanoscale morphology. KEYWORDS: organic solar cell, photovoltaic, organic field-effect transistor, n-type organic semiconductor, naphthalene diimide, donor
acceptor oligomer, non-fullerene electron acceptor, charge transport

1. INTRODUCTION Organic photovoltaics (OPVs) are promising for developing low cost solar energy conversion technologies.1
10 Because light absorption in OPVs initially generates strongly bound excitons,2 an electron donor
acceptor interface with suitable HOMO/LUMO energy level offsets is necessary for efficient charge photogeneration.1,3 Among the different OPV device architectures explored to date, including the Schotky barrier4 and bilayer heterojunction5 cells, the bulk heterojunction (BHJ) cell6 with an active layer consisting of a blend of a donor polymer and an acceptor material provides the most efficient example of these excitonic solar cells.7
10 Although other acceptor materials such as inorganic nanocrystals (CdSe,11 PbS,12 TiO2,13 etc), carbon nanotubes,14 graphene sheets,15 n-type conjugated small molecules,16
21 and n-type polymer semiconductors6,22
26 have been explored, fullerene-based acceptors have demonstrated excellent electron-accepting27 and electron-transport28 properties and have thus dominated the field of OPVs.1
10 Indeed, much progress has been made in polymer/fullerene BHJ solar cells, including substantial improvements in the power conversion efficiency to current maximum levels of 6
8%7 as well as understanding of the critical role of the BHJ nanoscale morphology9,29
32 r 2011 American Chemical Society

and the origin of the open circuit voltage (Voc).33 However, fullerene acceptors have important drawbacks in OPVs: (i) they have negligible light absorption in the visible-near IR region;1,34 (ii) they have relatively poor photochemical and chemical stability,35 requiring extensive encapsulation for the operation of the OPVs in air; (iii) their much higher rates of molecular diffusion and crystallization compared to those of the donor polymer component pose challenges for the control of the phase-separation kinetics and the BHJ film morphology;1b,e (iv) their accessible LUMO energy levels are in a very narrow range, limiting the accessible photovoltage; (v) the available chemistry for the synthesis of fullerene derivatives is limited;36 and (vi) the cost of fullerene synthesis and purification is high.22 There is thus a major need to investigate and discover new n-type organic semiconductors that could potentially replace fullerene acceptors in OPVs. Small-molecule organic semiconductors 16
21 and various conjugated polymers6,22
26 have been explored as non-fullerene acceptor materials in OPVs of bilayer heterojunction and bulk Received: July 10, 2011 Revised: September 5, 2011 Published: September 30, 2011 4563

dx.doi.org/10.1021/cm2019668 | Chem. Mater. 2011, 23, 4563–4577

Chemistry of Materials

Figure 1. (a) Schematic of a typical donor/acceptor interface in a BHJ solar cell with associated HOMO/LUMO energy level offsets. (b) Molecular structures of oligothiophene-functionalized NDIs.

heterojunction (BHJ) architectures. Among these n-type organic semiconductors that have been studied in non-fullerene OPVs are cyano-poly(phenylenevinylene) (CN-PPV),6b,24 poly(benzobisimidazobenzophenanthroline) (BBL),23 polyfluorene(benzothiadiazole)) (F8TBT),26 perylene diimides,18,19,25 vinazene derivatives,20 cyanopentacenes,16,21 bifluorenylidene,17a and diketopyrrolopyrrole derivatives.17b,c The highest power conversion efficiency of BHJ solar cells using these n-type small molecules varies from 1.87% PCE in perylene diimides derivatives,19d 1.29% PCE in cyanopentacene,21b and 1.4% PCE in vinazene derivatives20c to 1% PCE in diketyopyrrolopyrrole derivatives.17b,c Among the major current challenges in developing more efficient nonfullerene BHJ polymer solar cells is the lack of suitable n-type organic semiconductors that combine solution processability with suitable HOMO/LUMO energy levels, good charge transport, and other properties. Indeed, the factors essential to the design and development of new electron acceptor materials for the realization of more efficient non-fullerene OPVs remain to be elucidated. In this paper, we explore the design of new, non-fullerene, n-type organic semiconductors for use as efficient electron acceptors and electron-conducting materials in OPVs. Toward this end, we identify the following major considerations and design criteria: (1) The HOMO/LUMO energy levels of the acceptor material relative to those of the donor polymer semiconductor should offer sufficient offsets to facilitate photoinduced electron/charge transfer and efficient charge separation while maximizing the photovoltage (Figure 1a). A common LUMO/LUMO offset (ΔELUMO) thought to be necessary in the case of a donor polymer and a fullerene acceptor is 0.12
0.3 eV;1,37 this value is still a relevant guide in the case of new non-fullerene acceptors. Although the HOMO/HOMO offset (ΔEHOMO) has not been an important consideration with fullerene acceptors and donor polymers because of the rather deep HOMO level of fullerene acceptors (HOMO =
6.0 eV),38 a ΔEHOMO at 0.2
0.3 eV or

ARTICLE

larger is necessary to inject holes and confine them to the donor polymer. As is already well-known, the maximum possible open circuit voltage (Voc) is directly related to the HOMO/LUMO offset (ΔEHOMO/LUMO) of the active donor and acceptor in the OPV.1,3,33 (2) To facilitate good charge transport and efficient charge collection at the electrodes, the acceptor material should have a high electron mobility (μe > 10
4
10
3 cm2/(V s)). (3) The absorption band and optical band gap (EgA) of the acceptor material should contribute to light harvesting and exciton generation, and ideally, they should be complementary23 to those of the donor polymer (EgD) in the visible-near IR spectral range. (4) To facilitate favorable acceptor material/donor polymer blend phase separation thermodynamics and kinetics in forming the BHJ active layer film, the acceptor material should have a sufficiently large molar mass. (5) The solubility of the acceptor material in common organic solvents is essential to realize solution-based processing and the fabrication of OPVs. (6) The purity of the acceptor material should be sufficiently high (electronic grade) to enable high performance OPVs. To evaluate the criteria for the design of new acceptor materials for OPVs, we targeted a homologous series of π-conjugated oligomers with a donor
acceptor (D
A) architecture containing a central naphthalene diimide (NDI) electron-withdrawing group and appended electron-donating oligothiophenes of varying size (Figure 1b). We selected to focus on oligomers because they have a discrete molecular size, which should enable reproducible synthesis and ready tunability of HOMO/LUMO energy levels and other properties necessary to realize many of the design criteria. The D
A architecture of the oligothiophene-functionalized NDI molecules (NDI-nTH, n = 1, 2, 3, 4; NDI-nT, n = 2, 3) by virtue of intramolecular charge transfer (ICT) interactions39a can provide a facile means of tuning the optical band gaps, HOMO/ LUMO energy levels, and charge carrier mobilities.39b,c NDI-based small molecules (evaporated thin films) and polymers (spin coated thin films) are known to exhibit n-channel organic fieldeffect transistor properties with high carrier mobilities.40
44 In this paper, we report the synthesis, crystal structures, photophysics, electrochemical redox properties, and charge transport properties of the novel oligothiophene-functionalized naphthalene diimides shown in Figure 1b. The initial use of the materials as acceptors in bulk heterojunction solar cells in combination with a model donor polymer, poly(3-hexylthiophene) (P3HT), is also reported. The results are discussed in the context of the identified criteria for the design of acceptor materials toward more efficient nonfullerene OPVs.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthetic Procedures. Regioregular poly(3-hexylthiophene) (P3HT) was purchased from Rieke Metals with weight-average molecular weight (Mw) of 35.40 KDa and polydispersity index (PDI) of 2.45. All commercially obtained reagents were used without further purification. N,N0 -Bis(2-ethylhexyl)-2,6-bis(5-hexyl-thiophen-2-yl)-1,4,5,8-naphthalene Diimide (NDI-1TH). 2,6-Dibromonaphthalene diimide (1) (350 mg, 0.540 mmol), 5-hexyl-2-trimethylstannylthiophene (2a) (540 mg, 1.63 mmol), and palladium-tetrakis(triphenylphosphine) Pd(PPh3)4 (31 mg, 0.0268 mmol) were added to a flask under argon. The flask was degassed with argon three times, followed by the addition of 15 mL of anhydrous toluene, and the mixture was refluxed overnight. The product was purified by column flash chromatography using a 4564

dx.doi.org/10.1021/cm2019668 |Chem. Mater. 2011, 23, 4563–4577

Chemistry of Materials dichloromethane and hexane solvent mixture. The pure product was recrystallized from the dichloromethane/methanol mixture to give a red solid (212 mg, 48% yield). HRMS (FAB) (m/z) calcd for C50H66N2O4S2 [(M + H)+] 823.4464, found 823.45709. 1H NMR (CD2Cl2, 300 MHz, ppm): δ 8.7680 (s, 2H), 7.203 (d, J = 3.6 Hz, 2H), 6.8812 (d, J = 3.5 Hz, 2H), 4.0801 (d, J = 5.9 Hz, 4H), 2.9351 (t, J = 7.7 Hz, 4H), 1.934 (m, 2H), 1.797
1.719 (m, 4H), 1.424
1.274 (m, 29H), 0.956
0.865 (m, 18H). 13C NMR (CD2Cl2, 500 MHz, ppm): δ 162.554, 162.516, 146.647, 140.925, 139.135, 139.093, 136.245, 134.009, 129.793, 127.528, 125.430, 125.067, 124.073, 123.047, 122.806, 44.439, 37.855, 31.592, 31.577, 30.647, 30.164, 28.765, 28.613, 23.917, 23.123, 22.608, 13.886, 13.861, 10.348. N,N0 -Bis(2-ethylhexyl)-2,6-bis(50 -hexyl[2,20 ]bithiophenyl-5-yl)-1,4,5, 8-naphthalene Diimide (NDI-2TH). 2,6-Dibromonaphthalene diimide (1) (318 mg, 0.490 mmol), 5-hexyl-2-trimethylstannylbithiophene (2b) (890 mg, 2.15 mmol), and Pd(PPh3)4 (28 mg, 0.024 mmol) were added to a flask under argon. The flask was degassed with argon three times, followed by addition of 20 mL of anhydrous toluene, and the mixture was refluxed for 18 h. The product was purified by column flash chromatography using a dichloromethane and hexane mixture. The pure product was collected and redissolved in chloroform and precipitated from methanol. The pure product was obtained as a dark blue solid (384 mg, 79% yield). HRMS (FAB) (m/z) calcd for C58H70N2O4S4 [(M + H)+] 987.43015, found 987.43403. 1H NMR (CD2Cl2, 300 MHz, ppm): δ 8.779 (s, 2H), 7.307 (d, J = 3.8 Hz, 2H), 7.230 (d, J = 3.8 Hz, 2H), 7.139 (d, J = 3.6 Hz, 2H), 6.798 (d, J = 3.6 Hz, 2H), 4.125 (d, J = 7.6 Hz, 4H), 2.872 (t, J = 7.4 Hz, 4H), 1.940 (m, 2H), 1.771
1.721 (m, 4H), 1.475
1.297 (m, 29H), 0.980
0.894 (m, 18H). 13C NMR (CD2Cl2, 500 MHz, ppm): δ 162.554, 162.516, 146.647, 140.925, 139.135, 139.093, 136.245, 134.009, 129.793, 127.528, 125.430, 125.067, 124.073, 123.047, 122.806, 44.439, 37.855, 31.592, 31.577, 30.647, 30.164, 28.765, 28.613, 23.917, 23.123, 22.608, 13.886, 13.861, 10.348. Compound 5. 2,6-Dibromonaphthalene diimide (1) (627 mg, 0.967 mmol), 2-tributylstannylthiophene (1.08 g, 2.89 mmol), Pd(PPh3)4 (56 mg, 0.048 mmol), and 20 mL of anhydrous toluene were added to a flask under argon. The mixture was refluxed for 18 h. The crude product was purified by column flash chromatography using a dichloromethane and hexane mixture (3:2). The pure product was obtained as a red solid (546 mg, 86% yield). LC-MS (m/z) [(M + H)+] found 655.4, required 654.88. 1H NMR (CDCl3, 300 MHz, ppm): δ 8.768 (s, 2H), 7.203 (d, J = 3.6 Hz, 2H), 6.881 (d, J = 3.5 Hz, 2H), 4.118 (d, J = 7 Hz, 4H), 2.921 (t, J = 7.7 Hz, 4H), 1.932 (m, 2H), 1.475
1.268 (m, 16H), 0.891 (m, 12H). N,N0 -Bis(2-ethylhexyl)-2,6-bis([2,20 ]bithiophenyl-5-yl)-1,4,5,8-naphthalene Diimide (NDI-2T). 2,6-Dibromonaphthalene diimide (1) (560 mg, 0.87 mmol), 2-tributylstannylthiophene (4) (1.19 g, 2.61 mmol), Pd(PPh3)4 (50 mg, 0.043 mmol), and 20 mL of anhydrous toluene were added to a flask under argon. The mixture was refluxed for 18 h. The product was purified by column flash chromatography using a chloroform/hexane solvent mixture (3:2). The purified product was collected and redissolved in chloroform and precipitated from methanol. The pure product was obtained as a blue solid (615 mg, 86% yield). HRMS (FAB) (m/z) calcd for [(M + H)+] C46H47N2O4S4 819.24159, found 819.24212. 1H NMR (CD2Cl2, 300 MHz, ppm): δ 8.795 (s, 2H), 7.375
7.316 (m, 8H), 7.128 (d, 2H), 4.131 (d, 4H), 1.942 (m, 2H), 1.4607
1.3066 (m, 20H), 0.9828
0.8838 (m, 12H). 13C NMR (CD2Cl2, 500 MHz, ppm): δ 162.507, 162.488, 140.157, 139.815, 139.123, 136.714, 136.272, 129.671, 128.057, 127.568, 125.508, 125.248, 124.361, 123.866, 123.067, 44.466, 37.853, 30.634, 28.596, 23.915, 23.110, 13.869, 10.345. Compound 6. Compound 5 (373 mg, 0.569 mmol) was dissolved in 15 mL of chloroform and 5 mL of DMF. In a different flask, NBS (240 mg, 1.35 mmol) was dissolved in 5 mL of chloroform and 10 mL of DMF

ARTICLE

and added slowly via an addition funnel to compound 5. The reaction mixture was stirred in the dark for two days. The product was purified by column chromatography using a dichloromethane and hexane mixture. The pure product was collected as a red solid (443 mg, 96% yield). LCMS (m/z) (M
H+) found 812.2, requires 812.67. 1H NMR (CDCl3, 300 MHz, ppm): δ 8.739 (s, 2H), 7.174 (d, 2H), 7.109 (d, 2H), 4.114 (d, 4H), 1.919 (m, 2H), 1.398
1.294 (m, 16H), 0.956 (m, 12H). Compound 7. NDI-2T (490 mg, 0.60 mmol) was dissolved in 17 mL of chloroform and 5 mL of DMF. NBS (220 mg, 1.23 mmol) was dissolved in 5 mL of chloroform and 10 mL of DMF and added slowly to NDI-2T. The reaction mixture was stirred in the dark for two days. The product was extracted with dichloromethane, washed with water and brine solution, and the organic solvent was evaporated. The blue solid was purified by column chromatography using a dichloromethane/ hexane (5:1) solvent mixture to give the final product (496 mg, 85% yield). 1H NMR (CD2Cl2, 300 MHz, ppm): δ 8.776 (s, 2H), 7.317 (dd, 4H), 7.091 (m, 4H), 4.114 (d, 4H), 1.932 (m, 2H), 1.457
1.320 (m, 18H), 0.977
0.777 (m, 12H). N,N0 -Bis(2-ethylhexyl)-2,6-bis(500 -hexyl[2,20 ;50 ,200 ]terthiophen-5-yl)1,4,5,8-naphthalene Diimide (NDI-3TH). Compound 6 (440 mg, 0.545 mmol), 5-hexyl-2-trimethylstannylbithiophene (2b) (670 mg, 1.62 mmol), and Pd(PPh3)4 (31 mg, 0.027 mmol) were added to a flask under argon. The flask was degassed with argon three times, followed by addition of 20 mL of anhydrous toluene, and the mixture was refluxed for 24 h. The product was purified by column flash chromatography using a chloroform and hexane mixture. The pure product was collected and redissolved in chloroform and precipitated from methanol. The pure product was obtained as a dark green solid (377 mg, 60% yield). HRMS (ESI) (m/z) calcd for C66H75N2O4S6 1151.4046, found 1151.4059. 1H NMR (CD2Cl2, 300 MHz, ppm): δ 8.793 (s, 2H), 7.336 (dd, J = 3.8 Hz, 4H), 7.225 (d, J = 3.8 Hz, 2H), 7.114 (dd, J = 3.8 Hz, 4H), 6.776 (d, J = 3.6 Hz, 2H), 4.129 (d, J = 7.1 Hz, 4H), 2.854 (t, J = 7.6 Hz, 4H), 1.953
1.933 (m, 2H), 1.756
1.703 (m, 4H), 1.470
1.297 (m, 29H), 0.985
0.891 (m, 18H). 13C NMR (CD2Cl2, 500 MHz, ppm): δ 162.546, 162.502, 146.238, 140.063, 139.744, 139.028, 137.683, 136.269, 134.721, 134.137, 129.893, 127.600, 125.493, 125.010, 124.995, 123.686, 123.670, 122.946, 44.479, 37.861, 31.603, 31.566, 30.643, 30.137, 28.751, 28.604, 23.919, 23.123, 22.595, 13.884, 13.848, 10.351. N,N 0 -Bis(2-ethylhexyl)-2,6-bis(500 -hexyl-[2,20 ;50 ,200 ;500 ,2000 ]quaterthiophen-5-yl)-1,4,5,8-naphthalene Diimide (NDI-4TH). Compound 7 (623 mg, 0.638 mmol), compound 2b (681 mg, 1.66 mmol), and Pd(PPh3)4 (37 mg) were added to a flask under argon. The flask was degassed with argon three times, followed by addition of 18 mL of anhydrous toluene, and the mixture was refluxed for 12 h. The organic phase was washed with water and concentrated. The product was purified by column flash chromatography using a dichloromethane and hexane mixture. The product was collected and redissolved in chloroform and precipitated from methanol. The pure product was obtained as a dark green solid (265, 32% yield). MALDI-TOF MS (m/z) calcd 1314.3727, found 1313.844. 1H NMR (CD2Cl2, 300 MHz, ppm): δ 8.792 (s, 2H), 7.322 (m, 4H), 7.237 (dd, 2H), 7.17 (t, 4H), 7.075 (m, 4H), 6.764 (m, 2H), 4.115 (dd, 4H), 2.846 (m, 4H), 1.749 (m, 4H), 1.469
1.333 (m, 18H), 0.989
0.893 (m, 12H). 13C NMR (CDCl3, 500 MHz, ppm): δ 162.570, 145.903, 140.494, 139.498, 139.266, 137.309, 137.045, 136.526, 135.392, 134.958, 134.335, 130.194, 127.586, 125.402, 125.154, 124.909, 124.515, 124.198, 123.952, 123.635, 123.564, 122.538, 44.673, 37.812, 31.591, 30.687, 30.236, 28.791, 28.623, 23.961, 23.149, 22.604, 14.150, 14.112, 10.645. N , N 0 -Bis(2-ethylhexyl)-2,6-bis([2,20 ;50 ,200 ]terthiophen-5-yl)-1,4,5,8naphthalene Diimide (NDI-3T). Compound 7 (496 mg, 0.508 mmol), compound 3 (564 mg, 1.51 mmol), and Pd(PPh3)4 (29 mg) were added to a flask under argon. The flask was degassed with argon three times, followed by addition of 24 mL of anhydrous toluene, and the mixture was refluxed for 12 h. The organic phase was washed with water and 4565

dx.doi.org/10.1021/cm2019668 |Chem. Mater. 2011, 23, 4563–4577

Chemistry of Materials

ARTICLE

Scheme 1. Synthesis of Oligothiophene-Functionalized Naphthalene Diimidesa

a

Conditions: (i) Pd(PPh3)4, toluene, reflux; (ii) NBS, CHCl3, DMF, 48 h, rt.

concentrated. The product was purified by column flash chromatography using a dichloromethane and hexane mixture. The pure product was collected and redissolved in chloroform and precipitated from methanol. The pure product was obtained as a dark green solid (216 mg, 43% yield). MALDI-TOF MS (m/z) calcd 982.2095, found 983.086 [(M + H)+]. 1H NMR (CD2Cl2, 300 MHz, ppm): δ 8.887 (s, 2H), 7.331
7.318 (m, 6H), 7.291 (dd, 2H), 7.242 (dd, 2H), 7.194 (dd, 2H), 7.105 (t, 2H), 4.108 (dd, 4H), 1.953
1.933 (m, 2H), 1.469
1.333 (m, 16H), 0.989
0.893 (m, 12H). 13C NMR (CDCl3, 500 MHz, ppm): δ 162.566, 140.474, 139.495, 139.305, 137.158, 136.976, 136.544, 135.517, 130.117, 127.981, 127.583, 125.417, 125.074, 124.789, 124.472, 123.961, 122.600, 79.528, 44.670, 37.804, 30.676, 28.614, 23.953, 23.144, 14.147, 10.641. 2.2. Characterization. 1H NMR spectra were recorded on a Bruker AV300 at 300 MHz, whereas 13C NMR spectra were recorded on a Bruker AV 500 at 500 MHz using CDCl3 or CD2Cl2 as the solvent. Mass spectra were obtained with a Bruker Esquire LC/ion trap mass spectrometer, JEOL/HX-110, or a matrix-assisted laser desorption ionization time-of-flight (MALDI/TOF) instrument. Differential scanning calorimetry (DSC) analysis was performed on a TA Instruments Q100 under N2 at a heating rate of 10 °C/min. Second-heating DSC scans are reported for all molecules except for NDI-1TH and NDI-2TH, where first-heating scans are reported. Thermogravimetric analysis of the NDI-nTH and NDI-nT was conducted on a Perkin-Elmer instrument, TGA 7. A heating rate of 5 °C/min under flow of N2 was used with runs conducted from room temperature to 600 °C. X-ray diffraction (XRD) data were collected from a Bruker-AXS D8 Focus diffractometer with Cu Kα beam (40 kV, 40 mA, λ = 0.15418 nm). Thin films for powder XRD were prepared by drop-casting from a concentrated (10 mg/mL) solution in chloroform onto glass

slides or Si-wafers, followed by film annealing at 150 °C for 5 min. Cyclic voltammetry was measured on an EG&G Princeton Applied Research Potentiostat/Galvanostat (model 273A). Data were analyzed by model 270 electrochemical analysis system software on a PC computer. A three-electrode cell was used, using platinum wire electrodes as both the counter electrode and the working electrode. Silver/silver ion (Ag in 0.1 M AgNO3 solution, Bioanalytical System, Inc.) was used as a reference electrode. Ferrocene/ferrocenium (Fc/Fc+) was used as an internal standard. The potential values obtained in reference to Ag/Ag+ were converted to the saturated calomel electrode (SCE) scale. All solutions were purged with N2 for 20 min before each experiment. UV
vis absorption spectra were collected on a Perkin-Elmer model Lambda 900 UV/vis/near-IR spectrophotometer.

2.3. Fabrication and Characterization of Field-Effect Transistors. Organic field-effect transistors (OFETs) were fabricated in standard bottom-contact and bottom-gate geometry. Heavily n-doped silicon with 200 nm thermal oxide (C0 = 17 nF/cm2) acted as a gate electrode with a dielectric layer, as well as the substrate. Gold source and drain electrodes (∼50 nm) with a thin chromium layer (∼2 nm) were photolithographically patterned on the substrate. The width (W) and length (L) of the transistor channel are 800
1000 μm and 40
100 μm, respectively. The silicon dioxide surface was treated with octyltrichlorosilane (OTS-8). Thin films of NDI-nTH (or NDI-nT) were coated on the substrates by either spin-coating or thermal evaporation. NDI-nTH (or NDI-nT) solution (10 mg/mL) in 1,2-dichlorobenzene was spincoated onto substrates at 1000 rpm for 60 s. The films were annealed at 120
200 °C for 10 min under a flow of nitrogen (