Solution-processable low-molecular weight extended arylacetylenes: versatile p-type semiconductors for field-effect transistors and bulk heterojunction solar cells

Published on Web 04/08/2010

Solution-Processable Low-Molecular Weight Extended

Arylacetylenes: Versatile p-Type Semiconductors for

Field-Effect Transistors and Bulk Heterojunction Solar Cells

Fabio Silvestri,^†,§Assunta Marrocchi,*^,‡Mirko Seri,^‡Choongik Kim,^†

Tobin J. Marks,*^,†Antonio Facchetti,*^,†and Aldo Taticchi*^,‡

Department of Chemistry, the Argonne-Northwestern Solar Energy Research Center, and the

Materials Research Center, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois

60208-3113, and Department of Chemistry, UniVersity of Perugia, Via Elce di Sotto 8,

I-06123 Perugia, Italy

Received December 10, 2009; E-mail: assunta@unipg.it (A.M.); t-marks@northwestern.edu (T.J.M.);

a-facchetti@northwestern.edu (A.F.); taticchi@unipg.it (A.T.)

Abstract: We report the synthesis and characterization of a series of ﬁve extended arylacetylenes, 9,10-

bis-{[m,p-bis(hexyloxy)phenyl]ethynyl}-anthracene (A-P6t, 1), 9,10-bis-[(p-{[m,p-bis(hexyloxy) phenyl]ethynyl}

phenyl)ethynyl]-anthracene (PA-P6t, 2), 4,7-bis-{[m,p-bis(hexyloxy)phenyl]ethynyl}-2,1,3-benzothiadiazole

(BTZ-P6t, 5), 4,7-bis(5-{[m,p-bis(hexyloxy)phenyl]ethynyl}thien-2-yl)-2,1,3-benzothiadiazole (TBTZ-P6t, 6),

and 7,7′-({[m,p-bis(hexyloxy)phenyl]ethynyl}-2,1,3-benzothiadiazol-4,4′-ethynyl)-2,5-thiophene (BTZT-P6t,

7), and two arylvinylenes, 9,10-bis-{(E)-[m,p-bis(hexyloxy)phenyl]vinyl}-anthracene (A-P6d, 3), 9,10-bis-

[(E)-(p-{(E)-[m,p-bis(hexyloxy)phenyl]vinyl}phenyl)vinyl]-anthracene (PA-P6d, 4). Trends in optical absorption

spectra and electrochemical redox processes are ﬁrst described. Next, the thin-ﬁlm microstructures and

morphologies of ﬁlms deposited from solution under various conditions are investigated, and organic ﬁeld-

effect transistors (OFETs) and bulk heterojunction photovoltaic (OPV) cells fabricated. We ﬁnd that

substituting acetylenic for oleﬁnic linkers on the molecular cores signiﬁcantly enhances device performance.

OFET measurements reveal that all seven of the semiconductors are FET-active and, depending on the

backbone architecture, the arylacetylenes exhibit good p-type mobilities (µ up to ∼0.1 cm²V^-1s^-1) when

optimum ﬁlm microstructural order is achieved. OPV cells using [6,6]-phenyl C₆₁-butyric acid methyl ester

(PCBM) as the electron acceptor exhibit power conversion efﬁciencies (PCEs) up to 1.3% under a simulated

AM 1.5 solar irradiation of 100 mW/cm². These results demonstrate that arylacetylenes are promising hole-

transport materials for p-channel OFETs and promising donors for organic solar cells applications. A direct

correlation between OFET arylacetylene hole mobility and OPV performance is identiﬁed and analyzed.

Introduction

use in place of conventional inorganic semiconductors offers

the prospects of low manufacturing costs (i.e., casting or printing

technologies using solution-processable materials), large area

coverage, and compatibility with ﬂexible substrates. Among

Semiconducting materials based on π-conjugated organic

small molecules and polymers have been extensively investi-

gated for applications in a variety of optoelectronic devices such

as light-emitting diodes (OLEDs),¹ﬁeld-effect transistors

(OFETs),²photovoltaic cells (OPVs),³sensors, optical ampli-

ﬁers, and lasers.⁴One key attraction of organic semiconductors

is the tunability of their physical and chemical properties by

rational sequential structural modiﬁcation. Additionally, their

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^†Northwestern University.

^‡University of Perugia.

^§Current address: ETH Zu¨rich, Laboratory of Organic Chemistry,

Department of Chemistry and Applied Biosciences, HCI G 304 Wolfgang-

Pauli-Strasse 10, CH-8093 Zu¨rich, Switzerland.

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6108 J. AM. CHEM. SOC. 2010, 132, 6108–6123

10.1021/ja910420t  2010 American Chemical Society

Arylacetylenes for OFETs and BHJ Organic Solar Cells

A R T I C L E S

Chart 1. Representative Semiconducting (Poly)arylacetylenes

these materials, members of the (poly)arylacetylene family are

particularly attractive,⁵because of their ﬂexible molecular orbital

energetics, known⁶to be tunable Via appropriate skeletal

functionalization. Moreover, the availability of efﬁcient synthetic

protocols⁷allows the effective π-conjugation length of these

shape-persistent rod-like structures to be easily varied by

controlling the number of arylacetylene repeat units. Note also

that alkyne linkages are more accommodating than alkenes to

steric and conformational constraints due to the quasi-cylindrical

electronic symmetry.⁸

The results of Roy et al.^5don OFET-determined charge

transport indicate that vacuum-deposited thin ﬁlms of arylacety-

lene oligomers can exhibit ﬁeld-effect hole mobilities (µ) as

high as 0.3 cm²V^-1s^-1and current on-off ratios (I_on/I_off) ≈

10⁵. Yasuda et al.^5calso reported high-performance OFETs using

vacuum-deposited diethynyl-naphthalene derivatives, and a

maximum ﬁeld-effect hole mobility of 0.12 cm²V^-1s^-1and

I_on/I_off≈ 10⁵were reported (BH-202). Very recently, Meng et

al.^5areported that ﬁlms of soluble conjugated thiophene-

ethynylphenylene semiconductors exhibit charge carrier mobili-

ties as high as 0.084 cm²V^-1s^-1with I_on/I_off) 10⁵. A promising

approach to combining good transistor characteristics and high

solar energy conversion efﬁciencies has been reported by Baek

et al.⁹Here it was shown that, for solution-processable metalated

polyarylacetylenes, ﬁeld-effect mobilities and I_on/I_offratios

approach 0.01 cm²V^-1s^-1and 10⁴, respectively, and solar cell

power conversion efﬁciencies (PCEs) reach 3.73% (P4). Spin-

coated metal-containing polyarylacetylenes have also been

successfully used by other groups as active layers in bulk

heterojunction solar cells.¹⁰An important result of Cremer et

al.¹¹is the ﬁnding that poly(ethynylene-bithienylene) (PEBT)/

PCBM devices exhibit signiﬁcantly greater V_ocvalues (∼1.0

V) than PH3T/PCBM solar cells (V_oc≈ 0.62 V),¹²highlighting

the attraction of incorporating electron-withdrawing ethynylene

units into polymer backbones (Chart 1).

Taking all of these results into account, the development of

new arylacetylene structures should afford interesting and

instructive new hole-transporting materials for OFETs as well

as for OPVs. Furthermore, very few solution-processable

arylacetylenes suitable for OPVs are known,¹³and those which

have been reported generally exhibit modest power conversion

efﬁciencies. Small-molecule donor materials offer attractions

over polymeric materials in terms of ease of synthesis and

puriﬁcation, which greatly improves fabrication reproducibility,

as well as exhibiting a greater tendency to self-assemble into

ordered domains, affording high charge carrier mobilities. Most

important, small molecules do not suffer from batch-to-batch

property variations or end-group contamination as do their

polymeric counterparts. Finally, parameters such as polydis-

persity and regioregularity are not an issue.

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A R T I C L E S

Silvestri et al.

Chart 2. Structures of Anthracene-Based Derivatives 1-4

Chart 3. Structures of 2,1,3-Benzothiadiazole-Based Derivatives 5-7

In a recent communication¹⁴we brieﬂy described the

synthesis of compounds A-P6t and PA-P6t (Chart 2) and their

implementation in bulk heterojunction (BHJ) solar cells, yielding

PCEs up to ∼1.2%. We also showed recently¹⁵that OFETs

based on extended arylacetylenes PA-P6t and TBTZ-P6t

(Charts 2 and 3) exhibit substantial hole mobilities, approaching

0.1 cm²V^-1s^-1with on/off ratios up to ∼10⁷.

Additionally, high ﬁeld-effect mobilities are relevant for solar

cell applications since this can enhance exciton diffusion lengths,

remove photogenerated electron-hole pairs before recombina-

tion, and efﬁciently convey carriers to the electrodes for

collection.^12b,22Furthermore, unsubstituted acenes typically pack

in a herringbone motif^23,24awhereas appropriate arene substitu-

tion may promote face-to-face π-stacking.^24-26In several cases,

In this contribution, we report the synthesis and characteriza-

tion of previously unknown members of this extended aryl-

acetylene series, to further explore and elucidate architecture-

electronic structure relationships in this semiconductor class.

Solution-processable OFETs and BHJ photovoltaic cells (Figure

1) are fabricated and optimized for compounds 1-7, and their

device responses characterized. The design strategy in the

present semiconductor family aims at achieving high solubility

in common organic solvents, efﬁcient hole transport (required

for optimum OFET and OPV response), and broad NIR-shifted

optical absorption, to capture maximum solar light. Enhanced

solubility is achieved by introducing alkoxy chains onto the

delocalized core peripheries. Additionally, this approach pro-

motes molecular self-organization in the solid state.¹⁶

Note that anthracene-based compounds A-P6t (1), PA-P6t

(2), A-P6d (3), PA-P6d (4) are designed bearing in mind that

n-acenes exhibit some of the highest known OFET mobilities

(µ ) 5.5 cm²V^-1s^-1for pentacene).^17-19However, useful

chemical modiﬁcation of large acenes is challenging because

of the generally low solubility. Therefore, we employed

anthracene,²⁰which is more readily derivatized to afford greater

solubility and chemical stability.²¹It is known that anthracene

oligomer-based OFETs can exhibit high hole mobilities,^21c

suggesting that anthracene π-extension by interposed arylene

fragments can afford efﬁcient charge transporting structures.

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Figure 1. Schematic representation of (A) ﬁeld-effect transistor and (B)

bulk heterojunction solar cell structures employed in this study.

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6110 J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010

Arylacetylenes for OFETs and BHJ Organic Solar Cells

A R T I C L E S

e.g., going from pentacene and oligothiophenes (herringbone)

to TIPS pentacene or mixed ﬂuoroarene-thiophene oligomers

(cofacial π-stacking), respectively, this approach resulted in

enhanced carrier mobilities.^2m,21a

the potential for enhanced intramolecular charge transfer

character, thereby inﬂuencing solid state packing and carrier

transport.

OFETs based on arylacetylenes 1-7 are optimized here in

terms of solvent selection and ﬁlm processing methodology. It

will be seen that materials having hole mobilities approaching

0.1 cm²V^-1s^-1with I_on/I_off> 10⁶can be obtained with devices

fabricated by simple spin-coating techniques. Solution-process-

able BHJ solar cells are also fabricated and optimized, and their

device responses analyzed in terms of the interplay between

molecular electronic structure and ﬁlm microstructural/morpho-

logical characteristics. Furthermore, CtC triple bonds are shown

here to be effective CdC replacements and to afford both high

OPV open circuit voltage (V_oc) and good short circuit current

(J_sc) metrics, as well as power conversion efﬁciencies of ∼1.3%.

Finally, this study reveals close, instructive correlations between

semiconductor FET mobility and BHJ OPV performance.

These considerations suggest the potential of derivatives 1-4

for a cofacial π-stacking and for elucidating^14,27the effects of

replacing oleﬁnic with acetylenic linkers in organic semiconduc-

tors. Additionally, we report here 2,1,3-benzothiadiazole (BTZ)-

containing arylacetylene derivatives BTZ-P6t (5), TBTZ-P6t

(6), and BTZT-P6t (7). It is known that 2,1,3-benzothiadiazoles

have high electron afﬁnities and are amenable to facile ring

modiﬁcation.²⁸Moreover, BTZ-containing compounds generally

afford well-organized crystal structures due to their signiﬁcant

polarizability, leading to intermolecular interactions such as

heteroatom· · ·heteroatom contact and π-π interactions.²⁹For

these reasons, we synthesized new arylacetylene BTZ-P6t as

an analogue of compound A-P6t in which a BTZ unit replaces

the anthracene core. However, in compound TBTZ-P6t the

benzothiadiazole core is substituted with a donor-acceptor-

donor (D-A-D) motif by introducing thienyl groups on either

side of a 2,1,3-benzothiadiazole unit.¹⁵It is well-established that

intramolecular charge transfer from electron-rich to electron-

deﬁcient moieties lowers the band gap,³⁰desirable for OPV light

absorption at longer wavelengths.^9,31It is also known³²that such

D-A-D motifs enhance molecular core coplanarity due to

intramolecular charge transfer, which is further supported by

computation.³³In designing compound TBTZ-P6t, we also

recognized the possibility of strong dipole-dipole-induced

intermolecular interactions.^28a,34,35Finally, novel compound

TBTZ-P6t was created with an extended conjugation length

by introducing a donor-acetylene-acceptor unit.^28c,36This offers

Experimental Section

Materials and Methods. All reagents were purchased from

commercial sources and used without further puriﬁcation unless

otherwise noted. PCBM was purchased from American Dye Source,

Inc. (ADS) and was further puriﬁed by several cycles of sonication

in toluene followed by ﬁltration and then sonication in pentane,

followed by centrifugation. Anhydrous THF was distilled from Na/

benzophenone, and toluene was distilled from LiAlH₄. Petroleum

ether was used as the 40-60 °C boiling fraction. Solution optical

spectra were recorded on a Cary Model 1 UV-vis spectrophoto-

meter. NMR spectra were recorded on a Varian Associates VXR-

400 multinuclear spectrometer (internal Me₄Si). Elemental analyses

were performed on a Fisons EA 1108 instrument.

Synthesis of 4,7-Bis-{[m,p-bis(hexyloxy)phenyl]ethynyl}-2,1,3-

benzothiadiazole (BTZ-P6t, 5). Dry toluene (6 mL), 4,7-dibromo-

2,1,3-benzothiadiazole 9 (0.15 g, 0.51 mmol), CuI (0.004 g, 0.02

mmol), Pd(PPh₃)₄(0.025 g, 0.02 mmol), and diisopropylamine (3

mL) were placed in a ﬂask and degassed with argon at 0 °C. Next,

3,4-(bishexyloxy)-ethynylbenzene 8 (0.33 g, 1.1 mmol) was added

and the mixture was kept at 50 °C for 18 h. The solvent was then

evaporated to dryness, and the crude product was puriﬁed by column

chromatography (silica gel, petroleum ether/dichloromethane 3:2)

to afford BTZ-P6t (80% yield, yellow crystals); mp 139-140 °C

(26) Sun, Y.-Q.; Tsang, C.-K.; Xu, Z.; Huang, G.; He, J.; Zhou, X.-P.;

Zeller, M.; Hunter, A. D. Cryst. Growth Des. 2008, 8, 1468.

(27) (a) Egbe, D. A. M.; Wild, A.; Birckner, E.; Grummt, U.-W.; Schubert,

U. S. Macromol. Symp. 2008, 25. (b) Egbe, D. A. M.; Nguyen, L. H.;

Mu¨hlbacher, D.; Hoppe, H.; Schmidtke, K.; Sariciftci, N. S. Thin Solid

Films 2006, 511-512, 486. (c) Egbe, D. A. M.; Nguyen, L. H.;

Carbonnier, B.; Mu¨hlbacher, D.; Sariciftci, N. S. Polymer 2005, 46,

9585. (d) Hoppe, H.; Egbe, D. A. M.; Mu¨hlbacher, D.; Sariciftci, N. S.

J.Mater.Chem. 2004, 14, 3462.

1

(ethyl acetate); H NMR (CDCl₃): δ 0.85 (m, 12 H), 1.26-1.44

(m, 24H), 1.77 (m, 8H); 4.02 (m, 8H), 7.20 (m, 4H), 7.10 (s, 2H),

7.68 (s, 2H); ¹³C NMR(CDCl₃): δ 154.4, 150.3, 148.7, 132.2, 125.6,

117.1, 116.6, 114.4, 112.9, 98.1, 84.0, 69.3, 69.1, 31.6, 29.2, 29.1,

25.7, 22.6, 14.1; UV-vis (CHCl₃) [λ_maxnm (log ꢀ)] 311 (4.6), 447

(4.4). Anal. Calcd for C₄₆H₆₀N₂O₄S: C, 74.96; H, 8.21; N, 3.80; S,

4.35. Found: C, 75.33; H, 8.20; N, 3.77; S, 4.32%.

(28) (a) Sonar, P.; Singh, S. P.; Sudhakar, S.; Dodabalapur, A.; Sellinger,

A. Chem. Mater. 2008, 20, 3184. (b) Kono, T.; Kumaki, D.; Nishida,

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Chem. Mater. 2007, 19, 1218–1220. (c) Akhtaruzzaman, M.; Tomura,

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1997, 1851. (b) Yamashita, Y.; Tomura, M.; Imaeda, K. Chem.

Commun. 1996, 2021. (c) Ono, K.; Tanaka, S.; Yamashita, Y. Angew.

Chem., Int. Ed. 1994, 33, 1977. (d) Suzuki, T.; Fujii, H.; Yamashita,

Y.; Kabuto, C.; Tanaka, S.; Harasawa, M.; Mukai, T.; Miyashi, T.

J. Am. Chem. Soc. 1992, 114, 3034.

Synthesis of 4-{[m,p-Bis(hexyloxy)phenyl]ethynyl}-7-bromo-

2,1,3-benzothiadiazole (10). 4,7-Dibromo-2,1,3-benzothiadiazole (9)

was coupled with 3,4-(bishexyloxy)-ethynylbenzene 8 (1:0.5 ratio),

following the above procedure. Reaction time: 10 h. The crude

product was puriﬁed by column chromatography (silica gel, hexane/

chloroform 7:3). Yield: 51% (yellow solid); mp: 94-95 °C (ethyl

(30) (a) Park, Y. S.; Kim, D.; Hoosung, L.; Moon, B. Org. Lett. 2006, 8,

4702. (b) Jaykannan, M.; Van Hal, P. A.; Jannsen, R. A. J. Polym.

Sci., Part A: Polym. Chem. 2001, 40, 251. (c) van Mullekom, H. A. M.;

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Soc. 2008, 130, 16144. (b) Zaumseil, J.; Sirringhaus, H. Chem. ReV.

2007, 107, 296. (c) Blouin, N.; Michaud, A.; Leclerc, M. AdV. Mater.

2007, 19, 2295. (d) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.;

Moses, D.; Heeger, A. J.; Bazan, J. C. Nat. Mater. 2007, 6, 497. (e)

Mu¨hlbacher, D.; Scharberm, M.; Morana, M.; Zhu, Z.; Waller, D.;

Gaudiana, R.; Brabec, C. J. AdV. Mater. 2006, 18, 2884.

(34) (a) Melucci, M.; Favaretto, L.; Bettini, C.; Gazzano, M.; Camaioni,

M.; Maccagnani, P.; Ostoja, P.; Monari, M.; Barbarella, G.

Chem.sEur. J. 2007, 13, 10046. (b) Kono, T.; Kumaki, D.; Nishida,

J.-I.; Sakanoue, T.; Kakita, M.; Tada, H.; Tokito, S.; Yamashita, Y.

Chem. Mater. 2007, 19, 1218.

(35) Neto, B. A. D.; Lopes, A. S. A.; Ebeling, G.; Goncalves, R. S.; Costa,

V. E. U.; Quina, F. H.; Dupont, J. Tetrahedron 2005, 46, 10975.

(36) (a) Lehmann, M.; Seltmann, J.; Auer, A. A.; Prochnow, E.; Benedikt,

U. J. Mater. Chem. 2009, 19, 1978. (b) Yasuda, T.; Imase, T.;

Nakamura, Y.; Yamamoto, T. Macromolecules 2005, 38, 4687. (c)

Akhtaruzzaman, M.d.; Tomura, M.; Nishida, J.-i.; Yamashita, Y. J.

Org. Chem. 2004, 69, 2953. (d) Kato, S.-i.; Matsumoto, T.; Ishi-i, T.;

Thiemann, T.; Shigeiva, M.; Gorohmaru, H.; Maeda, S.; Yamashita,

Y.; Mataka, S. Chem. Commun. 2004, 2342.

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Tomura, M.; Yamashita, Y. Chem. Commun. 2005, 3183.

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9

J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010 6111

A R T I C L E S

Silvestri et al.

acetate); ¹H NMR (CDCl₃): δ 0.86 (m, 6H), 1.25-1.51 (m, 12H),

1.81 (m, 4H), 3.96 (m, 4H), 6.81 (d, 1H, J ) 8.0 Hz), 7.09 (d, 1H,

J ) 3.0 Hz), 7.16-7.20 (m, 1 H), 7.61 (d, 1H, J ) 8.0 Hz), 7.80

(d, 1H, J ) 8.0 Hz); ¹³C NMR (CDCl₃): δ 154.1, 153.0, 150.4,

148.7, 132.4, 132.0, 125.6, 116.7, 114.2, 114.0, 113.0, 97.6, 83.2,

69.3, 69.0, 31.5, 29.1, 25.6, 22.6, 14.0. Anal. Calcd for

C₂₆H₃₁BrN₂O₂S: C, 60.58; H, 6.06, N, 5.43; S, 6.22. Found: C,

60.91; H, 6.05; N, 5.46; S, 6.18%.

Device Fabrication and Measurements. OFET Devices. Prime-

grade n-doped silicon wafers having 300 nm thermally grown oxide

(Process Specialties Inc.) were used as device substrates. Before

ﬁlm deposition, the substrates were cleaned by ultrasonic treatment

with ethanol and then in an oxygen plasma cleaner for 5 min under

vacuum. Films of compounds 1-7 were spin-coated from 0.5%

(w/v) CHCl₃, toluene, or chlorobenzene solutions and, if annealed,

heated under nitrogen at various temperatures from 40° to 120 °C

for 3 h. Spin-coated ﬁlms were 25-30 nm thick as determined by

proﬁlometry (Tencor P10). For FET device fabrication, top contact

gold electrodes (50 nm thickness) were deposited by thermal

evaporation through a shadow mask. TFT device measurements

were carried out at 21-23 °C in a customized high-vacuum probe

station (1 × 10^-6Torr) or in air. Coaxial and/or triaxial shielding

was incorporated into Signaton probes to minimize noise levels.

TFT characterization was performed with a Keithley 6430 sub-

femtoamperometer (drain) and Keithley 2400 (gate) source meter,

operated by a locally written Labview program and GPIB com-

munication. OFET response parameters were extracted from I-V

data using a standard ﬁeld-effect transistor equation.³⁷Mobilities

(µ) were calculated in the saturation regime by the standard

relationship: µ_sat) (2I_DSL)/[WC_i(V_G- V_T)²], where I_DSis the

source-drain saturation current, C_iis the gate dielectric capacitance

(per area), V_Gis the gate voltage, and V_Tis the threshold voltage.

Synthesis of 4-{[m,p-Bis(hexyloxy)phenyl]ethynyl}-7-[(trimeth-

ylsylyl)ethynyl]-2,1,3-benzothiadiazole (11). This compound was

prepared by coupling of 10 with trimethylsilylacetylene (1:4 molar

ratio), following the above procedure. Reaction time: 10 h. Reaction

temperature: 25 °C. The crude product was puriﬁed by column

chromatography (silica gel, hexane/chloroform 65:35). Yield: 93%

1

(yellow solid); mp: 53-54 °C (ethyl acetate); H NMR (CDCl₃):

δ 0.28 (s, 9H), 0.86 (m, 6H), 1.26-1.51 (m, 12H), 1.86-1.75 (m,

4H), 3.98 (m, 4H), 6.81 (d, 1H, J ) 8.0 Hz), 7.11-7.18 (m, 2H),

7.70 (m, 2H); ¹³C NMR (CDCl₃): δ 154.3, 150.4, 148.7, 133.4,

131.8, 125.6, 117.9, 116.7, 116.3, 114.3, 113.0, 103.2, 100.1, 98.4,

83.9, 69.3, 69.0, 31.5, 29.2, 29.1, 25.6, 22.6, 14.0, -0.1. Anal. Calcd

for C₃₁H₄N₂O₂SSi: C, 69.88; H, 7.57; N, 5.26; S, 6.02. Found: C,

70.23; H, 7.56; N, 5.29; S, 6.05%.

Synthesis of 4-{[m,p-Bis(hexyloxy)phenyl]ethynyl}-7-ethynyl-

2,1,3-benzothiadiazole (12). To a solution of 11 (0.47 g, 0.88 mmol)

in dry THF (15 mL) was added tetrabutylamonium ﬂuoride (1.06

mL, 1.06 mmol). The reaction mixture was kept under nitrogen

and magnetically stirred at 25 °C for 1 h. The solvent was then

removed in Vacuo. The residue was puriﬁed by column chroma-

tography (silica gel, hexane/dichloromethane 3:1) to obtain com-

pound 12 (88% yield) as a yellow solid; mp: 64-65 °C (ethyl

acetate); ¹H NMR (CDCl₃): δ 0.86 (m, 6H), 1.26-1.51 (m, 12H),

1.75-1.86 (m, 4H), 3.26 (s, 1H), 3.98 (m, 4H), 6.81 (d, 1H, J )

8.0 Hz), 7.11-7.18 (m, 2H), 7.72 (m, 2H); ¹³C NMR (CDCl₃): δ

148.7, 144.3, 142.6, 133.6, 131.7, 125.7, 116.7, 114.2, 113.0, 98.5,

97.9, 84.7, 69.3, 69.1, 31.6, 29.2, 25.7, 22.6, 14.0. Anal. Calcd for

C₂₈H₃₂N₂O₂S: C, 73.01; H, 7.00; N, 6.08; S, 6.96. Found: C, 73.40;

H, 7.01; N, 6.11; S, 6.99%.

The latter can be estimated as the intercept of the linear section of

1/2

the plot of V_Gvs (I_DS

) .

OPV Devices. These devices were fabricated by spin-coating a

blend of 1-7 + PCBM (D:A wt:wt ratio, D ) 1, 2; A ) 1, 3),

sandwiched between a transparent anode and a cathode. The anode

consisted of glass substrates precoated with indium-tin oxide (ITO,

Delta Technologies; R_S) 8-12 Ω0). Before device fabrication,

the ITO-coated (150 nm) glass substrates were cleaned by ultrasonic

treatment with deionized water + detergent, deionized water,

isopropyl alcohol, methanol, and acetone sequentially, and ﬁnally

in a UV-ozone cleaner for 30 min under ambient atmosphere. A

thin layer (30 nm) of PEDOT/PSS (Baytron P VP A1 4083) was

spin-coated on (4000 rpm, 1 min) to modify the ITO surface. After

baking at 150 °C for 15 min, the active layer was obtained by spin-

coating the D:A blends under air at 6000 rpm (60 s) for A-P6t,

PA-P6t, A-P6d, and PA-P6t (solution concentration: 16 mg/mL

for A-P6t, PA-P6t, and PA-P6t, and 8 mg/mL for A-P6d); 2000

rpm (60 s) for BTZ-P6t and BTZT-P6t (8 mg/mL); and 4000 rpm

(60 s) for TBTZ-P6t (8 mg/mL). Dry chloroform was used as the

solvent. Contact areas were cleaned with dry toluene and a cotton

swab; the thickness of the active layer ﬁlm was measured by

proﬁlometry (Tencor, P10). The cathode consisted of LiF (∼0.12

nm, Acros, 99.98%) capped with Al (150 nm, Sigma-Aldrich,

99.999%). The deposition rates used were 0.1 Å/s for LiF and ∼2

Å/s for Al, with a chamber pressure of 1.1 × 10^-6Torr. Device

characterization³⁸was performed at 298 K using a Class A Spectra-

Nova Technologies solar cell analyzer having a xenon lamp that

simulates AM1.5G light from 400-1100 nm. The instrument was

calibrated with a monocrystalline Si diode ﬁtted with a KG3 ﬁlter

to bring spectral mismatch to unity. The calibration standard was

calibrated by the National Renewable Energy Laboratory (NREL).

Four-point contacts were made to the substrate with Ag paste and

copper alligator clips. Individual devices were isolated by a mask

during testing to avoid current collection from adjacent devices and

edge effects. Power conversion efﬁciencies were calculated from

the following equation: PCE ) (J_scV_ocFF)/P₀, where J_sc(mA/cm²)

Synthesis of 7,7′-({[m,p-Bis(hexyloxy)phenyl]ethynyl}-2,1,3-ben-

zothiadiazol-4,4′-ethynyl)-2,5-thiophene (BTZT-P6t, 7). This com-

pound was prepared by coupling 12 with 2,5-dibromothiophene

(2.2:1 molar ratio) following the procedure described for BTZ-

P6t (5). Reaction temperature: 55 °C. The crude product was

puriﬁed by column chromatography (silica gel, petroleum ether/

dichloromethane 1:1). Yield: 89% (orange solid); mp 188-189 °C

(ethyl acetate); ¹H NMR (CDCl₃): δ 0.86 (m, 12H), 1.24-1.49 (m,

24 H), 1.75-1.83 (m, 8H), 3.97 (m, 8H), 6.81 (d, 2H, J ) 8.0

Hz), 7.10 (m, 2H), 7.18-7.20 (m, 2H), 7.32 (s, 2H), 7.70-7.80

(m, 4H); ¹³C NMR (CDCl₃): δ 154.4, 154.1, 150.6, 148.8, 133.2,

132.6, 131.9, 125.8, 125.0, 118.1, 116.8, 115.8, 114.4, 113.1, 98.8,

90.7, 89.8, 84.0, 69.3, 69.1, 31.6, 29.2, 29.1, 25.6, 22.6, 14.0;

UV-vis (CHCl₃) [λ_maxnm (log ꢀ)] 319 (4.8), 328 (4.9), 472 (5.0).

Anal. Calcd for C₆₀H₆₄N₄O₄S₃: C, 71.97; H, 6.44; N, 5.60; S, 9.61.

Found: C, 72.01; H, 6.43; N, 5.63; S, 9.55%.

Electrochemistry. Cyclic voltammetry measurements were

performed on a C3 Cell Stand electrochemical station equipped

with BAS Epsilon software (Bioanalytical Systems, Inc., Lafayette,

IN) with a 0.1 M tetrabutylammonium hexaﬂuorophosphate

(Bu₄N⁺PF₆^-) electrolyte in dry CH₂Cl₂. Platinum wire electrodes

were used as both working and counter electrodes, and a Ag wire

was used as the pseudoreference electrode, unless otherwise noted.

Oxygen was removed from the working solutions by purging with

nitrogen gas. A ferrocene/ferrocenium redox couple was used as

an internal standard, and the potential values obtained in reference

to the silver electrode were converted to the vacuum scale.

Thermal Characterization. Thermogravimetric analysis (TGA)

was performed on a TA Q50 instrument at a ramp rate of 10 °C/

min under N₂(ramp: 25 to 800 °C) at atmospheric pressure in a

platinum crucible.

(37) Sze, S. M. Physics of Semiconductor DeVices, 2nd ed.; John Wiley &

Sons: Taipei, Taiwan, 1981.

(38) (a) Silvestri, F.; Lopez-Duarte, I.; Seitz, W.; Beverina, L.; Martinez-

Diaz, M. V.; Marks, T. J.; Guldi, D. M.; Pagani, G. A.; Torres, T.

Chem. Commun. 2009, 30, 4500–4502. (b) Irwin, M. D.; Buchholz,

B.; Hains, A. W.; Chang, R. P. H.; Marks, T. J. Proc. Natl. Acad. Sci.

U.S.A. 2008, 105, 2783. (c) Hains, A. W.; Marks, T. J. Appl. Phys.

Lett. 2008, 92, 023504.

9

6112 J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010

Arylacetylenes for OFETs and BHJ Organic Solar Cells

A R T I C L E S

Scheme 1. Synthesis of Benzothiadiazole-Based Compounds

BTZ-P6t and BTZT-P6t^a

temperatures are found to be between 230 and 316 °C.

Compared to many other molecular semiconductors used in

organic electronics,⁴¹this entire set of compounds is sufﬁciently

thermally stable to be processed at typical OFET/OPV solution

fabrication temperatures.

The anthracene-based semiconductors A-P6t and PA-P6t

exhibit well-deﬁned decomposition temperatures, comparable

to those of the oleﬁnic analogues A-P6d and PA-P6d. In

contrast, 2,1,3-benzothiadiazole-based derivative BTZ-P6t ex-

hibits a lower decomposition temperature (T_d≈ 230 °C) than

semiconductors TBTZ-P6t and BTZT-P6t.

In addition to information about thermal decomposition

processes, melting points provide insight into the cohesive

energetics of intermolecular interactions in the solid state. As

might be expected, compounds PA-P6t and PA-P6d exhibit

higher melting temperatures than the related derivatives A-P6t

and A-P6d, reﬂecting the increased molecular weight and

extended π-conjugation (Chart 4). Within the pair A-P6t and

A-P6d, the melting point decrease for fully trans¹⁴oleﬁnic

compound A-P6d relative to the acetylenic analogue can be

ascribed to a sterically related^27atwist of the conjugated

backbone (Vide infra), which results in less effective molecular

packing, despite the greater polarizability of double over triple

bonds. For the PA-P6t + PA-P6d couple, we tentatively ascribe

the increased melting point of fully trans¹⁴oleﬁnic PA-P6d

to the fact that in this case intermolecular cohesive forces due

to the double bond polarizability⁴²are greater than those in

parent structure A-P6d, due to the presence of two coplanar

phenylene-vinylene moieties, which may overcome the afore-

mentioned steric repulsions. Compounds BTZ-P6t and TBTZ-

P6t exhibit similar melting points (Chart 5); such behavior may

be ascribed to similar structural dimensions. Indeed, the

relatively low melting point of compound TBTZ-P6t may

indicate that it is conformationally more ﬂexible than might be

predicted from the number of possible repulsive intramolecular

interactions.⁴²Compound BTZT-P6t has the highest melting

point within the BTZ-based semiconductor series, reﬂecting the

collective effects of increased molecular weight, extended

effective conjugation, and enhanced donor-acceptor interactions

(Chart 5).

Optical Absorption Spectroscopy. Characterizing the optical

absorption cross sections of compounds 1-7 is essential for

photovoltaic studies and was performed both in chloroform

solution and as thin ﬁlms (Figure S2). Charts 4 and 5 summarize

the data for all compounds. The thin ﬁlm spectra of 1-7 are

red-shifted relative to the solution spectra, which can be

attributed to greater structural organization in the solid state.

Note from Chart 4 that the λ_solnand λ_ﬁlmlongest wavelength

absorption maxima for the anthracene-based semiconductors fall

in the order PA-P6t > A-P6t > A-P6d > PA-P6d and PA-P6t

> A-P6t > PA-P6d > A-P6d. Clearly, anthracene spacing with

double vs triple bonds causes a signiﬁcant twist of the

conjugated backbone due to signiﬁcant H(oleﬁnic)-H(4 an-

thracene) nonbonded repulsions. This explains the ∼40 nm blue

shift of the absorption maximum of A-P6d and PA-P6d versus

A-P6t and PA-P6t.^27a,43The coplanar structure and electron-

withdrawing nature²⁷of the sCtCs moieties are in accord

with the smaller optical band gaps (E_g^opt) of compounds A-P6t

and PA-P6t.

^a(i) Pd(Ph₃P)₄/CuI, diisopropylamine, PhMe, 50°C, 80%; (ii) Pd(Ph₃P)₄/

CuI, diisopropylamine, PhMe, 50°C, 51%; (iii) trimethylsilylacetylene,

Pd(Ph₃P)₄/CuI, diisopropylamine, PhMe, 50°C, 93%; (iv) Bu₄NF, THF,

25°C, 89%; (v) 2,5-dibromothiophene, Pd(Ph₃P)₄/CuI, diisopropylamine,

PhMe, 50°C, 55%.

is the short circuit current, V_oc(V) the open circuit voltage, FF the

ﬁll factor, and P₀the power of incident light source (mW/cm²).

Thin Film Characterization. Thin ﬁlms were analyzed by wide-

angle X-ray ﬁlm diffractometry (WAXRD) on a Rigaku ATX-G

instrument using standard θ-2θ techniques with Cu KR₁radiation.

Atomic force microscopy was performed on a Jeol 5200 SPM

instrument with silicon cantilevers in the tapping mode. Optical

absorption spectra were acquired on a Shimadzu 1601 spectropho-

tometer.

Results

In the following sections, we report the synthesis as well as

physical and electronic structure properties of semiconductors

1-7 as deﬁned by thermal analysis, UV-vis optical spectro-

scopy, and cyclic voltammetry (CV). Next, the fabrication and

characterization of 1-7-based thin ﬁlm transistors and bulk

heterojunction solar cell devices are reported, and device

response trends discussed in terms of molecular and electronic

structure as well as ﬁlm microstructure and morphology.

Semiconductor Synthesis. Compounds A-P6t, PA-P6t,

A-P6d, and TBTZ-P6t were synthesized as described previ-

ously,^14,15while Scheme 1 details the synthetic routes to new

compounds BTZ-P6t and BTZT-P6t. Pd(Ph₃P)₄/CuI-catalyzed

Sonogashira coupling of 4,7-dibromo-2,1,3-benzothiadiazole 9³⁹

with terminal alkyne 8⁴⁰affords compound BTZ-P6t in high

yield (80%). The synthesis of compound BTZT-P6t begins with

selective coupling between 4,7-dibromo-2,1,3-benzothiadiazole

9 and the alkyne 8 in toluene/diisopropylamine. The resulting

bromo derivative 10 is then subjected to additional coupling

with trimethylsilylacetylene, followed by deprotection of TMS-

derivative 11 to provide intermediate 12. Further coupling

between 2,5-dibromothiophene and 12 affords target compound

BTZT-P6t in good yield (55%).

Molecular Characterization. Thermal Properties. Thermo-

gravimetric analysis (TGA, heating ramp rate ) 10 °C/min

under N₂) was used to assess the thermal stability of compounds

1-7 (Charts 4 and 5, Figure S1). The thermolysis onset

(39) Edelmann, M. J.; Raimundo, J.-M.; Utesch, N. F.; Diederich, F.;

Boudon, C.; Gisselbrecht, J.-P.; Gross, M. HelV. Chim. Acta 2002,

85, 2195.

Regarding the benzothiadiazole-based semiconductors (Chart

5), both λ_solnand λ_ﬁlmsystematically fall in the order TBTZ-

P6t > BTZT-P6t > BTZ-P6t. Introduction of a D-A-D motif

in TBTZ-P6t and BTZT-P6t clearly results in red-shifted

(40) Valentini, L.; Bagnis, D.; Marrocchi, A.; Seri, M.; Taticchi, A.; Kenny,

J. M. Chem. Mater. 2008, 20, 32.

9

J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010 6113

A R T I C L E S

Silvestri et al.

Chart 4. Melting Point (m.p.), Decomposition Temperature Onset (T_d), Solution (CHCl₃) and Thin-Film (Glass Substrate) Optical Absorption

Maxima (λ), and Optical Energy Gap (E_g^opt) for Compounds A-P6t, PA-P6t, A-P6d, and PA-P6d^a

^aData from ref 14.

Chart 5. Melting Point (m.p.), Decomposition Temperature Onset (T_d), Solution (CHCl₃) and Thin-Film (Glass Substrate) Optical Absorption

Maxima (λ), and Optical Energy Gap (E_g^opt) for Compounds BTZ-P6t, TBTZ-P6t, and BTZT-P6t^a

^aData from ref 15.

absorption and reduces the optical band gap.⁴⁴Note here also

the red-shifted absorption maxima characteristic^13b,45of an-

thracene versus phenylene cores, leading to greater solar photon

harvesting (Vide infra). The origin of this shift is

9

6114 J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010

Arylacetylenes for OFETs and BHJ Organic Solar Cells

A R T I C L E S

Table 1. Electrochemical Data^aand MO Energies for Compounds

1-7

1/2

on

Compound

E_ox(V) (∆E)

E_red(V)

E_HOMO(eV)

E_LUMO(eV)

A-P6t (1)^b

PA-P6t (2)

A-P6d (3)

PA-P6d (4)

BTZ-P6t (5)

TBTZ-P6t (6)

BTZT-P6t (7)

+0.64 (0.1)

-

+0.21 (0.2)

+0.41 (0.1)

-

+0.48 (0.06)

+0.69 (0.1)

-

-1.57

-

-5.51

-5.44

-5.01

-5.21

-5.52

-5.33

-5.49

-2.98

-3.04

-2.50

-2.70

-3.09

-3.21

-3.35

-

-1.71

-

^aDetermined vs Fc/Fc⁺. ^bElectrochemistry performed on carbon

working electrode.

π-topological^45bsanthracene has a lower optical gap than

benzenesand reﬂects slightly increased π-delocalization along

the principal molecular axis. Similarly, replacement of phenylene

with a BTZ unit is expected to red-shift the absorption spectra.⁴⁶

Electrochemical Characterization. Cyclic voltammetry (CV)

experiments with semiconductors 1-7 (Figure S3) were per-

formed under N₂in 0.1 M CH₂Cl₂/TBAPF₆solutions. The redox

potentials were measured versus the ferrocene/ferrocenium redox

couple (Fc/Fc⁺), used as the internal reference. With the

exception of compound PA-P6t, all systems exhibit reversible

and/or quasi-reversible oxidation/reduction waves within the

span of the solvent/electrolyte window. In cases where the

voltammograms are (quasi)reversible, it is possible to extract

half-wave potentials (E^1/2); data are summarized in Table 1.

Not surprisingly, acetylenic systems A-P6t and PA-P6t are

more difﬁcult to oxidize than corresponding oleﬁnic A-P6d and

PA-P6d, because the triple bond is less electron-rich.⁴⁷Indeed,

the oxidation potentials for A-P6t and PA-P6t are ∼0.4 eV

higher than those of the oleﬁnic analogues (Figure S3).

Moreover, the resistance of A-P6t and PA-P6t to oxidation not

unexpectedly increases with decreasing conjugation length.⁴⁸

Correspondingly, it is somewhat more difﬁcult to reduce A-P6d

and PA-P6d, which is consistent with the greater electron

richness. The HOMO and LUMO energies of semiconductors

1-7 were estimated from optical absorption and cyclic volta-

mmetry data using the standard approximation that the Fc/Fc⁺

HOMO level is -4.8 eV.⁴⁹These data are compiled in Table 1

and indicate that the HOMO energies of compounds 1-7 are

within the required range to ensure very large V_ocvalues in

Figure 2. FET transfer (A) and output (B) plots for ﬁlms of semiconductor

PA-P6t spin-cast from chloroform onto Si/SiO₂substrates and annealed at

120 °C, and semiconductor TBTZ-P6t (C and D, respectively) spin-cast

from chloroform onto Si/SiO₂substrates and annealed at 40 °C. Measure-

ments made in vacuum.

PCBM BHJ photovoltaic cells. Note also that the LUMO energy

levels for BTZ-based semiconductors 5-7 lie signiﬁcantly

below those of anthracene derivatives 1-4, reﬂecting the strong

electron-accepting character of the BTZ fragment.

Optoelectronic Device Characterization. Field-Effect Transis-

tors. The structure of the typical top-contact OFETs fabricated

in this study is shown schematically in Figure 1A. These bottom-

gate top-contact devices consist of an n⁺²-Si gate-substrate, a

300 nm SiO₂dielectric layer, an optimized surface treatment

with a self-assembled monolayer, a spin-coated 1-7 semicon-

ductor ﬁlm (∼30-40 nm), and vacuum-deposited 50 nm thick

Au source and drain electrodes. Device fabrication and mea-

surement details are given in the Experimental Section. Rep-

resentative output and transfer characteristics of devices fabri-

cated with compounds 1-7 are shown in Figure 2 and Figures

S4-S11. Typical p-type organic semiconductor characteristics

are observed for all compounds. Tables 2 and 3 summarize the

FET performance response for the present semiconductors,

including the charge carrier mobility (µ), on-off current ratio

(I_on/I_off), turn-on voltage (V_ON), and threshold voltage (V_T).

To elucidate optimum semiconductor thin ﬁlm growth condi-

tions, chloroform solutions of 1-7 were spin-coated onto Si/

SiO₂substrates and then annealed at varying temperatures for

3 h before device completion. Thermal annealing from 40 to

80 °C generally enhances TFT performance. However, higher

annealing temperatures (120 °C) are found to have detrimental

effects (except for compounds PA-P6t and PA-P6d), although

the T_d’s of 1-7 are greater than 230 °C (Chart 4). Device

response maximizes at an annealing temperature of 80 °C for

semiconductors PA-P6t, BTZ-P6t, TBTZ-P6t, and BTZT-P6t

and at 60 °C for semiconductors A-P6t, A-P6d, and PA-P6d.

The data in Tables 2 and 3 reveal that compounds PA-P6t

and TBTZ-P6t exhibit the highest FET mobilities, 0.07 and

0.02 cm²V^-1s^-1, respectively,¹⁵while all other semiconductors

have signiﬁcantly lower mobilities (<10^-3cm²V^-1s^-1). The

I_on/I_offratios of PA-P6t and TBTZ-P6t are generally ∼10⁵-10⁷

whereas those of the others semiconductors are lower

(∼10²-10⁴). In general, semiconductor molecular architecture

is the most critical factor in determining the nature and the

degree of solid state order. However, for ﬁlm fabrication from

solution, the solvent properties greatly affect ﬁlm nucleation

(41) (a) Gao, J.; Li, L.; Meng, Q.; Li, R.; Jiang, H.; Li, H.; Hu, W. J.

Mater Chem 2007, 17, 1421. (b) Xiao, K.; Liu, Y.; Qi, T.; Zhang,

W.; Wang, F.; Gao, J.; Qiu, W.; Ma, Y.; Cui, G.; Chen, S.; Zhan, X.;

Yu, G.; Qin, J.; Weping, H.; Zhu, D. J. Am. Chem. Soc. 2005, 127,

13281. (c) Li, X.-C.; Sirringhaus, H.; Garnier, F.; Holmes, A. B.;

Moratti, S. C.; Feeder, N.; Clegg, W.; Teat, S. J.; Friend, R. H. J. Am.

Chem. Soc. 1998, 120, 2206.

(42) (a) Karthikeyan, M.; Glen, R. C.; Bender, A. J. Chem. Inf. Model.

2005, 45, 581. (b) Katritzky, A.; Lobanov, V.; Karelson, M. Chem.

Soc. ReV. 1995, 24, 279.

(43) Garay, R. O.; Naarmann, H.; Mu¨llen, K. Macromolecules 1994, 27,

1922.

(44) For other example of this phenomenon, see: Kron, R.; Lenes, M.;

Hummelen, J. C.; Blom, P. W. M.; De Boer, B. Polym. ReV. 2008,

48, 531.

(45) (a) Egbe, D. A. M.; Carbonnier, B.; Birckner, E.; Grummt, U.-W.

Prog. Polym. Sci. 2009, 34, 1023–1067. (b) Schenning, A. P. H. J.;

Tsipis, A. C.; Meskers, S. C. J.; Beljonne, D.; Meijer, E. W.; Bredas,

J. L. Chem. Mater. 2002, 14, 1362.

(46) Lu, S.; Yang, M.; Luo, J.; Cao, Y. Synth. Met. 2004, 140, 199.

(47) (a) Thomas, T. D. J. Chem. Phys. 1970, 52, 1373. (b) Patai, S., Ed.

The Chemistry of the Carbon-Carbon Triple Bond; J. Wiley and Sons:

1978.

(48) LaRue, T. A.; Kurz, W. G. W. Plant. Physiol. 1973, 51, 1074.

(49) Wu, C.-C.; Sturm, J. C.; Register, R. A.; Tian, J.; Dana, E. P.;

Thompson, M. E. IEEE Trans. Electron DeVices 1997, 44 (8), 1269.

9

J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010 6115

A R T I C L E S

Silvestri et al.

Table 2. FET Performance Parameters Measured under Vacuum and Air for Anthracene-Based Arylacetylenes A-P6t, PA-P6t, A-P6d, and

PA-P6d Films Spin-Cast from the Indicated Solvents

µ (cm²V^-1s^-1

)

I_on/I_off

V_on(V)

V_T(V)

Compound

Solvent

T_a(°C)^a

Vac

Air

Vac

Air

Vac

Air

Vac

Air

A-P6t (1)

CHCl₃

40

60

80

1 × 10^-5

4 × 10^-4

no

-

3 × 10³

-

-10

-3

-

-10

-

-9

-8

-

-15

-

3.9 × 10^-4

10⁴

-

10²

-

activity

Toluene

CB

80

40

60

80

120

80

40

60

80

40

60

80

120

80

2 × 10^-6

8 × 10^-7

2 × 10^-2

3 × 10^-2

7 × 10^-2

3 × 10^-2

2 × 10^-6

2 × 10^-7

3 × 10^-4

2 × 10^-5

1 × 10^-6

2 × 10^-6

3 × 10^-4

8 × 10^-5

6 × 10^-4

no

-

10²

-

-16

-12

-30

-12

0

-13

-19

-27

-9

-32

-10

-15

-9

-14

-26

-10

-13

-

-23

-15

-35

-15

-8

-

10²

PA-P6t (2)

CHCl₃

Toluene

CB

CHCl₃

Toluene

CB

CHCl₃

Toluene

10⁷

2 × 10^-2

10⁵

7 × 10⁴

-14

-22

-

-14

-

-23

-

-20

-

-20

-22

-

-16

-

-25

-

-20

-

7 × 10^-2

4 × 10⁶

2 × 10²

-

10⁵

-

-17

-20

-28

-25

-32

-12

-17

-13

-20

-27

-14

-18

-

10²

<10

A-P6d (3)

8 × 10²

2 × 10^-4

10⁴

2 × 10³

-

2 × 10⁴

10²

-

10²

PA-P6d (4)

9 × 10³

2 × 10^-5

10⁴

6 × 10⁵

-

2 × 10^-4

-

5 × 10⁴

2 × 10⁴

-

2 × 10⁶

-

activity

CB

80

6 × 10^-7

-

5 × 10¹

-

-38

-

-45

-

^aT_a) annealing temperature.

Table 3. FET Performance Measured Under Vacuum and Air for Benzothiadiazole-Based Arylacetylenes BTZ-P6t, TBTZ-P6t, and

BTZT-P6t Films Spin-Cast from the Indicated Solvents

µ (cm²V^-1s^-1

Vac

)

I_on/I_off

V_on(V)

V_T(V)

Compound

Solvent

T_a(°C)^a

Air

Vac

Air

Vac

Air

Vac

Air

BTZ-P6t (5)

CHCl₃

40

no

-

activity

3 × 10^-6

4 × 10^-5

2 × 10^-2

1 × 10^-2

2 × 10^-2

8 × 10^-7

3 × 10^-6

no

CHCl₃

Toluene

CB

60

80

40

60

80

40

-

4 × 10³

10³

-

-10

-44

-13

-12

-3

-47

-14

-

-15

-50

-20

-13

-7

-53

-17

-

7 × 10^-6

10²

-46

-

-52

-

TBTZ-P6t (6)

BTZT-P6t (7)

-

10⁷

-

10⁵

2 × 10^-2

6 × 10⁵

10

5 × 10³

-19

-25

-

10²

CHCl₃

-

activity

7 × 10^-6

2 × 10^-4

no

CHCl₃

60

80

120

-

8 × 10¹

-

-29

-24

-

-12

-

-30

-

-36

-

2 × 10^-5

10²

-

1 × 10⁴

-

activity

Toluene

CB

80

no

-

activity

4 × 10^-7

10

-51

-55

^aT_a) annealing temperature.

and growth mechanism and morphology, and thus charge

transport properties.⁵⁰

most continuous, and most electrically uniform ﬁlms were

obtained here from chloroform solutions, generally resulting in

the greatest FET performance (Tables 2 and 3). The poor quality

of toluene and CB spun ﬁlms appears to result from the lower

solubility of these compounds in those solvents. In fact, in most

cases, the compounds precipitate during solvent evaporation

resulting in irregular ﬁlms with areas of heavy deposition

distinguishable by eye as islands of crystalline material.

Thus, Sirringhaus et al.⁵¹reported that P3HT TFT perfor-

mance can be enhanced by using high-boiling solvents instead

of chloroform for ﬁlm growth. This is the result of the slower

solvent evaporation, which facilitates the slow growth of highly

crystalline ﬁlms, thereby enhancing interchain interactions and

charge mobility. For these reasons, solutions of 1-7 were also

spin-coated using other solvents such as toluene (bp ) 111 °C)

and chlorobenzene (CB, bp ) 131 °C). However, the smoothest,

OFET devices based on compounds PA-P6t and TBTZ-P6t

were also fabricated by drop-casting 0.5% (w/v) chloroform

solutions and then annealing at 80 °C under N₂for 3 h. Using

this methodology, the semiconductor ﬁlms are irregular and

discontinuous, displaying a negligible FET response (not

shown). Finally, the devices affording the maximum perfor-

(50) Banach, M. J.; Friend, R. H.; Sirringhaus, H. Macromolecules 2004,

37, 6079.

(51) Chang, J. F.; Sun, B.; Breiby, D. W.; Nielsen, M. M.; Solling, T. I.;

Giles, M.; McCulloch, I.; Sirringhaus, H. Chem. Mater. 2004, 16, 4772.

9

6116 J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010

Arylacetylenes for OFETs and BHJ Organic Solar Cells

A R T I C L E S

Figure 3. AFM images (1 µm × 1 µm) of (A) A-P6t (60 °C), (B) PA-P6t (80 °C), (C) A-P6d (60 °C), and (D) PA-P6d (120 °C) ﬁlms spin-cast from

chloroform on Si/SiO₂substrates and annealed at the indicated temperatures.

Figure 4. AFM images (1 µm × 1 µm) of (A) BTZ-P6t, (B) TBTZ-P6t, and (C) BTZT-P6t ﬁlms spin-cast from chloroform on Si/SiO₂substrates and

annealed at 80 °C.

Figure 5. AFM images (1 µm × 1 µm) of ﬁlms of PA-P6t spin-cast from chloroform on Si/SiO₂substrates and annealed at (A) 40 °C, (B) 60 °C, and (C)

120 °C.

mance were also evaluated in air, revealing no substantial

changes in carrier mobilities (except for compound BTZ-P6t),

whereas the I_on/I_offratio generally falls, likely due to O₂doping

as typically observed for electron-rich semiconductors such as

oligothiophenes.⁵²

Atomic force microscopy (AFM) images were recorded to

examine the morphologies of the 1-7 semiconducting ﬁlms,

and the results correlate well with the diffraction (Vide infra)

and TFT data. Figures 3 and 4 show AFM images of 1-4 and

5-7 ﬁlms, respectively, deposited on bare SiO₂and annealed

at temperatures yielding the highest carrier mobilities. The

A-P6t-derived ﬁlms spin-cast from chloroform solution contain

large circular holes in otherwise featureless surfaces (∼20 nm

Figure 6. AFM images (5 µm × 5 µm) of PA-P6t- and TBTZ-P6t-derived

ﬁlms spin-cast from toluene (A and C, respectively) and chlorobenzene (B

and D, respectively).

rms roughness, σ; Figure 3A). In contrast, arylacetylene PA-

P6t-derived ﬁlms exhibit large crystal-like features¹⁵and an rms

roughness of ∼3.2 nm (Figure 3B). Finally, ﬁlms of compounds

A-P6d, PA-P6d, TBTZ-P6t,¹⁵and BTZT-P6t are smooth

(σ ) 1.27, 1.61, 2.05, and 1.19 nm, respectively) whereas ﬁlms

of BTZ-P6t are far rougher with a σ ) 8.55 nm, which may be

ascribed to the formation of segregated phases that inhibit

efﬁcient charge transport⁵³and afford a signiﬁcantly lower

mobility.

V^-1s^-1, respectively). These results argue that crystallization

processes result in enhanced semiconductor ﬁlm molecular self-

organization. However, a different morphology is observed at

higher annealing temperature (120 °C), with decreased ﬁlm

continuity, although the grain size increases, which explains the

depressed mobility.

As another example of ﬁlm microstructure/morphol-

ogy-mobility correlations, note how PA-P6t-based ﬁlm rough-

ness evolves with the annealing temperature (Figures 3B and

5). Increasing T_afrom 40 to 60 to 80 °C induces a progressive

increase in σ (2.34 f 2.68 f 3.55 nm, respectively) which

correlates with increasing mobility (0.02 f 0.03 f 0.07 cm²

The morphologies of 1-7-derived ﬁlms spin-cast from

chloroform are substantially different from those from toluene

and CB. For example, Figure 6A shows the AFM image of PA-

P6t-derived ﬁlms spin-cast from toluene (σ ) 6.0 nm), featuring

grains with dimensions of ∼200 nm and heights of tens of

nanometers. The PA-P6t-derived ﬁlms spin-cast from CB

(σ ) 7.8 nm) are similar, exhibiting taller as well as wider

clusters (Figure 6B). Similarly, TBTZ-P6t-derived ﬁlms spin-

cast from toluene (σ ) 1.63 nm) and CB (σ ) 1.75 nm) exhibit

large crystal-like features (Figure 6C and 6D, respectively). Note

that ﬁlms of TBTZ-P6t from CB solution exhibit more highly

interconnected microstructures versus those from toluene. These

(52) (a) Meijer, E. J.; Detcheverry, C.; Baesjou, P. J.; van Veenendaal, E.;

de Leew, D. M.; Klapwijk, T. M. J. Appl. Phys. 2003, 93, 4831. (b)

Bao, Z.; Dodabalapur, A.; Lovinger, A. J. Appl. Phys. Lett. 1996, 69,

4108.

(53) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Frechet,

J. M.; Toney, M. F. Macromolecules 2005, 38, 3312.

9

J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010 6117

A R T I C L E S

Silvestri et al.

Table 5. Bulk Heterojunction Photovoltaic Response Properties of

Compounds BTZ-P6t, TBTZ-P6t, and BTZT-P6t

Active layer [wt:wt]

V_oc[V] J_sc[mA/cm²] FF [%] PCE [%] R_sh[ΚΩ/cm²]

BTZ-P6t/PCBM (1:1)^{a d}

0.66

0.53

0.45

0.28

0.16

0.12

2.90

0.97

1.76

0.47

0.45

0.37

27

38

51

21

25

33

30

33

0.05

0.03

0.56

0.21

0.28

0.13

0.11

0.08

71.1

101.2

146.7

13.5

14.9

8.3

30.7

41.5

42.8

,

BTZ-P6t/PCBM (1:3)^{a d}

,

BTZ-P6t/PCBM (2:1)^{a d}

,

TBTZ-P6t/PCBM (1:1)^{b d}0.89

,

TBTZ-P6t/PCBM (1:3)^{b d}0.83

,

TBTZ-P6t/PCBM (2:1)^{b d}0.64

,

BTZT-P6t/PCBM (1:1)^{c d}0.67

,

BTZT-P6t/PCBM (2:1)^{c d}0.80

,

BTZT-P6t/PCBM (1:3)^{c d}0.66

,

^ad ) 50. ^bd ) 80. ^cd ) 50 nm. ^dNot annealed.

Chart 6. Structure of r,r-DH6TDPP

Figure 7. Current density (J)-Voltage (V) characteristics under illumination

of optimized 1-7/PCBM based BHJ devices. A-P6t/PCBM 2:1 wt/wt (black),

PA-P6t/PCBM 2:1 (green), A-P6d/PCBM 2:1 (violet), PA-P6d/PCBM 1:1

(red), BTZ-P6t/PCBM 1:1 (cyan), TBTZ-P6t/PCBM 1:1 (pink), BTZT-

P6t/PCBM 1:1 (blue).

Table 4. Bulk Heterojunction Photovoltaic Response Properties of

Compounds A-P6t, PA-P6t, A-P6d, and PA-P6d

Active layer [wt:wt]

V_oc[V] J_sc[mA/cm²] FF [%] PCE [%] R_sh[ΚΩ/cm²]

A-P6t/PCBM (1:1)^{a d}

0.96

0.81

0.74

0.93

0.89

2.62

0.86

0.76

2.92

3.10

2.63

0.65

0.82

2.57

2.02

0.26

0.45

0.59

0.49

0.76

1.16

1.05

1.44

0.23

0.28

45

26

45

41

38

40

48

20

24

26

24

34

32

26

30

33

1.17

0.18

0.15

1.20

1.27

1.02

0.17

0.21

1.20

0.88

0.04

0.08

0.10

0.08

0.17

0.34

0.22

0.31

0.05

29.4

28.0

16.1

20.4

20.8

11.9

33.1

28.9

38.9

27.6

39.9

30.9

24.0

27.1

31.7

17.3

13.7

11.2

18.0

26.1

,

A-P6t/PCBM (1:3)^{a d}

,

0.81 and 0.66 V, respectively), thus resulting in lower power

conversion efﬁciencies (0.18% and 0.17%, respectively; Table

4 and Figure 4A). A similar trend in ﬁlm composition vs

performance is not uncommon. For example, Durrant et al.⁵⁴

studied the effect of blend composition for BHJ devices with

ﬁlms of regioregular P3HT + PCBM and found that the highest

external quantum and power conversion efﬁciencies are achieved

for a 1:1 D:A w:w composition. Further addition of PCBM (1:2

and 1:4 wt/wt) signiﬁcantly reduces the efﬁciency. This was

tentatively attributed to disruption of P3HT chain packing so

that mobility and light absorption at longer wavelengths were

decreased. More recently, Nguyen et al.⁵⁵reported high-

efﬁciency solar cells based on a r,r-DH6TDPP/PCBM BHJ

structure in which the optimal donor:acceptor wt/wt ratio is 70:

30. Also in this case, increasing amounts of PCBM (50:50 and

30:70 wt/wt) depress the device performance (Chart 6).

A-P6t/PCBM (1:3)^{a e}

,

A-P6t/PCBM (2:1)^{a d}

,

A-P6t/PCBM (2:1)^{a e}

,

PA-P6t/PCBM (1:1)^{b d}0.93

,

PA-P6t/PCBM (1:3)^{b d}0.66

,

PA-P6t/PCBM (1:3)^{b e}0.65

,

PA-P6t/PCBM (2:1)^{b d}0.95

,

PA-P6t/PCBM (2:1)^{b e}0.89

,

A-P6d/PCBM (1:1)^{a d}

0.79

0.76

0.63

0.72

0.65

,

A-P6d/PCBM (1:3)^{a d}

,

A-P6d/PCBM (1:3)^{a e}

,

A-P6d/PCBM (2:1)^{a d}

,

A-P6d/PCBM (2:1)^{a e}

,

PA-P6d/PCBM (1:1)^{c d}0.88

,

PA-P6d/PCBM (1:3)^{c d}0.78

,

PA-P6d/PCBM (1:3)^{c e}0.81

,

PA-P6d/PCBM (2:1)^{c d}0.65

,

PA-P6d/PCBM (2:1)^{c e}0.56

,

^ad ) 100 nm. ^bd ) 50 nm. ^cd ) 80 nm. ^dNot annealed. ^eAnnealed

at 60 °C/1 h.

From the optical absorption spectra, AFM analysis, and

mobility measurements of the various blends, Nguyen et al.

concluded that a high donor concentration enhances light

absorption by the chromophore and order in the microstructure,

in addition to balancing hole-electron charge transport properties.

microstructural and morphology ﬁndings are in good agreement

with the corresponding OFET data.

Bulk Heterojunction Solar Cells. The potential of semicon-

ductors 1-7 as donor (D) materials in OPV devices was

investigated in bulk-heterojunction (BHJ) cells using (6,6)-

phenyl-C₆₁butyric acid methyl ester (PCBM) as the electron

acceptor (A). The device structure utilized, glass/ITO/PEDOT:

PSS/DONOR:PCBM/LiF/Al, is illustrated in Figure 1B. The

active layer ﬁlms were spin-coated from dry chloroform

solutions (chloroform was found to be the optimum solvent).

Device fabrication details are given in the Experimental Section.

OPV current density-voltage (J-V) plots under illumination

for the optimized devices based on arylacetylenes 1-7 are

shown in Figure 7.

Tables 4 and 5 summarize the photovoltaic response data.

The performance of these OPVs is clearly dependent on the

donor material, blend composition, and ﬁlm annealing temper-

ature. For cells based on A-P6t and PA-P6t/PCBM in a 1:1

w:w ratio, power conversion efﬁciencies are 1.17 and 1.02%,

respectively.¹⁴When the D:A blend ratio is increased to 2:1,

efﬁciencies rise to as high as 1.20%. However, further increasing

the PCBM content in the A-P6t and PA-P6t blends (1:3 wt/

wt) leads to a signiﬁcant fall in the short circuit current densities

(to 0.86 and 0.65 mA/cm², respectively) as well as in V_oc(to

The present devices based on oleﬁnic structures A-P6d and

PA-P6d exhibit low power conversion efﬁciencies (0.04%

and 0.34%, respectively) using a 1:1 wt/wt ratio with PCBM,¹⁴

and the efﬁciency could not be signiﬁcantly improved by varying

the acceptor loading (Table 4, Figure 8A). These additional,

further optimized measurements supplement our previous

preliminary report on acetylene-based BHJ blends,¹⁴which are

signiﬁcantly more PV-efﬁcient than those based on the oleﬁnic

donors.

The effect of donor-acceptor blend thermal annealing at

various temperatures (50-120 °C) and for different times before

device completion was also investigated. It is known that thermal

annealing of polymer-based bulk heterojunction solar cells can

signiﬁcantly affect the blend morphology and microstructure

and generally leads to an increased J_scand ﬁll factor, hence an

(54) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.;

Durrant, J. R. J. Mater. Sci. 2005, 40, 1371.

(55) Tamayo, A. B.; Walker, B.; Nguyen, T.-Q. J. Phys. Chem. C 2008,

112, 11545.

9

6118 J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010

Arylacetylenes for OFETs and BHJ Organic Solar Cells

A R T I C L E S

Figure 9. Dark J-V measurements for the 1-7/PCBM BHJ best devices.

A-P6t/PCBM 2:1 wt/wt (black), PA-P6t/PCBM 2:1 (green), A-P6d/PCBM

2:1 (violet), PA-P6d/PCBM 1:1 (red), BTZ-P6t/PCBM 1:1 (cyan), TBTZ-

P6t/PCBM 1:1 (pink), BTZT-P6t/PCBM 1:1 (blue).

images of Donor/PCBM blends having wt/wt ratios affording

the best OPV performance. Films of A-P6t/PCBM and PA-

P6t/PCBM in a 2:1 wt/wt ratio exhibit higher degrees of order

than do A-P6d and PA-P6d/PCBM ﬁlms having the same blend

composition. These results are in accord with the signiﬁcant

torsional differences in the conjugated backbone when an-

thracene cores are connected with double versus triple bonds

(Vide supra), and this may inhibit self-organization. It is

reasonable to argue that cooperative interactions between A-P6t

(or PA-P6t) and PCBM occur, leading to a greater ordering,

rather than aggregation of the donor molecules; this is in

agreement with the similar solution and thin-ﬁlm optical spectra

observed for the two pristine compounds as well as the high

V_ocvalues observed in A-P6t-PA-P6t/PCBM-based OPVs.⁵⁸In

contrast to the solution optical absorption spectra of pristine

A-P6d and PA-P6d, their ﬁlms are red-shifted, qualitatively

indicating greater aggregation tendencies.⁵⁸This may account

for the lower V_ocvalues exhibited by the corresponding OPVs.

Figure 10 shows AFM images of A-P6t/PCBM ﬁlms as a

function of the D/A ratio. Comparing the morphologies it is

evident that ﬁlms of A-P6t/PCBM 2:1 wt/wt ratio (Figure 10A)

exhibit signiﬁcantly higher order than do ﬁlms of A-P6t/PCBM

at 1:1 and 1:3 wt/wt ratios (Figure 10B and 10C, respectively).

A similar trend is observed for PA-P6t -PA-P6d/PCBM blend

ﬁlms (Figures S12 and S13), with the 2:1 wt/wt ratio ﬁlm

revealing a higher degree of ordering.

Figure 8. (A) OPV power conversion efﬁciencies for compound 1-7:

PCBM BHJ cells as function of Donor:PCBM weight ratio. (B) Efﬁciencies

for compound 1-4:PCBM (2:1 wt/wt) BHJ devices as function of thermal

annealing at 60 °C/1 h.

increased PCE.⁵⁶Indeed, for some of the materials investigated

here, similar behavior is observed (Table 4). Thus, devices based

on arylacetylenes BTZ-P6t, TBTZ-P6t, and BTZT-P6t, i.e.,

the systems containing benzothiadiazole units, exhibit lower

PCEs vs those based on semiconductors A-P6t and PA-P6t.

The 1:1 weight ratio with respect to PCBM is found to be

optimum; TBTZ-P6t-based devices exhibit better overall per-

formance, especially in terms of the PCE approaching ∼0.6%

(Table 5, Figure 8A). After annealing at 60 °C, the short circuit

current density for devices based on arylacetylene A-P6t (2:1

wt:wt with respect to PCBM) increases slightly from 2.92 to

3.1 mA/cm²and the ﬁll factor (FF) from 26% to 45%. V_oc

decreases slightly on annealing. As a result, the PCE increases

from 1.20% to 1.27% (Table 4, Figure 8B). The PCEs of the

devices based on the present acetylenic structures demonstrate

how simple molecules can be efﬁcient and competitive donor

materials for small-molecule BHJ solar cells.^55,57

Thermal annealing of devices based on anthracenes PA-P6t

and PA-P6d does not signiﬁcantly affect the overall perfor-

mance, whereas the A-P6d-based OPV performance is improved

marginally upon annealing, to yield PCE ≈ 0.2%. In contrast,

annealing of devices based on benzothiadiazoles BTZ-P6t,

TBTZ-P6t, and BTZT-P6t leads to dramatic decreases in

overall PV performance. When a higher PCBM ratio (1:1 or

1:3 wt/wt) is used, thermal annealing generally results in

extensive ﬁlm degradation, adversely affecting device perfor-

mance.

Thermal annealing does not signiﬁcantly affect the ﬁlm

morphology of either A-P6t/PCBM and PA-P6t/PCBM or

A-P6d/PCBM and PA-P6d/PCBM as 2:1 wt/wt ratio blends

(AFM images not shown). The slight effects observed on PCEs

for 2:1 wt/wt A-P6t/PCBM and A-P6d/PCBM OPVs may be

related to the energetic accessibility of molecular reorganization

processes due to the shorter molecular length.

(57) (a) Liang, Y.; Feng, D.; Guo, J.; Szarko, J. M.; Ray, C.; Chen, L. X.;

Yu, L. Macromolecules 2009, 42, 1091. (b) Ma, C.-Q.; Fonrodona,

M.; Schicora, M. C.; Wienk, M. M.; Janssen, R. A. J.; Ba¨uerle, P.

AdV. Funct. Mater. 2008, 18, 3323. (c) Lincker, F.; Delbosc, N.; Bailly,

S.; De Bettignies, R.; Billon, M.; Pron, A.; Demadrille, R. AdV. Funct.

Mater. 2008, 18, 3444. (e) Kronenberg, N. M.; Deppisch, M.; Wu¨rtner,

F.; Lademann, H. W. A.; Deing, K.; Meerholz, K. Chem. Commun.

2008, 6489. (f) Silvestri, F.; Irwin, M. D.; Beverina, L.; Facchetti,

A.; Pagani, G. A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 17640.

(g) He, C.; He, Q.; Yi, Y.; Wu, G.; Bai, F.; Shuai, Z.; Li, Y. J. Mater.

Chem. 2008, 18, 4085. (h) Lloyd, M. T.; Mayer, A. C.; Subramanian,

S.; Mourey, D. A.; Herman, D. J.; Bapat, A. V.; Anthony, J. E.;

Maillaras, G. G. J. Am. Chem. Soc. 2007, 129, 9144. (i) Kopidakis,

N.; Mitchell, W. J.; Lagemaat, J. V. d.; Ginley, D. S.; Rumbles, G.;

Shaheen, S. E. Appl. Phys. Lett. 2006, 89, 103524. (j) Walker, B.;

Tamayo, A. B.; Dang, X.-D.; Zalar, P.; Seo, H. J.; Garcia, A.;

Tantiwiwat, M.; Nguyen, T.-Q. AdV. Funct. Mater. 2009, 19, 1–7.

(58) Perez, M. D.; Borek, C.; Forrest, C.; Thompson, M. E. J. Am. Chem.

Soc. 2009, 131, 9281.

The present OPVs generally exhibit good diode characteris-

tics. For example, the A-P6t/PCBM blend cell, the current-

voltage characteristics under illumination of which are shown

in Figure 7, exhibit high rectiﬁcation ratios under dark conditions

(10²-10³at 2.0 V, Figure 9). The other cells display similarly

good rectiﬁcation ratios (Figure 9).

It is known that ﬁlm microstructure and morphology greatly

inﬂuence OPV efﬁciency.^12b,56Figures 10A and S12 show AFM

(56) (a) Liu, J.; Shi, Y.; Yang, Y. AdV. Funct. Mater. 2001, 11, 420. (b)

Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. AdV. Funct. Mater.

2003, 13, 85.

9

J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010 6119

A R T I C L E S

Silvestri et al.

Figure 10. AFM images (500 nm × 500 nm) of A-P6t/PCBM in (A) 2:1, (B) 1:1, (C) 1:3 wt/wt ratios.

Figure 11. Thin ﬁlm optical spectra of (A) A-P6t/PCBM and (B) PA-

P6t/PCBM in 2:1 (blue line), 1:1 (black line), and 1:3 (red line) wt/wt ratios.

As judged by AFM, the 2:1 wt/wt A-P6t/PCBM and PA-

P6t/PCBM blend ﬁlms in general exhibit higher degrees of

Figure 12. XRD plots for ﬁlms of semiconductor PA-P6t (2) annealed at

40 °C (green line), 60 °C (blue line), 80 °C (red line), and 120 °C (black

line).

organization than 5-7/PCBM BHJ ﬁlms. Anthracene-based

arylacetylenes A-P6t and PA-P6t possess higher structural

symmetry than do BTZ-based arylacetylenes 5-7, which may

enhance supramolecular organization in BHJ blends. Within the

BTZ-derivative series (Figures S12 and S14), the presence of

the coplanar D-A-D motif in TBTZ-P6t may promote better

self-organization, which is consistent with the greater current

densities observed in the TBTZ-P6t-containing blends (Table

5). Additionally, the high V_ocvalues may also reﬂect the lower

aggregation tendency of the pristine donor material, also evident

in the similarity of the solution and thin ﬁlm optical spectra⁵⁸

(Vide infra).

The optical absorption spectra of the 1-7/PCBM blend ﬁlms

were also examined in detail. By increasing the blend D:A

weight ratios, enhancement in the long wavelength optical

absorption is observed (for representative spectra see Figures

11 and S15). This is beneﬁcial for sunlight harvesting, since

the photon ﬂux reaching the earth’s surface is maximum at ∼1.8

eV (700 nm).⁵⁹The enhanced long wavelength absorption by

the A-P6t/PCBM and PA-P6t/PCBM blends (Figure 11)

correlates with higher device efﬁciencies. At higher PCBM

content (1:3), the absorption at wavelengths longer than 400

nm is strongly suppressed, while absorption in the blue region

is maintained, suggesting that donor molecule dilution by the

additional PCBM disrupts donor molecular packing and reduces

the density of aggregates giving rise to red absorption.⁵⁴These

results correlate with the poor light harvesting at longer

wavelengths observed for the high-PCBM content blends (Vide

infra).

Table 6. Summary of Diffraction-Derived d-Spacings (d),

Computed Molecular Lengths (l), and Calculated Molecular Long

Axis Tilt Angles (ꢀ) in Semiconducting Films of Compounds 1-7

Spin-Cast from Chloroform Solution

d-spacing

(d, Å)^a

molecular length^b

(l, Å)

tilt angle^c

(ꢀ, deg)

Semiconductor

A-P6t (1)

15.24

34.20

26.34

33.42

26.74

31.10

30.43

33.25

46.33

32.03

42.76

32.50

38.27

47.05

62.72

42.42

34.68

38.60

34.64

35.65

49.70

PA-P6t (2)^d

A-P6d (3)

PA-P6d (4)

BTZ-P6t (5)

TBTZ-P6t (6)^d

BTZT-P6t (7)

^aMinority phases are not reported. ^bSemiempirical AM1

c

calculations. With respect to the substrate normal. ꢀ ) cos^-1(d/l).

^dData from ref 15.

polycrystalline ﬁlm grain connectivity. For the OPVs, the effect

of donor molecular orbital energetics on key OPV parameters

is studied in detail.

FET Performance vs Semiconductor Film Microstructure

and Morphology. The highest mobility (hole mobility ) 3 ×

10^-2cm²V^-1s^-1and I_on/I_offratio of ∼10⁶, Table 3) is found

for ﬁlms of semiconductor PA-P6t deposited from chloroform

solution on Si/SO₂and annealed at 80 °C. This behavior

correlates well with high ﬁlm crystallinity, indicated¹⁵by the

relatively high peak intensities in the out-of-plane X-ray

diffraction data (Figure 12). The molecular long axis is tipped

with respect to the substrate normal with a tilt angle θ ≈ 40°

(Table 6), providing a plausible pathway for charge transport.

Compound PA-P6t has almost the same core and side-chain

molecular structure as A-P6t except that two phenylene-

ethynylene units extend the backbone conjugation, which may

favor more extensive π-π stacking in the solid state.⁶⁰

Discussion

In this section we discuss OFET and OPV response charac-

teristics within the context of the foregoing molecular structure

discussion as well as the active semiconductor/blend ﬁlm

microstructure as assayed by wide-angle X-ray diffraction

(WAXRD). For the OFETs, particular attention will focus on

the semiconductor molecular orientation in the ﬁlm and on

(60) (a) Hutchinson, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc.

2005, 127, 16866. (b) Hutchinson, G. R.; Ratner, M. A.; Marks, T. J.

J. Phys. Chem. B 2005, 109, 3126.

(59) Moliton, A.; Nunzi, J.-M. Polym. Int. 2006, 55, 583.

9

6120 J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010

Arylacetylenes for OFETs and BHJ Organic Solar Cells

A R T I C L E S

Figure 13. θ-2θ X-ray diffraction data for ﬁlms of (A) semiconductor PA-P6t (2) and (B) semiconductor TBTZ-P6t (6) spin-coated from toluene (black

line) and chlorobenzene (red line) and annealed at 80 °C.

Interestingly, the FET response data reveal very different

electrical behaviors compared to the A-P6t-based FETs. The

highest measured A-P6t ﬁlm mobility is 100× lower than that

of PA-P6t. The A-P6t ﬁlm XRD scans (Figure S16) exhibit a

single low-intensity reﬂection corresponding to a d-spacing of

15.24 Å. The estimated molecular or long axis tilt angle ꢀ with

respect to the surface normal, deﬁned by the d-spacing and

computed molecular length (Table 6), is ∼63°, indicating that

the molecules of A-P6t are tipped signiﬁcantly from the surface

normal. This is consistent with the observed poor FET mobili-

ties. Upon replacing acetylenic for oleﬁnic spacers in the PA-

P6t molecular skeleton, a marked decrease in the carrier

mobilities as well as I_on/I_offratio is observed for optimized PA-

P6d ﬁlm-based TFTs (CHCl₃, T_a) 120 °C) relative to those of

PA-P6t, while the performance of A-P6d is only slightly lower

than that of A-P6t (Table 2).

correlates with the similar mobilities observed for PA-P6t- and

TBTZ-P6t-derived ﬁlms.

The out-of-plane ﬁlm microstructure is poor for benzothia-

diazole-based materials BTZ-P6t and BTZT-P6t (Figure S16).

BTZ-P6t ﬁlms exhibit a single major reﬂection at 2θ ) 3.30°,

corresponding to a d-spacing of 26.74 Å. Arylacetylene BTZT-

P6t exhibits multiple diffraction features after annealing at 80

°C, with a major feature at 2θ ) 2.90° corresponding to a

d-spacing of 30.43 Å. With the calculated molecular lengths

and XRD data taken into account (Table 6, Figure S16),

arylacetylene BTZT-P6t exhibits a poorly ordered, edge-on

orientation relative to the substrate surface with a tilt angle ꢀ

≈ 50°.

Thermal annealing clearly affects performance (Tables 2 and

3), as exempliﬁed by the results for the PA-P6t-derived ﬁlms

in Figure 12. On increasing the annealing temperature from 40

to 80 °C, the crystallinity and ﬁeld-effect mobility of the ﬁlms

increase (µ by ∼3.5%; Table 2). Further increase of the

annealing temperature to 120 °C lowers the crystallinity and,

consequently, the mobility to the 40 °C value. Thus, annealing

at ∼80 °C is optimum for this particular arylacetylene. The FET

performance of the 1-7-derived ﬁlms generally exhibits marked

casting solvent dependence (Tables 2 and 3). For example, TFTs

fabricated from PA-P6t-derived ﬁlms spin-coated from toluene

solution show hole mobilities 4 orders of magnitude lower than

those from ﬁlms spin-coated from chloroform solution. Com-

pound PA-P6t exhibits the lowest measured mobility of 2 ×

10^-7cm²V^-1s^-1with I_on/I_off< 10 for ﬁlms spin-coated from

CB solution. Similarly, a very poor performance is obtained

from devices fabricated with semiconductor TBTZ-P6t spin-

coated from chlorobenzene and toluene, with hole mobilities

10⁴-10⁵× lower, respectively, than those for devices spin-coated

from a chloroform solution. The very low mobilities of thin

ﬁlms of PA-P6t and TBTZ-P6t spin-coated from toluene or

CB correlate well with the XRD data (Figure 13), which exhibit

extremely weak diffraction features, thus revealing little or no

long-range order. These results argue that the degree of

crystallinity is, not unexpectedly, a major factor in determining

the measured carrier mobility in these compounds.

It is known⁶¹that constraining conformational mobility arising

from bond rotation is a viable molecular design concept for

enhancing organic semiconductor mobility. The rotational

degrees of freedom in acetylenes A-P6t and PA-P6t are

minimal,⁶²compared to the corresponding oleﬁns A-P6d and

PA-P6d, and this should favor closer molecular packing and

partially accounts for the superior FET performance. Addition-

ally, the twisted conjugated backbones in A-P6d and PA-P6d

(Vide supra) may sterically hinder π-π stacking in the solid

state. The diffraction data indicate that PA-P6d ﬁlms have poor

crystallinity, with only a broad second-order Bragg peak (Figure

S16). Film quality is also poor for semiconductor A-P6d (Figure

S16), which exhibits multiple ﬁlm diffraction features, and a

major reﬂection at 2θ ) 3.35° (d-spacing ) 26.34 Å, Table 6).

The estimated tilt angles of these molecular structures are similar

to that of PA-P6t, suggesting predominant edge-on molecular

orientation on the substrate surface with a modest inclination.

The hole mobilities and the I_on/I_offratios for arylacetylenes

A-P6t and PA-P6t are generally greater than those of the

benzothiadiazole-based materials, yielding the best performing

OFETs, excepting compound TBTZ-P6t (Figure S17) for which

the optimized performance (CHCl₃, T_a) 80 °C) is similar to

that of PA-P6t. This difference may be attributed to the higher

structural symmetry of A-P6t and PA-P6t, which should a priori

induce closer packing. The core geometry in TBTZ-P6t is to

some degree ﬁxed due to short intermolecular S · · ·N contacts

which should minimize any disorder caused by the benzothia-

diazole ring rotation and favor close intermolecular π-π

interactions. The regular structural organization in TBTZ-P6t

is evident¹⁵in the X-ray diffraction data (Figure 12) and

OPV Device Performance, Donor MO Energetics, and

1-7/PCBM Blend Morphology. Regarding OPV responses, the

most efﬁcient semiconductor donors discovered here are an-

thracene-based arylacetylenes A-P6t and PA-P6t, with PCEs

up to ∼1.3%. These high efﬁciencies reﬂect good J_sc(∼3.0 mA/

cm²) and exceptionally high V_ocvalues (∼0.9 V). In contrast,

the PCEs for A-P6d/PCBM and PA-P6d/PCBM (2:1 and 1:1

wt/wt, respectively)-based devices are poor, with lower J_scand

V_ocvalues. As previously communicated,¹⁴the lower J_scvalues

for the oleﬁnic semiconductors A-P6d and PA-P6d relative to

(61) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953.

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5427.

9

J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010 6121

A R T I C L E S

Silvestri et al.

Figure 14. HOMO/LUMO energetics of donor derivatives 1-7 and the PCBM acceptor estimated from CV and optical data.

Figure 16. Correlation between semiconductor FET mobility (left y-axis,

blue line) and BHJ OPV Photon to Current Efﬁciency (PCE, right y-axis)

for the 1-7/PCBM based series of materials (green and orange lines indicate

PCE values for D:A ratio ) 1:1 and 2:1 wt/wt, respectively).

Figure 15. Energy difference (∆E) between the HOMO level of donors

1-7 and LUMO level of PCBM Vs V_ocvalues of BHJ OPVs made with

blends having the indicated D:A weight ratios.

cm²V^-1s^-1is generally considered necessary for the donor

conjugated molecule/polymer.⁶⁴The hole mobilities of semi-

conductors 1-7 correlate well with their OPV performance,

except for compound A-P6t. We tentatively ascribe the different

behavior of A-P6t/PCBM-based devices to the predominant

effects of the active layer morphology (see above), as well as

the donor MO energetics. Figure 16 shows how enhanced

mobility correlates well with a higher photovoltaic response.

Spectral response data for the most efﬁcient of the present

OPVs are shown in Figures 17 and S18. Plots for devices

fabricated using nonoptimal PCBM content are included for

comparison. For A-P6t/PCBM 2:1 wt/wt ﬁlms (Figure 17A)

the EQE maximum is lower (∼13% at 550 nm) than that

obtained for devices based on a 1:1 wt/wt blend (∼27% at 430

nm);¹⁴however the 2:1 spectral response exhibits a broader

proﬁle. This may account^12bfor the appreciably high J_scresponse

of the 2:1 vs 1:1 device (3.1 vs 2.6 mA/cm², respectively).

Additionally, signiﬁcantly lower EQE values in the 400-500

nm wavelength range are exhibited for devices based on A-P6t:

acetylenic A-P6t and PA-P6t correlate with the OFET hole

mobilities, while the lower V_ocvalues may be attributed at least

in part to the HOMO energies, which are ∼0.2 eV higher for

the oleﬁnic donors (Figure 14). Within the BTZ-series, the

TBTZ-P6t-based devices afford the greatest OPV performance,

with the PCE approaching 0.6%. No signiﬁcant correlations

emerge from the HOMO/LUMO energetics-PCE metrics within

this donor series (Figure 14) whereas V_occlearly scales with

the HOMO energy. We ﬁnd here that V_ocfor donors 1-7/PCBM

BHJ OPVs is correlated with the energy difference (∆E)

between the donor HOMO and the PCBM LUMO, as shown

in Figure 15, rather with the oxidation potentials.⁶³Indeed, by

changing the D/A weight ratio, it is necessary to consider the

inﬂuence of other parameters such as morphology, photon

absorption/loss, and exciton diffusion length.

As already noted, good active layer transport properties are

important for efﬁcient photovoltaic responses. To avoid impor-

tant photocurrent loss by recombination, a hole mobility g10^-3

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1558.

9

6122 J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010

Arylacetylenes for OFETs and BHJ Organic Solar Cells

A R T I C L E S

Figure 17. EQE spectra of BHJ donors A-P6t (A), PA-P6t (B), TBTZ-P6t (C) + PCBM; 1:1 wt/wt ratio (black line), 2:1 wt/wt ratio (pink line), 1:3 wt/wt

ratio (blue line).

PCBM ) 1:3 (maximum EQE ∼7% at 450 nm). Spectral

response data for the PA-P6t/PCBM-based OPVs track the

progression 1:1 > 2:1 . 1:3 (Figure 17B), consistent with the

J_scvalues measured in the corresponding solar cells (2.63, 2.57,

and 0.82 mA/cm², respectively). Devices based on semiconduc-

tors A-P6d, PA-P6d, BTZ-P6t, TBTZ-P6t, and BTZT-P6t

generally exhibit low EQE responses, except in the case of 2:1

A-P6d:PCBM (∼10% at 420 nm, Figure S18A), 1:3 PA-P6d:

PCBM (∼15% at 410 nm, Figure S18B), and 1:1 TBTZ-P6t:

PCBM (∼12% at 510 nm, Figure 17C) wt/wt blends.

PCBM as the electron acceptor were also fabricated with these

new extended arylacetylenes. Power conversion efﬁciencies

range from ∼0.6% to ∼1.3%. The highest PCE, ∼1.3% under

standard AM 1.5 conditions, is obtained for anthracene-based

arylacetylene A-P6t. A direct correlation is identiﬁed here

between OFET hole mobility and OPV performance. These

PCEs rank among the highest reported for cells based on

nonpolymeric conjugated small molecules. Because the absorp-

tion, redox, and ﬁlm-forming properties of arylacetylenes can

be easily tuned, signiﬁcant advances are anticipated in the near

future for this class of semiconductors.

Conclusions

The design, synthesis, and characterization of a new family

of soluble extended arylacetylene optoelectronic materials,

A-P6t, PA-P6t, BTZ-P6t, TBTZ-P6t, and BTZT-P6t, is

reported. For comparison, arylvinylenes A-P6d and PA-P6d

were also synthesized and characterized. All of the new extended

arylacetylenes exhibit excellent thermal stability. Field-effect

transistor measurements demonstrate that all are FET-active,

exhibiting signiﬁcant p-type mobilities. Arylacetylenes PA-P6t

and TBTZ-P6t have well-ordered ﬁlm microstructures and are

the highest mobility p-type materials within this series, with a

Acknowledgment. We thank MIUR (Ministero dell’Istruzione,

dell’Universita` e della Ricerca, Rome - Italy), DOE (DE-

SC0001059) and AFOSR (FA9550-08-1-0331) for support of this

research and the Northwestern MRSEC (NSF Grant DMR-0520513)

for providing clean room and characterization facilities.

Supporting Information Available: Thermogravimetric analy-

sis (TGA) plots of the arylacetylenes 5 and 7; UV/electrochemi-

cal/FET/OPV data for 1-7; XRD/AFM images. This material

is available free of charge via the Internet at http://pubs.acs.org.

µ_has high as 0.07 cm²V^-1s^-1and 0.01 cm²V^-1s^-1

respectively, under ambient. Organic BHJ solar cells using

,

JA910420T

9

J. AM. CHEM. SOC. VOL. 132, NO. 17, 2010 6123

Article Doi

Solution-processable low-molecular weight extended arylacetylenes: versatile p-type semiconductors for field-effect transistors and bulk heterojunction solar cells

DOI: 10.1021/ja910420t

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Article abstract of DOI:10.1021/ja910420t

Full text of DOI:10.1021/ja910420t

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