Tuning orbital energetics in arylene diimide semiconductors. Materials design for ambient stability of n-type charge transport

Published on Web 11/14/2007

Tuning Orbital Energetics in Arylene Diimide Semiconductors.

Materials Design for Ambient Stability of n-Type Charge

Transport

Brooks A. Jones, Antonio Facchetti, Michael R. Wasielewski,* and Tobin J. Marks*

Contribution from the Department of Chemistry, Materials Research Center, and

Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern UniVersity,

EVanston, Illinois 60208-3113

Received July 13, 2007; E-mail: t-marks@northwestern.edu; m-wasielewski@northwestern.edu

Abstract: Structural and electronic criteria for ambient stability in n-type organic materials for organic field-

effect transistors (OFETs) are investigated by systematically varying LUMO energetics and molecular

substituents of arylene diimide-based materials. Six OFETs on n⁺-Si/SiO₂substrates exhibit OFET response

parameters as follows: N,N′-bis(n-octyl)perylene-3,4:9,10-bis(dicarboximide) (PDI-8): µ ) 0.32 cm²V^-1

s^-1, V_th) 55 V, I_on/I_off) 10⁵; N,N′-bis(n-octyl)-1,7- and N,N′-bis(n-octyl)-1,6-dibromoperylene-3,4:9,10-bis-

(dicarboximide) (PDI-8Br₂): µ ) 3 × 10^-5cm²V^-1s^-1, V_th) 62 V, I_on/I_off) 10³; N,N′-bis(n-octyl)-1,6,7,12-

tetrachloroperylene-3,4:9,10-bis(dicarboximide) (PDI-8Cl₄): µ ) 4 × 10^-3cm²V^-1s^-1, V_th) 37 V, I_on/I_off

)

10⁴; N,N′-bis(n-octyl)-2-cyanonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-8CN): µ ) 4.7 × 10^-3cm²V^-1

s^-1, V_th) 28, I_on/I_off) 10⁵; N,N′-bis(n-octyl)-1,7- and N,N′-bis(n-octyl)-1,6-dicyanoperylene-3,4:9,10-bis-

1

(dicarboximide) (PDI-8CN₂): µ ) 0.13 cm²V^-1s , V_th) -14 V, I_on/I_off) 10³; and N,N′-bis(n-octyl)-2,6-

dicyanonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-8CN₂): µ ) 0.15 cm²V^-1s^-1, V_th) -37 V, I_on/I_off

)

10². Analysis of the molecular geometries and energetics in these materials reveals a correlation between

electron mobility and substituent-induced arylene core distortion, while V_thand I_offare generally affected

by LUMO energetics. Our findings also indicate that resistance to ambient charge carrier trapping observed

in films of N-(n-octyl)arylene diimides occurs at a molecular reduction potential more positive than ∼ -0.1

V (vs SCE). OFET threshold voltage shifts between vacuum and ambient atmosphere operation suggest

that, at E_red1< -0.1 V, the interfacial trap density increases by greater than ∼1 × 10¹³cm^-2, while, for

semiconductors with E_red1> -0.1 V, the trap density increase is negligible. OFETs fabricated with the

present n-type materials having E_red1> -0.1 V operate at conventional gate biases with minimal hysteresis

in air. This reduction potential corresponds to an overpotential for the reaction of the charge carriers with

O₂of ∼0.6 V. N,N′-1H,1H-Perfluorobutyl derivatives of the perylene-based semiconductors were also

synthesized and used to fabricate OFETs, resulting in air-stable devices for all fluorocarbon-substituted

materials, despite generally having E_red1< -0.1 V. This behavior is consistent with a fluorocarbon-based

O₂barrier mechanism. OFET cycling measurements in air for dicyanated vs fluorinated materials

demonstrate that energetic stabilization of the charge carriers results in greater device longevity in

comparison to the OFET degradation observed in air-stable semiconductors with fluorocarbon barriers.

been the subject of an extensive worldwide research effort.¹¹

Furthermore, recent developments in organic-based light-

emitting diodes (OLEDs), photovoltaics (OPVs), and field-effect

transistors (OFETs) have opened up many new and exciting

challenges and opportunities in the field of thin-film optoelec-

tronics.¹²

One of the major challenges confronting the field of organic

electronics has been the development of high-mobility and

environmentally stable electron-transporting (n-type) organic

semiconductors for thin film device structures to complement

the high efficiency/robust nature of current generation p-type

materials. The availability of both n- and p-type conductors is

Introduction

The ongoing interest in organic semiconducting materials is

driven by the scientific challenges they present as well as by

the promise of real-world applications in a multitude of products

such as “plastic”/flexible displays, sensors, and RF-ID tags, as

recently reviewed.^1-10The mechanistic, energetic, and structural

phenomena governing charge transport in molecular solids have

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J. AM. CHEM. SOC. 2007, 129, 15259-15278

15259

A R T I C L E S

Jones et al.

essential for diodes and complementary circuits, structures

offering high operating speeds and low power consumption.^13-20

The operational performance and stability of organic n-type

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terparts; however in recent years major advances have signifi-

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and have suggested strategies for improved materials design.^21-40

One of the major hurdles remaining is the vulnerability of n-type

charge carriers to ambient conditions. The capability to design

and realize n-type semiconductors where the charge carriers are

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15260 J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007

Arylene Diimide Semiconductors/Charge Carrier Stability

A R T I C L E S

Chart 1. Structures of Some Known Air-Stable n-Type Organic Semiconductors, Their Reduction Potentials, and Reported FET Mobilities:

CuF₁₆Pc, NDI-F, NDI-XF, PDI-FCN₂, PDI-CN₂, and NDI-8CN₂

Chart 2. Structures of Some High Mobility Air-Unstable n-Type Organic Semiconductors, Their Reduction Potentials, and Reported FET

Mobilities: C₆₀, PDI-13, FT(Th)₂TF, DFHCO-4T, and DFCO-4T

fluoroacyl oligothiophenes (DFCO-4TCO),³³N,N′-fluorocarbon-

substituted naphthalene diimides (NDI-F, NDI-XF),^61-63cyano-

substituted perylene diimides (PDI-CN₂, PDI-FCN₂),^{14,15,31,64,65}

and cyano-substituted naphthalene diimides (NDI-8CN₂)⁶⁶

(Chart 1). With the exception of the NDI and PDI derivatives

which are imide-substituted oligo-naphthalenes, collectively

known as arylene diimides or rylene diimides for the larger

homologues,⁶⁷the electrical performance of TFT devices

fabricated with these air-stable materials does not approach that

of high mobility but of air-sensitive materials such as C₆₀,^68-70

PDI-8,^43,71PDI-13,⁷²FT(Th)₂TF,^73,74DFHCO-4T,^33,58DFCO-

4T,³²and DCMT^53,75(Chart 2).⁷⁶

For these reasons, members of the arylene imide family are

a particularly attractive class of target materials to explore

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A R T I C L E S

Jones et al.

or (ii) addition of highly electron-withdrawing core substituents,

thereby lowering the energies of the LUMO/charge carriers

below that of most atmospheric trapping species.^21,42,78

In the case of NDI-F or asymmetric derivatives with a single

R^Fchain, the addition of fluorocarbon substituents to the N,N′

positions does not significantly lower the molecular reduction

potentials relative to the fluorine-free analogues; however,

OFETs fabricated with films of the fluorinated materials are

reported to exhibit air-stable device operation while the fluorine-

free analogues do not.^48,77Since the R^Fchains do not signifi-

cantly alter the LUMO energies of these NDI derivatives (∆E

e 0.15 eV), a steric barrier to atmospheric penetration created

by the densely packed R^Fgroups is thought to be responsible

for the charge carrier air stability.^6,48,62Furthermore, in the case

of NDI-XF, the reduction potential is also nearly identical to

that of the non-fluorinated derivative.⁴⁸In this case as well, only

the fluorinated materials exhibit ambient OFET stability.

Recently, PDI-based materials with fluoro-substituted phenyl

groups at the N,N′ positions were shown to yield air-stable

OFET operation despite having similar reduction potentials to

the non-fluorinated parent.^79,80Interestingly, the air stability was

also shown to be dependent on the fluorination level and fluorine

substitution pattern.⁷⁹To explain this general behavior, Katz et

al. proposed that the larger fluorine van der Waals radius relative

to hydrogen leads to a decreased available spacing between the

N,N′-chains from ∼4 Å to ∼2 Å, thus preventing O₂intrusion⁶²

(Figure 1).

In the case of CuF₁₆Pc, the air-stable device behavior is

thought to reflect a combination of a close-packing O₂barrier

and the low-lying LUMO of the material.⁵⁴The analogous

chlorinated material, CuCl₁₆Pc, is also reported to support air-

stable OFET operation because of the low-energy charge

carriers.⁸¹Non-fluorinated air-stable organometallic- and fullerene-

based semiconductors with LUMO energies more negative than

∼ -4.0 eV also show resistance to air-derived electron traps.⁸²

Recently, we reported ambient operational stability and high

carrier mobilities in the cyanated perylene diimides, PDI-CN₂

and PDI-FCN₂,³¹and the cyanated naphthalene diimide, NDI-

8CN₂.⁸³While PDI-FCN₂has fluorinated functionalities that

can provide a barrier analogous to the aforementioned air-stable

materials, the N-hydroalkyl functionalized structures of PDI-

CN₂and NDI-8CN₂are unique among air-stable n-type organic

semiconductors because the N-alkyl groups (R^H) cannot provide

the same kind of barrier to atmospheric electron traps as the

N-R^Fchains.

Figure 1. Space-filling models of PDI-based dimers taken from the

published crystal structures. The model of air-unstable N,N′-n-pentyl-PDI

(a) depicts the channels between hydrogen atoms (purple) of adjacent

hydrocarbon N,N′-groups that is postulated to facilitate ambient-derived

charge carrier trap penetration into the film. The model of air-stable PDI-

FCN₂(b) depicts the substantial contraction of the channels between the

fluoroalkyl chains (green), relative to the hydrocarbon case in (a) that has

been proposed to present a barrier to penetration of ambient-derived charge

carrier traps in fluorinated phthalocyanines as well as in naphthalene and

perylene diimides.

In addition to these results, work from other groups has

documented the air instability of N-alkyl PDI and NDI deriva-

tives. Thus, Malenfant et al. demonstrated that OFETs fabricated

with N,N′-n-octyl PDI derivatives possess excellent electrical

properties but that the performance degrades rapidly in air.⁷¹

Furthermore, the same behavior is observed in N,N′-n-octyl NDI

derivatives.^48,62More recently, Bao et al. reported that N,N′-

cyclohexyl PDI devices also fail in air.⁸⁴Frisbie et al. shed light

on the mechanism of such device failures by increasing the O₂

partial pressure in the operating atmosphere for OTFT devices

fabricated with N,N′-n-pentyl PDI derivatives and reported a

positive shift in threshold voltage with increasing O₂partial

pressure during operation. This shift can be assigned to the

(77) Katz, H. E.; Otsuki, J.; Yamazaki, K.; Suka, A.; Takido, T.; Lovinger, A.

J.; Raghavachari, K. Chem. Lett. 2003, 32, 508-509.

(78) Laquindanum, J. G.; Katz, H. E.; Dodabalapur, A.; Lovinger, A. J. J. Am.

Chem. Soc. 1996, 118, 11331-11332.

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Mater. 2007, 19, 816-824.

(80) Hosoi, Y.; Tsunami, D.; Ishii, H.; Furukawa, Y. Chem. Phys. Lett. 2007,

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9

15262 J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007

Arylene Diimide Semiconductors/Charge Carrier Stability

A R T I C L E S

creation of metastable PDI/O₂trap states.⁸⁵Given that O₂-

derived trap formation in arylene diimide semiconductor films

is found to occur when the N,N′ substituents are either saturated

or aromatic hydrocarbon functionalities, it would appear that

the mechanism for air stability in the analogous cyano-

substituted materials (PDI-CN₂and NDI-8CN₂) is markedly

different from that invoked in the case of the aforementioned

N-fluorocarbon materials.

In this contribution, we investigate the effects on OFET

performance, in both vacuum and ambient, of increasing

molecular electron affinity in a series of arylene diimide

semiconductors. The energetics, thin film morphology and

microstructure, and crystal packing in a homologous series of

similar derivatives are systematically analyzed to understand

the differences in transistor performance for the series, and from

these data a value for the overpotential to ambient charge carrier

oxidation in this family of n-type semiconductors can be

estimated.

conditions was investigated by cycling the devices immediately after

breaking vacuum at V_d) +100 V between a V_gslightly more positive

than the “off” state and a V_gin the “on” state for 1000 cycles using a

gate delay of 0.1 s. Transfer plots were taken before (cycle 0) and

after (cycle 1001) to compare electrical parameters before and after.

The entire cycling measurement takes approximately 1 h and 30 min.

The “on” V_gand “off” V_gare (+100 V/-25 V) for PDI-8CN₂, (+100

V/+25 V) for PDI-F, and (+75 V/-25 V) for PDI-FCN₂.

Electronic structure calculations were performed in SpartanPC.^89,90

Initial geometry optimizations were performed using semiempirical PM3

methods. The resulting geometry was used as a starting point for

geometry optimization and frontier molecular orbital energy calculations

using DFT/B3LYP with a 6-31G* basis set.

Results

In the following sections we first discuss the synthesis and

characterization of the semiconductors used in this study,

including modifications to the previously reported synthetic

methodologies, and present an analysis of the regioisomer

distributions associated with disubstituted PDIs. We then present

reduced pressure thermogravimetric/differential thermal analysis

characterization of the volatility and phase transitions, followed

by electronic structure characterization via electrochemistry,

optical spectroscopy, and DFT computations. Finally, the

microstructures and transport properties of thin films are

compared using X-ray diffraction, tapping-mode atomic force

microscopy, and OFET measurements. The OFET measure-

ments are performed both under high vacuum and in ambient

atmosphere.

Synthesis. A series of 10 arylene diimide semiconductors

was synthesized for this study (Chart 3). Details of the NDI-

8CN, NDI-8CN₂, PDI-FBr₂, and PDI-FCN₂syntheses have

already been reported.^31,66The synthesis and purification of

semiconductors PDI-8, PDI-8Br₂, and PDI-8Cl₄were carried

out with modifications of previously reported procedures.^86-88

PDI-8 and PDI-F were synthesized by condensation of perylene

dianhydride (PDA) with n-octylamine or 1H,1H-perfluorobu-

tylamine, respectively, in molten imidazole;⁸⁶however, the

reaction was performed in a sealed tube to achieve a reasonable

yield of PDI-F, presumably due to the volatility of the

fluorinated amine. PDI-8Br₂, PDI-FCl₄, and PDI-8Cl₄were

prepared by refluxing a mixture of the halogenated dianhydride

and the corresponding amine, n-octyl amine or 1H,1H-perfluo-

robutylamine, in propionic acid to prevent amine substitution

at the halogenated positions.⁸⁷All semiconductors were purified

by multiple gradient vacuum sublimations, with the exception

of PDI-8Br₂, PDI-8Cl₄, and PDI-8CN₂to avoid thermal

decomposition during sublimation. These three materials were

purified by chromatography on silica followed by multiple

recrystallizations.

Experimental Section

The synthesis and purification of all arylene diimide materials were

accomplished using slight modifications of literature procedures; see

Supporting Information for further details.^{86-88 1}H NMR spectra (400

MHz) were measured in CDCl₃on a Varian Mercury 400 instrument,

with chemical shifts referenced to the internal CHCl₃resonance. The

600 MHz ¹H NMR spectrum of PDI-8Br₂in CHCl₃was acquired on a

Varian INOVA 600 instrument to obtain the dibromo 1,6-/1,7- PDI

substitutional isomer ratio. Optical absorption spectra were acquired

on a Shimadzu 1601 spectrophotometer. Emission spectra were acquired

on a PTI single photon counting spectrofluorimeter in a right angle

configuration. Cyclic voltammetry was conducted under N₂in dry 0.1

M TBAPF₆in dichloromethane solutions and was referenced to Fc/

Fc⁺(0.475 vs SCE).

Films of the semiconductors were grown from the vapor phase (2

× 10^-6Torr, 0.2 Å/s, ∼50 nm thick) on n⁺-Si (001) wafers having

300 nm of thermally grown SiO₂as the dielectric layer (Montco Silicon

Tech). The wafers were cleaned by rinsing with acetone, methanol,

and isopropanol followed by a 5 min plasma cleaning in a Harrick

Plasma Cleaner/Sterilizer PDC-32G. Substrates for PDI-FBr₂and

PDI-FCl₄thin film deposition were exposed to hexamethyldisilzane

(HMDS) vapor for 3 days prior to semiconductor film deposition,

because films grown on SiO₂did not yield active OFETs.

Tapping-mode AFM images were obtained on a Digital Instruments

Multimode Nanoscope IIIa instrument with Nanoprobe TESP-70 tips

(cantilever length ) 125 µm, freq ) 278-338 kHz). X-ray diffraction

(Θ/2Θ and ω scans) was performed on a Rigaku ATXG thin film

diffractometer in slit-configuration with a Ni-filtered Cu source.

Reduced pressure TGA/DTA data were obtained on a TA Instruments

SDT 2960 unit interfaced with a mechanical vacuum pump.

Top-contact TFTs were fabricated by vapor depositing Au (3 × 10^-6

Torr, 0.3 Å/s, ∼50 nm thick) onto the semiconductor thin films through

a shadow mask to obtain devices with a channel width of 100 µm and

length of 5 mm. Electrical measurements were performed with a

Keithley 6430 subfemtoammeter and a Keithley 2400 source meter in

ambient atmosphere or in a vacuum probe station at 10^-6Torr, as

described previously.³²Care was taken to protect the OFETs from

ambient light prior to and during electrical measurements. The durability

of PDI-8CN₂, PDI-F, and PDI-FCN₂TFTs operating under ambient

The current, improved PDI-8CN₂synthesis involves signifi-

cant modifications to the previously reported procedure (see

Supporting Information for details).^31,64The bromination and

imide condensation were accomplished via known methodolo-

gies; however, cyanation was achieved by heating a DMF

solution of the dibromo-PDI to 130 °C with a 10-fold excess

of CuCN. Previously, we reported that CuCN did not achieve

efficient cyanation of the PDI-core;⁶⁴however with the new

methodology recently utilized for NDI-8CN₂synthesis,⁶⁶we

(85) Chesterfield, R. J.; McKeen, J. C.; Newman, C. R.; Frisbie, C. D. J. Appl.

Phys. 2004, 95, 6396-6405.

(86) Demmig, S.; Langhals, H. Chem. Ber. 1988, 121, 225-230.

(87) Chen, Z.; Debije, M. G.; Debaerdemaeker, T.; Osswald, P.; Wu¨rthner, F.

ChemPhysChem 2004, 5, 137-140.

(88) Wu¨rthner, F.; Stepanenko, V.; Chen, Z.; Saha-Moller, C. R.; Kocher, N.;

Stalke, D. J. Org. Chem. 2004, 69, 7933-7939.

(89) Spartan ’06; Wavefunction, Inc.: Irvine, CA.

(90) Shao, Y. et al. Phys. Chem. Chem. Phys. 2006, 8, 3172-3191.

9

J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007 15263

A R T I C L E S

Jones et al.

Chart 3. Chemical Structures of the N,N′-n-Octyl and

N,N′-1H,1H-Perfluorobutyl Perylene Diimide Semiconductors and

N,N′-n-Octyl Naphthalene Diimide Semiconductors Investigated in

This Study

stability of PDI-8, PDI-8Br₂, PDI-8Cl₄, and PDI-8CN₂(Figure

S2). The degree of source material decomposition products in

a vapor-phase deposition can have significant effects on impurity

levels in the resulting thin films. Chemical contaminants can

contribute to unintentional doping, thereby increasing off-

currents, depressing on-off current ratios (I_on/I_off), and/or

creating carrier traps which reduce mobility.⁹¹At this relatively

low-pressure level, PDI-8, PDI-8Cl₄, and PDI-8CN₂exhibit

minimal decomposition, as indicated by nonvolatile residues of

∼1, ∼6, and ∼8 wt %, respectively. PDI-8Br₂undergoes

substantial decomposition at 10 Torr with ∼20 wt % nonvolatile

residue, meaning that impurities/traps from decomposed material

created during film deposition likely compromise device

performance. To minimize impurities from decomposition of

the source material, fresh semiconductor samples were used for

every film growth experiment to minimize film contamination

by dehalogenated or decyanated species.

In addition to information about decomposition, the melting

points from the DTA scans provide insight into the relative

cohesive energetics of the intermolecular interactions in the solid

state. In general, strong intermolecular interactions in molecular

solids correlate with greater intermolecular orbital overlap,

resulting in greater carrier mobilities.⁴The DTA reveals melting

points of 200, 233, and 300 °C for PDI-8Br₂, PDI-8Cl₄, and

PDI-8CN₂, respectively. Moreover, the DTA data for PDI-8

exhibit two transitions at 220 and 375 °C, where the first

transition is in good agreement with the previously reported

liquid crystalline (LC) phase transition for this derivative.⁹²The

increased ordering in LC phases of N-alkyl PDI semiconductors

has been previously invoked to explain the exceptionally large

electron mobilities in these materials.^92,93While LC phases of

core-substituted PDIs have been reported, these molecules do

not have N-alkyl functionalization.^94-96

realize efficient cyanation with CuCN instead of the Pd-

catalyzed methodology.^31,64

As reported previously,³¹the synthesis of significant quantities

of isomerically pure disubstituted PDIs remains problematic, a

consequence of perylene dianhydride bromination leading to a

mixture of 1,6 and 1,7 isomers.^31,88Separating the 1,6 and 1,7

isomers of PDI-8Br₂and PDI-8CN₂presents a challenge, and

repeated fractional recrystallizations⁸⁸involving the slow dif-

fusion of MeOH into methylene chloride solutions of PDI-8Br₂

Electronic Structure Characterization. The electronic

properties of all the present semiconductors were characterized

by cyclic voltammetry (CV), UV-visible optical absorption

spectroscopy (UV-vis), and photoluminescence spectroscopy

(PL). DFT level electronic structure calculations were performed

for each core to probe orbital energetics and the most stable

geometry. The relative HOMO and LUMO energetic positions

are crucial in determining the majority charge carrier^26,29,30and

charge carrier stability in typical organic semiconductors.²⁵In

the case of organic n-type materials where charge transport

occurs predominantly by hopping through low-lying LUMOs,

energies < -3 eV vs vacuum are generally thought necessary

for efficient electron injection into the semiconductor.^21,26,29,30

Additionally, the lower the LUMO energy, the lower the bias

required to inject carriers should be and the less susceptible

the electrons should be to trapping. The LUMO energies for

the semiconductors in this study were estimated by conventional

1

isomeric mixtures yields products enriched to ∼65% (by H

NMR) in the 1,7 isomer and analogous procedures yield ∼73%

1

enrichment (by H NMR) in the 1,7-isomer for PDI-8CN₂.

Attempts to separate the two isomers by either normal or

reversed-phase HPLC did not yield acceptable separations. The

effects on OFET performance of having multiple regioisomers

present in these samples are unclear because the synthesis of

sufficient quantities of regioisomerically pure materials for thin

film depositions has proven difficult. However, we have reported

the crystal structure of a PDI-FCN₂sample in which both

regioisomers cocrystallize,³¹suggesting that in the dicyanated

case the presence of two isomers does not significantly affect

crystal packing. Also note that the 1H,1H-perfluorobutyl

substituted semiconductors were also synthesized and character-

ized to compare reduction potentials and air-stable OFET

operation versus the n-octyl materials. Consequently, the 1H,1H-

perfluorobutyl materials and their thin films were not exhaus-

tively optimized and characterized.

(91) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359-369.

(92) Struijk, C. W.; Sieval, A. B.; Dakhorst, J. E. J.; van Dijk, M.; Kimkes, P.;

Koehorst, R. B. M.; Donker, H.; Schaafsma, T. J.; Picken, S. J.; van de

Craats, A. M.; Warman, J. M.; Zuilhof, H.; Sudho¨lter, E. J. R. J. Am. Chem.

Soc. 2000, 122, 11057-11066.

(93) An, Z.; Yu, J.; Jones, S. C.; Barlow, S.; Yoo, S.; Domercq, B.; Prins, P.;

Siebbeles, L. D. A.; Kippelen, B.; Marder, S. R. AdV. Mater. 2005, 17,

2580-2583.

(94) Fuller, M. J.; Sinks, L. E.; Rybtchinski, B.; Giaimo, J. M.; Xiyou, L.;

Wasielewski, M. R. J. Phys. Chem. A 2005, 109, 970-975.

(95) Fuller, M. J.; Walsh, C. J.; Zhao, Y.; Wasielewski, M. R. Chem. Mater.

2002, 14, 952-953.

Thermal Characterization. Reduced pressure (10.0 Torr

under N₂, 2 °C/min temperature ramp) simultaneous thermo-

gravimetric analysis-differential thermal analysis (TGA-DTA)

scans were performed to evaluate the volatility and thermal

(96) Fuller, M. J.; Wasielewski, M. R. J. Phys. Chem. B 2001, 105, 7216-

7219.

9

15264 J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007

Arylene Diimide Semiconductors/Charge Carrier Stability

A R T I C L E S

Table 1. Frontier Molecular Orbital Energies of the Organic

Semiconductors Studied in This Contribution Estimated from

Cyclic Voltammetry^a,band Optical Absorption/Emission^c

Measurements

Changes in the absorption/emission maxima from UV-visible

absorption spectroscopy and photoluminescence spectroscopy

can be utilized to confirm molecular orbital energetics and/or

structural relationships in the solution-phase compounds (Table

1 and Figure S1). The shift in absorption maximum (λ_abs) for

the core-substituted perylenes reflects both the degree of

conjugation in the perylene-core and the electron-withdrawing

nature of the substituents. PDI-8, PDI-8Br₂, and PDI-8CN₂have

nearly identical λ_absvalues of 521, 520, and 522 nm respectively,

which reflects the relatively constant optical transition energy.

The blue shift relative to PDI-8 of PDI-8Cl₄to λ_absof 514 nm

reflects the severe distortion of the perylene core by the four

chloro substituents, which results in an energetic increase in

the optical transition. Similarly, the fluorinated derivatives

exhibit similar trends with PDI-F, PDI-FBr₂, and PDI-FCN₂

exhibiting similar λ_absvalues of 524, 526, and 523 nm,

respectively. PDI-FCl₄shows a blue shift to 519 nm relative to

PDI-F, a similar shift compared to the n-octyl case. In comparing

the n-octyl substituted vs 1H,1H-perfluorobutyl substituted

materials, the fluorinated materials exhibit a similar 4 nm (PDI-

R), 6 nm (PDI-RBr₂), and 4 nm (PDI-RCl₄) red shift relative to

the n-octyl materials, most likely due to the slight LUMO energy

depression upon addition of the electron-withdrawing fluorous

chains. The PDI-RCN₂derivatives show negligible λ_absshifts

upon fluorination.

The Stokes shift between the absorption and emission

spectrum maxima of a molecule reflects the ability of the excited

state to reorganize to a lower energy state prior to photoemission,

and thus more rigid molecules tend to have smaller Stokes

shifts.¹⁰¹The Stokes shifts here are found to have little

dependence on the N,N′-substitution, with derivatives of similar

cores only showing ∼1-2 nm differences in Stokes shifts.

However, core substitution has a larger effect on Stokes shifts

with relatively rigid PDI-R and PDI-RCN₂cores showing small

shifts of ∼7 and ∼10 nm, respectively. The slightly larger shift

of the PDI-RCN₂derivative reflects the small perylene-core

distortion introduced upon cyanation, as elaborated upon in the

Discussion. In contrast, the halogenated derivatives exhibit much

larger shifts of ∼20 nm (PDI-RBr₂) and ∼29 nm (PDI-RCl₄),

which reflects the greater excited-state reorganization of the

more distorted perylene cores.

d

LUMO^b

(eV)

HOMO^c

(eV)

λ_abs

λ_em

E_g

a

semiconductor

E_red1

(nm)

(eV)

PDI-8

PDI-8Br₂

PDI-F

PDI-8Cl₄

PDI-FBr₂

NDI-8CN

PDI-FCl₄

PDI-8CN₂

PDI-FCN₂

NDI-8CN₂

-0.46

-0.36

-0.33

-0.25

-0.24

-0.22

-0.13

-0.06

+0.03

+0.08

-3.9

-4.0

-4.1

-4.2

-4.3

-4.5

-6.3

-6.4

-6.5

-7.2

-6.6

-6.7

-6.8

-7.5

521

520

524

514

526

386

519

522

523

380

528

541

532

543

545

392

547

533

532

446

2.4

2.3

3.0

2.3

2.4

2.3

3.0

^a0.1 M TBAPF₆in CH₂Cl₂: vs SCE. ^bEstimated vs vacuum level from

E_LUMO) 4.4 eV - E_red1. ^cEstimated from HOMO ) LUMO^b- E_g. ^dFrom

optical absorption/emission data in dichloromethane. E_g) optical gap.

electrochemical techniques,^97-99and HOMO energies were

estimated from the optical gaps (E_g) determined from the overlap

of the normalized UV-vis and PL spectra (Figure S1). These

data are summarized in Table 1.

An incremental displacement of the reduction potentials to

more positive values correlates with increasing relative energetic

ease of negatively charging a species in solution and, thus, the

relative stability of the electron charge carriers in the solid state.

The electrochemically derived LUMO energies, central to n-type

charge transport characteristics, vary substantially and system-

atically across the series from PDI-8 (-0.46 V vs SCE) to NDI-

8CN₂(+0.08 V vs SCE), which is consistent with the number

of arylene-core electron-withdrawing substituents and their

respective Hammett coefficients.¹⁰⁰Importantly, N,N′-function-

alization with 1H,1H-perfluorobutyl groups only decreases the

reduction potential by ∼ -0.13 V relative to the N,N′-n-octyl

derivative, which should allow us to make comparisons between

air stability due to fluorination (packing) and air stability due

to orbital energetics.

Electronic structure calculations were employed to estimate

molecular geometries due to the difficulty in obtaining diffrac-

tion-quality crystals for many members of the series and as

further confirmations of the experimentally derived orbital

energetic trends in this semiconductor series. The gas-phase

energetics of the molecules presented here were calculated by

DFT at the B3LYP level with a 6-31G* basis set in Spartan

PC. The computed energies reported are for N,N′-dimethyl (R

) Me) models, geometry-optimized with semiempirical PM3,

followed by DFT/B3LYP/631-G* geometry optimization. Cal-

culated in this manner, we find the LUMO energy versus

vacuum to be -3.46 eV for the PDI-R-core, -3.62 eV for the

PDI-RBr₂-core, -3.73 eV for the PDI-RCl₄-core, -3.85 eV for

the NDI-RCN-core, -4.10 eV for the PDI-RCN₂-core, and

-4.27 for the NDI-RCN₂-core. These LUMO energies obtained

from the calculations are generally in close agreement with the

experimental trends in reduction potentials (Table 1).

Thin Film X-ray Diffraction. X-ray diffraction experiments

are crucial to understanding microstructure-function relation-

ships between ordering over various length scales in organic

thin films and charge transport properties. Conventionally, Θ/2Θ

diffraction scans are utilized to obtain out-of-plane d-spacings

in the films, which allows determination of the molecular

orientations relative to the substrate surface. In most bottom-

gate OFET configurations, the optimal orientation for efficient

charge transport between source and drain electrodes requires

that the π-conjugated cores be aligned approximately perpen-

dicular to the dielectric surface.^3,7Additionally, the presence

of multiple Bragg reflections and Laue oscillations around the

first-order diffraction peak provides information regarding long-

range film order, which in turn indicates the crystalline quality

of these polycrystalline organic thin films.^102,103Finally, rocking

(97) Cervini, R.; Li, X.-C.; Spencer, G. W. C.; Holmes, A. B.; Moratti, S. C.;

Friend, R. H. Synth. Met. 1997, 84, 359-360.

(98) Li, Y.; Cao, Y.; Gao, J.; Wang, D.; Yu, G.; Heeger, A. J. Synth. Met. 1999,

99, 243-248.

(101) Turro, N. J. Modern Molecular Photochemistry; University Science

Books: Sausalito, CA, 1991.

(102) Cullity, B. D.; Stock, S. R. Elements of X-Ray Diffraction; Prentice Hall:

Upper Saddle River, NJ, 2001; p 664.

(103) Du¨rr, A. C.; Schreiber, F.; Munch, M.; Karl, N.; Krause, B.; Kruppa, V.;

Dosch, H. Appl. Phys. Lett. 2002, 81, 2276-2278.

(99) Smestad, G. P.; Spiekermann, S.; Kowalik, J.; Grant, C. D.; Schwarzberg,

A. M.; Zhang, J.; Tolbert, L. M.; Moons, E. Sol. Energy Mater. 2003, 76,

85-105.

(100) Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry;

University Science Books: Sausalito, CA, 2006.

9

J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007 15265

A R T I C L E S

Jones et al.

Figure 2. Representative Θ/2Θ X-ray diffraction scans in logarithmic scale of vapor-deposited PDI-8 (T_d) 130 °C), PDI-8Br₂(T_d) 70 °C), PDI-8Cl₄(T_d

) 110 °C), PDI-8CN₂(T_d) 130 °C), NDI-8CN (T_d) 130 °C), and NDI-8CN₂(T_d) 110 °C) films. The inset in the PDI-8 diffraction depicts a close-up

of the Laue oscillations around the PDI-8 001 reflection. Also note the presence of other phases/orientations (P2) in addition to the primary phase (P1) in

PDI-8Cl₄and NDI-8CN₂films. All plots have a break in the intensity axis between 10 and 20 C.P.S.; the scale is logarithmic at intensities above 20 C.P.S.

between arylene diimide layers.^31,104Exceptions are PDI-8Cl₄

Table 2. Thin Film X-ray Diffraction Data for the Various

Semiconductors of This Study, Indicating d-Spacing Related to the

and NDI-8CN₂films which contain a second thin film phase

Molecular Long-Axis, Estimated Tilt Angle of the Semiconductor

or orientation having a slightly larger d-spacing. Since crystal

structures of the n-octyl derivatives are not known, the family

of diffraction peaks are assumed to be 00l as assigned in other

n-alkyl PDIs on the basis of single-crystal diffraction data of

PDI-5.^43,71,85

Relative to the Substrate Plane Normal, and Rocking Curve Full

Width at Half-Maximum

T_d

C)

d-spacing

(Å)

tilt angle

(deg)

ω

_fwhm(001)

semiconductor

(

°

(deg)

PDI-8

PDI-8Br₂

PDI-8Cl₄

PDI-8CN₂

NDI-8CN

NDI-8CN₂

PDI-F

PDI-FBr₂

PDI-FCl₄

PDI-FCN₂

130

70

20.8

22.8

18.0 (19.4)

19.8

18.2

18.2 (20.2)

16.1

15.5

n/a

19.5

43

48

36 (40)

41

44

44 (50)

45

43

n/a

59

0.03

0.06

0.03 (0.03)

0.03

0.03 (0.03)

0.03

n/a

0.05

In addition to providing information on molecular orientation,

XRD techniques were utilized to elucidate the microstructural

quality of the polycrystalline thin films. The occurrence of (00l)

progressions is indicative of long-range ordering,^103,105and all

films, except the PDI-8Br₂films, exhibit five or more reflections

for the sole or predominant phase. The PDI-8Br₂films only

exhibit a first-order reflection, meaning that the microstructural

order is not comparable to that in the films of the other five

n-octyl semiconductors. Rocking curves of the first-order

reflections for all samples (Figure S3) indicate a high degree

of texturing (fwhm ∼0.03), meaning that the layers of the film

microstructure are uniformly oriented relative to the substrate

plane.^103,105Again, films of PDI-8Br₂exhibit substantially less

texturing (fwhm ∼0.06), further demonstrating the modest film

quality. Finally, the Laue oscillations around the first-order

diffraction peaks of PDI-8, PDI-8Cl₄, NDI-8CN, PDI-8CN₂, and

NDI-8CN₂films indicate that the crystallite spacings in the films

are rather uniform,¹⁰³and the spacing associated with the

oscillations (∼50 nm) indicates that the coherence is nearly

uniform throughout the film thickness (∼50 nm).

a

110

130

110

130

a

^aMinority phases are reported in parentheses.

curves evaluate the out-of-plane texturing in the films. All

samples exhibit diffraction features in the Θ/2Θ scans as

depicted in Figure 2, while data are compiled in Table 2.

The present XRD data for the n-octyl derivatives indicate

highly ordered/layered microstructures in all films,⁶which are

grown on Si/SiO₂by high vacuum physical vapor deposition at

relatively slow growth rates (0.2 Å/s) and at elevated substrate

temperatures (T_d). From the d-spacings calculated in these

experiments, molecular tilt angles of ∼45° are estimated from

the PM3 geometry-optimized molecular lengths of PDI-8 (30.3

Å) and NDI-8 (26.3 Å). This value reasonably assumes the same

basic crystal structures established for straight chain-substituted

arylene diimides, such as N,N′-n-pentyl-PDI, NDI-F, and PDI-

FCN₂, with negligible interdigitation of the N,N′-R^H/R^Fchains

(104) Hadicke, E.; Graser, F. Acta Crystallogr., Sect. C 1986, 42, 189-195.

(105) Wie, C. R. Mater. Sci. Eng., R 1994, R13, 1-58.

9

15266 J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007

Arylene Diimide Semiconductors/Charge Carrier Stability

A R T I C L E S

OFETs,³is challenging by conventional techniques; therefore,

scanning probe microscopies are often used to evaluate the thin

film surface morphology as an approximation to the active

region at the interface with the dielectric.^4,106AFM characteriza-

tion of the semiconductor films grown in the present study

reveals highly crystalline surface microstructures for PDI-8, PDI-

8Cl₄, NDI-8CN, PDI-8CN₂, and NDI-8CN₂(Figure 4). The

surface morphologies of PDI-8, PDI-8CN₂, and NDI-8CN₂films

have similarly structured ribbon-like grains, while NDI-8CN

and PDI-8Cl₄films exhibit plate-like grains. The unique surface

morphology of PDI-8Br₂does not appear to be as crystalline

or as smooth as that of the other films, with large features

protruding from the film. In contrast, there are few discontinuous

surface features for PDI-8, PDI-8Cl₄, and PDI-8CN₂. Since there

is no evidence in the XRD for multiple phases or orientations

in the films when these surface features are present, the

microstructural/morphological origin of these growth features

presumably does not involve additional phases or growth

orientations.

The surface morphologies of PDI-F, PDI-FBr₂, PDI-FCl₄, and

PDI-FCN₂films appear substantially different from those of the

n-octyl substituted materials (Figure 5). In the case of PDI-F

and PDI-FBr₂, the films exhibit large crystal-like features

protruding from the thin films, most similar to the PDI-8Br₂

case in the n-octyl materials. PDI-FCl₄films exhibit an unusual

honeycomb-like surface morphology, consistent with the unique

XRD pattern also observed. PDI-FCN₂forms smooth uniform

films with only a few smaller crystals/protrusions; however, the

surface morphology is substantially different from the n-octyl

case.

Field-Effect Transistor Characterization. Organic field-

effect transistors provide a simple device structure that allows

detailed analysis of materials charge transport characteristics

via evaluation of the current-voltage response. The function

of the OFET is to modulate the semiconductor conductivity

between the source and drain electrodes as a function of the

gate voltage. A top-contact/bottom-gate configuration device

in which the source and drain are vapor-deposited on top of

the semiconductor film is used in this study (Figure 6a). When

the bias (V_g) between the drain electrode (D) and the gate

electrode (G) is 0.0 V, the transistor is “off” and little current

should pass between source and drain. When V_gis applied,

charges collect on either side of the dielectric, leading to an

increase in the mobile n-type charge carrier density; hence

current flows in the channel between source (S) and drain (D)

electrodes when a source-drain bias (V_d) is applied. As a result,

the current (I_d) between the S and D increases and the device

is said to be “on”. The performance parameters that can be

extracted from the FET I-V response curves include the charge

carrier mobility (µ), current on-off ratio (I_on/I_off), and threshold

voltage (V_th). These performance parameters are extracted within

the assumptions of conventional transistor equations.¹⁰⁷For

organic transistors to become a viable technology, the compo-

nent semiconductors must ideally have electrical properties as

follows: µ ≈ 0.1-1 cm²V^-1s^-1, V_th≈ 0 V, and I_on/I_off≈ 10⁶,

with the availability of both environmentally stable p- and n-type

Figure 3. Representative Θ/2Θ X-ray diffraction scans in logarithmic scale

of vapor-deposited PDI-F (T_d) 130 °C), PDI-FBr₂(T_d) 130 °C), PDI-

FCl₄(T_d) 130 °C), and PDI-FCN₂(T_d) 130 °C) films. All plots have a

break in the intensity axis between 10 and 20 C.P.S.; the scale is logarithmic

at intensities above 20 C.P.S.

Thin film XRD measurements on the 1H,1H-perfluorobutyl

materials (Figure 3) reveal strikingly different film microstruc-

tures compared to the n-octyl substituted materials. The mo-

lecular orientation in these materials relative to the substrate

was estimated assuming a molecular long-axis length of 22.8

Å, taken from the crystal structure of PDI-FCN₂.³¹Given the

d-spacings in the thin films, PDI-F and PDI-FBr₂have ∼45°

tilt angles with respect to the substrate, similar to the n-octyl

materials. PDI-FCN₂exhibits a slightly larger d-spacing, and

thus the molecular tilt angle is nearly 60° relative to the

substrate. The diffraction peaks associated with PDI-FCl₄are

at significantly larger 2Θ values compared to all other materials,

and the lack of crystal structure data makes unambiguous

assignment of these diffraction peaks problematic.

The out-of-plane film quality as evaluated by thin film XRD

is significantly poorer for 1H,1H-perfluorobutyl materials

compared to n-octyl materials. The diffraction peak intensity is

noticeably less for these materials. Furthermore, the film

microstructure appears more complex for the core substituted

and fluorinated materials. PDI-F films exhibit one family of

four reflections (2Θ ) 5.5°, 10.9°, 16.5°, and 27.6°) that likely

correlate with the molecular long axis, whereas PDI-FBr₂, PDI-

FCl₄, and PDI-FCN₂exhibit multiple families of diffraction

features suggestive of different phases or orientations. PDI-FBr₂

and PDI-FCN₂films exhibit a family of peaks similar to those

in PDI-F which correlate with the molecular long axis; however,

these two semiconductors also exhibit reflections around 2Θ

) 13.5° and 18.0°, respectively. Note that PDI-FCl₄films do

not exhibit a diffraction peak at 2Θ ≈ 5° that corresponds to

the molecular long axis, and PDI-FCl₄films only exhibit

reflections at 2Θ ) 13.5°, 16.5°, 18.0°, and 25.3°, which are

also observed in PDI-FBr₂films. In films of these materials,

there are no Laue oscillations associated with the diffraction

peaks. In films of all the fluorocarbon-substituted materials, there

are at least two families of diffraction features observed, which

may correspond to different phases/orientations.

(106) DeLongchamp, D. M.; Sambasivan, S.; Fischer, D. A.; Lin, E. K.; Chang,

P.; Murphy, A. R.; Frechet, J. M. J.; Subramanian, V. AdV. Mater. 2005,

17, 2340-2344.

(107) Sze, S. M. Semiconductor DeVices; John Wiley & Sons: New York, 1985;

p 523.

Thin Film Morphology by Atomic Force Microscopy.

Characterization of the thin film microstructure at the substrate-

thin film interface, the active region for charge transport in

9

J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007 15267

A R T I C L E S

Jones et al.

Figure 4. Tapping-mode AFM images of (a) PDI-8 (T_d) 130 °C), (b) PDI-8Br₂(T_d) 70 °C), (c) PDI-8Cl₄(T_d) 110 °C), (d) NDI-8CN (T_d) 130 °C),

(e) PDI-8CN₂(T_d) 130 °C), and (f) NDI-8CN₂(T_d) 110 °C) films. The surface morphologies of PDI-8, PDI-8CN₂, and NDI-8CN₂films are similar with

ribbon-like grains. The surface morphology of PDI-8Cl₄and NDI-8CN films exhibit plate-like grains. PDI-8Br₂film morphology features a smooth background

with protruding surface structures.

ratio for all devices. To compare the electrical properties across

the series, all parameters were calculated for a V_densuring that

the device was operating in the saturation regime (V_d> V_g).

N,N′-n-Octyl Substituted Arylene Diimide OFETs. We first

consider OFETs fabricated with thin films of the present n-octyl

arylene diimides, deposited at the substrate temperatures (T_d’s)

indicated in Table 3, characterized under both vacuum and

ambient atmosphere. We have chosen to discuss films deposited

on bare SiO₂at T_d’s that yield the highest average carrier

mobilities because dielectric surface treatments and unoptimized

thin films give larger device-to-device variation. The electrical

properties of the n-octyl semiconductor-based OFETs will first

be discussed for operation under vacuum. Output plots showing

I_dvs V_dfor different gate biases measured under vacuum reveal

that each material has well-defined linear and saturation regimes

(Figure 6). Additionally, bidirectional transfer plots reveal

minimal hysteresis for all nonhalogenated materials (Figure 7).

Figure 5. Tapping-mode AFM images of (a) PDI-F (T_d) 130 °C), (b)

The hysteresis of the dicyanated semiconductors is exceptionally

PDI-FBr₂(T_d) 130 °C), (c) PDI-FCl₄(T_d) 130 °C), and (d) PDI-FCN₂

small for n-channel OFETs. The two halogenated semiconduc-

(T_d) 130 °C) films.

tors, PDI-8Br₂and PDI-8Cl₄, consistently exhibit the onset of

semiconductors for organic-CMOS.^1,6,9,41,108For the devices

described below, the gate is highly doped silicon, while the

dielectric is 300 nm SiO₂. The S/D contacts are gold. While

silanol groups on SiO₂dielectrics have been shown to act as

electron traps for materials with E_red1< ∼ -0.6 V vs SCE,^58,109

the linear regime at V_dvalues positive of V_d) 0 V, which is

usually indicative of non-ohmic contacts.¹¹⁰

The mobility trends in Table 3 reveal that PDI-8, PDI-8CN₂,

and NDI-8CN₂have the highest mobilities of 0.32, 0.13, and

0.15 cm²V^-1s^-1, respectively, while PDI-8Br₂, PDI-8Cl₄, and

NDI-8CN have significantly lower mobilities (e10^-3cm²V^-1

s^-1). These findings are generally in good agreement with

previously published results for PDI-8^43,71and cyano-PDI and

-NDI derivatives.³¹For PDI-8Cl₄films, the average electron

mobility of 4 × 10^-3cm²V^-1s^-1is lower than those reported

for other PDI-RCl₄-based semiconductors.^108,110From micro-

wave conductivity measurements (PR-TRMC) on bulk samples,

a mobility of ∼0.14 cm²V^-1s^-1for an aryl-n-alkyl PDI-RCl₄

the molecular energetics of the semiconductors with E_red1

>

-0.5 vs SCE utilized in this study should minimize trapping

contributions from these functionalities. OFET parameters (µ,

V_th, I_on/I_off) are averages of all measurements taken on the

specified devices, and standard deviations are given only when

greater than 5%. Transfer plots of I_dvs V_gwere used to calculate

the saturation mobility, threshold voltage, and current on-off

(108) Printed Organic and Molecular Electronics; Kluwer Academic Publish-

ers: New York, 2004; p 695.

(109) Chua, L.-L.; Zaumseil, J.; Chang, J.-F.; Ou, E. C.-W.; Ho, P. K.-H.;

Sirringhaus, H.; Friend, R. H. Nature 2005, 434, 194-199.

(110) Shen, Y.; Hosseini, A. R.; Wong, M. H.; Malliaras, G. C. ChemPhysChem

2004, 5, 16-25.

9

15268 J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007

Arylene Diimide Semiconductors/Charge Carrier Stability

A R T I C L E S

Figure 6. Representative OFET output characteristics of (a) PDI-8 (T_d) 130 °C), (b) PDI-8Br₂(T_d) 70 °C), (c) PDI-8Cl₄(T_d) 110 °C), (d) NDI-8CN

(T_d) 130 °C), (e) PDI-8CN₂(T_d) 90 °C), and (f) NDI-8CN₂(T_d) 110 °C) films in a vacuum. Diagram (a) depicts a top-contact OFET geometry. The

gate biases (V_g) are indicated immediately above the corresponding I-V curves.

Table 3. OFET Electrical Properties with Standard Deviations for the Present Arylene Diimide Semiconductor Series Measured in Vacuum

or Air on a Si/SiO₂Substrate^a

vacuum

air

1

semiconductor

T_d

(°

C)

µ

(cm²V^-1s^-

)

V

_th(V)

I

_on/I_off

µ

(cm²V^-¹s^-

)

V

_th(V)

I_on/I_off

PDI-8

130

70

0.32

55 (4)

62 (8)

37 (3)

-14 (3)

28 (2)

-37 (2)

38 (2)

13 (7)

25 (2)

-22 (4)

10⁵

10³

10⁴

10³

10⁵

10²

10⁴

10³

2 (9) × 10^-4

147 (6)

97 (40)

95 (19)

-21 (2)

165 (9)

-55 (5)

30 (3)

28 (11)

41 (16)

-19 (3)

10⁴

10³

10²

10³

10⁴

10²

10³

PDI-8Br₂

PDI-8Cl₄

PDI-8CN₂

NDI-8CN

NDI-8CN₂

PDI-F

PDI-FBr₂

PDI-FCl₄

PDI-FCN₂

3 (3) × 10^-5

4 × 10^-3

0.13 (0.02)

9 (10) × 10^-6

5 (2) × 10^-5

0.12

110

130

110

130

4.7 (0.4) × 10^-3

2.8 × 10^-4

0.15 (0.01)

0.11 (0.01)

1.4 (0.2) × 10^-3

1.4 (0.4) × 10^-3

2.8 (0.1) × 10^-5

0.27 (0.09)

9.5 (0.1) × 10^-4

8.8 (0.2) × 10^-4

1.1 (0.2) × 10^-5

0.24 (0.07)

a

^aElectron carrier mobility (µ) is given in cm²V^-1s^-1, and threshold voltages (V_th) are given in V. For the present dicyano semiconductors, current

on-off ratios (I_on/I_off) are given for V_g(+100 V/-100 V) under both vacuum and air. For all other semiconductors, I_on/I_offfor vacuum operation are for V_g

(+100/-100 V) and for air V_g(+200 V/0 V). If standard deviations are less than 5%, they are not reported. ^bDevice parameters reported here are for films

grown on hexamethyldisilazane vapor-treated substrates because films grown on bare SiO₂did not yield active OFETs.

has been reported.¹¹¹These measurements, unlike OFETs,

include coulombic interactions between electrons and holes and

are not influenced by contacts and dielectric/thin film interface

charge trapping, which reasonably explains the difference.¹¹²

Moreover, N-H functionalized PDI-RCl₄derivatives show

similarly low electron mobilities (10^-5-10^-3cm²V^-1s^-1) on

unfunctionalized SiO₂substrates.¹¹³The carrier mobilities for

the four perylene diimide semiconductors are best explained

by slight differences in crystal packing and thin film quality as

discussed below. The mobility of the naphthalene diimide-based

films will be discussed within the context of XRD, AFM, and

calculated molecular geometries.

The threshold voltage (V_th) is used to evaluate the gate bias

(V_g) at which electron trap states have been filled and the carriers

become mobile. V_thexhibits device-to-device variations char-

acteristic of OFETs, which can be influenced by many factors

including materials purity, atmosphere during operation, film

morphology, surface dipoles, film thickness, and light expo-

sure.^{55,69,85,114-116}Generally speaking, the V_thof the n-octyl

(111) Wu¨rthner, F.; Stepanenko, V.; Chen, Z.; Saha-Mo¨ller, C. R.; Kocher, N.;

Stalke, D. J. Org. Chem. 2004, 69, 7933-7939.

(112) Warman, J. M.; de Haas, M. P.; Dicker, G.; Grozema, F. C.; Piris, J.;

Debije, M. G. Chem. Mater. 2004, 16, 4600-4609.

(114) Horowitz, G.; Hajlaoui, R.; Bouchriha, H.; Bourguiga, R.; Hajlaoui, M.

AdV. Mater. 1998, 10, 923-927.

(115) Pernstich, K. P.; Goldmann, C.; Krellner, C.; Oberhoff, D.; Gundlach, D.

J.; Batlogg, B. Synth. Met. 2004, 146, 325-328.

(113) Ling, M.-M.; Erk, P.; Gomez, M.; Koenemann, M.; Locklin, J.; Bao, Z.

AdV. Mater. 2007, 19, 1123-1127.

9

J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007 15269

A R T I C L E S

Jones et al.

Figure 7. Bidirectional transfer plots for OFETs fabricated from films of (a) PDI-8 (T_d) 130 °C), (b) PDI-8Br₂(T_d) 70 °C), (c) PDI-8Cl₄(T_d) 110 °C),

(d) NDI-8CN (T_d) 130 °C), (e) PDI-8CN₂(T_d) 90 °C), and (f) NDI-8CN₂(T_d) 110 °C) OFETs operated under vacuum. The solid blue trace is the

positive V_gsweep, and the dotted red trace is the reverse V_gsweep.

semiconductor films systematically shifts more negative as the

molecule becomes thermodynamically easier to reduce, indicated

by a positive shift in reduction potential, as seen in Table 3

and Figure 9.

PDI-8Br₂, PDI-8Cl₄, and NDI-8CN films exhibit large response

variabilities when operated in ambient atmosphere, doubtless

in part due to the stress of the very high operating voltages and

also in part due to variations in the way(s) that ambient

atmosphere interacts with the films due to variations in film

microstructure/morphology. Bidirectional transfer plots for all

OFETs measured in air are shown in Figure 8. The hysteresis

for all these semiconductors is dramatic indicating substantial

charge trap densities, with the exception of PDI-8CN₂and NDI-

8CN₂. The hysteresis of PDI-8CN₂devices in air is negligible

while NDI-8CN₂has small hysteresis, similar to the OFET

response in vacuum.

Current on-off ratios are used to characterize the efficiency

with which the device can modulate current flow between the

S and D contacts. The I_on/I_off(V_g) +100 V/-100 V) ratio is

generally ∼10⁴-10⁵for PDI-8, PDI-8Cl₄, and NDI-8CN cases.

PDI-8Br₂, PDI-8CN₂, and NDI-8CN₂exhibit a lower I_on/I_offratio

of ∼10³. The relatively low I_on/I_offvalues are common for high

electron affinity materials,^21,31,54,78which often require taking

the I_on/I_offfor enhancement-depletion mode operation (V_g< 0

V/V_g> 0 V) to achieve values comparable to low electron

affinity materials.^31,54The primary reason for the low I_on/I_offis

the substantial current flow at V_g) 0 V. In this semiconductor

series, the I_dat V_g) 0 V is ∼10^-12A for PDI-8; ∼10^-11A for

PDI-8Br₂, PDI-8Cl₄, and NDI-8CN; ∼10^-7A for PDI-8CN₂;

and ∼10^-6A for NDI-8CN₂.

Comparing the ambient atmosphere mobilities of the different

semiconductors examined in this study, the PDI-8CN₂and NDI-

8CN₂OFET mobilities are approximately 2-3 orders of

magnitude greater than those of the other n-octyl materials. Upon

exposure to air, the average mobility decreases by a factor of

1600× for PDI-8, 3× for PDI-8Br₂, 80× for PDI-8Cl₄, and 17×

for NDI-8CN (Table 3). Contrastingly, changes to the PDI-8CN₂

carrier mobility in air are negligible within the standard

deviation, while NDI-8CN₂mobilities decrease by a factor 1.4×

upon air exposure. The relatively small decrease in mobility of

PDI-8Br₂reflects the modest film quality as evidenced by the

XRD and AFM data, and which results in poor semiconductor

performance under vacuum as well. Thus, the inclusion of

ambient-atmosphere-based traps does not substantially alter the

trap density magnitude relative to that under vacuum.

All TFT characterization measurements performed in vacuum

were also performed in ambient atmosphere. The measurements

reported here are for exposure to air for 24 h after measurement

under vacuum, although device electrical performance degrada-

tion for most of the present semiconductors is nearly instanta-

neous upon breaking vacuum. We will first consider the n-octyl

functionalized materials. The output and bidirectional transfer

plots for the semiconductors reveal relatively poor device

response characteristics, with the exception of PDI-8CN₂and

NDI-8CN₂(Figure 8). Since exposure to ambient atmosphere

greatly increases the trap density in the films, extremely large

gate biases are required to produce mobile charge carriers in

the non-cyanated semiconductors (Table 3). The modest quality

of the I-V data is likely due to a combination of high trap

density and physical degradation of the devices during operation

under such extreme biases. The devices fabricated with PDI-8,

The changes in V_thfor samples in air compared to devices

characterized under vacuum are very large for all n-octyl

semiconductors except PDI-8CN₂and NDI-8CN₂(Table 3). In

devices fabricated with films of PDI-8, PDI-8Br₂, PDI-8Cl₄,

and NDI-8CN, the V_thincreases relative to vacuum operation

by 92 ( 7, 35 ( 41, 58 ( 19, and 137 ( 9 V, respectively,

upon exposure to ambient atmosphere. The huge variation

associated with PDI-8Br₂is probably due to the poor film quality

as reported in the XRD, AFM, and Discussion sections. The

V_thdecreases slightly (<10 V) relative to vacuum operation in

(116) Pernstich, K. P.; Haas, S.; Oberhoff, D.; Goldmann, C.; Gundlach, D. J.;

Batlogg, B.; Rashid, A. N.; Schitter, G. J. Appl. Phys. 2004, 96, 6431-

6438.

9

15270 J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007

Arylene Diimide Semiconductors/Charge Carrier Stability

A R T I C L E S

Figure 8. Bidirectional transfer plots and output plots (insets) for OFETs fabricated from films of (a) PDI-8 (T_d) 130 °C), (b) PDI-8Br₂(T_d) 70 °C), (c)

PDI-8Cl₄(T_d) 110 °C), (d) NDI-8CN (T_d) 130 °C), (e) PDI-8CN₂(T_d) 130 °C), and (f) NDI-8CN₂(T_d) 110 °C) measured in ambient atmosphere,

demonstrating the stability of PDI-8CN₂. The gate bias (V_g) in the output plots is indicated directly above the corresponding I-V trace. The solid blue trace

of the bidirectional transfer plot is the positive V_gsweep, and the dashed red trace is the reverse V_gsweep.

Figure 9. V_thfor optimized OFETs of the indicated materials with standard deviations as a function of electrochemical reduction potential. The decrease

in V_th(vacuum) with positive E_red1shift demonstrates increasing ease of charge carrier generation with increased ease of reduction. The red and blue lines

are drawn as guides to the eye. Additionally, the shift in threshold voltage from vacuum to air operation demonstrates that materials with E_red1g ∼ -0.1

V exhibit the largest increases in trap densities due to their higher charge carrier energies.

PDI-8CN₂and NDI-8CN₂-based devices, which is likely due

to adventitious dopants common in high electron affinity

materials.^31,54,83The shift in threshold voltage from devices

operated in vacuum vs ambient can be used to estimate, via

established approaches,⁸⁵the increase in trap density (N_t). Based

on the shift in threshold voltage (Figure 9) for a given material,

N_tis found to be (6 ( 0.5) × 10¹³cm^-2for PDI-8, (2 ( 2) ×

10¹³cm^-2for PDI-8Br₂, (4 ( 1) × 10¹³cm^-2for PDI-8Cl₄,

and (9 ( 0.6) × 10¹³cm^-2for NDI-8CN. Similar to the other

electrical parameters, while I_on/I_offis affected adversely by ∼10²

for air exposure in PDI-8, PDI-8Cl₄, and NDI-8CN devices,

I_on/I_off(V_g) -100 V/100 V) for PDI-8CN₂(10³) and PDI-

8Br₂(10³) are unchanged and NDI-8CN₂exhibits a slightly

higher current modulation of 10⁴. The improvement of NDI-

8CN₂I_on/I_offupon air exposure likely reflects trapping of dopant-

induced charge carriers giving lower I_off. The lack of change in

9

J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007 15271

A R T I C L E S

Jones et al.

Figure 10. I_dat V_d) + 100 V when the indicated OFET is cycled in air between (a) V_g) -25 V and V_g) + 100 V for PDI-8CN₂, (b) V_g) + 25 V

and V_g) + 100 V for PDI-F, and (c) V_g) -25 V and V_g) 75 V for PDI-FCN₂.

the I_on/I_offfor PDI-8Br₂reflects the poor quality of the

semiconducting film leading to large trap densities in a vacuum;

therefore, there is little change upon exposure to air, as will be

discussed below.

unchanged in air vs vacuum with values of 10⁴and 10³,

respectively.

Durability Evaluation of N,N′-n-Octyl vs N,N′-1H,1H-

Perfluorobutyl Semiconductor-Based TFTs. The durability

of PDI-8CN₂, PDI-F, and PDI-FCN₂TFTs operating under

ambient conditions was investigated by cycling the devices in

air for 1000 cycles (∼1.5 h) immediately after breaking vacuum

(Figure 10). Cycling PDI-8CN₂devices between V_g) -25 V

and V_g) + 100 V reveals very stable source-drain on-currents

(I_on) with an ∼10% (2 × 10^-5A) increase. PDI-F devices cycled

between V_g) +25 V and V_g) 100 V exhibit exponential-like

decay in I_onof ∼92% (2 × 10^-5A) over the test. PDI-FCN₂

devices cycled between V_g) -25 and 75 V exhibit very stable

N,N′-1H,1H-Perfluorobutyl Substituted Perylene Diimide

OFETs. As a comparison to the n-octyl functionalized materials,

OFETs fabricated at T_d) 130 °C with films of the 1H,1H-

perfluorobutyl-substituted perylene diimide semiconductors were

evaluated in a vacuum and air. These analogues should exhibit

fluorocarbon barrier-induced behavior similar to the previously

reported air-stable naphthalene diimide materials, extensively

studied by Katz et al., and the recently reported fluorophenyl-

substituted materials of Bao and Furukawa et al.^{48,62,63,77,79,80}

The OFET data for the fluorinated materials are compiled in

Table 3 and Figures S6 and S7. Importantly, and consistent with

previously reported work, the 1H,1H-perfluorobutyl chain

functionalization extends air-stable OFET operation to all these

semiconductors, despite the small positive shift in reduction

potential of ∼ +0.13 V.

I

_onwith an ∼11% increase (4 × 10^-5A) over the measurement.

These data indicate that the environmental stability of the PDI-F

semiconductor is subject to substantial variation over time and

operating cycles. Previously reported and promising air-stable

n-type organic semiconductors containing fluorocarbon groups

have also been reported to be fragile, with fluoroalkyl-NDI

derivatives undergoing device failure after cycling in air, which

was ascribed to contact delamination,⁶²and F₁₆CuPc devices

undergoing a marked mobility erosion over ∼1000 h.⁵⁶In

contrast, PDI-8CN₂and NDI-8CN₂devices stored in a laboratory

ambient atmosphere over the course of 18 months exhibit

slightly positive (∼10 V) drifts in V_thand electron mobility ≈

0.10 cm²V^-1s^-1. Additionally, previously reported PDI-FCN₂

devices³¹measured in air after more than 3 years of storage in

ambient atmosphere exhibit transistor electron mobilities as high

The electron carrier mobility in a vacuum of the 1H,1H-

perfluorobutyl substituted semiconductors is significantly greater

than the n-octyl counterparts for the disubstituted PDIs, PDI-

FBr₂((3 ( 3) × 10^-5cm²V^-1s^-1) and PDI-FCN₂((0.27 (

0.09) cm²V^-1s^-1).¹¹⁷In the case of PDI-F and PDI-FCl₄, the

devices prepared with films of the fluoroalkyl material exhibit

lower mobilities relative to the n-octyl derivative of (1.4 ( 0.2)

× 10^-3cm²V^-1s^-1and (2.8 ( 0.1) × 10^-5cm²V^-1s^-1

,

respectively. The I_on/I_offratios of OFETs fabricated with the

R^F-substituted materials and measured in vacuum are 10⁴for

PDI-F, 10²for PDI-FBr₂, 10³for PDI-FCl₄, and 10³for PDI-

FCN₂. These numbers are generally lower than those for the

n-octyl counterparts, with the exception of PDI-8CN₂/PDI-FCN₂

which show comparable I_on/I_offvalues.

as 0.45 cm²V^-1s^-1

.

Inspection of the device performance parameters at cycle 0

and 1001 for the three materials reveals minimal change for

the two dicyanated derivatives. For the OFETs fabricated with

a PDI-8CN₂film, the electron mobility at cycle 0 and 1001

was 0.10 cm²V^-1s^-1, while, for the PDI-FCN₂device, the

carrier mobility was 0.2 cm²V^-1s^-1before and after the

cycling. For the OFETs fabricated with a PDI-F thin film, the

field-effect mobility decreases from 1.9 × 10^-2cm²V^-1s^-1

before cycling to 1.4 × 10^-2cm²V^-1s^-1after cycling. The

threshold voltage for the PDI-8CN₂TFT reveals a negligible

shift from -3 V before cycling to -2 V after cycling, while

the PDI-FCN₂TFT exhibits an increase from -26 V before

cycling to -19 V after cycling. The shifts of 1 V for PDI-

8CN₂and 7 V for PDI-FCN₂correspond to an ∼6 × 10¹⁰cm^-2

and ∼4 × 10¹¹cm^-2increase in trap density for both cases.

The V_thfor PDI-F devices, shifts from 47 V before cycling to

65 V after cycling, which corresponds to an increase in V_thof

18 V or in N_tof ∼1 × 10¹²cm^-2. The variation in current on-

off ratios before and after cycling is generally negligible in all

The electrical parameters for the fluorocarbon series measured

in air reveal that all four materials afford air-stable OFETs.

Within the estimated standard deviations, the change in average

electron mobility of the R^F-functionalized materials in air

relative to vacuum are very small or negligible. Slight or

statistically insignificant V_thshifts relative to vacuum operation

are observed in the fluoroalkyl substituted semiconductor films

upon exposure to ambient. Since these V_thshifts are insignificant,

the increase in trap density upon exposure to ambient atmo-

sphere vs vacuum are relatively small. I_on/I_offis lower by 1 order

of magnitude for PDI-FBr₂and PDI-FCl₄in air, which have

I_on/I_off) 10². I_on/I_offvalues for PDI-F and PDI-FCN₂remain

(117) Note that PDI-FCN₂-based OFETs can exhibit electron mobilities as high

as 0.6 cm²V^-1s^-1on hexamethyldisilazane (HMDS) treated substrates.

However, surface treatments significantly shift V_th, one of the significant

parameters in comparing materials within this study.

9

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Arylene Diimide Semiconductors/Charge Carrier Stability

A R T I C L E S

Figure 11. Molecular geometries of the four PDI cores investigated in this study. Structures a-d are derived from DFT calculations on N,N′-methyl

derivatives (R ) Me), while structures e-h are taken from previously published X-ray diffraction studies. The top projection is the face-on view of the

molecule, and the view directly below is the view along the molecular long axis. The torsional angles between the mean-square planes of the two perylene

core naphthalene units are shown.

three cases, with I_on/I_offfor the PDI-F, PDI-8CN₂, and PDI-

solution-phase aggregates of similarly core-substituted perylene

diimides, confirming the importance of these relationships.¹¹⁸

Core Substitution Effects on Molecular Geometries. From

the aforementioned crystal structures, it can be seen that core

substitution of perylene diimides leads to torsion/buckling of

the core as illustrated in Figure 11. The PDI-R perylene core is

rigorously planar when unsubstituted; however when substitution

is introduced at the 1, 6, 7, or 12 positions, the torsional angle

(TA) between the two naphthalene mean-square planes of the

PDI core increases to 5.2° for PDI-RCN₂, 24° for PDI-RBr₂,

and 37° for PDI-RCl₄. The trend in TA distortion is paralleled

by the trend in Stokes shift which increases as follows: PDI-R

(7 nm), PDI-RCN₂(10 nm), PDI-RBr₂(20 nm), and PDI-RCl₄

(29 nm), reflecting significant excited-state reorganization in

the more distorted PDI-RBr₂and PDI-RCl₄structures. Thus,

nonbonded repulsions arising from the core substituents in the

halogenated molecules leads to very large twisting at the central

perylene C₆core, and the relatively small cyano group induces

significantly less distortion. Interestingly, the minor distortion

of the cyano-substituted perylene core is accompanied by a

decrease of the C(aromatic)-C-N bond angle from 180° to

173°, which partially relieves steric interactions with the

proximate C(aromatic)-H group. Similar C-C-N angle com-

pressions have been previously observed in ortho-substituted

benzonitriles.¹¹⁹

FCN₂having values of 10⁴, 10³, and 10, respectively.

Discussion

Crystal Structure Analysis. Diffraction-quality single crys-

tals of the specific arylene diimide derivatives investigated in

this study have proven problematic to grow. However, the

crystal structure of every perylene diimide core examined in

this study has been previously reported,^{31,87,88,104,111}and crystal-

lographic data for derivatives having slightly different N,N′-

substituents (R) were therefore employed as reasonable core

models for PDI-8, PDI-8Br₂, PDI-8Cl₄, and PDI-8CN₂(Figure

11). Unfortunately, there are no known crystal structures of core-

cyanated naphthalene diimide materials, so the present discus-

sion is restricted to perylene derivatives. In the crystal structures

of the core-substituted PDIs, the differences in packing motif

can be largely attributed to the introduction of intramolecular

nonbonded repulsions and the torsional distortions in the PDI

core which result. In the DFT-level calculations discussed

earlier, the optimized molecular geometries of the cores are very

similar to the core geometries found in the experimental crystal

structures (Figure 11). Therefore, comparisons between the

crystal packing of the present PDIs and those in the published

crystal structures should be informative. Note that our inter-

pretation of perylene core-substitution effects on solid-state

packing is similar to the interpretation of PDI packing in

(118) Chen, Z.; Baumeister, U.; Tschierski, C.; Wu¨rthner, F. Chem.sEur. J.

2007, 13, 450-465.

(119) Britton, D.; Cramer, C. J. Acta Crystallogr., Sect. C 2006, C62, o307-

o309.

9

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A R T I C L E S

Jones et al.

(Figure 12) and result in significantly altered packing relative

to PDI-R and PDI-RCN₂(Figure 13a and 13d). In the case of

PDI-RBr₂(Figures 13b), the molecular packing scheme is

uniquely complicated due to less extensive segregation of the

aromatic and alkyl sections of the semiconductor. Additionally,

the location of the Br substituents on the same side of the PDI

plane leads to dimer-like packing where the outlying bromine

atoms allow close packing of an adjacent perylene unit on only

one side. While PDI-RCl₄forms a more continuous network

(Figure 11c), the π-π overlap between nearest neighbors is

only between naphthalenic subunits rather than involving the

entire molecular π-system.

In addition to alterations in the relative crystallographic

molecular orientations, core substitution results in dramatic

differences in intermolecular contacts. In the case of PDI-RCN₂

and PDI-R, the minimum interplanar spacing is ∼3.4 Å for both.

The large PDI-RBr₂bromine substituents induce the greatest

interplanar spacing of 3.64 Å, and the smaller PDI-RCl₄chlorine

substituents lead to a closer packing (3.51 Å), despite a greater

central C₆core torsional angle (Figure 12). From these observa-

tions, it is evident that the design of core-substituted PDI-based

semiconductors that maintain the advantageous crystal packing

properties of the parent PDI molecule requires that the substit-

uents be sterically unencumbering to minimize core torsion and

consequently enhance intermolecular π overlap.

Carrier Mobilities of Substitued Arylene Diimides in a

Vacuum. The differences in carrier mobilities in vacuum

between the six n-octyl subject semiconductors can be explained

primarily on the basis of molecular packing and film micro-

structural trends. PDI-8Br₂is a particularly poor OFET material.

The crystal structure suggests that the π-π orbital overlap

between adjacent PDI-RBr₂cores is minimal because of the

sizable intermolecular distances and the discontinuity of the

“dimer”-based packing motif which should depress carrier

mobility (Figure 13).^29,30The weakness of the orbital overlap

in the solid is also consistent with PDI-8Br₂having the lowest

melting point of the perylene-based semiconductors (Figure S2).

The film microstructure as assessed by XRD shows that the

crystalline quality/order of the PDI-8Br₂film is unfavorable for

efficient charge transport (Figure 2), as evidenced by the absence

of higher-order diffraction peaks which would indicate long-

range order and the relatively large fwhm values in the rocking

curve, signifying poor out-of-plane texturing (Table 2 and Figure

S3). Additionally, the film morphology assessed by AFM does

not reveal the distinctive crystalline features associated with the

other n-octyl derivative-based polycrystalline thin films exam-

ined in this study (Figure 4). The poor film quality and

discontinuous molecular packing motif of this material are

arguably the primary reasons for the substantially lower mobility

observed in PDI-8Br₂-based OFETs. Additionally, the large

degree of decomposition (∼20% residual mass) in the PDI-8Br₂

reduced pressure TGA (Figure S2) suggests that the OFET films

are likely to contain decomposition products which may lead

to trap states. To simplify subsequent data interpretation, PDI-

8Br₂will not be discussed further because the poor quality of

the semiconductor growth leads to an erratic OFET response

and likely large trap densities (N_t) even while operating under

vacuum.

Figure 12. Molecular structure of PDI skeleton depicting atom numbering

schemes and angles discussed. The long-axis (red LA) runs from N to N.

The torsional angle (TA) between naphthalene units is taken from the four

bay region carbons indicated in blue. The interior angles (magenta IA) and

outer angles (green OA) across the perylene unit are also depicted.

Additional distortions of the naphthalene subunits in the

substituted PDIs are also observed, in contrast to the planar

geometry of the unsubstituted PDI-R skeleton (Figure 11a and

11e). In an undistorted perylene diimide, the naphthalene

subunits should be planar. However, when the PDI skeleton is

substituted, the naphthalene units fold slightly. We will define

these folding angles in terms of the inner angles (IA), C₁-

C_â-C₆and C₇-C_â-C₁₂, while the outer angles (OA) are

C₂-C_R-C₅and C₈-C_R-C₁₁, Figure 12. In the case of PDI-

RBr₂, the skeleton increasingly twists from the imide group

toward the central C₆-unit with an OA distortion of 1.8° and an

IA distortion of 3.2°. While the twist around the central C₆-

unit of the PDI-RCl₄core is severe, as discussed above, the

distortion of the naphthalene rings is minimal with IA and OA

distortions of 1.2°. Conversely, for PDI-RCN₂, the torsion

around the central C₆-unit is minor, but the distortion of the

naphthalene units is the most severe in this case, with an IA

torsion of 4.9° and an OA torsion of 3.7°. Thus, the degree of

IA and OA folding of the naphthalene subunits is inversely

proportional to the torsional strain at the perylene central C₆

core.

The interplay between the substituent-derived steric-based

strain at the central C₆core vs that seen in the naphthalene

subunits has interesting consequences for positioning of the

substituents relative to the molecular planes. In PDI-RBr₂,

because the naphthalene units remain relatively planar and the

distortion is primarily associated with the central C₆-unit, the

bromo substituents are displaced to the same side of the PDI

plane, eliminating the molecular inversion center. Similarly in

PDI-RCl₄, the chloro-substituents at positions 1,7 and 6,12 are

displaced to the same side of the PDI plane. However, the

symmetry of the tetra-substituted bay region does not lead to

the asymmetry observed in PDI-RBr₂. For PDI-RCN₂, the

relatively large distortion of the naphthalene units versus the

central C₆core results in the cyano substituents being displaced

to opposite sides of the perylene plane, thus maintaining the

molecular inversion center.

Core Substitution Effects on Crystal Packing. Upon

examining the packing motifs of the four published PDI

derivative crystal structures, the similarity between the unsub-

stituted and cyanated structures stands out (Figure 13). In both

cases, the molecules are relatively planar with a slip-stacked

packing arrangement, where the long axes (LAs) of the cores,

defined by the N(imide)-N(imide) vector, are nearly in register

relative to the LA of the adjacent molecule. Previous compu-

tational studies from this laboratory have already indicated that

this packing motif is efficacious for efficient charge transport.²⁹

In the case of PDI-RBr₂and PDI-RCl₄, the highly distorted

perylene skeletons reflect severe substituent-originated non-

bonded repulsions in the bay-region (1, 6, 7, 12) positions

OFET carrier mobilities in PDI-8Cl₄and NDI-8CN are also

modest. The crystal structure of PDI-RCl₄suggests that in-

9

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Arylene Diimide Semiconductors/Charge Carrier Stability

A R T I C L E S

26

Figure 13. Crystal structures of (a) PDI-R,⁷⁸(b) PDI-RBr₂,⁶⁶(c) PDI-RCl₄,⁶⁵and (d) PDI-RCN₂showing the packing relationships among the four

molecules. The molecular structure to the left of each packing diagram depicts the N,N′ substituents for that particular crystal structure, which have been

deleted for ease of viewing.

tramolecular π-orbital overlap is less than that in PDI-8 and

PDI-8CN₂because the intermolecular spacing is greater and

molecular π-π overlap is only between naphthalenic monoim-

ide subunits rather than involving the entire PDI π-system

(Figure 13). Additionally, the melting point of PDI-8Cl₄is

substantially lower than those of PDI-8 and PDI-8CN₂(Figure

S2), suggesting weaker intermolecular interactions. While PDI-

8Cl₄thin film morphology (Figure 4) and microstructure

(Figures 2 and S3, and Table 2) are comparable to those of

PDI-8 and PDI-8CN₂films, the presence of two distinct phases/

crystallographic orientations may depress mobility by creating

trap states at the interfaces between phases/orientations.¹²⁰Since

crystal structure data on NDI-8CN has yet to be obtained,

assumptions based on thin film XRD data are most relevant to

explaining the low electron mobilities. The striking similarity

in thin film XRD data between low mobility NDI-8CN and high

mobility NDI-8CN₂suggests that the difference in mobility is

likely due to in-plane ordering. Assuming that the crystal

structure of NDI-8CN packs in a layered motif with segregated

aromatic and aliphatic regions, as suggested in the thin film

XRD (Figure 2), the molecular overlap between NDI-8CN units

is likely rather different from that between NDI-8CN₂units.

microstructures as deduced by AFM (Figure 4) and XRD (Figure

2) suggest that, all other things being equal, the carrier mobilities

should be comparable. However, the slightly smaller mobility

of PDI-8CN₂relative to PDI-8 may be due in part to the slight

torsion of the PDI-8CN₂core (Figure 12), resulting in weaker

intermolecular interactions, consistent with the lower melting

point of PDI-8CN₂relative to PDI-8 (Figure S2). Additionally,

the slight decomposition of PDI-8CN₂during film deposition

may introduce contaminant-derived traps in the film (Figure S2).

The DFT level electronic structure calculations performed

here on NDI-8CN₂suggest that dicyanation of the NDI core

does not lead to the core torsion observed in the perylene case

(Figure 14), most likely because the cyano functionality in the

NDI case does not have a proximate hydrogen atom with which

to interact. Thus it seems reasonable, given this lack of core

distortion upon cyanation and the similar electron mobility

relative to the uncyanated derivative, that the crystal packing

in NDI-8 and NDI-8CN₂is very similar.

Within the 1H,1H-perfluorobutyl series, the core-halogenated

materials still exhibit substantially lower mobilities relative to

the unsubstituted parent. This can be attributed in part to the

crystal packing effects discussed above (Figure 13). However,

the presence of more than one family of XRD diffraction peaks

for these films suggests a less favorable microstructure vs the

high mobility materials (Figure 3).

The electron mobilities of PDI-8, PDI-8CN₂, and NDI-8CN₂

thin films measured in a vacuum are similar. The correlation

between the high mobility of N-alkyl PDI materials and the

excellent crystal packing/film-forming properties has been

documented previously.^43,71,85The similarities in the slip-stacked

crystal packing motifs (Figure 13) and film morphologies/

The AFM images also exhibit large surface features protrud-

ing from the PDI-FBr₂film (Figure 5), similar to the n-octyl

analogue that has similarly low carrier mobility (Figure 4). The

PDI-FCl₄surface morphology is unique compared to the other

materials, which is consistent with this being the only material

(120) Dimitrakopoulos, C. D.; Brown, A. R.; Pomp, A. J. Appl. Phys. 1996,

80, 2501-2508.

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A R T I C L E S

Jones et al.

values as a function of electron affinity (Figure 9). However,

the relative degradation in OFET mobility measured in air

relative to vacuum operation does generally parallel trends in

electron affinity for the n-octyl materials (Table 3). The decrease

in mobility by a factor of 1600× for PDI-8, 80× for PDI-8Cl₄,

and 17× for NDI-8CN demonstrates the extreme sensitivity of

these materials to ambient-based charge traps, as well as

decreasing sensitivity with increasing electron affinity. The

materials with highest electron affinity, PDI-8CN₂and NDI-

8CN₂, show far less degradation in carrier mobility measured

in air vs vacuum. PDI-8CN₂mobility degradation is minimal,

and the factor of 1.4× OFET mobility decrease for NDI-8CN₂

is slight. For the air-unstable materials PDI-8, PDI-8Cl₄, and

NDI-8CN, the decrease in OFET mobility is nearly instanta-

neous upon breaking vacuum; however, after 18 months of air

exposure the air-stable devices fabricated with PDI-8CN₂and

NDI-8CN₂exhibit electron mobilities ∼0.10 cm²V^-1s^-1. The

long-term stability of materials with E_red1positive of -0.06 V

vs SCE suggests that the charge carrier stability is largely

responsible for the resistance of the electrons to trapping.

Figure 14. Calculated molecular geometries for N,N′-methyl derivatives

NDI-RCN (a and c) and NDI-RCN₂(b and d). The face-on views of (a)

NDI-RCN and (b) NDI-RCN₂show the distortion of the C(aromatic)-CN

bond angle to ∼172°, while the view along the N-N′ axes in (c) and (d)

reveal the rigorously planar aromatic core.

not exhibiting a first-order diffraction feature in the XRD having

the molecular long axis dimension (Figure 3). Interestingly, in

the R^F-substituted case, the dicyanated PDI-FCN₂derivative

exhibits much greater FET mobility than the unsubstituted PDI-

F, in contrast to the n-octyl case which displays the opposite

trend (Table 3). The XRD of PDI-F only exhibits a single family

of diffraction peaks as in the high mobility n-octyl materials;

however, the AFM shows large protrusions from the film, similar

to the low mobility PDI-Br₂-based materials. PDI-FCN₂exhibits

an unusually large mobility for a material with multiple families

of diffraction peaks; however the AFM evidences a much

smoother and homogeneous film morphology. The high mobility

of PDI-FCN₂can likely be explained by a favorable orientation

for charge transport as indicated by the ∼60° tilt angle from

the XRD (Table 2), which is significantly more upright than

the other semiconductors in this study.

For ambient OFET operation, V_this very large for all

semiconductors with E_red1more positive than -0.06 V vs SCE

(Figure 9 and Table 3). In ambient, the I_on/I_offratios are roughly

the same for PDI-8, PDI-8Cl₄, and PDI-8CN₂OFETs; however

the decrease in this ratio for PDI-8 and PDI-8Cl₄relative to

their vacuum performance is due to the lower I_on(Figures 7

and 8) which reflects the trapping tendencies of ambient species.

Moreover, the molecular electron affinities of these molecules

do affect the magnitudes of the change in their vacuum vs

ambient response properties. The electron mobility of the lowest

electron affinity material PDI-8 decreases by a factor of 1600×

in air relative to vacuum, while PDI-8Cl₄with a higher electron

affinity decreases by a factor of 80×, and NDI-8CN with a still

higher electron affinity only decreases by a factor of 17×. For

the very high electron affinity materials PDI-8CN₂and NDI-

8CN₂, the decrease in electron mobility of devices operated in

air is small or negligible relative to the standard deviation.

Furthermore, the shift in V_thupon operation in ambient is ∼100

V for PDI-8 and NDI-8CN, ∼70 V for PDI-8Cl₄, and negligible

for PDI-8CN₂and NDI-8CN₂, the trend reflecting the decrease

in trap state density for dicyanated derivatives (Vide infra) and

paralleling the trend in molecular electron affinity (Figure 9).

This decrease in threshold voltage shift with increasing electron

affinity suggests that the charge carriers become less susceptible

to higher energy trap states. Interestingly, the average V_thshifts

by ∼ -20 V for the most electron-deficient semiconductor, NDI-

8CN₂, suggesting that doping is significant in this material.

When comparing devices operated under ambient conditions,

the electron carriers in semiconductors with lower LUMO

energies (higher electron affinities) should be more stable and

less affected by atmospheric species. This explains the general

decrease in interfacial trap density with increasing N,N′-n-octyl

PDI electron affinity as follows: PDI-8 (∼6 × 10¹³cm^-2), PDI-

8Cl₄(∼4 × 10¹³cm^-2), NDI-8CN (∼9 × 10¹³cm^-2), PDI-

8CN₂(immeasurably low), and NDI-8CN₂(immeasurably low).

Thus, the negligible vacuum to ambient trap density increases

for PDI-8CN₂and NDI-8CN₂films explain the ambient OFET

stability. In contrast, the trap density increases dramatically (to

>10¹³cm^-2) for PDI-8Cl₄, NDI-8CN, and PDI-8 upon exposure

of the films to ambient.

Effects of Molecular Electron Affinity on OFET Perfor-

mance in Vacuum. Given that the packing and film-forming

properties of the semiconductors investigated in this study vary

greatly, analysis of electron affinity effects on OFET response,

although necessarily qualitative, provides useful guidelines for

understanding electrical properties and ambient-stable n-channel

materials. For OFETs characterized under vacuum, the observed

effects on the present arylene diimide OFET response properties

of electron affinity as expressed by solution-phase reduction

potentials are primarily restricted to V_thand I_on/I_off, which

generally decrease with increasing electron affinity for all of

the present materials (Table 3 and Figure 9). For V_th, the

decrease reflects the relative ease of creating mobile n-type

carriers in the material, which correlates with mobile carrier

creation at lower gate bias. Moreover, the depression in I_on/I_off

with increasing electron affinity is a reflection of the larger

source-drain current observed at V_g) 0 V indicating high I_off

values. The greater carrier density at V_g) 0 V for the larger

electron affinity semiconductors likely reflects greater suscep-

tibility to charge carrier-generating impurities (doping).

Effects of Ambient Environment on OFET Performance.

When operating the present OFETs in ambient atmosphere, the

electrical properties of the air-unstable materials exhibit large

device-to-device variations (Table 3), presumably also reflecting

minor differences in film microstructure/morphology (Figures

2 and 4) and degrees of O₂intrusion. Direct comparison of the

electrical properties still indicates no obvious trend in mobility

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Arylene Diimide Semiconductors/Charge Carrier Stability

A R T I C L E S

In comparison to the above results, all of the present 1H,1H-

perfluorobutyl-substituted semiconductor thin films afford air-

stable OFETs (Table 3). The electron mobility degradation upon

exposure of these materials to ambient atmosphere is minimal,

similar to the cases of PDI-8CN₂and NDI-8CN₂. However,

PDI-F and PDI-FBr₂have E_red1significantly negative of -0.06

V vs SCE (Table 1), which implicates a fluorocarbon barrier

mechanism discussed further below. Additionally, the fact that

the XRD of the fluorocarbon materials exhibits multiple families

of reflections (Figure 3) and the AFM shows substantially

different film morphologies than those of the fluorine-free

n-octyl analogues (Figures 4 and 5) suggests that the thin film

morphology/microstructure in the 1H,1H-perfluorobutyl deriva-

tives changes significantly in comparison to that of the n-octyl

materials, consistent with the model proposed by Katz et al.

However in the case of PDI-F, the device cycling in ambient

atmosphere demonstrates the kinetic sensitivity of I_dto degrada-

tion processes (Figure 10), which is also reflected in the

substantial electron mobility decrease between cycles 0 and

1001. The sensitivity of PDI-F over repeated operation suggests

that the crystal packing related barrier to atmospheric penetration

(Figure 1) allows atmosphere-based charge trap inclusion over

time/device operational stress, which leads to charge carrier

trapping. The increase in trap density on repeated device cycling

is further confirmed by the substantial positive shift in V_thover

the course of the measurement. In contrast, the significant

stabilization of electrons in the low LUMOs of PDI-8CN₂

relative to PDI-F leads to very stable device I_dand electron

mobility despite the duration of atmospheric exposure/device

operational stress, and the negligible electron trap density

increase in these materials is reflected in the stable V_th. Further

confirmation that the charge carrier stabilization in films of the

high electron affinity dicyanated materials is responsible for the

air-stability is found in the device cycling characteristics of PDI-

FCN₂, which remain stable in I_d, carrier mobility, and V_thdespite

the use of the same 1H,1H-perfluorobutyl substituents as PDI-

F.

very similar film microstructure/morphology to air-unstable

NDI-8CN devices (Figures 2 and 4), suggests that film

characteristics are not responsible for the environmental stability

of the NDI-8CN₂. In further support of the contention that the

dicyanated semiconductors do not owe their air stability to a

crystal packing barrier, the bulk of the cyclohexyl group at the

N,N′-positions of the air-stable semiconductor PDI-CN₂prevents

close packing (Chart 1), which is consistent with the somewhat

lower maximum mobility of 0.1 cm²V^-1s^-1for this material.³¹

Since the energetic stabilization of the electrons in the low

lying LUMOs of PDI-8CN₂and NDI-8CN₂materials is likely

the major contributing factor to the air stability of their OFETs,

it follows that a reduction potential more positive that ∼ -0.1

V vs SCE correlates with resistance to atmosphere-based

electron traps in arylene diimide thin films (Figure 9). In

comparison to the commonly studied materials discussed in the

introduction (Charts 1 and 2), the electrochemical air-stability

window defined by this study appears to pertain for other

materials as well. Specifically, air-stable perhalogenated ph-

thalocyanines F₁₆CuPc and Cl₁₆CuPc exhibit E_red1≈ -0.1 V

vs SCE. In OFETs fabricated with thin films of arylene diimides

NDI-8CN and PDI-8Cl₄having reduction potentials slightly

more negative than -0.1 V vs SCE, the electrons are clearly

vulnerable to charge trapping by ambient-atmosphere-based

species (Table 3, and Figures 8 and 9). Similarly, materials such

as DCMT with slightly more negative E_red1values of ∼ -0.2

V vs SCE exhibit ambient-sensitivity (Chart 2).

For 1H,1H-perfluorobutyl substituted semiconductors, the

reduction potential vs SCE of PDI-F (-0.33 V) and PDI-FBr₂

(-0.24 V) clearly places the LUMO/charge carrier energetics

within the regime of air-unstable n-octyl derivatives (Table 1

and Figure 9). Thus, it appears that the fluorocarbon-based

packing mechanism proposed by Katz et al. for naphthalene

diimides^48,62,63,77is operative in perylene diimides as well. The

case of PDI-FCl₄is borderline since the reduction potential lies

at -0.13 V vs SCE, which corresponds to the region of the

energetic window where the transition from air-unstable to air-

stable in the n-octyl substituted materials is observed. The

fluorinated material PDI-FCN₂(E_red1) +0.03 vs SCE) probably

derives its air stability from a combination of a fluorocarbon-

based mechanism and orbital/carrier energetic stabilization.

The V_thshifts and thus trap state density increases in these

materials on going from vacuum to ambient OFET operation

are statistically insignificant for all four N,N′-fluorocarbon

substituted materials, despite having E_red1values more negative

than those of air-unstable NDI-8CN and PDI-8Cl₄(Table 3).

I_on/I_offratios for PDI-F and PDI-FCN₂devices in air remain

the same relative to the values in vacuum, consistent with a

negligible increase in atmospheric-based traps. The slight

decrease for PDI-FBr₂and PDI-FCl₄could reflect a slight

increase in trap state density.

If the model proposed earlier for n-type air stability is indeed

operative,⁴²based on the results of the n-octyl-substituted

arylene diimides, the overpotential for charge carrier trapping

by O₂in ambient air for arylene diimide semiconductor films

should be approximately 0.6 V. Additionally, the air stability

of PDI-F, PDI-FBr₂, and PDI-FCl₄OFETs strongly suggests

that the previously proposed crystal-packing-based air-stability

mechanism is related to the presence of fluorocarbon chains,

as opposed to the minimal electron-withdrawing effects on the

arylene imide semiconductors.

Analysis of Ambient Stability Mechanisms. The stability

of PDI-8CN₂and NDI-8CN₂OFETs in ambient, compared with

the other air-unstable materials investigated here, suggests that

the origin of the OFET air stability is largely due to the energetic

stabilization of electrons in the extremely low-lying LUMOs

of the semiconductor. Since PDI-8CN₂has slightly weaker

intermolecular cohesive interactions than PDI-8, as evidenced

by the slightly lower carrier mobility in a vacuum (Table 3)

and the melting point (Table S2), and similar packing motif/

film microstructure properties (Figures 13 and 2), the hypothesis

that a crystal packing-based barrier is primarily responsible for

the charge carrier air-stability seems unlikely in this case.

Similarly, the case of air-stable NDI-8CN₂devices, exhibiting

While it appears that energetic stabilization of the electron

carriers plays a major role in the ambient OFET stability of the

n-type PDI-CN₂and NDI-8CN₂semiconductors, the role of

other factors which are not directly and quantitatively accessible

with the present measurements cannot be discounted. The

present evaluation of carrier energetics in these films is

extrapolated from solution phase measurements on solvated

single molecules. In solution, we previously showed that

cyanated-PDI radical anions are stable in partially aerated

9

J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007 15277

A R T I C L E S

Jones et al.

solutions; however, we also showed that deaeration enhances

the long-term stability of the solvated radical anions.⁶⁴This

observation suggests that the thin film morphology/microstruc-

ture, in addition to the aforementioned film energetics, contrib-

utes to electron stabilization. Additionally, the large device-to-

device variation in FET electrical parameters, µ and V_th, for

the air-unstable devices could be indicative of subtle thin film

microstructure/morphology/crystal packing effects that modulate

atmospheric charge carrier trap penetration into the semiconduc-

tor thin films. Therefore, design criteria for air-stable n-channel

OFET materials should account for both carrier energetics and

solid-state/thin film packing and barrier properties.

Further studies on additional materials families should prove

informative in determining how broadly this overpotential can

be generalized to thin film organic electronic materials. Com-

parison of OFETs with semiconductor air-stability based on

electron energetic stabilization vs a fluorocarbon-based crystal

packing barrier leads us to conclude that semiconductors

providing energetic carrier stabilization lead to long-term

resistance to OFET electrical parameter degradation, while

semiconductors owing air stability to ambient-based trap exclu-

sion show significantly more OFET parameter erosion over air

exposure duration/device stressing.

Acknowledgment. We thank DARPA (HR0011-05-1-0012),

AFOSR (STTR FA9550-05-0167), and ONR (grants N00014-

05-1-0021 and N00014-05-1-0766) for support of this research,

and the NSF-MRSEC program through the Northwestern

Materials Research Center (Grant DMR-0520513) for access

to characterization facilities, Dr. Y. Zhang for assistance with

NMR experiments, and Dr. M. J. Ahrens for helpful discussions.

Conclusions

The results of this study indicate that core substitution of

arylene diimide semiconductors induces large variations in

electron mobility due to geometric distortions of the arylene

skeleton and consequent weakening of intermolecular π-π

overlap. Therefore, in order to achieve substantial OFET

mobilities with core-functionalized arylene diimides, the sub-

stituents must be chosen to minimize such distortions and their

effects on close molecular packing. Additionally, we also

estimate for the first time that the overpotential to reaction of

PDI and NDI n-type charge carriers with ambient O₂in

aryleneimide films is ∼0.6 V, meaning that air-stable OFET

materials should be accessible with properly designed arylene

cores having the correct electron-withdrawing substituents.

Supporting Information Available: Experimental details

including synthesis, absorption/emission spectra, XRD of all

films, AFM of all films, and logarithmic/square root transfer

data. This material is available free of charge via the Internet

at http://pubs.acs.org.

JA075242E

9

15278 J. AM. CHEM. SOC. VOL. 129, NO. 49, 2007

Article Doi

Tuning orbital energetics in arylene diimide semiconductors. Materials design for ambient stability of n-type charge transport

DOI: 10.1021/ja075242e

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Article abstract of DOI:10.1021/ja075242e

Full text of DOI:10.1021/ja075242e

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