Xiaowei Zhan et al- Rylene and Related Diimides for Organic Electronics

8/3/2019 Xiaowei Zhan et al- Rylene and Related Diimides for Organic Electronics

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2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv.Mater.2011, 23, 268284wileyonlinelibrary.com268

Xiaowei Zhan,* Antonio Facchetti,* Stephen Barlow,* Tobin J. Marks,*

Mark A. Ratner,* Michael R. Wasielewski,* and Seth R. Marder*

Rylene and Related Diimides for Organic Electronics

Prof. X. ZhanBeijing National Laboratory for Molecular SciencesInstitute of ChemistryChinese Academy of Sciences, Beijing 100190, ChinaE-mail: [email protected]

Prof. A. Facchetti, Prof. T. J. Marks, Prof. M. A. Ratner,Prof. M. R. WasielewskiDepartment of ChemistryMaterials Research Center, and Argonne-Northwestern Solar EnergyResearch CenterNorthwestern UniversityEvanston, Illinois 60208-3113, USAE-mail: [email protected]; [email protected];[email protected]; [email protected]

Prof. A. FacchettiPolyera CorporationSkokie, Illinois 60077, USA

Dr. S. Barlow, Prof. S. R. MarderSchool of Chemistry and BiochemistryCenter for Organic Photonics and ElectronicsGeorgia Institute of TechnologyAtlanta, Georgia 30332-0400, USAE-mail: [email protected];[email protected]

DOI: 10.1002/adma.201001402

1. Introduction

Organic charge-transporting materials are -conjugated mole-cular or polymeric compounds in which charge carriers migrateunder the influence of an electric field. These materials canbe classified as hole- or electron-transport (HT or ET) mate-rials according to whether the majority charge carriers, undera given set of conditions, arise from removal of electrons fromthe manifold of filled molecular orbitals or from the addition of

electrons to empty orbitals, respectively.[1]Organic HT and ET materials differ fromclassical inorganic p- and n-type semicon-ductors in that they are generally undoped,and so that very few charge carriers are typ-ically present except under an applied field,in which case carriers can be injected fromelectrodes, from other proximate organicmaterials, or are generated via photoexci-tation. Charge transport can be describedas a series of successive electron-transferreactions between neutral and chargedmolecular or polymeric repeat units. Inthe hopping transport regime this processinvolves essentially localized radical cat-ions (HT) or anions (ET) and the corre-

sponding neutral species, while the orbitals of a -conjugatedpolymer chain can facilitate intrachain electron transfer in thesuperexchange or coherent tunneling regime.[2] The tendencyof the holes (electrons) to migrate under the influence of a fieldcan be described by the hole (electron) mobility, , of the mate-rial; this has units of velocity per unit field and is, in general,dependent on both the electric field and temperature. An addi-

tional class of materials, ambipolar materials, have similar holeand electron mobilities and can act as either HT or ET mate-rials, depending on the dominant injection processes occurringunder the experimental conditions of interest.

In general, development of high-performance (environmen-tally stable, high-mobility) organic ET materials has laggedbehind that of HT materials despite their importance for fabri-cating organic photovoltaic (OPV) cells and n-channel organicfield-effect transistors (OFETs), which are particularly valuableas components of organic complementary logic circuits, whichrequire both p- and n-channel transistors.[3,4] To achieve accept-able performance, ET materials must have: i) high electronaffinity (ideally greater than 3 eV, but not exceeding 5 eV) tofacilitate injection from contacting electrodes in OFETs or to facil-itate exciton separation in conjunction with typical HT materialsfor OPV applications; ii) good intermolecular electronic orbitaloverlap to facilitate high mobility; and iii) good air stability, ide-ally both as neutral and radical anion materials and, as discussedin more detail below, under device operating conditions.[1] Forgeneral reviews on ET materials, see references [1,3] and [4].

The criteria for useful ET materials described above can oftenbe met by appending strong electron-withdrawing substituents,such as fluoro, cyano, or acyl, to -conjugated cores such asacenes and oligothiophenes, which, in the absence of these sub-stituents, exhibit HT properties. Other classes of ET materials,such as the fullerenes, which have been widely studied for a

Organic electron-transporting materials are essential for the fabrication of

organic p-n junctions, photovoltaic cells, n-channel field-effect transistors,

and complementary logic circuits. Rylene diimides are a robust, versatile class

of polycyclic aromatic electron-transport materials with excellent thermal and

oxidative stability, high electron affinities, and, in many cases, high electron

mobilities; they are, therefore, promising candidates for a variety of organic

electronics applications. In this review, recent developments in the area of

high-electron-mobility diimides based on rylenes and related aromatic cores,particularly perylene- and naphthalene-diimide-based small molecules and

polymers, for application in high-performance organic field-effect transistors

and photovoltaic cells are summarized and analyzed.


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2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv.Mater.2011, 23, 268284 wileyonlinelibrary.com 269

Rational strategies for enhancing rylene ambient sta-bility must prevent the trapping species from reaching thecharge-transporting film area and/or involve the design ofmolecules or polymers in which the mobile electrons are

variety of applications, have inherently moderate to high elec-tron affinities in the absence of electron-withdrawing substitu-ents. Another extensively studied materials class, which is thefocus of this review, is that of rylene tetracarboxylic diimides.Rylenes are hydrocarbon families that can be regarded as naph-thalene oligomers, with bonds between the 1 and 1 positions

and between the 8 and 8 positions of adjacent naphthaleneunits, i.e., they are oligo(peri-naphthalene)s. The rylene tetracar-boxylic diimides considered here all bear two six-membereddicarboxylic imide rings fused to the terminal naphthaleneunits; the three simplest rylene diimide systems, naphthalene-1,8:4,5-tetracarboxylic diimide (2, NDIs), perylene-3,4:9,10-tet-racarboxylic diimide (4, PDIs), and terrylene-3,4:11,12-tetracar-boxylic diimide (5), are shown in Figure 1. Interest in rylenediimides and other aromatic diimides, such as those based onbenzene and anthracene (1 and 3, respectively, Figure 3), stemsfrom early observations of ET behavior and the ability to tunemolecular electronic properties by well-established organicchemistry, through either variation of substituents on the imidenitrogen atoms or on the rylene skeleton. Rylene diimides canexhibit relatively high electron affinities, high electron mobili-ties, and excellent chemical, thermal, and photochemical stabil-ities. These molecules and their derivatives have been used notonly as building blocks for electronic and optoelectronic devicessuch as organic light-emitting diodes,[5] dye lasers,[6] opticalswitches,[7] and photodetectors,[8] but also as electron acceptorsfor studying photoinduced energy- and electron-transfer proc-esses.[9] Several accounts have summarized the synthesis andorganization for various applications of PDI and NDI mate-rials.[1014] In the present review, we focus on the most recentdevelopments on high-mobility rylene-diimide-based smallmolecules and polymers for high-performance OFETs andOPVs (see Figure 2 for device structures and operation prin-

ciples). The high electron affinities of rylene diimides makesthem attractive candidates for ET materials in OFET and OPVdevices; however, as will be seen in the following sections, chal-lenges in achieving optimum ambient device operation remain.

2. Empirical Relationships Between ElectronicStructure and Device Operation UnderAmbient Conditions

Before discussing specific materials in detail, it is important tostress one generic issue that is important for all ET materials:their air stability. In most cases, air instability is not due to deg-radation of intrinsically chemically unstable neutral materials,but arises from the vulnerability of the corresponding radicalanions to reaction with atmospheric H2O or O2, an issue firstdiscussed by de Leeuw et al.[15] This vulnerability of the chargecarriers to trapping under ambient conditions, which can fre-quently be precluded to some degree by operation under inertconditions (e.g., vacuum or encapsulation),[16] can seriouslyimpair ET. Given that the ultimate promise of organic semi-conductors is in inexpensive, large-area, solution-processed, orprinted electronics, compatible with high-throughput reel-to-reel manufacture, developing air-stable materials is crucial tominimize costly vacuum- or inert-atmosphere-based fabricationsteps and device encapsulation.[3,4]

Antonio Facchetti is a co-founder and the CTO ofPolyera Corporation. Heobtained a Ph.D in ChemicalSciences from the U. Milan

(Italy) and carried outpostdoctoral research atthe University of California,Berkeley and at NorthwesternUniversity, where he is cur-rently an Adjunct Professor.Dr. Facchetti has published

about 200 research articles, 6 book chapters, and holdsabout 30 patents. He received the 2009 Italian ChemicalSociety Research Prize and the team IDTechEx PrintedElectronics Europe 2010 Award.

Xiaowei Zhan obtained aPh.D. degree in chemistry

from Zhejiang University in1998. He was then a post-doctoral researcher at theInstitute of Chemistry at theChinese Academy of Sciences(ICCAS) from 19982000,and in 2000 he was promotedto associate professor atICCAS. Dr. Zhan worked atthe University of Arizona

and Georgia Institute of Technology from 20022006 asresearch associate and research scientist. He has been afull professor at ICCAS since 2006. His research interests

include the development of organic and polymeric mate-rials for organic electronics and photonics.

Tobin J. Marks is the VladimirN. Ipatieff Professor ofChemistry and Professorof Materials Science andEngineering at NorthwesternUniversity. He received hisB.S. from the University ofMaryland (1966) and Ph.D.from MIT (1971), and cameto Northwestern immedi-ately thereafter. In 2006, he

was awarded the NationalMedal of Science, the highest scientific honor bestowed bythe United States Government. Marks is on the editorialboards of 9 major journals, consultant or advisor for 6major corporations and start-ups, and has published 935research articles and holds 93 U.S. patents.


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Recent experimental and theoretical studies empirically identi-

fied this energetic threshold at approximately 4.0 to 4.3 eV(Figure3), which scales to an overpotential versus O2 reductionof approximately 0.9 to 0.6 V and is applicable to several rylenediimide and other families of OFET materials.[16,1922]

In the following sections, we will see that ambient operationalstability for rylene and other aromatic diimide ET materials, par-ticularly in FET devices, can be achieved by following two designstrategies: 1) the introduction of (per)fluorinated substituentsat the imide N,N positions to create close-packed solid-stateatmospheric barriers and 2) the introduction of highly electron-withdrawing substituents such as cyano (CN) and F on the aro-matic core, thereby lowering substantially the LUMO energies.

3. Rylene and Other Aromatic Diimide SmallMolecules for OFET Devices

3.1. Phenylene, Naphthalene, and Anthracene Diimides

In this section we review small molecules based on thesimplest rylene diimide system, naphthalene-1,8:4,5-tetracarboxylic diimide (Figure 1, 3). In addition, we considerdiimides based on other small aromatic cores, benzene-1,2:4,5-tetracarboxylic diimide (pyromellitic diimide, 1) and anthra-cene-2,3:6,7-tetracarboxylic diimide (3), both of which differ

kinetically or thermodynamically resistant to trapping. It is

generally believed that H2O and O2 can be excluded either byutilizing crystalline materials with sufficiently dense molecularpacking to resist penetration by these species or by appropri-ately encapsulating the devices in inert atmosphere, although,as noted above, additional encapsulation steps should ideally beavoided. Thermodynamic stability is a complex issue. A solu-tion phase electrochemical reduction potential more positivethan 0.66 V vs the saturated calomel electrode (SCE) is gen-erally thought necessary to stabilize the charge carriers in ETmaterials with respect to H2O reduction.[17] This means that thelowest unoccupied molecular orbital (LUMO)[18] of this mole-cule should lie at an energy below approximately 3.7 eV withrespect to vacuum. Several materials satisfy this requirement,including the perylene and naphthalene diimide cores. How-ever, molecular materials in which negatively charged carriersare not thermodynamically susceptible to O2 oxidation wouldrequire a far more daunting reduction potential of greater than+0.57 V (vs SCE), and thus, LUMO energies less than 4.9 eV.Materials with such large electron affinities are rare; moreover,suitable partner HT materials for use in applications suchas OPV cells would require very low-lying highest occupiedmolecular orbitals (HOMOs), possibly raising additional sta-bility issues due to the susceptibility of the holes to reductionby water. However, an overpotential to the charge carrier + O2reaction could, in principle, prevent ambient trapping in mate-rials where the LUMO energies are considerably less negative.

Figure 1. Chemical structures of some rylene diimide small molecules.

NN RR

O

OO

O

NN

O

O

O

O

RR

CH2CH2C8F171a

NR

O

O

N

O

O

R

2a

2b

2c

R R

H

C8H17

CH2C7F15

CF32e

CH32d

2f

CH2CH2C6F5

CH2CH2C8F17

2g C6F5

2h

2i

C6H132j

N

O

O

C8H17 N

O

O

C8H17

X

X

3a

3b H

CN

X

4a

4b

4c

R

C6H5

C8H17

NC8H17

O

O

N

O

O

C8H17

X

X

C13H27

4d C18H37

4eOC12H25

OC12H25

OC12H25

NN RR

O

OO

O

4g

4h

4i

R

4j

X

X

Y

Y

4f CH2C3F7

CH2C3F7

X

CN

Y

H

CN H

CH2C3F7

CH2C3F7

CH2C3F7

F H

F F

4k

4l

4m CH2C3F7

CH2C3F7

Cl Cl

Br

HBr

Br

C6H5 Cl Cl

4n

C8H17 HCN4o

NN C5H11C5H11

O

OO

O

5a

NN CH2C7F15C7F15CH2

O

OO

O

X

X

(H2C)3OOC12H25

OC12H25

OC12H25

O

X =

6a

CF31b

R

C8H17

C8H17

3c

3d CN

BrCH2C3F7

CH2C3F7

2k

CF3

CF3

XX

O

OO

OCl

Cl

Cl

Cl

4p C6F6 F H

4q

4r

X

NH

O

2l

2m

X

CN

Br

Cl

Cl

Cl

Cl


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attained in NDIs by varying N,N substitution,Shukla et al. reported that compared to linearn-hexyl chains, cyclohexyl substituents assistin directing intermolecular stacking,affording a dramatic increase in mobilityfrom 0.70 (2j) to a reported 6.2 cm2 V1 s1

(2i) in vacuum.[

29

]

Compared to work on NDIs, there havebeen very few reports on pyromellitic diim-ides and anthracene-2,3:6,7-tetracarboxylicdiimides. Katz et al. reported the onlyexample of OFETs based on a pyromelliticdiimide:[23]1a (Figure 1) exhibits a mobility of0.054 cm2 V1 s1 in air, an order-of-magnitudelower than the NDI analogue with the sameN,N substituents (2f),[27] however com-pound 1b exhibits a mobility approaching0.1 cm2 V1 s1 in vacuum and 0.4 cm2 V1 s1in the air. Marks et al. reported the only exam-ples of OFETs based on anthracene-2,3:6,7-tetracarboxylic diimides.[31] Devices based on3b exhibit a mobility of 0.02 cm2 V1 s1 invacuum, 10 times lower than its NDI counter-part with the same substituents (2b). However,OFETs based on 3b do not operate in air dueto the relatively low electron affinity. Introduc-tion of cyano groups at the 9,10 positions ofthe anthracene ring significantly increases theelectron affinity and, therefore, improves airstability; devices based on 3a exhibit a mobilityof 0.02 cm2 V1 s1 in air, which is somewhatlower than that found for an NDI analoguehaving the same substituents (2l).[32]

The optical properties and crystal structuresof these diimides provide interesting insightsinto their comparative electronic structures,solid-state packing, and potential for applica-

tions. As expected, the optical absorbtion maximum shifts tolonger wavelengths upon proceeding from benzene (e.g., for1amax 310 nm) to naphthalene (e.g., for 2lmax 350 nm,

from the other diimides considered here in that the cores arenot strictly rylenes and in that they contain five-memberedimide rings.[2331] The earliest attempt to fabricate OFETs fromnaphthalene dimides (NDIs; Figure 1) yielded a mobility of104 cm2 V1 s1 for compound 2a.[24] Later, Katz et al. dem-onstrated that the performance and air stability of the OFETscould be greatly improved by varying the NDI N,N substitu-ents. Replacing hydrogen with an n-octyl group at the N,Npositions of 2a led to a 103 times enhancement in mobilityto 0.16 cm2 V1 s1 measured in vacuum for 2b,[25] althoughalmost no FET activity was measurable in air. It was found thatthe n-CH2C7F15N,N substituents in 2c significantly improvedevice air stability, with mobilities of 0.050.1 cm2 V1 s1 meas-ured in air.[25,26] Replacing the methyl substituents on the N,Nbenzyl groups of2d with CF3 groups in 2e leads to 105 timesenhancement of the mobility in air,[25] while elongating CF3 ton-CH2CH2C8F17 further enhances the mobility from 0.12 (2e) to0.57 cm2 V1 s1 (2f)[27]. Strikingly, inserting an ethylene bridgebetween the nitrogen atoms and the perfluorophenyl substitu-ents of2g to give 2h (Figure 1) leads to a crystalline, rather thanan amorphous, material and boosts the mobility from 0 V for n-typetransistors) the device turns on and charge carriers are accumulated at the semiconductordielectric interface, resulting in a gate-controlled Id. Principal FET figures-of-merit include field-effect mobility (), current on-to-off ratio (Ion:Ioff), threshold voltage (Vth) defining, respectively,the average charge carrier drift velocity, drainsource current ratio between on and offstates, and the gate voltage at which the injected carriers are mobile. b) OPV cell material com-ponents (top) and energetics of the light absorption and charge-collection processes involvinga rylene diimide as the acceptor material. In an OPV device, the light passing through the trans-parent contact (typically indium tin oxide, ITO) is absorbed in the active layer to form excitons(hole-electron pairs), which dissociate into free charges at the interface between the donor andthe acceptor layer. Holes and electrons flow in the donor and acceptor (rylene diimide) regions,respectively, and are collected at the electrodes, resulting in the generation of electrical power.Principal figures-of-merit include the power conversion efficiency (PCE), short-circuit current(Jsc), and the open-circuit voltage (Voc), fill factor (FF), defining, respectively, the ratio between

the output device electrical energy versus the input solar energy, the device current when noreverse bias is applied, and the device voltage when no current flows through the cell, and theratio between maximum power of the device and JscVoc, respectively.

Dielectric

Source Drain

Gate

Semiconductor

Id , Vd

Vg

energy[eV]

e-LUMO----

ITO anode

Donor-Acceptor Blend

Metal cathodeI

LUMO

HOMO

hv

energy[eV]

Contact Rylene diimideITO MetalDonor Rylene

diimide

Field-Effect Transistor Photovoltaic Cell

HOMO

a b

e-

h+


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Figure4) to anthacene (e.g., for 3amax 420 nm) diimides.Thus, the negligible optical absorption of these semiconduc-tors through most of the visible region of the spectrum mayenable their application in optically transparent devices. Katzand Marks have independently demonstrated transparent FETsbased on 2h[28] and 2l,[30] which exhibit electron mobilities of0.2 cm2 V1 s1 and 0.1 cm2 V1 s1, respectively (Figure 4).

The crystal structure of at least one member each rylene familyhas been reported (Figure5). The crystal structure of1b exhibitsa unique cofacial packing between the phenyl substituentring and the -pyromellitic diimide core. The majority of crystalstructures reported of NDI derivatives exhibit variations of theclassic herringbone packing motif, characterized by edge-to-faceinteractions.[32] Indeed, in the case of a single trifluoromethylsubstituent at the 4 position of the benzyl group (2e), packing

occurs in a herringbone fashion, whereas in the case with trif-luoromethyl groups at the 3 and 5 positions (2k), the NDI coresare oriented parallel to each other, with -stacking along thea-axis (out of the page) and the closest intermolecular spacingin the -stacking direction equal to 3.147 (O2-C1). [27] Clearly,slight changes in structure induce considerably different packingmotifs for these molecules. For compounds of family 3, twocrystal structures have been determined.[33] The conjugated back-bone of3c adopts a substantially planar molecular configurationwith a negligible interplanar twist angle. The molecular length ismeasured as 11.63 along the molecular backbone. The neigh-boring molecules are -stacked in a slipped cofacial orientationalong the a-axis with an interplanar stacking distance ofonly 3.1401 . As expected, the perfluoroalkyl end chains adopt atrans configuration with respect to each other. Furthermore, therelatively large fluorine atoms are densely packed, which could inprinciple inhibit air penetration to the cores and thus serve as akinetic barrier to improve air stability for these devices. Similarly,the anthacene core of molecule 3d is planar and extended stacking interactions are observed along the a-axis with an inter-planar distance of 3.183 , whereas along the b-axis, two solventmolecules (toluene) are inserted between neighboring 3d mol-ecules, so that the minimum distance between the closest arenerings is 6.387 . The planar cores, combined with the smallinterplanar distances observed in families 13, explain the effi-cient charge transport achieved in several of these compounds.

Figure 4. Optical absorption of a core-cyanated naphthalene (a) and an optically transparent thin film transistor (TFT) (b) based on compound 2l.Reproduced with permission.[30] Copyright 2007, American Chemical Society.

Table 1. OFET device data for vacuum-deposited films of naphthalene,benzene, and anthracene tetracarboxylic diimide small molecules.

Ts (C) Max e

(cm2 V1 s1)

Ion/Ioff Vth (V) Device structure Ref.

1 115 0.079 (vac.) 106 14 TC gold on Si/SiO2 treated

with OTS; tested in

vacuum or air

23

0.054 (air) 104 15

2a 104 BC gold on Si/SiO2; tested

in vacuum

24

2b 70 0.16 (vac.)

106 (air)

2be: TC gold on Si/SiO2;

tested in vacuum or air

25

2c 70 0.05 Tested in air 25

0.1 105 26

2d 70 106 Tested in air 25

2e 98 0.12 Tested in air 25

2f 140 0.57 107 1348 TC gold on Si/SiO2 treated

with OTS; tested in air

27

2g 70


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Figure 5. Crystal structures of several benzene, naphthalene, and anthracene perylene diimides. 1b, reproduced with permission.[23] Copyright 2008,American Chemical Society. 2e, reproduced with permission.[32] Copyright 2001, Wiley-VCH. 2k, reproduced with permission.[27] Copyright 2008, Amer-ican Chemical Society. 2m, reproduced with permission.[30] Copyright 2007, American Chemical Society.


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on/off ratio of 107, and threshold voltages of 1015 V can beobtained using 4b by coating the SiO2 gate dielectric withpoly(-methylstyrene).[36] Coating of the dielectric with poly-mers also considerably improves the air stability of deviceoperation for 4b, presumably by passifying acidic silanol groupsthat can act as electron traps on the SiO2 surface.[37] Ichikawa

et al. demonstrated that the mobility of OFETs based on N,N-bis-tridecyl PDI 4c can be increased 103 times to 2.1 cm2 V1 s1by thermal annealing;[38] the thermal treatments improve boththe thin film crystallinity and morphology.

Discotic liquid crystals (LCs) that can self-assemble into highlyordered one-dimensional columnar stacks are promising candi-dates for use as charge-transport materials in organic electronics;intermolecular orbital overlap within the stacks is antici-pated to enhance mobility compared to that in amorphous mate-rials. Mobilities of 0.1 cm2 V1 s1 have been measured by pulse-radiolysis time-resolved microwave conductivity (PR-TRMC)techniques for LC N,N-dioctadecyl-3,4:9,10-perylene diimide(4d).[39] Marder et al. found that at room-temperature, LC PDI(4e) displays a space-charge-limited current (SCLC) mobilityas high as 1.3 cm2 V1 s1 under ambient conditions,[40] whileeven higher values (up to 6.7 cm2 V1 s1 for 6a) were found forclosely related columnar discotic coronene-2,3:8,9-tetracarboxylic diimides.[41,42] However, attempts to exploit the high mobilitiesof LC 4e and 6a have so far been unsuccessful, presumably atleast in part due to the difficulty of obtaining the desired align-ment of the columnar stacks along the sourcedrain axis.

Bao, Wrthner, and co-workers reported that OFETs basedon N,N-bis(2,2,3,3,4,4,4-heptafluorobutyl)-3,4:9,10-perylenediimide (4f) can exhibit mobilities as high as 0.72 cm2 V1 s1,which decrease only slightly after air exposure and remain stablefor more than 50 days.[43] Since the partial fluorination has onlya small effect on the redox potential (LUMO energy) relative to

N,N-dialkyl analogues, the stability was attributed to hindranceof O2 and H2O diffusion by dense packing of the cores and thefluoroalkyl chains. On the other hand, 1,7-dicyano PDIs, firstsynthesized by Wasielewski et al., are significantly more readilyreduced than their unsubstituted analogues (by ca. 0.36 V);[44] theassociated high electron affinity is believed to be a factor contrib-uting to the high electron mobility (0.10 cm2 V1 s1) achieved inair for OFETs based on 4g.[45] Combining partial fluorination ofthe N,Nsubstituents and 1,7-dicyano substitution in 4h affordsa still higher electron mobility, 0.64 cm2 V1 s1.[45] The crystalstructure of4h provides a rationale for the high mobility of thismaterial both in vacuum and in ambient conditions. Singlecrystals of4h were grown by sublimation and the crystal struc-ture (Figure6) reveals a minimally twisted polycyclic core (tor-sion angle of about 58) with slip-stacked face-to-face molecularpacking and a minimum interplanar spacing of 3.40 . Thismotif allows considerable intermolecular overlap, whichresults in good charge transport properties. The effects of 4hfilm growth conditions on n-channel OFET performance havealso been investigated;[46] dramatic enhancements of the on/off ratio and the mobility are obtained with increased sub-strate temperature (Ts) during film growth, with the increasedmobility being correlated to higher levels of molecular orderand to minimization of film-surface irregularities.[46] In addi-tion, the effects modifying the SiO2 surface of the gate dielectricwith octadecyltrichlorosilane- or hexamethyldisilazane-derived

To our knowledge, thin film transistors (TFTs) based on singlecrystals of families 13 have never been investigated.

3.2. Perylene and Higher Rylene Diimides

Table2 provides a summary of selected n-channel OFET data forthe small-molecule PDIs in Figure 3.[3454] Horowitz et al. firstdemonstrated electron mobilities of 105 cm2 V1 s1 with anN,N-diphenyl-substituted example (4a).[34] In 2001, Malenfantet al. reported n-channel OFETs based on N,N-dioctyl PDI (4b)having a substantial mobility of 0.6 cm2 V1 s1 under nitrogen,but with a high threshold voltage of 75 V, which was attributedto a large trap density.[35] Later, Chesterfield et al. demonstratedthat devices with a maximum mobility of 1.7 cm2 V1 s1, an

Table 2. OFET device data for vacuum-deposited films of perylene tet-racarboxylic diimide small molecules.

Ts (C) Max e

(cm2

V1

s1

)


4a 105 BC gold on glass with Al gate

and PMMA dielectric; tested

in air

34

4b 0.6 >105 75 BC gold on Si/SiO2; tested

in N2

35

4b 75 1.7 107 1015 BC Ag on Si/SiO2 coated with

poly(-methylstyrene); tested

in 104 Torr of H2

36

4b 0.11 105 11 TC gold on Si/SiO2 coated with

PMMA; tested in air

37

4c 140 2.1 105 60 TC gold on Si/SiO2; annealing

at 140 C; tested in vacuum

38

4f 125 0.51 106

2843 TC gold on Si/SiO2 treated withOTS; tested in air

43

4g 90 0.1 105 15 4gh: TC gold on Si/SiO2

treated with HMDS; tested

in air

45

4h 110 0.64 104 20 45

4h 6 (vac.) 103 5 +5 TC gold on Si/SiO2 coated with

PMMA; SC; tested in vac. or air

47

3 (air) 104

4i 125 0.58 106 1129 4im:TC gold on Si/SiO2

treated with OTS; tested in air

50

4j 125 0.052 106 422 50

4k 125 3.3 105 103 1425 50

4l 125 0.27 107 1733 50

4m 125 3.9 105 103 1123 50

4n 130 8.8 104 102 28 TC gold on Si/SiO2; tested

in air

21

4q 125 0.82 108 28 TC gold on Si/SiO2 treated

with OTS; tested in air

51

5a 140 0.07 104 20 TC gold on Si/SiO2; tested

in vac.

54

Ts= substrate temperature; Vth= threshold voltage; TC = top contact; BC = bottom

contact; OTS = octadecyltrichlorosilane; HMDS = hexamethyldisilazane; PMMA =

poly(methyl methacrylate); SC = single crystal; vac. = vacuum.


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derivatives being slightly more readily reduced than their 1,7-dihalo analogues, the more highly substituted examples tendto exhibit lower mobilities. This can be attributed to disruptionof core planarity and, therefore, of effective overlap, dueto steric interactions.[50] The crystal structure of this homolo-gous series combined with those of previously characterized

PDIs provides important and general insights into the chargetransport properties of perylene derivatives, and it is worth dis-cussing them in detail.

Figure 8 shows the crystal structures of N,N-bis(heptafluorobutyl) functionalized perylenes 4f, 4i, and 4j andthe N,N-bis(perfluorophenyl) derivative 4p (molecular structuressketched in Figure 1). Similar to other core-unsubtituted peryl-enes, the crystal structure of a 4fshows a nearly planar -core(the torsion angles defined by the planes encompassing theatoms in positions 1, 12a, 12b and 12 and those in positions 6,6a, 6b, and 7, i.e., the atoms that define the PDI bays, are only1.5). The crystal packing is characterized by having 4fmole-cules arrayed in a slipped -stack with the shortest interplanardistance only 3.31 (similar to that in graphite) and closeperylene H-to-imide O contacts between stacks (2.46 ). The dif-luorinated derivative 4i also exhibits a substantially planar core(torsion angles 3.0), but significantly different herringbonemolecular packing (the shortest interplanar spacing is 3.33 within each stack and the closest intermolecular interaction is aperylene H-to-alkyl F contact of 2.49 ), resulting in considerablyreduced face-to-face overlap. The presence of four bay fluorineatoms in molecule 4j strongly distorts the core from planarity,resulting in a twisting of the two naphthalene imide subunits(the torsion angles defined as defined above, are 19.8 and 25.1).Although the naphthalene subunits of neighboring moleculesare still orientated parallel to each other, two different minimuminterplanar distances (3.18 and 3.28 ) are observed along the

molecular stack. Because of the very high displacement param-eters, negligible contact between neighboring molecules isobserved. Finally, the solid-state packing of the difluoro core sub-stituted PDI with N-perfluorophenyl groups (4p) differs signifi-cantly from that observed for the corresponding molecule 4i that

monolayers, as well as with polystyrene, were investigated for4h films deposited at Ts= 130 C; the SiO2 surface treatmentssubstantially modulate the mobility and growth morphology of4h films.[46]

Recently, Morpurgo et al. fabricated OFETs based on 4hsingle crystals (Figure 7), with poly(methyl methacrylate) asthe gate dielectric, that exhibit electron mobilities approaching6 cm2 V1 s1, 10 times greater than those of the correspondingthin-film devices, both in air and in vacuum.[47] Furthermore,these devices exhibit near-zero threshold voltage and sub-threshold slopes and current on/off ratios (103104) compa-

rable to the very best p-channel single-crystal devices when thesame gate dielectric is employed. In related work, Weitz et al. [48]reported air-stable n-channel OFETs based on five dicyano PDIswith fluorinated linear and cyclic N,N-substituents (mobilitiesup to 0.1 cm2 V1 s1) and investigated the relationships betweenmolecular structure, thin-film morphology,substrate temperature during growth, deviceperformance, and air stability. Interestingly,the mobility degradation rate in air wasfound to be similar for all compounds andat all growth temperatures, raising the ques-tion of whether air stability can always beexplained on the basis of kinetic barriers toO2/H2O diffusion formed by densely packedfluorocarbon substituents.[48]

In addition to cyano groups, halo sub-stituents can act as as perylene baysubstituents. Wrthner et al. reported thata 1,6,7,12-tetrachloro N,N-didodecyl PDIexhibited PR-TRMC mobilities as highas 0.1 cm2 V1 s1.[49] More recently, Bao,Wrthner et al. studied a series of corehalogenated N,N-bis(heptafluorobutyl) andN,N-bis(perfluorophenyl) PDIs. While intro-duction of halogens in the bay positionsfacilitates reduction, with 1,6,7,12-tetrahalo

Figure 7. Device structure (a), optical image (b), and electrical performance (c) of a single-crystal TFT based on perylene 4h. Reproduced with permission.[47]Copyright 2009, AmericanChemical Society.

0 10 20 300.00

0.25

0.50

0.75

1.00

1.25

1.50

ISD

(A)

VSD

(V)

+5 V

+10 V

+15 V

VacuumAir

c.Au (Source)

SiO2

Doped Si (gate)

PMMA(Dielectric)

Single-crystalAu (Drain)a.

b.

Figure 6. Crystal structure of compound 4h viewed along two differentcrystallographic directions. Reproduced with permission.[45] Copyright2004, Wiley-VCH.


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these crystals contain solvent molecules in the lattice, limitingany discussion of how stacking affects charge transport in thevapor-deposited films. As in the case of4f, molecule 4m crystal-lizes with benzene solvent molecules between the perylene cores.Interestingly, the torsion angle (37.2) increases only slightly com-pared to that of the tetrachlorinated compound 4k, in agreementwith molecular orbital computations, which predict saturation ofthe core torsional angle with tetrachloro substitution.

The structures of these derivatives, combined with X-ray evi-dence that vapor-deposited films of these PDIs are polycrystal-line, help rationalize the mobility trends in these PDI families.Thus, the PDI cores of both the difluoro compound 4i and theparent 4fare more or less planar (torsion angles


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counterpart 4j, presumably due to the increased bulk of theheavy halogen substituents.[50] Interestingly, replacing theN,N-fluoroalkyl substiutents of the 1,6,7,12-tetrachloro PDI4k with N,N-pentafluorophenyl groups in 4l lead to an 104times increase in the mobility (Table 2).[50]

Very recently, Bao and Wrthner showed that PDI core

planarity is not essential to achieve high field-effect mobilityby comparing two octachloro-substituted derivatives, 4q and 4r(Figure 1).[51] Compound 4q, which has free NH imide groups,was envisioned to enable close and directional hydrogenbonds between adjacent molecules. The crystal structure of 4q(Figure10) exhibits a highly twisted perylene backbone with adihedral angle of 37.28 between the naphthalene imide planes,in agreement with results for other octachloro-substitutedperylene diimides. These molucules form NHO hydrogenbonds of length 2.95 . In the crystal, the molecules arrange ina slipped 2D -stacked layers with the shortest interactionsof about 3.4 . This hydrogen-bond-enforced brickwall packingprovides substantial overlap between each molecule andits four neighbors. The LUMO level of4q, despite the heavilytwisted core, is remarkably low (4.23 eV) and well within theair-stability window for ET materials.. Top-contact bottom-gateOTFTs on OTS-treated SiO2/Si substrates exhibit mobilities ashigh as 0.91 cm2 V1 s1 under nitrogen and 0.82 cm2 V1 s1 inair for films vapor-deposited at 125 C. It is striking that suchhigh mobilities are obtained from such nonplanar -conjugatedmolecules. Interestingly, hydrogen bonding apparently playsan important role in achieving high mobility since the corre-sponding octachloroperylene-3,4:9,10-tetracarboxylic dianhy-dride (4r) exhibits much lower mobility (105 cm2 V1 s1) inthe same device structure.

Most of studies of PDI-based FETs have been carried outusing films deposited by vacuum evaporation. However, recent

studies demonstrate the great potentialof these materials for solution-processedor printed FETs (Figure 11). For example,Dodabalapur et al. reported the first organiccomplementary circuits (CMOS) fabricatedon Si-SiO2 substrates using solution-depositedfilms of4o for the n-channel FETs and poly-3-hexylthiophene (P3HT) for the p-channelFETs.[55] They reported ring oscillators oper-ating at a frequency of2 kHz without passi-vation or packaging. More recently, Facchettiet al. reported solution-processed FETs of 4owith mobilities 0.01 cm2 V1 s1 on Si-SiO2,as well as CMOS inverters and ring oscilla-tors on plastic substrates operating at 50 Hzand using 4o and P3HT in combination withsolution-processed polymer dielectrics.[56] Loiet al.[57,58] showed that fluorocarbon func-tionalization of the aromatic skeleton in 4hleads to an enhancement of the electronmobility by one order of magnitude (up to0.133 cm2 V1 s1) versus hydrocarbon-func-tionalized 4o for films processed from solu-tion. Such a difference between the solution-processed transistors is far larger than thedifferences reported for the corresponding

Figure 11. Optical images of several devices based on rylenediimide derivatives. a) Ring oscil-lator fabricated on PET by spin-coating compound 4o ( n-type FET) and P3HT ( p-type FET).b) FETs fabricated by inkjet printing compound 4o. c) FETs fabricated by gravure printingpolymer 7d. d) An inkjet-printed inverter based on a perylene derivative. Panel (a) reproducedwith permission.[56] Copyright 2008, Wiley-VCH. Panel (c) reproduced with permission. [65]Copyright 2009, Nature Publishing Group. Panel (d) reproduced with permission.[59] Copyright2009, American Institute of Physics.

Figure 10. Crystal structure of perylenedimide 4q. Reproduced with per-mission.[51] Copyright 2010, Wiley-VCH.


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for greater morphological stability (for example, with respect tocrystallization and electrically non-connected crystallites), poly-mers are ideally suited for high-throughput processing meth-odologies, such as gravure and flexographic printing, whichrequire viscosity ranges that are difficult to achieve with anysmall molecules. While the application of rylene diimide poly-mers (Figure14) in OFET applications is in an earlier stage ofdevelopment than their small molecule counterparts, the devel-opment of main-chain and side-chain rylene diimide ET poly-mers is a rapidly advancing field. Table3 summarizes n-channel

vacuum-processed devices. To explain these differences, theauthors carried out both morphological and spectroscopicanalyses on the spin-coated films of the two materials. By usingtime-resolved spectroscopy, they highlighted that the intermo-lecular interactions (excimer-like excitations) are more domi-nant in 4h than in 4o (Figure 12). The face-to-face molecular

organization responsible for the excimer formation is also theoptimum intermolecular organization motif for high charge car-rier mobility due to the increased overlap, explaining theenchanced FET performance of the fluorocarbon functionalizedderivative. In related work, Arias et al. demonstrated inkjet-patterned FETs and complementary inverters based on a PDIderivative and a poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene), and analyzed details of the bias stress response (aparameter related to FET operation in a circuit) for digital andanalog electronic circuit applications.[59] Very recently, Salleoet al. investigated the effects of molecular-level structure on grainboundaries by engineering the growth microstructure of theN,N-dioctyl perylene diimide 4o and analyzed the consequencesfor charge transport.[60] A combination of advanced X-ray scat-tering, first-principles computation, and transistor characteri-zation applied to 4o films reveals that grain-boundary orienta-tion modulates carrier mobility by approximately two orders ofmagnitude (Figure13). Anisotropic 4o films were grown using adrop-casting technique where nucleation of4o crystallites occursat the pinned edge of the evaporating solution at the top of anincline. The slow-moving evaporation front of the high-boiling-point solvent and a saturated solvent atmosphere promote thegrowth of elongated, oriented crystalline domains. For 4o it wasshown that the molecular packing motif (that is, herringboneversus slip-stacked) plays a decisive role in grain-boundary-induced transport anisotropy. These results show that the com-plete elimination of grain boundaries or, conversely, introduction

of many grain boundaries, is unnecessary for device uniformity.Instead, device performance can be optimized by control-ling grain-boundary orientation or reducing the energetic bar-rier associated with transport across less favorable boundaries,such as those of necessarily high misorientation in slip-stackedsystems.

Finally, Mllen et al. pioneered the synthesis of higher rylenediimide dyes and other species based on extended PDI cores,investigating in detail their thermotropic behavior and opticalproperties as well as the details of their microstructure.[52,53]However, exploitation of higher rylene diimides in organic elec-tronics is limited to a report by Petit et al. on the FET propertiesof vapor-deposited films of N,N-dipentylterrylene-3,4:11,12-tetracarboxylic diimide derivative (5a) on Si-SiO2 substrates;[54] amaximum electron mobility of0.07 cm2 V1 s1 and an on/offratio in excess of 104 is obtained.

4. Rylene Diimide Polymers in OFET Devices

Although the highest organic transistor performance is typicallyobtained using highly purified vacuum-deposited small mol-ecules, solution-processing methods such as spin-coating andinkjet printing are attractive alternatives due to their low cost, asnoted above. These solution phase deposition methods are alsocompatible with polymeric ET materials. Beyond the potential

Figure 12. Intensity images of the spectrally and time resolved PL spectraof4h (a) and 4o (b) thin films (the intensity scale for the two images is thesame). Rectangles delimit the temporal and spectral integration windowover which the corresponding profile displayed on the right (spectra)and below (decay) are integrated. Right panels: emission spectra time-integrated over the initial 0.2 ns after excitation (dark gray) and between1.2 and 4.2 ns (light gray). Bottom panels: PL decays spectra integratedbetween 580 and 610 nm (dark gray) and between 720 and 820 nm(light gray). Reproduced with permission.[58] Copyright 2009, Springer.


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have reported a PDI/bithiophene copolymer 7c with a mobilityof 2 103 cm2 V1 s1.

Thelakkat et al. reported that for OFETs based on polymerscontaining PDIs as pendant groups,[64] after thermal annealingat 210 C for 60 min, threshold voltage declines to 7 V, whilethe current and charge carrier mobility both increase by 100-fold, with electron mobilities as high as 1.2 103 cm2 V1 s1obtained in a device based on 7e. However, OFETs based onthese PDI polymers are unstable under ambient conditions.

Facchetti et al. have developed improved polymeric mate-rials based on NDIs (Figure 15). Thus, a copolymer with bithi-ophene 7d achieves an electron mobility of 0.06 cm2 V1 s1when measured in vacuum.[63] Furthermore, the devices exhibitgood air stability and function well under ambient conditionsfor at least four weeks after fabrication ( 0.01 cm2 V1 s1).Later, top-gate bottom-contact transistors were fabricated from

OFET data for solution-processed perylene and naphthalenediimide polymers.

Zhan et al. reported the first synthesis of soluble rylene-based fully conjugated polymers, i.e., polymers in which therylene cores are bridged by other -conjugated units, ratherthan polymers in which rylenes are linked through the imidegroups. This copolymer of PDI and dithienothiophene, 7a,exhibits a saturation electron mobility of 1.3 102 cm2 V1 s1and a low threshold voltage of 4 V (Figure15).[61] Very recently,

a dithienopyrrole analgue, 7b, was reported to show an elec-tron mobility of 7.4 104 cm2 V1 s1, which increases to1.2 103 cm2 V1 s1 on annealing at 100 C for 60 min underinert atmosphere.[62] The lower mobility observed for 7b may berelated to dilution of the PDI ET units by the additional N-sub-stituents of the dithienopyrrole donors or to the disruption ofPDIPDI interactions caused by these groups. Facchetti et al.[63]

1 2 3 4

10-5

10-4

10-3

10-2

10-1

Batch #

Mobility(cm

2/V

s)

10-7

10-6

10-5

10-4

10-3

10-2

10-1

001002003004

0.004 0.006 0.008 0.010

Temperature (K)

Mobility(cm

2/V

s)

1/T (K-1)

EA=125 meV

EA=468 meV

||

Channel length = 600um

Spin-coated, isotropic films

Perpendicular alignment

Parallel alignment

||

a b c

Figure 13. Mobility and activation energy anisotropy for 4o films. a) Optical micrographs of parallel (top), perpendicular (middle), and isotropic(bottom) TFT devices fabricated from 4o films. Channel lengths are all 600 m. b) Derived mobility of aligned and isotropic 4o films. Batch-to-batchvariations are probably due to differences in device geometry and the time elapsed between substrate ultravioletozone treatment and film growth, asthis affects film quality. c) Arrhenius plot of the temperature dependence of the TFT mobilities. In all cases blue corresponds to parallel devices, red toperpendicular, and black symbols to isotropic. Reproduced with permission.[60] Copyright 2009, Nature Publishing Group.

Figure 14. Chemical structure of some rylene diimide polymers.

N

N

C12H25

C10H21

C10H21

C12H25

S

Z

S

O O

OO n

N

N

C8H17

C10H21

C10H21

C8H17

O O

OO n

Z

7a S

7b N-C12H25

SS

7c

N

C8H17

C10H21

O O

n

SS

7d

N

C10H21

C8H17

OO

N

N

(CH2)11

C7H15

O O

OO

n

C7H15

OO

O

Ph

N Ph

iPr

tBu

7e

N

O

N

N

N

O

n8


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in bulk-heterojunction organic photovoltaic cells (OPVs), PDI-based small molecules and polymers have attracted interestas alternative ET materials since they exhibit large opticalabsorptivities, high electron mobilities, and electron affini-ties similar to those of fullerenes. The work of Wasielewskiet al. has shed light on photoinduced energy, charge, and spin

transport, and elucidated the timescales of these processesin PDI derivatives; this understanding is necessary for opti-mizing their use in solar energy conversion. [67,68] Efficientphotoinduced charge separation is observed in blends of asmall molecule PDI (4e) with a variety of polythiophene donors,even when rather low thermodynamic driving forces areanticipated.[69]

Many early studies of OPVs incorporating PDIs consisted oflayered structures fabricated by vapor deposition; for example,a three-layer OPV consisting of the structure: phthalocyaninelayer, phthalocyanine/9a blend layer, and 9a layer (Figure16),yielded a power conversion efficiency (PCE) of 0.7%. [70] How-ever, bulk heterojunction cells based on solution-processedblends of PDI-based materials with appropriate donor materialsare attracting increasing attention (Table4). Thus, Friend et al.used a discotic liquid-crystalline hexaperihexabenzocoronenederivative in combination with PDI 9b to fabricate solution-processed OPVs having a PCE of 1.95% at 490 nm. [71] Shinet al. fabricated OPVs using a blend of poly(3-hexylthiophene)(P3HT) with 9c.[72] A maximum PCE of 0.18% under simulatedsolar irradiation (AM1.5, 100 mW cm2) was achieved with aP3HT: 9c ratio of 1:4 after annealing at 80 C for 1 h. Thus far,however, PCEs achieved for solution-processed OPVs based onblends of small molecule PDIs with small molecule or polymerdonors have been modest.

Bulk-heterojunction solar cells incorporating diimide-basedacceptor polymers are among the most efficient all-polymer

solar cells to be reported. The PDI-based polymer acceptor(7a, Figure 14) synthesized by Zhan et al. and discussed in theprevious section was also used as an acceptor in conjunctionwith a bis(thienylenevinylene)-substituted polythiophene donorand a PCE of 1% was achieved (Figure 15).[61] More recently,OPVs based on a related donor (a tris(thienylenevinylene)-sub-stituted polythiophene) and a related acceptor (9d) were found,by optimizing the donor:acceptor ratio, to exhibit PCEs ashigh as 1.5% (AM1.5, 100 mW cm2).[73] Using an alternating

PDI-phenylenevinylene copolymer accceptor(9e) and poly(3-phenyl hydrazone thiophene)donor in OPVs, Mikroyannidis et al. obtaineda PCE of 2.3% under white-light illumina-tion calibrated to an AM1.5 intensity of30 mW cm2 after annealing at 80 C for10 min.[74] Very recently, Loo et al. investigated7d in combination with P3HT, achievingPCEs approaching 0.6%; they also foundthat PCEs of these blends are more sen-sitive to the active layer film morphologythan are P3HT-PCBM blends.[75] The ladderpolymer 8 (Figure 14) has also been usedto fabricate efficient bilayer cells (PCE =1.1% at 80 mW cm2 AM1.5) in conjunc-tion with a poly(phenylene vinylene)donor.[76]

7d and polymeric dielectrics by spin-coating; these exhibitelectron mobilities of 0.100.85 cm2 V1 s1 under ambientconditions with on/off ratios >105. Furthermore, it was shownthat this ET material can be processed using gravure, flexo-graphic, and inkjet printing, as well as by spin-coating, dem-onstrating great processing versatility.[65] In this manner, thefirst spin-coated and gravure-printed polymeric semiconductorcomplementary inverters on PET substrates exhibiting largegains (>2560) and operating in ambient conditions wererealized.[65]

Finally, it is important to note that, although not strictly adiimide, ladder polymer 8 is clearly very closely related to

NDIs and annealed films of this material, processed frommethanesulfonic acid, exhibit OFET electron mobilities up to0.1 cm2 V1 s1 under ambient conditions.[66]

5. Rylene and Aromatic Diimides for OPV Devices

Although fullerenes, particularly the solution-processible deriva-tives known as PCBMs, are the most commonly used acceptors

Table 3. OFET device data for solution-processed films of perylene andnaphthalene tetracarboxylic diimide polymers.

Max e

(cm2 V1 s1)


7a 0.013 >104 4 7a and 7b: TC Al on Si/SiO2 treated

with OTS; tested in vacuum; noannealing

61

7b 1.2 103 >103 9 annealing at 100 C 62

7c 2 103 7c and 7d: TC Au on Si/SiO2; tested

in vacuum; annealing at 110 C

63

7d 0.06 63

7d 0.85 106107 510 BC gold on glass with ActivInk

D2200 dielectric; tested in air

65

7e 1.2 103 104 7 TC gold on Si/SiO2 treated with

HMDS; annealing at 210 C; tested

in vacuum

64

Vth= threshold voltage; TC = top contact; BC = bottom contact; OTS = octadecyl-

trichlorosilane; HMDS = hexamethyldisilazane.

Figure 15. a) Current-voltage characteristics at several values of the gate voltage for a top con-tact device based on polymer 7a. b) Optical absorption spectra of films of7a (solid line) andof a 7a blend with a donor thiophene polymer (dashed line), spin-coated from chlorobenzenesolution, and plot of incident photon to current conversion efficiency (IPCE) as a function ofwavelength. Reproduced with permission.[61] Copyright 2007, American Chemical Society.


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6. Conclusions and Outlook

Rylene diimides can exhibit impressive electron affinities andhigh electron mobilities, often arising from favorable molecularpacking motifs, as well as good environmental and thermalstability. Rylene diimides are among the most promising ET

materials for organic electronics. Progress in the past decadehas been substantial but continued development of ET mate-rials will require a far better understanding of the relationshipsbetween molecular architecture, electronic structure, materialsmicrostructure, and optoelectronic properties than what iscurrently available. With this objective in mind, we have sur-veyed and analyzed what is currently known concerning rylenediimide architecture, electronic structure, and charge-transportproperty relationships. To summarize:

(1) The -electron aromatic core in rylene diimides plays a keyrole in determining the charge transport properties of thecorresponding molecular solids. Thus far, the rylene tetracar-boxylic diimides have been found to exhibit much higher mo-bilities than pyromellitic diimides and anthracene tetracar-boxylic diimides.

(2) The solid-state packing and electron-transport properties ofrylene diimides can be tailored by substituents in the imideN,N positions or on the aromatic core.

(3) Ambient stability in rylene diimide-based devices can beachieved through the introduction of fluorocarbon substitu-ents at the N,N positions and/or introduction of electron-withdrawing core substituents, such as CN and F.

(4) Compared to their small molecule counterparts, rylene diim-ide polymers exhibit better solution processability but some-what lower electron mobilities.

(5) Compared to simple small molecule PDIs, more complexsystems such as PDI-functionalized polymers and oligom-

ers have generally given higher power conversion efficiencieswhen used as acceptors in solution-processed OPVs.

Currently, a specific challenge to materials chemists is to designsolution-processable air-stable organic semiconductors that havemuch better all-around performance in OFETs than amorphoussilicon. To this end, the discovery of high-performance,air-stable, and solution-processable ET polymers wouldrepresent a major breakthrough towards polymeric comple-mentary circuit technologies, where the combination of p- andn-channel transistors results in far greater circuit speeds, lowerpower dissipation, and more stable operation.

Acknowledgements

Work at CAS was supported by NSFC (Grants 21025418, 50873107,20721061), MOST (Grants 2006AA03Z220, 2006CB932100) and CAS.Work at Ga. Tech. was supported by ONR (Grants N00014 03-1 0793,N00014 04-1 0120), Solvay, and the NSF through the STC Program(Grant DMR-0120967). Work at Northwestern was supported by AFOSR(Grant FA9550 08-1 0331), ONR (Grants N000140810923, N00014 05-1 0766, N00014 05-1 0021), Polyera Corp., ETRI, BP Solar, andthe Northwestern MRSEC (NSF Grant DMR-0520513).

Received: April 18, 2010Published online: December 10, 2010

hundreds of nanometers due to the low entropy of mixing,while in the case of polymer/PCBM systems, the phase sep-aration length scale is only tens of nanometers. Thus, thedonoracceptor interfacial area in the polymer/PCBM sys-tems is far larger than that in polymerpolymer systems.Given the fact that typical exciton diffusion lengths in disor-

dered blend layers are ca. 10 nm, the greater phase separationlength scale and smaller donoracceptor interfacial areas inpolymerpolymer system is likely responsible for inefficientexciton dissociation and lower PCEs in these systems.[80] Veryrecently, Shuai et al. carried out a dynamic Monte Carlo sim-ulation for the all-polymer solar cells based on a 7a/2TV-PTblend (Figure 17). The simulations indicate that a 5% PCEcould be achieved with an optimum phase separation mor-phology (feature sizes 10 nm).[81]

Compared to the spherical/ellipsoidal molecular shapes offullerenes, the planar shapes of PDIs may lead to enhanced-stacking and more quasi 1D exciton and electron transport.As a result, if PDI-rich phases do not have high degrees oflong-range order, there can be multiple orientations of -stacked phases, which may lower long-range mobilities andtherefore act as traps and recombination centers. In con-trast, the roughly spherical shapes of the fullerenes mayenable greater 3D exciton and electron transport, creatingmore extended pathways for the efficient exciton and electrondiffusion.

One significant challenge in organic-based bulk-heterojunc-tion OPVs involves the issue of splitting of photogeneratedexcitons into separated holes and electrons.[8284] The higherrylenes may prove helpful in this regard since it is expected[85]that their permittivities will increase with increased -systemdimensions. Increased permittivity should decrease the energypenalty associated with the exciton dissociation into carriers,

increasing the overall efficiency of the carrier photogenerationprocess.

Figure 17. Contour plots showing the calculated power conversion effi-ciency (PCE) for a 7abased blend versus the charge mobility and thecharacteristic feature size in the blend. The VOC and the fill factor used inthe calculation are 0.63 V and 0.65, respectively. Reproduced with permis-sion.[81] Copyright 2010, American Chemical Society.


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[30] B. A. Jones, A. Facchetti, T. J. Marks, M. R. Wasielewski, Chem.Mater.2007, 19, 2703.

[31] Z. Wang, C. Kim, A. Facchetti, T. J. Marks, J. Am. Chem. Soc.2007,129, 13362.

[32] H. E. Katz, T. Siegrist, J. H. Schn, C. Kloc, B. Batlogg, A. J. Lovinger,J. Johnson, ChemPhysChem2001, 2, 167.

[33] A. Facchetti, T. J. Marks, unpublished results.

[34] G. Horowitz, F. Kouki, P. Spearman, D. Fichou, C. Nogues, X. Pan,F. Garnier,Adv. Mater.1996, 8, 242.

[35] P. R. L. Malenfant, C. D. Dimitrakopoulos, J. D. Gelorme,L. L. Kosbar, T. O. Graham, A. Curioni, W. Andreoni,Appl. Phys. Lett.

2002, 80, 2517.[36] R. J. Chesterfield, J. C. McKeen, C. R. Newman, P. C. Ewbank,

D. A. da Silva Filho, J. L. Brdas, L. L. Miller, K. R. Mann,C. D. Frisbie,J. Phys. Chem. B2004, 108, 19281.

[37] F.-C. Chen, C.-H. Liao,Appl. Phys. Lett.2008, 93, 103310.[38] S. Tatemichi, M. Ichikawa, T. Koyama, Y. Taniguchi,Appl. Phys. Lett.

2006, 89, 112108.[39] C. W. Struijk, A. B. Sieval, J. E. J. Dakhorst, M. van Dijk, P. Kimkes,

R. B. M. Koehorst, H. Donker, T. J. Schaafsma, S. J. Picken,A. M. van de Craats, J. M. Warman, H. Zuilhof, E. J. R. Sudhlter,J. Am. Chem. Soc.2000, 122, 11057.

[40] Z. An, J. Yu, S. C. Jones, S. Barlow, S. Yoo, B. Domercq, P. Prins,L. D. A. Siebbeles, B. Kippelen, S. R. Marder,Adv. Mater.2005, 17, 2580.

[41] Z. An, J. Yu, B. Domercq, S. C. Jones, S. Barlow, B. Kippelen,S. R. Marder,J. Mater. Chem.2009, 19, 6688.

[42] Neither PR-TRMC nor SCLC techniques can unambiguously distin-guish between hole and electron mobilities; however, considerationof estimated carrier injection barriers suggests that these SCLCvalues are electron mobilities.

[43] J. H. Oh, S. Liu, Z. Bao, R. Schmidt, F. Wrthner, Appl. Phys. Lett.

2007, 91, 212107.[44] M. J. Ahrens, M. J. Fuller, M. R. Wasielewski, Chem. Mater.2003, 15,

2684.[45] B. A. Jones, M. J. Ahrens, M. H. Yoon, A. Facchetti, T. J. Marks,

M. R. Wasielewski,Angew. Chem. Int. Ed.2004, 43, 6363.[46] B. A. Jones, A. Facchetti, M. R. Wasielewski, T. J. Marks,Adv. Funct.

Mater.2008, 18, 1329.[47] A. S. Molinari, H. Alves, Z. Chen, A. Facchetti, A. F. Morpurgo,

J. Am. Chem. Soc.2009, 131, 2462.[48] R. T. Weitz, K. Amsharov, U. Zschieschang, E. B. Villas,

D. K. Goswami, M. Burghard, H. Dosch, M. Jansen, K. Kern,H. Klauk,J. Am. Chem. Soc.2008, 130, 4637.

[49] Z. Chen, M. G. Debije, T. Debaerdemaeker, P. Osswald, F. Wrthner,ChemPhysChem.2004, 5, 137.

[50] R. Schmidt, J. H. Oh, Y. S. Sun, M. Deppisch, A.-M. Krause,K. Radacki, H. Braunschweig, M. Knemann, P. Erk, Z. Bao,F. Wrthner,J. Am. Chem. Soc.2009, 131, 6215.

[51] M. Gsnger, J. H. Oh, M. Knemann, H. W. Hffken, A.-M. Krause,Z. Bao, F. Wrthner,Angew. Chem. Int. Ed.2010, 49, 740.

[52] F. Nolde, W. Pisula, S. Mueller, C. Kohl, K. Mllen, Chem. Mater.

2006, 18, 3715.[53] F. Nolde, J. Qu, C. Kohl, N. G. Pschirer, E. Reuther, K. Mllen,Chem. Eur. J.2005, 11, 3959.

[54] M. Petit, R. Hayakawa, Y. Shirai, Y. Wakayama, J. P. Hill, K. Ariga,T. Chikyow,Appl. Phys. Lett.2008, 92, 163301.

[55] B. Yoo, B. A. Jones, D. Basu, D. Fine, T. Jung, S. Mohapatra,A. Facchetti, K. Dimmler, M. R. Wasielewski, T. J. Marks,A. Dodabalapur,Adv. Mater.2007, 19, 4028.

[56] H. Yan, Y. Zheng, R. Blache, C. Newman, S. Lu, J. Woerle,A. Facchetti,Adv. Mater.2008, 20, 3393.

[57] C. Piliego, D. Jarzab, G. Gigli, Z. Chen, A. Facchetti, M. A. Loi,Adv.Mater.2009, 21, 1573.

[58] C. Piliego, F. Cordella, D. Jarzab, S. Lu, Z. Chen, A. Facchetti,M. A. Loi,Appl. Phys. A2009, 95, 303.

[1] A. Facchetti, Mater. Today2007, 10, 28.[2] A. Nitzan,Ann. Rev. Phys. Chem.2001, 52, 681.[3] J. Zaumseil, H. Sirringhaus, Chem. Rev.2007, 107, 1296.[4] R. N. Christopher, C. D. Frisblie, A. Demetrio, S. da Filho,

J-L. Brdas, P. C. Ewbank, K. R. Mann, Chem. Mater. 2004, 16,4436.

[5] C. Ego, D. Marsitzky, S. Becker, J. Zhang, A. C. Grimsdale, K. Mllen,

J. D. MacKenzie, C. Silva, R. H. Friend,J. Am. Chem. Soc.2003, 125,437.

[6] M. Sadrai, L. Hadel, R. R. Sauers, S. Husain, K. Krogh-Jespersen,J. D. Westbrook, G. R. Bird,J. Phys. Chem.1992, 96, 7988.

[7] M. P. ONeil, M. P. Niemczyk, W. A. Svec, D. Gosztola,G. L. Gaines III, M. R. Wasielewski, Science1992, 257, 63.

[8] K.-Y. Law, Chem. Rev.1993, 93, 449.[9] A. P. H. J. Schenning, J. v. Herrikhuyzen, P. Jonkheijm, Z. Chen,

F. Wrthner, E. W. Meijer,J. Am. Chem. Soc.2002, 124, 10252.[10] F. Wrthner, Chem. Commun.2004, 1564.[11] J. A. A. W. Elemans, R. van Hameren, R. J. M. Nolte, A. E. Rowan,

Adv. Mater.2006, 18, 1251.[12] L. Zang, Y. Che, J. S. Moore,Acc. Chem. Res.2008, 41, 1596.[13] S. V. Bhosale, C. H. Jani, S. J. Langford, Chem. Soc. Rev.2008, 37,

331.

[14] Z. Chen, A. Lohr, C. R. Saha-Mller, F. Wrthner, Chem. Soc. Rev.2009, 38, 564.

[15] D. M. de Leeuw, M. M. J. Simenon, A. R. Brown, R. E. F. Einerhand,Synth. Met.1997, 87, 53.

[16] A. Facchetti, M.-H. Yoon, C. L. Stern, G. R. Hutchison, M. A. Ratner,T. J. Marks,J. Am. Chem. Soc.2004, 126, 13480.

[17] The relationship between orbital energy and redox potential wasfirst suggested by Pearson as: 0.0 V vs NHE approximately corre-sponds to a 4.5 eV electron binding energy. See: R. G. Pearson,J. Am. Chem. Soc.1986, 108, 6109.

[18] Following the standard usage in discussing organic electronics,we use the term LUMO energy to indicate the affinity-level energy.This is actually quite misleading, because as soon as this level isoccupied, a combination of Coulomb, exchange and vibronic inter-actions will modify the energy substantially. In comparing a seriesof similar molecules, such as the rylenes, it is reasonable to assumethat these three energy changes will be similar across the series, sothat computed LUMO energies are useful for qualitative compartivedescriptions.

[19] H. Usta, C. Risko, Z. Wang, H. Huang, M. K. Deliomeroglu,A. Zhukhovitskiy, A. Facchetti, T. J. Marks, J. Am. Chem. Soc.2009,131, 5586.

[20] J. A. Letizia, M. R. Salata, C. M. Tribout, A. Facchetti, M. A. Ratner,T. J. Marks,J. Am. Chem. Soc.2008, 130, 9679.

[21] B. A. Jones, A. Facchetti, M. R. Wasielewski, T. J. Marks, J. Am.Chem. Soc.2007, 129, 15259.

[22] M.-H. Yoon, S. A. DiBenedetto, M. T. Russell, A. Facchetti,T. J. Marks, Chem. Mater.2007, 19, 4864.

[23] Q. Zheng, J. Huang, A. Sarjeant, H. E. Katz,J. Am. Chem. Soc.2008,

130, 14410.[24] J. G. Laquindanum, H. E. Katz, A. Dodabalapur, A. J. Lovinger,J. Am. Chem. Soc.1996, 118, 11331.

[25] H. E. Katz, J. Johnson, A. J. Lovinger, W. Li,J. Am. Chem. Soc.2000,122, 7787.

[26] H. E. Katz, A. J. Lovinger, J. Johnson, C. Kloc, T. Siegrist, W. Li,Y.-Y. Lin, A. Dodabalapur, Nature2000, 404, 478.

[27] K. C. See, C. Landis, A. Sarjeant, H. E. Katz, Chem. Mater.2008, 20,3609.

[28] B. J. Jung, J. Sun, T. Lee, A. Sarjeant, H. E. Katz, Chem. Mater.2009,21, 94.

[29] D. Shukla, S. F. Nelson, D. C. Freeman, M. Rajeswaran,W. G. Ahearn, D. M. Meyer, J. T. Carey, Chem. Mater. 2008, 20,7486.


17/17



[72] W. S. Shin, H.-H. Jeong, M.-K. Kim, S.-H. Jin, M.-R. Kim, J.-K. Lee,J. W. Lee, Y.-S. Gal,J. Mater. Chem.2006, 16, 384.

[73] Z. A. Tan, E. J. Zhou, X. W. Zhan, X. Wang, Y. F. Li, S. Barlow,S. R. Marder,Appl. Phys. Lett.2008, 93, 073309.

[74] J. A. Mikroyannidis, M. M. Stylianakis, G. D. Sharma, P. Balraju,M. S. Roy,J. Phys. Chem. C2009, 113, 7904.

[75] L. Loo. Presented at the 4th ACS-MRS-IEEE Meetings (2009).

[76] M. M. Alam, S. A. Jenekhe, Chem. Mater.2004, 16, 4647.[77] S. King, M. Sommer, S. Huettner, M. Thelakkat, S. A. Haque,

J. Mater. Chem.2009, 19, 5436.[78] Q. Zhang, A. Cirpan, T. P. Russell, T. Emrick, Macromolecules2009,

42, 1079.[79] L. Buo, X. Guo, B. Yu, Y. Qu, Z. Xie, D. Yan, Y. Geng, F. Wang,J. Am.

Chem. Soc.2009, 131, 13242.[80] G. Sang, Y. Zou, Y. Huang, G. Zhao, Y. Yang, Y. Li, Appl. Phys. Lett.

2009, 94, 193302.[81] L. Meng, Y. Shang, Q. Li, Y. Li, X. Zhan, Z. Shuai, R. G. E. Kimber,

A. B. Walker,J. Phys. Chem. B2010, 114, 36.[82] D. Veldman, S. C. J. Meskers, R. A. J. Janssen, Adv. Funct. Mater.

2009, 10, 1939.[83] C. R. McNeill, S. Westenhoff, C. Groves, R. H. Friend,

N. C. Greenham,J. Phys. Chem. C2007, 111, 11953.

[84] H. Ohkita, S. Cook, Y. Astuti, W. Duffy, S. Tierney, W. Zhang,M. Heeney, I. McCulloch, J. Nelson, D. D. C. Bradley, J. R. Durrant,J. Am. Chem. Soc.2008, 130, 3030.

[85] K. Hummer, C. Ambrosch-Draxl, Phys. Rev.2005, B71, 081202.

[59] T. N. Ng, S. Sambandan, R. Lujan, A. C. Arias, C. R. Newman,H. Yan, A. Facchetti,Appl. Phys. Lett.2009, 94, 233307.

[60] J. Rivnay, L. H. Jimison, J. E. Northrup, M. F. Toney, R. Noriega,S. Lu, T. J. Marks, A. Facchetti, A. Salleo, Nat. Mater.2009, 8, 952.

[61] X. Zhan, Z. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li,D. Zhu, B. Kippelen, S. R. Marder,J. Am. Chem. Soc.2007, 129, 7246.

[62] X. Zhan, Z. Tan, E. Zhou, Y. Li, R. Misra, A. Grant, B. Domercq,

X. Zhang, Z. An, X. Zhang, S. Barlow, B. Kippelen, S. R. Marder,J. Mater. Chem.2009, 19, 5794.

[63] Z. Chen, Y. Zheng, H. Yan, A. Facchetti,J. Am. Chem. Soc.2009, 131, 8.[64] S. Httner, M. Sommer, M. Thelakkat, Appl. Phys. Lett. 2008, 92,

093302.[65] H. Yan, Z. Chen, Y. Zheng, C. Newman, J. R. Quinn, F. Dtz,

M. Kastler, A. Facchetti, Nature2009, 457, 679.[66] A. Babel, S. A. Jenekhe,J. Am. Chem. Soc.2003, 125, 13656.[67] E. M. Giacobbe, Q. Mi, M. T. Colvin, B. Cohen, C. Ramanan,

A. M. Scott, S. Yeganeh, T. J. Marks, M. A. Ratner, M. R. Wasielewski,J. Am. Chem. Soc.2009, 131, 3700.

[68] E. A. Weiss, M. J. Ahrens, L. E. Sinks, A. V. Gusev, M. A. Ratner,M. R. Wasielewski,J. Am. Chem. Soc.2004, 126, 5577.

[69] S. Shoaee, Z. An, X. Zhang, S. Barlow, S. R. Marder, W. Duffy,M. Heeny, I. McCulloch, J. R. Durrant, Chem. Commun.2009, 5445.

[70] M. Hiramoto, H. Fujiwara, M. Yokoyama,Appl. Phys. Lett.1991, 58,1062.

[71] L. Schmidt-Mende, A. Fechtenktter, K. Mllen, E. Moons,R. H. Friend, J. D. MacKenzie, Science2001, 293, 1119.

Xiaowei Zhan et al- Rylene and Related Diimides for Organic Electronics

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