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DOI: 10.1002/adma.200703066

Grain-Boundary Evolution in a Pentacene Monolayer**

By Jian Zhang,* Jurgen P. Rabe, and Norbert Koch

Organic thin-film transistors (OTFTs) based on pentacene

polycrystalline films, for applications ranging from flat-panel

displays to radio frequency identification tags, have achieved

field-effect mobilities over 5 cm2 V�1 s�1,[1–5] which is com-

parable to that of competing amorphous silicon devices. Based

on studies of morphology and structure of organic semicon-

ductors, the structural quality of thin films was recognized as

the key for achieving high field-effect mobility values.[6–8]

Significant attention has been paid to the growth dynamics of

pentacene films in order to achieve even higher mobilities in

thin organic films, since work done on structurally perfect

single crystals indicates that several tens of cm2 V�1 s�1 can

be reached in principle.[9–12] For example, a mobility of

35 cm2V�1 s�1 was reported for a pentacene single crystal at

room temperature.[12] However, self-driven polycrystallization

has been directly observed from a single nucleus in the case of

epitaxial pentacene growth on Si(111)-H terminated surfaces

in ultrahigh vacuum, which indicates that polycrystallinization

might be an intrinsic property of pentacene films when growth is

governed predominantly by diffusion-limited aggregation.[13]

The macroscopic properties of polycrystalline films are

governed by both grains and grain boundaries (GBs). While

carrier mobility generally increases with decreasing tempera-

ture in high-quality organic single crystals, indicative of band-

like transport, mobility values increase for elevated tempera-

tures in thin-film samples, where hopping transport prevails.[14]

Significant charge-carrier trapping defects have been shown to

exist at GBs in polycrystalline pentacene films, and these GBs

are the principal bottleneck for charge transport in OTFTs and

also affect their overall lifetime.[1–5,15–19] Understanding the

formation and evolution of GBs in pentacene films will help to

better understand the growth of organic polycrystalline films,

and controlling the number of GBs in these films might be

facilitated. As charge transport inOTFTs is confined to the first

few molecular layers on the gate insulator,[20] our study is

focused on the evolution of GBs in the monolayer at the

organic semiconductor/dielectric (SiO2) interface. Using

transverse shear microscopy, we directly observed polycrys-

tallization within single pentacene topographical islands and

[*] Dr. J. Zhang, Prof. J. P. Rabe, Dr. N. KochInstitut fur PhysikHumboldt-Universitat zu BerlinNewtonstrasse 15, 12489 Berlin (Germany)E-mail: zhang@physik.hu-berlin.de

[**] We thank Jorg Barner for providing the program to measure grainboundaries. This work was supported by the Sfb448 (DFG). J.Z.acknowledges the Alexander von Humboldt Foundation for anindividual research fellowship. N.K. acknowledges financial supportby the Emmy Noether Program (DFG).

� 2008 WILEY-VCH Verlag Gmb

derived the GB evolution in the pentacene monolayer. In

addition, the effect of post-fabrication thermal annealing on

the GB density (j) in the first pentacene layer was studied.

The early stages of pentacene film growth on SiO2 have been

investigated earlier by photoelectron emission microscopy,[9]

atomic force microscopy (AFM),[21] and grazing-incidence

X-ray diffraction,[22–24] where themolecules were shown to stand

vertically or almost vertically on the substrate. Transverse shear

microscopy is a variant of lateral force microscopy, tracking the

twisting of the cantilever owing to lateral forces acting

perpendicular to the scan vector. Transverse shear microscopy

can be used to visualize the relative orientation of grains in thin

films with high contrast.[25,26] Figure 1a shows a contact-mode

topography image of a pentacene film with a coverage (u) of

0.17 monolayer (ML) in which islands are separated. The

transverse shear image (Fig. 1b) shows remarkable contrast

between individual islands, which is not evident in the corres-

ponding topography image. Apparently, the islands have different

in-plane crystal orientations, rotated around the direction of the

surface normal. In addition to pentacene single-crystal islands

(e.g., island b1) there are several islands that consist of two or

even more grains (e.g., island b2). These polycrystalline

pentacene islands comprise about 11% of all pentacene islands

at u¼ 0.17ML. Consequently, the existence of GBs within

single pentacene islands (not visible with standard contact or

tapping-mode AFM) indicates that ‘‘single-grain’’ transport

measurements should be carefully reviewed.[27–29] Polycrys-

tallization was observed before for epitaxial pentacene layers

grown on a hydrogen-terminated Si(111) surface, and was

attributed to kinetic growth processes in conjunction with the

intrinsic anisotropy of the pentacene crystal structure and

energy.[13] This anisotropy and the pentacene epitaxial relation

with the substrate resulted in preferential growth along the

b axis of the pentacene unit cell. Such type of intrinsic,

self-driven polycrystallization may apply also for pentacene

grown on SiO2. However, as no epitaxial relation exists for this

system (SiO2 is amorphous) and only a relatively low

percentage of all islands exhibits GBs at u¼ 0.17 ML, an

alternative mechanism for early intraisland GB creation may

be the simultaneous formation of two stable nuclei at substrate

defects (of morphological and/or chemical nature).

Subsequently, pentacene films with increasing u varying up

to a full monolayer were studied to assess GB evolution at the

early stage of the film growth. Figure 2 shows topography images

and corresponding transverse shear images of four pentacene

films with u equal 0.32ML, 0.53ML, 0.89ML, and 1.24ML,

respectively. The island density in the topography images

(Fig. 2a–d) is 2.76, 1.92, 0.01, 0.01 islands mm�2, respectively.

The values for the highest two coverage are upper bounds as

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Figure 1. Contact-mode AFM images of a pentacene film on SiO2 withcoverage of 0.17 ML (scan size: 10� 10mm2). a) Topography image.b) Transverse shear image, with zoom-in image b1) for a single-crystalisland and b2) for a polycrystal island.

island coalescence resulted in only one interconnected island

per image. The island density of the u¼ 0.17ML pentacene film

(Fig. 1a) is 2.78 islands mm�2. The chosen coverages reveal

characteristic steps of film growth: separated islands, island

coalescence, and formation of a continuous film. At low

coverage (<0.53ML), only intraisland GBs are observed. Both

intra- (Fig. 2f1) and interisland GB (Fig. 2f2) were first

observed in the 0.53ML sample, clearly indicating the u range

in which island coalescence sets in. Consequently, these two

types of GBs exist in a closed pentacene layer. The j in these

pentacene films was evaluated by measuring the length of GBs

between two touching grains (all island borders were

excluded). The dependence of j on u is shown in Figure 3.

The data clearly allow to differentiate between two stages of

film growth: island growth (u< 0.53 ML) and island coales-

cence (u� 0.53 ML). The data in the low-coverage regime

Figure 2. Topography images and transverse shear images of pentacene films0.53 ML, with zoom-in image f1 for an intra-island GB and f2 for an inter-is

Adv. Mater. 2008, 20, 3254–3257 � 2008 WILEY-VCH Verl

suggest a linear increase of j with u. This increase is stronger

than is for the case for a constant number of polycrystalline

islands during island growth, where j�ffiffiffi

up

is expected (as j

scales linearly with the island radius, while u scales with the

island area). Our data therefore suggest that new GBs within

single topographical islands are formed even during island

growth before island coalescence becomes important. The

superlinear increase of j for u> 0.53ML is a consequence of

island coalescence. Extrapolating the linear fit in Figure 3 to j

of zero yields u¼ 0.05 ML. Consequently, the formation of

intra-island GBs at the very beginning of pentacene film

growth can be concluded.

From Figure 2h it can be seen that pentacene multilayer

islands can span over several grains in the monolayer.

Consequently, the apparent island density observed by regular

scanning force microscopy (e.g., in tapping mode) imaging on

thick films may not be representative of the GB density in the

monolayer in direct contact with the gate dielectric in OTFTs.

This lack of information on the GB distribution in buried

organic layers can thus lead to unexpected trends of the charge

carrier mobility in relation to thick-film island density, as, for

example, observed by Shtein et al.[30]

Post-production thermal annealing of pentacene films in

various environments has been widely studied.[31–34] These

studies mainly concentrated on the improved bulk structural

order and on the increase of carrier mobility. Information on

the effect of thermal annealing on the first few pentacene layers

on dielectrics is not available although major charge-carrier

transport takes place in these layers in OTFTs. Pentacene films

with u¼ 1.24 ML were annealed at 110 8C for 30 minutes and

150 minutes in anAr atmosphere at ambient pressure. At these

conditions pentacene molecules do not sublimate. Figure 4a

on SiO2 with different coverage (scan size: 10� 10mm2). a,e) 0.32 ML; b,f)land GB; c,g) 0.89 ML; and d,h) 1.24 ML.

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Figure 3. The dependence of the GB density j in pentacene film growthwith coverage u. The dashed vertical line indicates the transition fromisland growth to island coalescence.

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gives a transverse shear image of a pentacene film annealed at

110 8C for 30min. Compared with Figure 2h, the GBs appear

less meandered and smoother, and the number of GBs is

reduced after annealing. The corresponding decrease of j as

function of annealing time is shown in Figure 4b. The data

could be fitted to single exponential decay. After annealing for

Figure 4. a) Transverse shear image of ca. monolayer pentacene on SiO2

after annealing at 110 8C for 30min in Ar atmosphere (scan size:10� 10mm2). b) Change of the GB density for different annealing timesof a pentacene monolayer on SiO2.

www.advmat.de � 2008 WILEY-VCH Verlag GmbH &

150 minutes j of the film decreased by ca. 30%. This

demonstrates that post-production thermal annealing not only

improves the bulk crystal structure, but significantly reduces

the GB density of a pentacene monolayer right at the organic

semiconductor/dielectric interface.

In conclusion, GBs within single pentacene topographical

islands on SiO2 were directly observed by transverse shear

microscopy. These intraisland GBs form at very early stages of

pentacene film growth (in the present case at a coverage of

0.05ML). Consequently, charge-carrier mobility values from

studies on single topographical islands may still include

contributions from GBs. During island growth the intraisland

GB density increases linearly, suggesting a continued forma-

tion of new GBs also before islands coalesce. Post-fabrication

thermal annealing significantly reduces the GB density in the

pentacene layer in direct contact with the dielectric, which

should lead to a considerable decrease of charge-carrier

trapping sites.

Experimental

Thin-film Deposition: Pentacene (99.5%; Aldrich Chemical Co.)was used as received. SiO2 substrates were treated by standardsolvent-cleaning procedures before use. Pentacene films wereevaporated in a high vacuum chamber with a pressure duringevaporation of 2� 10�4 Pa. The deposition rate was monitored witha quartz crystal microbalance and was kept constant at 1 Amin�1 for allexperiments. The substrate was held at room temperature duringdeposition.

Scanning Force Microscopy/Transverse Shear Microscopy: Allsamples were investigated using a Veeco Metrology NanoscopeLFM (lateral force microscopy)-3 atomic force microscope underatmospheric conditions. Lateral force microscopy is a contact-modetechnique, where the local variations in the sliding friction between thetip and the sample can be imaged in conjunction with topography,enabling a direct correlation between the two [22]. For transverse shearmicroscopy, the twisting of the cantilever was tracked, resulting from alateral force acting perpendicular to the scan vector while simulta-neously recording contact-mode topography. The grain specificity intransverse shear images results from an anisotropic transverse shearstress field in different crystalline domains. Transverse shear micro-scopy was done using tips fabricated by Veeco Metrology, USA(Triangular silicon nitride contact mode tips, model NP, force constant:0.06–0.58 Nm�1). The GB density was evaluated with a customsoftware program. Only boundaries between two grains weremeasured; island borders were excluded.

Received: December 10, 2007Revised: February 12, 2008

Published online: July 14, 2008

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