Simultaneous Surface and Bulk Imaging of Polymer Blends with X-ray Spectromicroscopy

Communication

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Simultaneous Surface and Bulk Imaging ofPolymer Blends with X-ray Spectromicroscopya

Benjamin Watts, Christopher R. McNeill*

We demonstrate the utility of soft X-ray spectromicroscopy to simultaneously image thesurface and bulk composition of polymer blend thin films. In addition to conventionalscanning transmission X-ray microscopy that employs a scintillator and photomultiplier tubeto measure the transmitted X-ray flux, channel-tron detection of near-surface photoelectrons isemployed to provide information of the compo-sition of the first few nanometers of the film.Laterally phase-separated blends of two poly-fluorene co-polymers are studied, with the struc-ture of both wetting and capping layers clearlyimaged. This new information provides insightinto the connectivity of bulk and surface struc-tures that is of particular relevance to the oper-ation of such blends in optoelectronic devices.

Introduction

The solution processing of thin-film polymer blends is a

convenient route toproducefilmswith tunable functionality

and morphology.[1] Thin-film blends of semiconducting

polymers in particular are of interest for applications in

organic solar cells, light emitting diodes (LEDs) and non-

volatilememories.[2] Filmdeposition is typicallyachievedvia

spin-coating, a process that results in complex, non-

equilibrium phase-separated structures. Our ability to

correlate device performance and morphology relies

C. R. McNeillCavendish Laboratory, University of Cambridge, J J Thomson Ave,Cambridge CB3 0HE, United KingdomFax: (þ44) 1223 764515; E-mail: [email protected]. WattsPaul Scherrer Institut, 5232 Villigen PSI, Switzerland

a : Supporting information for this article is available at the bottomof the article’s abstract page, which can be accessed from thejournal’s homepage at http://www.mrc-journal.de, or from theauthor.

Macromol. Rapid Commun. 2010, 31, 1706–1712

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonline

critically on our ability to characterise film microstructure.

Traditionally, techniques such as atomic-force micro-

scopy[3,4] and electron microscopy[5] have been employed

to image blend structure. However both AFM and TEM lack

chemical sensitivity, making it difficult to decouple surface

topographyandsurfacecompositionortodeterminedomain

purity. Soft X-ray spectromicroscopy provides a chemically

sensitive alternative, whereby differences in X-ray absorp-

tionspectraat the carbonK-edgeof theconstituentmaterials

provides contrast that derives from differences in anti-

bondingelectronicstructure.[6]Thuscontrastcanbeachieved

for materials with no difference in electron density or

constituent atoms. The incident X-ray beam is focused via a

zone-plate to a spot-size of �30nm and the intensity of the

transmitted light recorded as the sample is rastered with

respectto thebeam.Figure1presentsaschematicdiagramof

a typical scanning transmission X-ray microscope.

To date, soft X-ray spectromicroscopy has been employed

in transmissionmodewhich samples the bulk of the film.[7]

Recently, the ability to use soft X-ray spectromicroscopy to

also imagethesurfaceofthinfilmsvia thetotalelectronyield

(TEY) detection method has been demonstrated.[8] Electrons

library.com DOI: 10.1002/marc.201000269

Simultaneous Surface and Bulk Imaging of Polymer Blends with X-ray . . .

Figure 1. Schematic diagram of the scanning transmission X-raymicroscopy geometry showing the positioning of the channeltronand photomultiplier tube (PMT) facilitating simultaneous surfaceand bulk imaging.

Figure 2. (a) Chemical structures of PFB and F8BT. (b) NEXAFSspectra of neat films of PFB and F8BT with inset showingdifferences in the p� region.

are collected (without kinetic energy discrimination) by a

channeltron detector mounted next to the photomultiplier

tube as shown in Figure 1. Surface sensitivity of the TEY

detectionmode is afforded through the shortmean free path

of electrons in solids. While photo-, Auger and secondary

electrons are generated throughout the sample volume due

to photoabsorption, subsequent atomic relaxation and

inelastic scatteringprocesses, only thoseelectronsgenerated

very close to the sample surface are able to emerge from the

sample to be detected. The TEY depth sensitivity for

conjugated polymers has been determined to be of order

2.5nm (for 63% signal threshold) by Chua et al.[9]

Imaging the surface structure of thin-film polymer blends

is important since during film formation the substrate/film

and film/air interfaces can impose a direction on phase

separation leading to the formation of wetting and capping

layers that form in competition with bulk phase separa-

tion.[3,10,11] Wetting and capping layers can be particularly

important when the length-scale of bulk phase separation

exceeds that of the film thickness. The polymer blend studied

here is that of laterally phase separate blends of

the polyfluorene co-polymers poly(9,90-dioctylfluorene-co-

bis(N,N0-(4,butylphenyl))bis(N,N0-phenyl-1,4-phenylene)-

diamine) (PFB) and poly(9,90-dioctylfluorene-co-benzothia-

diazole) (F8BT), see Figure 2(a) for chemical structures.

PFB:F8BT blends have been well studied in prototype solar

cells[12] and the morphology of such blends has been

characterised previously by a number of experimental

techniques including scanning transmission X-ray micro-

scopy,[13] scanning Kelvin probe microscopy (SKPM)[14],

Raman microscopy[15] and environmental scanning elec-

tronmicroscopy (ESEM).[16] Blends of F8BTwith the related

co-polymer TFB (poly(9,9-dioctylfluorene-co-N-(4-butyl-

phenyl)diphenylamine)) areused inefficient light-emitting

diodes[17] and show very similar morphologies to PFB:F8BT

blends.[18,19] Previous X-ray photoelectron spectroscopy

(XPS) measurements averaging an area significantly larger

than the length scale of phase separation of TFB:F8BT

blends have provided evidence for awetting layer of TFB at

the film/substrate interface and a partial capping of the


� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

F8BT domains with TFB at the film/air interface. SKPM

measurements have also provided evidence for a partial

capping layer of PFB on top of the F8BT domain in PFB:F8BT

blends through imaging of surface photovoltage. Blends of

other polyfluorene co-polymers have been observed to

show similar structures.[11,20] Here we provide a direct

imaging of the partial PFB capping layer, and reveal the

structure of the PFB wetting layer that has not previously

been imaged. In particular we show that the F8BT droplets

enclosed in the PFB-rich phase penetrate the PFB-wetting

layer connecting the top andbottom surfaces of the film.As

these films are incorporated into devices by sandwiching

between two electrodes, being able to determine which

parts of the film are connected to which electrodes is

important in order to understand device operation and to

develop morphologically correct device models.

Experimental Part

Microscopy Details

Experiments were performed at the PolLux beamline at the Swiss

Light Source, Paul Scherrer Institut, Villigen, Switzerland.[21] As

described above and illustrated schematically in Figure 1, the

www.mrc-journal.de 1707

B. Watts, C. R. McNeill

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PolLux microscope was initially developed to operate in a

transmission geometry with a zone plate (outer zone width of

35nm) used to focus monochromatic X-rays from the beamline

(bending magnet beamline[22] with monochromator and higher

order suppressor) on the sample. An order sorting aperture (OSA) is

used to block zero order light that is not diffracted by the zoneplate

and orders higher than first order. Free-standing filmsmounted on

copperTEMgridsareheld in the focal positionof thebeam,with the

TEM gridsmounted on an earthedmetal sample plate. The sample

is rastered with respect to the focused soft X-ray beam using an

interferometer-controlled piezo stage. Transmitted light is mea-

sured using a scintillator and photomultiplier tube. Detection of

surface photoelectronswas achievedusing a channeltron biased at

2.5 kV (model no. KBL10RS, Dr. Sjuts Optotechnik GmbH) with the

chamber pumped down to �10�6mbar using a turbo pump and

coldfinger. Due to thegeometry of themicroscope, the channeltron

was mounted adjacent to the photomultiplier tube as shown in

Figure 1 and detects photoelectrons emitted from the surface of

the film facing the detectors. A bias of þ400V was applied to the

metal OSA to suppress photoemission of electrons from the OSA

that otherwise swamps the signal from the sample. Themeasured

channeltron signal is dependent on the number of photons present

at the surface layer from which the photo-emitted and resulting

secondary electrons are able to escape the sample surface and

hence reach the channeltron detector. Since the photonsmust pass

through the sample before reaching the electron signal’s selvedge,

someof the photons are absorbedbefore reaching the selvedge and

hence the measured electron signal is coupled to film thickness.

However, since the selvedge is very thin (�2.5 nm)[9] compared to

the attenuation length of the photons (varies stronglywith photon

energy, but is at least 100nmfor thematerials andphotonenergies

studied in thiswork), photonabsorptionwithin the selvedgecanbe

considered tobean insignificant fractionof the totalbeamandthus

the transmitted photonflux can be considered to be representative

of the photon flux within the selvedge. Therefore, the surface

signal may be normalised simply by dividing by the measured

transmitted photonflux.[8]While not necessary in thiswork, due to

concentrating on the surface signal measured at a single photon

energy, a fully quantitative normalisation should also take into

account the photon energy dependence of the detector efficiencies.

The surface structure of the film/air interface (the ‘top surface’)

and substrate/film interfaces (the ‘bottomsurface’)were examined

separately by measuring films mounted with the top surface and

then the bottom surface facing the detectors in turn. Experiments

were performed at 285 eV corresponding to the strong p� peak

of PFB compared to F8BT that provides strong contrast,[13] see

Figure 2(b). Images were also taken at 280 eV (pre-edge), 284.5 eV

(F8BT resonance) and 320eV (chemically insensitive), provided in

the Supporting Information.

Figure 3. (a) AFM image of the surface topography of the PFB:F8BTblend. (b) and (d) STXM-derived PFB composition images of twodifferent regions of the film corresponding to the two differentregions presented in Figure 4. (c) and (e) Corresponding STXM-derived film thickness images.

Sample Preparation Details

Poly(9,90-dioctylfluorene-co-bis(N,N0-(4,butylphenyl))bis(N,N0-phenyl-

1,4-phenylene)diamine) and F8BT were supplied by Cambridge

Display Technology, Ltd and were used as received. PFB had a

molecularweight (Mn)andpolydispersityindex(PDI)of135kg �mol�1

and2.8,respectively,whileforF8BTMn ¼100kg �mol�1andPDI¼2.1.

Blendswerepreparedbydissolving inp-xylenewitha1:1weight ratio



with a total solution concentration of 16.5mg �mL�1 and spin-coated

onto glass substrates at 2 000rpm to give films of �150nm average

thickness. Filmswere transferred to copper TEM grids by floating the

film off in water and picking up with grids.

Results and Discussion

Figure 3(a) presents anAFM image of the topography of the

top surface with �70nm differences in height between

phases foranaveragefilmthicknessof�150nm.The raised,

continuous domain corresponds to an F8BT-rich domain

while the lower-lying enclosed domain is PFB-rich, as

demonstrated in the STXM derived images of similarly

prepared blend films shown in Figure 3(b)–(e) as well as in

DOI: 10.1002/marc.201000269


previous measurements.[13] (The percentage composition

images of Figure 3(b)–(e) were acquired by imaging at four

different energies and performing singular value decom-

position, with reference to known NEXAFS spectra for the

component materials, as described in our previous pub-

lication.[19])WithinthePFB-richdomain, enclosedF8BT-rich

droplets can be clearly seen in both the AFM and STXM

images. The surface of the F8BT-rich domain shows

additional structure in the AFM image in the form of a

slightly raised lip. While this surface feature is invisible in

the conventional STXM transmission measurements

shown in Figure 3(b)–(e), measurements via SKPM, XPS

and Raman spectroscopy have established that the lower

lying regions of the F8BT domain surface correspond to a

partial PFB[14] (or TFB[18]) capping layer. On the other hand,

the STXM measurements also reveal sub-surface PFB

droplets in the F8BT-rich domain that are not observed

by AFM.

Figure 4 presents X-ray microscopy images taken at

285 eV (corresponding to strong absorption by PFB) of the

Figure 4. Transmission (a) and (b); channeltron TEY (c) and (d) andnormalised TEY (e) and (f) images of the PFB:F8BT blend. (a) and(c) were acquired simultaneously with the top of the film facingthe detectors while (b) and (d) were taken simultaneously of adifferent region of the film with the bottom of the film facing thedetectors. Images (e) and (f) were computed by normalising tothe transmitted photon flux acquired in (a) and (b), respectively.Scale bars record photon or electron counts in (a)–(d) and the ratioof collected electrons to transmitted photons in (e) and (f).



film regions shown in Figure 3(b)–(e). Surface images

were taken at other energies (albeit at lower dwell

time, see Supporting Information) however potential

differences in the electron escape depth of PFB and

F8BT and the additional time required to image the

surface (causing increased beam damage) presently

limits a full quantitative analysis of surface composition.

Complete quantification of the surface composition

requires significant effort beyond what has been

measured in this work and is a topic for further study.

Figure 4(a) and (b) presents raw transmission images of

the two different regions of the film studied. Figure 4(c)

presents the raw TEY image of the top surface (i.e. imaged

with this surface facing the detectors). Since Figure 4(a)

displays the number of photons that are transmitted

by the film, dark regions correspond to areas of strong

absorption, either by preferential absorption by PFB or

by the film being thicker at a given point. Similarly,

since Figure 4(c) displays the number of electrons emitted

by the surface, bright regions correspond to preferential

absorption by PFB at the surface and/or regions that

are thinner and so allow more photons to reach the

detector-side sample surface. Figure 4(e) presents a surface

image that has been corrected for the transmission of

light through the film by dividing the raw TEY image

of Figure 4(c) by the transmission image of Figure 4(a).

While the structure of the F8BT-rich domain as imaged

in transmission is dominated by the sub-surface

PFB droplets, the surface image clearly reveals the

partial PFB capping layer matching the structure seen

withAFM.Wenote, however, that theAFMmeasurements

themselves are not able to identify this surface region

as being PFB-rich, but rely on a combination of other

measurements. (See also Figure S2 for TEY images taken

at other energies but with lower dwell time that

confirm our assignment of surface composition.) The

bulk/surface X-ray microscopy images here in contrast

provide bulk and surface images with high resolution

and unambiguous chemical contrast. Interestingly,

there does seem to be a correlation between the

structure of the capping layer imaged in Figure 4(c) and

the PFB-rich droplets imaged in Figure 4(a), with the

edges of the capping layer appearing to be pinned to

PFB droplets. Areas of the capping layer immediately

above the PFB-rich drops also appear slightly brighter,

indicating that the PFB capping layer is both slightly

thinner than the sampling depth of the electron-yield

measurement and that the capping layer is connected

to underlying PFB droplets. Furthermore, these observa-

tions suggest that the formation of the capping layer

and PFB droplets are likely to be inter-related. Dark

regions within the mesoscale PFB domain, corresponding

to enclosed F8BT droplets, are observed in both the

surface and bulk images and that are also imaged by



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AFM. Further, the intensity of the surface image of

Figure 4(e) at the positions of the enclosed F8BT droplets

is similar to that in the uncapped areas of the mesoscale

F8BT domains, demonstrating that the enclosed F8BT

droplets extend to the surface.

Figure 4(b), (d) and (f) present images of a different

region of the film takenwith the bottom surface oriented

toward the detectors. In general there is less contrast

seen in the structure of the bottom surface, with

the underside of the F8BT-rich domains appearing as

bright as the bottom of the PFB-rich domain, consistent

with the presence of a PFB wetting layer. However,

the F8BT-rich droplets enclosed in the PFB-rich phase

do show strong contrast, appearing much darker than

the surrounding region, demonstrating that the PFB

wetting layer is penetrated by these columnar F8BT-rich

droplets. In combination with the information from the

top side of the film in Figure 4(e), we conclude that

the F8BT-rich droplets enclosed in the PFB-rich phase

extend completely from the top to bottom surfaces.

In the bottom surface image of Figure 4(f), we again

see bright spots in the F8BT-rich domains that correlate

with the dark PFB droplets in the bulk image of

Figure 4(d). These features mirror those observed in

the topside images of Figure 4(a), (c) and (e) and similarly

indicate that the PFB wetting layer has a similar

thickness to the TEY depth sensitivity and that the

PFB droplets are connected to this wetting layer. This

hypothesis is also consistent with our previous observa-

tions of the evolution of the morphology of PFB:F8BT

films in a solvent saturated atmosphere where the

enclosed PFB-rich droplets were observed to drain

into a surface layer.[23]

Another interesting feature of the surface images of

Figure 4(e) and (f) are the bright rings at the mesoscale

domain boundaries. While this is mirrored by a similar

dark ring in the transmission images of Figure 4(a) and (b),

the composition images in Figure 3(b) and (d) demonstrate

that these features do not originate from an enhanced

PFB concentration in the bulk of the film at the mesoscale

domain boundaries. The dark ring in the transmission

images of Figure 4(a) and (b) are explained by variations

in film thickness at the mesoscale domain boundaries,

as shown in Figure 3(c) and (e). While it is possible that

the surface electron yield is enhanced by the sloping

surface of the sample (via increased illuminated surface

area), the magnitude of this effect would be insufficient

to fully account for the bright domain boundary feature

seen on the top surface in Figure 4(e) and cannot explain

the similar feature in Figure 4(f) since the bottom of

the film is kept flat by the presence of a substrate during

film formation. Therefore these bright features must

correspond to a real PFB concentration at both film

surfaces.



Further information can be gleaned from the STXM

images by examining the surfaces of the mesoscale

domains shown in Figure 4(e) and (f) with reference to

the measured compositions that are shown in Figure 3(b)

and (d). The F8BT domains are straightforward: the

top surface is partially capped with PFB, with the

remainder likely showing the exposed bulk mixture

and the bottom surface totally coveredwith a PFBwetting

layer. Comparing the brightness of the surface of the

PFB domain to the brightness of the PFB capping layer

(Figure 4(e)) reveals the surface of the PFB domain to be

less pure. While difficult to quantify, given that the

bulk purity of the PFB domain is measured to be �70%[13]

and assuming a pure PFB capping layer, the surface

of the PFB domain is observed to be less pure than the

bulk. Therefore the PFB domain can be considered to

have a vertical composition gradient, with F8BT enrich-

ment at the top, although the intensity signal of

the PFB wetting layer is consistent with the bulk

composition. Given that F8BT is observed to segregate

to the top of the PFB phase, the origin of the PFB capping

layer on the F8BT phase is not clear, though it may

arise due to a continuous capping layer of PFB that

forms initially[3] then dewets following the earlier

solidification of F8BT.

For device applications where such films are sand-

wiched between two electrodes, connectivity of one

phase from one interface to the other represents a

direct pathway for a carrier type (electrons, e.g. in F8BT)

to transit fromelectrode to electrode. For LED applications,

such as with TFB:F8BT blends, such pathways are not

desirable as it affords the opportunity for charges to

travel from one electrode to the other under forward

bias without meeting an interface with the other phase

and hence the opportunity to recombine radiatively.

For the blend studied here, both the F8BT-rich

droplets in the PFB phase and the F8BT-rich droplets

in the PFB-rich phase appear to be connected to both

surfaces that would correspond to losses under LED

operation. Ideally the hole transporting phase, in this

case PFB, would completely wet the substrate/film inter-

face while the electron transporting phase, in this case

F8BTwould completely cover the film/air interface. At the

film/air interface, the partial PFB capping layer on top of

the F8BT-rich phase prevents electron injection into the

F8BT phase except for the uncapped region around the

boundaries of the domain. Thus while electrolumines-

cence has been observed to occur preferentially at the

interfaces of the mesoscale domains (and interpreted in

terms of charge recombination across the mesoscale

interface),[17] a more likely explanation is preferential

electron injection at these boundaries with injected

electrons recombining with holes at the PFB wetting

layer. Thus, the lateral morphologies produced by spin-

DOI: 10.1002/marc.201000269


coating fromxylenedonot produce idealmorphologies for

LED operation with the PFB capping layer preventing

electron injection into large parts of the F8BT domain, and

direct pathways between electrodes allowing charges to

traverse the film without recombining. For solar cell

operation, knowledge of film structure and the connec-

tivity of each phase to the electrodes is important for

understanding device operation. The PFB-capping layer

observed here can prevent electrons from reaching the

aluminium electrode, causing charge trapping and

recombination.[14] Our previous studies found that the

photovoltaic performance of xylene-processed PFB:F8BT

blends could not be well predicted from the bulk STXM

measurements of domain composition[13] with unfavour-

able surface layers likely playing a role. Additionally,

assessing interconnectivity of phases to electrodes is

important as direct pathways between electrodes

produce increased dark currents under forward bias and

may limit open circuit voltage particularly at low light

intensities. Next generation models are beginning to

incorporate blend structure with device physics[24] and

our ability to image blend surface structure will help

in informing these models. Thus, improved blend

imaging will assist in our understanding of device

performance and in the development of new, controlled

approaches for morphology control required to optimise

device operation.

Conclusion

We have demonstrated the utility of X-ray spectromicro-

scopy to simultaneously image surface and bulk structure

of polymer blends with high spatial resolution and

chemical selectivity. For the PFB:F8BT blends studied here,

a PFB capping layer was clearly imaged on top of the F8BT-

rich phase which appeared to be pinned to the underlying

PFB-rich droplets. Imaging of the bottom surface showed

that the F8BT-rich droplets in the PFB phase penetrate

through the PFB wetting layer connecting the top and

bottom surfaces. Similarly, PFB-rich droplets in the F8BT-

richphasewereobserved toconnect toboththePFBcapping

andwetting layers. Theability to image surface structure in

this fashion will be useful for further characterisation of

semiconducting polymer blends improving our under-

standingof structure/function relationships, andbeneficial

for determining the structure of organic thin films in

general.

Acknowledgements: The authors thank the SLS for beamtime,Cambridge Display Technology Ltd. for the supply of PFB andF8BT and T. Schuettfort and X. He, University of Cambridge,for assistance with sample preparation and data acquisition.



This work was supported by the Engineering and PhysicalSciences Research Council, UK (Advanced Research FellowshipEP/E051804/1). PolLux is funded by the BMBF (project no.05KS7WE1).

Received: May 1, 2010; Published online: July 27, 2010; DOI:10.1002/marc.201000269

Keywords: blends; conjugated polymers; NEXAFS; organic elec-tronics; X-ray microscopy

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DOI: 10.1002/marc.201000269

Simultaneous Surface and Bulk Imaging of Polymer Blends with X-ray Spectromicroscopy

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Transcript of Simultaneous Surface and Bulk Imaging of Polymer Blends with X-ray Spectromicroscopy