Research Article Characterization by Confocal Laser ...downloads.hindawi.com/journals/jpol/2014/541248.pdf · the blend/air interfaces. Comparing the average PL intensity foreachphaseatthetwointerfaces,onecaneasilyandquickly

Research ArticleCharacterization by Confocal Laser ScanningMicroscopy of the Phase Composition at Interfaces in ThickFilms of Polymer Blends

Sandro Lattante1 Andrea Perulli2 and Marco Anni1

1 Dipartimento di Matematica e Fisica ldquoEnnio De Giorgirdquo Universita del Salento via per Arnesano 73100 Lecce Italy2 Dipartimento di Ingegneria dellrsquoInnovazione Universita del Salento via per Monteroni 73100 Lecce Italy

Correspondence should be addressed to Sandro Lattante sandrolattanteunisalentoit

Received 27 December 2013 Revised 25 February 2014 Accepted 26 February 2014 Published 30 March 2014

Academic Editor Cornelia Vasile

Copyright copy 2014 Sandro Lattante et alThis is an open access article distributed under the Creative CommonsAttribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Confocal Laser Scanning Microscopy (CLSM) has been used as a fast user-friendly and noninvasive tool for characterizingthe phase composition differences at the substrate and air interfaces in thick films of polymer blends A clearly different phasecomposition at the blendglass interface and at the blendair interface has been detected We show that PCBM preferentiallyaccumulates at the glassblend interface while P3HT preferentially accumulates at the blendair interface by comparing theintegrated signal intensity of the luminescence coming from both interfaces Our results demonstrate that CLSM can be usedconveniently for the fast identification of a preferential phase segregation at interfaces in polymer blends This is useful in theresearch field on devices (like sensors or planar waveguides) that are based on very thick layers (thickness higher than 1120583m)

1 Introduction

In the last few years it has been widely demonstratedusing several complex experimental techniques that manysystems composed of organic blends undergo a verticalphase stratification For instance polymers blended withfullerenes like poly[(99-dioctylfluorenyl-27-diyl)-alt-55-(4070-di-2-thienyl-201030-benzothiadiazole)] (APFO-3)[1 2] where the dynamic secondary ion mass spectroscopy(SIMS) technique has been used for the characterizationor poly[(441015840-bis(2-ethylhexyl)dithieno[32-b2101584031015840-d]silole)-26-diyl-alt-(213-benzothiadiazole)-47-diyl] (PSBTBT) [3]using XRD and small-angle Neutron Scattering or Poly[[9-(1-octylnonyl)-9H-carbazole-27-diyl]-25-thiophenediyl-213-benzothiadiazole-47-diyl-25-thiophenediyl] (PCDTBT)[4] as well as all-polymer blends like poly(99-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) and poly(99-di-n-octylfluorene-alt-(14-phenylene-((4-sec-butylphenyl)imino)-14-phenylene)) (TFB) [5] The vertical stratificationof the prototype poly(3-hexylthiophene) (P3HT) blendedwith [6 6]-phenyl-C61-butyric acid methyl ester (PCBM)

has been characterized by several experimental techniquesTsoi et al studied thin films of RR-P3HT PCBM and RRa-P3HT PCBM by UV and X-ray photoelectron spectroscopy(UPS and XPS) [6] demonstrating that the P3HT chains arepreferentially ordered at the blend-air interface while PCBMtends to accumulate at the substrate-blend interface Xueet al [7] investigated the effect of the annealing process onthe vertical stratification of P3HT PCBM blend by meansof X-ray diffraction (XRD) and near-edge X-ray absorptionfine structure spectroscopy (NEXAFS) Karagiannidiset al [8] also studied the annealing effects on the verticalphase rearrangement in P3HT PCBM thin films by XRDconfirming the accumulation of PCBM on the bottominterface and of P3HT at the top interface Ellipsometry hasbeen used too by Madsen and coworkers [9] for monitoringthe vertical phase separation in P3HT PCBM blend findingresults compatible with that given by XPS characterization

Confocal Laser Scanning Microscopy (CLSM) can notattain the spatial resolution proper of the above mentionedtechniques (in the nm scale) being limited by the diffractionof light Moreover it can not be used for a morphological

Hindawi Publishing CorporationJournal of PolymersVolume 2014 Article ID 541248 10 pageshttpdxdoiorg1011552014541248

2 Journal of Polymers

vertical gradient characterization of very thin films (likethe ones usually employed in fields such as Organic LightEmitting Diodes or Polymer Solar Cells [10]) Neverthelessit is a simple time-preserving diffused and user friendlytechnique to be exploited for a fast and preliminary checkof experimental parameters that can be of importance indesigning a particular experiment where a very thick (ge1 120583m)polymer film is desirable For instance the preferential phasesegregation at interfaces in polymer blends is a key parameterto be known in order to properly understand and charac-terize the physics and behavior of organic sensor or planarwaveguides where a thick organic layer is employed Forexample the ideal thickness of bulk heterojunction organicbased photodiodes for image sensing should be sim4 120583m inorder to drastically reduce the reverse bias dark current[11] Thick films (sim1 120583m) are also needed to obtain highilluminance detection from ink-jet printed photosensorsbased on thick luminescent polymer blends [12] Since theusual device structure is the diode-like one (where the activelayer is sandwiched between two conductor layers) it shouldbe desirable to know if there is a difference in the blendcomposition at the interfaces in order to choose the propertop and bottom conductive layers improving the chargeinjectionextraction by matching the interfaces between thehole (electron) injectionextraction conductive layer with thehole (electron) transporting phase inside the active layerblend Thick samples are also requested when studying thewaveguide-assisted Amplified Spontaneous Emission (ASE)fromorganic layers composed of luminescent species embed-ded into polymer inert matrix [13] or lasing effects on thickbilayers [14] The optical gain in the waveguide is stronglydependent on the thickness of the active layer and on theoverlap between the gain media the pump laser beamand the electromagnetic field inside the waveguide [15ndash17]Therefore it should be desirable to know if there is anaccumulation on one side of the sample of the luminescentphase inside the wave-guiding layer for thicknesses in themicrometric range

Although P3HT PCBM is a standard system used inthe field of organic solar cells here it has been chosen asa mere testing system for the proposed technique due tothe fact that it is already well known that a thick blendof P3HT PCBM undergoes a vertical phase segregationduring the film deposition process typically forming a thinfullerene-rich layer at the bottom interface and a polymer-rich layer at the top interface [18] We point out that wehave no intention to extend our findings to solar energyconversion based on organic thin filmsMoreovermost of thecited results related on very thin films can not be extendedby extrapolation to thicker films for it is known that thephase morphology inside the layer strongly depends on theincreasing thickness up to the micrometric range [19]

The CLSM allows to detect and discriminate the photolu-minescence (PL) coming from the substrateblend and fromthe blendair interfaces Comparing the average PL intensityfor each phase at the two interfaces one can easily and quicklydetect if there are component differences at substrate and airinterfaces and at what extent that is the aim of this work

2 Materials and Methods

21 Samples Preparation Three chlorobenzene solutions ofP3HT PCBM have been realized with different relativecomponent ratio namely 1 2 1 1 and 2 1 (in P3HT relativecontent increasing order) each solution with a total con-centration of 25mgmL Highly regioregular P3HT (SepiolidP200) has been purchased from Rieke Metals and 995purity PCBMhas been purchased fromSESResearchmateri-als have been used as receivedThe solutions have been stirredfor several days at room condition in dark before using themfor the thick films preparation

Ultrathin microscopy cover glass slides (from Bresser22 times 22mm certified thickness of 130ndash170120583m) have beencarefully cleaned in an ultrasonic bath sequentially withacetone (10min) ethanol (10min) isopropanol (10min) anddistilled water (10min) and then gently dried by a nitrogengun Part of the cleaned slides has been monitored by opticalmicroscopy (40x) in order to check for possible damagesdue to the cleaning process no damages have been detectedFew drops of the solutions have been gently placed onto theglass slides inside a Petri dish The samples were then partlycovered in order to protect them from external perturbationsThe solutions have been used as stirred for the unfilteredsamples realization while they have been filtered by a PTFEfilter (045120583m pore size) for the filtered samples The filter-ing process allows obtaining samples without macroscopicfeatures that can be easily identified in a photoluminescencemap thus giving the opportunity to test the technique alsofor very uniform systemsThe samples have been left in placefew hours until all the solvent was evaporated All the aboveoperations have been performed at room condition inside achemical fume hood

22 Photoluminescence Photoluminescence (PL) spectrahave been measured by exciting the samples with a He-Cd laser (325 nm wavelength) filtered by a variable neutraldensity filter The PL signal was dispersed by a TRIAX 320monochromator with a 150 Lmm diffraction grating blazedat 500 nm and detected by a Peltier cooled Si-charge coupleddevice (CCD) The spectral resolution was sim2 nm

All the above measurements have been performed atroom condition

23 Morphological Characterization Immediately after thePL measurements the samples have been morphologicallycharacterized by Confocal Laser Scanning Microscopy witha Nikon Eclipse C1 inverted confocal microscope A 40xDIC Plan Apochromat objective (095 numerical aperture)mdashequipped with a manually adjustable correction ringmdashhasbeen used The samples have been excited by the 488 nm lineof an argon laser and the PL signal has been detected by acouple of photomultipliers (PMTs) The signal was detectedsimultaneously at 530 plusmn 20 nm using a bandpass filter andabove 770 nm using a high pass filter Incident excitationpower was set to 28120583W measured by a power meter placedon the objective focal plane Single pixel acquisition time wasfixed to 42120583s

Journal of Polymers 3

Table 1 Samples average thickness (range bar plusmn03 120583m)P3HT PCBM ratio in increasing P3HT relative content

Sample 1 2 Sample 1 1 Sample 2 1Unfiltered samples

11 120583m 11 120583m 12 120583mFiltered samples

12 120583m 4 120583m 12 120583m

Samples thicknesses have been measured by a DektakAlpha Step profilometer after all the optical and microscopyexperiments thus preventing any preliminary sample dam-age andor manipulation Multiple scratches have been real-ized all over the samples and thickness values obtained foreach scratch have been then averaged The mean thicknessvalues are reported in Table 1

In order to characterize the airblend interface and theglassblend interface the samples have been simply tippedover in between the measurement carefully placing thesample in such a way that the same region is analyzed on boththe surfaces Anyway no detectable differences have beenrevealed repeating the measurements on different regionsof the given surface being the sample morphology andthickness homogeneous Thus the ldquotipping overrdquo is not anintrinsic limitation since image shifts are not correlated withany measurable differences in the results Objective focus hasbeen checked and adjusted at every measurement Duringeach CLSM experiment the airblend interface and theglassblend interface have been measured keeping constantall the measurement parameters (PMT gain incident laserpower magnification and pixel dwell time)

3 Results and Discussion

31 Photoluminescence

Unfiltered Samples PL spectra of the unfiltered samples arereported in Figure 1 normalized to the film thicknessesThe PL peaks at around 650 nm and 700 nm are typical ofthe P3HT emission [20] while the band around 550 nmcan be attributed to residual PCBM emission due to theintermolecular interactions (absorption and luminescencefrom charge transfer excited states) between not well sepa-rated fullerene clusters [21] Moreover there is no evidenceof P3HT emission around 550 nm (pure P3HT PL is alsoreported as a dashed line in Figure 1 scaled down in orderto be comparable with the blend signal) We notice that therelative intensity of the peak at 550 nm increases with thePCBM content increase confirming the previous conclusion

It is well known that the strong PL quenching in thissystem is related to the efficient charge transfer from P3HTto PCBM after excitation [22 23] The spectral shape ofthe samples is almost the same irrespective of the relativeconcentration ratio As it should be expected the majorP3HT emission quenching effect has been detected in the 1 2sample (almost all the P3HT phase participate to the chargetransfer process to the overwhelming PCBMphase) while on

Wavelength (nm)350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

Blend 2 1

BlendPristine P3HT

1 1

Blend 1 2

Cou

nts (

au)

times103

Unfiltered3

2

1

0

5

4

3

2

1

0

2

1

0

times103

times104

Figure 1 Photoluminescence spectra of (from the top) 1 2 1 1and 2 1 P3HT PCBM unfiltered samples (increasing P3HT relativecontent) The data have been normalized to the sample thicknesses

the contrary the P3HT emission is clearly evident in the 2 1sample

Filtered Samples PL spectra of filtered samples are reportedin Figure 2

The main feature that is clear from the spectral shapeis still the presence of the band around 550 nm with nonnegligible intensity relative to the P3HT emission similarto the one of the unfiltered samples previously assigned toCT emission from PCBM clusters These results then suggestthat some of the PCBM clusters responsible for the emissionhave dimensions lower than 045 120583m (the filter pore size)On the other side the quenching effect appears to be lesspronounced (compare the 1 2 and 1 1 spectra) that couldbe an indication that the filtering process mainly removednot well dissolved PCBM from the solution However thethickness of the 1 1 sample in this case is higher thusother possible morphological and photophysical effects onthe overall PL intensity can not be excluded However themain scope of PLmeasurements here is to give strong support


Wavelength (nm)

18

12

06

0350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

Filtered

Blend 2 1

Blend 1 1

Blend 1 2times10

3

times104

Cou

nts (

au)

25

2

15

1

05

0

15

1

05

0

times103

Figure 2 Photoluminescence spectra of (from the top) 1 2 1 1and 2 1 P3HT PCBM filtered samples (increasing P3HT relativecontent) The data have been normalized to the sample thicknesses

to the following conclusions about the phase origin of thedetected signals during the CLSM experiments

32 Confocal Laser Scanning Microscopy In order tocheck for possible differences between the sample surfaces(airblend or glassblend interfaces) it is necessary that theinstrument can section the observed specimen thereforeavoiding optical overlap of signals detected on the two surfaceplanes According to the Nikon instrument specification andto the general confocal laser microscopy principles [24 25]the actual calculated total depth of field of the used system is064 120583m (microscope pinhole was set to 12 Airy Unit) Thismeans that there is no optical overlap between the regionsat the two interfaces during the measurements (the opticalslices on both sides do not overlap each other when thesample interfaces are on focus)

The collecting filters have been chosen in order to sepa-rate the emission from the two material phases The 530 nmband pass filter allows to collect the emission generatingfrom the PCBM rich phase only since there is no emission

of P3HT below sim600 nm (see Figures 1 and 2) while the770 nm high pass filter should account mainly for the P3HTrich phase emission (the very weak PCBM emission around700ndash750 nm [21] is cut off by the filter) The PL countshave been integrated over the whole images for the totalsignal intensity comparison in the following discussionThe reported confocal images in the following have beenprocessed with a median filter for signal noise removing Nonormalization has been performed

Figure 3 reports the confocal images (collected at 770 nmand above) of the unfiltered samples (the important parame-ter in this case is the depth of field of 0640 120583m This meansthat the microscope can collect without any overlap the lightthat comes from the focus plane plusmn a slice of 320 nm thick)

Looking at the confocal images at 770 nm channel (Fig-ure 3) where only the P3HT signal is expected it is observedthat for the 1 2 and 1 1 sample the total signal intensity(PL counts integrated over the image area) at the glassblendinterface is lower than that at the airblend interface con-firming a P3HT-phase preferential distribution on top ofthe specimen The 1 2 and the 1 1 samples show at 530 nm(Figure 4) distinguishable PCBM signal at the glassblendinterface and a very weak signal at the airblend interfaceconfirming a preferential arrangement of the PCBMphase onthe bottom of the sample

The averaged signal intensity 770 nm is on the contraryless different between the two interfaces for the (P3HT-richer) 2 1 sample meaning that the P3HT span all overboth surfaces On the contrary looking to the total PL signalcounts at the 530 nm channel (Figure 4(g)) where onlyPCBM-originating emission is expected the signal is verylow on both surfaces This indicates that most of the PCBMemission could be absorbed by the overwhelming P3HTphase (see also the very weak PL 530 in Figure 1)

We can exclude a possible degradation of the polymerblend at the airblend interface (respect to the glassblendinterface) due to oxygen interaction with the materials withpotential photooxidation process - influencing the lumines-cence signal intensity In fact the oxygen diffusion constantin the blend is in the range 015ndash12 120583m2sminus1 [26ndash28] thatmeans that oxygen penetrate the whole films studied herein 1-2 seconds up to the glassblend interface Thus possibleinfluences on luminescence should be the same on bothsurfaces Moreover the P3HT emission is not decreased byoxygen exposure in the 700 nm and above spectral region[26] Furthermore the ultrathin cover glass slice transmitsmore than 90 of the visible light (transmission spectrummeasured by a Cary Varian 500 spectrophotometer notshown) We can therefore neglect any possible glass effectson the blend absorption of laser pump on both interfaces aswell as on the transmitted luminescence Finally we ruledout any optical interference since the glass thickness is by fargreater than the pump and emission wavelength

We point out that the phase preferential arrangement atthe interfaces is a process driven by the differences in thesurface energy between P3HT and PCBM (being the P3HTone smaller than that of PCBM [8]) and by the interaction ofthe blend with the substrate and the air [29 30] Therefore


(a) (b)

(c) (d)

(e) (f)

Figure 3 Confocal images of as casted samples (a) and (b) 1 1 glassblend interface and blendair interface respectively (c) and (d) 1 2glassblend interface and blendair interface respectively (e) and (f) 2 1 glassblend interface and blendair interface respectively imagesare 2304 times 2304 120583m collected 770 nm (resolution 009 120583mpx 512 times 512 px) (color online)

the possible thickness variation among the samples in thepresent range limit greater than 1 120583m does not influencethe phase composition at the two interfaces [31] Moreoverthe scope of this work is to provide a fast detection ofdifferent phase behavior at the interfaces not claimingmdashon the contrarymdashto provide a tool for a complete verticalgradient segregation since the layer thickness is too low fora convenient multiple slicing of the system In our opinionthis is not a limitation if we consider the aim of this workthat is surface phases characterization It is possible that

multiple stratifications could occur but taking into accountthe observations given in the introduction about the dept offield and the optical slice overlap it appears evident that evenin the case of a multilayered structure the main contributionto the signal would come from the prevalent phase in theinterface subsystem

In order to investigate the dependence of the phaseseparation on possible PCBM aggregatesclusters content wehave repeated the experiments on fresh films deposited afterfiltering the solutions


(a) (b) (c)

(d) (e) (f)

1 2 2 11 1

Tota

l PL

coun

ts530

nm

Glassblend interfaceBlendair interface

107

106

Concentration ratio (P3HT PCBM)

(g)

Figure 4 Confocal images collected 530 nm of as casted unfiltered samples (a) and (b) 1 1 glassblend interface and blendair interfacerespectively (c) and (d) 1 2 glassblend interface and blendair interface respectively (e) and (f) 2 1 glassblend interface and blendairinterface respectively (2304 times 2304 120583m 009 120583mpx 512 times 512 px) (color online) (g) The corresponding total PL counts (total countsintegrated all over the image area) as detected by the instrument

Figure 5 reports the confocal images collected at 770 nm(glassblend interfaces (a) (c) and (e) airblend interface (b)(d) and (f))

It is clear that there is a macroscopic micrometer-sizedphase separation for all the samples at the glassblend

interface where the brighter regions (percentage of signalhigher than the average is of 3320 4182 and 3462respectively calculated by digital image analysis) should bericher in polymer On the contrary no clear macroscopicstructures are visible on the airblend interface that shows


(a) (b)

(c) (d)

(e) (f)

Figure 5 Confocal images of samples casted by filtering the initial solutions (a) and (b) 1 1 glassblend interface and blendair interfacerespectively (c) and (d) 1 2 glassblend interface and blendair interface respectively (e) and (f) 2 1 glassblend interface and blendairinterface respectively images are 2304 times 2304 120583m collected 770 nm (resolution 009 120583mpx 512 times 512 px) (color online)

a quite uniform luminescence coming from P3HT all overthe surface with almost no difference among the samples(for all a coverage higher than 85 over the average signalintensity is calculated) Again what is to be noted here is theaveraged PL counts differences between the two interfacesThe PL intensity at the airblend interfaces is indeed higherthan that at the glassblend interfaces implying that there isa majority of P3HT on the top surface

The confocal images in the PCBM aggregates emissionrange 530 nm (Figure 6) show again more PCBM PL signalat the glassblend interface while the signal at the airblend

interface is definitively lower (see PL counts plot in the samefigure) The fullerene phase lies mainly at the bottom of thesample

From the results on filtered samples we can concludethat the phase composition at the interfaces does not dependon possible clustersagglomerates of one of the materialsThese results demonstrate that the differences in the phasecomposition at the two interfaces can be revealed byCLSMbycomparing the PL intensity even in fine-mixed samples thatdo not show any particular big cluster or evidentmacroscopicfeatures


(a) (b) (c)

(d) (e) (f)

1 2 2 11 1


Tota

l PL

coun

ts530

nm


1E7

(g)

Figure 6 Confocal images collected530 nmof filtered samples (a) and (b) 1 1 glassblend interface and blendair interface respectively (c)and (d) 1 2 glassblend interface and blendair interface respectively (e) and (f) 2 1 glassblend interface and blendair interface respectively(2304 times 2304 120583m 009 120583mpx 512 times 512 px) (color online) (g)The corresponding total PL counts (total counts integrated all over the imagearea) as detected by the instrument

Although the presented results are related to aP3HT PCBM thick film casted on glass the used techniqueis generally valid for different thick filmsThe conclusions arevalid even when thinking to a structure geometry in which

the upper layer of the substrate is composed for exampleby commonly used materials like PEDOT PSS or ZnO orTiO2when realizing diode-like structures Indeed even if it

is not possible to directly perform the here reported CLSM


experiment comprising such additional layersmdashbecause thephotoluminescence counts detected at the two interfaceswould no more be comparable since the surface comprisingthe additional layers would suffer from increased scatteringand reduced transmittancemdashin the limits of the presentexperiment the parameter that has a strong influence onthe interface morphology is the hydrophilic or hydrophobicnature of the substrates interacting with the blend [29]It is known that the PCBM tends to accumulate towardshydrophilic substrates [30] as confirmed by our results asthe standard glass is hydrophilic The accumulation wouldthen be even more pronounced if instead of on glass theblend was casted on the high hydrophilic PEDOT PSS oron the hydrophilic ZnO or TiO

2(ZnO and TiO

2can be

easily made hydrophilic by simple UV irradiation [32 33])Since many blend systems used in the cited application fieldsare composed by species with different hydrophobicphilicproperties and free surface energies the generality of ourreports is evident All the results cited in the Introductionabout the vertical phase stratification have been obtainedusing experimental techniques and facilities not commonlyavailable and that require a great expertise and time-consuming implementation Moreover at this stage (to justknow if there is a majority of one phase on one particularinterface) it is not mandatory to have a nm scale resolutionalong the sample plane the important parameter to bedetected is the average content on one surface of one of thematerials and not the nm scale fine structure of the blend

4 Conclusions

TheConfocal Laser Scanning Microscopy has been exploitedas a common simple fast and user-friendly technique tocharacterize the phase composition differences at interfacesof polymer blends A clear difference in the macroscopicphase content at the bottom and at the top of the sampleshas been detected The novel idea of this work is to applythe CLSM to thick films in fields where the effects of externalinterfaces are important

Even if the results on thick films can not be extendeddirectly on thinner films and consequently to those applica-tion that require thin films these results are certainly usefulin the preliminary blend morphology characterization fieldwhen the phase content at the interfaces is an importantparameter to be considered and the thickness of the layermust be of fewmicrometers Using the CLSM instead of othertechniques like XRD XPS UPS SIMS or Neutron Scatteringcould indeed allow a fast in-lab checking of the influence ofmixing ratio solvent annealing and preparation process ina quick few-step experiment Experiments on different andmore interesting systems as well as experiments to comparemore precisely the morphology of both phases of variousblend systems are actually in progress

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

Dr Mauro Lomascolo and Dr Roberto Rella of IMM-CNR of Lecce are kindly acknowledged for providing thephotoluminescence experimental setup the chemical fumehood and the spectrophotometer Professor Massimo DiGiulio is kindly acknowledged for providing the profilometerSandro Lattante has received financial support from the FP7ALHsolar project-SP3-People Marie Curie ERG (PERG04-GA-2008-239342)This work has been supported by RegionePuglia through the projects SensampMicroLAB and SMILE

References

[1] C M Bjorstrom S Nilsson A Bernasik et al ldquoVertical phaseseparation in spin-coated films of a low bandgap polyflu-orenePCBM blend-Effects of specific substrate interactionrdquoApplied Surface Science vol 253 no 8 pp 3906ndash3912 2007

[2] C M Bjorstrom A Bernasik J Rysz et al ldquoMultilayerformation in spin-coated thin films of low-bandgap polyfluo-renePCBM blendsrdquo Journal of Physics Condensed Matter vol17 no 50 pp L529ndashL534 2005

[3] H Lu B Akgun and T P Russel ldquoMorphological charac-terization of a low-bandgap crystalline polymer PCBM bulkheterojunction solar cellsrdquo Advanced Energy Materials vol 1pp 870ndash878 2011

[4] P A Staniec A J Parnell and A D F Dunbar ldquoThenanoscale morphology of a PCDTBTPCBM photovoltaicblendrdquo Advanced Energy Materials vol 1 pp 499ndash504 2011

[5] K-H Yim Z Zheng R H Friend W T S Huck and J-SKim ldquoSurface-directed phase separation of conjugated polymerblends for efficient light-emitting diodesrdquo Advanced FunctionalMaterials vol 18 no 19 pp 2897ndash2904 2008

[6] W C Tsoi S J Spencer L Yang et al ldquoEffect of crystallizationon the electronic energy levels and thin film morphology ofP3HTPCBM blendsrdquoMacromolecules vol 44 no 8 pp 2944ndash2952 2011

[7] B Xue B Vaughan C-H Poh et al ldquoVertical stratificationand interfacial structure in P3HTPCBM Organic solar cellsrdquoJournal of Physical Chemistry C vol 114 no 37 pp 15797ndash158052010

[8] P G Karagiannidis D Georgiou C Pitsalidis A Laskarakisand S Logothetidis ldquoEvolution of vertical phase separationin P3HTPCBM thin films induced by thermal annealingrdquoMaterials Chemistry and Physics vol 129 no 3 pp 1207ndash12132011

[9] M V Madsen K O Sylvester-Hvid B Dastmalchi et alldquoEllipsometry as a nondestructive depth profiling tool for roll-to-roll manufactured flexible solar cellsrdquo Journal of PhysicalChemistry C vol 115 no 21 pp 10817ndash10822 2011

[10] A J Moule J B Bonekamp and K Meerholz ldquoThe effect ofactive layer thickness and composition on the performance ofbulk-heterojunction solar cellsrdquo Journal of Applied Physics vol100 Article ID 094503 2006

[11] T N Ng W S Wong R A Lujan et al ldquoCharacteriza-tion of flexible image sensor arrays with bulk heterojunctionorganic photodiodesrdquo in Organic Field-Effect Transistors VIIand Organic Semiconductors in Sensors and Bioelectronics ZBao IMcCulloch R Shinar andGGMalliaras Eds vol 7054of Proceedings of SPIE August 2008


[12] L L Lavery G L Whiting and A C Arias ldquoAll ink-jetprinted polyfluorene photosensor for high illuminance detec-tionrdquo Organic Electronics Physics Materials Applications vol12 no 4 pp 682ndash685 2011

[13] J Mysliwiec L Sznitko S Bartkiewicz et al ldquoAmplified spon-taneous emission in the spiropyran-biopolymer based systemrdquoApplied Physics Letters vol 94 no 24 Article ID 241106 2009

[14] J Mysliwiec L Sznitko A Sobolewska S Bartkiewicz and AMiniewicz ldquoLasing effect in a hybrid dye-doped biopolymerand photochromic polymer systemrdquoApplied Physics Letters vol96 no 14 Article ID 141106 2010

[15] M Anni A Perulli and G Monti ldquoThickness dependence ofthe amplified spontaneous emission threshold and operationalstability in poly(99-dioctylfluorene) active waveguidesrdquo Jour-nal of Applied Physics vol 111 Article ID 093109 2012

[16] V Navarro-Fuster I Vragovic E M Calzado et al ldquoFilmthickness and grating depth variation in organic second-orderdistributed feedback lasersrdquo Journal of Applied Physics vol 112Article ID 043104 2012

[17] EM CalzadoM G Ramirez P G Boj andM A Diaz-GarcialdquoThickness dependence of amplified spontaneous emission inlow-absorbing organic waveguidesrdquo Applied Optics vol 51 pp3287ndash3293 2012

[18] J Y Oh W S Jang T I Lee J-M Myoung and H K BaikldquoDriving vertical phase separation in a bulk-heterojunction byinserting a poly(3-hexylthiophene) layer for highly efficientorganic solar cellsrdquo Applied Physics Letters vol 98 Article ID023303 2011

[19] A D F Dunbar P Mokarian-Tabari A J Parnell S J MartinM W A Skoda and R A L Jones ldquoA solution concentrationdependent transition from self-stratification to lateral phaseseparation in spin-cast PSd-PMMA thin filmsrdquo EuropeanPhysical Journal E vol 31 no 4 pp 369ndash375 2010

[20] B Gholamkhass T J Peckham and S Holdcroft ldquoPoly(3-hexylthiophene) bearing pendant fullerenes aggregation vsself-organizationrdquo Polymer Chemistry vol 1 no 5 pp 708ndash7192010

[21] S Cook H Ohkita Y Kim J J Benson-Smith D D C Bradleyand J R Durrant ldquoA photophysical study of PCBM thin filmsrdquoChemical Physics Letters vol 445 no 4ndash6 pp 276ndash280 2007

[22] C J Brabec G Zerza G Cerullo et al ldquoTracing photoinducedelectron transfer process in conjugated polymerfullerene bulkheterojunctions in real timerdquo Chemical Physics Letters vol 340no 3-4 pp 232ndash236 2001

[23] F Padinger R S Rittberger and N S Sariciftci ldquoEffectsof postproduction treatment on plastic solar cellsrdquo AdvancedFunctional Materials vol 13 no 1 pp 85ndash88 2003

[24] S Wilhelm B Grobler M Gluch and H Heinz Confocal LaserScanning Microscopy Principles 2003

[25] R H Webb ldquoConfocal optical microscopyrdquo Reports on Progressin Physics vol 59 p 427 1996

[26] A Sperlich H Kraus C Deibel H Blok J Schmidt and VDyakonov ldquoReversible and irreversible interactions of poly(3-hexylthiophene) with oxygen studied by spin-sensitive meth-odsrdquo Journal of Physical Chemistry B vol 115 no 46 pp 13513ndash13518 2011

[27] M S A Abdou F P Orfino Y Son and S Holdcroft ldquoInter-action of oxygen with conjugated polymers charge transfercomplex formationwith poly(3-alkylthiophenes)rdquo Journal of theAmerican Chemical Society vol 119 no 19 pp 4518ndash4524 1997

[28] L Luer H-J Egelhaaf D Oelkrug et al ldquoOxygen-inducedquenching of photoexcited states in polythiophene filmsrdquoOrganic Electronics Physics Materials Applications vol 5 no1ndash3 pp 83ndash89 2004

[29] G Li Destri T F Keller M Catellani F Punzo K D Jandt andG Marletta ldquoInterfacial free energy Driven nanophase separa-tion in poly(3-hexylthiophene)[66]-phenyl-C61-butyric acidmethyl ester thin filmsrdquo Langmuir vol 28 no 11 pp 5257ndash52662012

[30] Z Xu L Chen and G Yang ldquoVertical phase separationin poly(3-hexylthiophene) fullerene derivative blends and itsadvantage for inverted structure solar cellsrdquo Advanced Func-tional Materials vol 19 pp 1227ndash1234 2009

[31] H Ogawa T Kanaya K Nishida and G Matsuba ldquoPhaseseparation and dewetting in polystyrenepoly(vinyl methylether) blend thin films in a wide thickness rangerdquo Polymer vol49 no 1 pp 254ndash262 2008

[32] T Charinpanitkul W Suthabanditpong H Watanabe et alldquoImproved hydrophilicity of zinc oxide-incorporated layer-by-layer polyelectrolyte film fabricated by dip coating methodrdquoJournal of Industrial and Engineering Chemistry vol 18 no 4pp 1441ndash1445 2012

[33] V Kekkonen A Hakola T Kajava et al ldquoSelf-erasing andrewritable wettability patterns on ZnO thin filmsrdquo AppliedPhysics Letters vol 97 no 4 Article ID 044102 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



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CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


vertical gradient characterization of very thin films (likethe ones usually employed in fields such as Organic LightEmitting Diodes or Polymer Solar Cells [10]) Neverthelessit is a simple time-preserving diffused and user friendlytechnique to be exploited for a fast and preliminary checkof experimental parameters that can be of importance indesigning a particular experiment where a very thick (ge1 120583m)polymer film is desirable For instance the preferential phasesegregation at interfaces in polymer blends is a key parameterto be known in order to properly understand and charac-terize the physics and behavior of organic sensor or planarwaveguides where a thick organic layer is employed Forexample the ideal thickness of bulk heterojunction organicbased photodiodes for image sensing should be sim4 120583m inorder to drastically reduce the reverse bias dark current[11] Thick films (sim1 120583m) are also needed to obtain highilluminance detection from ink-jet printed photosensorsbased on thick luminescent polymer blends [12] Since theusual device structure is the diode-like one (where the activelayer is sandwiched between two conductor layers) it shouldbe desirable to know if there is a difference in the blendcomposition at the interfaces in order to choose the propertop and bottom conductive layers improving the chargeinjectionextraction by matching the interfaces between thehole (electron) injectionextraction conductive layer with thehole (electron) transporting phase inside the active layerblend Thick samples are also requested when studying thewaveguide-assisted Amplified Spontaneous Emission (ASE)fromorganic layers composed of luminescent species embed-ded into polymer inert matrix [13] or lasing effects on thickbilayers [14] The optical gain in the waveguide is stronglydependent on the thickness of the active layer and on theoverlap between the gain media the pump laser beamand the electromagnetic field inside the waveguide [15ndash17]Therefore it should be desirable to know if there is anaccumulation on one side of the sample of the luminescentphase inside the wave-guiding layer for thicknesses in themicrometric range

Although P3HT PCBM is a standard system used inthe field of organic solar cells here it has been chosen asa mere testing system for the proposed technique due tothe fact that it is already well known that a thick blendof P3HT PCBM undergoes a vertical phase segregationduring the film deposition process typically forming a thinfullerene-rich layer at the bottom interface and a polymer-rich layer at the top interface [18] We point out that wehave no intention to extend our findings to solar energyconversion based on organic thin filmsMoreovermost of thecited results related on very thin films can not be extendedby extrapolation to thicker films for it is known that thephase morphology inside the layer strongly depends on theincreasing thickness up to the micrometric range [19]

The CLSM allows to detect and discriminate the photolu-minescence (PL) coming from the substrateblend and fromthe blendair interfaces Comparing the average PL intensityfor each phase at the two interfaces one can easily and quicklydetect if there are component differences at substrate and airinterfaces and at what extent that is the aim of this work

2 Materials and Methods

21 Samples Preparation Three chlorobenzene solutions ofP3HT PCBM have been realized with different relativecomponent ratio namely 1 2 1 1 and 2 1 (in P3HT relativecontent increasing order) each solution with a total con-centration of 25mgmL Highly regioregular P3HT (SepiolidP200) has been purchased from Rieke Metals and 995purity PCBMhas been purchased fromSESResearchmateri-als have been used as receivedThe solutions have been stirredfor several days at room condition in dark before using themfor the thick films preparation

Ultrathin microscopy cover glass slides (from Bresser22 times 22mm certified thickness of 130ndash170120583m) have beencarefully cleaned in an ultrasonic bath sequentially withacetone (10min) ethanol (10min) isopropanol (10min) anddistilled water (10min) and then gently dried by a nitrogengun Part of the cleaned slides has been monitored by opticalmicroscopy (40x) in order to check for possible damagesdue to the cleaning process no damages have been detectedFew drops of the solutions have been gently placed onto theglass slides inside a Petri dish The samples were then partlycovered in order to protect them from external perturbationsThe solutions have been used as stirred for the unfilteredsamples realization while they have been filtered by a PTFEfilter (045120583m pore size) for the filtered samples The filter-ing process allows obtaining samples without macroscopicfeatures that can be easily identified in a photoluminescencemap thus giving the opportunity to test the technique alsofor very uniform systemsThe samples have been left in placefew hours until all the solvent was evaporated All the aboveoperations have been performed at room condition inside achemical fume hood

22 Photoluminescence Photoluminescence (PL) spectrahave been measured by exciting the samples with a He-Cd laser (325 nm wavelength) filtered by a variable neutraldensity filter The PL signal was dispersed by a TRIAX 320monochromator with a 150 Lmm diffraction grating blazedat 500 nm and detected by a Peltier cooled Si-charge coupleddevice (CCD) The spectral resolution was sim2 nm

All the above measurements have been performed atroom condition

23 Morphological Characterization Immediately after thePL measurements the samples have been morphologicallycharacterized by Confocal Laser Scanning Microscopy witha Nikon Eclipse C1 inverted confocal microscope A 40xDIC Plan Apochromat objective (095 numerical aperture)mdashequipped with a manually adjustable correction ringmdashhasbeen used The samples have been excited by the 488 nm lineof an argon laser and the PL signal has been detected by acouple of photomultipliers (PMTs) The signal was detectedsimultaneously at 530 plusmn 20 nm using a bandpass filter andabove 770 nm using a high pass filter Incident excitationpower was set to 28120583W measured by a power meter placedon the objective focal plane Single pixel acquisition time wasfixed to 42120583s





12 120583m 4 120583m 12 120583m







Wavelength (nm)350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

Blend 2 1

BlendPristine P3HT

1 1

Blend 1 2

Cou

nts (

au)

times103

Unfiltered3

2

1

0

5

4

3

2

1

0

2

1

0

times103

times104






Wavelength (nm)

18

12

06

0350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

Filtered

Blend 2 1

Blend 1 1

Blend 1 2times10

3

times104

Cou

nts (

au)

25

2

15

1

05

0

15

1

05

0

times103












(a) (b)

(c) (d)

(e) (f)






(a) (b) (c)

(d) (e) (f)

1 2 2 11 1

Tota

l PL

coun

ts530

nm


107

106


(g)






(a) (b)

(c) (d)

(e) (f)







(a) (b) (c)

(d) (e) (f)

1 2 2 11 1


Tota

l PL

coun

ts530

nm


1E7

(g)







2(ZnO and TiO

2can be


4 Conclusions





Acknowledgments


References










































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







12 120583m 4 120583m 12 120583m







Wavelength (nm)350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

Blend 2 1

BlendPristine P3HT

1 1

Blend 1 2

Cou

nts (

au)

times103

Unfiltered3

2

1

0

5

4

3

2

1

0

2

1

0

times103

times104






Wavelength (nm)

18

12

06

0350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

Filtered

Blend 2 1

Blend 1 1

Blend 1 2times10

3

times104

Cou

nts (

au)

25

2

15

1

05

0

15

1

05

0

times103












(a) (b)

(c) (d)

(e) (f)






(a) (b) (c)

(d) (e) (f)

1 2 2 11 1

Tota

l PL

coun

ts530

nm


107

106


(g)






(a) (b)

(c) (d)

(e) (f)







(a) (b) (c)

(d) (e) (f)

1 2 2 11 1


Tota

l PL

coun

ts530

nm


1E7

(g)







2(ZnO and TiO

2can be


4 Conclusions





Acknowledgments


References










































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




Wavelength (nm)

18

12

06

0350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

350 400 450 500 550 600 650 700 750 800 850

Filtered

Blend 2 1

Blend 1 1

Blend 1 2times10

3

times104

Cou

nts (

au)

25

2

15

1

05

0

15

1

05

0

times103












(a) (b)

(c) (d)

(e) (f)






(a) (b) (c)

(d) (e) (f)

1 2 2 11 1

Tota

l PL

coun

ts530

nm


107

106


(g)






(a) (b)

(c) (d)

(e) (f)







(a) (b) (c)

(d) (e) (f)

1 2 2 11 1


Tota

l PL

coun

ts530

nm


1E7

(g)







2(ZnO and TiO

2can be


4 Conclusions





Acknowledgments


References










































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)

(c) (d)

(e) (f)






(a) (b) (c)

(d) (e) (f)

1 2 2 11 1

Tota

l PL

coun

ts530

nm


107

106


(g)






(a) (b)

(c) (d)

(e) (f)







(a) (b) (c)

(d) (e) (f)

1 2 2 11 1


Tota

l PL

coun

ts530

nm


1E7

(g)







2(ZnO and TiO

2can be


4 Conclusions





Acknowledgments


References










































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b) (c)

(d) (e) (f)

1 2 2 11 1

Tota

l PL

coun

ts530

nm


107

106


(g)






(a) (b)

(c) (d)

(e) (f)







(a) (b) (c)

(d) (e) (f)

1 2 2 11 1


Tota

l PL

coun

ts530

nm


1E7

(g)







2(ZnO and TiO

2can be


4 Conclusions





Acknowledgments


References










































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)

(c) (d)

(e) (f)







(a) (b) (c)

(d) (e) (f)

1 2 2 11 1


Tota

l PL

coun

ts530

nm


1E7

(g)







2(ZnO and TiO

2can be


4 Conclusions





Acknowledgments


References










































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b) (c)

(d) (e) (f)

1 2 2 11 1


Tota

l PL

coun

ts530

nm


1E7

(g)







2(ZnO and TiO

2can be


4 Conclusions





Acknowledgments


References










































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





2(ZnO and TiO

2can be


4 Conclusions





Acknowledgments


References










































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials

































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Documents

Research Article Characterization by Confocal Laser ...downloads.hindawi.com/journals/jpol/2014/541248.pdf · the blend/air interfaces. Comparing the average PL intensity foreachphaseatthetwointerfaces,onecaneasilyandquickly