Solar Energy Materials & Solar Cells 117 (2013) 15–21

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Solar Energy Materials & Solar Cells

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Photobleaching of MEH-PPV thin films: Correlation between opticalproperties and the nanoscale surface photovoltage

Elisa Palacios-Lidon a,n, Elisa Escasain a, Elena Lopez-Elvira b,Arturo M. Baro b, Jaime Colchero a

a Departamento de Física, Edificio CIOyN, Campus Espinardo, Universidad de Murcia, E-30100 Murcia, Spainb Department of Surfaces and Coatings, Instituto de Ciencia de Materiales de Madrid—CSIC, Campus Cantoblanco, E-28049 Madrid, Spain

a r t i c l e i n f o

Article history:Received 19 February 2013Received in revised form3 May 2013Accepted 9 May 2013Available online 3 June 2013

Keywords:Surface photovoltageElectro-optical propertiesConjugated polymerKelvin probe microscopyPhotochemical reactions

48/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.solmat.2013.05.022

esponding author. Tel.: +34 868 888 551; fax:ail address: elisapl@um.es (E. Palacios-Lidon).

a b s t r a c t

The electro-optical properties of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) thin films have been studied while the sample is illuminated with blue light. By combining Kelvinprobe microscopy with classical optical techniques it has been possible to monitor the evolution of thenanoscale contact potential, the surface photovoltage and the optical properties of the materialestablishing a correlation between them. Contrary to what is usually accepted, it has been found thatthe nanoscale surface photovoltage is larger when the polymer is already photobleached. In addition, ithas been possible to distinguish between the photophysical processes that govern the working principleof a polymer-based device and the photochemical reactions that degrade the material. Although complexcharges trapping phenomena take place during the irradiation, the surface photovoltage effect dependsonly on the number of photons that have reached the polymer.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

The study of semiconducting polymers has become an impor-tant field in the recent years. From a technological point of view,they bring the possibility of fabricating low-cost, flexible andeasily tunable optoelectronic devices such as organic solar cells,OLEDs or transistors [1,2]. In addition, from a fundamental point ofview, these materials present new properties which lies inbetween the classical inorganic semiconductors and the molecularsolids that have attracted the attention of the scientific community[3]. In fact, both technological and fundamental aspects are closelyrelated since to optimize the device performance a completeunderstanding of the underlying electronic processes is required[4]. A typical large area organic optoelectronic device is composedof thin films of one or several semiconducting polymer blendswhere the polymer chains are packed to form an active layer. It iswell known, that the nanostructure of the films plays a crucial rolein the final device efficiency. In a solid state polymer filminterchain and intrachain interactions that govern the electro-optical properties of the material take place at the nanoscale[5–7]. However, there are several aspects such as the optimumpolymer nanomorphology [8], the nanoscale charge generationand charge transport mechanisms within the polymer as well as

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+34 968 398 568.

between interfaces [9], or how the photodegradation modify thepolymer photophysical properties [10] that remain unknown.Hence studies of such aspects at this nanoscale are of vitalimportance.

The surface photovoltage (SPV) method, based on analyzing theillumination-induced changes on the surface voltage, has beenused for a long time to characterize the macroscopic optoelec-tronic properties of semiconducting materials. With this methodthe semiconductor doping type, the band bending near thesemiconductor surface and/or the built in voltage at the buriedinterface inside a heterojunction, the surface dipole, the carrierdensity , the carrier diffusion length, etc. can be obtained [11]. Acommon approach consists in using the classical Kelvin probetechnique with controlled illumination. Measuring the light-induced changes of the contact potential (CP) the SPV can beinferred [12,13]. Classical SPV techniques lack lateral resolution;however this problem can be solved by using scanning probetechniques such as electrostatic force microscopy (ESFM) andKelvin probe microscopy (KPM) [14]. Several works have provedthat the combination of these techniques with external sampleillumination leads to valuable information of the opto-electronicproperties at the nanoscale [15–22]. In a recent work we proposeda novel measuring methodology, called “two traces method” basedon the combination of KPM and external illumination [23]. Thismethod together with appropriate data analysis allows studyingthe temporal and spatial evolution of the CP acquired in darknessand under illumination as well as the SPV. From these data the

E. Palacios-Lidon et al. / Solar Energy Materials & Solar Cells 117 (2013) 15–2116

nanoscale photophysical reversible phenomena that take place inthe semiconducting polymer Poly[2-methoxy-5-(2-ethylhexy-loxy)-1,4-phenylenevinylene] (MEH-PPV) under green light illu-mination were reported. However, it is well known that non-reversible photochemical processes such as photo-oxidation, chainscission, cross-linking, etc. can also occur under certain illumina-tion conditions [10,24]. In fact, photophysical and photochemicalphenomena are usually mixed and it is difficult to discriminatebetween them leading to contradictory results [25]. The photo-chemical processes that strongly depend on the wavelength andintensity of the light and on the device environment degrade thepolymer rendering it useless [14]. Macroscopically, the conse-quence of this degradation is the photobleaching of the polymerthat becomes transparent [26,27].

In this work, we have taken the advantage of the greatversatility of the KPM/optical combined microscope to measurenot only the CP under illumination, as usually, but also to recordsimultaneously the optical properties of the sample. In this way itis possible to establish a correlation between them. With thispurpose, Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenyleneviny-lene](MEH-PPV) thin film samples have been irradiated with bluelight (λ¼480 nm) while the light-induced evolution of the nanos-cale CP and the SPV together with the optical properties have beenmonitored. This conducting polymer has been chosen due to itsapplications in optoelectronic devices. Even though its rich elec-tronic structure has been extensively studied in pristine as well asin blended heterojunction samples [28,29], there are still manyphoto-induced mechanisms closely correlated with the nano-scopic polymer structure that remain controversial. In addition,the present results can be generalized to other semiconductingpolymers and the proposed methodology can be useful to theorganic photovoltaic community as well as to those interested inthe nano-scale physical–chemical properties of materials.

2. Materials and methods

Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene](MEH-PPV) (Sigma-Aldrich) with average Mn 150,000–250,000was used without further purification. MEH-PPV thin film sampleswere prepared by spin-coating (2500 rpm) from chloroform solu-tion (30 mg/ml) on indium tin oxide coated PET (ITO:PET) (SigmaAldrich) previously cleaned with isopropanol. Thin MEH-PPV filmswith an average thickness of 200 nm are obtained. Furtherdescription of the sample surface can be found in Ref. [23].

The morphology and electro-optical properties of the thin filmswere studied at room temperature and ambient conditions using ahome-made Scanning Force Microscopy (SFM) equipped withKelvin probe microscopy (KPM) capabilities. The SFM was oper-ated in frequency modulation (FM) mode using Platinum coatedsilicon tips (Budget Sensors, k¼3 N/m and f¼75 kHz). Kelvinprobe microscopy (KPM) images were acquired in frequencymodulation KPM (FM–KPM) with an AC voltage of 500 mV at7 kHz. More details of KPM set up are described elsewhere [30].Freely available WSxM software has been used for image acquisi-tion and processing [31].

The SFM is mounted on an inverted Optical microscope (NikonEclipse TE2000-E) and has been designed so that none of theelements blocks the optical path of the inverted microscopeallowing illumination from “top” and “bottom”. In the presentwork “top” illumination is used to irradiate the free sample surfacewhile in the “bottom” illumination the polymer is illuminatedthrough the transparent electrode (ITO: PET). The system isequipped with a LEDs source (Prizmatix) used as external illumi-nation. This system allows controlling the illumination intensity aswell as the illumination wavelength. Blue light (λ¼480 nm) has

been used. The optical inverted microscope is equipped with 10�(numerical aperture NA¼0.3 and working distance WD¼16 nm)and 40� (NA¼0.6, WD¼3.6−2.8 nm) Nikon objectives (Plan Fluortype). The illumination area is controlled with a diaphragm and itis typically of 0.2–1 mm2 [32]. The light intensity has beencalibrated with a photodiode. In addition, the system is equippedwith a reflex camera (Cannon EOS500) that allows the acquisitionof optical micrographs during the sample illumination [33].

To study the photo-induced processes a line by line two tracesmethod has been used. The working principle, data processing andcapabilities of this method are explained in detail elsewere [23].Briefly, in this method the tip scans each fast scan line twice. Thefirst trace of a scan line is performed in darkness while forthe second pass the external illumination is switched on, beingthe only difference between the two traces the external illumina-tion. Then the tip moves to the next horizontal line and theprocess is repeated. In this way, two simultaneous images arerecorded, one in darkness (“off” image) and the other one underillumination (“on” image) for each interaction channel. In this workwe will focus on the analysis of the topography and KPM signaleven though oscillation amplitude and frequency shift images arealso recorded. The time evolution of the SPV, defined as SPV¼−ΔCP¼−(CPon−CPoff) is obtained from the difference between thecorresponding “on”/“off” KPM lines respectively [11].

3. Results and discussion

To characterize the nanoscale phenomena induced under blueillumination (λ¼480 nm), two different experiments have beencarried out. In the first one, the polymer–air interface is irradiatedfor a certain period of time and the permanent light inducedchanges are studied once the light is switched off. In the secondset of experiments, the two traces method is used while thesample is illuminated from the back side, that is, through thetransparent ITO:PET electrode, as in an organic solar cell config-uration. In this case, the evolution for increasing illuminationtimes of both “off” and “on” states are measured and informationabout blue light induced photophysical and photochemical pro-cesses are obtained [34].

3.1. Polymer–air surface irradiation

In the first kind of experiments the initial polymer state ischaracterized by acquiring topography and CP images of a freshlyprepared sample never illuminated before. Then the SFM scanis stopped, the tip is placed at a fixed position and the light isswitched on for certain period of time. Afterwards the light isswitched off and topography and CP images are again acquired indarkness. Repeating this procedure several times, the evolution ofthe topography and CP due to increasing irradiation time isobtained. In our experimental set up, when the illumination isperformed from the top the tip acts as a mask and the regionbelow the tip is not exposed to the light. This set up has theadvantage that in a single SFM image an irradiated and a non-irradiated region (created by the tip shadow) can be directlycompared. Therefore any change in the irradiated region can beassociated to a light induced process discarding SFM artifacts suchas tip changes. In addition, optical micrographs are recordedduring the irradiation periods to follow the evolution of the opticalappearance of the polymer. Since the illumination is done fromthe top of the sample while the light collection is performed fromthe bottom, the optical micrographs contain the information of thelight transmitted throughout the sample.

A representative selection of optical micrographs acquired atspecific irradiation times and the topography and CP images

Fig. 1. Optical micrographs (top panel) recorded at different irradiation time (t) with an irradiation intensity of I¼3.2�1017 photons s−1 cm−2 and the correspondingtopography images (middle panel) and KPM images (bottom panel) acquired in darkness after different irradiations. t¼0 (a),(h) and (o), t¼360 s (b),(i) and (p), t¼840 s (c),(j) and (q), t¼1440 s (d),(k) and (r), t¼2040 s (e),(l) and (s), t¼3000 s (f),(m) and (t), t¼4200 s (g),(n) and (u). Inset in (a) shows the SFM scanned area that is marked with asquare. The solid and empty circle and the solid triangle in (h) mark the low, high and below tip regions respectively that are discussed in the main text. The z scale isz¼800 mV for KPM images and z¼500 nm for all the topography images except (n) where z¼50 nm to highlight the small step of 4 nm between the non-degraded and thedegraded regions produced by the photoablation.

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acquired in darkness in between intervals of illumination areshown in Fig. 1. At the initial state, the optical micrograph(Fig. 1a) reveals a structure with bright (higher transmittance)and dark (lower transmittance) regions. From the SFM topographyimages these bright and dark regions can be correlated to low andhigh topography regions respectively (Fig. 1h) of the polymersurface with a height difference of about 85710 nm. Measuringthe intensity directly from the optical micrograph in low (Ilow) andhigh (Ihigh) regions and using Beer–Lambert′s law with typicalvalues of the MEH-PPV absorption coefficient (α0¼16.8�104 cm−1) [35]

Δd¼ −1α0

lnIlowIhigh

ð1Þ

a height difference (Δd) between high and low regions of about80 nm is obtained, in good agreement with the values measureddirectly from the topography SFM images. Hence, at the initialstate, the differences in the transmittance are not due to hetero-geneities in the polymer composition but due to different localfilm thicknesses. This is confirmed with the corresponding CPimage, which is almost flat (Fig. 1o) as expected for a homoge-neous material. For the present study, these kinds of samples areideal since thickness dependent effects can be studied with onesingle experiment. However it should be noticed that the thick-ness differences in our samples are larger than the ones typicallyfound on MEH-PPV thin films. This is mainly due to the ITO:PETsubstrate used in this work that has a larger roughness than thetypical ITO:glass substrates. Flatter samples can be also obtainedusing larger spinning speeds during the sample preparation.

For increasing irradiation times the sample becomes moretransparent and the optical contrast vanishes. The SFM imagesprove that topography is not strongly affected; therefore noselective etching is taking place. Nevertheless, after 70 min ofexposure a height difference of about 4 nm can be measuredbetween the irradiated and non-irradiated regions. Therefore, itcan be concluded that the blue light is producing a small homo-geneous photoablation. On the contrary, the CP images present awell defined tendency. At the first stages the CP decreases until

a minimum value and then it increases, being the final CP valuehigher than the initial one. It should be noticed, that at inter-mediate irradiation times (Fig. 1s) a CP difference between highand low regions is apparent. This CP difference tends to disappearfor longer irradiation times.

All this data are analyzed in Fig. 2 where the optical density O.D.¼−ln(I(t)/I0) with I0 the intensity of the incident light directlyobtained from the optical micrographs together with the CP valuesof the low and high irradiated regions and the non-irradiated oneare represented. In both low and high regions the O.D. decreasesfor increasing illumination time until it saturates at a value close tozero. This is the typical tendency found in MEH-PPV duringphotobleaching in which the absorption band intensity decreaseswhile it is blue shifted [26]. To quantify the changes in the O.D. as afunction of the degradation the quantitative model proposed byStaring et al. [36] was used. In this model the polymer stability ischaracterized by the photobleaching quantum yield (γ), describedas the inverse of the average number of photons that a singlecromophore can absorb before it becomes bleached. This approachis general in the sense that it does not enter in the details of theunderlying chemical reactions that photobleach the chromophore,being the only assumption that the photoproducts resulting fromthe degradation do not absorb at the irradiation wavelength. Basedon this model, the experimental O.D. data are fitted to thefollowing expression.

O:D:¼ −lnIðtÞI0

� �¼ −ln

T∞

ð1þ ðexp ðα0dÞ−1Þexp ð−ðI0α0=N0ÞγtÞÞ

� �

ð2Þwith α0 and N0 the initial absorption coefficient and cromophoresdensity respectively, I0 the irradiation intensity, d the local filmthickness, t the irradiation time, and T∞¼ I(∞)/I0 the transmittanceat t¼∞. Taking α0¼16.8�104, N0¼1021 cm−3 and dlow¼135 nm,dhigh¼218 nm for the low and high regions respectively a γ−1 of2.8�104 photons is obtained on both low and high regions.

From the corresponding CP images several features can beinferred. Firstly, the CP does not change in the non-irradiatedregion. This means that the degradation is directly induced by the

Fig. 2. (a) Optical density as a function of the illumination time measured from theoptical micrographs in the low (solid circles) and high (empty circles) regionsmarked in Fig. 1. The solid lines correspond to the data fit to Eq. (2) for the low andhigh regions respectively as described in the main text. (b) CP measured indarkness after a certain illumination time in the same low (solid circles) and high(empty circles) regions as in (a). The CP of the non-degraded region produced bythe tip shadow (solid triangles) has been included as a reference. The dashed linesare a guide for the eyes.

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photons hitting the polymer and not due to ozone generation orsample heating. Therefore only the regions that are directlyreached by photons become degraded. Secondly, the CP evolutionis closely correlated with the transmittance. The minimum in theCP and the inversion from a decreasing to an increasing tendencytakes place in the low and high regions exactly when the polymerO.D. reaches the saturation value. At intermediate and largeirradiation times, even if the overall behavior is the same for lowand high regions, the CP is sensitive to the film thickness. Finally, itshould be noticed that once the polymer is photobleached, the CPstill evolves, indicating that the degradation process is not com-pletely finished.

The low photo-stability of PPV derivates polymers is the maindrawback that limits their use in optoelectronic devices. To over-come this problem, a wide variety of degradation studies havebeen performed in presence or absence of oxygen in solution andon solid state films [23,26,37–41]. Different degradation mechan-isms have been proposed by identifying the photoproducts result-ing from the polymer irradiation. Although the underlyingphotoreactions still remain controversial [10,39] it is well acceptedthat in air atmosphere the polymer suffers photo-oxidation. Inaddition, it is known that the absorption band at 500 nm decreasesat the same rate as the ether and vinylene group disappear. Thisimplies that the conjugation length is reduced leading to thepolymer photobleaching [42]. Recently, the role of the singletoxygen as principal reactive intermediate in the photodegradationof the MDMO-PPV has been questioned. On the contrary, it wasshown that the oxygen initially favors the photoinduced electron

transfer leading to the formation of the radical cation MDMO-PPV�+

and the superoxide anion O�− [43]. In addition, the photoproductswere shown to acts as an electron acceptors that propagate thepolymer oxidation process [44]. This agrees with the CP decreasefound in the present experiments during the polymer photobleach-ing. A lower CP implies that the amount of more electronegativespecies increases with the illumination time [45]. However, theintermediate photoproducts are not photochemically stable andthey still develop [43]. This explains the inversion in the CPtendency that saturates when the degradation products becomephoto-stable.

3.2. Back polymer surface irradiation

In the second kind of experiments the goal is to study not onlylight-induced permanent processes once the light is switched off,but also what is happening during the illumination. As explainedin the experimental section, combining the two traces methodwith bottom illumination, that is, through the ITO: PET backelectrode, the CP can be recorded both in darkness (CPoff) andunder illumination (CPon). With an appropriate data analysis, theirevolution as a function of the illumination time can be obtained.From these signals the ΔCP(t)¼−SPV¼−(CPon(t)−CPoff(t)) is easilyinferred [34] In addition, during the “on” trace, optical micrographsare simultaneously acquired to monitor changes in the opticalappearance of the sample. In this configuration, both the illumina-tion and the light collection are performed from the bottom;therefore they can be interpreted as reflectance images. Resultsobtained from a typical two traces experiment with relatively lowillumination intensity (I¼5�1017 photons s−1 cm−2) are summar-ized in Figs. 3 and 4.

Optical micrographs acquired at different irradiation times areshown in Fig. 3a–l. The change of optical appearance due toirradiation with blue light is obvious in the reflectance images.Initially, just after switching on the light (Fig. 3a), some brightstructures on a dark background can be distinguished. As theillumination time increases the background becomes slightlydarker (lower reflectance) until a certain illumination instant(Fig. 3b–f). For longer exposure times, the brighter areas start togrow while the dark background becomes brighter (higher reflec-tance). In addition, note that up to this illumination time thesample becomes more transparent and then it remains essentiallyconstant, in good agreement with results of Section 3.1. This canbe inferred by taking the cantilever, that acts as a mirror, on theopposite side of the sample as a reference; at the beginning it canbe hardly distinguished (Fig. 3a–e) and from that illumination timeit can be seen throughout the sample (Fig. 3f–l). The quantitativeexplanation of the optical contrast in the reflectance images is notstraightforward. The samples are a composed system made ofseveral thin films interfaces and the reflectance depends on thecomplex refractive index at each degradation stage. Nevertheless,it can be qualitatively seen that at the beginning, when thepolymer thin film is highly absorbing, the main contribution tothe optical micrograph is given by the flat ITO/polymer backsurface while once the sample is nearly transparent they repro-duce the topography of the free polymer surface. In the later case,the optical micrographs contrast is due to interference effectsbetween both front and back polymer surfaces, being the brighterand darker regions the lower and higher topography regions,respectively. However, it is important to notice that, in contrastof what was found in the transmission images, the opticalmicrographs still evolve once the polymer has reached its mini-mum optical density and they do not remain constant until muchlonger exposure times (Fig. 3g–l). This means that at a certainirradiation time even if the absorption coefficient is very small

Fig. 4. (a) CPoff and CPon vs. time curves obtained during two trace method. (b) ΔCPvs. time obtained from (a). The larger gray solid circles correspond to the timewhere the optical micrographs of Fig. 3 were acquired. The dashed line indicatesthe moment when the blue light is switched on.

Fig. 3. Optical micrographs acquired during the “on” trace of two traces method at different illumination times: (a) 0 s, (b) 65 s, (c)127 s, (d) 189 s, (e) 272 s, (f) 355 s,(g) 479 s, (h) 520 s, (i) 644 s, (j) 1203 s, (k) 1720 s and (l) 2258 s. Light intensity I¼5�1017 photons s−1 cm−2.

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(but not zero) the dielectric constant of the polymer still changesfor longer exposures.

The CPon and CPoff as well as the ΔCP evolution as a function ofthe illumination time are shown in Fig. 4a and b. It can be seenthat the behavior of the “on” and “off” curves is very different. Atfirst, the CPon stays essentially constant and similar to the initialCPoff and then it quickly increases up to a saturation value. On thecontrary, the CPoff curve resembles the one shown in Section 2.1; itdecreases at the beginning reaching a minimum value and then itincreases until CPon and CPoff curves overlap. The saturation CPvalue, which is higher than the initial one, is reached at largerillumination time compared to the CPon signal. The CPon curve isalways above the CPoff as expected for a p-type semiconductor, andthey never cross. The behavior of the ΔCP curve is mainlydominated by the CPoff signal; it has a peak shape with amaximum that coincides in time with the CPoff minimum. Atlonger exposure times, the ΔCP decreases and goes to zero.

It is not the aim of this paper to review the basic concepts ofthe semiconductors theory and the SPV principle. An excellent reviewcan be found in Ref. [11]. However, some basic concepts should bekept in mind to the following discussion. The semiconductor work

function Ws is defined as:

Ws ¼ ðEc−EF Þb þ χ−Δϕs−eVs ð3Þwhere Ec is the energy of the conduction band bottom, EF is theFermi level, χ is the electron affinity, Δϕs is due to the surfacedipole and Vs is the band bending. The CP measured with the KPMtechnique is CP¼Wtip−Ws, where Wtip is the metallic tip workfunction. Assuming that Wtip is constant and light-independent,any variation on the Ws during or after irradiation can be ascribedto light induced changes in any of the four terms of Eq. (3). Usually,changes in the CPoff and the ΔCP signal are explained with lightinduced modifications of the surface dipole and/or band bending[21], since it is assumed that only surface photoreactions takeplace, and the first two terms in Eq. (3) are inherent bulk proper-ties. In the present situation the interpretation is not straightfor-ward. It has already been observed that the light inducesphotobleaching in the entire polymer volume. This means thatthe bulk polymer nature is being modified and therefore Ec, EF andχ can also change in Eq. (3). Note that in these kinds of experi-ments the CPoff is acquired in darkness but in a polymer regionthat has been already illuminated for an extended amount of time.Hence, the evolution of this signal records the permanent photo-chemical changes or reversible photophysical processes with arelaxation time longer than under darkness during the “on” and“off” cycles. Under blue irradiation, the initial CP is never recoveredonce the light is switched off, independently of the waiting time.This means that the main contribution to the CPoff signal is due topolymer degradation. This is in contrasts with previous resultsfound for similar experiments but using green light where onlyreversible photophysical phenomena were observed [23].

Comparing the ΔCP curve with the optical micrographs it isinteresting to notice that the ΔCP maximum and the momentwhen the polymer becomes less absorbing take place at about thesame irradiation time. Moreover, at this stage the polymer pre-sents the larger SPV even thought the photon absorption is muchlower than in the initial state. In addition, for longer exposuretimes, the ΔCP still evolves changing its increasing tendency to adecreasing one, indicating that the polymer degradation is notcompleted and it still remains photoactive. This behavior is closelycorrelated with the one shown in the reflectance images (Fig. 3). Infact, it is found that the evolution of the optical micrographs stopswhen the ΔCP is null (Fig. 3j–l). A zero ΔCP can be due to twoeffects. On one hand, the polymer band gap can be shifted tohigher energies, which implies that it does not absorb blue lightanymore and there is no photocharge generation. On the otherhand, the SPV effect only occurs if the surface net charge variesduring the illumination. This means that after a neutral excitongeneration, it should be split into the electron and hole counterpart

Fig. 5. (a) CPoff (solid circles) and CPon (empty circles) and (c) ΔCP curves obtained with the two traces method in which “off–on” traces (unshaded regions) are alternatedwith “off–off” traces (shadow regions), as explained in the main text. (b) CPoff (solid circles) and CPon (empty circles) and (d) ΔCP curves, obtained from (a) and (c) respectivelybut cutting the “off”/”off” periods.

E. Palacios-Lidon et al. / Solar Energy Materials & Solar Cells 117 (2013) 15–2120

and due to internal build in fields a spatial charge redistributiontakes place. In absence of Dember potentials this happens in thesurface charge regions of the polymer interfaces [11]. In the presentcase, the first option can be discarded, since it has been seen thatthe polymer photobleaching already took place before the ΔCPbecomes null. Hence, the degradation process finished when thepolymer band bending is completely absent.

In order to study in more detail the time evolution of the ΔCPas well as the dependence of the P with the darkness periods, anexperiment where the sample was kept in darkness not onlyduring the “off” cycle of the two pass method, but for a longerperiod of time was performed. The corresponding data are shownin Fig. 5. Technically, this was achieved by alternating typical “off/on” cycles during the two pass method (unshaded regions in Fig. 5)with “off/off” cycles, that is, intervals where the illumination wasoff during the two passes (shaded regions in Fig. 5). Fig. 5 isequivalent to Fig. 4, however, due to the fact that the illuminationis kept off during some intervals, the corresponding CPon and CPoffare much more complex. When the light is turned on after a longdarkness period a CP decay of about 100 mV is observed. Similarly,when the light is turned off after an illumination period anincrease of CP of also 100 mV is observed. The decay and increaseof the CP do not follow an exponential law. Another interestingfeature found in the data is a fluctuation of the CP which isdefinitely not experimental (typical noise level about 10 mV). Thenon-exponential decay as well as the higher noise, are related tocharging/discharging photophysical processes with a long relaxa-tion time and are not yet well understood by the authors. They arestill being investigated and will be explained in a future work.However it should be noticed that once the light is switched off,the CP tends to a value different to the initial one, confirming thepolymer degradation.

Fig. 5b and d show the data corresponding only to the normal“on”/“off” cycles of the same experiment; that is, the off–offintervals have been removed. Therefore the time axis of Fig. 5band d corresponds to the total illumination time, while the timeaxis of Fig. 5a and c corresponds to the total experimental time.Interestingly, the ΔCP curve (Fig. 5d) presents essentially the same

peak shape as the curve shown in Fig. 4b, although the experi-mental conditions are quite different. The corresponding “onlyilluminated” CPon and CPoff curves (Fig. 5b) are different from theones shown in Fig. 4a only in the non-exponential decay discussedabove. However, since this decay is very similar for the CPon andCPoff, it only appears as spikes in the ΔCP. We therefore concludethat after a fast transient state, the ΔCP value is associated to adefined polymer degradation stage that only depends on theprevious illumination time.

4. Conclusions

The evolution of the electro-optical properties of MEH-PPV thinfilms samples induced by blue light illumination has been studied.In a first step, permanent photochemical processes have beeninvestigated by irradiating the sample thought the polymer:airinterface. It has been found that under blue illumination thepolymer is degraded in two stages. At low and intermediatedirradiation times, while the optical density (O.D.) decreases the CPshifts to lower values reaching a minimum when the polymer iscompletely photobleached. For larger illumination times, althoughthe O.D. remains essentially constant, the CP still evolves indicat-ing that the degradation is not finished. The irradiation does notinduce appreciable changes in the nanomorphology of the poly-mer and only a small homogeneous photoablation is found. In asecond step, the evolution of SPV for increasing illumination timeshas been measured with the two traces method irradiating theback ITO:polymer interface. This allows the study of the polymerelectro-optical properties at different degradation stages. It hasbeen found that the ΔCP increases to a maximum value at amoment when the polymer becomes photobleached. For largerillumination time, the ΔCP decreases until it vanished. Finally, thecontribution of the photophysical and photochemical processes tothe ΔCP has been determined. It has been found that even ifcomplex charging and discharging processes takes place, the ΔCPsignal is correlated with a defined degradation stage and onlydepends on the previous illumination time.

E. Palacios-Lidon et al. / Solar Energy Materials & Solar Cells 117 (2013) 15–21 21

Acknowledgments

This work was supported by the MICINN thought the projectsForce for Future CSD2009-Force For Future-00024, FIS2009-07657,MAT2010-21267-C02-01, the Comunidad Autónoma de la Regiónde Murcia through the project “Células solares orgánicas: de laestructura molecular y nanométrica a dispositivos operativosmacroscópicos“and the Fundacion Seneca 15324/PI/10. EE thanksthe MICINN for the FPI program and EPL thanks the Ramon y Cajalprogram for financial support. This work has also been co-fundedby the European Union (Fondos FEDER).

References

[1] S. Gunes, H. Neugebauer, N.S. Sariciftci, Conjugated polymer-based organicsolar cells, Chemical Reviews 107 (2007) 1324–1338.

[2] M.A Ruderer, E. Metwalli, W. Wang, G. Kaune, S.V. Roth, P. Müller-Buschbaum,Photoactive nanostructures of polypyrrole, ChemPhysChem 10 (2009)664–671.

[3] V. Burtman, A. Zelichonok, A.V. Pakoulev, Molecular photovoltaics in nanos-cale dimension, International Journal of Molecular Sciences 12 (2011) 173–225.

[4] I.G. Scheblykin, A. Yartsev, T. Pullerits, V. Gulbinas, V.J. Sundstroem, Excitedstate and charge photogeneration dynamics in conjugated polymers, Journalof Physical Chemistry B 111 (2007) 6303–6321.

[5] T.Q. Nguyen, I.B. Martini, J. Liu, B.J.J. Schwartz, Controlling interchain interac-tions in conjugated polymers: the effects of chain morphology on exciton–exciton annihilation and aggregation in MEH-PPV films, Physical Chemistry B104 (2000) 237–255.

[6] B.J. Schwartz, Conjugated polymers as molecular materials: how chain con-formation and film morphology influence energy transfer and interchaininteractions, Annual Review of Physical Chemistry 54 (2003) 141–172.

[7] T.E. Dykstra, V. Kovalevskij, X.J. Yang, G.D. Scholes, Excited state dynamics of aconformationally disordered conjugated polymer:a comparison of solutionsand film, Chemical Physics 318 (2005) 21–32.

[8] S. Cataldo, B. Pignataro, Polymeric thin films for organic electronics: propertiesand adaptive structures, Materials 6 (2013) 1159–1190.

[9] F. Terzi, L. Pasquali, R. Seeber, Studies of the interface of conducting polymerswith inorganic surfaces, Analytical and Bioanalytical Chemistry 405 (2013)1513–1535.

[10] M. Jorgensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solarcells, Solar Energy Materials and Solar Cells 92 (2008) 686–714.

[11] L. Kronik, Y. Shapira, Surface photovoltage phenomena: theory, experiment,and applications, Surface Science Reports 37 (1999) 1–206.

[12] J.H. Yang, I. Shalish, Y. Shapira, Photoinduced charge carriers at surfaces andinterfaces of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene]with Au and GaAs, Physical Review B 64 (2001) 035325.

[13] M. Eschle, E. Moons, M. Gratzel, Construction of the energy diagram of anorganic semiconductor film on SnO2: F by surface photovoltage spectroscopy,Optical Materials 9 (1998) 138–144.

[14] M. Nonnenmacher, M.P. O´Boyle, H.K. Wickramasinghe, Kelvin probe forcemicroscopy, Applied Physics Letters 58 (1991) 2921–2923.

[15] H. Hoppe, T. Glatzel, M. Niggemann, A. Hinsch, M.Ch. Lux-Steiner, N.S. Sariciftci, Kelvin probe force microscopy study on conjugated polymer/fullerene bulk heterojunction organic solar cells, Nano Letters 5 (2005)269–274.

[16] H. Hoppe, T. Glatzel, M. Niggemann, W. Schwinger, F. Schaeffler, A. Hinsch, M.Ch. Lux-Steiner, N.S. Sariciftci, Efficiency limiting morphological factors ofMDMO-PPV: PCBM plastic solar cellsThin Solid Films 511 (2006) 587–592.

[17] J. Cermak, B. Rezek, V. Cimrova, A. Fejfar, A. Purkrt, M. Vanecek, J. Kocka, Time-resolved opto-electronic properties of poly(3-hexylthiophene-2,5-diyl) full-erene heterostructures detected by Kelvin force microscopy, Thin Solid Films519 (2010) 836–840.

[18] M. Chiesa, L. Bürgi, J.S. Kim, R.H. Friend, H. Sirringhaus, Correlation betweensurface photovoltage and blend morphology in polyfluorene-based photo-diodes, Nano Letters 5 (2005) 559–563.

[19] C. Groves, O.G. Reid, D.S. Ginger, Heterogeneity in polymer solar cells: localmorphology and performance in organic photovoltaics studied with scanningprobe microscopy, Accounts of Chemical Research 45 (2010) 612–620.

[20] E.J. Spadafora, R. Demadrille, B. Ratier, B. Grevin, Imaging the carrier photo-generation in nanoscale phase segregated organic heterojunctions by Kelvinprobe force microscopy, Nano Letters 10 (2010) 3337–3342.

[21] A. Liscio, V. Palermo, P. Samori, Nanoscale quantitative measurement of thepotential of charged nanostructures by electrostatic and Kelvin probe force

microscopy: unraveling electronic processes in complex materials, Accounts ofChemical Research 43 (2010) 541–550.

[22] K. Maturova, M. Kemerink, M.M. Wienk, D.S.H. Charrier, R.A.J. Janssen, Scan-ning Kelvin probe microscopy on bulk heterojunction polymer blends,Advanced Functional Materials 19 (2009) 1379–1386.

[23] E. Escasain, E. Lopez-Elvira, A.M. Baro, J Colchero, E. Palacios-Lidon, Nanoscalesurface photovoltage of organic semiconductors with two pass Kelvin probemicroscopy, Nanotechnology 22 (2011) 375704.

[24] F. Mohammad, H.S. Nalwa (Eds.), Handbook of organic Molecules and Poly-mers, 3, John Wiley & Sons Ltd, West Sussex, England, 1997, Ch. 16.

[25] E. Escasain, E. Lopez-Elvira, A.M. Baro, J. Colchero, E. Palacios-Lidon, Nanoscaleelectro-optical properties of organic semiconducting thin films: from indivi-dual materials to the blend, Journal of Physical Chemistry C 116 (2012)17919–17927.

[26] A. Rivaton, S. Chambon, M. Manceau, J.L. Gardette, N. Lamaitre, S. Guillerez,Light-induced degradation of the active layer of polymer-based solar cells,Polymer Degradation and Stability 95 (2008) 278–284.

[27] A.E. Ozimova, V.V. Bruevich, D.Y. Paraschuk, Measurement of the photobleach-ing kinetics of semiconducting polymer films by the pump-probe method,Quantum Electronics 41 (2011) 1069–1072.

[28] T.E. Dykstra, E. Hennebicq, D. Beljonne, J. Gierschner, G. Claudio, E.R. Bittner,J. Knoester, G.D. Scholes, Photogeneration and ultrafast dynamics of excitonsand charges in P3HT/PCBM blends, Journal of Physical Chemistry B 113 (2009)656–667.

[29] I. Hwang, G.D Scholes, Electronic energy transfer and quantum-coherence inpi-conjugated polymers, Chemistry of Materials 23 (2011) 610–620.

[30] E. Palacios-Lidón, B. Pérez-García, J. Colchero, Enhancing dynamic scanningforce microscopy in air: as close as possible, Nanotechnology 20 (2009)085707.

[31] I. Horcas, R. Fernandez, J.M. Gomez-Rodriguez, J. Colchero, J. Gomez-Herrero,A.M. Baro, WSXM: a software for scanning probe microscopy and a tool fornanotechnology, Review of Scientific Instruments 78 (2007) 013705.

[32] E. López-Elvira, E. Escasain, A. Baro, J. Colchero, E. Palacios-Lidon, Wavelengthdependence of nanoscale photodegradation in poly(3-octylthiophene) thinfilms, Polymer Degradation and Stability96 (2011) 1279–1285.

[33] S.K. Kohl, J.D. Landmark, D.F.J. Stickle, Demonstration of absorbance usingdigital color image analysis and colored solutions, Journal of ChemicalEducation 83 (2006) 644–646.

[34] E. Fefer, L. Kronik, M. Leibovitch, Y. Shapira, W. Riedl, In-situ monitoring ofsurface chemistry and charge transfer at semiconductor surfaces, AppliedSurface Science 104 (1996) 61–67.

[35] H. Hoppe, N.S. Sariciftci, D. Meissner, Optical constants of conjugated polymer/fullerene based bulk-heterojunction organic solar cells, Molecular Crystals andLiquid Crystals 385 (2002) 233–239.

[36] E.G.J. Staring, A.J.M Berntsen, S.T.R. Romme, G.L.J.A. Rikken, P. Urbach, On thephotochemical stability of dialkoxy-PPV; a quantitative approach, Philosophi-cal Transactions of the Royal Society of London A 355 (1997) 695–706.

[37] B.H. Cumpston, I.D. Parker, K.F.J. Jensen, In situ characterization of theoxidative degradation of a polymeric light emitting device, Journal of AppliedPhysics 8 (1997) 3716–3720.

[38] N. Dam, R.D. Scurlock, B.J. Wang, l.C. Ma, M. Sundahl, P.R. Ogilby, Singletoxygen as a reactive intermediate in the photodegradation of phenylenevi-nylene oligomers, Chemistry of Materials 5 (1999) 1302–1305.

[39] R.D. Scurlock, B.J. Wang, P.R. Ogilby, J.R. Sheats, R.L. Clough, Singlet oxygen as areactive intermediate in the photodegradation of an electroluminescentpolymer, Journal of the American Chemical Society 117 (1995) 10194–10202.

[40] S. Chambon, A. Rivaton, J.L. Gardette, M. Firon, Durability of MDMO-PPV andMDMO-PPV: PCBM blends under illumination in the absence of oxygen, SolarEnergy Materials and Solar Cells 92 (2008) 785–792.

[41] S Chambon, M. Manceau, M. Firon, S. Cros, A. Rivaton, J.L. Gardette, Photo-oxidation in an O-18(2) atmosphere: a powerful tool to elucidate themechanism of UV–visible light oxidation of polymers—Application to thephotodegradation of MDMO-PPV, Polymer 49 (2008) 3288–3294.

[42] S. Chambon, A. Rivaton, J.L. Gardette, M. Firon, Photo- and thermal degrada-tion of MDMO-PPV: PCBM blends, Solar Energy Materials and Solar Cells 91(2007) 394–398.

[43] S. Chambon, A. Rivaton, J.L. Gardette, M.J. Firon, Reactive intermediates in theinitiation step of the photo-oxidation of MDMO-PPV, Polymer Science A 47(2009) 6044–6052.

[44] S. Chambon, A. Rivaton, J.L. Gardette, M. Firon, Photo- and thermo-oxidation ofpoly(p-phenylene-vinylene) and phenylene-vinylene oligomer, Polymerdegradation and Stability 96 (2011) 1149–1158.

[45] O.G. Reid, G.E. Rayermann, D.C. Coffey, D.S. Ginger, Imaging local trapformation in conjugated polymer solar cells: a comparison of time-resolvedelectrostatic force microscopy and scanning Kelvin probe imaging, Journal ofPhysical Chemistry C 114 (2010) 20672–20677.