Charge Transfer States in Merocyanine Neat Films and Its Blendswith [6,6]-Phenyl‑C61-butyric Acid Methyl EsterDomantas Peckus,† Andrius Devizis,† Ramu nas Augulis,

† Steven Graf,‡ Dirk Hertel,*,‡ Klaus Meerholz,‡

and Vidmantas Gulbinas*,†

†Center for Physical Sciences and Technology, Savanoriu 231, LT-02300 Vilnius, Lithuania‡Department of Chemistry, Physical Chemistry, University of Cologne, Luxemburgerstrasse 116, 50939 Cologne, Germany

ABSTRACT: Excited-state relaxation processes in neatmerocyanine MD376 films and their blends with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have beeninvestigated by means of steady-state and time-resolvedabsorption and fluorescence spectroscopy. Formation ofcollective states with intermolecular charge transfer character(CTM−M) during the initial several picoseconds after excitationdetermines the excited-state dynamics of neat MD376 films.Blending of MD376 with PCBM leads to quenching of itsintrinsic fluorescence and to the appearance of a newfluorescence band caused by interfacial charge transfer states between MD376 and PCBM molecules (CTM−F). Generation offree charge carriers in films with high PCBM concentration causes quenching of all fluorescent states.

1. INTRODUCTION

The growing need for energy stimulates scientific researchaiming to develop efficient, low-cost energy sources, and one ofthem may be photovoltaic devices. Organic semiconductorshave emerged as a new class of materials, which hold promisefor the development of low-cost and large-area solar cells. Thebulk heterojunction (blend) concept1 has turned out to be themost promising approach to highly efficient organic solar cellsbecause it features an extended interface between the phases ofthe electron donor and the electron acceptor compounds andleads to enhanced generation of free charge carriers.2,3 A broadvariety of organic conjugated polymers4 and small moleculescan be used as electron donors,5 while fullerene derivatives arethe most common electron acceptors. Small molecules may bedeposited under high vacuum conditions by thermal evapo-ration. On the other hand, soluble materials like polymers canbe spin-coated from solution. Many semiconducting polymershave been successfully applied in solution processed solarcells.6,7

Solution processing is a relatively fast and low-cost thin layerfabrication method. However, stacking of multiple layers on topof each other becomes a challenge because each new layertends to dissolve in the previous one.8,9 Nevertheless, theefficiencies of solar cells made from organic polymers bysolution processing exceed 7%,6,7 while the efficiencies of smallmolecule-based cells reach up to 7%.10,11

Polymer blends have been widely investigated, and chargegeneration and transport are comparatively well under-stood.12,13 Understanding of photophysical properties andcharge generation in blends of small molecules is lessthorough.14−16 It is conceivable that solar cells of small organicmolecules have a higher potential to meet challenges of

commercialization due to the ease of synthesis, purification, andprocessing as demonstrated by 10% in tandem solar cells.17

Besides this tremendous improvement over the last 10 years,little is known about relevant photophysical processes in smallmolecule materials and their blends as compared to polymers;thus more detailed research is necessary to unravel unusedpotential of these materials. It is very likely that sometechnological improvement of vacuum deposition or solutionprocessing may increase the efficiency of small molecule-basedsolar cells dramatically.Recently, a number of low-molecular-weight dyes have been

introduced as new absorbers and electron-donating compoundsin blend solar cells, among them squaraines,18,19 thiophenes,14

donor−acceptor thiadiazoles,11 and merocyanines20,21 leadingto efficient devices. It is promising in terms of application thatmerocyanine molecules can be either thermally evaporated invacuum or processed from solution.22

In this Article, we address the excited-state dynamics afteroptical excitation in blend films processed from solution. Wehave chosen MD376 (see Figure 1) for our investigationbecause it performs well in solar cells and can be processedfrom solution and vacuum. Pure MD376 films and blends withfullerene derivatives at various concentration ratios have beeninvestigated. MD376 solutions in dichloromethane and toluenewere also studied.The main goal of our investigation is to reveal the properties

of merocyanine films and their blends with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) related to the dipolar nature

Received: November 16, 2012Revised: February 19, 2013Published: March 1, 2013

Article

pubs.acs.org/JPCC

© 2013 American Chemical Society 6039 dx.doi.org/10.1021/jp311336v | J. Phys. Chem. C 2013, 117, 6039−6048

of merocyanine molecules. The charge transfer (CT) excitonformation peculiarities in the films of MD376 are of specialinterest because the CT exciton can either facilitate or suppressgeneration of free charge carriers. This is due to the fact thatdipolar molecules can form CT excitons within the donor phaseas well as the donor−acceptor phase. It is expected that itdepends on the peculiar constitution and arrangement of thedonor molecules. To draw a complete picture of photophysicalprocesses, we will discuss results of absorption, fluorescence,and transient absorption studies of MD376 solutions, neatfilms, and blends with PCBM. It will be shown that CT statesare present in neat films of MD376; however, CT statesbetween MD376 and PCBM prevail in blends and play animportant role in free carrier generation.

2. EXPERIMENTAL SECTIONThe syntheses of merocianine MD376 have been described inref 23. For investigations of MD376 properties in solutions, itwas desolved in toluene, chlorobenzene (CB), or dichloro-methane (DCM) at concentrations between 10−3 and 10−6

mol/L.The deposition of merocyanine layers on glass substrates was

performed by spin coating from MD376 solution inchlorobenzene. It was solved with a concentration of 15 mg/mL and spin coated to obtain film thicknesses of ca. 60 nm. Forblend films, a MD376 solution was mixed with PCBM solutionin various ratios using the same solvent and identicalconcentrations.Fluorescence spectra and fluorescence decay kinetics with

subnanosecond time resolution were investigated by means of afluorescence spectrometer F900 (Edinburgh Instruments). Adiode laser EPL-375 emitting 50 ps pulses at 375 nm with arepetition rate of 20 MHz was used for the sample excitation.The average excitation power was about 150 μW/mm2.Streak-camera Hamamatsu C5680 with synchroscan unit

M5676 was used for the measurement of fluorescence dynamicswith picosecond time resolution. For the excitation, afemtosecond Yb:KGW oscillator (Pharos, Light ConversionLtd.) was employed. The oscillator produced 80 fs 1030 nmlight pulses at 76 MHz repetition rate, which were frequencytripled to 343 nm (HIRO harmonics generator, LightConversion Ltd.), attenuated, and focused into ∼100 μmspot on the sample, resulting in about 1 mW/mm2 averageexcitation power. The maximum time resolution of the wholesystem was about 3 ps.

For temperature-dependent photoluminescence spectra, thesamples were kept in a customized Cryostat (CryoVac) undernitrogen atmosphere and were excited at 566 nm using thebeam of an InnoLas Spitlight300 Nd:YAG laser with ca. 4 nspulse width and 20 Hz repetition rate to operate an OPO(GWU versaScan). PL spectra were recorded with a PI-MAX2CCD camera from Roper Scientific after passing an Acton2300i spectrograph with appropriate gratings.Transient absorption investigations have been performed by

means of conventional broadband femtosecond absorptionpump−probe spectroscopy. The spectrometer was based on anamplified femtosecond Ti:sapphire laser (Quantronix Integra-C) generating pulses at 805 nm of approximately 130 fsduration at 1 kHz repetition rate. Pulses of optic parametricgenerator TOPAS C at 500 nm wavelength were used for theexcitation of the sample. White light continuum for probing wasgenerated in a 2 mm thick sapphire plate. The excitation beamwas focused into a spot of about 500 μm diameter, while thediameter of the probe spot was about 300 μm. The differentialabsorption (ΔA) is defined as the negative logarithm of theratio of transmitted light with and without optical excitation(pump), respectively. Thus, positive signals correspond toinduced (excited state) absorption, and negative signals are dueto absorption bleaching and/or stimulated emission.

3. RESULTS AND DISCUSSION

3.1. Solutions. Absorption spectra of merocyanine MD376in solutions (c = 4 × 10−6 mol/L, ε = 66 400 M−1 cm−1 (at 576nm)) are presented in Figure 1.The absorption spectra of solutions in weakly polar solvent

toluene (μ = 0.3D, ET30 ≈ 33.9 kJ/mol) show two clearlyexpressed absorption bands. The main absorption band (575nm) corresponding to the S0→S1 transition is accompanied bya vibronic satellite roughly 142 meV higher in energy, typical ofa C−N stretching mode. On the short wavelength side, anadditional absorption band (∼475 nm) is observed. It is shiftedby about 0.5 eV relative to the main transition; therefore, it cannot be related to a vibrational satellite. We attribute thistransition to an electronic state above S1. The energeticposition of the absorption transition does not change if themore polar solvent dichloromethane (DCM, μ = 1.1D, ET30 ≈40.7 kJ/mol) is used. This seems a little surprising becauseMD376 molecules have a large ground-state dipole moment of6.2 D.24 However, the absorption band shows only someincreased inhomogeneous broadening instead. All together, it isconceivable that we observe absorption of monomeric MD376in solution; otherwise, the absorption would split into twotransitions as observed for more polar merocyanines withchanging solvent polarity.25,26

Figure 1 also shows fluorescence spectra of MD376 intoluene and DCM solutions at a concentration of 10−5 mol/L.In toluene the fluorescence band is red-shifted to the longwavelength side by about 100 meV relative to the absorption,which is a typical Stokes-shift for large organic moleculesindicating that fluorescence originates from the same electronictransition as the lowest energy absorption band. It should benoted that fluorescence spectra of solutions are much moresensitive to solvent polarity: the fluorescence band shifts muchmore (160 meV) to the long wavelength side, becomes muchbroader, and the intensity decreases in more polar solventDCM. Altogether, this hints to an emitting state having moreCT character. The MD376 fluorescence decays exponentially

Figure 1. Absorption (thick lines) and fluorescence (thin lines)spectra of MD376 in toluene (red) and DCM (black).

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with about 300 ps time constant in toluene solution and with a600 ps time constant in DCM (not shown).Transient absorption spectra of MD376 solution in DCM

measured at various delay times are presented in Figure 2.

The spectra show a negative band around 580 nm (0.2 ps),which closely resembles the sum of the absorption andfluorescence bands; thus it is caused by absorption bleachingand stimulated emission. The negative signal in the shortwavelength absorption band region (ca. 500 nm, 0.2 ps) isweak, and even weak induced absorption appears at about 475nm at longer delays (50 ps). Thus, excited-state absorptionapparently compensates absorption bleaching in this spectralregion (450−520 nm). The transient absorption decaysexponentially with about a 600 ps time constant (notpresented), coinciding with the fluorescence decay time, andshows no evident spectral evolution. Consequently, it revealsno dynamics other than excited-state decay, which indicatesthat no major changes of electronic or conformational structureof molecules take place in the excited state within picosecondto nanosecond time scales. On the other hand, relatively lowcontribution of stimulated emission hints at some conforma-tional changes reducing transition dipole moment, which takeplace faster than the time resolution of our setup (∼130 fs).3.2. Neat Films. Absorption and Fluorescence Spectra.

Freshly prepared neat MD376 films have a main absorptionband split into two bands (A1 and A2 in Figure 3) of almostthe same strength separated by about 160 meV.

The first absorption band is located at 610 nm as comparedto 574 nm in solutions, that is, red-shifted by 140 meV. Thepositions and relative intensities of both bands slightly dependon sample preparation. First, it should be noted that the ratiobetween integrated intensities of the higher energy and thelower energy bands remains approximately the same insolutions and in films. It indicates that no new absorptionbands appear in films, and just a long wavelength band is splitinto two bands. The simplest interpretation of the absorptionband splitting would be an assumption that the absorption bandshifts to the long wavelength side by about 140 meV incomparison with solutions and the vibronic satellite gainsintensity. Although we cannot completely rule out such apossibility, it is not very likely because the position of the longwavelength absorption band is insensitive to the solventpolarity, and therefore environment influence can hardly beso strong in the solid state. Moreover, the band splitting slightlyvaries in different samples, which is inconsistent with thevibrational origin. The two bands may be also attributed todifferent molecular species present in the films, for example, toaggregate or microcrystalline structures surrounded byamorphous material as it has been suggested for someconjugated polymers.29 This is also unlikely because the twobands have very similar relative intensities in differentlyprepared samples, which would be surprising given that relativeconcentrations of amorphous and aggregate structures wouldnot be sensitive to sample preparation. We suggest that theabsorption band splitting is caused by excitonic coupling as aresult of formation of interacting dimers, aggregates, ormicrocrystallites. According to the excitonic theory,27 theabsorption transition splits into two transitions if the aggregatestructure is composed of two inequivalent molecules in the unitcell, that is, molecules with different orientations of theirtransition dipole moments. The relative intensities of bothbands depend on the mutual orientation of nonequivalentmolecules, while the amount of splitting depends on theinteraction energy. Therefore, relative band intensities and theirpositions may slightly vary depending on the moleculararrangement in aggregates. The arrangement of MD376molecules in single crystals supports our assignment.24,26

Below we show that this attribution is also consistent withthe transient absorption results.On the other hand, the films are not completely ordered or

crystalline; at least a fraction of the material is expected to beamorphous. The absorption of amorphous material isapparently hidden under the high intensity bands of theaggregates. It should be noted that neat MD376 films are proneto crystallization; clearly visible polycrystalline structure appearsin the films during a period of several weeks. It depends largelyon solvent used: solvents of lower vapor pressure producestable films, whereas solvents of higher vapor pressure tend toform films prone to crystallization. Aged crystallized films haveflattened spectra and slightly different peak positions. Bandflattening is a natural consequence of formation of largecrystallites nontransparent to light of any wavelength withinabsorption band region.28

Fluorescence spectra of the MD376 films (Figure 3) arecomposed of two main bands peaking at approximately 720 and650 nm (1.72 and 1.9 eV); however, their intensities, positions,and widths vary significantly depending on the film preparationconditions, aging, and particularly on temperature. The highenergy fluorescence band (F1) at about 650 nm gains intensityupon aging of the sample, but disappears at low temperatures.

Figure 2. Transient absorption spectra of MD376 in DCM at variousdelay times. The steady-state absorption (gray line) and PL spectrum(green line) are shown for comparison.

Figure 3. Absorption spectrum and fluorescence spectra measured atindicated temperatures of neat MD376 film processed fromchlorobenzene (CB) solution.

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Judging by a typical Stokes shift, this band originates from thedominating aggregated species responsible for the lowestenergy absorption band at 600−620 nm. The long wavelengthfluorescence band (F2) at about 720 nm has a very large Stokesshift; therefore, it should be attributed to some minor structuralspecies with low energy electronic states or to formation of newelectronic states in the excited state inactive in absorption.Zviterionic character of MD376 molecules suggests for-

mation of low energy intermolecular charge transfer states uponsome optimal arrangement of different fragments of neighbor-ing molecules. Thus, these states are most likely responsible forthe low energy fluorescence band. Hereafter, we label them asCTM−M. Other explanations of the low energy fluorescenceband such as molecular isomerization or intersystem crossing totriplet states can be ruled out because both fluorescence andtransient absorption show no signs of their formation insolutions. Moreover, triplet states are not consistent with thefast formation of the long wavelength band, as we will discussbelow, and also with the fact that the intensity of this band isnot sensitive to the film exposure to oxygen.The temperature dependence of relative intensities of the

two bands approximately follows the occupation probability ofthe two states determined by a Boltzmann distribution. Itindicates that excitations relax to the low energy excited statesat low temperatures, while thermal activation repopulates thehigh energy excited states.Fluorescence Decay Dynamics. Fluorescence dynamics of

the neat films was measured in two time domains by means of astreak-camera (picosecond domain) and by a TCSPCspectrometer (nanosecond domain). Figure 4 shows fluo-rescence kinetics of the neat film measured at the maxima ofthe two fluorescence bands.Fluorescence intensity on a nanosecond time scale

investigated by a TCSPC spectrometer decays with about a550 ps time constant at both wavelengths at room temperature,and with about a 3800 ps time constant at 725 nm at 80 K (notpresented). The fluorescence decay measured with a streak-

camera reveals an additional nonexponential fast decay. Fittingof the early time fluorescence decay with biexponential functiongives additional relaxation components with decay timeconstants of 7 and 60 ps at 650 nm, and of 20 and 200 ps at725 nm.The fast decay component at 725 nm has a lower relative

contribution than at 650 nm and should be attributed to the tailof the short wavelength fluorescence band still present at 725nm. The fluorescence intensity of F1 band at 650 nm decaysapproximately 100 times during this fast relaxation phase, whichlasts for about 200−300 ps until fluorescence reaches the slow,independent of detection wavelength decay phase. It indicatesthat population of the S1 state decreases about 100 times, whenexcitations relax to CTM−M states. Assuming that only the highenergy states are initially optically excited and total density ofexcitations decays much slower than excitations relax to the lowenergy states, we obtain that about 1% of excitations remain onthe high energy states after the thermalization process, whilethe remaining 99% relax to the low energy CTM−M states. Onthe other hand, after thermalization, relative populations of high(n1) and low (n2) energy states should be determined by aBoltzmann distribution: n1/n2 = (ρ1/ρ2) exp(−ΔE/kT), whereρ1/ρ2 determines relative densities of high and low energystates and ΔE is the energy difference between the two states ofabout 0.2 eV. This energy difference gives that occupationprobabilities of the two states must differ by about 3000 times(exp(−ΔE/kT) = 1/3000). Taking into account that thefluorescence intensity drops about 100 times, we obtain relativedensities of states ρ1/ρ2 = 30. Thus, this estimation gives thatthe density of the low energy CTM−M states is about 3% of thetotal density of molecules. This estimation is in a reasonableagreement with the tens of picosecond exciton trapping time.As we will show below, 10% of PCBM causes several timesfaster fluorescence quenching. However, apparently the densityof the CTM−M states strongly depends on the samplepreparation conditions and aging causing significant variationof the fluorescence spectra; thus this estimation shall beconsidered only as a rough estimate.Figure 5 shows the energy scheme for neat MD376 films

based on the above presented experimental data.Transient Absorption of Neat MD376 Films. Several

samples of neat MD376 films prepared from MD376 solutionsin different solvents have been investigated. Despite strongvariation of absorption and particularly fluorescence spectra ofdifferent films, the transient absorption spectra were quite

Figure 4. Fluorescence decay kinetics of neat MD376 films at 650 and725 nm measured by means of streak-camera (a) and TCSPCspectrometer (b). The inset shows the initial part of the kinetics on alinear scale normalized to equal amplitudes of the slow relaxationcomponent.

Figure 5. Energy level scheme of neat MD376 films. S1 shows twoexcitonically split excited singlet states responsible for A1 and A2absorption bands, and CTM−M shows the intermolecular chargetransfer state. Singlet excitons relax to the CTM−M; however,subsequently the lower energy singlet state is thermally repopulated.

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similar. Figure 6 shows the transient absorption spectra atvarious delay times for the film processed from chlorobenzenesolution obtained under excitation intensity of 60 μJ/cm2 perpulse.

At zero delay time, the transient absorption spectrum revealsbleaching of the absorption spectrum (500−650 nm) partlycompensated by the excited-state absorption. The negativesignal at λ > 650 nm is attributable to the stimulated emissionbecause steady-state absorption and consequently absorptionbleaching are almost absent in this spectral region.Evolution of the transient absorption of films is more

complex in comparison with solutions. The intensity of thelong wavelength part (>630 nm) of the negative bandsignificantly decays within 50 ps, while the short wavelengthpart (∼570 nm) of the absorption bleaching band decays muchslower, remaining almost constant up to 5 ps. Figure 7 showsthe transient absorption kinetics at various wavelength probingthe different energies, that is, 550, 580, 640, and 700 nmmeasured at low excitation intensity of 24 μJ/cm2 whennonlinear relaxation processes play no essential role (seebelow).

The kinetics at 640 nm is much faster than at otherwavelengths during the initial 50 ps supporting its attribution tothe stimulated emission related to the F1 fluorescence band,which shows identical relaxation kinetics. Transient absorptionrelaxation becomes identical at all wavelengths when excitationtrapping to CTM−M states terminates at about 50 ps.The transient absorption decay also reveals a subpicocecond

relaxation process, which was not observed in fluorescenceexperiments due to the limited time resolution of about 3 ps ofthe streak-camera. Once the stimulated emission decays, thetransient absorption spectrum is almost identical to the invertedabsorption spectrum (Figure 6, thick line). It shows that themajority of the molecules remain in the excited state, while onlylosing the stimulated emission property, in agreement withrelaxation to the weakly fluorescent low energy state asdiscussed above. Surprisingly, absorption bleaching experiencesno changes during this process. It unambiguously shows thatthe low energy sites, dominantly occupied after the relaxationprocess, have an absorption spectrum identical to or verysimilar to that of the major molecular species, which wereinitially excited. The only difference is that the low energyspecies show no stimulated emission and their fluorescencespectrum is shifted to the long wavelength side (Figure 3).These properties are consistent with the charge transfer (CT)character of the low energy states where electron transferbetween differently charged fragments of neighboring mole-cules takes place in the excited state. Such CT states are verycommon in molecular solids composed of polar molecules. CTstates usually have small oscillator strength; thus the absence ofthe stimulated emission is a natural consequence of the CTstate formation. The CTM−M states are most probably formedbetween MD376 molecule pairs having some particular mutualarrangement, supported by the single crystal structure.24 Thesemolecular pairs may be excited directly; however, more likelythey are dominantly excited during exciton migrationdetermining tens of picoseconds relaxation component of theF1 fluorescence band. Even a small difference in the CTM−Mstate energy caused by some variations of arrangement ofmolecular pairs may cause significant differences in relativepopulations of Frenkel and CTM−M exciton states. Morphologydifferences may also cause differences in CTM−M state oscillatorstrength. Thus, these factors evidently cause strong sensitivityof the fluorescence spectra to the sample morphology.The shape of the transient absorption spectrum changes

dramatically at 2500 ps delay time: a new induced absorptionband appears at the long wavelength absorption band slope(630−670 nm). We assign this spectrum to the absorbancechanges induced by the local heating of the film followingoptical excitation. To support this assignment, we havemeasured the changes of the steady-state absorption spectrumupon sample heating. The sample was heated to 80 °C andcooled to room temperature several times. The spectrumexperienced almost reversible changes, which in the 500−700nm region were very similar to the transient absorptionspectrum measured at 2500 ps (see Figure 6). The values of thethermally and optically induced absorbance changes were alsoin good agreement with the local heating values estimatedassuming typical heat capacity of organic solids of 0.5 cal g−1

K−1. It should be noted that the local heating contribution doesnot disappear by reducing the excitation intensity; its intensitydecreases approximately proportionally to the transientabsorption intensity at zero delay time. This is due to thefact that almost all absorbed excitation energy is converted to

Figure 6. Transient absorption spectra of a neat MD376 film atvarious delay times. The solid purple line shows inverted absorptionspectrum of the film. The spectrum on top shows the absorbancechanges induced by the sample heating by 50 K.

Figure 7. Transient absorption kinetics at various probe wavelengthsfor the neat MD376 film, measure at 24 μJ/cm2 per pulse excitationintensity.

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the thermal energy within about 1 ns after excitation, whilethermal energy transfer to the glass substrate is much slower.The transient absorption relaxation at all probe wavelengths

becomes progressively faster with increasing excitationintensity. Figure 8 shows kinetics at 630 nm where the fastrelaxation component is almost absent at low intensity.

It indicates that exciton−exciton annihilation, common inmolecular solids, shortens the excited-state lifetime at highexcitation intensities. It is interesting to note that oscillatingfeatures become clearly observable at high excitation intensitiesfor times >100 ps. The oscillations are particularly strong at thelong wavelength slope of the absorption band, and may beunambiguously attributed to acoustic oscillations induced bylocal heating, which were observed in thin films of otherorganic materials.30 Strong oscillations in MD376 films indicatethat the long wavelength absorption band is very sensitive tothe density of the material. In agreement with the excitonicsplitting, the long wavelength absorption band shifts toward thelong wavelength side when intermolecular interactions increasedue to decreasing intermolecular distances.We have also investigated the dependence of the transient

absorption on excitation wavelength (not shown). No cleardifference of spectra and their evolution was observed underexcitation of the films at 500, 550, and 610 nm, correspondingto different absorption peaks. It supports the assumption thatall three absorption bands belong to the same molecularspecies.3.3. Blends. Absorption spectra of blends correspond to the

sum of MD376 and PCBM absorption spectra (see Figure 9).Absorbance of the films at long wavelength, where PCBM

absorption is weak, is approximately proportional to theconcentration of MD376, while PCBM absorption dominatesin the short wavelength region. We do not observe any newbands, which could be related to formation of MD376/PCBMcomplexes.Addition of PCBM causes strong quenching of the MD376

fluorescence and appearance of a new low energy fluorescenceband F3 with maximum at about 740 nm (see Figure 10),which is only slightly red-shifted and broader than F2 band (seeFigure 3).Even 1% of PCBM quenches the F1 fluorescence band by

about 30%. MD376 fluorescence intensity drastically drops athigher PCBM concentrations, and, interestingly, intensity of theF3 fluorescence band drops as well.

Fluorescence decay kinetics at 650 and 725 nm for blendfilms are presented in Figure 11.Fluorescence of MD376 at 650 nm in the blend with 10%

PCBM dominantly decays within about 20 ps. At 50% PCBMconcentration, the decay at 650 nm is limited by the resolutiontime of the streak-camera (∼3 ps). Kinetics on a nanosecondtime scale shows that the F1 band was not completelyquenched on a picosecond time scale but has also a slowlyrelaxing component, which was not observed with streak-camera because of lower sensitivity. Decay kinetics at 725 nmhas an identical fast decay component, which should be alsoattributed to the MD376 fluorescence band tail. Fluorescence at725 nm also has a slow relaxation component, which is strongerthan at 650 nm and therefore was also observed with streak-camera. Fluorescence at both detection wavelengths decaysalmost identically on a nanosecond time scale, weakly dependson PCBM concentration, and may be characterized by about 1ns (46%) and 3 ns (54%) decay time constants. These decaytimes are much longer than fluorescence decay time in neat filmof about 550 ps, thus confirming attribution of the slowlyrelaxing fluorescence component to a new F3 band formed inblends.Conventionally, quenching of the inherent material (i.e.,

MD376) fluorescence by acceptors such as PCBM is due tothree main processes: (a) energy transfer from donor to theacceptor, (b) electron transfer from the donor to the acceptor,and (c) formation of complexes between donor and acceptorand energy transfer to these complexes. Consequently, the newfluorescence band F3 may be attributed to fluorescence of newstates, probably of CT origin, formed between MD376 andPCBM. This has been observed many times for conjugatedpolymer/PCBM blends.31−33 Fluorescence of PCBM can be

Figure 8. Transient absorption kinetics at 630 nm for the neat MD376film measured at various excitation intensities. Oscillations observedon a long time scale at high excitation intensities are due to thegeneration of acoustic waves.

Figure 9. Absorption spectra of MD376:PCBM blend films withdifferent concentrations of PCBM.

Figure 10. Fluorescence spectra of MD376:PCBM blend films withdifferent PCBM concentrations.

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ruled out, because PCBM fluorescence peaks at about 1.7 eVwith a rather narrow half width and decays with time constantsof about 0.7 ns (58%) and 1.4 ns (42%).34 The F3 band ofMD376/PCBM blend peaks at slightly lower energy, is muchbroader, and decays about 2 times slower. Moreover, our recentinvestigations revealed that F3 band is very efficiently quenchedby electric field and the quenching efficiency closely correlateswith photocurrent.35 Therefore we attribute F3 to CTM−F statesformed between MD376 and PCBM. A 10-fold decrease of theF1 fluorescence band intensity in the film with 10% PCBMconcentration shows that quenching of MD376 fluorescence inthe film takes place with about 90% efficiency. Thus, only aminor increase of the CTM−F state formation is expected athigher concentrations. At high PCBM concentrations, otherfactors are likely to reduce CTM−F fluorescence intensity. Oneof them is free charge carrier generation, competing withformation of fluorescing CTM−F states or causing theirquenching.31,32 The carrier generation/collection efficiency ishigher at high PCBM concentration when PCBM clusters areformed as was demonstrated for polymer:PCBM blends.32,36−39

On the other hand, an identical slow relaxation componentobserved at 650 and 725 nm detection wavelength is mostlikely due to thermal repopulation of S1 states of MD376 inanalogy with repopulation of these states from CTM−M states in

neat MD376 films. It causes not complete quenching of the F1fluorescence band and shows that a small energy differencebetween S1 and CTM−F states is not sufficient to completelyprevent electron back transfer from PCBM to MD376.Figure 12 shows transient absorption spectra of solution

processed blends with 10% and 50% PCBM concentrations.

The initial spectrum of the 10% PCBM film is identical tothat obtained in neat films. This similarity is quite predictable,because PCBM weakly absorbs at the excitation wavelength;thus most of MD376 molecules are excited directly. Subsequentevolution of the transient absorption spectrum is alsoqualitatively similar to that of neat films; however, thestimulated emission decays significantly faster, in agreementwith the fluorescence decay, and a new induced absorptionband appears in the 650−750 nm region (compare Figure 6).This band should be attributed to the CTM−F states observed influorescence spectra. Similar near-IR absorption has beenobserved in P3HT/PCBM blends and attributed to theCoulombically bound charge transfer states or to free chargecarriers.40,41 Decay of the stimulated emission gives an estimateof the CTM−F state formation rate of several picoseconds.Transient absorption spectra of the film with 50% PCBM

(see Figure 12) show no stimulated emission even at zero delaytime: the maximum of the absorption bleaching band coincideswith the long wavelength steady-state absorption band, and thenegative signal in the fluorescence band region is absent. Itindicates that quenching of the MD376 excited states andformation of CTM−F states or free charge carriers is very fast athigh PCBM concentration, faster than the resolution time ofour setup of about 130 fs. Similar ultrafast energy transfer andcharge carrier generation has been observed in conjugated-polymer/PCBM films.40,41 The long wavelength inducedabsorption (650−750 nm) attributed to the CTM−F states isinitially about 2 times stronger in film with 50% PCBM than infilm with 10% PCBM; however, it partly decays during tens ofpicosecond (see inset in Figure 12). On the other hand, theabsorption bleaching decays very similarly in both films. We

Figure 11. Fluorescence decay kinetics in MD376:PCBM blend filmswith different PCBM concentrations measured at 650 and 725 nm bymeans of streak-camera (a) and TCSPC spectrometer (b).

Figure 12. Transient absorption spectra of MD376/PCBM blend filmswith 10:1 and 1:1 concentration ratios at various delay times. Insetsshow transient absorption kinetics at 560 and 725 nm correspondingto absorption bleaching of MD376 dyes and absorption of CTM−Fstates, respectively.

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conclude that in films with 50% PCBM, a fraction of molecularspecies absorbing in the 700 nm region are converted tononabsorbing species during tens of picosecond. Combiningthis property with the quenching of the CTM−F fluorescence athigh PCBM concentration, we attribute the induced absorptionto the CTM−F states, which are partly quenched by the freecharge carrier generation in the films with high PCBMconcentration. This attribution is based on the assumptionthat free charge carriers do not absorb in the visible spectralregion. Indeed, PCBM anion radicals do not absorb in thevisible spectral region.42 MD376 radical cations prepared bychemical oxidation have a wide absorption band in the near IRregion (see Figure 13); however, the intensity of this band is

about 30 times lower than the intensity of the visible absorptionband of neutral molecules, and therefore its intensity in thetransient absorption spectra should be very low.On the basis of the investigation results, we propose the

energy state model presented in Figure 14 for MD376:PCBMblends.

The main finding is the formation of a charge transfer stateCTM−F below the CTM−M, already present in pure MD376films. Because of similar energies and thus similar fluorescencespectra, we cannot clearly distinguish between CTM−M andCTM−F states in blends. However, there are some indicationsthat formation of CTM−F states dominates, at least in blendswith high PCBM concentration: much faster quenching of the

F1 fluorescence band and of the related stimulated emission inblends shows that formation of CTM−F states is much fasterthan formation of CTM−M states in pure MD379; several timeslonger fluorescence relaxation time in blends indicates that it isdetermined by CTM−F states absent in pure material. Theprevalent role of CTM−F states for the photophysics of blends isevidently mainly caused by a higher density of CTM−F statesrather than by their lower energy. Thus, most likely CTM−Mstates do not play an important role in charge carrier generationin MD376:PCBM blends and consequently in solar cellsfabricated from these blends.

4. SUMMARY AND CONCLUSIONS

Exciton relaxation and charge separation in neat films ofMD376 and its blends with PCBM have been investigated bymeans of steady-state and time-resolved spectroscopy.Absorption, transient absorption, and fluorescence spectrareveal different aspects of the photophysics of neat MD376films. MD376 molecules, which are in monomeric form insolution, most probably create aggregates with split lowestenergy absorption band in solid films. We attribute splitting ofthe absorption band to the excitonic interaction resulting fromformation of dimers or aggregate structures with twononequivalent molecules in a unit cell with different orientationof their transition dipole moments. Solid films show dualfluorescence with a high energy band disappearing at lowtemperatures. The dual fluorescence is determined byformation of low energy states CTM−M invisible in absorptionspectra, most probably of the intermolecular charge transfernature. Excitation energy trapping by the molecular speciesforming CTM−M states determines the excited-state dynamicsdominated by the stimulated emission decay on a time scale oftens of picosecond. Dual fluorescence observed at longer timesis determined by the thermal repopulation of the higher energyS1 fluorescent states.PCBM additives quench inherent MD376 fluorescence and

induce appearance of a new fluorescence band related toformation of charge transfer CTM−F states between MD376 andPCBM molecules. Both types of charge transfer states, CTM−Fand CTM−M, have similar energies; therefore, fluorescenceproperties of the blends are determined by CTM−F states, mostlikely because their higher density. All fluorescence bands arequenched in films with high (>10%) PCBM concentration,which we attribute to dissociation of a fraction of CTM−M statesto free charge carriers taking place on a tens of picosecond timescale.In combination with the high efficiency of MD376 solar

cells,20 our results indicate that formation of aggregates of CTcharacter within the donor phase is not detrimental to deviceperformance as long as the density is low enough, and theCTM−M is energetically above the CTM−F between donor andPCBM. Thus, free charge generation competes efficiently withCTM−M state formation.Further investigations are ongoing to reveal the influence of

the donor−acceptor strength in the DA type merocyanines andthe role of processing influence in these promising solar cellmaterials.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: dirk.hertel@uni-koeln.de (D.H.); vidgulb@ktl.mii.lt(V.G.).

Figure 13. Absorption spectra of MD376 cations in DCM obtained bychemical oxidation using indicated volumes of saturated FeCl3solution. The concentration of the MD376 solution is ∼4 × 10−4

mol/L in a 0.1 cm cuvette.

Figure 14. Excited-state relaxation scheme in neat MD376 film blendwith PCBM. Optically created S1 excitations form intermolecularCTM−M or interphase CTM−F states during several picoseconds;however, thermal reactivation causes a weak population of S1 stateduring the entire lifetime of CTM−F state. Generation of separatedcharge carriers from unrelaxed CTM−F state at high PCBMconcentrations causes reduction of CTM−F and S1 state population.

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NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We would like to acknowledge Prof. F. Wurthner (UniversitatWurzburg) for fruitful discussions and supply of merocyanine.We thank V. Steinmann for help during sample preparation.This research was partly funded by the European Social Fundunder the Global Grant measure. We are also indebted to BASFfor material supply and to BMBF (FKZ 03EK3503C) forfinancial support.

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