FTIR Study of the Impact of PC[60]BM on the Photodegradation of

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FTIR study of the Impact of PC[60]BM on the photo-degradation of the low band gap polymer PCPDTBT under O2 environment Ulf Dettinger, Hans-Joachim Egelhaaf, Christoph J Brabec, Florian Latteyer, Heiko Peisert, and Thomas Chassé Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00268 • Publication Date (Web): 13 Mar 2015 Downloaded from http://pubs.acs.org on March 30, 2015

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Chemistry of Materials

FTIR study of the Impact of PC[60]BM on the photo-degradation of the low band gap polymer PCPDTBT under O2 environment Ulf Dettinger(a), Hans-Joachim Egelhaaf(b), Christoph J. Brabec(b),(c), Florian Latteyer(a), Heiko Peisert*,(a), Thomas Chassé(a) (a)

Institute for Physical and Theoretical Chemistry, Eberhard-Karls-University, Auf der Morgenstelle 18, 72076 Tübingen, Germany

(b)

ZAE Bayern, Haberstr. 2a, 91058 Erlangen, Germany

(c)

Institute of Materials for Electronics and Energy, Technology (I-MEET), Department of Materials Science and Engineering, Friedrich-Alexander-University, Martensstr. 7, 91058, Erlangen, Germany

Organic solar cells, polymer, fullerene, blend, photo-oxidation, FTIR

ABSTRACT: The photo-degradation of the low band-gap polymer Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) during irradiation under white light (AM 1.5 conditions) has been studied in pristine polymer films as well as in blend films with [6,6]-Phenyl C61 butyric acid methyl ester (PC[60]BM). In order to gain insight into the degradation process, FTIR spectroscopy has been used to follow the evolution of different subunits of the polymer and to probe the chemical product formation. In contrast to other polymers, not the alkyl side chains but the π-conjugated system is preferentially oxidized during the first stages of degradation. Furthermore it has been shown that the subunits of the polymer backbone are differently affected by the degradation. Blending the polymer with PC[60]BM leads to a significantly larger impact on the stability of the cyclopentadiene group compared to the benzene ring of the benzothiadiazole group.

13, 14

1. Introduction: Organic photovoltaics (OPV) offer a promising lowcost technology for the increasing energy demand of the future. While present device efficiencies of about 1 10% are sufficient for market entry of this technology, device lifetime remains an important issue for the 2 commercial success of organic photovoltaics. As module lifetimes of at least 20 years are required, which can only be realized by encapsulating them into expensive barrier materials, it is indispensable to gain a detailed insight into the polymer degradation itself to identify the weak points of the conjugated polymers to achieve intrinsically more stable polymers. The degradation of the active layer of an organic solar cell is mainly driven by the lack of the materials resistances against molecu3 lar oxygen, water, temperature and light; the detailed degradation mechanism is often discussed controversially and depends on the material under considera4-12 tion. Already the initial degradation can have dramatic consequences on the electrical transport and

charge extraction in devices. Moreover, it was shown that the electron acceptor (EA) in bulkheterojunction solar cells can affect the stability of electron donor (ED) materials in different ways signifi15 cantly. The commonly used EA [6,6]-Phenyl C61 butyric acid methyl ester (PC[60]BM) exhibits several stabilizing effects on conjugated polymers, namely screening from UV light, radical scavenging, hydroper7, 15-18 oxide cleavage and excited state quenching, in which these stabilizing effects predominate for poly15 7, 16 15 mers like P3HT, MDMO-PPV and Si-PCPDTBT, PC[60]BM has been shown to have as well a destabiliz15 ing effect on the low band-gap polymer PCPDTBT. Also, destabilizing effects of other fullerenes on conjugated polymers have been reported, mainly for those 19 showing small electron affinities. In order to gain a detailed insight into the polymer and fullerene degradation, we apply FTIR spectroscopy monitoring chemical changes of the materials during the ageing process. We are able to study different parts of the molecules

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whose properties are essential for OPV devices, as i) the alkyl side-chains of the polymer that are crucial e.g. for the polymer solubility and ii) the π-conjugated backbone of the polymer as various vibration modes of the aromatic components are IR active. In addition, the evolving degradation products, namely various carbonyl species, can be tracked. Most studies on the degradation of π-conjugated polymers and blends are focused on traditional systems like poly[2-methoxy-5-(3’,7’-dimethyloctyloxy) 1,4-phenylenevinylene] (MDMO-PPV) and poly(37, 8, 16, 20-22 hexylthiophene-2,5-diyl) (P3HT). In these polymers, the photo-oxidation process seems to follow a radical chain based mechanism, which probably 8, 21 starts on the α-carbon atoms of the alkyl chains. The addition of PCBM to films of these polymers results in decreased degradation rates, probably due to 8, 15 the radical scavenging effect of the fullerene. However, addition of a fullerene to the polymer leads to a modified morphology and therefore to altered interface energetics, being also crucial issues for the poly23, 24 This work focuses on the investimer degradation. gation of the degradation process of the low band-gap polymer PCPDTBT, both in pristine films and blended with PC[60]BM. PCPDTBT is a representative of the class of cyclopentadithiophene (CPDT) based electron 25 donor materials for OPV, like PCPDTTBTT, PCPDTQ 26 and PCPDTTPD, which are built by alternating the electron rich subunit CPDT and an electron deficient subunit like benzothiadiazol (BT). We study the degradation kinetics of the polymer with focus on the oxygen induced photo-degradation by FTIR spectroscopy in order to identify the site of the polymer chain where degradation starts in the presence of oxygen under illumination. Furthermore, we investigate the impact of blending the polymer with the acceptor PC[60]BM on its degradation behavior. Regarding the degradation process of a blend sample, one should keep in mind that also the film morphology may 27 change during illumination and exposure to heat. Recent work has demonstrated that annealing of PCPDTBT:PCBM systems up to 200 °C for 2 h had no 28 significant impact on the morphologies of the films. Therefore, heat induced demixing promoted should not produce dominant effects in this case. This is corroborated by the results of Raman and fluorescence 23 studies of PCPDTBT films under laser irradiation. 2. Experimental section 2.1. Materials:

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PCPDTBT (MW=56000 gmol , PD=2.3) was obtained from Belectric OPV, PC[60]BM was purchased from Solenne. Ortho-Dichlorobenze (o-DCB, Chromasolv purity grade) was purchased from Aldrich. As substrates, optically polished KBr substrates (Korth Kristalle) were used. The utilized oxygen gas for the degradation experiments was of the purity grade 5.0. 2.2. Sample Preparation: Thin Films of PCPDTBT (~110 nm ± 10 nm in thickness), PCPDTBT:PC[60]BM and PC[60]BM were coated onto KBr substrates at 60 °C by doctor blading, using an Erichsen COATMASTER 509 Film Applicator. The 1% (w/w) polymer solutions in o-DCB were prepared under inert atmosphere inside a glove box. The PCPDTBT- PCPDTBT:PC[60]BM- (1:1 ratio) and PC[60]BM-solutions were stirred at 80 °C over night before use. The polymer to PCBM ratio of 1:1, instead of the 1:3 ratio which is the optimized blend for photovoltaics, was chosen in order to follow in detail the formation of the carbonyl species on the polymer during degradation, which would otherwise be masked by the band of the primary carbonyl group of PC[60]BM. As shown previously, the degradation kinetic of PCPDTBT is only slightly changed by this difference in poly15 mer:PCBM ratios. The o-DCB was bubbled with dry nitrogen to reduce the concentration of dissolved oxygen before use. After film preparation, the samples were annealed inside the glove box in the dark at 120°C for 5 minutes. 2.3. Photo-degradation: Photo-degradation was carried out using a LOT -2 LS0106 solar simulator (AM 1.5; 1000 Wm ). During illumination the samples were kept under pure oxygen atmosphere at a pressure of 1 bar in a home-made deg4 radation chamber. The samples were kept at a constant temperature around 21 °C during illumination by fan cooling to avoid temperature induced degradation. In the oxygen pressure range between 0.2 and 1 bar, oxygen has been shown to have an accelerating effect on the degradation without affecting the degradation 4 mechanism, which is a useful tool to reduce the expenditure of experimental time. As a control experiment, PCPDTBT:PC[60]BM was irradiated under pure oxygen, synthetic air and pure nitrogen atmosphere, respectively, while tracking the cyclopentadiene feature of PCPDTBT, shown in the supporting information in Figure 3S. The choice of the atmosphere under which the irradiation of the samples is carried out is a crucial issue, as it determines the photo-

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transformation properties (i.e. crosslinking, oligo- and polymerization) of the fullerene, being discussed later on more in detail. The experiment shows that photooxidation is the dominating degradation pathway under the conditions chosen in this work. In order to -2 check the AM 1.5 (1000 W m ) conditions, a calibrated reference solar cell (ReRa Solutions) was used. The Photo-degradation process was monitored using FTIR spectroscopy. Figure 1. Left: π-conjugated polymer Poly[2,6-(4,4-bis-(2ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt4,7(2,1,3-benzothiadiazole)] (PCPDTBT). Right: [6,6]-Phenyl C61 butyric acid methyl ester (PC[60]BM).

2.4. Spectroscopy: In order to study the degradation process a Bruker Vertex 70v FTIR spectrometer, equipped with a MCTand a DLaTGS-detector was used in transmission mode under vacuum conditions in order to exclude CO2 and H2O vibration bands. The absolute photon flux was recorded using a MAYA 2000 pro spectrome4 ter in absolute irradiance mode as reported elsewhere. 2.5. DFT calculations: The DFT calculations were done for a CPDTBT tri29 mer, using Gaussian 09 at the B3LYP/6-31G** level of theory. The wavenumbers, obtained from these calculations were scaled with a factor of 0.95 for the high -1 energy vibrational bands (3059 – 2854 cm ) and a factor of 0.96 for the low energy vibrational bands (below -1 1600 cm ). 3. Results and Discussion 3.1. Assignment PCPDTBT:

of

vibrational

frequencies

of

The photo-degradation of PCPDTBT results in distinctive changes of the FTIR spectrum. The analysis of these specific changes requires knowledge of the assignment of the PCPDTBT IR absorption bands to particular molecular vibrations. DFT calculations were used to assign the IR features to molecular vibrations. Figure 2 shows the polymer FTIR spectrum including the frequencies of the resonances.

The highest energy spectral features of the PCPDTBT FTIR spectrum in the wavenumber region between -1 4000 and 400 cm are weak =C-H stretching modes at -1 -1 3059 cm of the thiophene and at 3027 cm of the benzene ring. A strong asymmetrical stretching mode at -1 2955 cm (νas) and a strong symmetrical stretching -1 mode at 2869 cm (νs) from the methyl groups, as well as strong stretching modes from the methylene groups -1 -1 at 2923 cm (νas) and 2854 cm (νs) appear. All reso-1 nances between 2955 and 2854 cm originate from the 2-ethylhexyl side chains of the polymer. The C=C-C=N stretching mode of the benzothiadiazole unit is located -1 at 1571 cm and the asymmetric ring stretching mode -1 of the benzene ring can be found at 1561 cm ; in addition, an asymmetric ring stretching mode of the thio-1 phene groups is located at 1504 cm . Furthermore, ring stretching modes of the cyclopentadiene group at 1461 -1 -1 cm (symmetric ring stretching) and 1398 cm (asymmetric ring stretching) are observed. The band at 1184 -1 cm is assigned to the CH2 twisting mode and the band -1 at 823 cm is assigned to a skeletal vibration mode. The absorptions originating from the π-conjugated system generally contain vibrations over the whole molecule but with the strongest contributions of the assigned vibrations. All these spectral features are summarized in Table 1 and are in good accordance with the assign23 ment of the Raman bands.

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3.2. Photo-degradation experiments 3.2.1. Reaction spectra:

Figure 2. FTIR Spectrum of the PCPDTBT thin film. The highlighted areas are of special importance and will be discussed in detail later on within the article.

Table 1. Band assignment of the spectral features of pristine PCPDTBT. Experimental [cm-1]

ν

Calculated ν [cm1]

Assignment

3096

thiophene streching

=C-H

3059

3026

benzene stretching

=C-H

3027 2955

2955

CH3 νas

2923

2923

CH2 νas

2869

2884

CH3 νs

2854

2852

CH2 νs

1571

1550

-C=C-C=N-

1561

1514

C=C stretching of the benzene ring

1477

thiophene stretching, asymmetric

1469

cyclopentadiene ring stretching, symmetric

1412

cyclopentadiene ring stretching, asymmetric

1184

1184

CH2, twisting; overlapping with thiophene =C-H

823

817

Skeletal vibration

1504

1461

1398

ring

During irradiation of the pristine polymer and the blend, the IR features representing the cyclopentadi-1 ene ring stretching (Figure 3: a, b) modes at 1398 cm immediately start to decrease in intensity which is related to the destruction of the π-conjugated system of the polymer, as a disappearance of π-bonds in the cyclopentadiene ring goes hand in hand with disruptions of the π-conjugated backbone of the polymer. Besides the apparent destruction of the polymer backbone, a new band rises in the ring-stretching region at -1 about 1530 cm . This band is also observed during the 20 photo-degradation process of pure P3HT and can therefore be assigned to the degradation of the thiophene containing part of the molecule. Simultaneously with the degradation of the π-conjugated system, new -1 features in the carbonyl region (1900-1600 cm ) of the IR spectrum rise and increase further in intensity during the degradation process. In Figure 3 (c, d) the evolution of the envelope of several carbonyl bands is shown. Seven different bands at about 1775, 1720, 1713, -1 1694, 1678, 1643 and 1633 cm can be identified for the pure polymer degradation process. During advanced stages of the degradation process, the band around -1 1720 cm becomes the most intense one and overlaps with the others. The carbonyl region of the blend IR spectra exhibits very similar bands. However, also additional features are observed at 1854, 1784 and 1736 -1 cm . They can be assigned to the degradation of the PC[60]BM, as exactly the same bands rise during the degradation process of pure PC[60]BM. The reaction spectrum of pristine PC[60]BM can be found in the supporting information. -1

The band at 1720 cm rising during the degradation of the pure polymer cannot be clearly identified in the -1 blend film, as the strong band at 1736 cm is overlapping this region. The intensity of signals related to CH2 and CH3 groups, shown in Figure 3 (e, f) representing the alkyl side chains of the polymer do not decrease in intensity in the initial degradation process. In fact, a decrease of the CH2 and CH3 signals is not detected before the cyclopentadiene ring stretching intensity has lost about 50% of its intensity in case of the pure polymer and about 60% at the blend degradation. This indi20, 21 cates that in contrast to P3HT, no detachment of the alkyl side-chains in the initial steps of the degradation process is observed.

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Figure 3. FTIR spectra of PCPDTBT and PCPDTBT:PC[60]BM recorded during the photo-degradation process under pure oxygen. -2 Left column: degradation of the pure polymer. Incident photon doses (mol m ): 0 (1), 58 (2), 333 (3), 671 (4), 1569 (5), 2681 (6), -2 3958 (7). Right column: degradation of the polymer:fullerene blend. Incident photon doses (mol m ): 0 (1), 58 (2), 216 (3), 551 (4), 1293 (5), 1629 (6), 1913 (7). (a,b) stretching region of the π-conjugated system, (c,d) carbonyl region and (e,f) methyl and methylene stretching region.

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3.2.2. PCPDTBT Photo-degradation: The evolution of the different FTIR signals is summarized in Figure 4a, in which the areal densities of the alkyl-, the carbonyl-, and the cyclopentadiene features are shown as a function of the incident photon dose. The areal densities cf(D) of the different features with respect to the respective photon dose are calculated by cf(D)=Ef(D)×εf

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dominated by the degradation of the π-conjugated system, whereas stage III is dominated by the degradation of the alkyl side-chains. In order to get a quantitative insight into the degradation process during the different stages, the absolute changes of the areal densities are calculated for the π-conjugated system (Δ Areal density π-conjugated system), the carbonyl species (Δ Areal density carbonyl species) and the methylene signals (Δ Areal density methylene). The numbers are shown in table 2 and can be used to give a quantitative interpretation by comparing the areal densities of the single created and single depleted species during the different degradation stages.

-1

With Ef(D) as the absorbance of a feature f for a certain photon dose D. The molar extinction coefficient 2 -1 εPCPDTBT(700nm)=2200 m mol gained from combined UV/vis and AFM measurements was used to calculate the molar extinction coefficients for the vibrational 2 -1 bands located at 1561 cm-1 (ε1561=13,84 m mol ) and 2 -1 1398 cm-1 (ε1398=48,70 m mol ) representing the benzene and the cyclopentadiene stretching, respectively . The areal densities of the carbonyl species were calculated by dividing the absorbance values of the different carbonyl species (Ecarbonyl) by a molar extinction coeffi2 -1 cient of (εCarbonyl=30 m mol ), which is a typical value 30, 31 for carbonyl functions. Furthermore, the C60 areal densities are determined with a molar extinction coef2 -1 32 ficient of εC60,527cm-1=23,42 m mol . The initial areal density of the methylene groups is set to the 20-fold of the cyclopentadiene value and is calculated during the degradation process by multiplying the initial areal density with its absorbance values, normalized to the initial value. In Figure 4a the kinetics of the IR features of the carbonyl species, of the methylene groups as well as of the cyclopentadiene and benzene rings, representing the π-conjugated system are shown. The degradation of the π-conjugated system of the pure polymer continues for about 4000 mol of incident photons until almost no absorbance of the features is noticeable. By treating the formation of the carbonyl species as an indicator for the ongoing degradation process and comparing their kinetics to those of the methylene groups and the π-conjugated system leads to a hint, at which sites of the polymer chain the degradation is fastest at certain stages of degradation. We note that a possible formation of volatile reaction products during 33 the photo-oxidation is not considered. We divide the degradation process into 3 stages, which are marked in Figure 4a. Stages I and II are

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Chemistry of Materials clarity reasons. c) Absorbance vs. absorbance graph of πconjugated system degradation vs. carbonyl formation.

Table 2. Changes in areal density of the functional groups during the degradation process of PCPDTBT. Stage of degradation

Δ Areal density

Photon Dose

πconjugated system

[mol pho-2 tons m ]

-4

[*10 mol m 2 ]

Figure 4. a) Degradation kinetics of pristine PCPDTBT. Areal densities of various IR features: (□) cyclopentadiene ring stretching; (◊) benzene ring stretching; (■) methylene features from the 2-ethylhexyl side-chain; (▲) carbonyl species -1 in the region of 1900-1600 cm . The areal density values of the methylene and the carbonyl features were divided by 20 and 3, respectively to match the scale. b) Oxygen and light induced changes of the conformation of the CH2 Bands. Inset: Wavenumber of the CH2 Band vs. Photon Dose. The colors of the squares are matching to the respective spectra. Empty symbols correspond to spectra, not shown here for

Δ Areal density Carbonyl species -4

-

[*10 mol m ]

2

0-60

-0.6 ± 0.1

0.6 ± 0.1

60-1600

-2.1 ± 0.1

2 ± 0.1

1600-4000

-2 ± 0.1

6.3± 0.1

-

Δ Areal density methylene -4

[*10 mol m ]

-

2

-21.4 ± 0.1

The very initial step of degradation is dominated by an increase of carbonyl areal density, a decrease of areal density of the π-conjugated system and a significant increase of the methylene feature. In this early -4 stage of PCPDTBT degradation, around 0.6 ± 0.1 x 10 -2 [mol m ] of the polymer backbone have vanished, and -4 -2 about 0.6 ± 0.1 x 10 [mol m ] of carbonyl species have been formed. The comparison of these numbers points to a process of carbonyl formation taking place almost completely on the backbone of the polymer. Further hints to this degradation behavior are given by the πconjugated system absorbance vs. carbonyl absorbance graph (Figure 4c) showing for the π-conjugated system- compared to carbonyl trends similar kinetics of the evolving species. In addition, Figure 3c reveals during the initial degradation process a rising carbonyl -1 band at around 1694 cm , which is a typical value for 31 αβ-unsaturated ketones and ensuing a rising band at -1 1720 cm which can be assigned to the formation of 31 saturated ketones in polymer degradation. Figure 4S in the supporting information shows the correlation between the absorbance of the rising carbonyl band at -1 1694 cm and the depleting benzene band, as well as -1 between the rising band at 1713 cm and the depleting methylene band. These correlations permit to assign the formation of the αβ-unsaturated ketone to the oxidation of the benzene ring, which is in accordance with the oxidation behavior of 2,1,3-benzothiadiazole showing oxidation of the benzene ring during the reac34 tion with oxidizing reagents, as well as the formation of saturated ketones to the oxidation of the sidechains. Concomitantly to the oxidation of the benzene

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ring in the initial stage of degradation, the methylene bands increase in intensity and shift towards lower -1 frequencies by about 4 cm (Figure 4b) within the first approximately 60 mol of incident photons, whereas no changes in the methyl bands occur. This effect is observable under both, oxygen and nitrogen atmosphere, however to different extents. Furthermore, it could be shown to be almost completely reversible when heating the sample under vacuum (50 mbar) at 120 °C for 35, 36 60 minutes. A similar effect was observed before and was addressed to changes in the conformation of the CH2 containing parts of the molecules. The low -1 frequency values (here, about 2919 cm ), which are reached after a photon dose of 60 mol are referred to be characteristic for highly ordered side chains, whereas high frequency values are reported to be characteristic for conformationally disordered side chains includ37, 38 ing a higher content of gauche conformers. These results indicate that the packing of the polymer side chains is affected during the initial irradiation process, what could be of special importance as in donor materials for OPV blends alkyl side-chains have been shown 39, 40 to be crucial for the blend morphology. In stage II of degradation (cf. Figure 4a), the formation of the carbonyl species still follows the decrease of the π-conjugated system, whereas the areal densities of the methylene groups stay constant. Ex-4 -2 pressed in numbers, 2.1 ± 0.1 x 10 [mol m ] of the polymer backbone are degraded and simultaneously, -4 -2 2.0 ± 0.1 x 10 [mol m ] of carbonyl species rise. This 20, 21 indicates that in contrast to with the case of P3HT, PCPDTBT is almost exclusively oxidized on the πconjugated polymer backbone during the initial sequence of the degradation process. A preferred oxidation of the π-conjugated system was also observed by 33 x-ray photoemission. To describe this behavior more clearly, Figure 4c shows the correlation between the degradation of the π-conjugated system, expressed by the loss of absorbance and the rise of carbonyl absorbance. This correlation is kept until the end of the degradation stage II. In the last stage of the degradation process (III), the formation of carbonyl species does not correlate any more with decreasing features of the π-conjugated system, as observed for stages I and II. Figure 4c shows significantly more formation of carbonyl species than depletion of the π-conjugated system in this stage of degradation. More precisely, the kinetics of the carbonyl species appears to be closely related to the methylene kinetics. The formation of carbonyl species seems to become enhanced at the stage of degradation, when the signals of the alkyl sidechains start to decrease in intensity, which could hint to new degradation sites accessible on the polymer molecule at this stage of degradation. While during the

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final stage of the degradation process 6.3 ± 0.1 x 10 -2 -4 [mol m ] of carbonyl species form and 2.0 ± 0.1 x 10 -2 [mol m ] of the π-system disappear, the difference between these numbers indicates an oxidation process leading to a substantial loss of the methylene groups. However, the remaining methylene signals are still -1 located at 2919 cm , indicating that the high ordering of the side-chains is still intact and the remaining sidechains stay in all-trans conformation until the end of the degradation process. 3.2.3. PC[60]BM Photo-degradation: The degradation behavior of PC[60]BM as pristine material or in a blend with conjugated polymers was 16-18, 41, 42 recently studied by various techniques. Generally, it was shown that PC[60]BM forms photooxidation products during exposure to light and oxy16, 17 gen whereas different mechanism were suggested. It was found that PC[60]BM can be increasingly oxidized by subsequently adding oxygen atoms to the 17 molecule. It was concluded that a polymerization of 43-45 the PC[60]BM is unlikely, e.g. from oxygen quench43 ing experiments of the PC[60]BM triplet state. On the other hand, recent studies show that under inert at41, mosphere a light induced dimerization of PC[60]BM 27, 48 42, 46, 47 and oligomerisation has to be considered during the photo-degradation process. In contrast to PC[60]BM, C60 forms photo-polymerized products 45 when irradiated under oxygen-free atmosphere. In the presence of molecular oxygen, the photo-oxidation of neat C60 films can result in the formation of 49 C60(O2)x. In combination with polymers on the other hand, under illumination and oxygen atmosphere C60 can form “networks” resulting in film stabi49 lization. In order to study the degradation of pristine PC[60]BM more in detail, FTIR measurements were performed; the corresponding reaction spectra are shown in Figure 1Sb of the supporting information. Similar to the polymer, the evolution of methyl and methylene groups of the PC[60]BM and carbonyl moieties are monitored. The photo-degradation of PC[60]BM leads to carbonyl species with absorbance -1 maxima located at 1853 and 1783 cm as well as to at least one more species, overlapping with the methyl ester band of PC[60]BM. According to the literature, -1 the band at 1783 cm can be assigned to the degrada16 tion of the C60 moieties. Similar to the pristine polymer (see above) we classify the degradation process of the pure PC[60]BM into 3 main stages of degradation, indicated by dashed lines in Figure 5a.

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Chemistry of Materials Starting from about 1000 until 3000 mol of incident photons (stage II), a strong enhancement of the carbonyl formation is observed. During the same period a significant decrease of intensity of the methylene features occurs, which points to favored oxidation sites on or close to the side-chain of the PC[60]BM molecule. This decrease of methylene areal density can either be due to a chemical reaction of the specific groups during the irradiation process, or due to a detachment of parts of the molecule, including the methyl and methylene groups of the PC[60]BM molecule. Simultaneously with the degradation of methylene groups and the strong increase of the carbonyl areal densities, degradation of the C60 moieties still goes on with the same degradation rate as in the initial stages of the degradation process. From these results it looks implausible that this stage of PC[60]BM degradation is dominated by the decrease of the C60 moieties.

Figure 5. a) Degradation kinetics of pristine PC[60]BM. Areal densities of various IR features: (□) C60 fullerene; (■) methylene feature; (▲) carbonyl species at 1853 and 1736 cm-1; (♦) carbonyl band at 1783 cm-1. b) Oxygen and light induced changes of the conformation of the CH2 Bands during stage I. Inset: Wavenumber of the CH2 Band maximum vs. Photon Dose. The colors of the squares are matching to the respective spectra. Empty symbols correspond to spectra, not shown here for clarity reasons.

Up to about 1000 mol of incident photons (stage I), a slight increase of the carbonyl areal density can be observed, which is accompanied by a decrease of similar magnitude of the fullerene C60 areal density signal, which is due to a partial loss of conjugation in the buckyball. On the other hand, the methyl and methylene features of PC[60]BM do not decrease in intensity during the initial stage of the degradation process. Similar to the behavior of the CH2 bands of the polymer, the maximum of the asymmetric CH2 vibration -1 band shifts to about 2918 cm (Figure 5b). We again assign this behavior to a conformational change of the CH2 containing part of the molecule towards higher 35, 36 ordering. The kinetics observed in stage I indicate that the photo-oxidation process starts in the C60 part of PCBM while the substituent remains intact during this initial phase of oxidation.

In the final stage III of the PC[60]BM degradation process, after about 3000 mol of incident photons, all trends seem to reach saturation values even if small changes are visible. Judging from initial carbonyl areal -4 -2 densities of about 6.6 x10 mol m which corresponds to one carbonyl function per PC[60]BM molecule, -4 overall carbonyl areal densities of around 21.25 x10 -2 mol m in the end of the degradation process and around 15 percent of intact PC[60]BM molecules leads to an average number of around 3 carbonyl functions per degraded PC[60]BM molecule, with around one carbonyl function being located on the buckyball and two on the side-chain. It was shown that light induced oxidation of PC[60]BM leads to a series of degradation 17 products of differing amount of oxidation, where the abundance of highly oxidized PC[60]BM decreases with increasing oxidation. This brings us to the conclusion that the oxidation behavior of PC[60]BM reaches a boundary value for the distribution of C60 molecules with different oxidation levels and does not go on until each C60 molecule exhibits 8 oxygen atoms, which is in good agreement to the reported maximum level of C60 17 oxidation. 3.2.4. PCPDTBT:PC[60]BM Photo-degradation: Generally, the degradation behavior of the blend is very similar to that of the pristine polymer and can again be classified into the 3 main stages of degradation, as discussed before. However, the time scales are different: A complete destruction of the π-conjugated system of the polymer:fullerene blend is achieved after irradiation of about 2000 mol of photons (Figure 6a) which is a factor of 2 less than in the case of the pure polymer.

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clarity reasons. c) Absorbance vs. absorbance graph of πconjugated system degradation vs. carbonyl formation.

Similar to the pristine materials, the very initial step of degradation (stage I) is dominated by an increase of carbonyl areal density, a decrease of areal density of the π-conjugated system and a significant increase of the methylene feature. For the blend film we observe a -1 shift of the methylene band of about 3 cm towards lower frequencies (Figure 6b) during an incident Pho-2 ton Dose of about 10 mol m . The evolving band has its -1 maximum at around 2919 cm and hints to conformational changes of the methylene groups towards highly orders side-chains, like in the pristine polymer- and the pristine PC[60]BM-degradation. Stage II of the blend degradation process is dominated by the strong decrease of signals related to the πconjugated system whereas the alkyl side-chains are hardly affected. This degradation behavior is also illustrated by Figure 6c, showing the π-conjugated system vs. the carbonyl absorbance. A clear correlation between these signals is observed until about 670 mol of incident photons, pointing to a reaction of the polymer backbone producing carbonyl species; i.e. the degradation takes place predominantly on the π-conjugated backbone of PCPDTBT. In stage III of the degradation process (incident photons > 670 mol), the carbonyl formation mirrors the methylene kinetics rather than that of the πconjugated system. Comparing the losses of areal density and therefore the degradation trends of the π-conjugated system of the polymer and the C60 part of the fullerene in the blend provides information about the more stable part of the blend. Figure 7 shows the respective losses of areal density with respect to the incident photon dose.

Figure 6. a) Degradation kinetics of PCPDTBT:PC[60]BM. Areal densities of various IR features: (□) cyclopentadiene ring stretching; (◊) benzene ring stretching; (■) methylene features from the 2-ethylhexyl side-chain; (▲) carbonyl spe-1 cies in the region of 1900-1600 cm . The areal density values of the methylene and the carbonyl features were divided by 40 and 20 respectively to match the scale. b) Oxygen and light induced changes of the conformation of the CH2 Bands. Inset: Wavenumber of the CH2 Band vs. Photon Dose. The colors of the squares are matching to the respective spectra. Empty symbols correspond to spectra, not shown here due to

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Figure 7. Degradation trends of the π-conjugated system of PCPDTBT (●) and the PC[60]BM buckyball (■).

It is obvious that the degradation process of the πconjugated system proceeds faster than that of the buckyball, which means that within the polymer:fullerene blend, PC[60]BM is the more stable part. In both cases, the pristine polymer and the polymer:fullerene blend, the evolution of carbonyl species seems to follow the degradation of the π-conjugated system during early stages of the photo-degradation process until a noticeable decrease of alkyl CH2 intensity can be observed, accompanied by further enhanced carbonyl formation. This similarity in the behavior of the polymer and the polymer:fullerene sample may indicate that the underlying mechanisms of the degradation processes are similar, although the timescale for the polymer:fullerene sample is substantially shorter. To address this behavior, we compare in Figure 8 the ratios of the degradation trends of the -1 -1 rising carbonyl species at 1694 cm and 1713 cm , -1 whereat the carbonyl species at 1694 cm are assigned to the degradation of the benzene ring and therefore of the π-conjugated system, whereas the carbonyl species -1 at 1713 cm are assigned to the degradation of the sidechain of the polymer, as mentioned earlier.

cess are greater than unity (photon-dose ≥ 58 mol of incident photons), indicating a more pronounced formation of carbonyl species associated to the degradation of the π-conjugated system, whereas after this initial period the formation of carbonyl species on the side-chains predominates. The observation that the blend shows the same behavior as the neat polymer, just on a different time scale, indicates that the underlying degradation-mechanism is the same in both cases. The degradation trends of all rising carbonyl species of the polymer as well as of the polymer:fullerene blend are shown in Figure 5S in the supporting information, exhibiting similar trends, albeit on different time scales and absolute amounts. However, within the first two stages of the degradation process, the rise of all carbonyl signals related to the blend are more pronounced than those of the neat polymer.

Impact of blending on the π-conjugated polymer backbone: Since we have shown the π-conjugated system being the bottleneck of PCPDTBT stability in the early stages of the degradation process, it is essential to pay more attention to its degradation process. In the previous section we discussed the complete degradation process being classified into 3 main stages and observed the formation of carbonyl species following the vanishing of the π-conjugated system during the first two stages. In order to understand the degradation mechanism more in detail, we will consider now whether the πconjugated system has favored sites of attack, and the influence of the blending on reactive sites.

Figure 8. Ratio of the evolving Carbonyl species at 1694 cm-1 (product of the benzene degradation) and 1713 cm-1 (product of the side-chain degradation). Trend of the ratio for the pristine polymer (●) and polymer:fullerene blend (■) degradation.

These ratios mirror the preferred sites of oxidation during the degradation process. By comparing the trend of the ratios of the pristine polymer to that of the blend, the impact of adding PC[60]BM to the polymer can be evaluated. In both cases, the pristine polymer as well as the polymer:fullerene blend, the first ratios observed after initiation of the photo-degradation pro-

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PCPDTBT as a low-band-gap polymer is built from

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decreases during the degradation process until a value

Figure 9. Degradation trends of the cyclopentadiene group (CPDT, ●) and the benzene ring (BT, ■). Degradation trends of a) pristine PCPDTBT and b) PCPDTBT/PC[60]BM blend. Insets: initial stages of the photo-degradation process.

alternating electron-rich cyclopentadithiophene (CPDT) and electron-deficient benzothiadiazole (BT) subunits, to gain a more detailed insight into the degradation behavior of PCPDTBT, the respective molecule parts are observed separately. For the pure polymer, a loss of areal density of the π-conjugated system -4 -2 by 0.6 ± 0.1 x 10 [mol m ] within 60 mol of incident photons is observed. Judging from the areal density losses of the single subunits, this overall loss can be attributed by about 75 % to the degradation of the BT subunit (Figure 9a). In case of the PCPDTBT:PC[60]BM blend, the loss of areal density of the π-conjugated system during an incident photon dose of 30 mol of photons is caused to around 60 % by the degradation of the benzene ring (Figure 9b).

In order to track quantitatively the relative degradation behavior of the subunits during the complete process, we introduce a factor DBT/DCPDT. While DBT gives the relative loss of absorbance of the benzene ring stretching of the BT subunit, DCPDT gives the relative loss of absorbance of the cyclopentadiene ring stretching of the CPDT subunit. A factor of DBT/DCPDT 1 indicates a more pronounced degradation of the BT subunit. The development of DBT/DCPDT as a function of the incident photon dose is shown in Figure 9. For the very initial stage of degradation of the pristine polymer, a DBT/DCPDT factor of about 2.8 is calculated, while it

of about 1 is reached, when both units are destroyed completely. The observed trend of DBT/DCPDT confirms that the benzene ring of the BT subunit is attacked significantly faster than the CPDT subunit. DBT/DCPDT indicates that the BT subunit is the weak point in the stability of the pristine polymer during irradiation with white light.

Figure 10. Trends of the DBT/DCPDT ratios of the polymer and the blend films during the degradation process.

After blending the polymer with PC[60]BM, the degradation behavior of both subunits appears to be enhanced compared to the pristine polymer, as it was shown to occur for the degradation process of all functional groups. Again, the BT subunit seems to be more sensitive to light than the CPDT subunit, as shown in Figure 10. The DBT/DCPDT factor is about 2.5 within the first 2 mol of incident photons, but drops rapidly to about 1.8 after 30 mol of incident photons. The large

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initial value points to a high degree of selectivity in the early phases of degradation, as it was observed for the pure polymer. Impact of blending on the fullerene: It was shown that blending the low band-gap polymer PCPDTBT with the acceptor molecule PC[60]BM has a big impact on the degradation behavior of the polymer. In order to check whether the fullerene is affected in a similar way as part of the polymer:fullerene blend, the degradation kinetics of the C60-vibration was monitored for a film of pristine PCBM and as part in the blend, shown in Figure 11. -1 Here, the C60 vibration at 527 cm was used as a probe for the intactness of the C60 fullerene. The kinetic plot of the fullerene absorbance shows an almost linear degradation behavior until the slope of the graph gets smaller after an absorption loss of about 80%.

Figure 11. Degradation kinetics of the fullerene vibration at -1 527 cm as a function of incident photon dose for pristine PC[60]BM (□) and PC[60]BM in the blend (■).

The slope of the degradation kinetics plot, shown in Figure 11 can be expressed as a reaction rate of the fullerene degradation. The degradation rate of pristine -3 -4 PC[60]BM was determined to be (3.0 ± 0.2) x 10 [*10 -2 -2 mol m /mol photons m ]. In the blend, the degrada-3 tion rate of PCBM was observed to be (3.1 ± 0.1) x 10 -4 -2 -2 [*10 mol m /mol photons m ]. Thus, these rates -for the pristine and the blended PCBM- are almost equal. However, Tromholt et al. presented degradation kinetics of different electron acceptors, including 18 PC[60]BM and showed that the degradation rate of the fullerene is thickness dependent. PC[60]BM exhibits lower degradation rates, the thicker the films are. In our studies the amount of pristine PC[60]BM was significantly higher than in the blend film, which proba-

bly leads to an underestimation of the degradation rate of the pristine PC[60]BM, compared to the blend sample. On the other hand one has to take into account a filter effect caused by the presence of the polymer for the blend sample. Therefore a more detailed analysis of this effect is necessary. 4. Summary: The data obtained from FTIR measurements indicate that PCPDTBT and PC[60]BM as pure materials, as well as in their polymer:fullerene blend films undergo conformational changes in the alkyl parts of the molecules during very initial stages of the degradation and keep this conformation up during the complete process. The degradation kinetics of the essential functional groups were studied for the pristine polymer, PC[60]BM and the polymer - PC[60]BM blend. In the initial stage of degradation of the polymer PCPDTBT the areal densities of the units of the π-conjugated system decrease fast while the areal density of the sidechain signals remain almost constant. The side-chain signals start to decrease only after the polymer backbone has been strongly degraded. At this stage an increase of the rate of carbonyl product formation was observed, too. Within the π-conjugated system the BT units degrade significantly faster than the CPDT units during the initial stages of photo-oxidation. These results indicate that the weakest point regarding the photo-stability of the π-conjugated system of PCPDTBT is the electron deficient benzothiadiazole (BT) subunit. The degradation process of pure PC[60]BM can be classified into 3 main stages of degradation where the first stage is dominated by the loss of C60 areal density, e.g. due to oxidation . The intermediate stage is mainly affected by oxidation of the side-chains. In the third stage carbonyl as well as of the side-chain signals seem to reach saturation values, whereas the C60 degradation is still going on. The degradation in the polymer - PC[60]BM blend proceeds in similar stages as for the pristine polymer, including enhanced initial photo-degradation of the πconjugated system, a correlation of enhanced carbonyl production with alkyl chain degradation at later stages, and also a relatively higher stability of the CPDT unit compared to the BT unit. However, the degradation in the blend proceeds significantly faster compared to the pristine polymer during the different stages. These observations indicate that similar reactions and reaction products occur in the degradation processes, but the availability of additional electronic levels of the

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fullerene in the blend seems to enhance the degradation rates of the π-conjugated system of PCPDTBT significantly. Acknowledgement This work was financially supported by the University of Tuebingen [twinning project, within the frame of “Organische Photovoltaik & Farbstoffsolarzellen” of the BadenWuerttemberg Stiftung]. We gratefully thank the bwGRiD project for the computational resources. The „Solar Factory of the Future“ on the Energy Campus Nuremberg is acknowledged for financial support (Bavarian State Government Grant No. 20-3043.5). We thank Belectric OPV GmbH for a gift of PCPDTBT. Supporting Information Available: FTIR reaction spectra of PC[60]BM as pristine film as well as part of the blend during degradation with white light. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Heiko Peisert Email: [email protected] Fax: 0049 (0)7071 29 5490

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FTIR Study of the Impact of PC[60]BM on the Photodegradation of

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