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New Insights into the Mechanisms of Photodegradation/Stabilization of P3HT:PCBM Active Layers Using Poly(3-hexyl‑d13-Thiophene) Aurélien Tournebize,†,‡ Pierre-Olivier Bussière,†,‡,§ Agnès Rivaton,*,†,‡ Jean-Luc Gardette,†,‡ Hussein Medlej,# Roger C Hiorns,∇ Christine Dagron-Lartigau,*,# Frederik C. Krebs,○ and Kion Norrman*,○ †

Université Blaise Pascal, Institut de Chimie de Clermont-Ferrand, Clermont Université, , Equipe Photochimie, BP 10448, F-63000 Clermont-Ferrand, France ‡ ICCF, Equipe Photochimie, CNRS, UMR 6296, BP 80026, F-63171 Aubière, France § ENSCCF, LPMM, Clermont Université, , BP 10448, F-63000 Clermont-Ferrand, France # Université de Pau et des Pays de l’Adour, IPREM CNRS-UMR 5254, Hélioparc, 2 Avenue Président Angot, 64053 Pau Cedex 9, France ∇ IPREM, CNRS-UMR 5254, Hélioparc, 2 Avenue Président Angot, 64053 Pau Cedex 9, France ○ Department of Energy Conversion and Storage, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde, Denmark S Supporting Information *

ABSTRACT: The photo-oxidation mechanism of thin-film blends based on poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) upon irradiation with ultraviolet−visible light (UV-Vis) was studied. The use of deuterated P3HT, i.e., poly(3-hexyl-d13-thiophene) (P3HdT), permitted discrimination of carbon originating from the hexyld13 chain and carbon originating from PCBM and the nondeuterated thiophene unit. The photo-oxidation of both components of the blend was monitored using the combination of various analytical techniques to probe the bulk and the surface of the deposits. The results show that the stabilization of P3HT by PCBM is due to a morphological reorganization between P3HT and PCBM. This change occurs at a low temperature (ca. 42 °C) and increases the lifetime of the primary property, i.e., the ability of the active layer to absorb light. However, this is counterbalanced by the enhanced formation of oxidized PCBM molecules, which may act as electrons traps. It is shown that UV light is harmful for P3HT, PCBM, and P3HT:PCBM blend stabilities, even if PCBM provides a filter effect that is strongest at short wavelengths. It is proposed that the photochemical behavior of the chromophoric species involved in the chain radical oxidation of P3HT is a key characteristic in the underlying mechanism. The results obtained in this work advance the understanding of active layer stability and will help improve the design of long lifetime organic solar cells thanks to the use of cutoff filter in the substrate or encapsulation of the devices. KEYWORDS: poly(3-hexythiophene) (P3HT), phenyl-C61-butyric acid methyl ester (PCBM), P3HT:PCBM, isotopic labeling, photo-oxidation, photochemical behavior, morphology, organic photovoltaics

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INTRODUCTION Harvesting energy directly from sunlight using photovoltaic cells is an important route to addressing the growing global energy needs with a renewable resource while minimizing detrimental environment effects. The utilization of semiconducting conjugated polymers as active components in bulk heterojunction organic solar cells (OSCs) has advantages, such as ease of processing, formation of large surface areas, and low-cost production. Poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM)-based bulk heterojunctions, which are common active layers with a high commercial relevance, are currently capable of achieving 5% conversion efficiencies.1 There are still some improvements in lifetimes to be achieved before large-scale industrial production can become © 2013 American Chemical Society

significant, and a prerequisite to this is the knowledge of degradation mechanisms responsible for OSC performance deterioration. It has been established that performance decay is due to the combined effects of sequential and interrelated degradation mechanisms occurring at various rates and parts of the OSC.2 The main triggers of degradation have been clearly identified: oxygen, moisture, elevated temperatures, and light.3 The chemical and physical stability of the active layer is critical as one major problem associated with the application of polymers is their instability to weathering. The formation of Received: July 4, 2013 Revised: September 18, 2013 Published: September 18, 2013 4522

dx.doi.org/10.1021/cm402193y | Chem. Mater. 2013, 25, 4522−4528

Chemistry of Materials

Article

but also at the surface. The bulk analyses were performed by UV-Vis spectroscopy and infrared (IR) spectroscopy, and the surface analysis by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOFSIMS). Finally, the impact of the wavelength of the light on the photostability of P3HT and of the blend was considered.

oxidation products appears as the major cause of this instability in outdoor applications.4 The photodegradation of P3HT has been elucidated by Manceau et al.5,6 The degradation mechanism involves a chain radical oxidation process starting with the abstraction of an allylic hydrogen, leading first to side-chain oxidation and then to sulfur oxidation. This process is responsible for the breaking of the macromolecular backbone, and results in the loss of conjugation and a consequent bleaching of the material. Further studies have shown that intrinsic P3HT parameters, namely, regioregularity, molar mass, crystallinity, and purity, play important roles in photostability.7−10 P3HT photostability has also been shown to be more affected by radiation at lower wavelengths.7−10 Besides the photostability of P3HT, it is also important to study the P3HT:PCBM blend. The routes by which the active layer can evolve are diverse, and any decrease in the ability of the active layer to absorb light, generate and transport free charge carriers, will irremediably impair device operation. Concerning polymer:PCBM blends, the presence of the PCBM has been shown to slow degradation of the polymer, be it P3HT or poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene-vinylene] (MDMO-PPV).11−14 This behavior has been attributed both to the radical-scavenging properties of fullerene (C60) and to its ability to quench the polymer singlet state. Even though the polymer is more stable in the blend, it has been shown that both the substituent and C60 moiety degrade in MDMO-PPV:PCBM blends, the C60 moiety degrading faster. It has also been reported that oxidation of the PCBM C60 cage impacts its electronic properties.15 It was suggested that oxidized molecules act as deep traps, thus decreasing the electron mobility. It is clearly crucial to understand the complex photooxidation mechanisms of P3HT:PCBM as many routes can be involved: the impact of P3HT upon PCBM, and vice versa; how the photo-oxidation of P3HT and PCBM might concurrently evolve, and finally, fully apprehending the stabilizing role of PCBM. The wavelength of the irradiating light is most likely an essential parameter as P3HT and PCBM do not absorb in the same spectral region. In this report, we describe the photodegradation mechanism of the P3HT:PCBM blend using a deuterated P3HT, i.e., poly(3-hexyl-d13-thiophene) (P3HdT), in which the substituent of the thiophene group is totally deuterated (Figure 1). Since the alkyl side chain is the first site to be involved in the photooxidation processes, this deuteration made it possible to discriminate between photodegradation mechanisms caused by polythiophenyl- and fullerenyl-anchored alkanes. A set of experiments was developed to monitor the modifications during aging that occur not only in the bulk

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EXPERIMENTAL SECTION

Sample Preparation and Aging. Phenyl-C61-butyric acid methyl ester (PCBM) was obtained from Nano C. P3HdT, with a mass average molar mass (Mm) of 23200 g mol−1 and a dispersity (Đ = Mm/ Mn) of 1.45, was prepared as indicated in the Supporting Information. Thin films were prepared by spin coating (G3P-8 Spincoat, Cookson Electronics Equipment) from anhydrous chlorobenzene (HPLC grade) solutions on KBr or glass substrates. The thickness of the P3HdT and P3HdT:PCBM (1:1 in weight) films were adjusted in order to have the same quantity of P3HdT in the polymer and in the blend. Therefore, the thicknesses were ∼150 nm for the polymer and ∼350 nm for the blend. The amount of polymer was controlled by measuring the C−D absorption in both the CD polymer and the CD blend. Irradiations were carried out in ambient air in a SUNTEST (from Atlas) equipped with a xenon lamp. The aging intensity was 750 W m−2 (in the 300−800 nm domain) and the temperature of the samples was ca. 42 °C. Analytical Techniques. Bulk Analysis. Infrared spectra were recorded in transmission with a Nicolet 760-FTIR spectrophotometer working with OMNIC software. Spectra were obtained using 32 scan summations and a 4 cm−1 resolution. Changes in UV-Vis spectra were followed with a Shimadzu UV-2101PC spectrophotometer equipped with an integrating sphere. Surface Analysis. Prior to TOF-SIMS and XPS analyses, samples were stored in the vacuum chamber of the instruments in question for ∼24 h, to ensure the removal of molecular oxygen and P3HT (and thus P3HdT) charge transfer complexes. TOF-SIMS characterization was performed using a TOF-SIMS IV (ION-TOF GmbH, Münster, Germany), wherein 25 ns pulses of 25keV Bi+ (primary ions) were bunched to form ion packets with a nominal temporal extent of 20 h), an increase of the degradation rate of fullerene moieties is observed in Figure 3 and the rate of disappearance of the thiophene band is reduced by almost a factor of 5. These results unambiguously demonstrate that the reorganization of the blend (increase in the P3HdT/PCBM interface area) results in a stabilization of P3HdT by PCBM, but with an enhancement of the oxidation rate of the fullerene cage. However, oxidized PCBM molecules are known to act as deep traps, thus decreasing the overall device electron mobility.15 The decrease of the IR bands is accompanied by the appearance of new absorption bands in the carbonyl region, as previously reported in the case of P3HT and PCBM subjected to photo-oxidation.11,17 In order to compare the oxidation of P3HdT and P3HdT:PCBM, the increase in intensity of the IR band at 1772 cm−1 (oxidation of both PCBM and P3HdT) was measured, and the results are reported in Figure 4. No film thickness correction was made, and therefore oxidation measurement is related to the same quantity of P3HdT in

reorganization of the blend morphology via the diffusion of PCBM out of the polymeric matrix.16 P3HdT crystallinity is thus enhanced in PCBM poor regions, leading to an increase of the optical absorption of the film.13 Once the reorganization is complete, irradiation of the P3HdT:PCBM blend film results in a steady decrease of the absorbance (Figure 2b), as previously observed for P3HT films. Such an evolution was attributed to the oxidation of the thiophene rings.5,6 Focusing in Figure 2b, one can observe that P3HdT is five times more stable when mixed with PCBM, compared to pure P3HdT. In order to elucidate the mechanisms accounting for the decrease of the UV−Vis absorbance of the blend, IR spectroscopy, XPS and TOF-SIMS were employed to monitor the modification in the bulk and at the surface of solid films, respectively. The hexyl-d13 side chain of P3HdT is helpful in elucidating the photodegradation of P3HT:PCBM blends. This is because the aliphatic C−H stretching modes from the hexyl side chain in P3HT (3000−2850 cm−1) are shifted to the 2100−2050 cm−1 region in the IR spectra in the transmission mode of P3HdT. This permits a discrimination of the evolution of the P3HdT hexyl-d13 moiety from that of the alkane chain on PCBM when the blend is submitted to irradiation (see also Figure S1 in the Supporting Information (SI)). Upon irradiation, a progressive decrease of the various functional groups of the polymer was observed except for the thiophene ring band; the intensity of this last band increased during the first hours of exposure before decreasing. The modifications in absorbances from thiophene rings (1510 cm−1), P3HdT alkyld13 (2100−2050 cm−1), and the fullerene moieties of PCBM (526 cm−1) are shown in Figure 3. Three phases can be distinguished in Figure 3: (1) During the first phase (10 h), the intensity of the signals characteristic of the thiophene moieties increases, reflecting the increase in P3HdT crystallinity in the blend observed in the UV-Vis spectra (Figure 2).13 Within this period, the intensity of the signals characteristic of the alkyl moieties of P3HdT (C−D) and the PCBM fullerene moieties starts to decrease. (2) After this reorganization of P3HdT π-stacking, the intensity of the signal characteristic of the thiophene

Figure 4. Evolution of the IR intensity of the band at 1772 cm−1 in P3HdT (open square, □) and in the P3HdT:PCBM blend (open circle, ○), as a function of the incident light dose and irradiation duration. 4524



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other words, hexyl-d13 moieties from P3HdT are progressively replaced by PCBM molecules, which will consequently increase the C4H7 intensity and decrease the C4D7 intensity. The fact that the C4H7/C4D7 intensity ratio increases drastically at the same time the proposed stabilizing effect is observed, confirming that P3HdT becomes protected by “sacrificial” PCBM. Thus, the results presented in Figure 5 are in excellent agreement with the IR results, where a stabilization of the polymer is observed after 30 h of aging (Figure 3). Figure 6 displays the changes in oxygen concentration in the P3HdT and P3HdT:PCBM films as a function of aging time in

both samples. Indeed, as reported in the Experimental Section, the P3HdT:PCBM sample is three times thicker than the P3HdT sample, but the same quantity of P3HdT is present in both deposits (same intensity of the C−D bonds). As previously observed for the decrease of the IR bands of the materials, curves reported in Figure 4 show that there is no significant difference between the rate of formation of the product absorbing at 1772 cm −1 in P3HdT and in P3HdT:PCBM upon short irradiation durations. For longer exposures (tirr > 50 h), the band at 1772 cm−1 reaches a plateau in P3HdT while it keeps growing in P3HdT:PCBM to a plateau with a value three times higher than that of P3HdT. The higher quantity of oxidized species formed in the P3HdT:PCBM blend unambiguously confirms that photo-oxidation of PCBM also occurs in the blend. Surface Analysis (TOF-SIMS and XPS). Although IR spectroscopy is a powerful tool to monitor the degradation of the bulk of polymers,4 XPS (5−10 nm probe depth) and TOFSIMS (1−2 nm probe depth) are commonly accepted as the most powerful tools to monitor chemical changes at the surface of polymers.6,18 In addition, the use of mass spectral TOFSIMS data in this study of P3HdT distinguishes carbon from the hexyl-d13 chain and carbon originating from other sources such as PCBM and the thiophene unit (and possible contamination). Figure 5 displays the TOF-SIMS acquired C4H7/C4D7 intensity ratio as a function of aging time. The fragment ions

Figure 6. Change in oxygen content/intensity relative to no aging as a function of aging time measured by (a) the quantitative XPS technique and (b) the nonquantitative TOF-SIMS technique. Each point is an average of three measurements on different surface locations. The TOF-SIMS oxygen intensities were normalized against the sum of carbon signals (Cn−, n = 2−7) in order remove possible instrument effects. Figure 5. Change in C4H7/C4D7 intensity ratio (positive-ion mode) as a function of aging time measured by TOF-SIMS. Each point corresponds to a single measurement.

ambient air. The oxygen uptake, which is a result of oxidation, is displayed relative to the nonilluminated samples. Figure 6a shows the increase in oxygen measured by the quantitative XPS, and Figure 6b shows the increase in oxygen measured by the nonquantitative TOF-SIMS. The results shown in Figure 6 originate from two techniques that are based on completely different physical principles but produce equivalent qualitative results. P3HdT exhibits an increase in oxygen content/intensity with increasing aging time (i.e., degree of photo-oxidation). The increase is observed to level off at higher aging times, which is consistent with a decreasing availability of the virgin blend. The P3HdT:PCBM material follows the same trend up until ∼30 h of aging, and then the oxygen content appears to level out. These results suggest that the surface of the bulk heterojunction P3HdT:PCBM is more stable, in terms of photo-oxidation, after morphological reorganization/optimization has occurred, which is consistent with the results obtained by the IR and UVVis measurements. Furthermore, since the surface is initially P3HdT-rich, the photo-oxidation behavior for P3HdT and P3HdT:PCBM is expected to be similar up to 30 h of aging.

(C4H7 and C4D7) are representative of all the CxHy and CxDy fragment ions (i.e., the same trend is observed for close to all analogue fragment ions). For P3HdT, a subtle increase is observed with aging time, which is consistent with a mechanism whereby hexyl-d13 detaches and subsequently transfers to the gas phase, making the detached deuterated chain unavailable for detection using TOF-SIMS analysis of the solid film.4,6 The initial C4H7/C4D7 intensity ratio is zero for P3HdT and P3HdT:PCBM, which suggests that (nondegraded) PCBM contributes very little to the C4H7 fragment ion signal (the sulfur contents measured by XPS reveals that the surface consists of 97% P3HdT). Then, the intensity ratio increases drastically after ∼30 h of exposure, due to the formation of C4H7 during the ionization process of the TOF-SIMS analysis, which suggests that the degradation of PCBM is increasing. One other possible contributing phenomenon should be considered. As a consequence of the morphological reorganization/optimization, the surface becomes poorer in P3HdT. In 4525



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Wavelength-Dependent Pathways of P3HT:PCBM Photo-oxidation. Dupuis et al. showed that the photodegradation of P3HT is strongly enhanced upon UV irradiation, compared to visible light exposure.8 Similar results were recently reported by Hintz et al.;10 it is noteworthy that, in this paper, the kinetic rates are related to incident dose and not to the absorbed dose, and therefore the conclusions are qualitative, with respect to the photochemistry. It is important to determine whether a similar wavelength dependence is found in P3HdT:PCBM, which is a relevant question, since P3HdT and PCBM do not equally absorb in the same domain (also see Figure S4 in the SI). Using a band-pass filter or a high-pass filter in the Suntest apparatus, it was possible to selectively irradiate the samples with mainly UV photons, an irradiation hereafter denoted “Sun-UV”, or with visible light at wavelengths higher than 450 nm under conditions we call “Sun-Vis” (see Figure S5 in the Supporting Information). Importantly, both the highpass and band-pass filters attenuate the incident light. Photo-oxidation of P3HT. Under both Sun-UV and Sun-Vis irradiation conditions, a progressive decrease in absorbance was observed in the photo-oxidation of P3HT. The photodegradation rates have been calculated according to eqs 1 and 2:

The oxygen content/intensity is hereafter (tirr > 30 h) observed to decrease slightly, which is evident from both techniques, which is most likely a consequence of the competing processes involving photo-oxidation and morphological reorganization/ optimization. The absolute oxygen content (not shown) for higher aging times (tirr > 30 h) is lower for P3HdT:PCBM, which is due to the significantly elevated content of carbon originating from PCBM. The important observation is that the oxygen increase slows significantly after 30 h for P3HdT:PCBM but not for P3HdT. At first, this is not consistent with the data reported in Figure 4. However, the information in Figure 4 is based on a bulk analysis (IR spectroscopy), whereas Figure 6a originates from an XPS analysis that quantitatively monitors oxygen from oxygen functionalities at the extreme surface (5−10 nm) of the deposits. The change in slope observed in the blend at ∼30 h clearly demonstrates that the oxidability of PCBM is less than that of P3HdT. High-resolution C 1s and S 2p XPS spectra of P3HdT and P3HdT:PCBM were also acquired (also see Figures S2 and S3 in the SI), giving information supporting these conclusions (see the SI for a detailed discussion). For the S 2p XPS spectra, there seem to be two clusters of peaks merged in two broad peaks for both materials (see Figure S3 in the SI): the low binding energy peaks (162−167 eV) that represent nonoxidated sulfur (i.e., S) together with the slightly oxidated sulfur (i.e., SO); and the merged peaks at higher binding energies (167−172 eV) due to more oxidated sulfur (i.e., SO2, SO3, and SO4). The ratio between the peaks in these two binding energy windows is a measure of the degree of photo-oxidation specifically on the S atom in P3HdT. The corresponding ratio is shown for both P3HdT and P3HdT:PCBM in Figure 7, as a function of aging time.

⎛ dC ⎞ V = −⎜ ⎟ ⎝ dIa ⎠

(1)

Ia = I0(1 − 10−A)

(2)

where V is the photodegradation rate (m−1), C the thiophene concentration (mol L−1), Ia the absorbed photon flux (mol m−2), I0 the incident photon flux (mol m−2), and A the absorbance. In order to find the values of VUV (Sun-UV) and VVis (SunVis), we have plotted the absorbance of thiophen rings as a function of absorbed dose. As Ia decreases upon irradiation, Ia has been calculated for each measurement. Curves reported in Figure 8 show that irradiation with UV light induces a ∼10 times faster decay of the absorbance, confirming previous reports obtained under polychromatic and monochromatic irradiation.8,10 As previously reported, the loss of absorption is accompanied by the decrease of the IR bands of the polymer. The rates of thiophene ring (T) and C−D bonds (CD) decay are reported in Figure 9. We have defined Q20 as being the absorbed dose inducing a 20% decrease of the IR bands of the irradiated material. In this way, the Q20 values determined for the decay of

Figure 7. Ratio between the intensity of the accumulated S 2p peaks between 167 and 172 eV and the accumulated S 2p peaks between 167 and 172 eV, as a function of aging time. The ratio is a measure of the degree of photo-oxidation specifically on the S atom in P3HdT. Each point represents a single measurement.

Figure 7 provides valuable information in the sense that photo-oxidation is monitored specifically on the S atom in P3HdT, i.e., unaffected by the P3HdT/PCBM composition or possible instrument effects. It is evident from Figure 7 that photo-oxidation of the S atom in P3HdT becomes inhibited after 30 h of aging of the P3HdT:PCBM material. It should be emphasized that the S atom is not the only oxidation site in P3HdT (e.g., the side-chain is the primary oxidation site), but represents one specific site of oxidation.

Figure 8. Thiophene rings concentration (calculated from UV−vis data) during the photo-oxidation of P3HdT films in the two irradiation conditions: Sun-Vis and Sun-UV. 4526



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By applying this common mechanism to P3HT, the identification of the chromophoric species and their photochemical behavior permits the following inferences: • Initiation Step: The energy of the quanta of the UV radiation from sunlight is sufficient to break chemical bonds. It is not the case for less-energetic visible photons. Therefore, even if UV photons are less absorbed by P3HT, this absorption can lead to the formation of radicals that can initiate the oxidation of the polymer chain. Conversely, the absorption of visible photons by P3HT is more important, but a less energetically excited state is reached, and the deactivation mainly occurs via fluorescence. Furthermore, the initiation step does not govern the chain radical oxidation process as it was shown that the oxidation continues (formation of sulfinate groups), even when the absorbance of P3HT has totally vanished.5 • Propagation, Termination, and Branching Steps: Photooxidation of P3HT leads to the formation of hydroperoxides formed at the α-C atom of the side chain.5,6 These aliphatic hydroperoxides are known to be thermally unstable, the photochemical homolysis of their O−O bond occurring only at the shorter wavelengths in sunlight.4 Decomposition of ROOH gives aromatic ketones.4 It has been evidenced that aromatic ketones are photo-unstable when irradiated at λ < 400 nm5 and accumulate only under visible light.5,10 Hydroperoxide and aromatic ketone absorption of UV yields radicals, which participate in the chain oxidation process. The fact that the πconjugated system appears to be more affected than the alkyld13 chains is related to the presence of one S atom per thiophene unit and five CD2 groups and one CD3 on the alkyld13 chain. The oxidation of the S atom provokes the destruction of the thiophene, whereas the oxidation of the side chain only involves the oxidation of the α-CH2 group of the side chain. Under UV exposure, the gap between the degradation of these two groups is reduced due to Norrish type 1 reactions of aromatic ketones.4 They yield a chain radical further oxidized with aromatic carboxylic acid end-groups.5,6 This reaction also produces C5H11• alkyl radicals that are oxidized into low molecular aliphatic carboxylic acids.12 Their facile migration provokes the loss of the alkyl side chain, even if only one CH2 group is initially oxidized with UV. • P3HT:PCBM Photo-oxidation: The same analysis was performed for P3HdT:PCBM samples submitted to photooxidation under the two irradiation conditions, and data obtained for Q20 (thiophene) with and without PCBM are reported in Figure 9. In addition, Q20 values for fullerene decay are reported in Figure 9. Figure 9 shows the rapid degradation of PCBM in the blend upon UV light irradiation. In parallel, the higher stabilization of P3HdT (the alkyl side chain (CD) at a higher extent than the thiophene ring (T)) by PCBM is observed upon Sun-UV irradiation compared to Sun-Vis irradiation, but at a higher extent upon Sun-UV irradiation. As previously reported, the morphological reorganization of the active layer involves PCBM diffusion at the surface. Therefore, PCBM molecules which absorb in the UV region, act as anti-UV layers. The higher stabilization effect provided by PCBM observed in Figure 9 is in good agreements with surface/bulk analysis. The results also confirm that the stabilization of P3HdT by PCBM is unfortunately accompanied by the oxidation of PCBM.15 In addition, data reported in Figure 10 show that if the UV photons are filtered, there is a significant increase of the stability of the active layer; in the

Figure 9. (Top) Evolution of the IR absorbance of thiophene groups (T), C−D bonds (CD), and fullerene cages (F) of P3HdT:PCBM film under light exposure in air, and determination of the corresponding Q20 values. (Bottom) Q20 obtained for the P3HT and P3HT:PCBM films under Sun-UV and Sun-Vis irradiation.

thiophene groups (T), C−D bonds (CD) of P3HdT and the fullerene cages (F) of PCBM in the two irradiation conditions are reported in Figure 9. To explain such a significant effect of UV and visible photons on the alkyl side chain and on the thiophene rings, Hintz et al.7,10 suggested that the mechanisms of degradation are not the same under UV and visible light. Degradation under UV illumination proceeds via a mechanism involving a radical attack at the α-carbon atom of the alkyl side-chain, followed by the destruction of the π-conjugated system, as suggested by Manceau et al.5,6 Hintz et al. proposed that under visible illumination the primary attack would take place at the polymer backbone, and that singlet oxygen formation may be a possible reaction pathway under visible irradiation. However, another paper has ruled out the reaction with singlet oxygen,19 so one may propose another explanation. The two observationsi.e., different effects of UV and visible photons (a) on P3HT in general, and (b) on the alkyl side chain and on the thiophene ringscan be easily explained if one takes only the general photo-oxidation mechanism of polymers into account. Four steps depicted in Scheme 1 are involved in the polymer-based chain radical oxidation mechanism4 (see also the SI). Scheme 1. General Photo-oxidation Mechanism of Polymers (PH)

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(4) Gardette, J.-L. Handbook of Polymer Degradation, 2nd ed.; Halim Hamid, S., Ed.; Marcel Dekker: New York, 2000; Vol. 1, pp 671−698. (5) Manceau, M.; Rivaton, A.; Gardette, J.- L.; Guillerez, S.; Lemaître, N. Polym. Degrad. Stab. 2009, 94, 898−907. (6) Manceau, M.; Gaume, J.; Rivaton, A.; Gardette, J.-L.; Monier, G.; Bideux, L. Thin Solid Films 2010, 518, 7113−7118. (7) Hintz, H.; Egelhaaf, H.-J.; Lauer, L.; Hauch, J.; Peisert, H.; Chasse, T. Chem. Mater. 2011, 23, 145−154. (8) Dupuis, A.; Wong-Wah-Chung, P.; Rivaton, A.; Gardette, J.-L. Polym. Degrad. Stab. 2012, 97, 366−374. (9) Madsen, M. V.; Tromholt, T.; Böttiger, A.; Andreasen, J. W.; Norrman, K.; Krebs, F. C. Polym. Degrad. Stab. 2012, 97, 2412−2417. (10) Hintz, H.; Sessler, C.; Peisert, H.; Egelhaaf, H.-J.; Chasse, T. Chem. Mater. 2012, 24, 2739−2743. (11) Rivaton, A.; Manceau, M.; Chambon, S.; Gardette, J.-L.; Guillerez, S.; Lemaître, N. Polym. Degrad. Stab. 2010, 95, 278−284. (12) Dupuis, A.; Tournebize, A.; Bussière, P.-O.; Rivaton, A.; Gardette, J.-L. Eur. Phys. J.−Appl. Phys. 2011, 56 (3), 34104/1− 34104/9. (13) Chambon, S.; Rivaton, A.; Gardette, J. L.; Firon, M. Sol. Energy Mater. Sol. Cells 2008, 92, 785−792. (14) Manceau, M.; Chambon, S.; Rivaton, A.; Gardette, J.-L.; Guillerez, S.; Lemaître, N. Sol. Energy Mater. Sol. Cells 2010, 94, 1572− 1577. (15) Reese, M. O.; Nardes, A. M.; Rupert, B. L.; Larsen, R. E.; Olson, D. C.; Lloyd, M. T.; Shaheen, S. E.; Ginley, D. S.; Rumbles, G.; Kopidakis, N. Adv. Funct. Mater. 2010, 20, 3476−3483. (16) Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. C.; Nelson, J. Nat. Mater. 2008, 7, 158−164. (17) Chambon, S.; Rivaton, A.; Gardette, J.- L.; Firon, M. Sol. Energy Mater. Sol. Cells 2007, 91, 394−398. (18) Hintz, H.; Egelhaaf, H.-J.; Peisert, H.; Chasse, T. Polym. Degrad. Stab. 2010, 95, 818−825. (19) Manceau, M.; Rivaton, A.; Gardette, J.-L. Macromol. Rapid Commun. 2008, 29, 1823−1827.

same time, the absorption of light by the active layer is not significantly decreased (

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