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To make polymer solar cells (PSCs) a competitive market technology, integrated efforts are required toward the development of highly efficient light harvesting and charge transporting materials with good thermal and photochemical stability, and which can be processed from solution. Nowadays, a critical issue to be solved is enhancing the stability and durability of PSCs. Indeed, the photoactive material used in the active layer dictates the efficiency of the device on the one hand but, on the other hand, it is well known that organic materials are unstable when exposed to light irradiation, which provokes a degradation of their properties. Making long lifetime solar cells with polymers that are susceptible to degradation under light exposure could be an unrealistic challenge. Therefore, elucidating the mechanism of polymer photodegradation is a key point for developing strategies to decrease or prevent the loss of the functional properties of the material. In this paper, the basic concepts of polymer photo-aging are explained first. Then the photodegradation mechanisms of conjugated polymers currently used in PSCs are reported. Finally, as barrier materials able to cut off moisture and oxygen ingress are essential for the stability of PSCs, methods for designing coatings for PSC encapsulation are presented, based on recent publications.

This paper is devoted to the investigation of the phototransformation mechanisms of PVK exposed to long-wavelength radiations (λ > 300 nm) in the absence and in the presence of oxygen. PVK irradiated as thin films was analyzed by fluorescence, UV-vis, and IR spectroscopies. The photoproducts formed were identified by postirradiation treatments including chemical derivatization reactions and HPLC analysis. The cross-linking was evaluated by gel fraction measurements. PVK is known to give an intense excimer fluorescence and no monomer fluorescence. The phototransformations provoked by light absorption are shown to lead to a dramatic decrease of this fluorescence which can be correlated with the modification of the chemical structure of the macromolecules. The decrease of fluorescence is the result of the diminution of the local mobility of the macromolecular chains resulting from cross-linking reactions provoked by the recombination of the several radicalar species. A general mechanism accounting for the phototransformation of PVK in the presence and in the absence of oxygen is proposed.

This paper reports on the photochemical behavior upon exposure to UV-visible light of a poly(2,7-carbazole) derivative for use in high-performance solar cells. Poly[ N -9 ′ -hepta-decanyl-2,7-carbazole-alt -5,5-(4 ′ ,7 ′ -di-2-thienyl-2 ′ ,1 ′ ,3 ′benzothiadiazole)] (PCDTBT) is one of a relatively large class of push-pull carbazole-based copolymers that have been synthesized to better harvest the solar spectrum. The 2,7-carbazole building block of PCDTBT is also used with different electron-accepting units in a large variety of low-band-gap polymers. The photochemical and morphological behavior of PCDTBT thin fi lms is investigated from the molecular scale to the nanomechanical properties. The photo-oxidation mechanism is shown to be governed by chain-scission and cross-linking reactions. It results in dramatic evolution of the morphology, roughness and stiffness of thin PCDTBT fi lms. Based on the identifi cation of several photoproducts formed along the macromolecular chains or released into the gas phase, the main pathways of PCDTBT photochemical evolution are discussed. These processes fi rst involve the scission of the C-N bond between the carbazole group and the tertiary carbon atom bearing the alkyl side-chain. Modifi cations of the chemical structure of PCDTBT, the evolution of its UV-visible absorbance, and its nanomechanical properties initiated by light irradiation are shown to be closely related. 479

This study reports the effect of light on PLA/ZnO nanocomposites films produced by melt-extrusion. The attention focused on the discrimination between the photocatalytic degradation of PLA provoked by ZnO and the UV screening effect of the ZnO nanoparticles. The chemical modifications of PLA induced by UV light irradiation were analyzed using infrared spectroscopy and completed through the analysis of the low-molecular-weight photoproducts using IC and SPME and the characterization of chain scissions with SEC. A comprehensive mechanism for the photooxidation of PLA was then proposed. The results indicated that the photocatalytic activity of ZnO nanoparticles induces the oxidation of PLA. Because ZnO limits the penetration of light inside the samples, this effect mainly concerns the first micrometers at the surface of the exposed samples. Cross-sectional analysis using micro-IR and ATR-IR spectroscopies was performed to highlight the degradation profile in the PLA/ZnO nanocomposites.

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The photo-oxidation mechanism of thin-film blends based on poly(3-hexylthiophene):phenyl-C 61 -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-d 13 -thiophene) (P3HdT), permitted discrimination of carbon originating from the hexyld 13 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.

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