We present the simple synthesis of a star-shape non-fullerene acceptor (NFA) for application in organic solar cells. This NFA possesses a D(A)3 structure in which the electron-donating core is an aza-triangulene unit and we report the first crystal structure for a star shape NFA based on this motive. We fully characterized this molecule's optoelectronic properties in solution and thin films, investigating its photovoltaic properties when blended with PTB7-Th as the electron donor component. We demonstrate that the aza-triangulene core leads to a strong absorption in the visible range with an absorption edge going from 700 nm in solution to above 850 nm in the solid state. The transport properties of the pristine molecule were investigated in field effect transistors (OFETs) and in blends with PTB7-Th following a Space-Charge-Limited Current (SCLC) protocol. We found that the mobility of electrons measured in films deposited from o-xylene and chlorobenzene are quite similar (up to 2.70 × 10-4 cm2 V-1 s-1) and that the values are not significantly modified by thermal annealing. The new NFA combined with PTB7-Th in the active layer of inverted solar cells leads to a power conversion efficiency of around 6.3% (active area 0.16 cm2) when processed from non-chlorinated solvents without thermal annealing. Thanks to impedance spectroscopy measurements performed on the solar cells, we show that the charge collection efficiency of the devices is limited by the transport properties rather than by recombination kinetics. Finally, we investigated the stability of this new NFA in various conditions and show that the star-shape molecule is more resistant against photolysis in the presence and absence of oxygen than ITIC.

The synthesis of four non-fullerene acceptors (NFAs) with a "A-pi-D-pi-A" structure, in which the electron-donating core is extended, was achieved. The molecules differed by the nature of the solubilizing groups on the pi-spacer and/or the presence of fluorine atoms on the peripheral electron-accepting units. The optoelectronic properties of the molecules were characterized in solution, in thin film, and in photovoltaic devices. The nature of the solubilizing groups had a minor influence on the optoelectronic properties but affected the organization in the solid state. On the other hand, the fluorine atoms influenced the optoelectronics properties and increased the photo-stability of the molecules in thin films. Compared to reference ITIC, the extended molecules showed a wider absorption across the visible range and higher lowest unoccupied molecular orbital energy levels. The photovoltaic performances of the four NFAs were assessed in binary blends using PM6 (PBDB-T-2F) as the donating polymer and in ternary blends with ITIC-4F. Solar cells (active area 0.27 cm(2)) showed power conversion efficiencies of up to 11.1 % when ternary blends were processed from non-halogenated solvents, without any thermal post-treatment or use of halogenated additives, making this process compatible with industrial requirements.

Polycarbonate exposed to sunlight yellows, degrades and loses its usable properties. In order to increase its lifetime, it can be coated with nano-ceramic thin layers of ZnO and Al2O3 deposited by sputtering. The role of the ZnO is to absorb the UV photons that can damageable for the polycarbonate. However, one of the limitations in the use of ZnO is the photocatalytic oxidation that could occur at interface ZnO/PC as a consequence of the photocatalytic activity of this oxide. Insertion of Al2O3 between PC and ZnO could be a way to inhibit this interfacial oxidation. The photooxidation of the ceramic/polymer assemblies, in condition of artificial accelerated ageing, was measured by infra-red and UV-vis spectroscopies. The results show that the photocatalytic activity of ZnO occurring in monolayer coated substrates can be significantly reduced by insertion of Al2O3 and that, in addition, Al2O3 decreases the permeability to oxygen of the coating.

We have identified modifications of material distributions in inverted organic solar cell structures during accelerated aging processes that are potentially linked to the loss of the mechanical integrity of the devices. Non-encapsulated devices were treated with ISOS-D-3 ageing (the so called damp heat exposure at 85 degrees C/85% room humidity in the dark). After performing pull-off tests, the exposed surfaces were analyzed by X-ray photo-emission spectroscopy and atomic force microscopy. For fresh devices, the results revealed that mechanical failure occurs close to the PEDOT:PSS/P3HT:PCBM interface, in agreement with the literature. However, after a short exposure to damp heat, XPS investigations indicate an enrichment of PCBM at the PEDOT:PSS/P3HT:PCBM interface and a change in the PEDOT:PSS surface composition. Both phenomena may explain the rapid drop in device performances and the increased tendency towards delamination.

An innovative bottom-up approach was used to reveal the importance of traces of polymer oxidation products in altering solar cell efficiency.

This study is focused on the radiooxidative degradation of polymeric insulation of electric cables used in Nuclear Power Plants (NPPs). In order to investigate the degradation mechanisms of the insulation, model composites with ATH (Aluminium TriHydrate) filler and blends (without filler) based on a cross-linked mixture of EVA (Ethylene Vinyl Acetate) and EPDM (Ethylene Propylene Diene Monomer) were submitted to gamma-rays. In normal operating conditions of a NPP, the dose rate which electric cables are exposed to is around 0.1 Gy h -1 . In this work, artificial accelerated ageing test process has been applied at a relatively low dose rate of 7 Gy h -1 . Gamma-irradiations at higher dose rates typically used to accelerate the ageing, in the range 0.2 1 kGy h -1 , were also carried out. The first part of the study is focused on irradiations performed at relatively low dose rate and is devoted to the highlighting of the radiooxidative degradation mechanisms of EVA/EPDM blend with and without ATH filler. Correlations between the evolutions of the chemical, morphological and mechanical/electrical properties of the materials occurring after the ageing process are presented. It is shown that the degradation process is governed by radical oxidation mechanism involving chain scissions leading to the formation of carboxylic acids as end-groups. One of the main effects of the ATH filler is the progressive loss of the mechanical properties of the composite upon radiooxidation whereas they are maintained in the case of the unfilled sample. Despite the oxidation of the polymer, no change in the electrical properties of the blend and of the composite could be observed.

Increasing attention is being paid to the environmental fate and impact of plastics and their additives under sunlight exposure. We evaluated the photodegradation of polystyrene (PS) films (∼100 μm) containing brominated flame retardants (BFRs): decabromodiphenylether (BDE-209), tetrabromobisphenol A (TBBPA), and tetrabromobisphenol A-bis (2.3-dibromopropylether) (TBBPA-DBPE). Irradiations were performed in a solar simulator and outdoors. Infrared (IR) analyses indicated an acceleration of the photooxidation rate of fire-retarded PS films compared to pure PS with an enhancement factor of 7 for TBBPA-DBPE and TBBPA, and 10 for BDE-209. The accelerating effect was found to be correlated with the quantum yield for BFR photodegradation and its absorbance in the PS films. The presence of BFRs also modified the PS photooxidation mechanism and resulted in the formation of 14 brominated photoproducts via bromination and oxidation of PS. Furthermore, a drastic increase in chain scissions and loss of molecular weight was revealed by size exclusion chromatography. This enhanced degradation of PS led to significant leaching (15%) of oxidation products from PS films after immersion in water, and to the gas-phase emission of several volatile brominated products. Our findings suggest that fire-retarded plastics may be a source of potentially hazardous contaminants when exposed to sunlight.

Brominated flame retardants (BFRs) are widely used as additives in plastics, textiles and electronics materials. Here, we investigated the photodegradation of four BFRs including decabromobiphenylether (BDE-209), tetrabromobipsphenol A (TBBPA), tetrabromobisphenol A-bis(2,3-dibromopropylether) (TBBPA-DBPE) and tetrabromobisphenol A bis (allyl) ether (TBBPA-BAE). Experiments were carried out in polystyrene (PS) films using monochromatic and polychromatic irradiations. For comparison, irradiations were also carried in a solvent (tetrahydrofuran: THF). Monitoring of BFR degradation was performed using bulk and surface infrared (IR) measurements, as well as by extraction and HPLC-UV. Photoproducts were characterized using HPLC-high resolution electrospray ionization mass spectrometry (HPLC-ESI-Orbitrap-MS). All four BFRs underwent photochemical transformation in THF at 290 nm with a quantum yield (Phi) ranging from 0.05 for TBBPA to 0.27 for BDE-209, indicating an increase of photoreactivity with the number of Br atoms in BFRs. On the other hand, no major difference in the c values was observed when BFRs were embedded in PS films (Phi: 0.82-0.89). The higher photoreactivity in PS appears to be associated with a fast oxidation of PS as revealed by infrared (IR) analysis and yellowing of the films. Interestingly, the faster the yellowing occurred, the faster the BFR degradation was inhibited due to light screening effect. Several major photoproducts were identified for TBBPA and TBBPA-DBPE. Additional photoproducts possibly arising from PS oxidation and bromination by Br center dot were observed for the first time. This work provides a better understanding of the reactivity and fate of BFRs in polymers allowing for a better assessment of their environmental impacts. (C) 2018 Elsevier Ltd. All rights reserved.

The effect of gamma-irradiation and temperature to accelerate, in a representative way, the ageing of EVA (Ethylene Vinyl Acetate)/EPDM (Ethylene Propylene Diene Monomer) blends was investigated. These blends constitute model materials of cable insulation used in Nuclear Power Plants (NPPs). Physicochemical evolutions of the blends were monitored by InfraRed (IR) spectroscopy, gel fraction measurements and Differential Scanning Calorimetry (DSC). After having pointed out the main similarities between the thermooxidative degradation (100 degrees C) and the radiooxidative degradation at room temperature, both stresses were combined to accelerate the ageing. It is shown that combining gamma-irradiation and high temperature, or subsequent thermooxidation after radiooxidation, accelerates the ageing but may cause distortion within the degradation mechanisms. These limitations of representativeness encountered are particularly due to an increase in chain scission reactions and formation of new crystallites in the blend.

This study focuses on the thermal oxidation of model composites of insulating materials based on a cross-linked mixture of EVA (Ethylene Vinyl Acetate) and EPDM (Ethylene Propylene Diene Monomer) highly loaded (60 wt %) with ATH (Aluminium TriHydrate) filler. A thorough analysis of the material is performed, focusing not only on the polymer but also on the ATH filler. This preliminary study is essential firstly to determine the possible influence of a large amount micrometric filler incorporation on the structure of crosslinked EVA/EPDM materials, and secondly on the thermooxidative degradation mechanisms at different levels and scales (chemical structure, microstructure and architecture, degradation profile, functional properties...). The structure of both the polymer and the filler are modified during the processing step of the composite. It is shown that the reactivity of the ATH fillers leads to the probable intercalation of some segments of the polymer into the interlayer space of the ATH, and to a much less dense polymer network in the composite compared to the unfilled material. Then, the role of the added ATH filler on the thermooxidative degradation and on the resistance of the composite to ageing is studied. After thermooxidative ageing, there is only a very limited accelerator effect of ATH on the oxidation rate of the polymer within the composite compared to the unfilled material, but significant difference between materials with or without ATH filler in terms of oxidation profile or insulating properties. Nevertheless, a noticeable effect on the mechanical properties is highlighted: the mechanical properties of the unfilled materials are retained all through the oxidation process while these properties reduce drastically in the case of filled composites. The significant loss of mechanical properties occurring in the filled composites can be explained by the combination of three factors: only 40 wt % of polymer in the composite, a low density polymer network within the composite, and de-cohesion process between the ATH filler and the polymer matrix upon thermooxidative degradation. (C) 2018 Elsevier Ltd. All rights reserved.

Upon exposure to photooxidative conditions, organic materials are susceptible to undergo degradation via processes involving radical oxygen species and/or reaction with singlet oxygen (1O2). In this frame, the work herein presents a new and straightforward methodology to clarify the role of highly-reactive 1O2 in the photodegradation mechanism of conjugated materials applied in organic electronics. The general methodology consists in the comparison of the infrared signatures of the conjugated materials after the materials are exposed to photooxidative and thermooxidative conditions and in situ generated 1O2. The methodology was validated by analysing the behaviour of four donor materials commonly used in organic solar cells. Analysis of the degradation mechanism of these materials allowed exemplifying the three possible case scenarios, namely (1) both 1O2 and radical oxygen species are involved in the general photooxidation mechanism of the studied material, (2) the material is unreactive towards 1O2 and thus this species plays no role in the photooxidation process, and (3) the conjugated material is reactive towards chemically produced 1O2 but this species is not the main responsible for the photooxidative degradation of the material. In the latter two cases, a free-radical oxidation process accounts for the photooxidation of the investigated materials. The results derived from this simple, yet enlightening, methodology provide fundamental understanding about the degradation pathways of conjugated materials, which is a key point to develop not only efficient but also stable organic electronic devices.

Fullerene derivatives have been ubiquitous as an electron-accepting material in organic photovoltaic solar cells (OSCs). We consider whether and why traces of PCBM oxidation products should be seen as electronic defects impairing the performance of OSCs. Thin PCBM deposits were first illuminated under ambient air for a few minutes, thus revealing the extraordinary easiness of oxidizing PCBM. The charge transfer in polymer:PCBMox bulk heterojunctions was then studied. As a result of a few minutes of PCBM photooxidation, the electron transfer from the polymer to two types of PCBMox species was shown to occur at the expense of the transfer to pristine PCBM. Such modifications to the molecular structure of PCBM and to the charge transfer at the nanoscale were finally correlated with a dramatic loss in the device’s photovoltaic performance at the macroscale. This study clearly indicates the need to integrate photooxidation-resistant electron-accepting materials into OSCs to extend their lifetime.

This study is focused on the radiooxidative degradation of polymeric insulation of electric cables used in Nuclear Power Plants (NPPs). In order to investigate the degradation mechanisms of the insulation, model composites with ATH (Aluminium TriHydrate) filler and blends (without filler) based on a cross-linked mixture of EVA (Ethylene Vinyl Acetate) and EPDM (Ethylene Propylene Diene Monomer) were submitted to gamma-rays. In normal operating conditions of a NPP, the dose rate which electric cables are exposed to is around 0.1Gyh−1. In this work, artificial accelerated ageing test process has been applied at a relatively low dose rate of 7Gyh−1. Gamma-irradiations at higher dose rates typically used to accelerate the ageing, in the range 0.2–1kGyh−1, were also carried out.The first part of the study is focused on irradiations performed at relatively low dose rate and is devoted to the highlighting of the radiooxidative degradation mechanisms of EVA/EPDM blend with and without ATH filler. Correlations between the evolutions of the chemical, morphological and mechanical/electrical properties of the materials occurring after the ageing process are presented. It is shown that the degradation process is governed by radical oxidation mechanism involving chain scissions leading to the formation of carboxylic acids as end-groups. One of the main effects of the ATH filler is the progressive loss of the mechanical properties of the composite upon radiooxidation whereas they are maintained in the case of the unfilled sample. Despite the oxidation of the polymer, no change in the electrical properties of the blend and of the composite could be observed.The second part of the study focuses on the dose rate effect. It is shown that one of the main consequences of an increase of the dose rate from 7Gyh−1 to 0.2–1 kGy h−1 is a reduction of the chain scission process yield by a factor of about 20. Therefore, an important and consistent finding is that there are some limits that must not be exceeded in terms of dose rate to ensure a global satisfactorily representativeness of the accelerated ageing process.

A series of eight low band gap polymers based on benzodithiophene - diketopyrrolopyrrole (BDT-DPP) skeleton were designed with side chain variation on the benzodithiophene unit. The effect of different side chains, including -H, alkyl, alkoxy, and aryl moieties, over polymer stability is examined. Thin films were processed and submitted to photooxidative degradation. We experimentally confirm that alkoxy side chains are the most promising candidates for designing soluble and photo-resistant polymers. This conclusion is modulated by the polymer dispersity which also plays an important role: a low dispersity value being synonymous of higher photostability. The reasons why the side chain structure and dispersity value drive the polymer photostability are discussed. For the design of photo-resistant polymers and devices, side chain selection must go hand in hand with the control of the synthesis meaning a low dispersity.

The photooxidation of a polymer blend film used in efficient solar cells based on poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-b:20,30- d]silole)-2,6-diylalt-(2,1,3-benzothiadiazole)-4,7-diyl], (Si-PCPDTBT) and [6,6]-phenyl-C71-butyric acid methyl (PC70BM) has been investigated. A set of experiments from complementary techniques was developed to monitor the modifications during ageing that occur not only in the bulk but also at the surface. The surface analyses were performed by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), and the bulk analyses by UV–visible spectroscopy and infrared (IR) spectroscopy. The results reveal that the silicon bridge atom is the first target of degradation. We have identified the existence of a photooxidation profile within the 300nm thick film. Such a heterogeneous degradation was confirmed by time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling.