A novel stable bisazide molecule that can freeze the bulk heterojunction morphology at its optimized layout by specifically bonding to fullerenes is reported. The concept is demonstrated with various polymers: fullerene derivatives systems enable highly thermally stable polymer solar cells.

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 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

We have investigated the impact of residual additives such as diiodooctane (DIO) and octanedithiol (ODT) on the photostability of state of the art P3HT:PCBM active layers. A series of active layers prepared with and without additives as well as neat additives were submitted to light irradiation in ambient air and analyzed by UV–vis and IR spectroscopy. We show not only that residues are sensitive to the combined action of light and oxygen but also that their presence can dramatically impact the polymer blend stability. DIO molecules are highly sensitive to light and can directly saturate the polymer conjugated backbone or be trapped by the fullerene moieties. ODT molecules can be photooxidized and may accelerate the intrinsic photooxidation of the active layer. Another important result is that the additives’ impact is directly linked to the presence of a top layer above the active layer. The confinement makes that additives react within the active layer, and thus accelerate its photodegradation, rather than decomposing in the gas phase (irradiation without top layer). Thus, a light-soaking step before top layer deposition could allow a clean removing of additives without affecting the optimized morphology and polymer blend stability. This process would be easily adaptable to industrial scale production.