Predicting the ageing and the long-term durability of organic polymer solar cells

Abstract

Organic solar cells based on conductive polymers exhibit a unique combination of properties which include low cost, flexibility and large surface processability. Organic photovoltaic could then prevail for some applications alongside silicon, such as nomad or indoor. To achieve this objective, the sustainability of the initial properties in conditions of use of the cell is required, since it could be a lock to the emergence of these devices in the market. The polymers used in solar cells are indeed known to exhibit low resistance to environmental constraints, in particular to the combined action of sunlight, oxygen and water.We present recent results on both the accelerated artificial and the natural outdoors ageing of MDMO‐PPV (Poly[2‐methoxy‐5‐(3’,7’‐dimethyloctyloxy)‐1,4‐Phenylenevinylene) and P3HT/PCBM blends poly(3‐hexylthiophene) (P3HT) (methano‐fullerene[6,6]‐phenyl C61‐butyric acid methyl ester) ([60] PCBM). The influence of various parameters such as the temperature and the presence of oxygen were studied. The modifications of the chemical structure of both the components of the blend were monitored by spectroscopic analysis (infrared, UV‐visible), the morphology of the blends was analysed by AFM and XRD and the photovoltaic performances all along the exposure were recorded.Two important results have been pointed out: on one hand, the Achilles heel of the chemical structure of MDMO‐PPV and P3HT under the impact of light has been evidenced. On the other hand, it has been shown that P3HT:PCBM blends are much more stable than MDMO:PCBM blends whatever the conditions of ageing are. Results show that a convenient encapsulation can ensure a promising lifetime of P3HT/PCBM blends in real conditions of use. This work also focuses on this last point and proposes to study and try to understand the behavior of the materials used in the active layer when submitted to photoaging and thermal aging in the absence of oxygen. To fulfil very good encapsulation, glass substrates can be used. However, as a result, organic devices lose some of their attractive properties, such as flexibility. Organic PV cells finally need a complex layer stack as a permeation barrier to be protected from water and oxygen of the environment. Multilayer barrier coatings are required: on the one hand, there are inorganic materials applied as thin films. Those films show a preferential permeation at defects. For the most part, these inorganic materials are then used in combination with a polymeric substrate. Polymers can be considered as being homogeneous with regard to the usually utilized substrate or film thicknesses. Their barrier properties can be reinforced by nanoclays. The impermeable clay layers force a tortuous pathway for a permeant transversing the nanocomposite. It is reported that gas permeability through polymer films can be reduced by 50–500 times even with small loadings of nanoclays.