CEPHORCAS

The project entitled CEPHORCAS is submitted to the theme of "innovative concepts and procedures", itself aimed at the development of solar photovoltaic materials (axis 3) and specifically concerned with the development of organic devices that will allow low cost production from readily available raw materials. The targets are to increase device efficiencies and lifetimes. The proposed project is concentrated on the latter target as it is a critical parameter that must be confronted so that these devices can be used in industrial and private spheres. The active layer in organic polymer devices is based on two components: a polymer donor which acts as the dominant chromophore, giving rise to excited electronic states; and an electronic acceptor such as fullerene derivative which permits these states to become separate positive and negative charges. The importance of the morphology of the active layer at this point cannot be understated, as it determines the route that these charges once formed take in reaching the electrodes. Great steps have been taken to understad the morphology at a nanometric scale; it has been realised that the long term changes that a structure can undergo, such as axcessive phase separation, can be detrimental to the operation of the device. This project aims to improve the stability of the morphology of the donor/acceptor blend by using cross-linkable materials in the active layer. These will form covalent bonds that will fix the structure into place. It is envisaged that the cross-linking technique that will be used will be as simple as possible (i.e. thermally or photochemically provoked) so that device preparation remains undemanding. The ideal scenario will be that sunlight which provokes the cross-linking. The current literature shows that, so far, this route has been little explored, making it strategically important. Materials to be developped include polymer donors based on polythiophenes that can be cross-linked with each other within the device, the same but to be crss-linked with the acceptor in the device, and finally derivatives of fullerenes that may be cross-linked with each other in the formed device. It is essential that these three different routes be explored so that by the half-way point in the project a clear understanding of the most appropriate technique can be made. On the basis of the choice that will be made at this point, the second part of the project will be spent fully characterising the materials and optimising their use in photovoltaic devices. Ink formulation optimisation will be adressed with our industrial partner RHODIA. Above all, the latter will include extensive characterisation of device aging so as to understand the profile of the materials in hand with respect to materials that have not been cross-linked. This will be performed using various conditions, including exposure to inert conditions and controlled amount of oxygen and water, so as to be able to define the reasons for any eventual device degradation. Finally, AM 1.5 conditions and accelerated aging will be used to give a complete profile of the cross-linked devices.