Incorporation of fullerene into polymers for photovoltaic applications

Abstract

Photovoltaic technology (PV) makes it possible to directly convert sunlight into electricity and it is seen as a very promising solution to the current energy crisis. Although the PV market is dominated by inorganic-based devices, those systems present high production costs and deployment issues that limit their application. Polymer-based organic solar cells (OPVs) are promising sources of renewable energy due to their facile, low cost production, and formable nature. Due to its electronic properties and high electron mobility, small molecule fullerene (C60) derivatives are widely used in large scale OPVs. However the morphological properties of C60 derivatives decrease device stability as C60 easily undergoes self-aggregation during OPV use. The processibility of C60 can be improved by incorporating it into a polymer. These systems are already described in literature but have in general a multistage synthesis that could affect the electronic properties of C60 as well as give insoluble products due to reticulative reactions. The objective of the work here presented was to prepare innovative polymers based on C60 for photovoltaic and electronic devices using reliable, well-known C60 chemistry. At the University of Pau (EPCP Lab), after the syntheses of small molecules to be used as co-monomers, two synthetic routes were used in order to obtain main- chain oligo- and polyfullerenes. The first route is based on the atom transfer radical addition polymerization (ATRAP), which has already been used for the preparation of main-chain polyfullerenes. With this method, soluble compounds with various molecular weights were prepared. The second route was discovered in this work and exploits a well-known fullerene chemistry to prepare soluble polyfullerenes with reasonably high molecular weights. Preliminary studies to understand the effect of the reaction parameters (reagents, reagents concentration, temperature, time and solvent) and the kinetics of the polymerisation were performed. Material characterisations were carried out via GPC chromatography, NMR spectroscopy, and UV-visible and IR spectroscopies. Thermal analyses (TGA and DSC) were also run to complete the characterisations. Both C60 and its derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), have been exploited as monomers in the reactions. A nine-month (cotutelle) stay at Tübingen University, Germany, permitted a study of the synthesized materials by means of XPS and UPS spectroscopies. The aim of these studies was to obtain a better understanding of the energy levels pictures of the oligo- and polyfullerenes. Thin films of the compounds were deposited on different substrates via solution processes (doctor blade or spin coating) to obtain ex-situ samples for characterisation. Samples of polyfullerene-containing active layers were prepared during a short stay (1 week) at BELECTRIC OPV GmbH, Nuremberg (Germany), and analysed by optical microscopy and AFM microscopy during thermal-degradation studies at Tübingen University. These studies were completed thanks to collaborations with researchers at BELECTRIC OPV GmbH, who have incorporated the compounds into devices and performed complementary and comparable experiments. As general trends, the compounds are found to improve the stability of the devices upon thermal stresses.