New light into fundamental mechanisms in organic solar cells
Organic photovoltaics is a very active field of research that, as part of the larger field of organic electronics, faces challenges in understanding of the material structure and resulting properties. Research will help discover advanced organic materials to build improved solar cells that will be lighter, cheaper and more flexible, creating new possibilities for solar cells and electronics. While modelling work is a major drive towards design of improved materials, computational tools that bridge the length and timescales pertaining to electronic processes in organic materials are not yet widely available. Addressing this need, researchers within the project BEST successfully developed an integrated multi-scale modelling platform that more accurately describes the fundamental electronic processes taking place in organic semiconductors. A major project goal was to explore how active materials that typically consist of a blend of an electron fullerene-based acceptor and a polymer-based electron donor get optically excited, and how this excitation spreads out over the molecules and gets converted to separate charges that generate electricity. Research results show that the electronic states in soluble fullerene derivatives are characterised by localised charge carriers at room temperature that sustain high-energy states. By increasing the electron-hole distances, delocalisation of excited states helps reduce energy losses through charge recombination, a process that strongly limits the efficiency of organic solar cells. Another part of BEST’s study focused on the largely unexplored mechanism of singlet fission in organic materials. Solar cells sensitised with a singlet fission material can significantly lift the power-conversion efficiency. Combining modelling work and time-resolved spectroscopy measurements, the team obtained further insight into the complex electronic structure and excited-state properties of pentacene-perfluoropentacene co-crystals. Lack of clear understanding of the molecular doping mechanism in organic semiconductors is one of the main factors hampering widespread adoption of organic solar cells. Researchers proposed a general model for the study of molecular doping that can be fully parameterised from first-principle calculations and that more accurately describes structure-property relationships in doped semiconductors. Theoretical and computational work led by BEST should help design advanced organic materials to build more efficient solar cells.
Keywords
Organic solar cells, electronic processes, BEST, organic semiconductors, singlet fission
