IIT- SEED 2009 "HELYOS"

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IIT–SEED 2009 "HELYOS" :High pErformance modeLling of Hybrid Organic Solar Cells (understanding the mechanism, improving the efficiency)

The HELYOS project has the ambitious, yet feasible, target of providing the theoretical and computational basis for the efficient and accurate computer simulation of hybrid organic solar cells, in particular Dye-Sensitized Solar Cells (DSC). DSC have reached high solar to electric power conversion efficiency currently exceeding 11%. The DSC low cost coupled to their high performance have raised substantial impulse to industrialization of this technology. A key advantage of DSC is that their conversion efficiency does not decrease under non-ideal illumination conditions. The operation of DSC mimics photosynthesis in that the processes of light harvesting and charge carrier transport are spatially separate. Sunlight is absorbed by a dye monolayer located at the junction between electron and hole transporting phases. Upon photo-excitation, the dye injects an electron and a hole into the n- and p-type materials, respectively, generating free charge carriers. The key to efficient light-harvesting is the high internal surface area of the mesoporous nanostructured oxide film.
The computer simulation of DSC is a formidable task: the complex heterointerface constituted by the dye sensitizer, the nanostructure semiconductor oxide and the solution containing the liquid electrolyte or the amorphous phase containing the solid state hole transporter need to be effectively and accurately simulated. The situation is further complicated by the need to simulate the electronic and optical properties of the combined systems, thus requiring a balanced description of the system properties on both its ground and excited states. To solve this very challenging issue, we gather together the most skilled theoretical and computational chemists working in the field of DSC and related phenomena. Only a multi scale computational approach capable of dealing on the same footing all the DSC elements and to provide a unified description of their properties can be successful. We propose the use of an integrated computational strategy based on a hierarchy of computational approaches ranging from correlated ab initio methods, such as coupled cluster or multi-configurational techniques, to Density Functional Theory (DFT) and its Time Dependent extension, to approximate DFT methods (tight binding DFT) and integrated quantum mechanics/molecular mechanics (QM/MM). Once a proper computational set up will be operative, we will extensively investigate realistic model systems representative of the open question in DSC technology. We initially wish to gain fundamental insight into the operational mechanism of DSC using various dyes and nanostructured semiconductor oxides. A proper modelling of both dyes and nanostructured oxides (TiO2, ZnO) will therefore be pursued. We will then utilize the information obtained from such studies to design and test “in silico” new dye sensitizers with improved characteristics. The fundamental phenomena occurring on the semiconductor surfaces will then be investigated, with emphasis on intermolecular interactions and on the dye interactions with the electrolyte, which are the main factors currently limiting the DSC efficiency.

The obtained results and predictions will be immediately translated into active experimental testing, which will be carried out as part of the collaborations with the DSC technology academic and industrial world leaders. If funded, the HELYOS project has the potential to revolutionize the way to approach DSC device physics, leading to realistic DSC performances approaching 13 %. The investigated phenomena and the proposed computational set up is complementary, albeit perfectly fit, to what outlined in the IIT Platform 7, providing access and insight into further information which we believe are of fundamental importance for the rapid development and diffusion of DSC technology.