Testing two concepts to increase the efficiency of p-type dye-sensitized solar cells

• Applicants: Professor Dr. Harald Krautscheid; Dr. Stefan Zahn
• Subject Area: Theoretical Chemistry: Molecules, Materials, Surfaces ; Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2015 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 280093010

Cheap solar energy sources are one of the key factors to reduce carbon dioxide emission and prevent global warming. Dye-sensitized solar cells (DSCs) are cheap, possess a short energy payback time, and show a good performance at diffuse light, which make them very attractive for commercial applications. So far, DSCs do not exceed 13% light-to-electricity conversion efficiency. While DSCs with a photoanode (n-DSCs) are suitable for commercial applications, the most efficient DSC with a photocathode (p-DSCs) shows only about one-tenth power conversion efficiency, significantly too low for commercial applications. Building a high efficient p-DSC would not only provide a new type of DSC for commercial applications, it is also a milestone in the construction of high-performance DSCs with a photoactive anode and a photoactive cathode (np-DSC). These devices can possess a significantly higher power conversion efficiency than their individual modules. Within this application, two concepts are addressed to increase the light-to-electricity conversion efficiency of p-DSCs.The first concept is the usage of ionic liquids in combination with transition metal redox mediators in p-DSCs. It seems reasonable that the anions of ionic liquids should be closer to injected holes in the semiconductor than cations, like positively charged transition metal complexes which serve as redox mediator. Thus, the usage of ionic liquids might decrease the high dark current of p-DSCs which limits the power conversion efficiency of p-DSCs. The second concept is a temporary docking of the redox mediator at the dye. A dye with a Lewis basic group as building block can coordinate at a transition metal complex. The close contact of the dye and the positively charged redox mediator should enhance the preferred electron transfer rate from the dye to the redox mediator while the undesired electron recombination rate between dye and semiconductor should be increased. If the electron transferred from the dye to the redox mediator resides in an antibonding d-orbital of the transition metal complex, the bond between the dye and the redox mediator should be weakened. As a result, the reduced redox mediator coordinated at the dye can be replaced by the oxidized form of the redox mediator in solution if the dye offers a strongly coordinating ligand. This replacement will restore the initial configuration in the device.If the concept of temporary redox mediator docking at the dye or the usage of ionic liquids in combination with transition metal redox mediators can be shown as general concept to increase the efficiency of a p-DSC, a follow-up project will focus on device performance optimizations to construct highly efficient p-DSCs and a new generation of DSCs, np-DSCs exceeding 13% light-to-electricity conversion efficiency.

• DFG Programme: Research Grants