Testing two concepts to increase the efficiency of p-type dye-sensitized solar cells
Inorganic Molecular Chemistry - Synthesis and Characterisation
Final Report Abstract
The efficiency of dye-sensitized solar cells with a photoactive cathode is still below 5 % so far and thus hinders the construction of high-performance tandem dye-sensitized solar cells. The scope of our work was to investigate two possibilities which might increase the performance of p-type dye-sensitized solar cells. Firstly, can the usage of electrolytes with high anion concentrations as solvent decrease the interception rate between reduced transition metal redox mediators and nickel oxide? This is based on the assumption that anions prefer to reside close to positively charged holes in the semiconductor and thus might prevent close contacts between semiconductor and reduced redox mediator. We have employed tris(1,2diaminoethane)cobaltII/III and mixtures of choline chloride and an organic compound as solvent-electrolyte-system. Unfortunately, the solubility of tris(1,2diaminoethane)cobaltII/III was poor in the investigated mixtures even when ethylene diamine was selected as organic compound. To understand this behavior, we have investigated mixtures of choline chloride and organic compounds which are known as deep eutectic solvents. These mixtures possess a significantly decreased melting point compared to the pure compounds. We found that the hydrogen bond strength of the organic compound plays a crucial role for the charge located on the organic compound. Thus, the development of a polarizable force field is strongly recommended. Furthermore, we observe similar interactions beyond hydrogen bonds. For example, the oxygen atom of urea can even push the anion off-center of the cation in the deep eutectic solvent composed of choline chloride and urea. It seems that ethylene diamine can not perturb the cation-anion-interaction of choline chloride in a similar way which might result in the poor miscibility. Secondly, we have studied if a temporary link between dye and redox mediator can increase electron transfer rate from dye to the redox mediator. Based on a computational screening, transition metal complexes based on Cu, Ni and Co were selected for experimental studies. Unfortunately, a working device with the Cu or Ni complexes could not be obtained which might be attributed to the low stability of these complexes. In case of the Co complex, the quenching of the excited state was much faster than the injection of an electron from nickel oxide to the dyetransition metal complex system. Therefore, follow-up studies will focus on tuning the electronic structure towards a push-pull structure where the HOMO resides close to the semiconductor and the LUMO is located at the transition metal. Furthermore, transition metal complexes should be selected which possess a life time of the photo-excited state beyond the ps timescale.
Publications
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Charge Spreading in Deep Eutectic Solvents, ChemPhysChem, 17, (2016), 3354–3358
S. Zahn, B. Kirchner, D. Mollenhauer
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Deep eutectic solvents: similia similibus solvuntur? Phys. Chem. Chem. Phys., 19, (2017), 4041–4047
S. Zahn
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Can a temporary bond between dye and redox mediator increase the efficiency of p-type dye-sensitized solar cells?, J. Mol. Model. (2018)
S. Merker, H. Krautscheid, S. Zahn
