Mechanistic analysis of dye regeneration and recombination processes at dye-sensitized solar cells using microelectrochemical experiments
Final Report Abstract
This collaborative project of one group in Physical Chemistry and one group in Applied Physics was dedicated to the study of photoelectrochemical kinetics in dye-sensitized solar cells (DSSCs). Dye regeneration and recombination was analyzed in detail since these reactions often limit the performance of such cells. We used and advanced techniques such as electrochemical impedance spectroscopy and more specialized techniques like scanning electrochemical microscopy (SECM) for this application. As semiconductors, electrodeposited ZnO and nanoparticulate TiO2 reference samples were used. The semiconductors were sensitized by all-organic dyes, free of noble metals. Welladapted mediators such as complexes of CoII/III and CuI/II were applied. These complexes generally exhibit faster redox kinetics than the traditional iodide/triiodide couple used in DSSCs. The faster redox kinetics bear the potential for desired faster dye regeneration but may also enhance loss processes of conduction band electrons. The project showed that the combination of specific additives, namely Li+ and tert-butyl-pyridine can increase the built-in potential, slow down the unwanted back transfer processes and improve the performance. These effects showed up in impedance measurements, SECM characterization and solar cell characteristics. Another consequence of using transition metal complexes for DSSC operation is the increased importance of the mass transport of the mediator inside the porous semiconductor electrode. The complexes have a bulkier structure and can only be used in lower concentrations compared to traditional redox electrolytes. When they are used with porous ZnO, which has significantly smaller pores than nanoparticulate TiO2 electrodes, these effects become more significant. It was especially found that the transport coefficients of CoII/III are decreased compared to the values in the bulk solution much stronger than what would be expected from the geometry of the porous electrodes indicating the significance of additional solute-wall interactions that retard the transport. This indicates a need for new, more quantitative modelling that links the mass transport processes in the liquid-filled pore volume and in the solution above the photoanode, the electron transport in the semiconductor and the transfer processes between them that avoid simplifying assumptions that were justified for traditional DSSCs but may not yield good descriptions of the new cell types. Such models are also required for a quantitative interpretation of SECM current transients that could be measured with high reproducibility in the same cell as the conventional steady state SECM for the first time. Attempts to establish flexible DSSCs that include microscopic probe electrodes and would allow measurements described above in completely seals cells have met several unexpected difficulties due to incompatibility of the employed materials with the organic redox electrolyte solution. After changing to an aqueous electrolyte system, the general concept could be demonstrated. This approach requires further refinement and development and could ultimately allow to acquire microscopic and detailed kinetic information on exactly the same test cell, from which solar cell characteristics are obtained.
Publications
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´Dye-sensitized solar cells with electrodeposited ZnO and Co(bpy)3 redox electrolyte: Investigation of mass transport in the electrolyte and interfacial charge recombination´ Electrochim. Act., 2017, 258, 591-598
R. Ruess, S. Haas, A. Ringleb, D. Schlettwein
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´Metal Complexes as Redox Shuttles in Dye-Sensitized Solar Cells Based on Electrodeposited ZnO: Tuning Recombination Kinetics and Conduction Band Energy´ J. Electrochem. Soc., 2018, 165, H3115- H3121
R. Ruess, T. H. Q. Nguyen, D. Schlettwein
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'Efficient Electron Collection by Electrodeposited ZnO in Dye-Sensitized Solar Cells with TEMPO+/0 as the Redox Mediator' J. Phys. Chem. C, 2019, 123, 22074-22082
R. Ruess, J. Horn, A. Ringleb, D. Schlettwein
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'Influence of Crystal Facets (102) or (100) on Photoelectrochemical Kinetics of ZnO Nanocrystals in Dye-Sensitized Solar Cells' J. Electrochem. Soc. 2019, 166, B3290-B3294
H. Sun, R. Ruess, D. Schlettwein, T. Yoshida
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'Local studies of photoelectrochemical reactions at nanostructured oxides' Curr. Opin. Electrochem., 2019, 13, 25-32
G. Wittstock, S. Rastgar, S. Scarabino
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´Adjusting Porosity and Pore Radius of Electrodeposited ZnO Photoanodes´ J. Electrochem. Soc., 2019, 166, B3040- B3046
T. H. Q. Nguyen, R. Ruess, D. Schlettwein
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´Diverging Surface Reactions at TiO2- or ZnO-based Photoanodes in Dye-sensitized Solar Cells´ Phys. Chem. Chem. Phys., 2019, 21, 13047-13057
R. Ruess, S. Scarabino, A. Ringleb, K. Nonomura, N. Vlachopoulos, A. Hagfeldt, G. Wittstock, D. Schlettwein
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´Modeling of Dendrite Formation as a Consequence of Diffusion-Limited Electrodeposition´ J. Electrochem. Soc., 2019, 166, D3182-D3189
C. Lupo, D. Schlettwein
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'Effect of TiO2 photoanodes morphology and dye structure on dye-regeneration kinetics investigated by scanning electrochemical microscopy' Electrochem. 2020, 1, 329– 343
S. Scarabino, K. Nonomura, N. Vlachopoulos, A. Hagfeldt, G. Wittstock
