Diverging surface reactions at TiO₂- or ZnO-based photoanodes in dye-sensitized solar cells

Abstract

Surface reactions of electrolyte additives and consequences for cell properties are studied and assigned to characteristics specific for both semiconductors.

Efficient Electron Collection by Electrodeposited ZnO in Dye-Sensitized Solar Cells with TEMPO+/0 as the Redox Mediator

“…In the bulk material, both band edges are situated at −0.5 V versus SHE . This position can be assumed to be the same as in contact to the I 3 – /I – reference electrolyte because in this case no shift was observed relative to an inert electrolyte …”

“…Because both the TEMPO +/0 and the I 3 – /I – redox mediator solutions contain the same concentration of additives and the same solvent, it can be concluded that the TEMPO +/0 redox couple is responsible for this downward shift of the CB edge of ZnO. It has been observed in preliminary work that adsorbed cations induce a significant downward shift of the ZnO CB energy. ,, In TiO 2 -based DSSCs, such shifts are not observed if alkaline molecules are present in the redox electrolyte solution, such as the presently used MBI . These molecules control the amount of surface-adsorbed cations that would otherwise stabilize energy levels also in TiO 2 . , Because the surface of ZnO is alkaline by itself, the control of surface-adsorbed cations by alkaline molecules is mitigated .…”

“…It has been observed in preliminary work that adsorbed cations induce a significant downward shift of the ZnO CB energy. ,, In TiO 2 -based DSSCs, such shifts are not observed if alkaline molecules are present in the redox electrolyte solution, such as the presently used MBI . These molecules control the amount of surface-adsorbed cations that would otherwise stabilize energy levels also in TiO 2 . , Because the surface of ZnO is alkaline by itself, the control of surface-adsorbed cations by alkaline molecules is mitigated . Therefore, we conclude that TEMPO + cations adsorb onto the ZnO surface, stabilize the electronic levels, consistent with changes in the trap distribution, and lead to the observed downward shift of the CB (Figure a), while on TiO 2 , the adsorption of TEMPO + is hindered by the MBI.…”

Effect of TiO2 Photoanodes Morphology and Dye Structure on Dye-Regeneration Kinetics Investigated by Scanning Electrochemical Microscopy

“…The setup for scanning electrochemical microscopy has been detailed before [32,34,42] and was run by the program SECMx [43]. The setup is shown in Figure 3 and included a piezoelectric actuator and the control electronics (P780.20 and E665, Physik Instrumente, Karlsruhe, Germany) for vertical approach, a digital potentiostat (Ref600, Gamry Instruments, Warminster, PA, USA), three SPI motors (SPI Robot, Oppenheim, Germany) to coarsely move the microelectrode (ME) relative to the sample and the light beam, and a two-dimensional positioning system (Nexcact N661.21A with controller E681, Physik Instrumente) for moving the sample horizontally at a fixed relative position between the microelectrode and the light beam.…”

“…In this regard, scanning electrochemical microscopy (SECM) provides an evaluation of the dye regeneration rate constant (k ox ) for process 4 in Figure 1, that measures the ability of the mediator to reduce the photo-oxidized dye adsorbed on the photoanode [29,31,32]. In this work, SECM analysis is applied to passivated TiO 2 photoanodes sensitized with the all-organic dye molecules D358 or DN216 (Figure 2) [33,34]. These molecules have an aliphatic chain terminated by a carboxylic group that attaches it to the semiconductor surface.…”