The Impact of the Electrical Nature of the Metal Oxide on the Performance in Dye-Sensitized Solar Cells: New Look at Old Paradigms

Abstract

The process of primary charge separation in Ru-based dye-sensitized solar cells (DSSC) based on both TiO 2 and ZnO photoanodes has been studied in fully working devices by time-resolved measurements, including emission and transient absorption (from femtoseconds to microseconds), and electrochemical techniques (current-voltage characteristics and impedance spectroscopy). By studying the effect of different electrolyte compositions using potentialdetermining additives (Li + ions, 4-tert-butylpiridine) we have been able to provide novel insights into the mechanisms of electron injection and dye regeneration across the oxide/dye/electrolyte system. In this respect, the shift of the conduction band that is commonly reported for TiO 2 in the presence of additives is not observed for ZnO. In addition, both injection and regeneration for ZnO-based cells is shown to be much slower than for TiO 2 and independent of the electrolyte composition. The slower injection and regeneration for ZnO and its lower sensitivity with respect to the addition of potential-determining additives strongly suggests that the electrical nature of the oxide is crucial to facilitate charge-separation across the oxide/dye/electrolyte interface. In addition, electron injection from singlet and triplet states has been identified for both metal oxides. The former process occurs on the ultrafast time scale (<100 fs) while the latter extends over many time scales from single ps to single ns. The present work provides novel insights about the processes of electron injection and dye regeneration in DSSC as well as the reasons for the lower performance of ZnO-based dye solar cells.

Optimizing CuO p-type dye-sensitized solar cells by using a comprehensive electrochemical impedance spectroscopic study

“…In n-type TiO 2 -and ZnO-DSSCs this behavior has widely been noted, 21 but never for p-type NiO-DSSCs. 22 It should be noted that a number of different models, which include additional parameters, such as transmission line and Warburg diffusion elements, have frequently been used for EIS measurements of DSSCs.…”

Organic dyes for the sensitization of nanostructured ZnO photoanodes: effect of the anchoring functions

“…Thus, for the preparation of the sensitized photoanodes later used in DSSCs, the sensitization time was restricted to only one hour, where a shorter period usually to sufficient adsorption of dyes on the mesoporous ZnO electrodes. Bearing in mind the insensitivity of ZnO under the presence of potential determining additives reported recently, 19 the employed electrolyte in this work consists of only a redox pair (0.05M I 2 + 0.5M BMII) in acetonitrile as solvent electrolyte. Figure 4A shows current-voltage characteristics of DSSCs employing a thin film (≈4 μm) of ZnO as photoanode sensitized with the different dyes.…”

“…18,19 In addition, ZnO electrodes show a high instability in the presence of ruthenium dyes, such as N719 or other acidic dyes. At this point it is important to mention the limitations of the application of ZnO electrodes as photoanodes in DSSC.…”

Mass Synthesis in Polyol of Tailored Zinc Oxide Nanoparticles for Photovoltaic Applications

“…The origin of this striking difference in performance is attributed to the factors affecting light absorption and electron injection, as well as the transport and recombination rates in the operating device. Mechanisms of electron injection and dye regeneration across the oxide/dye/electrolyte system have been recently studied by Idígoras et al [22].…”