Electrodeposited ZnO-Based, Organic-Dye Sensitized Solar Cells: Advantages and Limitations

Abstract

The dye-sensitized solar cell (DSSC) is one of the most promising third generation photovoltaic technologies, which nowadays shows conversion efficiencies as high as 14 %. The attractive qualities of DSSCs include promising efficiencies, but also the low cost projected for their large-scale production for niche applications, in particular, for semi-transparent windows and indoor lighting conditions. This implies the optimization of all processing steps and components used, and a better understanding of the mechanisms of generation and loss of electrons involved in the solar cell functioning. Based on these observations, we have selected ZnO as the electron transporting material, not only for its much better electrical properties (when comparing with the commonly used TiO2), but also for the simple synthesis methods by which it can be obtained, such as electrodeposition (a low temperature an scalable technique), which offers to move toward the fabrication of more economically profitable devices. Hence, part of this work presents a detailed study on the effect of the bath composition, additives and deposition conditions used on the morphological and structural features of materials electrodeposited from ZnCl2 and Zn(NO3)2 solutions. The combination of this material with organic dyes with high absorption coefficient in the solar spectrum regime, promise an increase in cell current and voltage. For that reason, we have prepared solar cells with nanostructured, mesoporous ZnO films electrodeposited from an optimized aqueous ZnCl2 solution, an organic fluorenyl-thiophene dye (OD-8) as sensitizer, and an electrolyte solution with either the I-/I3 - or the [Co(2,2'-bipyridyl)3]2+/3+ redox couple. An improvement in efficiency may be expected, related to the more positive redox potential of the [Co(2,2'-bipyridyl)3]2+/3+ couple and faster dye regeneration kinetics, but this can only be achieved if the recombination rate at the semiconductor/electrolyte interface is sufficiently slow. Hence, we have evaluated the effect of CDCA (chenodeoxycholic acid) as coabsorbent in the dye solution, with the purpose to diminish the amount of uncovered area of the electrode surface, in order to minimize recombination. We use I-V curves in combination with intensity-modulated photovoltage spectroscopy, charge extraction measurements, and surface photovoltage spectroscopy to gain insight in the factors involved into the photochemical charge separation in ZnO-based organic-DSSCs.