Electrodeposited ZnO with squaraine sentisizers as photoactive anode of DSCs

The adsorption behavior of two symmetrical indolenine-based squaraines, indicated with VG1-C2 and VG1-C10, sensitizing electrodeposited mesoporous zinc oxide (ZnO), was studied and compared with that of di-tetrabutylammonium cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium (II) (N719). The choice of squaraines as metal-free dye-sensitizers was motivated by their far-red NIR sensitivity with respect to the traditional Ru complex-based dyes, the ease of their preparation and their higher molar extinction coefficient. The electrochemically grown ZnO here described were porous due to the high volume yield of electrodeposition (about 10⁴ cm³ of ZnO per unit charge of electrolysis). In the present analysis the process of sensitization of TiO₂ with the same set of dyes and the performance of the corresponding DSCs were also considered for sake of comparison. VG1-C2 sensitizer proved to be particularly effective electron injector in ZnO electrodes giving larger photocurrents in VG1-C2-sensitized ZnO with respect to TiO₂. The latter system presented higher kinetic stability of the photoinjected charges as evidenced by the larger photovoltages of TiO₂-based DSCs with respect to ZnO-based devices with the same sensitizer. The long alkyl substituents in squaraine VG1-C10 inhibit electron injection in ZnO and a specific effect of electrical passivation of the ZnO surface introduced by the bulky apolar groups was claimed. Overall efficiencies in the order of 1% were measured with the ZnO-based DSCs under AM 1.5 solar simulator when the photoactive area and the thickness of ZnO films were 0.5 cm² and 2 μm, respectively. These results on ZnO-based DSCs with the oxide prepared at temperatures below 260 °C are particularly interesting considering the relatively large area and small thickness of the related electrodes, which makes them in principle useful for application in weakly absorbing devices utilizing flexible substrates. (papers)