Carbazol-phenyl-phenothiazine-based sensitizers for dye-sensitized solar cells

Heteroaromatic units are commonly used as pi-spacers for sensitizers in dye-sensitized solar cells (DSSCs). Here, we investigated the influence of the pi-spacer on the photophysical properties, electrochemical properties and photovoltaic performance of carbazol-phenyl functionalized phenothiazine donor based organic dyes, i.e., CPPC-No (without a pi-spacer), CPPC-Th (thiophene as the pi-spacer), CPPC-Fu (furane as the pi-spacer), CPPC-Ph (phenyl as the pi-spacer) and CPPC-Py (pyridine as the pi-spacer). The solar to electric power conversion efficiency (PCE) achieved with these dyes strongly depends on the type of pi-spacer, increasing in the order of CPPC-Ph < No < Py < Th < Fu. The width of the absorption band and the photocurrent density increased with the electron richness of the pi-spacer. Theoretical calculations indicated that the planar molecular configuration of CPPC-Fu promotes the intramolecular charge transfer process. As a result, the CPPC-Fu based device showed a PCE of 7.4% under one sunlight illumination. The dyes using thiophene and pyridine as pi-spacers showed slightly lower performance, however, still significantly higher than the PCE achieved with the control compound lacking a pi-spacer. By contrast, introducing a phenyl group as the pi-spacer led to a decrease in device efficiency. These results give deeper insight into the effect of different pi-spacers on DSSC performance, showing the importance of molecular engineering in developing high efficiency organic dyes.