Molecular Engineering of Zinc Phthalocyanine Sensitizers for Efficient Dye-Sensitized Solar Cell

Dye-sensitized solar cells (DSSCs) have been expected as low-cost, lightweight, and flexible energy conversion devices. To date, the highest conversion efficiency above 12 % has been achieved with p-conjugated macrocyclic zinc porphyrin sensitizer YD2-o-C8 using cobalt polypyridine redox shuttles. Designed p-conjugated macrocycles such as porphyrins and phthalocyanines have attracted special attention as versatile molecular platforms of highly efficient dyes for DSSCs. While a lot of porphyrin-based sensitizers have been designed and synthesized to enhance the conversion efficiency in DSSCs, the efficiencies of DSSCs employing red/near-IR absorbing phthalocyanine-based sensitizers had not been impressive due to their strong tendency to aggregate and the lack of directionality of electron transfer in the excited states. To overcome these drawbacks of phthalocyanine-based sensitizers, several approaches such as steric suppression of aggregation, electronic push-pull structures through asymmetrical substitutions, and optimization of adsorption sites have been reported. Recently, we showed that the structural modification of peripheral bulky substituents and adsorption site around the phthalocyanine core induced the improvement of incident photon-to-current efficiency (IPCE), and thereby achieved a solar-to-electric power-conversion efficiency (PCE) of 5.9% from solar cells employing PcS18 under one sun condition. However, while the maximum value of IPCE was more than 80 %, the values around 520 nm was about 30 %. Since the absorption coefficient of the sensitizers at the wavelength is very low, in order to increase the efficiency, higher dye adsorption density is desired. This can be done by decreasing the molecular size. In addition to IPCE spectrum, open circuit voltage of the PcS18 cell was not high. This could be improved by modifying the size of peripheral substituents attached to the phthalocyanine core to cover the TiO₂ surface by the dye molecules. In this presentation, we examine the effect of the length and number of alkoxy groups around the phthalocyanine moiety on the photovoltaic properties. We now expect the alkoxy groups to have three functions, preventing aggregation, filling the space among dyes on TiO₂ surface and increasing dye adsorption density. We found that an asymmetrical zinc phthalocyanine (ZnPc) PcS20 bearing propoxy groups in the 2 and 6 positions of peripheral phenoxy units showed a record PCE value of 6.4 % under simulated air mass 1.5 global sunlight.