Mesoscopic titania solar cells with the tris(1,10-phenanthroline)cobalt redox shuttle: uniped versus biped organic dyes

The combination of some non-iodine one-electron outer-sphere redox shuttles with high-absorption-coefficient organic D-pi-A dyes has very recently brought forth a new opportunity towards efficiency enhancement of dye-sensitized solar cells (DSCs). Thereby a comprehensive understanding on the structure-property relationship of metal-free organic photosensitizers will play a pivotal role in further development of high-efficiency devices iodine-free. Herein, we scrutinize some general influences of the cyanoacrylic-acid-anchoring-group number of organic dyes upon the optoelectronic features of mesoscopic titania solar cells based on the tris(1,10-phenanthroline)cobalt(II/III) redox shuttle, by employing three pairs of uniped and biped dyes with thiophene, 3,4-ethylenedioxythiophene and 4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b']dithiophene as the pi-conjugated spacers. It is found that regardless of the spacer selection, a biped dye confers an improved light absorption coefficient of a stained titania film in comparison with its uniped analogue, which is highly desirable for cells with a one-electron outer-sphere redox shuttle, because in this type of cells a very thin titania film is always needed to satisfy the mass transport and electron collection requirements. However, our electrical impedance analyses have revealed that the alteration from a uniped chromophore to its biped congener evokes not only a downward displacement of the titania conduction band edge but also an acceleration of interfacial charge recombination of the titania electrons with cobalt(III) ions, leading to an attenuated open-circuit photovoltage, highlighting the future necessity to design a sterically bulky organic dye featuring superior light-harvesting capacity.