Donor-Substituted β-functionalized porphyrin dyes on hierarchically structured mesoporous TiO₂ spheres. Highly efficient dye-sensitized solar cells

Novel zinc porphyrin dyes for use in dye-sensitized solar cells (DSSCs) have been synthesized. These dyes are based on a molecular design that relies on donorπ-acceptor interactions, a concept implemented by introducing a bis(4-tert-butylphenyl)amino group at the meso position of the porphyrin opposite to what are 2-propenoic or 2,4-pentadienoic acid anchoring groups at the β-pyrrolic positions. Incorporating an electron-donating group (i.e., the diarylamine) on the porphyrin core serves as the considerable electronic coupling between the donor site and porphyrin core, and hence, the HOMO-LUMO energy gap is decreased. This change is reflected in the remarkable red shift and broadening of the absorption spectra relative to an unfunctionalized parent system. This substitution, in conjunction with functionalization with carboxylic acid moieties on the β-pyrrolic positions, also provides what is an effectively aligned donorπ-acceptor dipolar architecture. This, in turn, gives rise to advantageous charge-transfer properties, including what are significant improvements of the electron injection efficiency on titanium oxide (TiO₂) compared to our previous models without a donor substituent. The DSSCs of this study were composed of zero-dimensional hierarchical structured TiO₂ spheres with a diameter of 600-800 nm prepared from P-25 and anatase TiO₂ nanoparticles, which functioned as the photoelectrodes. With the anatase TiO₂-based DSSCs, the power conversion efficiencies (η) as well as the photocurrent action spectra were relatively enhanced, an effect ascribed to the characteristic mesoporous effect and associated electrophysical properties of the anatase TiO₂ spheres. Among the dyes prepared in the context of the present study, the doubly functionalized carboxylic acid derivative, tda-2b-bd-Zn, gave rise to the highest power conversion. The η value was 7.47%, and the maximum incident photon-to-current efficiency was 77.3% at the Soret band. The overall η value of tda-2b-bd-Zn is comparable to the performance of typical ruthenium-based dyes, such as N3 (η = 7.68%), under the same conditions.