Abstract
A series of porphyrin compounds, free base (H2P) and their Zn (II) metallated analogues (ZnP), bearing one, two or three carboxylic acid groups, have been synthesized, characterized, and used as sensitizers in dye sensitized solar cells (DSSCs). The performance of these devices has been analyzed, showing higher efficiencies of those sensitized with ZnP compounds. These results have been explained, on one hand, taking into account the electronic character of the metal ion, which acts as mediator in the injection step, and, on the other, considering the number of anchoring groups, which determines both the stereoelectronic character of the dye and the way it binds to TiO2 surface.
Highlights
Porphyrins [1] have been extensively used as key components in different organic optoelectronic applications
There is a general agreement on the structural requirements of porphyrins to be used in dye sensitized solar cells (DSSCs): 1) at least one anchoring group must be present for the covalent binding to the semiconductor surface [9], 2) metallic complexes (MP), especially the zinc ones, are preferred to free-base (H2 P), because of their longer-lived singlet excited states and much lower oxidation potentials [10], and 3) bulky electron-donor meso-substituents favor electron injection in the semiconductor, as they originate an intrinsic dipole moment [11]
Free base porphyrin compounds H2 P-CO2 H 1, H2 P-(CO2 H)2 2 and H2 P-(CO2 H)3 3 were prepared through a two-step synthetic sequence, as it is described in Scheme 1
Summary
Porphyrins [1] have been extensively used as key components in different organic optoelectronic applications. Thanks to their aromatic nature, porphyrins present interesting light absorption/emission and electroactive properties, tunable due to their chemical versatility [2,3]. With these elements, it is possible to find in the literature a myriad of different porphyrin compounds, chemically designed to fulfill technological requirements in a variety of fields [4,5]. High efficiencies have been achieved for TiO2 -based devices sensitized, for example, with free-base [12] and zinc porphyrin derivatives [13], presenting one or more carboxylic acid appends as anchoring groups, either in β [9,14]
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