Photovoltaic and spectroscopic studies of selected halogenated porphyrins for their application in organic solar cells

Three porphyrin dyes were investigated: basic tetraphenylporphyrin (TPP), fluorine tetraphenylporphyrin symmetrically substituted with twelve fluorine atoms (TPPF12) and chlorine tetraphenylporphyrin symmetrically substituted with twelve chlorine atoms (TPP Cl12). TPP without external atoms was used as a standard sample. Photoelectrochemical examinations (short-circuit photocurrent in the second time scale) combined with the spectroscopic methods (absorption, fluorescence, photoacoustics supported by the laser-induced optoacoustic spectroscopy) were applied. On the basis of absorption and fluorescence spectra some spectral parameters were estimated, whereas with photoacoustic methods the thermal parameters and kinetics of thermal relaxation in the microsecond time scale were evaluated. The studies of the dyes in the range from about 10−6 M up to 10−3 M show the absence of aggregated structures although dye fluorescence behavior could indicate the presence of weakly interacting dye molecules when the dyes are in their excited states. The results show that the changes in fluorescence behavior indicate redistribution of the electron density in the molecular macroring upon linkage of the fluorine and chlorine atoms to the main molecular core. The results of the photocurrent and spectral examinations were interpreted in terms of occurrence of mesomeric and inductive effects; photocurrent enhancement in the chlorinated porphyrin (TPPCl12) versus the free-based (TPP) and fluorinated (TPPF12) porphyrins support the occurrence of the mesomeric effect which dominates over the inductive one in TPPCl12, whereas in TPPF12 the inductive effect is essential and can lead to photocurrent declining.