PHOTOVOLTAIC PROPERTIES OF MESO-TETRAARYLPORPHYRINS

Abstract— The photovoltaic properties of meso -tetraphenylporphyrin and its derivatives with various para -substituents, meso-tetrakis(2,4-dimethoxyphenyl)porphyrin and meso -tetrakis (2-fluorenyl)porphyrin have been investigated. The forward dark current-voltage characteristics of Al/porphyrin/Ag cells are attributed to the MIS Schottky barriers consisting of Al/Al₂O₃/porphyrin. The barrier parameters such as the apparent diffusion potential V ₀ the barrier width w₀ and the density of ionized impurity N are estimated by using the capacitor discharge method. The action spectra of photocurrents closely follow the optical absorption spectra of the porphyrin films. The photocurrents vary as i _pα I ^γ, where I is the incident light intensity, and the Sight exponents y range between 0.83 and 1.0. The sublinear difference from unity could be related to the exponential distribution of hole traps in films of the porphyrins. No obvious correlation between the photocurrent and the fluorescence quantum yields of the sublimed porphyrin films is found. The electron donating substituents such as OCH₃ and CH₃ strikingly increase the photocurrents. The photocurrent quantum yields correlate exponentially with the first ring oxidation potentials of the porphyrins and also with the substituent constants for the Hammett linear free-energy relationship. The current quantum yields estimated for the porphyrins studied, range in the order of 10^-4–10^-2 with power conversion efficiencies 10^-7–10^-3.