Evaluation of exciton diffusion lengths and apparent barrier widths for metal/porphyrin Schottky barrier cells by using the optical filtering effect

Abstract

The optical filtering effect due to the porphyrin phase in the Al/porphyrin/Au (Schottky-type) photovoltaic cell is investigated in an attempt to estimate the exciton diffusion length L/sub exc/ and the apparent width of the Schottky barrier W/sub ap/. The observed filtering effects are interpreted in terms of a theoretical model in which the photogenerated charge carriers are attributed to the dissociation of excitons diffusing toward the Schottky barrier. The results show that the quantum efficiency of carrier generation from excitons is not constant over the porphyrin layer but exponential at the Schottky barrier. In other words, either ionized impurities (acceptors or donors) in the porphyrin film are distributed not uniformly but exponentially or the excitons dissociate into free carriers owing to interaction with exponentially distributed impurities in the barrier. The values of L/sub exc/ = 15 +/- 10 and 3 +/- 2 nm estimated for 5,10,15,20-tetraphenylporphyrin (H/sub 2/TPP) and its metalloporphyrin ZnTPP from the proposed model seem reasonable in comparison with the values of 24 and 13 nm, respectively, estimated by fluorescence spectroscopic measurements. In addition, the W/sub ap/ values of 39 +/- 2 and 14 +/- 1 nm obtained for H/sub 2/TPP and ZnTPP correspond fairly well to themore » depletion layer thickness determined by low-frequency capacitance measurements at zero bias.« less