Monolayer Studies of 5‐(4‐carb‐ Oxyphenyl)‐10,15,20‐tritolyl‐porphyrin‐ii. Photovoltaic Study of Multilayer Sandwich Cells*

The photovoltaic behavior of multilayer sandwich cells fabricated from the title compound has been examined. These multilayers are amorphous and contain a wide range of aggregated species. The action spectrum of the photocurrent agrees qualitatively with the absorption spectrum. This indicates that the photocurrent arises from all species in the film. The shape of the dark current‐voltage characteristics varied with the porphyrin thickness. For thin films, (1 and 3 monolayers) the current can be described by a tunnelling mechanism. For thick cells (29 monolayers) the current‐voltage characteristic can be fitted using either of two models: a modified Shockley equation (suggesting that the dye acts as a p‐type semiconductor) or a combination of Schottky and Frenkel–Poole emission (suggesting that the porphyrin acts as an insulator). The former interpretation is preferred because it is consistent with the observed rectification characteristics; nevertheless, the latter is in agreement with the magnitude of the conductivity of the films. The power‐conversion efficiencies of these cells were quite low (∼4 × 10‐4% for incident white light of intensity 2.72 W m‐2) owing to the very large internal resistance of the cells. The high resistance can be explained by observing that the molecules are in a very unfavorable orientation; maximum conductivity would be expected for the porphyrin ring systems perpendicular to the direction of current flow to give maximum overlap of the systems.