PHOTOCONDUCTIVITY AND PHOTOVOLTAIC EFFECTS IN LANGMUIR‐BLODGETT FILMS OF CHLOROPHYLL‐a

The DC photoconductivity and photovoltaic effect in Langmuir films of Chlorophyll‐a (Chi a) of precisely controlled thickness formed between Al and Au electrodes have been extensively investigated. The dark conductivity and dark voltage are almost abolished in a N2 atmosphere. The action spectrum of the photocurrent closely resembles the monolayer absorption spectrum of Chl indicating that the primary event in photoconduction is the generation of singlet excited states. Thicknessdependence studies on the photoconductivity indicate that carrier generation is a surface process and that the mean diffusion length of the excited state is approximately 20 nm. In short (˜1 s) light exposures the photocurrent always increases linearly with both the applied voltage and the light intensity. In continuous light the current‐voltage characteristics are highly non‐linear and the photocurrent shows a square‐root intensity dependence at high intensities and small applied potentials. These results are interpreted in terms of second‐order recombination and charge‐trapping processes at high carrier densities. The photovoltage usually shows a logarithmic intensity dependence at high intensities and its maximum value in thick films is 800 mV with the Al electrode acquiring a negative polarity. This behaviour, together with some observations on the asymmetry of photocurrent‐voltage characteristics and the effect of substituting an aqueous top electrode for Au, further suggest that the photoactive surface is a p‐n junction between the Chl and Al2O3 layers.