Photovoltaic and electrical properties of Al/Langmuir-Blodgett films/Ag sandwich cells incorporating either chlorophyll a, chlorophyll b, or zinc porphyrin derivative

The photovoltaic properties of sandwich cells of Langmuir-Blodgett films of chlorophyll a, chlorophyll b, and zinc porphyrin derivative have been measured. The rectifying devices were prepared by interposing the pigment multilayers between two semitransparent metal electrodes, i.e., aluminum and silver with work functions (φ) such that φ_Al < φ_pigment < φ_Ag. All sandwich cells exhibit small short-circuit photocurrents and large open-circuit photovoltages (as high as 1.1 V). The low-power conversion efficiency of all three pigments (0.001-0.030%) is related to the large internal resistance of the photovoltaic cells (30 to more than 1000 MΩ). This suggests that the poor photovoltaic properties of the sandwich cells are extrinsic to the molecular structure of pigments and should rather be related to the presence of the insulating layers of aluminum and chrome oxide and of the monolayer of cadmium arachidate. In addition, a theoretical treatment of the action spectra is presented. The multiple internal reflections of light between the semitransparent metal electrodes strongly modify the photovoltaic properties of the sandwich cells. The characterization of the metal/semiconductor junction was carried out with the low-frequency oscillographic method. Large variations in the relations between 1/C² and V were observed as a function of the light intensity, the frequency, and the amplitude of the applied bias. These variations have been related to the presence of the insulating layer which modifies the voltage applied to the junction.