PHOTOVOLTAIC EFFECT OF CHLOROPHYLL a–QUINONE SYSTEMS IN MULTILAYER ARRAYS

— Photovoltaic cells made of an array of chlorophyll a (Chi a) monolayers between an aluminum and a silver electrode have been analyzed. These cells are characterized by charge carrier production due to the dissociation of singlet excitons. The exciton diffusion length λ∼ 300 ± 100 A. The optimum thickness of these cells consists of an array of 44 monolayers for which a power conversion efficiency η= 0.038% and a quantum yield o= 0.49% has been measured at 678 nm using an incident light intensity of 0.3 W/m2. In these cells, about 60% of the collected charges are generated by the exciton dissociation in the bulk of the semiconductor and 40% by the exciton dissociation at the aluminum electrode. The behaviour of Chl a photovoltaic cells doped with two quinones has also been analyzed. The two quinones are vitamin K1 and N,N-distearoyl-1,4-diaminoanthraquinone (SAQ). The photovoltaic properties of Chi a remain practically unchanged when quinone is not in the same monolayer as Chi a. But when Chi a and quinone are in the same monolayer, up to a molecular ratio of 1:0.3, there is a drastic decrease in the efficiencies and quantum yields of the cells. The photovoltaic behaviour varies in parallel with the extinction of Chi a fluorescence by the quinones in contrast with what is reported for strong electron acceptors adsorbed on photoconductors. An electron transfer from the singlet state of Chi a to the quinones is proposed on the basis of the variation in the quantum yield for current production with the energy of the incident photon.