Photovoltaic properties of Aundash;merocyaninendash;TiO2sandwich cells. II. Properties of illuminated cells and effects of doping with electron acceptors

摘要

Photocurrent generation in thin films of a merocyanine photosensitizing dye sandwiched between a TiO2single crystal dopedntype and an Au overlayer has been studied using photovoltaic techniques. A theoretical model was developed to explain the observed photovoltaic properties. The model assumes that the principal route for the formation of charge carriers is via singlet excitons diffusing to the merocyaninendash;TiO2interface followed by dissociation of the excitons into electronndash;hole pairs, the electrons being injected into the TiO2conduction band and the holes into the merocyanine. The model also incorporates field dependence of the quantum efficiency for charge generation. An exciton diffusion length of 79 Aring; was determined by analyzing the short circuit action spectra using the theoretical model developed. The low fill factor of 0.35 for these cells was attributed to the field dependence of the quantum efficiency and the high series resistance of the undoped merocyanine films. Doping the merocyanine films with iodine was found to increase both the dark conductivity and the steady state photoconductivity, the latter by as much as a factor of 5. This resulted in a quantum yield of 12percnt; for a 500 Aring; thick film and an increase in the fill factor to 0.44 giving a monochromatic power conversion efficiency of 0.4percnt; at 520 nm. The carrier generation in iodine doped films is shown to result from a bulk process, possibly involving collisions between singlet excitons and acceptorndash;hole complexes resulting in activation out of the bound states formed by the chargehyphen;transfer complex. The quenching of excitons in the immediate vicinity of the metal surface was studied by monitoring the photoconductive response of a 200 Aring; merocyanine film with varying thickness of perylene sandwiched between the metal and the merocyanine. Perylene was shown to be able to transport the photoexcited holes from the merocyanine to the Au electrode. The quantum efficiency for photocarrier production increased to a maximum of 21percnt; for a 750 Aring; thick perylene layer.