Patterns of efficiency and degradation in dye sensitization solar cells measured with imaging techniques

A larger number of dye sensitization solar cells based on cis-Ru^II(LH₂)₂(NCS)₂ with LH₂=2,2′-bipyridyl-4,4′-dicarboxylic acid with an electrolyte consisting of 0.5 M LiI, 50 mM I₂, 0.2 M tert.-butyl pyridine in acetonitrile have been studied, using spatially resolved photocurrent imaging techniques. Measurements have been made after preparation and periodically during a longer period of simulated solar light illumination. The observed phenomena have been grouped into five categories. The first one concerns significant inhomogeneities reflecting the TiO₂-layer preparation technique used. The second category concerns an inhomogeneous deterioration of the dye sensitization cell during illumination. The third phenomenon involves photodegradation itself, which can be visualized by selectively illuminating the dye sensitization solar cell. Changes observed in the composition of the electrolyte, typically indicated by a bleaching of the iodide/iodine solution were also observed. Finally, the fifth category to be considered deals with a loss of electrolyte and the parallel appearance of gas bubbles in the solar cell. All these phenomena may coexist, being responsible for the overall process of degradation. The different mechanisms are discussed and analyzed in an effort to determine parameters critical for increasing efficiency and stability of dye sensitization solar cells.