Injection limitations in a series of porphyrin dye-sensitized solar cells
We report electron injection dynamics for a series of porphyrin sensitized nanocrystalline TiO2 films, comparing zinc and free base porphyrins with a conjugated or nonconjugated linker group to the carboxylate binding group. Injection dynamics are measured used time correlated single photon counting, using dye sensitized ZrO2 control films. These injection dynamics are correlated with molecular orbital calculations, electrochemical data and device photocurrent efficiencies. The injection dynamics, and overall injection efficiency is found to be increased by linker conjugation and by the use of a zinc central metal. The faster injection dynamics for the Zinc porphyrins is shown to be quantitative agreement with the higher singlet excited state energy of these dyes compared to free base porphyrins. For the most efficient dye studied, addition of a typical redox electrolyte to the dye sensitized film is observed to retard the injection dynamics. Moreover studies of sensitized ZrO2 control films indicated that the redox electrolyte resulted in a reduction of excited state lifetime, indicative of an additional decay pathway competing with electron injection. Overall, a close correlation is found between electron injection dynamics and photocurrent efficiency for this series of porphyrin sensitized solar cells, indicating that for such sensitizer dyes, electron injection is a key factor limiting device performance.