Electron injection in TiO2 films and quasi-solid state solar cells sensitized with a dipolar fluorene organic dye

In the present work, a contribution to the understanding of electron injection dynamics in fluorene dye sensitized TiO2 nanocrystalline films and complete dye sensitized solar cells (DSSCs) with a quasi-solid state electrolyte is attempted. This is realized using time resolved fluorescence up conversion spectroscopy with various detection wavelengths in femtosecond and picosecond timescales. A dipolar fluorene organic dye containing diarylamine and 2-cyanoacrylic acid as electron donor and acceptor respectively is used as sensitizer. For determining the electron injection efficiency, similar measurements have been taken in sensitized films as well as in cells fabricated with Al2O3 nanocrystalline substrates instead of TiO2. In addition, fluorescence dynamics of the organic sensitizer in solutions and in neat films are also studied to determine the excited state lifetimes. The time resolved fluorescence measurements in TiO2 films have shown that electron injection dynamics depend on the detection wavelength, having an average lifetime ranging from 0.16 to 3.0 ps. Electron injection dynamics in TiO2 quasi-solid solar cells show retardation in comparison to films, ranging from 0.32 to 75 ps, also depending on the detection wavelength. The electron injection efficiency from the dye to the TiO2 conduction band becomes maximum at intermediate detection wavelengths, being >0.90 in TiO2 films while it reduces significantly in TiO2 solar cells, reaching a value of 0.75.

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► Electron injection in organic dye sensitized TiO2 films and cells is studied.
► Femtosecond upconversion spectroscopy at various emission wavelengths is used.
► A quasi solid-state electrolyte is used.
► Slower dynamics and lower efficiency of injection is found in cells than in films.
► Higher injection efficiency is found at intermediate emission wavelengths.