Abstract
Efficient dye-sensitized solar cells are based on highly diffusive mesoscopic layers that render these devices opaque and unsuitable for ultrafast transient absorption spectroscopy measurements in transmission mode. We developed a novel sub-200 femtosecond time-resolved diffuse reflectance spectroscopy scheme combined with potentiostatic control to study various solar cells in fully operational condition. We studied performance optimized devices based on liquid redox electrolytes and opaque TiO2 films, as well as other morphologies, such as TiO2 fibers and nanotubes. Charge injection from the Z907 dye in all TiO2 morphologies was observed to take place in the sub-200 fs time scale. The kinetics of electron-hole back recombination has features in the picosecond to nanosecond time scale. This observation is significantly different from what was reported in the literature where the electron-hole back recombination for transparent films of small particles is generally accepted to occur on a longer time scale of microseconds. The kinetics of the ultrafast electron injection remained unchanged for voltages between +500 mV and –690 mV, where the injection yield eventually drops steeply. The primary charge separation in Y123 organic dye based devices was clearly slower occurring in two picoseconds and no kinetic component on the shorter femtosecond time scale was recorded.
Highlights
Model systems based on a transparent TiO2 thin film sensitized with various dyes and semiconductors in different environments were investigated so far[3,4,5,6,7]
The resulting light transmittance of the cell is less than 15% in the visible region and, conventional transient absorption spectroscopy in transmission mode cannot be applied in this case
We aim here to investigate the dynamics of charge carriers directly in fully functional devices, using potential control and state-of-the-art pump-probe diffuse reflectance spectroscopy
Summary
Model systems based on a transparent TiO2 thin film sensitized with various dyes and semiconductors in different environments (solid samples or in solution) were investigated so far[3,4,5,6,7] It should, be noted that the most efficient liquid-based solar cell devices are not transparent. We studied samples of anodized nanotubes on Ti foil[14,15] and nanostructured fibers[16,17] These samples have exhibited promising behavior in cell performance but are not optically transparent and are not suitable for investigation with pump-probe transmission based transient absorption technique. While this classifies as ultrafast, it is about one order of magnitude slower than for Ru-based dyes, which was measured to have features in femtosecond time scale
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