Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Here we report on the use time-resolved diffuse reflectance spectroscopy as a powerful technique to scrutinize the dynamics of electron transfer in opaque and highly absorbing materials, for which conventional transient absorption spectroscopy in transmittance mode cannot be applied.
For the first time, femtosecond time-resolved diffuse reflectance spectroscopy combined with potentiostatic control is applied to study the kinetics of electron injection in optimized, fully operational solar cells based on opaque TiO2 films and liquid redox electrolytes. An optical-electronic model based on Kubelka-Munk1 theory is proposed for light propagation in opaque DSCs and quantified analysis of transient species in opaque DSCs under bias voltage. The dynamics of charge separation and recombination in devices based on new opaque TiO2 nanostructures, TiO2 scattering particles, anodized nanotubes2 and TiO2 nanofibers3 are studied. An early charge recombination in the presence of the electrolyte is observed in 400 nm-diameter scattering TiO2 particles, TiO2 fibers and anodized TiO2 nanotubes, with different amplitudes. This feature was not observed for small particles and is rationalized in terms of different electron mobilities and trapping states in TiO2 film. The kinetics of ultrafast electron injection from the dye excited state into the oxide remains unchanged for applied biases between +500 mV and –690 mV, until the injection process is turned off.
One other good example of highly absorbing DSCs is constituted by the new emerging highly absorbing perovskite heterojunction solar cells. The (CH3NH3)PbI3 perovskite under study is characterized by a dark brown colour with absorption shoulder around 780 nm. The material is opaque, and the photocarrier dynamics can thus be monitored in diffuse reflectance mode. Some preliminary results on charge separation in highly absorbing perovskite based solar cells prepared by the sequential deposition4 technique is reported. Carrier dyamics observed in samples of (CH3NH3)PbI3 perovskite deposited either on TiO2 or Al2O3 films, with and without coating by the hole transporter material spiro‐ometad are compared.
1.Kortüm, G. Reflectance spectroscopy. (Springer- Verlag: New York, (1969). 2.Mohammadpour, R. et.al. Comparison of Trap-state Distribution and Carrier Transport in Nanotubular and Nanoparticulate TiO2 Electrodes for Dye-Sensitized Solar Cells. ChemPhysChem 11, 2140–2145 (2010). 3.Ghadiri, E. et.al. Enhanced electron collection efficiency in dye-sensitized solar cells based on nanostructured TiO2 hollow fibers. Nano Lett. 10, 1632–1638 (2010). 4.Burschka, J. et.al. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 499, 316–319 (2013).
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