Professor of Applied Physics
Departament de Ciències Experimentals
Universitat Jaume I
12071 Castelló de la Plana
bisquert@uji.es
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| Introduction | ||||
In all solar cell, the ligth generates excess carriers that have a probability to meet their conjugate carriers and recombine. Recombination means a dissipation of the free energy that was provided by the photons to heat into the solar cell. In solar cell devices recombination reduces the output of electrical power and it is a priority to keep the recombination processes at the minimum level compatible with the demands of the particular technology. |
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The electron transfer from TiO2 nanoparticles into electrolyte species is a complex phenomenon because it involves a combination of processes [J. Phys. Chem. B 106, 8774 (2002)]. First, there is a variety of electronic states from which electrons can be injected, both conduction band states and bandgap surface states that exhibit a wide distribution in energy. Second, electrolyte species are subjected to thermal fluctuation and in effect show a gaussian distribution as described in the Marcus model for charge transfer. |
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| Open-circuit voltage (Voc) decay | ||||
The Voc decay is a rather simple measurement technique dessigned in collaboration with Arie Zaban and co-workers [ChemPhysChem, 4, 859 (2003)] for studying the recombination processes in DSSC. When the illumination on a DSSC is interrupted, at open circuit, the excess electrons are removed by recombination only, and the rapidity of the decay of the photovoltage is directly related to the electron lifetime by the expression Later on it was shown with a detailed model [J. Am. Chem. Soc. 126, 13550 (2004)] that the Voc decay displays information on different regimes of recombination, as indicated in the following figure.
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Comparison of Voc decay measurement to the model.
A constant lifetime at high photovoltage is related to free electrons. An exponential increase due to internal trapping and detrapping is observed in the intermediate regime. At low photovoltage the lifetime decreases and again increases revealing the Marcus inverted region.
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The same type of physical model was applied to describe the steady state characteristics of the DSSC, i.e. the photovoltage dependence on illumination intensity, by Pedro Salvador et al. [J. Phys. Chem. B, 109, in press (2005)] |
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