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Atomistic effects in the Grätzel solar cell

J. Korppi-Tommola, J. Helbing, P. Hamm, L. Antila, K. Honkala, V. Koskinen, M. Kemell, M. Heikkilä, M. Leskelä, L. Kettunen, T. Kovanen
University of Jyvaskyla, FI

Keywords: DSSC, nanoparticle, TiO2, ALD, DFT, EIS


Millenium Prize 2010 was given to professor Michael Grätzel for his invention of the dye sensitized solar cell. Together with polymer solar cells the dye solar cells have a promise for excellent price to performance ratio in the emerging PV market. An intensive international research effort is going on to unravel the molecular level events that control the function of the dye cell [1]. We report new results on 1) the role of atomic layer deposition (ALD) coated barrier layers on nanocrystalline TiO2 electrodes in reduction of leak reactions of the dye cell [2] 2) role of the sensitizer dye, its binding geometries and dynamics in particular by two dimensional infrared spectroscopy (2DIR) [3] 3) from modeling of chemical composition in electric current generation of the cell [4]. Already a single ALD cycle of Al2O3 on the active electrode reduced recombination reactions but at the same time forward electron injection was more strongly affected and the over all performance of the cell reduced. State-of-the-art DFT calculations indicated atomic scale roughness of the first ALD layer with holes large enough to allow the oxidant I3- but not the sensitizer (Fig.1, left) to reach the underlying semiconductor. Two sensitizer binding geometries and chemical exchange between the two (Fig. 1 right) was observed in solvent filled nano-cavities of the active electrode in 2DIR experiments and in computer simulations. Current generation on the active electrode according to the developed chemical composition model is subject to hysteresis under rapidly changing illumination conditions. It is concluded that sensitizer binding geometries and flexibility, on the semiconductor surface are important aspects of dye solar cell efficiency.
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