Measurements and evaluation of dye-sensitized solar cell performance

Measurement and evaluation methods for the performance of dye-sensitized solar cells (DSCs), of which the mechanism for photocurrent generation is quite different from that of silicon-type solar cells, are reviewed here and a relevant method proposed. The slow response times and nonlinearity of DSC photocurrents against the light intensity (irradiance) at wavelengths of incident light are profoundly influenced by their characteristic working principles since photocurrent generation for DSCs is more complicated than for Si-type solar cells. DSCs work not only by the physical process of an electron in solid-state TiO2 but also diffusion processes in the fluid electrolytes in contrast to only the simple solid-state physical process of a charge separation at the p-n junction of the interface for Si-type solar cells. In addition, newly developed DSCs are prepared by such elemental materials as sensitizers, electrolytes and semiconductors of diverse morphologies. In this respect, it is essential to establish a comprehensive and relevant method for the correct spectral measurement of the responsivity and performance of a wide range of DSCs which may include cells involving various kinds of electrolytic media. In this review, DSC electrolyte media with such disparate viscosities as a typical organic solvent, 3-methoxypropionitrile (Cell A), or an ionic liquid (Cell B) are introduced and analytical methods such as the AC method is compared with the DC method to gauge spectral responsivity. IPCE measurements were carried out by adjusting the chopping frequency low enough to obtain a steady state current under illumination conditions similar to those under practical use. Our studies revealed that when sufficient time is allowed for complete photocurrent generation, especially for DSCs involving an ionic liquid, I-V measurements which take this time allowance into consideration show them to perform satisfactorily. In fact, in an extreme example, I-V measurements of a DSC with an ionic liquid electrolyte can take over 50 min before correct data can be obtained. Thus, standards for the evaluation of DSCs need to be established separately from those for Si-type solar cells to avoid incorrect and incomplete comparisons. (C) 2012 Elsevier B.V. All rights reserved.