The CELLO technique allows to measure local data for a large number of solar cell parameters like bulk life time, back side surface recombination velocity, diffusion coefficient, and serial resistance. Inhomogeneities of these parameters induce lateral current flow in forward direction, which mixes the effects of the different defect types. Even if all local parameters would be known, a complete simulation is needed to obtain information about the local efficiency. Direct CELLO measurement at e.g. the working of the solar cell or at open circuit allows extracting this combined information directly. Examples for this will be presented in this paper, dealing with multicrystalline silicon solar cells showing a variety of defect types.
Mainly to save measurement time several frequencies are applied simultaneously as a Laser intensity perturbation signal and analyzed by FFT deconvolution. The frequencies can be applied simultaneously to 3 confocal Lasers with different wavelength, i.e. penetration depths. Typically 4 frequencies are analyzed simultaneusly in order to maintain a good signal to noise ratio. This features will be discussed shortly as well. Especially the application of a deep penetrating Laser significantly increases the robustness of the fitting procedure and allows to separate reliably bulk and surface recombination and mobility dependencies (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)