Femtosecond optical pump-terahertz probe studies are performed on the ultrafast dynamical processes of photo-excited charge carriers in silicon solar cells with various nanostructured surfaces and in ternary organic solar cells. The pump-probe measurements provide a direct insight on the lifetime of photo-excited carriers and the charge transfer processes in both inorganic silicon solar cells and conjugated polymer based organic solar cells. In silicon solar cell, lifetime of photo-excited carries longer than tens of nanoseconds is identified from the pump-probe measurement. Moreover, the photoconductivity, which describes the charge transfer process, of silicon solar cells increases with the introduction of micron length nanowires on the surface. We identified that the surface defects induced by micron length nanowires enhances the coupling between electrons and the terahertz probe field via changing the momentum of electrons to fulfil momentum conservation. In contrast to the long lifetime of photo-excited carriers in silicon solar cells, few hundreds of femtoseconds lifetime of photoexcited free carriers and few picoseconds lifetime of excitons in organic solar cells are obtained from our ultrafast spectroscopy.
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