Optical Pump-Terahertz Probe Studies on Silicon and Organic Solar Cells

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[1]. 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 photo-excited free carriers and few picoseconds lifetime of excitons in organic solar cells are obtained from our ultrafast spectroscopy[2]. By increasing the weight ratio of the third component into the conjugated polymer based donor and fullerene derivative based acceptor system, we observe a reduction of the lifetime of excitons and an increasing of charge transfer rate from the pump-probe measurement. We attribute these behaviours to the energy level of the lowest unoccupied molecular orbital (LUMO) of the third component, which is between the LUMOs of the conjugated polymer based donor and the fullerene derivative based acceptor. The introduction of the third component adjusts the energy-level cascade and enhances the recombination rate of excitons and the charge transfer probability. Moreover, we notice that the photo-conductivity arrives its peak value when the weight ratio of the third component is 15%. With further increasing the ratio of the third component, the film morphology of the ternary organic solar cell is worse and results in a decrease of photo-conductivity. The characters of the photo-conductivity obtained from pump-probe measurement agrees well with the static conductivity measurements reported by literature[3]. Our results not only provide fundamental understanding of the ultrafast dynamical process of photo-excited carriers in semiconductors but also benefit to the design of high efficient solar cells.