Electron transfer in hydrogenated nanocrystalline silicon observed by time-resolved terahertz spectroscopy

We report on the ultrafast carrier dynamics in hydrogenated nanocrystalline silicon (nc$-$Si:H) using time-resolved terahertz spectroscopy. Photoexcitation at 407 nm primarily produces charge carriers in the $a-$Si phase, but they undergo a rapid electron transfer to the $c-$Si phase prior to complete thermalization into the band-tail states of $a-$Si. We studied the carrier dynamics on a range of nc$-$Si:H samples with varying crystalline volume fractions ($X_c$) and mapped out the carrier dynamics with sub-ps resolution. Our measurements are consistent with a model in which electrons are first trapped at interface states at the $a-$Si–$c-$Si boundary prior to being thermally emitted into the $c-$Si phase. Wavelength and temperature dependent measurements are consistent with our model. The phenomena observed here have implications toward solar cell structures that utilize an amorphous material as an absorber layer, previously thought to have a mobility value too low to attain effective charge transport in a device. DOI: http://dx.doi.org/10.1103/PhysRevB.87.081301 ©2013 American Physical Society