We have applied time-resolved terahertz (THz) spectroscopy to probe ultrafast conduction dynamics of photoexcited carriers in silicon nanocrystal films with a wide range of nanocrystal sizes and concentrations. The picosecond THz conductivity reveals microscopic photocarrier motion with significant interface scattering within the nanocrystals, as well as percolative transport between nanocrystals. In films with silicon filling fractions above the percolation threshold, we observe a transition from long-range internanocrystal transport immediately after photoexcitation to increased carrier localization over a 50-ps time scale due to accumulation of charges at interface defect sites. However, in films with silicon filling fractions below the percolation threshold, transport between nanocrystals is strongly suppressed at all times. Finally, we estimate effective carrier diffusion lengths of 60 to 130 nm for the silicon nanocrystal composites with silicon filling fractions above the percolation threshold, making such films promising candidates for active layers in photovoltaic devices.

• Received 25 August 2010

DOI:https://doi.org/10.1103/PhysRevB.83.085403