Ultrafast percolative transport dynamics in silicon nanocrystal films

Abstract

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.

Size-Dependent Ultrafast Charge Carrier Dynamics of WO3 for Photoelectrochemical Cells

“…for the more ordered Si/SiO 2 system where they found a similar trends of localization of photoinduced charge carriers at long time scales. 31 In contrast to the NPs, we were unable to detect mobile carriers beyond ~400 ps in the GP samples. In this case, the grain boundaries (deep traps) within the GPs have a more significant influence on the amount of long-lived carriers probed with THz radiation compared to surface traps.…”

Charge Transport in ${\hbox{TiO}}_{2}$ Films With Complex Percolation Pathways Investigated by Time-Resolved Terahertz Spectroscopy

“…We start our analysis with a simplified formula (19): (21) Only some of the parameters in (21) can be determined independently from the experiment. Taking into account the expression (20), the frequency dependence of -see [32, eq. (1)]-and that of , we rewrite formula (21) in the following form which was used for the fitting of the experimental data: (22) The relevant fitting parameters are described in Table I. The time represents the longest waiting time found in the system; it is well below the THz range and cannot be determined from our fits.…”

“…We express the result of the integration in (18) in a convenient form as the normalized transient sheet conductivity [cf. with (3), (4)] (19) where is the optical absorption coefficient in the th component and (20) Equation (19) is the crucial result of our theoretical analysis. It relates the directly measurable transient transmission at the left-hand side with the microscopic transport response functions (yield-mobility products of photoconductive components) and with the coefficients related to the equivalent circuit in Fig.…”

“…In a number of papers a different approach is used where the spectra of macroscopic are directly fit with the Drude-Smith model [6], [20]- [22]. The Drude-Smith parameters then carry information on the response of carriers as discussed in [17] or on the sample percolation.…”

Ultrafast carrier dynamics in BiVO4 thin film photoanode material: interplay between free carriers, trapped carriers and low-frequency lattice vibrations

“…A non-contact probe of microscopic conductivity, TRTS has already been successfully applied to uncover ultrafast carrier dynamics and conductivity mechanisms in a variety of bulk and nanostructured materials for solar energy conversion. [17][18][19][20][21][22][23][24][25][26][27] As the bandwidth of THz pulses spans from $0.2 to 2.4 THz (8-80 cm À1 ), corresponding to the energy range of 1-10 meV, transmission of THz pulses through samples is intrinsically sensitive to motion of free carriers over length scales of tens of nanometers as well as to low-frequency (<80 cm À1 ) phonon modes. As a result, TRTS simultaneously provides insights into the dynamics of photoexcited carriers and carrier-lattice interactions that are not accessible by other techniques.…”