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.

Figure 1

Schematic of the ET process. In steps 1-a and 1-c, light produces carriers in $a-$Si or $c-$Si. Hot carriers in $a-$Si are trapped at the $a-$Si–$c-$Si interface (2a) or cool into the band-tail states (2a’). Trapped electrons establish a thermal equilibrium with carriers in the CB of the nanocrystals. Finally, electrons eventually recombine (4).

Figure 2

(a) 300 K TRTS transient data for $X_c$ varying from 0 ($a-$Si) to 0.7. (b) TRTS data at early delay times. Solid lines represent a linear least-squares best fit of our model (see text) to the data. (c) The fraction of carriers in each state in our model as a function of delay. Inset: early time behavior. (d) The fraction of the total carriers remaining in $a-$Si calculated from this study (black circles) and PL data from Ref. 8. Red triangles represent the fraction of total photons absorbed in $a-$Si.

Figure 3

(a) Wavelength dependence of the carrier dynamics at 300 K for the $X_c$ $=$ 0.5 sample for 407 and 815 nm excitation. Inset: PDS measurements of the absorption coefficient of $a-$Si:H, $X_c$ $=$ 0.5 nc$-$Si:H, and literature data for $c-$Si.[19] (b) 300 and 77 K data for the $X_c$ $=$ 0.7 and 0.5 samples and their linear squares best fits (solid, 0.7; dashed, 0.5).