Free electron-driven photophysics in n-type doped silicon nanocrystals

Although there have been extensive speculation regarding the applicability of doped silicon nanocrystals (Si NCs) in optoelectronic technologies, a quantitative analysis on the photophysical workings of introduced free carriers remains elusive. Here, we present a comprehensive study on the photophysics of ∼7.5 nm heavily phosphorous-doped Si NCs, using a combination of spectroscopic techniques. We correlate the carrier dynamics with the location of the free carriers - which we tune from NC core to surface depending on the state of oxidation. The strength of the Coulomb interactions between the photoexcited electron-hole pairs and the doping-induced free carriers depends on (1) the concentration of free carriers, (2) the location of these carriers, and (3) the diameter of the NCs. In contrast to prior studies, the photoexcited carrier dynamics in these n-type doped Si NCs are dominated by strong Coulomb interactions with doping-induced free electrons, characterized by a negative trion lifetime of around 9 ns. While radiative recombination in doped direct bandgap NCs can often still compete with trion recombination (allowing emission to be present), emission in our doped Si NCs is completely quenched due to the relatively slow radiative recombination in these indirect bandgap NCs. Furthermore, multi-exciton interaction times are slightly shortened compared to those of intrinsic Si NCs, which we attribute to an increased number of free electrons, enhancing the oscillator strength of Auger recombination. These results constitute a framework for the optimization of doped Si NC synthesis techniques and device engineering directions for future doped-Si NC-based optoelectronic and photovoltaic applications.