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Silicon nanoparticles have the promise to surpass the theoretical efficiencylimit of single-junction silicon photovoltaics by the creation of a "phononbottleneck", a theorized slowing of the cooling rate of hot optical phononsthat in turn reduces the cooling rate of hot carriers in the material. Toverify the presence of a phonon bottleneck in silicon nanoparticles requiressimultaneous resolution of electronic and structural changes at shorttimescales. Here, extreme ultraviolet transient absorption spectroscopy is usedto observe the excited state electronic and lattice dynamics in polycrystallinesilicon nanoparticles following 800 nm photoexcitation, which excites carrierswith $0.35 \pm 0.03$ eV excess energy above the ${\Delta}_1$ conduction bandminimum. The nanoparticles have nominal 100 nm diameters with crystalline grainsized of about ~16 nm. The extracted carrier-phonon and phonon-phononrelaxation times of the nanoparticles are compared to those for a silicon (100)single crystal thin film at similar carrier densities ($2$ x $10^{19} cm^{-3}$for the nanoparticles and $6$ x $10^{19} cm^{-3}$ for the thin film). Themeasured carrier-phonon and phonon-phonon scattering lifetimes for thepolycrystalline nanoparticles are $870 \pm 40$ fs and $17.5 \pm 0.3$ ps,respectively, versus $195 \pm 20$ fs and $8.1 \pm 0.2$ ps, respectively, forthe silicon thin film. The reduced scattering rates observed in thenanoparticles are consistent with the phonon bottleneck hypothesis.

Characterization of Carrier Cooling Bottleneck in Silicon Nanoparticles by Extreme Ultraviolet (XUV) Transient Absorption Spectroscopy