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Strong interactions between excitons are a characteristic feature of two-dimensional (2D) semiconductors, determining important excitonic properties, such as exciton lifetime, coherence, and photon-emission efficiency. Rhenium disulfide (ReS2), a member of the 2D transition-metal dichalcogenide (TMD) family, has recently attracted great attention due to its unique excitons that exhibit excellent polarization selectivity and coherence features. However, an in-depth understanding of exciton-exciton interactions in ReS2 is still lacking. Here we used ultrafast pump-probe spectroscopy to study exciton-exciton interactions in monolayer (1L), bilayer (2L), and triple layer ReS2. We directly measure the rate of exciton-exciton annihilation, a representative Auger-type interaction between excitons. It decreases with increasing layer number, as observed in other 2D TMDs. However, while other TMDs exhibit a sharp weakening of exciton-exciton annihilation between 1L and 2L, such behavior was not observed in ReS2. We attribute this distinct feature in ReS2 to the relatively weak interlayer coupling, which prohibits a substantial change in the electronic structure when the thickness varies. This work not only highlights the unique excitonic properties of ReS2 but also provides novel insight into the thickness dependence of exciton-exciton interactions in 2D systems. 

Surface recombination is a major bottleneck for realizing highly efficient micro/nanostructure solar cells. Here, parametric studies of the influence of Si microwire (SiMW) surface‐facet orientation (rectangular with flat‐facets, {110}, {100} and circular), with a fixed height of 10 µm, diameter (D= 1.5–9.5 µm), and sidewall spacing (S= 2.5–8.5 µm), and mesh‐grid density (1–16 mm⁻²) on recombination and carrier collection in SiMW solar cells with radial p‐n junctions are reported. An effective surface passivation layer composed of thin thermally grown silicon dioxide (SiO₂) and silicon nitride (SiN_x) layers is employed. For a fixedDof 1.5 µm, tight SiMW spacing results in improved short‐circuit current density (J_sc= 30.1 mA cm⁻²) and sparse arrays result in open‐circuit voltages (V_oc= 0.552 V) that are similar to those of control Si planar cells. For a fixedS, smallerDresults in better light trapping at shorter wavelengths and higherJ_scwhile largerDexhibits better light trapping at larger wavelengths and a higherV_oc. With a mesh‐grid electrode the power conversion efficiency increases to 15.3%. These results provide insights on the recombination mechanisms in SiMW solar cells and provide general design principles for optimizing their performance.