Coupling of photoexcitation and grain-boundary disorder in the inverse Hall-Petch plasticity of nanocrystalline CdTe.

The mechanical reliability of nanocrystalline semiconductors limits the durability of photovoltaic and optoelectronic devices, yet how grain size and electron-hole (e-h) excitation jointly govern their plasticity remains poorly understood at the atomic scale. Here we use molecular dynamics (MD) driven by machine-learning interatomic potentials (MLIPs) to investigate the shear deformation of nanocrystalline CdTe across effective grain diameters of 4.33 to 13.86 nm in the ground state and under two photoexcited electronic states. The ultimate shear strength at the ground state increases from approximately 0.73 to 1.02 GPa with increasing grain size, indicating an inverse Hall-Petch response in this regime. The most effective excitation depends on grain size: ES 2% gives the highest strength at d = 4.33 and 6.50 nm, whereas ES 0.5% gives the highest strength at d = 6.93, 8.66, 10.39, and 13.86 nm. By quantifying the disordered grain-boundary volume, we find that the pre-peak evolution of interfacial disorder, rather than grain size alone, tracks the strength and can serve as a strain-resolved structural descriptor of grain-boundary-mediated plasticity. The descriptor exposes a grain-size-dependent crossover: in finer grains, where the initial grain boundary fraction is high, higher strength accompanies delayed interfacial disordering, whereas in coarser grains it accompanies the controlled activation of a connected grain boundary plasticity network. Electron-hole excitation modulates this pathway non-monotonically by shifting the onset and extent of interfacial disordering. These results recast inverse Hall-Petch behavior in nanocrystalline CdTe in terms of an evolving, e-h tunable interfacial descriptor and point toward mechanically robust semiconductor microstructures designed through combined control of grain size and e-h excitation.