Ceramics via Grain Size Engineering: A Correlative Study of Ferroelectric and Charge Transport Properties"*

This study elucidates the pivotal role of grain size engineering in enhancing the ferroelectric photovoltaic (FPV) performance of [KNbO₃]_1-x[BaNi_1/2Nb_1/2O_3-δ]_x [KBNNO] ceramics. A series of samples with controlled grain sizes, ranging from 10 nm to 200 nm, were synthesized via tailored sintering conditions. Structural analysis using Rietveld refinement of X-ray diffraction data confirmed single-phase perovskite structure and provided precise crystallite size estimation, which closely aligns with grain size evolution observed through scanning electron microscopy (SEM). SEM images further reveal a consistent increase in particle size with increasing grain size, reinforcing the effectiveness of the sintering strategy. Electrical and dielectric measurements demonstrate that grain growth significantly enhances ferroelectric polarization (from 10 to 25 µC/cm²) and increases the dielectric constant (from 1700 to ~4000 at 1 Hz) while maintaining the band gap (1.1 eV to 2.6 eV in different regions) in the visible region of solar spectrum. Concurrently, the reduction in grain boundary density suppresses charge carrier recombination and strengthens conductive pathways, as evidenced by the increase in AC conductivity from 120 µS/cm to over 600 µS/cm. These improvements directly contribute to superior photovoltaic performance, with photocurrent density increasing from 27 µA/cm² to60 µA/cm² that is highest obtained photocurrent density value which is more than twice the value obtained by Chen et al[1]. Collectively, these results establish grain growth—validated by both Rietveld refinement and SEM analysis—as a key design parameter for optimizing FPV performance, and provide a comprehensive multiscale framework linking structural, microstructural, and functional properties in electroceramics.