Enhanced Photovoltaic and Ferroelectric Properties of Na 0.5 Bi 0.5 TiO ₃ Thin Films via Na/Bi Excess and La Doping

Transition metal incorporation into semiconductors not only induces ferroelectric behavior but also supplies localized spins. The resulting spin interactions can give rise to ferromagnetism, offering a versatile route to multiferroic properties. Ferroelectric thin films exhibit intrinsic photovoltaic behavior due to their internal electric fields, which arise from spontaneous polarization and result in conduction and valence band tilting similar to the built-in fields in p–n junctions. In this work, we synthesized Na 0.5 Bi 0.5 TiO 3 (NBT) thin films with excess Na/Bi and La doping using pulsed laser deposition (PLD), and investigated their structural, ferroelectric, and photovoltaic properties. Three types of NBT films (NBT1, NBT2, and NBT3) were fabricated using Na 0.55 Bi 0.55 TiO 3 (+5% Na/Bi), Na 0.6 Bi 0.6 TiO 3 (+10% Na/Bi), and La 0.05 Na 0.6 Bi 0.55 TiO 3 (+10% Na/Bi, +5% La) targets, respectively. X-ray diffraction analysis confirmed that all thin films were preferentially c -axis-oriented with enhanced crystallinity, particularly in La-doped samples. A progressive increase in the c -lattice parameter was observed with excess Na/Bi and La incorporation, suggesting improved tetragonality and domain orientation. Ferroelectric measurements showed that the remanent polarization ( P r ) increased from 17.9 μC/cm 2 in the undoped NBT1 to 43.7 μC/cm 2 in NBT2 and reached 61.3 μC/cm 2 in NBT3. In parallel, leakage current was markedly reduced in the doped thin films. These improvements are attributed to the synergistic effects of enhanced ferroelectric properties and the asymmetric work functions of the ITO and Pt electrodes. Overall, the results demonstrate that compositional engineering of NBT thin films provides a promising strategy for realizing high-performance ferroelectric photovoltaic devices.