Bandgap narrowing and polarization enhancement in (K,Na,Li)(Nb,Sb,Ta)O 3+ x% Fe 2O 3 lead-free ceramics for photovoltaic applications

The need for ferroelectric materials with both narrow bandgaps (E_g) and large remanent polarization (P_r) remains a key challenge to the development of high-efficiency ferroelectric photovoltaic devices. In this work, [(K_0.43Na_0.57)_0.94Li_0.06][(Nb_0.94Sb_0.06)_0.95Ta_0.05]O₃ (KNLNST)-based lead-free ceramics with narrow E_g and large P_r are obtained via Fe₂O₃ doping. By optimizing the level of Fe₂O₃ doping, a KNLNST + 1.3% Fe₂O₃ ceramic is fabricated that simultaneously possesses a narrow E_g of 1.74 eV and large P_r of 27.05 μC/cm². These values are much superior to those of undoped KNLNST ceramics (E_g=3.1 eV, P_r=17.73 μC/cm²). While the large P_r stems from the increment of the volume ratio between the orthorhombic and tetragonal phases in KNLNST ceramics by proper amount of Fe³⁺ doping, the narrow E_g is attributed to the coupling interaction between the Fe³⁺ dopants and the B-site Sb³⁺ host ions. Moreover, a switchable photovoltaic effect caused by the ferroelectric depolarization electric field (E_dp) is observed in the KNLNST + 1.3% Fe₂O₃ ceramic-based device. Thanks to the narrower E_g and larger P_r of the doped ceramic, the photovoltaic performance of the corresponding device (V_oc = -5.28 V, J_sc = 0.051 μA/cm²) in a downward poling state is significantly superior to that of an undoped KNLNST-based device (V_oc = -0.46 V, J_sc = 0.039 μA/cm²). This work offers a feasible approach to developing ferroelectric materials with narrow bandgaps and large remanent polarizations for photovoltaic applications.