Bandgap narrowing and polarization enhancement in (K,Na,Li)(Nb,Sb,Ta)O3+x% Fe2O3 lead-free ceramics for photovoltaic applications

The need for ferroelectric materials with both narrow bandgaps (Eg) and large remanent polarization (Pr) remains a key challenge to the development of high-efficiency ferroelectric photovoltaic (FPV) devices. In this work, [(K0.43Na0.57)0.94Li0.06][(Nb0.94Sb0.06)0.95Ta0.05]O3 (KNLNST)-based lead-free ceramics with narrow Eg and large Pr 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 Eg of 1.74 eV and a large Pr of 27.05 μC/cm2. These values are much superior to those of undoped KNLNST ceramics (Eg = 3.1 eV and Pr = 17.73 μC/cm2). While the large Pr stems from the increment of the volume ratio between the orthorhombic and tetragonal phases (VO/VT) in KNLNST ceramics by proper amount of Fe3+ doping, the narrow Eg 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 (Edp) is observed in the KNLNST+1.3% Fe2O3 ceramic-based device. Thanks to the narrower Eg and larger Pr of the doped ceramic, the photovoltaic performance of the corresponding device (open-circuit voltage (Voc) = −5.28 V and short-circuit current density (Jsc) = 0.051 μA/cm2) under a downward poling state is significantly superior to that of an undoped KNLNST-based device (Voc = −0.46 V and Jsc = 0.039 μA/cm2). This work offers a feasible approach to developing ferroelectric materials with narrow bandgaps and large Pr for photovoltaic applications.