Bandgap narrowing and polarization enhancement in (K,Na,Li)(Nb,Sb,Ta)O <sub>3</sub>+ <i>x</i>% Fe <sub>2</sub>O <sub>3</sub> lead-free ceramics for photovoltaic applications
Jian Chen, Jiaxing Mao, Zihui Wang, Yanhui Dong, Jinming Guo, Mingkai Li, Yi Zhang, Yinmei Lu, Yunbin He
Abstract
The need for ferroelectric materials with both narrow bandgaps (<i>E</i><sub>g</sub>) and large remanent polarization (<i>P</i><sub>r</sub>) remains a key challenge to the development of high-efficiency ferroelectric photovoltaic (FPV) devices. In this work, [(K<sub>0.43</sub>Na<sub>0.57</sub>)<sub>0.94</sub>Li<sub>0.06</sub>][(Nb<sub>0.94</sub>Sb<sub>0.06</sub>)<sub>0.95</sub>Ta<sub>0.05</sub>]O<sub>3</sub> (KNLNST)-based lead-free ceramics with narrow <i>E</i><sub>g</sub> and large <i>P</i><sub>r</sub> are obtained via Fe<sub>2</sub>O<sub>3</sub> doping. By optimizing the level of Fe<sub>2</sub>O<sub>3</sub> doping, a KNLNST+1.3% Fe<sub>2</sub>O<sub>3</sub> ceramic is fabricated that simultaneously possesses a narrow <i>E</i><sub>g</sub> of 1.74 eV and a large <i>P</i><sub>r</sub> of 27.05 μC/cm<sup>2</sup>. These values are much superior to those of undoped KNLNST ceramics (<i>E</i><sub>g</sub> = 3.1 eV and <i>P</i><sub>r</sub> = 17.73 μC/cm<sup>2</sup>). While the large <i>P</i><sub>r</sub> stems from the increment of the volume ratio between the orthorhombic and tetragonal phases (<i>V</i><sub>O</sub>/<i>V</i><sub>T</sub>) in KNLNST ceramics by proper amount of Fe<sup>3+</sup> doping, the narrow <i>E</i><sub>g</sub> is attributed to the coupling interaction between the Fe<sup>3+</sup> dopants and the B-site Sb<sup>3+</sup> host ions. Moreover, a switchable photovoltaic effect caused by the ferroelectric depolarization electric field (<i>E</i><sub>dp</sub>) is observed in the KNLNST+1.3% Fe<sub>2</sub>O<sub>3</sub> ceramic-based device. Thanks to the narrower <i>E</i><sub>g</sub> and larger <i>P</i><sub>r</sub> of the doped ceramic, the photovoltaic performance of the corresponding device (open-circuit voltage (<i>V</i><sub>oc</sub>) = −5.28 V and short-circuit current density (<i>J</i><sub>sc</sub>) = 0.051 μA/cm<sup>2</sup>) under a downward poling state is significantly superior to that of an undoped KNLNST-based device (<i>V</i><sub>oc</sub> = −0.46 V and <i>J</i><sub>sc</sub> = 0.039 μA/cm<sup>2</sup>). This work offers a feasible approach to developing ferroelectric materials with narrow bandgaps and large <i>P</i><sub>r</sub> for photovoltaic applications.