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

Abstract

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 (FPV) devices. In this work, [(K 0.43 Na 0.57 ) 0.94 Li 0.06 ][(Nb 0.94 Sb 0.06 ) 0.95 Ta 0.05 ]O 3 (KNLNST)-based lead-free ceramics with narrow E g and large P r are obtained via Fe 2 O 3 doping. By optimizing the level of Fe 2 O 3 doping, a KNLNST+1.3% Fe 2 O 3 ceramic is fabricated that simultaneously possesses a narrow E g of 1.74 eV and a large P r of 27.05 μC/cm 2 . These values are much superior to those of undoped KNLNST ceramics (E g = 3.1 eV and P r = 17.73 μC/cm 2 ). While the large P r stems from the increment of the volume ratio between the orthorhombic and tetragonal phases (V O /V T ) in KNLNST ceramics by proper amount of Fe 3+ doping, the narrow E g is attributed to the coupling interaction between the Fe 3+ dopants and the B-site Sb 3+ host ions. Moreover, a switchable photovoltaic effect caused by the ferroelectric depolarization electric field (E dp ) is observed in the KNLNST+1.3% Fe 2 O 3 ceramic-based device. Thanks to the narrower E g and larger P r of the doped ceramic, the photovoltaic performance of the corresponding device (open-circuit voltage (V oc ) = −5.28 V and short-circuit current density (J sc ) = 0.051 μA/cm 2 ) under a downward poling state is significantly superior to that of an undoped KNLNST-based device (V oc = −0.46 V and J sc = 0.039 μA/cm 2 ). This work offers a feasible approach to developing ferroelectric materials with narrow bandgaps and large P r for photovoltaic applications.

Synthesis and investigation of structural, morphological and dielectric properties of Ba‐ordered BaTiO3 perovskite oxide

“…2 Adjusting material properties including polarization response, loss mechanisms, electro-mechanical responses, Curie temperature and permittivity stability, [3][4][5] is a common scientific practice. 1,6,7 The substitution of either homo-or heterovalent cations at B-site results in the conversion of FE to diffuse phase transition (DPT), as reported in previous studies. 1,[8][9][10] Due to the development of strong local fields, the effective charge of the dopant and the chargecompensating defect are anticipated to have a significant impact.…”

Distinguishable Separation of Artificial Electron–Oxygen Vacancy Pairs Using Heterovalent Doping Effect for Ferroelectric Photovoltaics

“…An obvious photocurrent response can be observed in the visible light range because phonons with energy hν trigger the band-edge transition. Thus, the superior device performance can be ascribed to the following factors: (i) The narrower band gap of the ferroelectrics contributes to the production of more photogenerated carriers and (ii) a larger P r results in a larger E dp , which is favorable for the separation and transport of photogenerated carriers . In this context, the surface state goes into equilibrium with the air by emitting oxygen to create more oxygen vacancies, which subsequently diffuse into the film.…”

“…Thus, the superior device performance can be ascribed to the following factors: (i) The narrower band gap of the ferroelectrics contributes to the production of more photogenerated carriers and (ii) a larger P r results in a larger E dp , which is favorable for the separation and transport of photogenerated carriers. 40 In this context, the surface state goes into equilibrium with the air by emitting oxygen to create more oxygen vacancies, which subsequently diffuse into the film. This process represents a "pump" that fills the capacitor volume with oxygen vacancies that start to degrade after saturation.…”