Band Gap Tuning of Solution-Processed Ferroelectric Perovskite BiFe_1–xCo_xO₃ Thin Films

Abstract

Ferroelectric perovskite oxides are emerging as a promising photoactive layer for photovoltaic applications because of their very high stability and their alternative ferroelectricity-related mechanism for solar energy conversion that could lead to extraordinarily high efficiencies. One of the biggest challenges so far is to reduce their band gap toward the visible region while simultaneously retaining ferroelectricity. To address these two issues, herein an elemental composition engineering of BiFeO 3 is performed by substituting Fe by Co cations, as a means to tune the characteristics of the transition metal–oxygen bond. We demonstrate by solution processing the formation of epitaxial, pure phase, and stable BiFe 1– x Co x O 3 thin films for x ≤ 0.3 and film thickness up to 100 nm. Importantly, the band gap can be tuned from 2.7 to 2.3 eV upon cobalt substitution while simultaneously enhancing ferroelectricity. As a proof of concept, nonoptimized vertical devices have been fabricated and, reassuringly, the electrical photoresponse in the visible region of the Co-substituted phase is improved with respect to the unsubstituted oxide.

Enhancement of phase stability and optoelectronic performance of BiFeO3thin filmsviacation co-substitution

“…Our recent work demonstrates that the optical gap of BFO can be reduced from 2.7 eV to 2.3 eV through cobalt doping while preserving a robust ferroelectricity. 21 The valence configuration of this cobalt-doped system is found to be Fe 3+ for iron and Co 3+ /Co 2+ for cobalt. 22 The presence of Co 2+ is expected to alter the charge defect scenario such as the ubiquity of oxygen vacancies and thus the leakage currents.…”

“…Following the deposition, a low-temperature treatment is performed in order to eliminate organic compounds. 21 Finally, the sample is subjected to a high temperature thermal treatment at 600 -700 • C for 30 min by which the phase crystallizes and grows on the substrate. This high-temperature process is carried out in a pre-heated tubular furnace to minimize the formation of Bi 2 Fe 4 O 9 and Bi 25 FeO 39 10 , with an O 2 flow of 0.6 l min −1 .…”

Towards photoferroic materials by design: recent progress and perspectives

“…Besides BFO-based materials, other possible photoferroic materials with appropriate band gaps include [43], Bi 2 ZnTiO 6 , [44] hexagonal ferrite (h-RFeO 3 , R=Y, Dy-Lu) thin films [45], Ni-doped SrBi 2 Nb 2 O 9 [46], and composite thin films of mixed BiMnO 3 and BiMn 2 O 5 [47].…”

The Microstructure, Electric, Optical and Photovoltaic Properties of BiFeO3 Thin Films Prepared by Low Temperature Sol–Gel Method

“…In recent years, the photovoltaic effect of ferroelectric materials, such as Pb(Zr,Ti)O 3 [6], BaTiO 3 [7,8] and BiFeO 3 [9,10,11], have attracted a lot of research for photovoltaic application. Among many ferroelectric materials, BiFeO 3 (BFO) is the most representative [12,13,14].…”