Unraveling the structural properties of Bi(Fe_1-xCr_x)O₃ thin films

Abstract text (incl. figure legends and references)

This work focuses on the structural, chemical, and optoelectronic properties of Bi(Fe_1-xCr_x)O₃ thin films synthesized by spin-coating for future ferroelectric solar-cell absorber layers. We want to use laterally polarized ferroelectric domains and their electric fields to drive the photogenerated charge carrier towards the domain walls, along which they can be extracted into the electrodes. Early results on the efficacy of this idea were achieved with methylammonium lead halide (MAPH) perovskites [1]. However, we strive for a non-toxic ceramic-based material with similar properties. A possible material system with desirable properties and suitable bandgap energy is Bi(Fe_1-xCr_x)O₃ (BFCO), where Bi2+ occupies the A site with alternating Fe2+ and Cr2+ on the B site in the ABO3 perovskite structure. In this contribution, the material properties are investigated in detail by different electron microscopic techniques on the nanoscale to understand the bulk material's optoelectronic properties and to improve material synthesis.

BFCO films with different Fe/Cr concentrations were deposited by spin coating an Fe-, Cr- and Bi- containing sol onto fluorine-doped tin oxide (FTO) substrates followed by a calcination step at 350 °C in air. We used several analytical scanning/transmission electron microscopy (S/TEM) techniques for comprehensive characterization, such as quantitative energy-dispersive X-ray spectroscopy (STEM-EDXS), electron energy loss spectroscopy (STEM-EELS), selected area electron diffraction (SAED), and other imaging modes. Nanoscale investigations were accompanied by X-ray diffractometry (XRD) and UV-Vis spectroscopy to analyze the films.

The BFCO thin films are polycrystalline with a thickness of about 400 nm. Bright-field S/TEM images reveal a homogeneous microstructure with 30-50 nm grain sizes. The crystal structure is predominantly tetragonal BiFeO3, however, we do note that occasional metal Bi nanoparticles and amorphous Cr-Fe oxide can be seen in some synthesis batches. Quantitative analysis of STEM-EDXS maps shows that the average composition of the thin film follows the expected stoichiometry Bi(Fe_1-xCr_x)O₃ . Furthermore, core loss STEM-EELS accompanied by an analysis of the fine structure of Fe and Cr confirms the 2+ oxidation state and shows that both Fe and Cr are located at the B-site in the perovskite. Analyses of the crystal structure with SAED, HRTEM and the coordination environment revealed with STEM-EELS fits with the bulk crystal structure determined by XRD. Further, the nanoscale secondary phases which occur in individual synthesis batches help explain additional contributions to the bulk the UV-Vis signal.

Based on comprehensive electron microscopic analyses, this study demonstrates that BFCO thin films can be successfully synthesized by tuning the sol-gel synthesis conditions. Looking ahead, we are aiming to fine-tune the ferroelectric domain structure.

[1] H. Röhm et al., Adv. Mater., 2019, 26, 1806661]

[2] Mustonen, O. et al Chem. Commun., 2019, 55, 1132-1135

[3] Herber, R-P and Schneider, G.A., J. Mater. Res., 2007, 22