Quaternary ferroelectric perovskite oxides for photovoltaics

2017 edition

Pamela Machado

Ferroelectric perovskite oxide based photovoltaic (PV) cells arise as an innovative and promising alternative to current PV technology [1]. Unlike traditional silicon based PV cells, perovskites provide unique routes to spontaneously separate charge carriers achieving extremely large, above bandgap voltages by a single phase material, abnormal photovoltaic effect. Most ferroelectric oxides have bandgaps in the range of 2.7-5 eV, which allow the use of less than 8-20% of the solar spectrum and show PV currents orders of magnitude lower than traditional PV cells, which have thus far prevented a wider impact (efficiencies < 1%). Enhancing the APV effect in ferroelectric perovskite oxide materials by judicious engineering of the bandgap offers unprecedented opportunities for this class of materials to build PV devices with increased power conversion efficiency [2,3].

Ferroelectric perovskites, sharing a general ABO3 structure, can accommodate a large variety of cations in the A and B positions. Importantly, small changes in the metal-oxygen bond and cation electronegativity leads to octahedral distortions and local strain and thus modify the ferroelectricity and lower the bandgap. Chemical deposition techniques offer great opportunities to nanoengineer ABO3 thin films into functional layers that enhance the electronic and optical properties required for the preparation of efficient solar cells. In this work, we are studying for the first time the A and B cation substitution in BiFeO3 thin films by low cost and scalable chemical solution deposition using non-toxic elements. Regarding the B cation substitution, epitaxial BiFe1-xCoxO3 thin films have been obtained even with challenging high cobalt concentration (up to x =0.7) identifying a strong decrease in the bandgap, from 2.7 to 1.4 eV, while preserving the ferroelectric behavior. Photoconductivity is observed under illumination from a 100 mW/cm2 white light source. On the other hand, we are studying the Bi substitution by rare earth cations as a robust approach to further improve film purity, epitaxy and surface morphology.

[1] P. Lopez-Var├│, M. Coll et al. Physics Reports, (2016) 653,1

[2] I. Grinberg et al., Nature, (2013)

[3] R. Nechache et al. Nature Photonics, (2014)