Recombination losses continue to be one of the major barriers to improving efficiencies in both solar photovoltaic and solar fuels devices. One strategy gaining interest for addressing this issue is the utilisation of materials with internal electric fields, due to the potential for efficient spatial separation of photogenerated charges. Indeed, enhancements in device efficiencies attributed to piezoelectric or ferroelectric effects have been reported for a broad range of photovoltaic, photoelectrochemical and photocatalytic devices.[1-3] The organohalide lead perovskite materials currently yielding very promising photovoltaic device efficiencies are also reported to be ferroelectric, although the importance of their ferroelectric properties in achieving such high efficiencies is currently unclear.[4–7] In almost all cases where improvements are attributed to ferroelectric material properties, reduced rates of bimolecular recombination are identified as an explanation for the observed enhancements. However, direct evidence of the effect of spontaneous polarisation on charge carrier lifetimes is currently lacking in the literature.

We have used transient absorption spectroscopy (TAS) to directly probe the correlation between ferroelectric behaviour and carrier lifetimes in a model material, BaTiO₃. We use nanostructuring and elevated temperatures to induce a crystal strucutre change and thus “turn off” the spontaneous polarisation. We reveal that carrier lifetimes in ferroelectric BaTiO₃ are remarkably long, on the order of 10^-1 s, in the absence of chemical scavengers or applied electrical bias.[8] In non-ferroelectric BaTiO₃, on the other hand, lifetimes are four orders of magnitude shorter. We attribute the long lifetimes to polarisation-induced band bending in ferroelectric BaTiO₃ acting as a thermal barrier to electron/hole recombination, which is absent in non-ferroelectric BaTiO₃. This study reveals that the spontaneous polarisation in ferroelectric materials can indeed facilitate significantly reduced recombination rates and could thus be used to enhance solar energy conversion device efficiencies. Carrier lifetimes observed here match well with rate constants for photoelectrochemical water oxidation, indicating significant potential for ferroelectric materials in solar water splitting systems.

[1]         S. Shoaee et al., Adv. Mater. 2013, 26, 1.

[2]        W. Yang et al., Nano Lett. 2015, 15, 7574.

[3]        Y. Cui et al., Chem. Mater. 2013, 25, 4215.

[4]        Y. Kutes et al., J. Phys. Chem. Lett. 2014, 5, 3335.

[5]        J. M. Frost et al., Nano Lett. 2014, 14, 2584.

[6]        Z. Fan et al., J. Phys. Chem. Lett. 2015, 1155.

[7]        M. Coll et al., J. Phys. Chem. C 2015, 6, 1408.

[8]        M. R. Morris et al., Adv. Mat. 2016, accepted