Effect of internal electric fields on charge carrier dynamics in ferroelectric materials for solar energy systems

Poster, Madeleine Morris, 039
Publication date: 16th April 2014

Both solar photovoltaic and solar fuels devices suffer major energy losses through the recombination of photogenerated charge carriers and species, and hence efforts to reduce these energy-loss mechanisms in solar energy systems are extremely important as they have the potential to lead to greatly improved power conversion efficiencies. It is well established that employing the use of an electric field will act as a driving force for charge separation and collection, and is the basis on which p-n junction and heterojunction solar cells work. Materials with internal electric fields, namely piezoelectric and ferroelectric materials, are being increasingly exploited in solar energy systems due to their potential to reduce such recombination by driving the spatial separation of both photogenerated charge carriers and of photo-oxidation and -reduction products. Existing reports provide empirical evidence that piezo- and ferro-electric materials can enhance performance of these systems, with the general consensus being that this is due to reduced recombination of photogenerated charge carriers.^1-5 However no detailed kinetic studies to fully quantify the influence of internal electric fields upon recombination dynamics currently exist. In order to develop more efficient solar energy conversion systems, an acute understanding of energy loss mechanisms must be understood so that device architecture can be modified constructively.
Studies employing transient absorption spectroscopy (TAS), a pump-probe technique, on picosecond-nanosecond and microsecond-second timescales have facilitated the performance of such kinetic studies. TAS allows the relaxation, trapping, recombination and reaction of photogenerated electrons and holes to be directly monitored. Barium titanate (BaTiO₃) a wide band-gap semiconductor with ferroelectric properties, has a modest Curie temperature of 120 °C. Below this critical temperature, the non-centrosymmetric structure results in an internal polarisation, i.e. ferroelectric properties. A phase change at 120 °C introduces an inversion centre to the unit cell, and hence the material is non-ferroelectric in this phase. By performing TAS studies below and above this critical temperature, the effect of the material’s ‘built in’ electric field on recombination dynamics can be clearly shown.
References
1. L. Li, P. A. Salvador, G. S. Rohrer; Nanoscale 2014, 6, 24-42      
2. Shoaee, S.; Briscoe, J.; Durrant, J. R.; Dunn, S. Adv. Mater. 2013, 1–6      
3. Stock, M.; Dunn, S. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 2011, 58, 1988–1993      
4. Giocondi, J.; Rohrer, G. J. Phys. Chem. B 2001, 105, 2–4      
5. Kalinin, S.; Bonnell, D.; Alvarez, T.; Lei, X. Nano Lett. 2002, 3, 589–593

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