From late 1970s to the early 2000s, the efficiency of traditional photovoltaics, PV, cells based on single crystal Si, GaAs or CdTe has evolved from 8-13% to 25% approximately. Since then, the improvement of the efficiency of these classic solar cells based on p-n junctions seem to have found a ceiling. The discouraging progress during the last decade stems from the limited photovoltage of solar cells based on p-n junctions due to the band gap of the semiconductor and the Shockley-Queisser, S-L, limit. In order to make this technology more competitive, significant improvements must be made in their efficiency by the discovery of new photovoltaic mechanisms that overcome the aforementioned issues. Ferroelectric materials, which present a spontaneous electric polarization, exhibit a different behavior under photo-excitation, providing an alternative way to separate carriers. This is called the bulk photovoltaic effect, which has reinvigorated efforts for the development of new solar cells, now based on ferroelectric materials.
Hybrid perovskite thin-films such as CH3NH3PbI3, MAPbI3, have changed the solar cell field in recent years because of their low cost and high efficiency. However, their low stability and durability have hampered their implementation in the market. Chalcohalides have been recently proposed as an alternative, but they usually exhibit a lower polarization than other ferroelectric materials. Oxide perovskites stand as the most promising candidates because their strong polarization and high thermal, mechanical and chemical stabilities are combined with very low costs for their synthesis. Oxide perovskites also present issues that will need to be overcome to design efficient PV devices, their wide band gap being the main obstacle.
Different strategies have been proposed to tailor the electronic properties of these oxides, reducing the band gap from 2 eV–3 eV to 1.5 eV. The epitaxial growth of strained thin-film oxides, promoting the rhombohedral-to-tetragonal phase transition or doping of both A and B sites are effective approaches to reduce the band gap of these oxides. Theoretical works, most of them based on density functional theory, DFT, calculations, have helped to understand the electronic and structural properties of ferroelectric oxides. However, the optimization of solar cells based on ferroelectric materials requires the screening of too many variables. This challenge can be tackled combining DFT calculations with high-throughput frameworks and the use of materials database.
HT-PHOTO-DB project has developed and used high-throughput frameworks and materials databases for accelerating the discovery of new materials for more efficient solar cells based on ferroelectric oxides.
