FERROVOLT · For a better understanding and design of ferroelectric photovoltaics: First-principles study of optical absorption and charge-carrier transport at ferroelectric domain walls in BiFeO3

For a better understanding and design of ferroelectric photovoltaics: First-principles study of opticalabsorption and charge-carrier transport at ferroelectric domain walls in BiFeO3

"Ferrovolt is a basic-research project in the field of solar-cell materials. Its goal is to determine how a specific nanostructure (a ferroelectric domain wall) can be exploited to boost solar-cell performance. A ferroelectric is a material that exhibits a spontaneous electric dipole in a similar way that ferromagnets exhibit a magnetic dipole. Ferroelectrics tend to spontaneously form domains with different directions of the ferroelectric polarization with nanoscopic interfaces (ferroelectric domain walls) between them. Standard silicon solar cells require a p-n junction to generate a photovoltage because of the crystal structure of silicon, which is inversion-symmetric. This means that photogenerated charge carriers flow equally into all directions and the net current and voltage are zero. In order to break the symmetry, one needs to create a p-n junction which causes electrons and holes to flow into opposite directions, resulting in a net photovoltage. In contrast, ferroelectric materials do not have inversion symmetry and can therefore provide a photovoltage without a p-n junction. This phenomenon is called the bulk photovoltaic effect, and it is technologically interesting because other than p-n junctions, ferroelectricity comes for free. It has been theorized [1] that ferroelectric domain walls could yield an even stronger photovoltaic effect. It was suggested that these create much stronger electric fields than p-n junctions do. This hypothesis was soon challenged [2]. Measurements of the photovoltaic effect in BiFeO3 (bismuth ferrite) and BaTiO3 (barium titanate), both prototypical ferroelectrics, showed that the photocurrent oscillates if the light polarization rotates, which it should in the case of the bulk photovoltaic effect, but not in the case of the domain-wall effect. This finding shows that the bulk photovoltaic effect exists, but it does not prove that the domain-wall effect does not. The difficulty to measure the size of the domain-wall effect lies in the nanoscopic width of the ferroelectric domain walls (a few atomic layers). To measure the photocurrent or photovoltage with this resolution is challenging. In the Ferrovolt project I model what cannot be measured, using atomistic, quantum-mechanical modelling (density-functional theory) to to study the electronic processes that determine electronic potential and current of photoelectrons at ferroelectric domain walls in BiFeO3. BiFeO3 is a model system for which there is a large body of experimental data to compare with, although as an inefficient light absorber it is not ideal for solar cells. What we learn about BiFeO3 will finally need to be transferred to other ferroelectrics that are better absorbers. The objectives of Ferrovolt were 1) to determine the photovoltage generated by ferroelectric domain walls by calculating optical absorption and spatial photocarrier distribution, 2) to determine whether the domain walls are considerably more conductive than the domain interior by calculating the electronic conduction, and 3) to determine the effect of defects (oxygen or bismuth vacancies) on these domain-wall properties. The overall goal of Ferrovolt was to understand how large the domain-wall photovoltaic effect is, and how it can be optimized. [1] Jan Seidel et al., ""Conduction at domain walls in oxide multiferroics"", Nature materials 8 (3) 229 (2009) (https://ir.nctu.edu.tw/bitstream/11536/7519/1/000263556800023.pdf) [2] Akash Bhatnagar et al., ""Role of domain walls in the abnormal photovoltaic effect in BiFeO3"", Nature communications 4, 2835 (2013) (https://www.nature.com/articles/ncomms3835). "

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