Realizing New Multiferroic Double Perovskites for Photovoltaic Applications—First-Principles Approach

Clean and renewable solar energy is regarded as one of the most reliable and abundant sources to tackle the prevalent energy crisis. The photovoltaic effect, directly converting sunlight into electricity, is an important way to harvest solar energy, and many efforts have been devoted to the development of photovoltaic technology. Toward the next-generation photovoltaics, multiferroic materials (MFs), with the coexistence of ferroelectricity and magnetism in the same phase, show promise as candidates for harvesting energy, especially from sunlight. The spontaneous polarisation of such type of non-centrosymmetric materials is the origin of charge carrier separation. On the other hand, magnetic ordering induces a small bandgap, typically due to electronic states of the magnetic atom in the Fermi energy region. One method to find these rare materials is to employ double perovskites (DPs) with general formula A2BB'X6, in which one of the sublattices is magnetic, and the other one is ferroelectric. This study intends to develop and apply theoretical tools to predict and improve the photovoltaic properties in new double perovskites with multiferroic properties. In that light, the structural, magnetic, electronic, and photovoltaic properties of new MF-DPs A2MnBO6 (A=Sn, Bi and B=V, Ta, Ti) using ab initio simulations based on DFT method are systematically investigated. The electronic band structure and density of states demonstrate the semiconducting behaviour of these double perovskites with the values of band gaps falling within the desired range for optimal photovoltaic performance. Structural calculations, including distortion parameters and bond angles, indicate that these DPs undergo a significant octahedral tilting and distortion which may result in a considerable value of electric polarization and hence, may be suggested as potential multiferroic materials. In the MnO6 octahedra, Jahn-Teller effect, which is related to the Mn3+ electron configuration, distorts the Mn-O bonds. Whereas, in Ta(Ti)O6 octahedra, the Second Order Jahn-Teller effect originating from d0 electron configuration of Ta5+(Ti4+) is responsible for the off-centering displacement of Ta(Ti) atoms and the distortion of Ta(Ti)O6 octahedra. Optical properties, including absorption coefficient (α) calculations, show that the value of α in the visible region is in the order of 105 cm-1, comparable to the absorption coefficient of some well-known photovoltaic materials. These results draw an intense theoretical research interest to explore the photovoltaic properties of new multiferroic double perovskites to characterize them as potential solar materials.