Electronic Properties of Materials seminar (Nanotalo). Speaker: Dr. Dong Jik Kim (Warwick University, UK).
The conventional bulk photovoltaic effect, which is completely distinct from that of p-n junction solar cells, hence free from the Shockley-Queisser detailed-balance limit, is attributed to quantum mechanical processes, such as photoexcitation, relaxation, recombination, and scattering, asymmetric in k-space and, therefore, arises only in noncentrosymmetric semiconductors. In the same time, any inhomogeneous or asymmetric deformation gives rise to an electric polarization due to a local symmetry breaking. This is generically known as flexoelectric effect and occurs in almost all materials including insulators, semiconductors, ionic materials and covalent materials (e.g., graphene).
We will show here that the same inhomogeneous or asymmetric deformation which give rise to flexoelectric effect combined with bulk photovoltaic effect can be a driving force to separate the photoexcited carriers. This new photovoltaic effect based on inhomogeneous strain is named flexo-photovoltaic effect. As the flexoelectric effect is a universal property, the flexo-photovoltaic effect would be a universal property in all semiconductors.
We demonstrate the flexo-photovoltaic effect by showing that a sharp tip of atomic force microscopy (AFM) or a probe needle of micro-indentation with a sufficient loading force enhances significantly the short-circuit photocurrent of centrosymmetric SrTiO3, TiO2 and Si. Local deformation and strain gradients are introduced using either an AFM or a micron-scale indentation system, creating a local bulk photovoltaic effect resulting in giant photovoltaic currents from centrosymmetric single crystals. The flexo-photovoltaic effect does not need any Fermi level gradient as in p-n junctions of a proper band alignment. Only simple strain gradient generator, such as a sharp probe with a sufficient loading force, is sufficient. This effect will extend significantly present solar cell technologies by boosting the solar energy conversion efficiency of a wide pool of established semiconductors. An important strain engineering playground for improving the final performance of solar cells and optoelectronic devices is now open.