Akash Bhatnagar

Bio: Akash Bhatnagar is an academic researcher from Martin Luther University of Halle-Wittenberg. The author has contributed to research in topics: Thin film & Anomalous photovoltaic effect. The author has an hindex of 9, co-authored 20 publications receiving 634 citations. Previous affiliations of Akash Bhatnagar include Max Planck Society & University of Warwick.

Role of domain walls in the abnormal photovoltaic effect in BiFeO 3

Abstract: Recently, the anomalous photovoltaic (PV) effect in BiFeO3 (BFO) thin films, which resulted in open circuit voltages (Voc) considerably larger than the band gap of the material, has generated a revival of the entire field of photoferroelectrics. Here, via temperature-dependent PV studies, we prove that the bulk photovoltaic (BPV) effect, which has been studied in the past for many non-centrosymmetric materials, is at the origin of the anomalous PV effect in BFO films. Moreover, we show that irrespective of the measurement geometry, Voc as high as 50 V can be achieved by controlling the conductivity of domain walls (DW). We also show that photoconductivity of the DW is markedly higher than in the bulk of BFO.

Persistent photoconductivity in strained epitaxial BiFeO3 thin films.

Abstract: A drastic change in the conductivity of strained BiFeO3 (BFO) films is observed after illuminating them with above-band gap light. This has been termed as persistent photoconductivity. The enhanced conductivity decays exponentially with time. A trapping character of the sub-band levels and their subsequent gradual emptying is proposed as a possible mechanism.

Enhancement of Local Photovoltaic Current at Ferroelectric Domain Walls in BiFeO 3

Abstract: Domain walls, which are intrinsically two dimensional nano-objects exhibiting nontrivial electronic and magnetic behaviours, have been proven to play a crucial role in photovoltaic properties of ferroelectrics. Despite this recognition, the electronic properties of domain walls under illumination until now have been accessible only to macroscopic studies and their effects upon the conduction of photovoltaic current still remain elusive. The lack of understanding hinders the developing of nanoscale devices based on ferroelectric domain walls. Here, we directly characterize the local photovoltaic and photoconductive properties of 71° domain walls on BiFeO3 thin films with a nanoscale resolution. Local photovoltaic current, proven to be driven by the bulk photovoltaic effect, has been probed over the whole illuminated surface by using a specially designed photoelectric atomic force microscopy and found to be significantly enhanced at domain walls. Additionally, spatially resolved photoconductive current distribution reveals a higher density of excited carriers at domain walls in comparison with domains. Our measurements demonstrate that domain wall enhanced photovoltaic current originates from its high conduction rather than the internal electric field. This photoconduction facilitated local photovoltaic current is likely to be a universal property of topological defects in ferroelectric semiconductors.

Optical properties of epitaxial BiFeO3 thin films grown on LaAlO3

Abstract: Highly strained and nearly pseudomorphic BiFeO3 epitaxial films were deposited on LaAlO3 and TbScO3 substrates, respectively. The symmetry of the tetragonal-like BiFeO3 films is discussed based on polarisation dependent Raman measurements and on the comparison with Raman spectra measured for rhombohedral films deposited on TbScO3. The evaluation of ellipsometric spectra reveals that the films deposited on LaAlO3 are optically less dense and the features in complex dielectric function are blue-shifted by 0.3 eV as compared to the rhombohedral films. Optical bandgaps of 3.10 eV and 2.80 eV were determined for the films deposited on LaAlO3 and TbScO3, respectively. The shift in the optical bandgap and dielectric function is nearly preserved also for thicker films, which indicates that the compressive strain is retained even in films with thicknesses above 100 nm as was confirmed also by XRD investigations.

Mutual Insight on Ferroelectrics and Hybrid Halide Perovskites: A Platform for Future Multifunctional Energy Conversion.

Abstract: An insight into the analogies, state-of-the-art technologies, concepts, and prospects under the umbrella of perovskite materials (both inorganic-organic hybrid halide perovskites and ferroelectric perovskites) for future multifunctional energy conversion and storage devices is provided. Often, these are considered entirely different branches of research; however, considering them simultaneously and holistically can provide several new opportunities. Recent advancements have highlighted the potential of hybrid perovskites for high-efficiency solar cells. The intrinsic polar properties of these materials, including the potential for ferroelectricity, provide additional possibilities for simultaneously exploiting several energy conversion mechanisms such as the piezoelectric, pyroelectric, and thermoelectric effect and electrical energy storage. The presence of these phenomena can support the performance of perovskite solar cells. The energy conversion using these effects (piezo-, pyro-, and thermoelectric effect) can also be enhanced by a change in the light intensity. Thus, there lies a range of possibilities for tuning the structural, electronic, optical, and magnetic properties of perovskites to simultaneously harvest energy using more than one mechanism to realize an improved efficiency. This requires a basic understanding of concepts, mechanisms, corresponding material properties, and the underlying physics involved with these effects.

Bandgap tuning of multiferroic oxide solar cells

Abstract: Tuning the bandgap of multiferroic solar cells made from Bi2FeCrO6 is achieved by cationic ordering and is shown to dramatically improve their performance.