Oxide materials for energy harvesting: In-silico study coupled with experiment

Resume : Solution processing of inorganic thin films has become an important thrust in material research community because it offers low-cost and high-throughput deposition of various functional coatings and macroelectronic devices such as thin film solar cells. There have been several examples of solar cells with solution (ink)-processed absorber materials, including Cu(In,Ga)Se2, Cu2ZnSn(S,Se)4, CdTe-CdS nanocrystal hybrids, a-Si, Cu2O-ZnO, and recently emerged organometallic halide perovskites. Solution processing of other functional layers like metal contacts and especially transparent contacts still remains a big challenge because a high-temperature annealing step is usually required. The talk will critically review solution approaches for the functional layers of inorganic thin film solar cells. As the main highlight, a proprietary low-temperature (<100 °C) chemical bath deposition route to fabricate highly conductive Al-doped ZnO (AZO) is used to fabricate transparent contacts for various types of thin film solar cells. Conversion efficiencies approaching 14% have been achieved for solar cells in which all functional layers (except the metal back contact) were processed from aqueous solutions.

Resume : Hybrid organic/inorganic perovskite material, CH3NH3PbI3 (=MAPbI3) has received a great deal of attention due to their intrinsic properties such as appropriate band gap (1.55 eV), high absorption coefficient, long hole-electron diffusion length (~100 nm) and excellent carrier transport in photovoltaic applications. Recently, great progress has been made in the perovskite solar cells consisting of electron transporting layer/perovskite materials/hole-transporting layer/Au. Despite of high performance, most of electron transporting layer such as mesoporous (mp)-TiO2-layer process at high temperature. Therefore, many approaches have been performed to avoid high-temperature manufacturing (>450 oC) for preparing a compact or mp-TiO2 layer. In this work, we have demonstrated highly efficient perovskite-PCBM heterojunction unit cells and modules (10 x 10 cm2). The optimization of PCBM layer thickness (with ~ 55 nm) and insertion of LiF interlayer, owing to the formation of extremely uniform perovskite layers, enabled the device performance to reach 14.1% in a unit cell and 8.7% in module, respectively, which are one of the highest values reported till date for a normal-type perovskite-PCBM heterojunction solar cells. This study provides new avenues for the fabrication of more efficient perovskite thin film solar cells by solution-processing at low temperature and great potential in large-area flexible photovoltaic devices.

Resume : La1-xSrxCo0.25Fe0.75O3-delta perovskite solid solutions containing reducible transition metals is known as one of the best cathode materials for permeation membranes and solid oxide fuel cells. The key properties controlling the quality of cathode materials are the concentration of oxygen vacancies and their migration energies. The first principles calculations were used in this study, in order to understand properties of oxygen vacancies in La1-xSrxCo0.25Fe0.75O3-delta and La1-xSrxFeO3-delta at two concentrations of Sr dopant, namely 12.5 and 50%. The standard GGA-type PBE exchange-correlation (ex-corr) functional and the DFT+U method, as implemented in plane wave code VASP, and hybrid ex-corr functional PBE0, as implemented in Gaussian basis set code CRYSTAL, were used for careful analysis of obtained results. The calculations were spin polarized in supercells with full structure relaxation. The analysis also included the phonon contribution to the formation energies of oxygen vacancies using direct method and supercells and, thus, temperature dependences. We give a particular emphasis to the comparison of calculated formation energies with those obtained in thermogravimetric measurements as well as permeation measurements. The role of ex-corr functional for the calculated formation energies is discussed from the viewpoint of Sr content and valence state of Fe.

Resume : The identification of alternative and renewable sources of energy is one of the main challenges that modern society faces. One of the most promising technologies is that of thermoelectric (TE) devices, which allows one to transform heat into electrical energy or vice-versa. Several basic technological problems still need to be solved before thermoelectricity becomes a competitive energy source for large scale applications. In particular, the performance of TE materials governed by the figure of merit ZT will have to be roughly doubled before large-scale applications can be envisaged (ZT=(S2σT)/(κe + κl), where S2σ is called the power factor PF, κe and κl are the electronic and lattice contributions to the thermal conductivity, and T is absolute temperature). In practice, ZT should be greater than 3 for TE devices to become fully competitive with other energy conversion systems. New perspectives on thermoelectrics have been opened by nanostructuring, which has generated materials with ZT’s ranging from 1 to 2.4. Using a new guidance idea, which exploits the highly-directional character of some orbitals to produce a low-dimensional transport, and the concept of electronic band structure engineering we predict from first-principles calculations impressive PF’s in LaVO3(KNbO3) nanostructures embedded in a SrTiO3 matrix. These impressive PF’s are realistically able to generate ZT’s>3. The origin of these impressive PF’s will be discussed in detail.

Resume : A device that can convert solar energy into a chemical fuel with good efficiency while also offering high stability and the capability to be constructed from widely available materials using inexpensive processing techniques is urgently needed. A practical, stable and inexpensive device could be fabricated with an n-type oxide photoanode in tandem with a p-type oxide photocathode. In this presentation I will present our progress in the development of solution-processed, stable oxide photoelectrodes and their application toward overall photoelectrochemical water splitting tandem cells. I will discuss the feasibility of employing BiVO4, WO3, or Fe2O3 as the n-type photoanode and Cu2O or CuFeO2 as the p-type photocathode. Combinations of these materials have potential solar-to-fuel conversion efficiencies higher than 10%. However, specific drawbacks have limited their performance in solar water splitting to date. I describe these limitations and our efforts to overcome them. In detail, by employing water oxidation and reduction catalysts (Co-Pi and RuOx, respectively) together with an operating point analysis we show that an unassisted solar photocurrent density on the order of 1 mA cm–2 is possible and moreover gain insight into routes for improvement. We further demonstrate the unassisted 2-electrode operation of a tandem cell in this system and show that light harvesting is an important limiting factor.

Resume : Electrochemical interfaces are present in many emerging energy applications, such as fuel cells, batteries, or solar fuel conversion devices. In most studies, experimental methods are used in order to characterize the interface and the processes taking place at the interface. However, a combined approach of experiments and modelling & simulations does significantly improve the understanding of these interfaces and gives guidelines for advanced processing of electrochemical interfaces. We will illustrate this with two examples: In solid oxide fuel cells, we have prepared patterned Ni thin film model electrodes and have characterized them in detail by electrochemical impedance spectroscopy as a function of pattern geometry. In combination with electrochemical modelling of the Ni, H2-H2O | YSZ interface, we could exclude seven possible reaction mechanisms to be limiting the kinetics at the interface and could pinpoint the rate limiting reaction step at the fuel cell anode to hydrogen spillover. In photoelectrochemical solar fuel conversion, we are pursuing a similar approach. We are fabricating and electrochemically characterizing well-defined, nanostructured photoelectrodes by impedance spectroscopy. In parallel, we are doing electrochemical modelling & simulations of the photoelectrode ? electrolyte interface. Combining both aspects will allow us to identify the electrochemical limitations at solar fuel conversion electrodes and to process highly efficient photoelectrodes in the future.

Resume : Interest in the study of lanthanum manganites with perovskite structure primarily associated with observed in them the effect of colossal magnetoresistance (CMR) at close to room temperature, which is promising in the practical application as highly sensitive magnetic field sensors, components of anodes of solid oxide fuel cells, catalysts of chemical reactions, etc. This work presents the results of studies of the effect of superstoichiometric manganese on density, grain size and structure as well as the phase composition and magnetoresistive properties of ceramics (La0.65Ca0.35)1-xMn1+xO3±Δ (LCMO) (x = 0, 0.2) sintered at 1000-1500 ° C. Powder manganite of LCMO had been obtained by sol-gel methods and compacted into preforms by cold isostatic pressing at a pressure of 0.1 GPa. Sintering was performed at 1000, 1200, 1300, 1400, 1450 and 1500 °C for 2-3 hours. After measuring a density the samples were split and the cleaved surfaces were thermally etched at 1000 °C for visualization of the grain and its internal structure. The x-ray diffractions were measured by the diffractometer Philips-1050/70 in CuKα radiation. The study of the microstructure of ceramics was carried out by the scanning electron microscope (SEM) JSM-6490LV. Measurements of the magnetoresistive effect was performed using four probe method in magnetic fields H=0 and 5 kOe in the temperature range 77-400 K. SEM studies were shown that with increasing the sintering temperature above 1300 °C the excess of manganese allocates on the grain boundaries in the oxides form of Mn3O4 and thus the composite material with clean LCMO and Mn3O4 grains is obtained. In addition, the presence of superstoichiometric manganese has a decisive influence on the structure of ceramics as a uniform grain growth and the emergence of nanoscale layered LCMO grain structure after sintering at 1450 °C, as well as, significantly effects on the magnetoresistive properties of ceramics. Experimental results of measurements shown that with rise in the sintering temperature in the ceramics with x=0.2 the peak value of colossal magnetoresistance increasing more in 2,5 times as compared with the ceramics with x=0. In addition, in the LCMO (x=0.2) ceramics sintered at 1450 °C having composite structure and less grain size the sloping low temperature trend of a magnetoresistance curve was detected which is typical for the single-crystal manganite.

Resume : Efficient solar-to-hydrogen conversion in photoelectrochemical (PEC) cell is an ultimate goal for scientific community in the field of energy generation. Surface disor¬dering at nano-level is a promising tool for altering optical absorption and carrier transport in metal oxide semiconductors which can be used an efficient photoelectrode in PEC cell for solar energy harvesting. RF-sputtering technique offers a viable solution for overcoming thermodynamic impediments involved in the growth of metal oxide thin film and surface engineering through hydrogen treatment. This talk will present how ionic and molecular hydrogen incorporation in sputter deposited metal oxide thin films alters the material properties, which can be transformed into advanced material technologies. Optical absorption, XPS and KPFM studies shows that changes in the band gap, shift in valence band maximum and work function values are dependent on method of hydrogen incorporation. Enhancement in photoelectrochemical response in hydrogen treated metal oxide thin films will be explained in terms of optical and electrical properties along with energy band diagrams drawn on the basis of work function, band gaps, and valence band offsets.

Resume : TiO2 photocatalysts have two key challenges: (1) to shift the TiO2 band gap to the visible region, allowing solar energy to be used and (2) enhancing charge separation after photoexcitation. We discuss our simulation driven work on a new mechanism for band gap modification in TiO2: surface modification of TiO2 with metal oxide nanoclusters. Modifying TiO2 with transition metal oxide nanoclusters induces visible light activity, which is achieved by introducing nanocluster derived electronic states above the original TiO2 valence band edge, to shift the VB edge to higher energy. A model of the photoexcited state confirms the band gap reduction which is controlled by the coverage of transition metal oxide nanoclusters. For tin oxide, experiment and simulation show that the change in the band gap is controlled by two factors: the tin oxidation state and the TiO2 crystal polymorph, with rutile and anatase showing different properties when modified with SnO2. The stereochemical lone pair in SnO and PbO induces a band gap reduction. Simple rules for modifying TiO2 to induce visible light absorption are presented. Models of the photoexcited state of modified TiO2 show that the photoexcited hole localises on low coordinated oxygen in the nanocluster and the electron in the TiO2 surface; the presence of low coordinated atoms is crucial. We present initial studies of the interaction of molecules at these structures.