First-principles study of the electrooptic effect in ferroelectric oxides

We present a method to compute the electrooptic tensor from first principles, explicitly taking into account the electronic, ionic and piezoelectric contributions. It allows us to study the non-linear optic behavior of three ferroelectric ABO_3 compounds : LiNbO_3, BaTiO_3 and PbTiO_3. Our calculations reveal the dominant contribution of the soft mode to the electrooptic coefficients in LiNbO_3 and BaTiO_3 and identify the coupling between the electric field and the polar atomic displacements along the B-O chains as the origin of the large electrooptic response in these compounds.Comment: accepted for publication in Phys. Rev. Let

Pyroelectric Trapping and Arrangement of Nanoparticles in Lithium Niobate Opposite Domain Structures

The particular ferroelectric domain structure of periodic opposite domain lithium niobate (ODLN) crystals has been used for the first time for structured nanoparticle trapping. The surface charge density produced by a temperature change in this pyroelectric material is the origin of the trapping forces: dielectrophoretic on neutral particles and electrophoretic on charged ones. Metallic and dielectric particles are trapped and structured. The results show that ODLN structures are efficient substrates for pyroelectric trapping. The different trapping behaviors are presentedThis work has been supported by the Spanish Ministerio de Economía y Competitividad under grant ref.: MAT2014-57704-C3-1-

Non-linear optical susceptibilities, Raman efficiencies and electrooptic tensors from first-principles density functional perturbation theory

The non-linear response of infinite periodic solids to homogenous electric fields and collective atomic displacements is discussed in the framework of density functional perturbation theory. The approach is based on the 2n + 1 theorem applied to an electric-field-dependent energy functional. We report the expressions for the calculation of the non-linear optical susceptibilities, Raman scattering efficiencies and electrooptic coefficients. Different formulations of third-order energy derivatives are examined and their convergence with respect to the k-point sampling is discussed. We apply our method to a few simple cases and compare our results to those obtained with distinct techniques. Finally, we discuss the effect of a scissors correction on the EO coefficients and non-linear optical susceptibilities

Photovoltaic R and D in the Federal Republic of Germany

Research and Development in the field of photovoltaics have found a steadily increasing support from the Federal Ministry of Research and Technology (BMFT) as well as from the Federal States. Since the Governmental grants for the private industry are on a cost-sharing basis the Governmental budgets are supplemented by appreciable contributions from that side. The aim of the German PV R and D programme is the development of technical solutions for the use of solar energy for the production of electricity as well as the development of cost-effective technologies in the fields of materials, solar cells, modules, systems and system components. In the long range PV is thought to be one of the most valuable energy options

Silicon sheets for solar cells grown from silicon powder by the SSP-technique

A thin layer of silicon powder is poured onto supporting plates made of quartz. In a second step the surface or this layer is melted by focussing light of halogen lamps onto the surface. The liquid silicon combines the powder particles. Thus a self-supporting pre-sheet is formed which can easily be removed from the quartz plates. In a third step this pre-sheet is converted into the final coarse-grained sheet by zone melting also using focussed light as heat source. The feasibility of the technique was demonstrated (1,2) by fabricating sheets of up to 80 x 150 qmm at a thickness of only 350 mym. Now, a new machine has been built up which allows continuous processing of ribbons of 100 mm width. The same sequence of process steps and the same heating technique is used. Starting material is a mixture of undoped and highly doped silicon granulate grown from a CVD process in a fluidized bed reactor. The sheets are characterized by metallographical and electrical methods. Grains extend to som e cm in length and some mm in width showing a predominance of in growth direction. Strong variations in the etch-pit-density (10 high 3-10 high 6/qcm) were correlated with variations in EBIC contrast. Test solar cells (20 x 20 qmm) from these demonstrated already the quality of the SSP-technique. Conversion efficiencies ranged up to 13,2 % (I sub SC = 29.8 mA/qcm, V sub OC = 563 mV, FF = 0.78). From I sub SC decay measurements local diffusion lengths of up to 250 mym were calculated

ELECTRON-SPIN-RESONANCE AND OPTICAL ABSORPTION OF Ni+ AND Ti3+ IN IB-III-S2 COMPOUNDS

On a étudié la résonance paramagnétique électronique (rpe) et l'absorption optique de Ni+(3d9) dans CuAlS2, CuGaS2 et AgGaS2. L'identification des centres rpe axiaux, comme provenant du Ni, a été confirmée par l'observation de la structure hyperfine de cristaux dopés au Ni61 (I = 3/2). A la fois les données optiques et celles de la rpe permettent de déduire de grandes séparations dues au champ axial provenant de la composante tétragonale du champ cristallin de la structure chalcopyrite. Tous les résultats expérimentaux peuvent être interprétés de manière consistante dans le cadre d'une théorie d'un champ cristallin statique. La rpe de Ti3+(3d1) a été observée dans CuAlS2 et AgGaS2. Pour CuAlS2, la raie de résonance se sépare en 21 composantes hyperfines bien résolues, résultant d'une interaction avec les ligandes des noyaux des quatre plus proches voisins équivalents Al27 (I = 5/2).Electron-Spin-Resonance (ESR) and optical absorption of Ni+(3d9) has been observed in CuAlS2, CuGaS2 and AgGaS2. Identification of the axial ESR centers as being due to Ni has been confirmed by the observation of hyperfine structure in crystals enriched with Ni61 (I = 3/2) Large axial field splittings due to the tetragonal crystal field component in the chalcopyrite structure are inferred both from the ESR and optical data. All experimental results can be consistently interpreted within the framework of static crystal field theory. ESR of Ti3+ (3d1) has been observed in CuAlS2. The resonance line is split into 21 well resolved hyperfine components arising from interaction with four equivalent next-nearest Al27 (I = 5/2) ligand nuclei

Silicon ribbons for solar cells grown from powder by the SSP method

Aim of the SSP project (Silicon Sheets from Powder) is to grow large area silicon substrates for solar cells of high conversion efficiency. A silicon powder layer is converted into a polycrystalline sheet by a zone melting process using incoherent focussed light as heat source. The feasibility of the method had been demonstrated before by the growth of 60 x 150 qmm sheets. Now the design of a new machine for semi-continous production of 100 mm wide silicon ribbons is presented. As the grain selection takes place at the beginning of zone melting, continous processing instead of single sheet processing is reasonable and leads to fairly wide grains (in the order of 10-20 mm) extending over the whole length of the ribbon. Grain structure and defects are revealed by etching techniques. Due to the high intra-grain defect density there is a significant effect of hydrogen passivation which is demonstrated by EBIC and LBIC measurements. The best solar cells made from SSP material showed effici encies above 13%. The problem of the starting material is solved now. High purity silicon particles of 150-600 mym diameter from a SiH4-CVD-process in a fluidized bed reactor are available now and are proved to be suitable for SSP