Publicaties

Reducing the absorber layer thickness below 1 μm for a regular copper indium gallium di-selenide (CIGS) solar cell lowers the minimum quality requirements for the absorber layer due to shorter electron diffusion length. Additionally, it reduces material costs and production time. Yet, having such a thin absorber reduces the cell efficiency significantly. This is due to incomplete light absorption and high Molybdenum/CIGS rear-surface recombination [1]. The aim of this research is to implement some innovative rear surface modifications on a 430 nm thick CIGS absorber layer to reduce both these affects: an aluminium oxide passivation layer to reduce the back-surface recombination and point contact openings using nano-particles for electrical contact. The impact of the implementation of all these rear-surface modifications on the opto-electrical properties of the CIGS solar cell will be discussed and analyzed in this paper.

Several aqueous solution-gel precursors for the Li ion battery cathode material LiFePO4, were synthesized. These differ in their composition, both regarding their Fe source, as well as in the complexing agent present. Fe(II) lactate hydrate is for the first time used as Fe2+ source. The ability to use an Fe3+ source (Fe(III) nitrate nonahydrate) for the synthesis of LiFePO4 is also investigated. Our results show that it is possible to reduce the Fe3+ to Fe2+, necessary to enable LiFePO4 phase formation, during annealing under specific conditions. The decomposition behavior for these precursors in dry air, as well as in an inert atmosphere, is shown. Raman spectroscopy is used to evaluate the structure of the carbon phases present after annealing of the precursor powders.

Spinel Li4Ti5O12 (LTO) is considered as a promising anode material for lithium ion batteries due to its high stability and its inherent safety. Since LTO is typically synthesized at high temperatures, a study of combustion synthesis of LTO is presented, wherein effects of oxidizer amount in the precursor and atmosphere during thermal decomposition are investigated. Combustion synthesis implies heating a precursor to a relatively low process temperature, after which the system generates the necessary energy for complete conversion and crystallization to the desired oxide. Hereto, the precursor and thermally treated powders were characterized by thermogravimetric analysis (TGA) coupled with differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray diffraction (XRD) and galvanostatic cycling. The study shows that the combustion process allows the synthesis of LTO at process temperatures as low as 300 °C, compared to around 800 °C for solid state reactions and sol-gel
routes. The product consists of crystalline LTO with minor impurities. The product was used as an electrode material in a lithium battery coin cell and demonstrated a high stability and a
capacity of 164 mAh g-1 at 0.1 C and 132 mAh g-1 at 2 C. This paper shows that combustion synthesis can considerably lower the temperature required for the synthesis of ceramic materials, after careful optimization of the precursor, since the mechanism of thermal degradation is complex and dependent on a large number of parameters.

The Pb2Fe2O5 compound with a layered intergrowth structure has been prepared by a solid-state reaction at 700 degrees C. The incommensurate compound crystallizes in a tetragonal system with alpha = 3.9037(2) angstrom, c = 3.9996(4) angstrom, and q = 0.1186(4)c star, or when treated as a commensurate approximant, alpha = 3.9047(2) angstrom, c = 36.000(3) angstrom, space group I4/mmm. The crystal structure of Pb2Fe2O5 was resolved from transmission electron microscopy data. Atomic coordinates and occupancies of the cation positions were estimated from high-angle annular dark-field scanning transmission electron microscopy data. Direct visualization of the positions of the oxygen atoms was possible using annular bright-field scanning transmission electron microscopy. The structure can be represented as an intergrowth of perovskite blocks and partially disordered blocks with a structure similar to that of the Bi2O2 blocks in Aurivillius-type phases. The A-cation positions at the border of the perovskite block and the cation positions in the Aurivillius-type blocks are jointly occupied by Pb2+ and Fe3+ cations, resulting in a layer sequence along the c axis: -PbO-FeO2-PbO-FeO2-Phys(7/8)Fe(1/8)-O1-x-Fe-5/8/Pb-3/8-O-2-Fe5/8Pb3/8-. Upon heating, the layered Pb2Fe2O5 structure transforms into an anion-deficient perovskite modulated by periodically spaced crystallographic shear (CS) planes. Considering the layered Pb2Fe2O5 structure as a parent matrix for the nucleation and growth of CS planes allows an explanation of the specific microstructure observed for the CS structures in the Pb-Fe-O system.

The particle surface of LiNi0.5Mn1.5O4-δ (LNMO), a Li-ion battery cathode material, has been modified by Ti cation doping through a hydrolysis-condensation reaction followed by an anneal in oxygen. The effect of different annealing temperatures (500-850oC) on the Ti distribution and electrochemical performance of the surface modified LNMO was investigated. Ti cations diffuse from the preformed amorphous ‘TiOx’ layer into the LNMO surface during annealing at 500oC. This results in a 2-4 nm thick Ti-rich spinel surface having lower Mn and Ni content compared to the core of the LNMO particles, which was observed with scanning transmission electron microscopy coupled with compositional EDX mapping. An increase in the annealing temperature promotes the formation of Ti bulk doped LiNi(0.5-w)Mn(1.5+w)-tTitO4 phase and Ti-rich LiNi0.5Mn1.5-yTiyO4 segregates above 750°C. Fourier-transform infrared spectrometry indicates an increasing Ni-Mn ordering withannealing temperature, for both bare and surface modified LNMO. Ti surface modified LNMO annealed at 500oC shows a superior cyclic stability, Coulombic efficiency and rate performance compared to bare LNMO annealed at 500oC when cycled at 3.4-4.9 V vs. Li/Li+. The improvements are probably due to suppressed Ni and Mn dissolution with Ti surface doping.