P.-T. Miclea

Upgraded Silicon Nanowires by Metal‐Assisted Etching of Metallurgical Silicon: A New Route to Nanostructured Solar‐Grade Silicon

Through metal-assisted chemical etching (MaCE), superior purification of dirty Si is observed, from 99.74 to 99.9884% for metallurgical Si and from 99.999772 to 99.999899% for upgraded metallurgical Si. In addition, large area of silicon nanowires (SiNW) are fabricated. The purification effect induces a ∼35% increase in photocurrent for SiNW based photoelectrochemical cell.

Structural phase transitions of barium halide nanocrystals in fluorozirconate glasses studied by Raman spectroscopy

Rare-earth-doped fluorochlorozirconate (FCZ) and fluorobromozirconate (FBZ) glasses developed for fluorescence applications are analyzed, with particular attention paid to their phonon energy spectra. After thermal processing of as-made Eu-doped FCZ and FBZ glasses, Raman measurements show additional phonon bands at low phonon energies, indicating the formation of BaCl2 and BaBr2 nanocrystals, respectively. The phonon bands can be assigned to hexagonal, orthorhombic, or a mixture of both phases of BaCl2 and BaBr2 depending on the annealing conditions. Apart from line broadening, the Raman spectra of the orthorhombic nanocrystals agree well with those of BaCl2 and BaBr2 bulk crystals, while the metastable hexagonal phases of BaCl2 and BaBr2 are investigated only in appropriately treated FCZ and FBZ glasses, respectively. The experimental Raman spectra are compared to first principle studies of the phonon frequencies of the hexagonal and orthorhombic phases of both barium halides, showing good agreement.

Few micrometers wide, perfectly isolating scribes in transparent conductive oxide layers prepared by femtosecond laser processing

Laser ablation for the preparation of isolating trenches in transparent conductive oxide films on glass has been studied comparing the use of nano- and femtosecond pulses at approximately 1 μm wavelength. By a detailed analysis of the obtained ablation craters and grooves, it can be shown that this processing, which is called P1 scribing in the field of thin film photovoltaics, can be considerably optimized with so-called “cold” ablation: Using femtosecond pulses, the P1 scribe can be done with almost rectangular cross section profiles, achieving the required electrical separation already at widths well below 10 μm, without thermal and mechanical stresses in the substrate or adjacent material. Analogous preliminary results for P2 scribes indicate that this technique allows reducing the total scribe region (optically inactive zone) to widths below 50 μm.

Dispersive elements for spectrum splitting in solar cell applications

Photovoltaic tandem and triple solar cells are currently being developed and produced with reasonable efficiencies at high technological cost. The concept of spectrum splitting has been proposed with the advantage of compatibility to all types of cells. Although additional optical efforts are to be made, external photon management can be achieved to match different solar cell combinations no matter which band gaps involved or how the cells are connected. We present an experimental study comparing optical devices based on either interference or diffraction for tandem and triple cell configurations. Whereas diffractive media such as gratings suffer intrinsically from higher order diffraction losses, devices based on interference such as Bragg filter can yield a significant efficiency increase. For a triple cell configuration consisting of GaInP/GaInAs/GaSb, a net efficiency gain of more than 30% is shown in a solar cell simulator compared to the best cell in direct light.

Optical characterization of TCO films on borate glasses for high efficiency solar cells

Two different transparent conductive oxides (TCO) were deposited by magnetron sputtering on borate glasses. The influence of sputtering conditions on optical, electrical and microstructural properties was much higher for indium tin oxide (ITO) than for aluminium-doped zinc oxide (AZO) films. Specific resistivity values obtained from simulation of the optical spectra are in good agreement with values obtained from four-point probe measurements.

Optical characterization of ITO films on fluorescent borate glasses for high efficiency solar cells

Solar cells with a cover glass have a poor response in the blue and near UV spectral range due to absorption in the glass and the front contact layer. The efficiency of these solar cells can be improved by replacing the top glass by a spectral converter, i.e. a frequency down-converter, without modifying the solar cell itself. Potential candidates are fluorescent, samarium-doped borate glasses. They convert the incident violet and blue part of the solar spectrum to visible red light, which is more efficiently absorbed by the solar cell. The optical properties of borate glasses with different network modifiers are investigated. Transmission spectra of undoped sodium, barium, and lead borate glass reveal significant differences in the absorption edge. Optical simulations were performed to determine the complex refractive indices of the glasses. In addition, the different borate glasses were used as substrates for deposition of a conductive indium tin oxide (ITO) film. The deposition is done by radio frequency magnetron sputtering. The layer thickness is subsequently analyzed by profilometry and scanning electron microscopy (SEM) of cross-section samples. Again, optical simulations provide the refractive indices of the ITO films. Apart from the optical parameters, the electrical ones such as resistivity and I-V characteristics are determined. Moreover, the influence of different sputter parameters, i.e. gas pressure, oxygen flow, and film thickness on the optical and electrical properties of the deposited films is investigated.

Size and structure of barium halide nano-crystals in optically active fluorozirconate-based glasses

Optically active fluorozirconate-based (FZ) glass ceramics offer a broad range of applications. The functionality of the glass ceramic can be modified by appropriate doping and thermal processing performed after the glass production. For use in digital radiography, for example, the FZ glass has been doped with europium and chlorine ions [1, 2]. Thermal processing by annealing in the vicinity of the glass transition temperature produces barium chloride nano-crystals in the glass. The glass ceramic can act either as a scintillator (able to convert ionizing radiation to visible light) [1], or as a storage phosphor (able to convert the radiation into stable electron-hole pairs, which can be read out afterwards with a scanning laser beam in a so-called “photostimulated luminescence“ process) [2].