Tobias Boström

Spectral density analysis of the optical properties of Ni-Al 2 O 3 nano-composite films

Thin films consisting of transition metal nanoparticles in an insulating oxide exhibit a high solar absorptance together with a low thermal emittance and are used as coatings on solar collector panels. In order to optimise the nanocomposites for this application a more detailed understanding of their optical properties is needed. Here we use a highly efficient recently developed numerical method to extract the spectral density function of nickel-aluminum oxide (Ni-Al2O3) composites from experimental data on the dielectric permittivity in the visible and near-infrared wavelength ranges. Thin layers of Ni-Al2O3 were produced by a sol-gel technique. Reflectance and transmittance spectra were measured by spectrophotometry in the wavelength range 300 to 2500 nm for films with thicknesses in the range 50 to 100 nm. Transmission electron microscopy showed crystalline Ni particles with sizes in the 3 to 10 nm range. The spectral density function shows a multi-peak structure with three or four peaks clearly visible. The peak positions are influenced by particle shape, local volume fraction distributions and particle-particle interactions giving rise to structural resonances in the response of the composite to an electromagnetic field.

Efficiency Improvement in Nonprime Crystalline Silicon Solar Cells by Chemical Isolation of Shunts Under Front Metallization

Yield loss due to the breakage and production of nonprime or off-spec cells in industrially produced crystalline silicon-based solar cells is around 1-2%. Nonprime cells identified based on their electrical properties are rejected after quality inspection. The cells that are rejected can be classified as nonprime for many reasons, such as poor edge isolation, presence of conductive paths through p-n junctions formed by print paste stains, paste-filled microcracks, inclusions, nonuniform emitter, and nonuniform antireflective coatings. Development of efficient and economically feasible repair methods for the repowering of nonprime cells will increase the overall yield of the solar cell industry and reduce costs. To isolate severe shunts under front metallization, we developed a two-step chemical etching process to remove front metallization and emitter. Removal of front metallization and emitter yielded the best isolation of shunts. Shunt isolation and efficiency gain achieved by the chemical etching process has been demonstrated on both mono- and multicrystalline silicon solar cells.

Optical and passivating properties of sol-gel derived silica and titania coatings on textured monocrystalline silicon

In standard solar cell fabrication, silicon nitride is applied to the front surface by plasma enhanced chemical vapor deposition. The layer serves as an antireflective coating and provides passivation, but is costly to produce. An interesting alternative of making thin films is the sol-gel technique. The process is simple, cheap and low-temperature. In the present study, silica and titania sol-gel coatings were applied to textured and polished monocrystalline silicon wafers and subsequently annealed in a furnace. On polished wafers, the solar averaged reflectance was decreased from 0.37 to 0.11 with a titania layer and to 0.25 with a silica layer. The deviation in reflectance did not translate onto textured wafers. The reflectances of these wafers were decreased from about 0.13 to about 0.08 independently of coating type. The layers were not found to passivate the surface. The layers are promising in terms of reflectance, but to improve passivation properties new approaches are necessary.