Lars Oberbeck

Recombination mechanisms in amorphous silicon/crystalline silicon heterojunction solar cells

This article investigates limitations to the open circuit voltage of n-type amorphous silicon/p-type crystalline silicon heterojunction solar cells. The analysis of quantum efficiency and temperature dependent current/voltage characteristics identifies the dominant recombination mechanism. Depending on the electronic quality of the crystalline silicon absorber, either recombination in the neutral bulk or recombination in the space charge region prevails; recombination at the heterointerface is not relevant. Although interface recombination does not limit the open circuit voltage, recombination of photogenerated charge carriers at the heterointerface or in the amorphous silicon emitter diminishes the short circuit current of the solar cells.

Electronic properties of silicon epitaxial layers deposited by ion-assisted deposition at low temperatures

Ion-assisted deposition enables epitaxial growth of high-quality Si films with high deposition rates at low growth temperatures. The present study investigates structural and electronic properties of monocrystalline Si epitaxial layers deposited at temperatures between 470 and 800 °C and deposition rates between 0.07 and 0.18 μm/min. The density of extended defects decreases with increasing deposition rate and saturates at values around 2×104 cm−2 for deposition rates above 0.16 μm/min. We apply methods of design of experiment to systematically investigate the influence of deposition parameters on the electronic properties of epitaxial layers. The majority carrier mobility in p- and n-type layers reaches values comparable to those in Czochralski Si and does not depend on deposition rate, deposition temperature, and sample pretreatment prior to epitaxy in the investigated parameter range. The minority carrier diffusion length strongly increases with rising deposition temperature and reaches about 22 μm in ...

Ion-Assisted Deposition of Silicon Epitaxial Films with High Deposition Rate Using Low Energy Silicon Ions

Ion-assisted deposition (IAD) enables low temperature (≥ 435°C), high-rate (≤ 0.5 μm/min) epitaxial growth of silicon films. Therefore, IAD is an interesting deposition technique for microelectronic devices and thin film solar cells. The Hall-mobility of monocrystalline epitaxial layers increases with deposition temperature T dep and reaches values comparable to those of bulk Si at T dep ≥ 540°C. Polycrystalline epitaxial layers exhibit inhomogeneous electrical properties, as shown by Light Beam Induced Current measurements. Recombination within the grains dominates over recombination at grain boundaries. Secco etching identifies an inhomogeneous density of extended structural defects in the polycrystalline epitaxial layers and in the substrate. A major part of the extended defects in the epitaxial layers originates from defects in the substrate.