Electron beam evaporation was employed in order to fabricate Al- and Tb-codoped Si oxide multilayers via the delta-doping approach. This methodology permits the control of the rare-earth (RE) separation along the growth direction with nanometric resolution. To investigate the control of the RE separation in the growth direction, different SiO2 thicknesses were studied. After deposition, the samples were submitted to different annealing processes for 1 h in N2, at temperatures ranging from 700 to 1100 °C. Photoluminescence experiments reveal narrow emissions ascribed to Tb3+ ions in all samples, with an intensity variation depending on the oxide thickness and annealing temperature. In addition, the incorporation of Al under different spatial configurations produced an enhancement of more than one order of magnitude in the photoluminescence intensity, in respect to the best sample without Al. Finally, time-resolved measurements were carried out in order to determine the 5D4→7F5 transition dynamics, obtaining a decay time of ~1.6 ms ascribed to the Tb3+ ions.
Light confinement strategies play a crucial role in the performance of thin-film (TF) silicon solar cells. One way to reduce the optical losses is the texturing of the transparent conductive oxide (TCO) that acts as the front contact. Other losses arise from the mismatch between the incident light spectrum and the spectral properties of the absorbent material that imply that low energy photons (below the bandgap value) are not absorbed, and therefore can not generate photocurrent. Up-conversion techniques, in which two sub-bandgap photons are combined to give one photon with a better matching with the bandgap, were proposed to overcome this problem. In particular, this work studies two strategies to improve light management in thin film silicon solar cells using laser technology. The first one addresses the problem of TCO surface texturing using fully commercial fast and ultrafast solid state laser sources. Aluminum doped Zinc Oxide (AZO) samples were laser processed and the results were optically evaluated by measuring the haze factor of the treated samples. As a second strategy, laser annealing experiments of TCOs doped with rare earth ions are presented as a potential process to produce layers with up-conversion properties, opening the possibility of its potential use in high efficiency solar cells.
The electrical and electroluminescence (EL) properties of Si-rich oxynitride (SRON)/SiO2 superlattices are studied for different silicon excess and layer thicknesses. The precipitation and crystallization of the Si excess present within the SRON layers is induced by a post-deposition annealing treatment, in order to form Si nanocrystals (Si-NCs). The electrical characterization performed in dark conditions allowed for deducing the charge transport mechanism through the superlattice structure, found to follow the Poole-Frenkel law. In addition, the EL investigation revealed the correlation between EL excitation and transport mechanisms, suggesting that impact ionization of high-energy conduction electrons dominates the whole frame. The reduction of the SiO2 barrier thickness and the increase in the Si excess were found to enhance the carrier transport through the superlattices due to the reduction of the electrons mean free path, which, in turn, modifies the EL properties.
A study of the non-linear optical properties of Si-nc embedded in SiO2 has been performed by using the z-scan method in the nanosecond and femtosecond ranges. Substoichiometric SiOx films were grown by plasma-enhanced chemical-vapor deposition (PECVD) on silica substrates for Si excesses up to 24 at. %. An annealing at 1250 °C for 1 hour was performed in order to precipitate Si-nc, as shown by EFTEM images. Z-scan results have shown that, by using 5-ns pulses, the non-linear process is ruled by thermal effects and only a negative contribution can be observed in the non-linear refractive index, with typical values around -10-10 cm2/W. On the other hand, femtosecond excitation has revealed a pure electronic contribution to the nonlinear refractive index, obtaining values in the order of 10-12 cm2/W. Simulations of heat propagation have shown that the onset of the temperature rise is delayed more than half pulse-width respect to the starting edge of the excitation. A maximum temperature increase of ΔT = 123.1 °C has been found after 3.5 ns of the laser pulse maximum. In order to minimize the thermal contribution to the z-scan transmittance and extract the electronic part, the sample response has been analyzed during the first few nanoseconds. By this method we found a reduction of 20 % in the thermal effects. So that, shorter pulses have to be used to obtain just pure electronic non-linearities.