Silicon nitride compounds emit photoluminescence all over the visible range. Recent studies ascribed this luminescence to quantum-size effects within silicon nanocrystals that were either shown or assumed to form inside the silicon nitride matrix; the luminescence of the matrix itself was ignored. In contrast, observing the same luminescence even without the presence of silicon crystallites, our work identifies the silicon nitride matrix itself as responsible for the photoluminescence. All experimental observations are well explained by band tail luminescence from the silicon matrix. In contrast to the silicon nanocrystal approach, our model explains all aspects of the luminescence. As a consequence, we conclude that silicon nitride films are inappropriate if one aims at investigating photoluminescence from silicon nanocrystals within such a matrix.
The piezoresistive response of n-and p-type hydrogenated nanocrystalline silicon thin films, deposited by hot-wire (HW) and plasma-enhanced chemical vapor deposition (PECVD) on thermally oxidized silicon wafers, has been studied using four-point bending tests. The piezoresistive gauge factor (GF) was measured on patterned thin-film micro-resistors rotated by an angle θ with respect to the principal strain axis. Both longitudinal (GF L) and transverse (GF T) GFs, corresponding to θ = 0º and 90º, respectively, are negative for n-type and positive for p-type films. For other values of θ (30º, 45º, 120º and 135º) GFs have the same signal as GF L and GF T and their value is proportional to the normal strain associated with planes rotated by θ relative to the principal strain axis. It is concluded that the films are isotropic in the growth plane since the GF-values follow a Mohr's circle with the principal axes 2 coinciding with those of the strain tensor. The strongest p-type pirezoresistive response (GF L = 41.0, GF T = 2.84) was found in a film deposited by PECVD at a substrate temperature of 250ºC and working pressure of 0.250 Torr, with dark conductivity 1.6 Ω-1 cm-1. The strongest n-type response (GF L =-28.1, GF T =-5.60) was found in a film deposited by PECVD at 150ºC and working pressure of 3 Torr, with dark conductivity 9.7 Ω-1 cm-1. A model for the piezoresistivity of nc-Si is proposed, based on a mean-field approximation for the conductivity of an ensemble of randomly oriented crystallites and neglecting grain boundary effects. The model is able to reproduce the measured GF L values for both n-and p-type films. It fails however to explain the transversal GF T data. Both experimental and theoretical data show that nanocrystalline silicon can have an isotropic piezoresistive effect of the order of 40% of the maximum response of crystalline silicon.
The monolithically integrated series connection of single solar cell stripes into complete photovoltaic (PV) modules is one of the key advantages of thin film PV technologies. Instead of the well established laser scribing for series connection, this contribution focuses on a novel in situ series connection technology, without breaking the vacuum during module manufacturing, and without the need of costly laser-scribing equipment. Metallic wires or other filaments aligned along the slightly bent substrate, sequentially pattern the solar cell layers for implementing the monolithic series connection, simultaneously with the consecutive evaporation, plasma deposition, and sputtering of the semiconductor and contact layers. In addition to a proof of concept by flexible PV modules, this paper for the first time investigates wire-shading on rigid glass substrates and by multiple adjacent filaments. The results of these studies demonstrate that the in situ series connection is a promising candidate for competing with laser scribing, not only in roll-to-roll production of flexible PV modules, but also in batch or inline processing of standard large-area glass plates. Applying the novel in situ series connection to a laboratory-scale solar cell process, yields 40 cm 2 sized PV modules, consisting of ten single junction amorphous silicon n-i-p cells on a flexible polymer foil. The modules' total area efficiency of 3 % is close to the non-optimized efficiency of reference cells of 3.3 %. Wire-shading with wire diameters down to 50 µm proves successful, and thereby projects total interconnection losses F < 5 %, whereas the first experimental modules exhibit F = 15 %.
711 685 67143 690 R. Merz et al.: Optimization of in situ series connection of aSi solar modules physica s s p status solidi a ß
This paper analyzes the impact of hydrogen on the photoluminescence (PL) efficiency of the three wide gap silicon alloys: silicon carbide (a-SiCx), silicon nitride (a-SiNx): silicon oxide (a-SiOx). All three materials behave similarly. The progression of the PL efficiency over the Si content splits into two regions. With decreasing Si content, the PL efficiency increases until a maximum is reached. With a further decrease of the Si content, the PL efficiency declines again. A comprehensive analysis of the sample structure reveals that the PL efficiency depends on the degree of passivation of Si and Y atoms (Y = C, N, O) with hydrogen. For samples with a high Si content, an effective passivation of incorporated Y atoms gives rise to an increasing PL efficiency. The PL efficiency of samples with a low Si content is limited due to a rising amount of unpassivated Si defect states. We find that a minimum amount of 0.2 H atoms per Si atom is required to maintain effective luminescence.
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