Multicrystalline silicon films with large grains on glass: preparation and applications

Abstract

Polycrystalline silicon films on glass with grain sizes exceeding 10 µm by far are of growing interest for thin film solar cells as well as for TFTs in flat panel displays. Grains as large as this arise preferably from solidification of the melt. The most successful method for preparing such films is laser melting and crystallization of amorphous silicon, which may be deposited at temperatures well below the softening point of glass substrates. By making use of a laser the silicon melting time can be short enough as not to damage the substrate. Moreover, by using an appropriate laser irradiation process, a temperature regime can be chosen so as to influence the nucleation rate and growth kinetics to get grain sizes over 100 µm in size. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystalline silicon on glass—interface passivation and absorber material quality

“…Recently, liquid‐phase crystallization (LPC) of silicon by means of laser radiation or electron beams (e‐beam) showed very promising results . Because of its low thermal impact, the fast LPC process allows the usage of float glass substrates . Many concepts originally developed for the production of other thin film solar cells on glass—for example, large area deposition, laser patterning, and printing methods—can be transferred to the manufacturing of LPC‐Si‐based solar cells.…”

Multicrystalline silicon thin film solar cells on glass with epitaxially grown emitter prepared by a two-step laser crystallization process

“…Starting from a multicrystalline seed layer consisting of grains sized in the 10–100 µm range, absorber and emitter layers were prepared epitaxially. The seed layers, a few 100 nm thick, were generated by aluminum‐induced crystallization or by diode laser crystallization . In the final solar cell, the highly doped seed layer acts as an electrode layer and, in addition, generates a back surface field or emitter.…”

Applicability of an economic diode laser emitting at 980 nm for preparation of polycrystalline silicon thin film solar cells on glass

“…For this purpose, a common approach of fabricating high quality absorber material is the laser crystallization of silicon deposited on glass substrates by a liquid phase crystallization process (LPC) : a continuous wave (cw) laser beam is scanned across the sample and heats up the silicon layer of some micrometer thickness to its melting temperature. If the cooling rate is slow enough during the subsequent solidification very large grains with sizes of several hundred micrometers, millimeters, and even centimeters could be achieved . Recently, the material quality of such polycrystalline silicon (pc‐Si) became comparable with conventional multicrystalline wafers, e.g., indicated by high open circuit voltages of the prepared solar cells .…”

“…The laser used for the crystallization process must fulfill some technical demands to gather optimal results: a very high and stable power output in cw operation mode is needed to heat up the silicon above 1687 K and let it cool properly. Typically intensities of 1–15 kW cm −2 and more are needed . Additionally the emitted radiation should have a very high homogeneity localized on the sample since the absorption of solid silicon rises strongly with temperature, and therefore self‐amplified heating occurs.…”