The microwave phase-shift (μW-PS) technique is used to determine the bulk lifetime (τb) of minority carriers. In this contactless technique, the phase-shift between a microwave beam (10 GHz) and a sine-modulated infrared excitation is related to τb and to the surface recombination velocity (S). Surfaces are passivated by means of an aqueous iodine solution or by short phosphorus diffusion (850°C for 20 min). By varying the excitation, the modulation frequency (S) can be deduced. The technique works at a quasi-constant excitation level, using either a large excitation beam or a focused beam (50 μm) to establish a lifetime scan map. The phosphorus-diffused samples are transformed into n+p diodes and light beam-induced current (LBIC) maps lead to a mapping of minority carrier diffusion length (L). From the τb and L maps, we can get a map of the minority carrier diffusion coefficient (or mobility). Comparison of the measured values of τb in surface-passivated samples leads to an evaluation of S, which is in agreement with that obtained directly by changing the excitation frequency. The technique proposed works for Si wafers containing oxygen precipitates and for multicrystalline Si wafers. Features of extended defect recombination strength, at the defects and far from them, as well as the local variation of S, can be deduced. In conclusion, using in association the lifetime maps obtained by μW-PS and the diffusion length maps obtained by LBIC leads to an enhanced knowledge of the properties of silicon wafer minority carriers.

High resolution lifetime mappings of excess minority carriers in silicon wafers are obtained by means of the phase-shift technique at practically constant injection level. This technique is contactless, based on reflected microwave power variations which occur when the sample is illuminated by a focused near infrared light. It measures the phase shift φ between a sinusoidal modulation of the excitation and the reflection power of microwaves. Surfaces are passivated by means of an aqueous iodine solution, whose passivating efficiency is remarkably constant. Scan maps are obtained with a lateral resolution of 50 μm by means of a coaxial cable which directs 9.4-GHz microwaves onto the wafer and a fiber coupled laser diode which generates minority carriers in excess. Lifetime mappings are in agreement with minority carrier diffusion length mappings obtained by the light beam induced current technique, especially for multicrystalline wafers.

Improved resolution lifetime scan map of minority carriers in excess in monocrystalline and multicrystalline silicon wafers are obtained by means of the phase-shift technique at a practically constant injection level. This technique is contactless, based on reflected microwave power variations which occur when the sample is illuminated by a focused near infrared light. Surfaces are passivated by means of an aqueous iodine solution, whose passivating efficiency is remarkably constant. Scan maps are obtained with a lateral resolution of 50 μm thanks to a coaxial cable which directs 9,4 GHz microwaves onto the wafer and a fiber coupled laser diode which generates minority carriers in excess. Lifetime scan maps are found to be in acceptable agreement with minority carrier diffusion length scan maps obtained by the light beam induced current technique.

Continuous casting with an electromagnetic cold crucible is a promising way of producing multi-crystalline (mc) silicon on an industrial basis for solar cell production. Casting equipment, capable of producing ingots of 130×130 mm2 cross-section and 600 mm length has been developed and installed. Thorough characterisation of the produced material revealed a relatively high defect density (grain boundaries, dislocations) and small grain sizes of 1–4 mm in diameter. Although the effective minority carrier diffusion length is high on as-cut wafers (>150 μm) it decreases during solar cell processing to values of 50 μm. This can be attributed to standard high temperature processing steps that lead to redistribution and agglomeration of residual impurities (e.g. metals, carbon) at extended crystallographic defects which then act as strong recombination centres. In order to passivate these recombination centres, a PECVD SiNx-layer is deposited which acts as a source of hydrogen and also as an anti-reflective coating. During the firing of the screen-printed metal contacts through the SiNx-layer, atomic hydrogen is released from this layer and diffuses into the bulk of the wafers where it saturates dangling bonds and passivates impurities at crystal defects. After this treatment the minority carrier diffusion length can be restored to values of around 100 μm on finished solar cells.

Multicrystalline-Si ingots were prepared by a cold crucible continuous pulling technique and 230 μm thick wafers were cut using a wire saw. By means of mapping techniques such as light beam induced current, it is shown that the electrical properties of the wafers are homogeneous. Impurities have a tendency to aggregate in the form of quasi neutral precipitates and/or inclusions. Minority carrier diffusion lengths are relatively high in the grains: around 90 μm due to a low recombination strength of extended defects. Conversely the recombination strength of grain boundaries is high. Solar cells made with this material lead to conversion efficiencies higher than 12%