Efficiency limiting crystal defects in monocrystalline silicon and their characterization in production

Abstract

Wafers and thicker slices of an entire n-type monocrystalline silicon ingot were studied using production-compatible electrical and optical characterization techniques. We investigated the capability of these techniques to detect efficiency limiting factors in the early phase of solar cell manufacturing. In addition to the standard characterization methods and parameters - carrier lifetime, resistivity, FTIR, photoluminescence (PL) - the OxyMap technique was used to evaluate the thermal history of the wafers, while the distribution of Bulk Micro Defects (BMD) was measured using Light Scattering Tomography (LST). PERT cells and samples with amorphous silicon surface passivation – deposited at low temperature – were produced from neighboring wafers to correlate silicon properties and solar cell efficiency results.An interesting indirect correlation was found between “as-grown” lifetime and cell performance, as both are influenced by the thermal history of the wafer.We also observed a very strong correlation between BMD position, measured by LST on as-grown samples, and defective areas on PERT cells localized by PL measurements. The LST measurements on heat treated samples (simulating the PERT cell process) showed the growth of BMDs in low efficiency areas. It indicates that the detection of harmful defects is possible even in the as-grown material using LST technique.