Electronic grade Czochralski and float zone silicon in the as grown state have a very low concentration of recombination generation centers (typically <1010 cm−3). Consequently, in integrated circuit technologies using such material, electrically active inadvertent impurities and structural defects are rarely detectable. The quest for cheap photovoltaic cells has led to the use of less pure silicon, multi‐crystalline material, and low cost processing for solar applications. Cells made in this way have significant extrinsic recombination mechanisms. In this paper we review recombination involving defects and impurities in single crystal and in multi‐crystalline solar silicon. Our main techniques for this work are recombination lifetime mapping measurements using microwave detected photoconductivity decay and variants of deep level transient spectroscopy (DLTS). In particular, we use Laplace DLTS to distinguish between isolated point defects, small precipitate complexes and decorated extended defects. We compare the behavior of some common metallic contaminants in solar silicon in relation to their effect on carrier lifetime and cell efficiency. Finally, we consider the role of hydrogen passivation in relation to transition metal contaminants, grain boundaries and dislocations. We conclude that recombination via point defects can be significant but in most multi‐crystalline material the dominant recombination path is via decorated dislocation clusters within grains with little contribution to the overall recombination from grain boundaries.
In the manufacturing of modern solar cells, the early-phase quality inspection is crucial to detect harmful impurities and to reduce production waste. Thus, the charge-carrier lifetime measurement is beneficial before slicing the silicon ingot into wafers. Optimization of the widely used eddy-current detected photoconductance decay (e-PCD) technique for lifetime measurements is necessary to keep the pace with the new challenges appearing in photovoltaics. We investigated the dependence of the accuracy of the measurable carrier lifetime on the excitation laser parameters in the entire relevant carrier lifetime range. The complex behaviour of surface-recombination phenomena and its time evolution is studied by computer simulation. We optimized the components and parameters of the measurement setup and found that a 1064 nm laser is rather optimal compared to the 980 nm laser used previously. Using a longer wavelength laser source with elevated photon flux results in an improved accuracy of carrier lifetime recorded at Δn = 1015 cm−3 excess carrier concentration. A convincing similarity is found by comparing results from as-sliced and passivated surfaces. The optimized laser e-PCD method was compared to the flash-lamp based PCD method (known as QSSPC). Results agree very well for longer carrier lifetimes for which both systems evaluates the PCD curves in transient mode. However, for lower carrier lifetimes the universal transient mode operation of the e-PCD method is advantageous in terms of consistency.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.