Effects of transition metal impurities on solar cell performance in polycrystalline silicon

Ti V, Mo and Cr have been deliberately introduced into the bulk melt during growth of polycrystalline silicon octagons by the edge-defined film-fed growth (EFG) technique, and their influence on solar cell performance is examined. Trap concentrations in solar cells have been monitored by DLTS. Cell efficiency is most depressed by Ti and V, and drops by 35% at electrically active concentrations of these impurities of between 10¹³ and 10¹⁴ atoms/cm³ . The degradation for Mo is at most about 15%, and for Cr is 10%. Known electron trap parameters for Ti account for the full degradation in cell performance for Ti-contaminated material when the DLTS-active trap concentration is assumed to provide the dominant recombination sites. Trap cross sections for V and Mo are estimated from DLTS concentrations and from diffusion lengths measured in the solar cells. Cr is found as Cr-B pairs in as-grown material, but their concentration drops below the DLTS detection limit in the finished solar cells. Cell efficiencies of contaminated cells increase with oxygen addition but decrease with boron dopant addition, essentially independently of the specific contaminating transition metal impurity that is present. Hydrogen passivation does not lead to solar cell efficiency recovery in the contaminated materials