The properties of polycrystalline silicon solar cells with controlled titanium additions

By coupling the results of electrical measurements, such as spectral response, lighted and dark I-V determinations, and deep-level-transient spectroscopy with optical and laser scan photomicroscopy, we have evaluated the effects of grain boundaries and impurities on silicon solar cells. Titanium which produces two deep levels,
E_{v} + 0.30
and
E_{c} - 0.26
eV, in silicon, degrades cell performance by reducing bulk lifetime and thus cell short-circuit current. Electrically active grain boundaries induce carrier recombination in the bulk as well as in the depletion region of the solar cell. Experimental data imply a small but measurable segregation of titanium into some grain boundaries of the polycrystalline silicon containing high Ti concentration (2 × 10
¹⁴
cm
^-3
). However for the titanium-contaminated polycrystalline material used in this study, solar cell performance is dominated by the electrically active titanium concentration in the grains. Microstructural impacts on the devices are of secondary importance.