High-efficiency multicrystalline silicon solar cells on gallium-doped substrate
Gallium-doped multicrystalline silicon of p about 5 omega cm is investigated with respect to it's application for solar cells. A high base resistivity is beneficial because of the higher contamination tolerance against defects which exhibit a large asymmetry in the capture cross sections sigma(ind n) > sigma(ind p), like interstitial iron in p-type silicon. In the gallium-doped multicrystalline silicon, the minority carrier lifetime changes after strong illumination with white light. Injection level dependent lifetime spectroscopy reveals a defect which can be described by the recombination kinetics of FeGa-pairs. The crossover point of the lifetime traces before/after illumination was experimentally observed between delta n(ind COP) = 4 - 7x10(exp 13) cm-3 which is slightly higher than the value of delta n(ind COP) = 2.5x10(exp 13) cm-3 calculated by Schmidt and Macdonald. Using their data and assuming a complete splitting/pairing of iron and gallium, the defect concentration is determined as [Fe(ind i)] about 8x10(exp 10) cm-3. Phosphorus diffusion gettering completely removes the defect and high minority carrier lifetimes of 200 µs are obtained. The high base resistivity requires more contact points than usually applied in the laser-fired contact process but effective rear surface recombination is still low at S(ind eff) about 300 cm/s for a dense contact spacing of 250 µm. Solar cell efficiencies in excess of 19 % are achieved for small cells of 1 and 4 cm2 and 18 % in average.