Multicrystalline Silicon Solar

The electronic quality of multicrystalline silicon (mc-Si) has improved significantly in recent years due
to a substantial reduction of impurity contamination and advanced gettering and bulk passivation techniques.
In consequence, the minority-carrier diffusion length easily exceeds wafer thickness, a fact
which is intensified by the general move to thinner substrates. This creates the need for a solar cell design with
excellent light confinement and high-quality surface passivation. Phosphorus diffused emitters with a sheet
resistance of around 100
/square contribute only little to recombination and therefore rear surface passivation
becomes increasingly important. Efficiencies of mc-Si solar cells exceeding 18% with aluminium back surface
field1 or silicon nitride2 have been reported, but the best result so far of 19
8% has been achieved by an oxidepassivated
rear surface with locally defined contacts.3 This superior performance of oxides, together with additional
restrictions arising from the other techniques mentioned above (i.e., limited passivation quality of aluminium
back surface field and shunting of the field effect for silicon nitride4) make thermal oxidation a
promising technology for the production of high-efficiency mc-Si solar cells. The major drawback so far has
been the significant reduction of minority-carrier lifetime in multicrystalline silicon during high-temperature
oxidation at more than 1000C.5,6 This thermal degradation can be prevented by lowering the process temperature
to 800C during wet oxidation of the rear surface. The rear contact pattern has to consist of closely spaced
contacts since the grain boundaries in mc-Si decrease lateral conductance. The contacts were fired through the