Study of injection effects on BSF silicon solar cells

The behavior of p⁺-n-n⁺ and n⁺-p-p⁺ silicon solar cells in terms of short-circuit current, open-circuit voltage, fill factor, and efficiency is studied as a function of base doping and illumination levels. A theoretical model that is valid for any injection level in the base region is used. Experimental results for cells of n-type base (in the range of 0.3 to 1000 Ω-cm) and a p-type base (0.4 to 300 Ω-cm) are presented. The theoretical model is able to explain phenomena such as the superlinearity of I_sc with concentration and the degradation of short-circuit current and efficiency at very high concentrations. These effects are seen as connected with the ohmic electric field in the base region. For the emitter saturation currents considered here, it can be concluded that, for p-type substrates, low base resistivities (≅1 Ω-cm) are necessary to achieve high efficiencies under concentrated light (≅100 suns), while for flat-array cells a particular resistivity is not required. For n-type substrates, it is found that any resistivity level can be used for both flat-array and concentrator cells