A further improvement involves making separate diffusions for the different regions since the requirements are so different (Figure 7.3c) : a heavily doped and thick region under the contacts, a thin and lightly doped region under the passivating layer. These structures, known as “selective emitters”, come at the expense of more complicate processing, usually involving photolithographic delineation and alignment of the diffusions.
If a very low SRV is possible, it would be best to have no emitter at all since doping always degrades bulk lifetime (Figure 7.3d). Examples are the back point contact solar cell and the point emitter design with bifacial contact , originally designed for concentration but capable of very high one-sun efficiency as well.
With localized contacts, surface recombination decreases, with the penalty of an increase in transport losses in the substrate: deeper gradients for minority carriers, or increased series resistance for majority carriers, because of current crowding near the contacts. The trading is more favorably solved as the contact size shrinks . Light and/or localized diffusions have also the drawback of decreased gettering action.