Many researchers have since worked on transistor-like solar cells, in most cases
with a n+pn+structure. The first devices having a creditable efficiency (12.7% for a 4 cm2 area) were fabricated by the author at the Polytechnical University of Madrid between 1977 and 1980 [9, 10]. The main advantage of this structure, shown in Fig. 2, lies in its tolerance of low quality material. The need for a third metallic contact implies, however, a considerable fabrication complexity. In a display of craftsmanship, Hitachi researchers fabricated in 2000 transistor-structure cells with a 21.3% front
and 19.8% rear efficiency [11]. The similarity between the recent sliver cells [12] and the first bifacial solar cell, shown in Fig. 1, is striking, although the fabrication processes are drastically different. The n+pn+ structure proved to be a useful research tool, thanks to the different operating modes that could be selected via its three terminals [13]. If the front junction (shown at the bottom in Fig. 2) is left in open-circuit or even without a metal contact, the device becomes the Tandem-Junction back
contact cell of ref 7. This approach was used in 1997 by Glunz et al. to produce 4cm2 devices with 20.6% front and 20.2% rear conversion efficiencies [14].