High efficiency silicon solar cells have been manufactured since the 80’s (Green, 1987). These cells were manufactured in labs with microelectronic technology steps and with ultrapure crystals, in order to allow for the maximal performances; efficiency in the order of 27% have been achieved for back contact solar cells under around 100x and in the order of 25% under around 250x for cells produced by Amonix Inc. (Yoon et al., 1994). However, the fabrication processes required for these cells is expensive, and the ultimate device cost is comparable to that for multijunction solar cells on III-V semiconductors. Mainly for this reason the back contact technology is no longer used for CPV under the mentioned value of concentration; Sunpower Corp. commercialized this kind of solar cells until the beginning of 2000th but moved forward and transferred the technology on low cost processes for one Sun module
production. The silicon cells are currently used in CPV systems with concentration up to around 100 Suns; the technology used in this range of concentration must not differ so much from that of standard solar cells, in order to allow for an economical convenience of the CPV solutions. One established technology is the laser grooved buried contact (LGBC), in which the metallic contacts of the frontal grid are buried into the bulk of the wafer, as in fig.(10); the high aspect ratio of the fingers allows for low resistance of the contacts, while the large area of metal-semiconductor interface permits to strongly reduce the electrical resistance at the interface of the Shottky energy barrier, keeping a low shadowing of the photo-active material.
|
Fig. 10. Cross section of the LGBC silicon solar cell (Cole et al., 2009) |
This LGBC concept is employed for the Saturn cells commercialized by BP Solar in flat plate PV modules (Bruton et al., 1994). For concentrated light BP Solar produced cells with this technology for the Euclides concentrators (40x) (Sala et al., 1998); at the Narec PV technology centre, these cells are manufactured and developed for different concentrating solutions, with efficiency approaching the 20% (Cole et al., 2009).
Standard solar cells obtained with screen printing technology and designed for one sun application strongly reduce their efficiency even at 2-3 suns because of ohmic losses due to series resistance; however, some improvements can be achieved through slight design modifications, varying doping concentrations, electroplating parameters, line pitches and other fabrication steps.
