Multiple-junction silicon concentrator solar cells fabricated by diffusion bonding

Abstract

02 have been under development now for many years. The most important problem make with high solar radiation concentration is to make the series resistance of the concentrator solar cell ~CSC! no higher than 10210 V•cm. In the development of conventional structures with p–n junctions oriented perpendicular to the incident light flux, mutually contradictory requirements inevitably arise for the layer resistance of the emitter, the spectral sensitivity, the shading produced by the contact grid, and so on. For this reason, for such a CSC design it is impossible to obtain small values of Rs and the efficiency of such cells increases with the illumination intensity approximately up to 60–80 suns and then decreases. Besides the conventional structures with ‘‘horizontal’’ p–n junctions, structures with multiple-junction CSCs with vertical p–n junctions have been proposed. It seems to us that these structures best meet the requirements imposed on CSCs. Besides the absence of the above-mentioned mutually contradictory requirements ~since there is no need for a contact grid and so on!, such structures have a number of advantages as well. For example, since there is no metallization on the front and back surfaces of these cells, they should be transparent in the long-wavelength part of the spectrum beyond the fundamental absorption edge. As a result of this equilibrium, the working temperature of such CSCs should be lower than in the case of the planar analog. Furthermore, multiple-junction elements are bilateral, since they can convert the light incident from the backside and they can serve as a constituent part of cascaded solar cells. The most important problem arising in applications of CSCs is to decrease the electrical losses in the series resistances of the contacts and the feed circuits. As a result of connection losses in high-current cells, the efficiency of a conventional battery is much lower than that of an individual cell. This problem is more easily solved in the case when multiple-junction CSCs are used, since in contrast to planar cells, they generate a high voltage as a result of the series resistance of the cells and a low current with the same output power. As a result, a battery assembled from such cells is more efficient. However, a number of technological problems must be solved in order to implement in practice multiple-junction CSCs with vertical p–n junctions. The most important problem is ensuring low series resistance and surface passivation. This explains the small number of works devoted to such CSCs as well as the poor results obtained by their authors. For example, in an early work, multiple-junction cells were obtained by soft-soldering, silicon wafers with a p–n junction, no passivation was performed, and the efficiency was