This invention relates generally to the field of solar cells, and more particularly to improvements in vertical junction solar cells.
The basic structure and operation of solar cells is well known, and the utilization of silicon solar cells for the generation of electricity for remote marine, terrestrial, and satellite applications is well documented. The development of the vertical junction solar cell answered the need for a radiation resistant cell having improved electrical conversion efficiency for operation in an ionizing radiation environment to which the cell may be subjected in extra-terrestrial applications. The state of the art relating to vertical junction solar cells may be exemplified by solar cell configurations disclosed by or referenced in U.S. Pat. No. 3,690,953 to Wise, entitled "Vertical Junction Hardened Solar Cell", and U.S. Pat. No. 3,985,579 to Rahilly, entitled "Rib and Channel Vertical Multijunction Solar Cell".
Vertical junction solar cells finding substantial prior use have structural fragility which has made them susceptible to failure when subjected to thermal cycling, or when subjected to mechanical stresses within a power system. Since desirable radiation resistance and good electrical conversion efficiency of the cell are related in part to channel depth, it is desirable to optimize channel depth, especially if structural integrity can be maintained. In existing vertical junction solar cells, optimum radiation resistance is substantially achieved for channel depths of about 75.mu. (75.times.10.sup.-4 cm). However, the ribs between channels of that depth in existing configurations are undesirably fragile, and, therefore, channel depth in fabricated cells has, for practical purposes, been limited to considerably less than that consistent with optimum radiation resistance and cell efficiency. Existing cell structures also have excessively large channel length-to-depth ratios and are characterized by limited control on the channel depth during fabrication, which precludes effective quality control during fabrication.
The present invention provides a novel vertical junction solar cell structure comprising a silicon chip having in one surface a plurality of short, wedge-shaped channels closely spaced in an array and separated by thin vertical ribs which are configured to substantially eliminate any rib fracture problems. The junction is diffused over the rib surfaces and chip surfaces between the ribs. The cell is fabricated from a chip having the surface parallel to the (110) crystallographic planes, and the channels are aligned along the (111) planes following conventional technique. Unconventional control of the length l of the channels forming the vertical junctions, however, allows substantial control of channel depth d, approximately according to the relationship d=l/2.sqroot.3. Vertical junction cells according to the present invention are thus characterized by deeper channel construction which results in substantially increased light utilization at end-of-life condition, and improved fabrication process control, as well as improved radiation resistance, conversion efficiency and mechanical integrity.
It is therefore an object of this invention to provide an improved efficiency silicon solar cell.
It is a further object of this invention to provide a vertical junction solar cell having improved mechanical strength and light-absorbing characteristics.
These and other objects of the present invention will become apparent as the detailed description of representative embodiments thereof proceeds.