PN Junctions and solar cells by ion implantation

Ion implantation provides a new PN junction formation technique that is at once more controllable and more versatile than either alloying or diffusion. It is based on doping by a coherent positive ion beam from a high energy accelerator. For convenient accelerator size, the technique is limited to devices having shallow junctions such as solar cells. The penetration depth of heavy ions in crystalline materials is extremely sharp and is a function of ion energy. Consequently voltage control of the accelerator allows placement of the junction with millimicron accuracy. Doping levels are dependent on beam current and time. Since ion energy and irradiation time are independent, the doping profile obtainable by the technique may be chosen at will. Boron was accelerated in a 2 million electron volt Van de Graaff generator to produce 1 × 1 cm solar cells with various junction depths and doping gradients. The effects of junction depth on cell efficiency were studied as was the effect of a built in drift field. Work was also conducted on the effect of carrier concentration on cell efficiency arid on the effect of annealing on carrier concentration.