Substrate for thin silicon solar cells

#The instant invention relates to thin layered silicon photovoltaic devices. Such devices typically have a substrate on which is grown a thin layer of high purity silicon. In the instant invention the substrate is formed of an alloy of silicon and boron.

#Photovoltaic devices or solar cells convert solar radiation into useable electrical energy. When solar radiation impinges on a solar cell it is absorbed by an active region of semiconductor material to generate an electric current. This is known as the photovoltaic effect.

#In the past, pure silicon has frequently been used for photovoltaic devices and to form such an active region.

#FIG. 1 shows a cross-section of a solar cell, (not to scale), made in accordance with the instant invention.

#FIG. 2 shows a cross-section of the apparatus used to make the substrate of the instant invention with the alloy materials in molten form.

#FIG. 3 shows a cross-section of the apparatus used to make the substrate of the instant invention with the alloy in solid form.

#The invention is directed to a substrate 3 for a solar cell 2 shown in FIG. 1. A thin layer of high purity silicon is grown on the substrate 3. The thin layer is shown generally at 4. The substrate 3 is formed of a metallurgical grade silicon and boron alloy. The solar cell has front grid metal contacts 5, a diffused p/n junction shown generally at 6, and a rear metal contact 7. A process for forming the substrate is shown specifically in FIGS. 2 and 3.

#A crucible 15 to hold the materials for the alloy is formed having walls 10 and bottom 12 as shown in FIGS. 2 and 3. The crucible contents are preferably heated by an induction heating method using a radio frequency coil 11 and susceptor 16 although other forms of heating can be used to form the melt. Typically, the crucible 15 is formed of graphite or quartz, and the susceptor 16 is formed of a conductive material such as graphite or silicon carbide that can be heated by the RF or radio frequency coil to transfer its heat to the crucible contents.

#A source of boron such as Boric oxide (B.sub.2 O.sub.3) is placed in the crucible in solid form along with metallurgical grade silicon. The relative amounts are such to achieve an alloy of 0.1 to 1.3 atomic percent boron relative to the silicon. Thus the percentage of silicon in the resulting alloy would be in a range from 99.9%-98.7%, respectively. The heating is done in an argon atmosphere to reduce any interaction with the alloy materials although the process can be performed in ambient air if a quartz crucible and silicon carbide susceptor are used.