What is the percent concentration of doping in n and p-type regions of typical solar cell?
Silicon has a density of \$2.33\ {\rm g/cm^3}\$. And its atomic weight is about 28. Knowing that 1 atomic mass unit is about \$1.66\times10^{-24}\ {\rm g}\$, we can find that there are about \$5\times10^{22}\$ silicon atoms per cubic centimeter of material.
If the doping density in a solar cell is about \$10^{17}\ {\rm cm^{-3}}\$ (I don't know if this is the most likely value, but it's a common value for fairly strongly doped material in other devices and the first one I came up with in a google search for &quot;solar cell dopant concentration&quot;), then the fraction of silicon atoms replaced by dopants in that material is about \$2\times10^{-6}\$, or 2 ppm.
Of course if you have a more accurate figure for the doping concentration in some particular solar cell you're interested in, you can easily calculate the value for that device yourself.