For anyone who is trying to see the solar spectrum that hits the Earth, look no further than the Air Mass 1.5 index:
This is the standard spectrum, at 1000 W/m^2 irradiance, used in the testing of solar energy devices -- namely photovoltaic panels. It could also be useful for modeling the spectrally selective materials used for solar thermal applications. They provide the data in a number of formats for modeling purposes. I'm probably the only person interested in this stuff but there it is, nonetheless. An entire report on the atmospheric physics used for the model is available here:
ETR appears to be the extra-terresterial radiation (totals to 1356 W/m2), tilt is a surface at 48.81o to the horizon (total 1000 W/m2). The Tilt numbers are what you would normally apply to a PV cell to determine its peak power output. The direct and circumsolar irradiance appears to be the irradiance on a horizontal surface directly illuminated plus the diffuse light that is scattered off a 'clear' atmosphere (total 887.65 W/m2). I'm not really sure however because the two data sets don't seem to be consistant. Here's a plot of the tilt data, showing the Si exciton peak at 1.12 eV = 1107 nm:
Silicon can't absorb the light to the right of the green line -- it doesn't have enough energy to push an electron from the valence band to the conduction band. It's only 148.7 W/m2 lost however. Silicon can absorb light to the left, but only in 1.1 eV chunks. High energy photons (E > 2.2 eV) can actually generate more than one exciton, however. If we look at the actual number of photons per wavelength we get the following:
If we assume every photon with a wavelength below 1107 nm can create one (and only one) 1.1 eV exciton-polariton then the maximum power a Si-photovoltaic cell can absorb is 451 W/m2, or 45 % efficiency. Some people may recognize this as the classic Shockley result, although I derived it numerically in about 15 minutes. With current commercial Si cells running around 12 - 14 % efficiency, you can see that there is a long way to improve even with this most basic design. The actual theorectical (entropy) limits for a solar device are about 95 %. No one is going to get there but it does show you the great potential in solar power, even if it is overly expensive at the moment.