The Sun as a Blackbody
The sun radiates approximately as a blackbody at 5778 K, producing a broad spectrum that peaks in the green-yellow region around 502 nm. Wien's displacement law governs this peak: hotter stars peak at shorter wavelengths. The total power output follows the Stefan-Boltzmann law, delivering roughly 1361 W/m² at Earth's orbital distance before atmospheric absorption reduces it to about 1000 W/m² at sea level under standard conditions.
Bandgap and Spectral Mismatch
A semiconductor with bandgap Eg can only absorb photons carrying energy of at least Eg electron-volts. For silicon (Eg = 1.12 eV), this means wavelengths shorter than about 1100 nm. Photons with less energy sail through the material. Photons with more energy excite electrons far above the conduction band edge, but the excess energy is immediately lost as lattice heat. This double penalty — sub-bandgap transparency and above-bandgap thermalization — is the core physics behind the Shockley-Queisser limit.
Atmospheric Filtering
Earth's atmosphere absorbs and scatters sunlight selectively. Ozone blocks most UV below 300 nm. Water vapor and CO₂ carve deep absorption notches in the infrared. Rayleigh scattering removes short wavelengths preferentially, which is why direct sunlight reddens at sunrise and sunset (high air mass). The AM1.5 standard spectrum accounts for all these effects to give solar engineers a consistent reference for rating cell performance.
From Spectrum to Current
This simulation lets you adjust the semiconductor bandgap and atmospheric conditions to see how much of the solar spectrum a cell can actually harvest. Move the bandgap slider to watch the usable fraction change. Increase air mass to simulate low sun angles. The Shockley-Queisser limit readout shows the theoretical ceiling for each configuration, revealing why material scientists obsess over bandgap engineering and why multi-junction architectures exist.