A hot emitter at temperature T radiates photons following Planck's law for spectral radiance:
B(λ,T) = (2hc²/λ⁵) · 1/(exp(hc/λkT) − 1)
Each photon's energy is E = hc/λ. A photovoltaic cell can only convert a photon into electricity if E ≥ Eg, the semiconductor's bandgap — and even then only Eg of that energy becomes usable electrical work; the rest thermalizes into heat within picoseconds. Photons with E < Eg can't excite an electron across the gap at all: in a plain cell they're simply absorbed as parasitic heat.
- Emitter temperature — shifts the Planck spectrum; by Wien's law the peak wavelength is λmax = 2898 μm·K / T, so hotter emitters push more photons above the bandgap.
- Bandgap Eg — sets the cutoff wavelength λg = 1240 eV·nm / Eg. Real TPV cells use low-bandgap III-V alloys (InGaAs ≈ 0.6 eV, GaSb ≈ 0.72 eV) to match emitter temperatures in the 1000–2000 K range that combustion, radioisotope, or concentrated-solar heat sources reach.
- Spectral filter (photon recycling) — real TPV systems place a selective emitter or a back-surface/interference filter between the emitter and cell that reflects sub-bandgap photons back to the emitter instead of wasting them. That returned energy reheats the emitter rather than escaping, which is what lets lab TPV systems exceed 30–40% efficiency instead of the single digits a graybody emitter alone would give.
This simulator samples real photon wavelengths from the Planck distribution at your chosen temperature, fires them at the cell, and tallies three outcomes live: useful conversion, thermalization loss (the excess energy of above-gap photons), and sub-bandgap loss — recycled back to the emitter when the filter is on, lost as heat when it's off.