Incoprea hypothesis tests · volume II
Single-junction solar efficiency is capped by the ratio of band gap to photon energy, producing an optimum gap rather than a monotonic improvement with absorption.
Efficiency should peak at an intermediate band gap; efficiency increasing with absorption (decreasing gap) would remove the trade-off.
Falsified by: Efficiency increasing with absorption would remove the trade-off.
Shockley-Queisser detailed balance. Source: 5778 K blackbody normalised to 1000 W/m2. Cell at 300 K; every photon above the gap absorbed, radiative recombination only; ideal fill factor. Band gap swept from 0.55 to 2.30 eV.
code/tests/shockley-queisser.py from the recorded run.| Optimum gap (eV) | 1.25 |
| Efficiency at the optimum | 31% |
| Ceiling at silicon 1.12 eV | 31% |
| Ceiling at GaAs 1.42 eV | 30% |
| Open-circuit voltage at optimum (V) | 1.00 |
| Fill factor at optimum | 0.88 |
Efficiency peaks at 31% at a gap of 1.25 eV, and falls to 19% at 0.6 eV and 19% at 2.2 eV. The trade-off is two-sided: below the optimum, excess photon energy is lost as heat; above it, photons pass through. Silicon at 1.12 eV has a ceiling of 31%, which is why its laboratory record near 27% is close to the single-junction limit for that gap.