Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2020 / Paper 1 / Section B / 12 / ii / b / Solution

The longest wavelength capable of producing an above-gap electron-hole pair isEvery transmitted wavelength is longer than $600\mathrm{nm}$, so every transmitted photon has energy below $2.4\mathrm{eV}$. Consequently the ideal single-photon absorber generates no photocurrent and $\boxed{\eta =0\%}$. The stated irradiance does not change this spectral-threshold conclusion. Defect-mediated or multiphoton absorption is outside the assumed ideal band-to-band model.