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

The Shockley-Queisser limit is the detailed-balance efficiency bound for an ideal single-junction solar cell with radiative recombination under specified illumination and optical conditions. Below-gap photons are not absorbed and excess energy of above-gap photons is thermalized; radiative emission further limits the output voltage. The often quoted unconcentrated terrestrial maximum is roughly one third, not an illumination-independent universal percentage.

A multi-junction solar cell places a high-gap junction before lower-gap junctions, dividing the spectrum so photons generate carriers in a better-matched absorber. This reduces transmission and thermalization losses and can exceed the single-junction bound without violating its assumptions: there is now more than one junction. In a two-terminal series stack, all junctions carry the same current, so the lowest photocurrent limits the whole device. Current matching uses bandgap choice, absorber thickness, optical management and sometimes lateral area to make the generated currents comparable for the intended spectrum. Independently contacted junctions need not share the same current.^[ref]

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.