A solar cell consists of two parts

1. Light absorption.  Photons are absorbed and create electron hole pairs.

2. Charge separation and collection.  The electrons and holes are separated through some built in electric field and collected through contact metals.

Solar cells are quantified through quantum efficiency.

The separation and collection of mobile charges (part 2) is quantified through the internal quantum efficiency.   The internal quantum efficiency is the ratio of electron-hole pairs collected for each photon absorbed.

The external quantum efficiency quantifies both 1 and 2 above.  It is the ratio of electron-hole pairs collected for each photon shined on the solar cell.

The external quantum efficiency is thus always less than the internal quantum efficiency.

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