External Quantum Efficiency (EQE) for a photovoltaic device is the number of extracted free charge carriers per incident photon. This ratio is obtained by measuring the photocurrent spectrum of the photovoltaic device under test and comparing it to the photocurrent spectrum of a calibrated photodetector, thereby removing the spectral characteristics of the test system. This video describes the process of obtaining an EQE measurement from a photovoltaic device.
More information on the photonics wiki:
http://photonicswiki.org/index.php?title=External_quantum_efficiency
