External quantum efficiency

Overview

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. Relative and arbitrary unit EQE measurements can be obtained quite easily when care is taken to ensure similar test conditions for each device under test. Quantitatively accurate measurements require the user to pay attention to details such as illumination intensity and flux distribution of the light source[ref], optical reflectance and interference in device and test chamber[refs], device temperature[ref], and also the definition of the effective active area[ref]. With all of these considerations in addition to the variability of device fabrication protocol in different laboratories, the standardized power conversion efficiency measurement may make the best universal benchmark for device performance. However, the EQE measurement can be broken up into different spectral components and allows for the comparison and analysis of device physics.

see Wikipedia External Quantum Efficiency

Operation

Ideally the calibrated photodiode should be the same size as the illuminated area of the test sample. Otherwise eventhough the photodiode is masked to the same area as the test pixel there is the possibility of measuring dark currents from the remaining area of the photodiode.