2. Optimization

The external quantum efficiency (η _{q, ext} ) of a light-emitting diode is defined as the ratio of the number of photons emitted to the number of electrons injected. It can be described as the product of the electron-hole recombination rate (η _r ), the radiative de-excitation probability (η _dr ), the photoluminescence efficiency (φ _PL ) and the fraction of photons extracted from the device (η _opt ) :