Quantifying OLEDs: Yield, efficiency and luminance
In the literature there are several definitions of the electroluminescence yield. The internal quantum yield ηiq is the ratio of the number of generated photons to the number of charges transported through the device (in %).
where: EEL – the number of generated photons, h – the Planck constant, n – the radiation frequency, j – the electrical current supplied to the diode, e – the electron charge.
The internal energetic yield ηie (in W/W) is the ratio of energy generated as light (radiant energy) to expended electrical energy, so it is smaller than the quantum yield hn/eV. The corresponding external yield ηee refers to photons that are emitted in the outer space.
One should not forget that these yields are not characteristic of the materials, or even of the device, because they depend on the operating conditions of the device, particularly the applied voltage.
The luminous efficiency (lumen per watt) is the photometric equivalent of the external energetic yield weighted by a standard spectral function V(λ) representing the eye response. The lumen (lm) is the unit of luminous flux; the radiant flux weighted by V(λ) (see the following figure). Another important photometric quantity is the luminous flux per solid angle unit (the candela). Taken per unit emitting surface, one gets the luminance (in cd/m2).
The quantities, which have a practical significance, are the luminous efficiency (lm/W) and the luminance L (cd/m2), connected with ηee. Many authors emphasize that the internal quantum yield is a fundamental quantity of interest for basic physical description, and its optimisation leads to higher performances of other quantities. This depends on the understanding and control of the elementary steps of the EL fabrication process.
Good LEDs operate at applied voltages of 3 to 9V and at luminance of ca. 10000 cd/m2, with current densities in the mA/cm2 range. The emitting materials used have fluorescence quantum yields of 15 to 70%, and corresponding external quantum yields (EL) of a few percent. All colours should be available, and the red, green and blue fundamental colours should be very close to those used in colour TV in the Commission Internationale de I’Eclairage (CIE) colour coordinates (in table).