External Quantum Efficiency (EQE) is the ratio of the number of photons emitted in the forward viewing direction to the number of injected electrons related to photoelectric effect. With higher the photons emitted, the better efficiency. EQE is usually related to the reflectivity or reflective area of the mirror inside the chip and the surface light extraction structure such as the surface roughness and photonic crystals. There are three loss channels, which the light is lost, internal reflection, absorption in layer, evanescent coupling mode. Then, this is influenced by various parameters, which impact to the efficiency. The emission-angle dependence of the EL is evaluated to be an element to measure the external quantum efficiency (EQE)

EQE vary depending on energy, so it is usually represented as a graph of efficiency versus the wavelength of light.

According to figure 1, it is easier for users to determine in each wavelength point. In other words, this can find out the best point to minimize the reflectivity in range of EQE based on the wavelength (below 100%).

By measuring EQE, find out the measure of the number of photons that are extracted to air per injected electrons. From that, we can minimum the loss related to energy, material, ...through the solar cell, OLED, LED… products. In other words, with the EQE measurement, it will optimize the power saving elements for the electronic products

Used for:

• Test structures and cellular materials, as well as to verify the reproducible production of modules
• Throughout the EQE measurement, it will know the suitable condition which optimize the approach efficiently
• To investigate the emission mechanism from the viewpoint of using devices for display applications
• It is an excellent diagnostic tool to improve efficiency
• Test structures and cellular materials, as well as to verify the reproducible production
• The external quantum efficiency gives the probability that photons will be absorbed to generate electron-hole pairs

Measurement:

• To calculate the EQE, it is necessary to find out the energy of one photon, total power of photon as well as charge of an electron:
• EQE =  =
• EQE can be adjusted because of the suppression of non-radiative recombination originating from the etching damage of the surface and the improvement of light extraction of the sidewalls
• To measure EQE for a light emitting materials, it is necessary to put the sample in integrating sphere with the stimulation of current and voltage, luminous efficiency, and chromaticity…. With the higher EQE, the stronger the current of the device.

Advantages:

• The most representative performance indicator quantity when comparing differently emitting OLEDs when there are three main device efficiencies (EQE, CE (current efficiency) and LE (luminous efficiency) are used and only EQE can measure the number of photons extracted to air per injected electrons.
• Total and wavelength-resolved external quantum efficiencies tend to be useful in understanding the fundamental physical mechanisms responsible for light emission within a device
• The ability to operate at a lower input power, at a given luminance, reduces the joule heating and thereby increases the device lifetime.
• By increasing and balancing the number of carriers, injected into the emissive layers, the efficiency also increases respectively

Application:

EQE measurement is used in modern photoelectrical devices or luminescent materials which is correlated photoelectrical effect such as solar cell, photodetectors (photodiodes, PD), avalanche photodiodes (APD), charge-couple device (CCD) sensors, CMOS image sensors (CIS), Light Emitting Diodes (LED), Organic Light emitting Diodes (OLED). ELQ series is evaluated to be a optical EQE measurement system with SNR ratio and low stray light.