Usually, the performance of OLED light-emitting materials and devices can be evaluated from two aspects: luminescence performance and electrical performance. Luminous performance mainly includes emission spectrum, luminous brightness, luminous efficiency, luminous chromaticity and lifetime; while electrical performance includes the relationship between current and voltage, luminous brightness and voltage, etc., which are the main parameters to measure the performance of OLED materials and devices.

Emission Spectrum

Emission spectrum refers to the relative intensity of the various wavelength components in the emitted fluorescence, also known as the distribution of the relative intensity of fluorescence with wavelength. Emission spectra are generally measured with various types of fluorescence measuring instruments, and the measurement method is: fluorescence is irradiated on a detector through a monochromatic emitter, the monochromatic emitter is scanned and the corresponding fluorescence intensity at various wavelengths is detected, and then the emission spectrum is obtained by recording the relationship curve of fluorescence intensity to emission wavelength through a recorder.

There are two types of luminescence spectra of OLEDs, namely photoluminescence (PL) spectra and electroluminescence (EL) spectra. PL spectra require excitation by light energy and keep the wavelength and intensity of the excitation light constant; EL spectra require excitation by electrical energy and can be measured at different voltages or current densities. By comparing the EL spectra of the device with the PL spectra of different carrier transport materials and luminescent materials, useful information about the location of the complex region and the actual luminescent material can be derived.

Luminescence

The unit of luminous brightness is cd/㎡, which indicates the luminous intensity per square meter, and the luminous brightness is generally measured with a luminometer. The brightness of the earliest made OLED devices has exceeded 1000cd/㎡, while the brightest OLED luminance can currently exceed 140,000cd/㎡.

Luminous Efficiency

The luminous efficiency of OLEDs can be expressed in terms of quantum efficiency, power efficiency and lumen efficiency. Quantum efficiency ηq is the ratio of the number of photons output Nf to the number of electron-hole pairs injected Nx. Quantum efficiency is further divided into internal quantum efficiency ηqi and external quantum efficiency ηqe. Internal quantum efficiency ηqi is the ratio of the number of photons radiated by the compound generated inside the device to the number of electron-hole pairs injected; in fact, the luminescence efficiency of the device is reflected by the external quantum efficiency ηqe, which can be expressed by the following equation. The external quantum efficiency can be used to measure the total luminous flux of the luminescent device per unit time with an integrating sphere photometer, which is calculated to derive the external quantum efficiency of the device. Excitation light photon energy is always greater than the energy of the emitted light photon, when the excitation light wavelength than the emitted light wavelength is much shorter, this energy loss is very large, and the quantum efficiency does not reflect this energy loss, the need to use the power efficiency to reflect. Power efficiency ηp, also known as energy efficiency, is the ratio of the output optical power Pf to the electrical power Px of the inflow. When measuring the function of a light-emitting device, more lumen efficiency this parameter. Lumen efficiency ηl, also known as luminous efficiency, is the luminous flux emitted L (in lumens) and the ratio of the input electrical power Px. Among them, S for the luminous area (㎡), B for the luminous brightness (cd / ㎡), I and V were added to measure the brightness of the bias current and voltage, J for the corresponding current density (A/ ㎡), the lumen efficiency in lm / W.

Luminous Chromaticity

Luminous chromaticity is expressed by color coordinates (x, y, z), x means red value, y means green value, Z means blue value, usually x, y two color products can be noted on the color.

Luminous Life

Lifetime is the time required for the brightness to reduce to 50% of the initial brightness. For commercial OLED devices require a continuous life of 10,000 hours or more, and a storage life of 5 years. One of the factors found in the study that affects the life of OLED devices is the presence of water and oxygen molecules, so it is important to isolate water and oxygen molecules when the device is packaged.

Current Density-Voltage Relationship

In OLED devices, the current density versus voltage curve reflects the electrical properties of the device, which is similar to the current density-voltage relationship of light-emitting diodes with rectification effects. At low voltages, the current density increases slowly with increasing voltage, and when exceeding a certain voltage the current density rises sharply.

Luminance-Voltage Relationship

The luminance-voltage relationship curve reflects the optical properties of OLED devices and is similar to the current-voltage relationship curve of the device, i.e., at low driving voltage, the current density increases slowly and the luminance increases slowly, and at high voltage driving, the luminance increases rapidly along with the sharp increase in current density. From the luminance-voltage relationship curve, we can also get the information of the start-up voltage. The starting voltage refers to the voltage at which the luminance is 1cd/㎡.