How to Evaluate the Performance of OLED?

The performance of OLED materials and devices is very important not only in scientific studies but also in applications to practical terminals and commercial goods. There are 7 key parameters to evaluate OLED performance.

In general, the performance of OLED light-emitting materials and components can be evaluated from two aspects: the luminescence property and the electrical property. 

Luminescence Property

The light-emitting properties mainly include emission spectrum, luminous brightness, efficiency, chroma, and lifespan. 

Emission Spectrum

The emission spectrum refers to the relative intensity of various wavelength sections in the emitted fluorescence, also known as the distribution of fluorescence relative intensity with wavelength.

The emission spectrum is measured by various types of fluorescence measuring instruments. 

The method is: fluorescence is irradiated on the detector through a monochromatic emitter, the monochromatic emitter is scanned and the corresponding fluorescence intensity at various wavelengths is detected, and then the relation curve between the fluorescence intensity and the emission wavelength is recorded by a recorder to obtain the emission spectrum.

Figure 1. Test comparison of the photoluminescence spectrum
Figure 1. Test comparison of the photoluminescence spectrum

There are two kinds of luminescence spectrum of OLED, photoluminescence (PL) and electroluminescence (EL) spectrum. 

  • PL spectrum needs excitation of light energy, and keeps the wavelength and intensity of excitation unchanged; 
  • EL spectrum requires the excitation of electrical energy, and can be measured under different voltage or current densities;

By comparing the EL spectra of the device with the PL spectra of different carrier transport materials and luminescent materials, can obtain useful information about the position of the composite region and the actual luminous substances.

Luminous Brightness

The unit of luminous brightness is cd/㎡, which represents the luminous intensity per square meter. The luminance is generally measured with a luminometer.

The brightness of the earliest OLED devices has exceeded 1000cd/㎡, at present, the brightness of the brightest OLED can exceed 140000cd/㎡.

Luminous Efficiency

The luminous efficiency of OLED can be expressed by quantum, power, and lumen efficiency. 

Quantum efficiency ηq refers to the ratio of the number of output photons Nf to the number of injected electron-hole pairs Nx.

When measuring the function of a light-emitting device, often used the parameter of lumen efficiency ηl, also called photometric efficiency, is the ratio of the emitted luminous flux L (in lumens) to the input electric power Px. 

Luminous Chroma

The luminous chromaticity is represented by color coordinates (x, y, z), “x” represents the red value, “y” represents the green value, and “z” represents the blue value. Usually, two chromaticities of “x” and “y” are enough to indicate the color.

Luminous Lifespan

The lifespan refers to the time required for the brightness to decrease to 50% of the initial brightness. 

For commercial OLED components, the continuous service lifespan is required to reach more than 10,000 hours, and the storage lifespan needs to be 5 years.

After the research, it was found that one of the important factors affecting the lifespan of OLED devices is the presence of water and oxygen molecules, so OLED must be isolated from them in the process of packaging.

Electrical Property

The electrical properties include the relation between current and voltage, luminous brightness, and voltage.

Relation of Current Density-Voltage

In the OLED device, the change curve of current density with voltage reflects the electrical properties of the device, it is similar to the relation between current density – voltage of LED, and has a rectification effect. 

When at a low voltage, the current density increases slowly with the increase of voltage. However, when exceeding a certain voltage, it jumps sharply. 

Relation of Luminous Brightness -Voltage

The change curve of brightness and voltage reflects the optical properties of the OLED device. 

It’s similar to the current-voltage relation curve of the device, that is, when driven at a low voltage, the current density increases slowly, and the brightness also increases slowly.  When driven at a high voltage, the brightness increases rapidly with a sharp increase in current density. 

From the correlation curve of brightness-voltage, the information of the starting voltage can also be obtained, which refers to the voltage with a brightness of 1cd/㎡.

To know about the technology and features of OLED, you can read the related articles

What is OLED Technology?

With the development of technology, large-size OLEDs have also made breakthroughs. Based on the advantages brought by self-illumination, OLED screens can not only be extremely thin within 1mm, but also realize the expectancy of the future displays, such as curling, folding, transparency, and double-sided.

In theory, OLED screens can be attached to the surface in transparent or irregular to create a future world of “display everywhere”.

Contact our engineer if you want to adapt OLED or LCD on your terminals, and we could work on the solution for you.

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