LED TECHNOLOGIES DEPARTMENT

Electro-optical problems

The main areas of electro-optical research conducted by Optogan NTL are as follows:

  1. Current spreading uniformity in the light-emitting diode (LED) chip analysis and efficiency of light extraction from the chip associated with it;
  2. Analysis of light extraction and radiation patterns of the LED as well as LED assemblies, including primary and secondary optics;
  3. Quality control of the LED light: color rendering index (CRI), correlated color temperature, and luminous efficiency.

The study of these aspects can provide for further improvement in light-emitting diodes (LEDs) optical properties, therefore making the importance of such research self-evident.

Efficiency of the LED chip (the ratio of output power to the power consumption) is one of the most important parameters. Efficiency of the LED is proportional to the external quantum efficiency, which is equal to internal quantum efficiency multiplied by the injection efficiency and light extraction efficiency.

The internal quantum efficiency ηint is the ratio of the number of photons emitted in the LED chip to the number of electrons injected in the LED chip. In modern LED chips ηint ≈ 1, that is, almost every electron produces a photon. The injection efficiency ηinj is the ratio of the number of electrons injected into the active region to the number of electrons injected into device; when using the effect of one-side injection ηinj≈1. Light extraction efficiency ηextris the ratio of the number of photons emitted into free space to the total number of photons emitted from active layer; due to the effects of light propagation in LED chip, only 10 - 30% of photons emitted from active layer are then emitted into free space.

Current spreading uniformity in the light-emitting diode (LED) chip

The light in the chip is generated for the most part in the area under the non-transparent metal contact which leads to low light extraction efficiency. This issue may be solved by using the spreading layer for increased carrier collection and by optimizing contacts configuration.

The optimal contacts configuration provides the best conditions for light extraction from the chip, current spreading and uniformity of the structure heating. For example, maximum efficiency of light extraction for the forked form contacts configuration (Fig. 1) is achieved by optimizing parameters a, b, c geometry of the electrodes. Fig. 2 shows the increase in current spreading uniformity after electrodes geometrical parameters optimization.

 

Fig. 1 Contacts configuration of the LED chip (top view)

Fig. 2 Current density distribution in the active region of the LED for parameters а=5 μm (left) and а=65 μm (right)

Increase in light extraction efficiency

Another problem is that a large amount of rays undergo total internal reflection on semiconductor-air interface and then they are partially absorbed by a semiconductor heating it. There are a number of approaches to solve this problem, for example texturing the LED chip surface or mounting the LED chip into encapsulating material with refractive index n, for which nair<n<nLED (fig.3).

 

Fig. 3 Schematic drawing of rays path in the LED chip: left – basic LED chip , right – LED chip with textured surface and LED chip mounted in encapsulating material  nair<nencapsulant<nLED

Primary optical elements can be used for further light extraction enhancement. These elements have optical contact with encapsulating material (fig. 4).

Fig. 4 Schematic drawing of the ray path for the COB: left – basic LED chip covered with encapsulating material, right - LED chip covered with encapsulating material with primary optical element

Once light was emitted from the LED, secondary optics is required for create the proper radiation pattern, which is determined by the luminary requirements. Radiation pattern is angle-dependent distribution of the luminous flux (fig.5).

Choice of primary and secondary optics and material properties is laborious work, therefore before implementing various ideas in production, there is a necessity to conduct detailed investigation of the processes and perform preliminary calculations using relevant software. In an effort to choose optimal characteristics of materials and forms of the optical elements, Optogan NTL performs light extraction simulations taking into account all the physical processes. For such simulations our company uses ZEMAX and COMSOL-MultiPhysics software.

 

Fig. 5 Secondary optics (standard spherical lens and Fresnel lens) and corresponding radiation patterns

Light Quality improvement

An important issue for white LEDs is the color quality control that is characterized by correlated color temperature Tcolor and color rendering index (CRI).

The color temperature of the light source Tcolor is characterized by the shade of white, it being cold (blue) or warm (red) (Fig. 6).

 

Fig. 6 Colors corresponding to different color temperatures

Fig. 7 Black body radiation (Planckian locus) in CIE1931 color space 

The color temperature of LEDs is determined through spectral analysis of visible light. The chromaticity coordinates for one of the color spaces such as CIE1931 (Fig. 7) can be seen on the spectral curve. The chromaticity coordinates have the same color temperature Tcolor if they lie in perpendicular to the Planckian locus line. However, the equality of the color temperature does not guarantee that color shade of the sources is the same. This is one of the reasons why LED producers use binning.

The color rendering index (CRI) is a quantitative measure that indicates the ability of a light source to reproduce the colors of various objects faithfully in comparison with a natural light source.

White LEDs use blue chips and yellow phosphor to generate white light (fig. 8). By adjusting the mixing proportion and concentration of phosphor, our company achieves required color temperature and CRI in a light source.

 

Fig. 8 Schematic drawing of generating white light with the blue light chip and yellow phosphor