OLED and LED technology - Generation of broad emission spectra

OLED and LED Emission Spectra
The crystal structure of inorganic LED materials tends to produce narrower electroluminescence (EL) spectra that appear to be purer in colour to the human eye. While many methods have been investigated for producing white illumination from inorganic LEDs, the most economical method is to combine the short blue wavelength electroluminescence (EL) emitted by the LED device with a yellow (and sometimes green and red) luminescent phosphor coated on the outer surface of the LED. The phosphor absorbs the light emitted by the LED and then re-emits it at a lower wavelength during the absorption process, a process known as photoluminescence (PL). The combination of blue EL from inorganic LEDs and phosphor PL builds the broad spectrum needed for white lighting. It is worth noting that it is difficult to achieve sufficient spectral power density in the red region with this method, which is why luminaires using inorganic LEDs can sometimes have a low colour rendering index (CRI). In addition, white lighting produced in this way may contain excessive amounts of short-wave blue light radiation, especially at more neutral and cooler colour temperatures, which can be harmful to human health.

In organic light-emitting diodes (OLEDs), the luminescent dopant material EL can be either narrow-spectrum or broad-spectrum depending on the molecular design, but in general, the luminescence of a single molecule is not sufficient for white solid-state lighting (SSL) applications. Although several methods have been demonstrated, the most effective way to generate efficient white light emission is to integrate blue, green, and red organic light-emitting host materials (Host materials) and light-emitting dopant materials (Dopant materials) into a single device. Typically, these systems have to be placed in different emissive layers to obtain good spectral control and high efficiency, which is very easy to achieve due to the amorphous layer structure of OLED materials. With one "stack" for blue emission and another for red and green (or yellow), there is flexibility in choosing the luminescent doping materials and emission layers to optimise the overall EL spectrum. It is worth emphasising that all emissive layers are directly excited by current, rather than relying on photon absorption and re-emission to achieve longer spectral wavelengths. This provides greater freedom to balance red, green and blue light for illumination, and as a result, broad-spectrum SSLs based on OLED technology have a high colour rendering index (CRI), typically in excess of 90. This flexibility is also the reason why OLED lighting is free of UV wavelengths, which are harmful to the eyes and skin, and of excessive blue light wavelengths, which can be disruptive to human health.

OLEDs and LEDs Wide Emission Spectrum

As diodes, inorganic LEDs and OLEDs are essentially direct current (DC) devices that can be run DC in SSL luminaires if desired. The advantage of DC operation is that there is no risk of flickering lights, which can cause eye strain and headaches. When using DC, the lighting output can be reduced by reducing the current at the output. However, with inorganic LED and OLED technologies, when the current is reduced, the emission spectrum may change due to differences in red, green and blue light emission efficiencies. Inorganic LED SSLs typically shift to cooler colour temperatures with dimming, while OLED SSLs shift to warmer colour temperatures, with the latter typically preferred at dimmer light levels. Both technologies can be dimmed with pulse width modulated (PWM) drivers and, with proper driver design, can produce low flicker lighting.

SSL's Operational Life Cycle
Of course, inorganic LED and OLED lighting technologies cannot be discussed without mentioning lifetime. With inorganic LED devices, lifetime is primarily affected by heat, which can be caused by the use of high current densities or insufficient heat dissipation in the luminaire. With proper design, lifetime can exceed 100,000 hours, depending on the conditions. Although inorganic materials are very robust, many electrical and thermal connections are used in building the array, which in turn increases the chance of failure, so manufacturers need to take appropriate design and reliability testing steps to ensure product quality.

The lifetime of an OLED is largely proportional to the amount of current it is subjected to. Longer lifetimes can be achieved by increasing the efficiency of the device, which reduces the amount of current required for the lighting output. In OLED lighting, this is achieved by correctly selecting the luminescent host material (Host material) and luminescent dopant material (Dopant material) and designing the device. In particular, in the same device OLED lighting manufacturers can again take advantage of the amorphous qualities of the organic layer structure to combine multiple blue and yellow electroluminescent (EL) stacks, which will result in an illumination output for a given amount of current roughly proportional to the number of stacks used. Currently, the latest technology for OLED SSL uses six stacks to achieve a lifetime of over 100,000 hours, which is more than sufficient for general lighting applications. Heat may also affect the lifetime of OLED SSLs, but it is worth noting that the technology is now commercially available and is being used for exterior automotive lighting functions such as tail lights, applications which need to be tested at temperatures up to 105 degrees Celsius in order to be approved.

Combination of OLED and inorganic LED technology
Inorganic LEDs and OLED SSLs offer different options for different general lighting experiences, giving designers more choices when realising the design of a lighting space. In some cases, the properties of these two SSL technologies are very complementary. A good example of this would be Acuity's Peerless Olessence and Nadarra's Rex 2, both of which are indoor lighting fixtures designed for offices, commercial venues, and hospitality spaces such as conference rooms, guest rooms, lounges, common areas, bars/restaurants, entrances, bathrooms, and hallways. These luminaires take full advantage of the soft and diffuse output characteristics of OLEDs for direct lighting and combine the power of inorganic LEDs for indirect lighting from the fixtures. The overall effect is a stylish and elegant lighting fixture that combines the best features of both technologies to provide a new lighting experience.

Over the past decade, the performance of inorganic LEDs and OLED SSLs has improved significantly and has met the market demand for a balance between performance and cost. This trend will continue with the introduction of new materials, device structures and manufacturing methods. With the widespread adoption of these technologies, we are moving into a whole new world of integrating inorganic LED and OLED lighting into our products and spaces in new and interesting ways.