What is an OLED light panel

An OLED light panel is a planar emission device that makes use of the electroluminescent properties of organic semiconductor materials to produce visible light. The operation principle of OLEDs is similar to that of LEDs but the light-emitting layer takes the form of an organic stack, rather than an epitaxial layer of inorganic compound semiconductors. When a DC bias is applied to the electrodes (anode and cathode), the injected electrons and holes can recombine in the light-emitting layer to form excitons. The excitons can then radiatively decay (relax down into the ground state) and release their energy through the emission of photons at wavelengths in the visible portion of the electromagnetic spectrum. The process of emitting light through radiative recombination of charge carriers is referred to as electroluminescence. The wavelength of the light emitted by the organic emissive layer and the color of the light may depend on the band gap energy of the material, which is the difference in energy between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).

Light-emitting stack

Depending on light-emission directions and whether one or both of the electrodes are transparent, OLED light panels can be divided into three types: bottom-emitting OLED, top-emitting OLED, and transparent OLED. In a bottom-emitting panel, the anode acts as a path that allows light to travel through and then exit from the substrate. In a top-emitting panel, light is emitted through the cathode. A transparent OLED panel emits light from both the substrate and the top of the device. Except for transparent panels that provide double-sided emission, OLED panels designed for lighting applications generally use the bottom emission structure.

A bottom-emitting OLED is formed on a glass or plastic substrate onto which an anode layer is deposited. The anode is made of a transparent conductive oxide (TCO) such as indium tin oxide (ITO). Over the anode are multiple organic layers including the hole injection layer (HIL), hole transport layer (HTL), emitting layer (EML), hole blocking layer (HBL) and then the electron transport layer (ETL). The top layer is the cathode, which is typically a layer of highly reflective aluminum foil or similar negatively charged material to complete the electrical connection. Following this layer deposition, thin-film encapsulation (TFE) is applied to protect organic layers again the degradation of atmospheric moisture and oxygen. A heat spreader made from a metal foil is attached to the rear side of the TFE layer to maximize heat transfer to the ambient atmosphere.

An exciting feature with OLED lighting is that organic layers can be laid onto a variety of flexible and bendable substrates to provide products that can be curved, rolled, or even folded.

Color quality

While LEDs produce white light through phosphor conversion (PC) of blue or violet light emitted by indium gallium nitride (InGaN) LED chips, OLED light panels typically make use of additive color mixing of the three primary colors of visible light (red, green, and blue) to generate white light.

Incorporating layers of red, green, and blue emitters into a single organic stack allows an OLED light panel to produce full-spectrum white light that has a spectral power distribution (SPD) uniformly spread throughout the visible spectrum. The wavelength distribution awards OLEDs the ability to accurately reproduce all colors in illuminated objects. The color rendering index (CRI) of OLED light panels usually exceed 90. The special color rendering index R9, which shows the response with deep reds, generally scores greater than 50 and can get to 80.

Another benefit of the RGB color mixing method is that there is no penalty in luminous efficacy for delivering white light at a lower color temperature. In contrast, the efficacy of phosphor-converted LEDs is compromised due to the Stokes loss when a large amount of short wavelength light is down-converted for red light. OLED light panels are offered in 2700K, 3000K, 4000K white light colors.

Luminous efficacy

OLED light panels have been produced with efficacies of 40-90 lm/W. Although there are laboratory demonstrations that the efficacies of OLEDs can be much greater than 100 lm/W, OLED lighting manufacturers are struggling with the efficiencies of light extraction and blue emitters. Light extraction efficiency (LEE) is the ratio of photons escaping from the device to the photons generated in the emissive region. Despite the use of internal light extraction layers and reflective cathodes (where silver replaces aluminum), current OLED panels still lose 30-50% of the light emitted by the organic stack. Another ongoing challenge is the efficiency of blue emitters.

Phosphorescent emitters are preferred over fluorescent emitters because they can achieve nearly 100% internal quantum efficiency (IQE), while the latter is only around 25% efficient due to the ratio of radiative singlet (25%) to non-radiative triplet (75%) states. While phosphorescent red and green emitter systems of long life and high efficiency are available, the lifetime of short wavelength (blue) emitters is currently still very short.

Commercial products rely on fluorescent blue emitters to achieve practical levels of reliability, at the cost of reduced efficacy. This also explains the lower efficacy of higher CCT OLED light panels. To improve the stability and efficiency of fluorescent blue emitters, the use of thermally activated delayed fluorescence (TADF) to harness both singlet and triplet excitons in light generation is being explored.

OLED driving and dimming

OLEDs run natively on DC power and need to be driven in a forward direction. The driver must provide a substantially constant current to the load under supply voltage or load variations. The operating temperature, in particular, has to be considered in load regulation because OLED forward voltage has a negative temperature coefficient. Switch mode power supplies are typically used to achieve low ripple output and high conversion efficiency.

The OLED driver need to suppress the transients so that no excessive electrical energy will be delivered to the load. The efficiency of OLEDs is reduced as the power is increased beyond the current saturation point. Operating the OLEDs at higher current densities for a higher lumen output will be accompanied by the complication of a remarkably shortened lifespan. A localized increase of current density, known as current crowding, can result in localized overheating as well as an inhomogeneous distribution of light. Due to the limited conductance of transparent electrodes, a long current path can cause a significant voltage drop. As a result, increasing the size of an OLED light panel remains a big challenge.

The preferred method for dimming OLEDs is to use the constant current reduction (CCR) method, which is sometimes called “analog dimming.” Pulse width modulation (PWM) dimming is not generally recommended for OLED lighting.