Standards Development

Scope

This part of IEC TS 62607 establishes a standardized method to determine the key control characteristic

• Luminance for quantum-dot based light emitting diodes (QLEDs) by

• integrating sphere for bottom emission OLED devices

• spectroradiometer for top emission OLED devices.

Top-emission devices use a spectroradiometer to record light intensity and spectrum directly. Bottom -
emission devices employ an integrating sphere with a silicon photodetector to measure light transmitted
through the ITO substrate.
– Luminance measurements are critical during the prototype development phase to ensure that the OLED
displays meet specific brightness and colour uniformity standards.
– Regular luminance testing in the production line helps detect deviations from design specifications,
ensuring consistent quality in mass-produced OLED screens.
– Luminance data is used to evaluate new materials or structures in OLED technology, aiding in the
development of more efficient and high-quality displays.
– Calibration of the photodetector is essential when measuring with an integrating sphere to ensure
accurate and reliable luminance readings of OLED devices.

Purpose

The technology of Quantum Dots Light Emitting Diodes (QLEDs) is emerging as one of the most promising next-generation displays due to its narrow full width at half maximum, excellent chemical and physical stability, better contrast, high response frequency, and great foldability. QLED technology has become a major contender in the market and is expected to revolutionize the display industry in the coming years.
Recently, QLED technology has made significant progress in terms of external quantum efficiency ( EQE) and device operational stability. Both luminance and device longevity have met the commercialization requirements, especially for red and green electroluminescence. Blue QLED progress has slightly lagged behind due to its wider band gap and limited candidates of materials, but with tireless research efforts, blue device performance is catching up. Currently, research is transitioning from small-scale spin-coating film deposition approaches to mass production inkjet printing. Once inkjet printing techno logy is optimized, most technological obstacles will be cleared. We foresee that QLEDs will be successfully commercialized in the next few years.
The performance of the QLEDs is mainly improved through the optimization of the QD materials and the QLED device structures. And luminance is the most important characteriastics of evaluating QLED devices.
so accurately measuring the luminance of the prototype device is essential for the development of QD
materials and QLED device structure. The device structures can be separated into two categories: bottom
emission and top emission.The bottom emission structure is the most widely used to evaluate the
performances of QDs and the device structure due to the ease of fabrication and non-incorporation of
microcavity effect. The light emission follows the lambertian distribution, but the device luminance often suffers from a nonuniformity issue. On the other hand, the top emission device, by properly adjusting the functional layer thickness, can intensify the light extracted from the device in the 90 degree angle, and the electroluminescence of the device is rather uniform. However, the top emission device does no t obey lambertian distribution because of the microcavity effect.
Generally speaking, there are currently three existing luminance measurement approaches:
spectroradiometer, silicon photodetector, and integrating sphere. The spectroradiometer method pinpoints a small spot of the electroluminescent device and derives the luminance of the whole device assuming it is uniform. This method is currently the most widely used due to the ease of operation and straightforward interpretation. However, for bottom emission prototype devices, this method may cause a large error bar since these devices are usually much larger than the spot that the spectroradiometer can cover, and the device luminance is usually not uniform. Moreover, the observer tends to pinpoint the bright spot to measure, which leads to an overestimation of the averaged luminance, affecting the device EQE and causing serious overestimation of device operational longevity since it is proportional to the luminance's power of 1.5 -1.7.The misuse of spectroradiometer is one of the major reasons for inconsistent device lifetime reports in the literature. To address the luminance measurement issue in QLED prototype devices, we propose using silicon photodetector equipment to measure the luminance of bottom emission devices. This method is more accurate than using a spectroradiometer because the silicon photodetector is designed to measure the average luminance of the entire QLED device, whose light emission follows a lambertian distribution . However, the silicon photodetector method can only provide relative luminance values. To obtain absolute results, calibration with an integrating sphere setup is necessary. Therefore, we recommend using a combination of integrating sphere and silicon photodetector to achieve more accurate luminance measurements for bottom emission prototype devices. For top emission devices, we suggest using a spectroradiometer because their luminance is relatively uniform compared to bottom emission devices, and their light emission does not follow a lambertian distribution, making the silicon photodetector method unsuitable. To further improve the accuracy of the measurement, we recommend performing multiple measurements at various positions of the same device and averaging the luminance values to represent the overall luminance of the device.