OLED lighting is panel lighting without bulbs.
The mechanism of OLED lighting is fundamentally different from that of conventional lighting. Some organic substances emit light when a voltage is applied. Such organic substances are deposited onto thin plastic or glass panels; light is emitted when a voltage is applied to the panels. A light-emitting layer is less than 0.1 µm thick; and an entire panel thickness is only 1–2 mm. In addition, OLED lighting panel emits light from its entire surface, therefore it can offer new designs and applications that cannot be achieved with conventional lighting solutions.
OLED lighting panels are characterized by high energy efficiency as well as low environmental impact because they are mercury-free, unlike fluorescent lamps. OLED lighting panels are also beneficial for our eyes because they do not emit ultraviolet rays.
OLEDs have been developed in various fields, such as displays for mobile phones, etc.. However, it would be noted that there are two types of luminescence: fluorescence and phosphorescence, and they have significantly different efficiency.
Theoretically, electric energy is converted into light with 100% efficiency in phosphorescence, whereas in fluorescence, only 25% of energy is converted into light; the rest is converted into heat. Thus, phosphorescence is preferable from the viewpoint of conserving electricity and reducing heat generation.
Phosphorescent products that generate less heat can be reliably applicable in areas close to people and goods (e.g. food), and help expand the scope of applications in areas where lighting was previously not possible.
OLED lighting seemed to be a magic bullet. However, in order to use OLEDs for general-purpose lighting, Konica Minolta found it a challenge to meet the requirements regarding light-emitting lifetime and cost reduction (i.e. OLED lighting should be comparable to that of fluorescent lamps and LEDs).
Konica Minolta became the first company in the world to commercialize all-phosphorescent lighting equipment in 2011. In general, white OLEDs can be produced by combining three colors of phosphorescent material (blue, green, and red). However, development of blue light-emitting phosphorescent materials had been considered as bottle-neck. Konica Minolta took the lead in the industry to develop and commercialize blue light-emitting phosphorescent materials.
Symfos OLED-010K, Konica Minolta’s OLED lighting panel sample kit, is made entirely from phosphorescent materials by employing blue light-emitting phosphorescent materials developed in-house. The luminescence efficiency is as high as 45 Lm/W for mass production; and its light-emitting lifetime is 8,000 hours – several times longer than that of incandescent lamps. Symfos OLED-010K is also characterized by its high-quality light. Specifically, the light produced is uniform due to (i) evenness on the light-emitting surface and (ii) minimal change in color and intensity in varying angles.
To emit light, OLEDs require not only light-emitting materials (dopants) but also host materials for transferring electrons and holes to light-emitting materials. In the light-emitting layer, a trace amount of light-emitting material is contained in the host material; these materials affect one another.
For example, when BE-1 (a blue light-emitting material) is used, the lifetime of OLED remains almost unchanged even if the host materials are changed (HOST-A to HOST-D), as shown in the figure on the right. When BE-2 (an improved blue light-emitting material) is used, the light-emitting lifetime changes dramatically depending on selected host material. Given the fact that the lifetime depends largely on combination of light-emitting material and host material, efforts to correctly identify and design the appropriate combinations of materials are the key to extending the lifetime of OLED.
The biggest feature of OLED material development in Konica Minolta is our unique molecular design technology: two-molecule simultaneous design for light-emitting materials and host materials. Though it may seem simple, this is an advanced development technique unrivaled by competitors. Notably, Konica Minolta’s expertise gained and refined in its color photograph business is fully utilized in developing light-emitting materials and host materials at the same time in an integrated flow from molecular design, to synthesis, and then to evaluation.
Even if a technology is established in a laboratory, its commercialization requires production technologies, including evaluation technologies, in order to mass-produce high-quality products.
In order to mass-produce OLED lighting panels, the key was to develop the technologies to evaluate reliability described below.
1.Predicting the light-emitting lifetime
To predict the light-emitting lifetime, Konica Minolta established a high-precision method for estimating the luminance attenuation in a short space of time, and employed it as an inspection method before shipment. Based on statistical analysis of lifetime behavior, Konica Minolta developed its proprietary accelerated test and estimation methods for lifetime evaluation. Luminance attenuation over thousands of hours can be estimated in a short evaluation time.
2.Predicting short circuits
Dielectric breakdown during operation results in a short circuit, causing non-luminescent defects. To prevent such defects, Konica Minolta established an inspection method for detecting panels with high probability of short circuit failures in advance by statistically identifying the correlation between the characteristics of the leaking current and the probability of short circuit failures.
Development of these objective and quantitative evaluation technologies enabled Konica Minolta to ensure quality evaluation of OLED lighting panels with high precision in a short space of time and thereby commercialize products.