OLEDs for Professional Lighting Applications
Organic light-emitting diodes (OLEDs) already represent a strong market segment in display applications such as mobile devices. In the lighting market, they are still in the beginning phase. To achieve a relevant market segment, OLED technology has to compete in terms of performance, cost, and other unique features against established light-source technologies.
by Jörg Amelung, Christian Kirchhof, Tino Göhler, and Michael Eritt
OLED-BASED DISPLAYS are well-established for use in smartphones and tablets. OLED TVs are still in the early stages but will enter higher volume markets soon. In the lighting field, however, OLEDs are still exotic.
To reach a larger lighting market segment, OLEDs have to compete against traditional lighting solutions and, additionally, fulfill lighting requirements for dedicated applications. To achieve this, synergies between displays and lighting architectures must be accomplished.
In 2011, the world residential lighting market was around €21 billion, which represents approximately 40% of the general lighting market.1 This number covers new fixture installations, including the full value chain, lighting-system control components, and light-source replacements. The professional market segment is €34 billion in total, which is the other 60% of the market. Additionally, the value per application in the professional market is higher than in the residential market; these factors make the professional market rather attractive for OLED applications (Fig. 1).
Fig. 1: The cost/volume ratio for different lighting applications shows that costs are high and volumes are low in the niche areas of art installations and decorative lighting.
Application Requirements
To date, OLED lighting solutions exist mainly in art installations and decorative lighting. These niche applications are tolerant of high costs and low performance. Actual OLED applications in high-end decorative luminaires or art installations are possible because the requirements for luminous flux or lighting performance are low. To enter a wider range of professional lighting applications, OLEDs must improve in technical parameters and in cost. Additionally, innovative integration concepts for large lighting areas are necessary.
Professional lighting describes the market segment for B2B (business-to-business) solutions, covering office, shop, hospitality, industrial, architectural, and outdoor lighting solutions. These application fields have many individual application requirements, but, in general, the applications can be summarized as shown in Table 1.
Table 1: Requirements are compared for various professional applications.
OLEDs could in a near-to-middle time frame cover only a part of these applications, based on their parameter field, which can be summarized as follows:
• Medium-to-low luminous flux due to limited brightness level. • Medium areas due to actual fabrication limitation. • High lighting quality based on a good lighting mixture (CRI > 80–90). • Medium-to-high cost aspect. • Very good design aspect based on slim format and homogenous lighting area.
Based on these criteria, the application fields of hospitality, shop, and office are the best candidates for OLED applications in the near future.
Office-related solutions are the largest market segment in general lighting; in 2011, they represented €8 billion worldwide.1 This market segment is very suitable for OLED-related lighting solutions because the applications are indoor, higher-cost related (especially in Europe), and require high-quality lighting generation.
Unfortunately, the largest applications inside the office market are downlights and recessed luminaires. Both applications are not really suitable for OLED integration. Downlights require very high brightness levels and luminous flux; recessed luminaires are mainly cost driven and do not need a slim form factor because in ceilings additional installations such as air conditioning are required.
Based on the limited luminous flux of OLEDs, potential applications for OLED lighting currently involve those that are in close proximity to the user. The most interesting applications are desk, wall-integrated, pendant, and free-standing luminaires (Fig. 2).
Fig. 2: The following office-lighting applications are the most likely to be commercially viable for OLED at this time: wall luminaires (top left), pendant luminaires (top right), light lines or slot lighting (bottom left), and free-standing luminaires (bottom right).
There are two ways of implementing new lighting technology for such applications: by retrofitting elements or by designing new luminaires. Retrofit solutions would require OLED elements that replace in size and luminous flux traditional fluorescent tubes such as T5, T8, or T-CL. Such retrofit solutions with LEDs have already entered the marketplace. They are ideal because there is no need to redesign the luminaire, which saves cost and time.
For OLEDs, these kinds of retrofit solutions will not work because of the form factors and brightness levels of traditional fluorescent solutions. New flat luminaire designs are the best way to integrate OLEDs into applications. In Table 2, the relationship between volume, price, and type of luminaire is shown. OLED applications will be implemented first into design-oriented luminaires and will follow the route to mass volume by reducing cost and improving performance.
The requirements in price and performance for OLEDs rise dramatically for functional and mass-volume luminaires because there OLEDs will be in direct competition with LED solutions.
Even for this first stage of application, the design-oriented luminaires, OLEDs will have to fulfill some requirements that are commonly restricted by the applications.
These requirements are:
• Luminous flux > 9000 lm/m² (> 3000 cd/m² if assuming lambertian emission). • Color-temperature standards; 3000 and 4000K. • Area sizes: 0.3 × 0.3 m or 0.6 × 0.6 m. • Simple integration.
None of the OLEDs now in the marketplace fulfil these requirements; in most cases, the luminous emittance is too low or the color temperature is lower than 3000K and/or the sizes are too small.
One additional important target is the cost aspect. Actual OLED solutions are about 100 times more expensive than LED-based ones, so OLED process costs have to be dramatically reduced. OLED display fabrication is well-positioned in the market, so there is the question of which synergies could be used between the display and lighting segment to improve performance and reduce fabrication costs.
With regard to OLED TVs, some of the main parameters are close to those of OLED lighting, so both industries use similar approaches. Normally, the display architecture for the two is completely different based on the required RGB pixel arrangement. Nevertheless, there is an alternative color arrangement in some displays that use white with color filters, which offers certain advantages, especially for large-area displays.2 In principle, this arrangement consists of a large OLED lighting area integrated into an active-matrix backplane. The fabrication synergies are maximal in that arrangement, so similar machine concepts could be used. The OLED stacks in such color-by-white arrangements are also similar; nevertheless, the white color point and brightness level are different. Thus, OLED displays and lighting fabrication technologies could enjoy synergies in the case of fabrication technologies and OLED stack architecture. This could make the market entrance for OLED lighting easier, especially in the professional sector.
Highly Efficient OLED Modules
OLED modules represent an important step toward achieving large-area lighting solutions with accepted and required lumen packages. OLED glass panels are not very easy to integrate. The thin-film electrodes on glass have to be contacted in a good manner to avoid voltage drops to the module. Additionally, the glass itself cannot be handled easily, and an additional optical out-coupling system has to be integrated. Based on the limited sizes of OLEDs (up to 15 × 15 cm²), large areas of OLED lighting have to be subdivided in several OLED lighting plates.
OLED lighting modules represent an import intermediate level for integration. Such modules could be easily mounted to cover large lighting areas via a mechanical support system. Important aspects of these OLED modules could be subdivided as follows:
• Small contact areas to maximize the OLED lighting area. • High-luminous-intensity homogeneity. • Minimal thickness. • Robust contact system.
To achieve a robust contact system, the OLED electrode contacts are bound by a flexible contact spring to a PCB connection plate, which results in a total contact resistance of < 1 Ω for a 99 × 99-mm OLED module.
A potential OLED module system is shown in Fig. 3. The module combines a large active area (> 80% lighting area) with a slim form factor (< 3 mm thickness) and a robust contact system for use in large-area applications.
Fig. 3: This OLED module system measures 99 × 99 mm per panel.
Based on very efficient OLED panels (fabricated by LG Chem), the OLED module system allows very efficient OLED lighting. The modules achieve a luminous efficacy of over 50 lm/W at 3000 cd/m² (averaged luminance at 230 mA) and a total luminous flux of over 100 lm for a 350-mA current, which results in a luminous emittance of over 12,000 lm/m² (Fig. 4).
Fig. 4: Module efficacy is plotted vs. average luminance of the module.
Mirror OLED Module Solutions
Besides performance, it is important to provide new features or form factors to users to earn new application scenarios. One interesting implementation for OLED modules inside professional applications is the off-state mirror surface. There are two solutions to achieve an off-state mirror. In the first, the out-coupling foil on top of the OLED is not applied; in the other, an additional mirror is used as secondary optics on top of the light-emitting surface.
An optical system with an additional mirror plate achieves a highly efficient mirror-like OLED in the off-state. In the on-state, the optimized mirror allows a stable color coordinate at different angles, with only a low shift of color compared to an OLED with no out-coupling or secondary optic system. The appearance is uniform even at the edges of the OLED module.
This solution also allows for a more efficient OLED module; the OLED efficacy is still 34 lm/W at a luminous flux of 59 lm (99 mm x 99 mm) based on an OLED panel with initially 40 lm/W and a milky diffuse surface (Fig. 5).
Fig. 5: Module efficacy is shown vs. driving current of the mirror-type OLED module.
An innovative luminaire solution based on such modules has been demonstrated by the companies Selux and Art+Com. The luminaire Manta Rhei is a pendant type with individual dimming of 140 mirror-type OLED modules and kinetic movement of the OLED panels that is enabled by the bending of the mounting metal sheet. Based on individual dimming, the illuminance level remains constant during movement (Fig. 6).
Fig. 6: The Manta Rhei luminaire by Selux and ART+COM consists of 140 mirror-type modules. (Source: Selux.)
The Future of OLED Lighting
To achieve a larger market segment, OLED’s best near-time possibility is in fulfilling product requirements for professional lighting solutions. High brightness and efficient OLED solutions could be achieved by optimized module solutions. Novel surface finishings such as a high-quality off-state mirror surface allow for new luminaire appearances, which may accelerate the OLED market penetration, although initially toward the high end. Last, synergies between OLED solutions in displays and lighting could generate advantages in terms of fabrication costs and OLED stacks. Such advantages could boost both industries.
Acknowledgment
Some of these OLED results were achieved inside the BMBF-funded OLED project SO-LIGHT (FKZ 13N10536).
References
1Lighting the way: Perspectives on the global lighting market, 2nd ed. (McKinsey & Company, Inc., August 2012).
2C-W. Han et. al., “55-inch FHD OLED TV Employing New Tandem WOLEDs,” SID Symposium Digest of Tech Papers 43, Issue 1, 279–281 (June 2012). •