Printed electronics for flexible solid-state lighting

Printed electronics technologies are being introduced as competitors to crystalline semiconductor technologies in several applications, including logic circuits, photovoltaic cells and light emitting diodes (LED).

This article originally appeared in Global SMT & Packaging magazine #10.11 (November 2010)

Printed electronics technologies are being introduced as competitors to crystalline semiconductor technologies in several applications, including logic circuits, photovoltaic cells and light emitting diodes (LED). Near term solid-state lighting (SSL) products based on LED technologies are entering the general illumination market. These SSL products use conventional SMT processes in a four-level assembly hierarchy to fabricate light engines for incorporation into luminaires. In parallel with LED device development, printed electronics technologies are also moving forward using ink-based systems to fabricate organic-LEDs (OLED). However, the OLED assembly hierarchy used to fabricate light emitting structures is significantly different from that for LED-based devices. This paper will compare LED and printed OLED assembly technologies, discuss available printing technologies and ink systems for OLED structures and address OLED fabrication and assembly issues.

Introduction
By various estimates, lighting is one of the largest U.S. consumers of electrical power1. Around the world, significant economic and environmental pressures are driving major energy saving efforts with respect to lighting technology. One area receiving a great deal of attention for replacing general lighting sources, i.e., incandescent lamps, fluorescent lamps and compact fluorescent lamps (CFL), is Solid State Lighting (SSL). The excitement with SSL arises from the rapid performance increase demonstrated by crystalline LED-based lighting technology.

Globally, industrial investments and government funded activities are supporting R&D and manufacturing development. The result has been SSL lamp performance that in some cases is equal to or better than existing incandescent and fluorescent lamps. LED-based lamps are now commercially available and in many cases is cost competitive with incandescent and fluorescent lighting systems based on life-cycle assessments. Furthermore, the long lived SSL product can produce additional cost savings through reduction of maintenance costs.

Figure 1. Historical and predicted light source efficacie
Figure 1. Historical and predicted light source efficacie
Following Holonyak’s2 1962 announcement of the first light-emitting diode, LED efficacy has dramatically increased over time, Figure 13. Efficacy, the ratio between the amount of light that a source generates relative to its energy input, is measured in lumens per watt, (lm/W). During this time, the incumbent incandescent lamp efficacy has plateaued at a relatively low level. While the various fluorescent technologies have demonstrated increased efficacy, their growth has not been as rapid as that for the LEDs.

Figure 2. 2010 version of Haitz’s Law
In addition to increased performance, LED cost has rapidly decreased. Haitz’s Law (Figure 2), analogous to Moore’s Law for ICs, demonstrates how the cost of LED lighting decreases (cost per lumen) while the efficacy increases (lumens per watt) over time4. Note that the “flux/package” trend is increasing faster than the historical trend.

Organic-LED (OLED) is also undergoing rapid development, but is several years behind the LED-based technology. In contrast to point source LED luminaires, OLED products are dispersed light sources, which promise new lighting paradigms. Developed by Kodak5 in the 1980s, OLED devices were initially fabricated using vapor deposition processes. The opportunity to use printing processes for OLED fabrication opens up new opportunities.

Presently, OLED efficacy is lower than that for LED product, but is rapidly increasing. It is too soon to know whether OLED products will follow Haitz’s Law.

SSL assembly hierarchies
LED assembly
LED-based SSL assembly follows a 4 level assembly hierarchy, Table 1.

Table 1. LED assembly hierarchy
Table 1. LED assembly hierarchy
LED assembly uses a broad range of SMT processes and unique materials. Following die singulation and light-emission testing, binned die (based on light quality) are assembled into a light source. A Level 1 LED assembly is depicted in Figure 36. Ultimately, the light engine is placed in a luminaire.

Figure 3. Level 1 LED package assembly
Figure 3. Level 1 LED package assembly
OLED assembly
In contrast to LED assembly, OLED assembly follows a very different assembly process, Table 2. While this is still a four-level assembly hierarchy, Levels 0, 1 and 2 are merged into a comprehensive assembly step since OLED fabrication intrinsically produces the light source (Level 0), and light engine (Level 1 and 2) in an interrelated process, not as a series of discrete assembly steps, which defines LED SSL assembly.

Table 2. OLED assembly hierarchy
Table 2. OLED assembly hierarchy
This OLED assembly hierarchy arises from the nature of the OLED device, which is different from that for a crystalline based LED. A typical OLED light source fabricated on a glass substrate, shown in Figure 47, is composed of a stack of thin films (usual total thickness of 100-200 nm) situated between planar electrodes, with at least one transparent electrode to enable light to exit the OLED structure. Descriptions of the various layers are described in Table 3.

Figure 4. Assembly structure for OLED-based device
Figure 4. Assembly structure for OLED-based device
Table 3. Description of OLED layers (from Figure 4)
Table 3. Description of OLED layers (from Figure 4)
Visible light is created when electrons and holes, driven by an applied voltage across the electrodes, combine in the OLED layers. A luminaire is formed when the OLED is assembled into a structure that protects the OLED from physical and environmental damage as well as supplying it with electrical energy and control functions.

SSL OLED technology began as a display technology. In addition to developing the materials and fabrication processes to produce displays, lithography capable of sub-millimeter resolution was also required for forming small pixels with the OLED material sets. While requiring similar OLED materials and substrates, SSL requires lower resolution lithography (i.e., the pixels are larger) so that a critical display manufacturing step is removed from the lighting path. The result is a significant fraction of the manufacturing cost is reduced for a luminaire relative to a display, while concurrently increasing manufacturing throughput and yields.

OLEDS for solid-state lighting
As noted in the OLED assembly hierarchy, Levels 0, 1 and 2 are strongly interrelated. This interrelation is made more complex by multiple options available for producing the OLED device. These include:

Vapor deposition vs. printed processes
Flexible substrate vs. rigid substrate
Reel-to-reel (R2R) vs. sheet fed processing
While some OLED material systems can be vapor deposited, this paper will generally focus on those technologies that use printing technologies for fabricating the OLED. We will discuss vapor deposited processes for comparison purposes. OLED materials have a rich developmental history which is described in other publications8. The materials span multiple spaces of the OLED fabrication process and continue to undergo rapid development. This development is intimately tied to the chosen deposition process and substrate properties. Furthermore, these processes are strongly correlated with the associated ink technology.

Figure 5. Vertically stacked OLED structures for producing white light
OLED light emitting material systems

Printable OLED inks are available from a number of commercial suppliers. Polymer OLED materials (polymer light emitting diode, or PLED) are a class of polymers that emit light and are solution proccesable. Alternatively, vacuum thermal processes are typically used to deposit small molecule OLED (SMOLED) materials.

In addition to the light emitting materials that form the OLED PN junction, additional materials are used to enhance OLED performance. For example, inks have been developed for Hole Transport and Electron Transport functions.

LED-based solid-state lighting produces white light by mixing light from red, green and blue LEDs, or by using phosphors to convert blue or UV LED light to white light. Similar schemes can be used with OLED devices. Various OLED constructions have been developed to realize white light sources. Figure 5 shows a vertically stacked “hybrid tandem” OLED structure for producing white light9, where the OLED structures are deposited on top of each other. Light is produced in the two junction regions and exits the OLED through the transparent anode side. An alternative structure that spreads the light emitting structures along the substrate is depicted in Figure 610.

Figure 6. Horizontally positioned OLED structures for producing white light
Figure 6. Horizontally positioned OLED structures for producing white light
In the horizontally arranged OLED structures, the red, green and blue light sources are positioned next to each other rather on top of each other. This reduces the number of vertical layer depositions.

OLED substrate material systems
The OLED light emitting structures are encased between two electrodes, one of which needs to be transparent so that the generated light can exit the OLED structure. The transparent electrode is usually the anode. Rigid OLED devices are typically fabricated on glass coated with a transparent conductive oxide (TCO), usually Indium Tin Oxide (ITO). Flexible anodes are fabricated on polymer substrates similarly coated on one side with ITO. PET (polyethylene terephthalate) and PEN (polyethylene naphthalate) polymers have been used for substrates. The cathode is typically a low work function metal foil, for example aluminum or calcium.

OLED light emitting materials are extremely sensitive to water vapor and oxygen. For commercial devices11, typical water vapor transmission rates (WVTR) are

Light extraction from the OLED device is a primary concern for increasing OLED panel efficiency and luminaire efficacy. In conventional OLEDs, it is estimated that only 20% of the generated light exits the OLED due to refractive index mismatch12. Films that redirect light between the substrate and the electrode can be used to enhance light output. Photonic crystals can provide improved coupling to enable light to exit the OLED.

OLED printing processes
SMT assembly makes use of various printing and deposition processes in the production of electronic products, e.g., solder paste screen printing, conformal coating, conductive adhesive dispensing, etc. Printing technologies for OLED technology embrace a number of printing technologies that move beyond the typical SMT space. The ability to print many of the materials that comprise an OLED SSL device makes such fabrication a compelling value proposition.

The OLED substrate will determine whether R2R or sheet fed processes will be feasible. Rigid glass substrate systems require sheet fed processes, while flexible substrates such as polymer and metal foils can be used with either process. While R2R printing produces long sheets of OLED material the OLED devices still need to be excised from the web. Depending upon the manufacturing flow and subsequent processing steps, sheet fed printing that produces near-net final shapes may be as cost effective as R2R printing with its subsequent slitting and excising steps. Engineering analysis is required to determine the optimum system.

OLED printing techniques can be differentiated by the method they use to feed the substrate to the printing process. R2R processes print on long continuous material films, while sheet fed printing uses individual sheets in discrete sizes. Relative to sheet fed processes, R2R processes minimize the load/un-load time between printing stations. However, sheet fed processes can be made to mimic R2R printing with inline techniques.

A simplified R2R process flow for producing an OLED light source is depicted in Figure 713. In this schematic diagram, the transparent substrate is printed with the anode layer and then successive layers of functional inks are in-line printed to produce the OLED structure. For white light production, red, green and blue emissive layers can be printed, but if a single light color is required then only printing that ink would be necessary. White light can also be produced by printing a blue light emissive layer and adding a phosphor layer that converts the blue light to white light, which is similar to crystalline LED SSL practices. Within this basic process flow numerous permutations are possible to address cost effective manufacturing. The encapsulation and completed module steps will be discussed in later sections.

Gravure, flexographic and slot-die coating processes can be used to print or coat the inks to produce an OLED device. An alternative to these printing processes are inkjet printing processes. While the first three processes require contract between dispensing equipment and the substrate, inkjet printing is a non-contact process. Inkjet printing processes have been developed for fabricating OLED displays14, Figure 8. An OLED suitable for SSL applications could be simpler to produce since it would not require the polysilicon TFT and the pixel sizes could be larger.

Figure 7. Schematic of a R2R OLED printing process
Figure 7. Schematic of a R2R OLED printing process
Figure 8. Schematic of an inkjet OLED printing process
Figure 8. Schematic of an inkjet OLED printing process
Screen-printing technology (which is an SMT staple) uses a masked screen to determine the location where the ink will be deposited. With both flat bed and rotary machines available, it has wide applicability because it can print on many types of surfaces and substrates and is compatible with a wide range of ink viscosities. It is suitable for printing relatively thick layers to produce electrical conductors and dielectric layers. These are required: a) to produce interconnects between the OLED light engine and power sources, b) to generate printed bus bars to bring electrical power to the OLED pixels, c) to provide conductors for control lines to offer unique features and d) to provide insulating layers where appropriate. Bus bars and interconnect conductors can be formed from silver or carbon inks. Silver inks have higher conductivity while carbon is lower cost. Bus bars and interconnect lines must be positioned so that they offer the lowest shadowing effects.

OLED encapsulation processes
In an OLED SSL device, the OLED light source generates the light (Assembly Level 1 and 2) while the light engine (Assembly Level 2) enables it to couple to the luminaire. Assembly Level 2 must provide the light engine environmental protection (e.g., humidity resistance, oxygen resistance, etc.) and physical protection (e.g., abrasion resistance, etc.). Assembly Level 2 also provides the opportunity for improving light extraction.

Printed OLEDs are thin structures with electrodes on the top and bottom surfaces. Since the separation distance between the electrodes is small and the flexible substrate materials tend to be soft, care must be taken when cutting and excising OLED devices from the web. Electrode surfaces cannot come in contact with each other (e.g. due to edge smearing) since they would generate an electrical short and cause device failure. Mechanical cutting tools or cutting lasers can be used if they don’t leave burrs.

Encapsulation of OLED light engines via hermetic barriers is highly advantageous to ensure long product lifetime. Most barrier systems follow a multilayer approach. Laminate films are comprised of thin polymer layers interspersed with thin ceramic layers to provide hermeticity and maintain flexibility. Coating processes also use a multilayer approach with the hermetic film built up with polymer layers and ceramic layers.

Numerous companies are developing barrier films that can be applied via printing/coating or film lamination processes. In addition to offering protection against water vapor and oxygen, these encapsulation layers must also offer abrasion resistance and provide physical protection to the light engine. If they can’t supply these attributes within one barrier film, then additional layers need to be applied that meet the product requirements.

OLED device surfaces have a higher refractive index than air, which coupled with a planar emitting surface leads to low light out-coupling. Techniques are being pursued to increase light out-coupling by modifying the OLED surface with micro lenses, surface roughening and quantum optics15.

OLED luminaire
Connecting power lines to the OLED device requires a mechanical/electrical connection stable over temperature, that is formed at low temperatures or localized high temperatures, and is mechanically compatible with a flexible substrate. The low temperature requirement arises because several of the OLED material systems cannot take high heat levels, so conventional reflow soldering processes are precluded. Localized contact soldering techniques such as thermodes or hear bars, or laser soldering are potential candidates. Conductive adhesives (anisotropic and isotropic may be useful in these circumstances. Mechanical connectors may offer an alternative approach.

Luminaires using vapor deposited OLED light engines are available from Osram16 and Philips17, Figure 9.

Figure 9. Left – OSRAM Orbeos product. Right – Philips Lumiblade product
Figure 9. Left – OSRAM Orbeos product. Right – Philips Lumiblade product
A cross-section schematic of the Philips Lumiblade device18 is shown in Figure 10. This luminaire is fabricated on a rigid glass substrate with a vapor deposited OLED and a back cover to seal the device. To help reduce oxygen and water vapor effects, a getter is encapsulated with the OLED.

Figure 10. Cross section of Philips Lumiblade device
Figure 10. Cross section of Philips Lumiblade device
The performance of printed OLED luminaires lags that for vapor deposited ones, but they are rapidly improving. A number of companies are working to develop printed OLED devices. For example, GE is developing a R2R process, depicted in Figure 11, for fabricating printed OLEDs19. They fabricate an OLED light engine by laminating together “half-devices” printed on flexible substrates.

Figure 11. Schematic diagram of GE lamination process for OLED fabrication
Figure 11. Schematic diagram of GE lamination process for OLED fabrication

Figure 12. Example of printed flexible OLED devices
An example of printed OLED devices fabricated via a printing process using flexible substrates is shown in Figure 1220.

OLED value chain
Most incandescent and fluorescent lamps are commodity products. LED and OLED SSL products will be able to command premium pricing in their initial roll-out, but they will ultimately participate in a commodity market. In this value chain, cost will be a critical factor as they compete against the incumbent technologies. Manufacturing technologies that drive toward low cost solutions will be critical toward ultimate market success. OLED printing technologies, based on commercial processes and equipment sets, can help realize a low manufacturing cost structure.

Also in this value chain, new supply chains and distribution channels are developing. Printing companies are learning how to design and manufacture electronic products and test their performance. These are skills that the SMT manufacturers have in abundance, but interestingly are not presently particularly active in this new value chain creation. Many of the assembly equipment sets, test equipment and manufacturing protocols that the SMT industry routinely uses could be transferred to the SSL industry and open up new sources of revenue.

Conclusion
SSL products based on LED technology are beginning to enter the commercial marketplace, while OLED technology is several years behind for broad market entry. The ability to print OLED devices on flexible substrates will provide a powerful impetus to realize low cost OLED luminaires. Numerous printing processes are amenable to OLED production, and in this nascent stage, value chains and supply chains are undergoing rapid development. SMT assembly processes could offer much support to this growing industry.

As distributed light sources, OLEDs will open up new lighting paradigms for the lighting designer, lighting engineer and architect. With widespread installation, they should reduce the energy required to light our society while providing new lighting sources to complement the traditional point source devices.

References
http://www.nglia.org
Holonyak and Bevaqua, Applied Physics Letter, Volume 1, 1962, pp 82-83
US DOE Solid-State Lighting Research and Development: Multi-Year Program Plan, March 2010, p.23
US DOE Solid-State Lighting Research and Development: Multi-Year Program Plan, March 2010, p.29
C. W. Tang, S. A. VanSlyke, Organic electroluminescent diodes, Appl. Phys. Lett. 1987, 51, 913
LUXEON Rebel Board Design and Assembly Application Brief AB32 (10/08)
https://www.msu.edu/~nobleda3/pages/How%20Oled%20Works.html
“Flexible Solid State Lighting: Technology, Manufacturing, and Market Assessment”, FlexTech Alliance Report, Released May 19, 2009
Eastman Kodak, SID Tech Symp 54-2 (2008)
Anil Duggal (GE Global Research), FlexTech Alliance Conference 2009
http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/clausen_vancouver09.pdf
Dr Jonathan Halls – “Polymer OLED Technology Fundamentals, Status & Prospects” CDT SID 2008, http://www.cdtltd.co.uk/resources/279.asp
“Organic Light Emitting Devices for Solid-State Lighting”, Franky So, Junji Kido, and Paul Burrows, MRS Bulletin, July 2008, Vol. 33, No. 7,
http://www.oled-display.net/oled-inkjet-printing
Dr Jonathan Halls – “Polymer OLED Technology Fundamentals, Status & Prospects” CDT SID 2008, http://www.cdtltd.co.uk/resources/279.asp,
http://www.osram-os.com/osram_os/EN/Press/Press_Releases/Organic_LED/ORBEOS-OLED-light-source.jsp
http://www.lighting.philips.com/in_en/global_sites/led_lighting/information/oled/index.php?main=gb_en&parent=1&id=in_en_led_lighting〈=en
http://www.lighting.philips.com/in_en/global_sites/led_lighting/information/oled_lumiblade.php?main=gb_en&parent=1&id=in_en_led_lighting〈=en
Anil Duggal (GE Global Research), FlexTech Alliance Conference 2009
http://www.grcblog.com/?cat=19