Vacuum Deposition

Abstract

Vacuum deposition technologies for organic thin films are described as an essential fabrication technology that has enabled today’s displays and lighting products based on organic light-emitting diodes (OLEDs). Fundamentals behind vacuum thermal evaporation are summarized, and requirements for fabrication of high-end OLED displays are presented. Key engineering and advances employed to meet those requirements are then introduced with examples.

1 Introduction

Displays and lighting devices based on organic light-emitting diodes (OLEDs) involve organic thin films sandwiched between two electrodes. Those organic thin films are merely a few hundred nanometers thick in total, yet they should be formed over a substrate that can often be a meter wide or more. Keeping the fidelity and uniformity of a thin film over such a large area indeed poses a technical challenge in fabrication from both optical and electrical perspectives. The pixel-to-pixel variation in thickness will result in variation in luminous efficiency and/or shift in resonant wavelength eventually to yield optical nonuniformities in brightness, colors, etc. Occasional pinholes and bumps in those films will induce leakage current and accelerate degradation due to local high electric field. Successful commercialization of OLED displays and lighting devices would thus have not been possible without organic thin-film deposition technology that can cope with those challenges. In that respect, the importance of vacuum deposition technique, particularly those based on vacuum thermal evaporation (VTE), cannot be overemphasized as a key enabling technology that led to “OLED quality” thin organic films and patterns.

This chapter will explore the various aspects of vacuum deposition methods relevant to fabrication of displays and lighting devices based on OLEDs. Fundamentals of VTE and its variations will be first introduced to those who are new to this field. Some of the detailed features critical to industry-scale realization of state-of-the-art active-matrix OLED displays will then be discussed, and their implementation in practice will be presented with some examples. Finally, white OLED technologies are briefly discussed considering their importance in large OLED TVs, which have recently become commercialized using a pixel architecture based on white OLEDs coupled with RGB color filters.

2 Fundamentals of Organic Thin-Film Deposition by Vacuum Thermal Evaporation

2.1 Fundamentals of Vacuum Thermal Evaporation

Vacuum thermal evaporation (VTE) refers to a deposition technique in which source materials are heated up in a vacuum and vaporized, transported by diffusion, and condensed at a “cold” substrate to form a film of the source material (Ohring 2002). Although VTE has some inherent disadvantages such as large amount of wasted material, it is widely regarded that the reliability and maturity of this well-established method are largely responsible for the emergence of OLED displays and lighting products. This section will briefly review the fundamentals of VTE methods from the perspectives of OLED fabrication and introduce variations of VTE that are being considered to address new challenges.

Figure 1a shows the most common configuration of a VTE system. It generally consists of a plurality of source units, a substrate- and mask-holding assembly, shutters, and thickness monitors connected with a deposition rate controller. A source unit is based on resistive heating in most cases, and thus the heating process can be easily controlled in an electrical manner. Source material to be deposited is placed in its solid form either in a crucible surrounded with a resistive heater coil or on a “boat” (see Fig. 1a and b). In the case of organic compounds, typically given in the form of powders, the crucible method is preferred because it is easier to confine the compounds without spill, and it ensures enhanced control over the deposition process.

Fig. 1

(a) Typical configuration of lab-scale vacuum thermal evaporation system. Organic material is typically deposited with a low-temperature effusion source in which a crucible surrounded with a resistive heating coil is placed inside a metallic liner with a small hole at the top. (b) Boat-type source used for deposition of metals. It is being held by the two copper posts (Photo courtesy of Angstrom Engineering)

Once the source material is sufficiently heated up, it goes through a phase change either from solid directly to vapor (called “sublimation”) or from solid to liquid and then to vapor (see Fig. 2a) (Woodbury 1997). Whether the source is sublimated directly or evaporated after melting depends on the material, as phase diagrams differ from material to material. It is worth noting that evaporation can occur at a lower temperature when ambient pressure is low. The pressure at which the vapor state is in equilibrium with either solid state or liquid state is called “vapor pressure (P_vap).” At a given temperature, evaporation takes place when ambient pressure (P_amb) becomes lower than P_vap. Crucibles or filaments are thus made of materials with ultralow P_vap to ensure evaporation does not take place from containers or heaters but from source materials.

Fig. 2

(a) Typical phase diagram of a material. (b) Vapor pressure of common materials important in OLEDs. Ca has a relatively high vapor pressure than Al or Ag. Tungsten (W) has such a low vapor pressure compared to the other materials and thus is a preferred choice as a material for a boat or filament in VTE sources

Assuming an ideal gas, P_vap is shown to be in the form of:

$$ \log \, \left({P}_{\mathrm{vap}}\right)=A-B/T $$

(1)

where T is absolute temperature and A and B are material-dependent coefficients, with A and B being positive (Ohring 2002). P_vap is thus a monotonically increasing function of T. Because of this temperature dependence, P_amb should be low in order to enable evaporation at a relatively low deposition temperature (Fig. 2b). This is one of the main reasons why a high vacuum is preferred for VTE. For fabrication of OLEDs, a vacuum level on the order of 10⁻⁸ torr – 10⁻⁶ torr is typically used. Another critical benefit of a high vacuum deposition is that it can minimize the possibility of involving contaminants. This is highly critical as it is directly related to the long-term reliability of the OLEDs made thereof.

Another important aspect to consider when designing a vacuum chamber is the distance (= h) between a source and a substrate. For uniform deposition, a large h value is generally preferred. Under ballistic assumption, the total mass of evaporated materials (=Δm_sub) arriving at a small areal portion ΔA on a substrate is determined by the projected solid angle subtended by ΔA seen from the source. Depending on whether the source can be regarded as an isotropic point source or a small area source as shown in Fig. 3a, Δm_sub per unit area at x and thus the corresponding local thickness (= d(x)) are given by:

$$ \frac{\Delta {m}_{\mathrm{sub}}(x)}{\Delta A}=\rho\;d(x)=\left\{\begin{array}{c}\left(\frac{m_0}{4\pi}\right)\frac{\cos^3\theta }{h^2}=\left(\frac{m_0}{4\pi {h}^2}\right)\frac{1}{{\left(1+{\eta}^2\right)}^{3/2}}\quad \left(\mathrm{isotropic}\right)\\ {}\left(\frac{m_0}{\pi}\right)\frac{\cos^4\theta }{h^2}=\left(\frac{m_0}{\pi {h}^2}\right)\frac{1}{{\left(1+{\eta}^2\right)}^2}\quad \left(\mathrm{surface}\right)\end{array}\right. $$

(2)

where m₀ is the total mass evaporated from the source, η = x/h, and ρ is the density of the material being deposited (Fig. 3b for θ-dependence) (Tsujimura 2012).

Fig. 3

(a) A source-substrate configuration to estimate the flux of evaporated particles incident on ΔA from a small area source. Shown with dashed lines is the projected area of the corresponding area with respect to the line connecting the source opening and ΔA. (b) Normalized thickness profile as a function of θ for area and point sources. The former follows cos⁴θ and the latter follows cos³θ law. To be within 5% [10%] difference from the thickest spot (x = 0), x/h should be within 0.161 [0.233]

To ensure thickness variation is within 5% over a 10 × 10 cm² substrate, for example, h larger than ca. 62 cm is required for a small areal source configured as shown in Fig. 3a. It should be noted that height h cannot be made arbitrarily large; too large an h not only tends to make the chamber too big but also will waste a large portion of the evaporated material. Furthermore, h should be less than the mean free path (l_free), which is inversely proportional to ambient pressure (P) by:

$$ {l}_{\mathrm{free}}=\frac{k_BT}{\sqrt{2}{\pi \sigma}^2P}, $$

(3)

where k_BT is the thermal energy and σ is the collision cross section (Chapman and Cowling 1990). For air molecules at room temperature, l_free is approximately given by 0.005 P⁻³, when the pressure P is given in torr (Ohring 2002). Meeting h < l_free will make evaporated particles less subject to contamination and ensure the ballistic assumption is valid so that the deposition behavior may remain predictable.

The deposition process is monitored by a temperature sensor placed on a heater as well as a micro-quartz balance. The latter measures deposition rate by monitoring the temporal change in resonant frequency due to added mass during the deposition process. As it is not the actual thickness of the film grown on the substrate, a geometry-dependent tooling factor should be obtained carefully through calibration processes done on a regular basis. The deposition rate can ideally be stabilized by a feedback loop containing these sensors/monitors and a heating controller. When co-deposition of two different materials (e.g., host and emitter) is desired, micro-quartz balances may be placed separately near respective sources. The relatively easy control of mixing or doping ratio done in this way is another advantage of VTE, as the performance of OLEDs benefits largely from doped host structure or doped transport layers.

Patterning in VTE can be done in situ during deposition via shadow mask or fine metal mask (FMM) techniques (Tian et al. 1999). It is noted that extra length (Δl_shadow) can be added to the desired pattern due to a spatial gap present between the topmost layer and the mask (t_gap). With simple line-of-sight consideration, Δl_shadow is given by:

$$ \Delta {l}_{\mathrm{shadow}}=\frac{t_{\mathrm{gap}}{W}_{\mathrm{source}}}{h+{t}_{\mathrm{mask}}}\approx \frac{t_{\mathrm{gap}}{W}_{\mathrm{source}}}{h}, $$

(4)

for a source that is located at a vertical distance of h from the bottom side of the mask that has a properly tapered pattern (Fig. 4).

Fig. 4

Non-ideal situation that can occur when patterning is done by a shadow-masking technique. Shadowing as well as shift of a pattern center can occur depending on various geometrical parameters

For example, Δl_shadow is estimated to be a few μm for h = 50 cm, t_gap = 100 μm, and W_source = 1 cm. This can become larger as the horizontal position from the opening of a source increases. In such a case, the corresponding pattern can become off-centered as well. Estimation of Δl_shadow thus carries a significant importance; together with mask pitch variation, slit tolerance, and positioning accuracy, Δl_shadow can eventually limit the minimum pixel size and the maximum pixel density that can be achieved. With a very thin mask t_gap, Δl_shadow may become larger at the center than near the edges due to mask sagging. This non-ideal behavior becomes particularly serious when the substrate size becomes larger. Hence, optimization should carefully consider the trade-off between an overall decrease in Δl_shadow and an increase in the spatial nonuniformity of pattern fidelity associated with a thinned mask.

2.2 Variations of Vacuum Thermal Evaporation

Vacuum thermal evaporation is well suited for OLED fabrication, but it does have several aspects that should be improved, in particular, for further scale-up. The most significant issues include poor scalability and a large amount of material that is deposited on to the chamber wall eventually to become wasted. These issues are becoming more and more important as large OLED displays such as TVs begin to enter mainstream markets. In this regard, scanning linear-type sources are being considered as a solution to these problems. In this scheme, an array of closely spaced sources is distributed linearly and scanned along the direction normal to the symmetry axis of the array as shown in Fig. 5a. With the optimal spatial period of individual sources, uniformity requirements may still be met without increasing h. If one further reduces h, materials lost to the wall of a chamber can also be minimized, and the chamber can be maintained relatively small even for a large substrate. One thing to be careful about when decreasing h is to keep heat transfer to underlying organic layers minimal so that their properties are not degraded due to excessive heat.

Fig. 5

Variation of vacuum thermal evaporation technology. (a) Linear source deposition method. (b) Small mask scanning system. (c) Hot-wall method. (d) Organic vapor phase deposition (OVPD) method

Another method to improve the scalability of VTE is based on small mask scanning (SMS) (Kwon 2013). In this scheme, a mask smaller than the substrate is fixed, and the substrate is scanned with respect to the mask (or vice versa) (see Fig. 5b). This provides a remedy to the sagging problem of a large FMM. The requirement for h is essentially the same as that corresponding to the size of the FMM instead of the size of a substrate. This enables the size of a deposition chamber to remain within a reasonable range. Both FMM and SMS technologies are described further in details in Sect. 3.

For reduction of wasted materials, a hot-wall deposition method had also been proposed (Tsujimura 2012). In this scheme, the inner wall of a vacuum deposition chamber is heated to a similar or higher temperature than the deposition temperature of the source materials, such that materials that would otherwise condense at the wall can reevaporate and eventually condense at the substrate, which is the only “cold spot” within the chamber (see Fig. 5c). High thermal load as well as contamination control could be an issue in this scheme.

The large thermal load of the hot-wall deposition method may be circumvented by localizing the path of the evaporated materials. Such localization can be realized with the help of carrier gas. In this scheme, thermally evaporated vapor can be picked up by inert carrier gas such as He, N₂, or Ar and transported along well-defined gas lines that are heated to a temperature higher than the evaporation temperature. The carrier gas plays a role similar to that of a solvent in solution-processing such as inkjet printing. This technique is called organic vapor phase deposition (OVPD) and is illustrated in Fig. 5d (Baldo et al. 1998). The OVPD method combined with a small nozzle is called “organic vapor jet printing (OVJP)” (Shtein et al. 2004). In this scheme, carrier gas picks up the organic vapor thermally produced in the source unit and transports it through its nozzle. OVJP is a mask-free direct jet printing method and thus uses far less organic materials and does not require any FMM or sophisticated post-patterning steps to obtain micro-patterns. This method does not use organic solvents, which can be potentially harmful to the environment, and can use well-established small-molecule-based technology that shows high performance as well as good reliability. It is thus regarded that OVJP combines the advantages of both inkjet printing (scalability and mask-free additive processing) and VTE (proven reliability and high performance). The performance of organic thin-film transistors (OTFTs) fabricated by OVJP has recently been shown to be as good as that of OTFTs made by conventional vacuum thermal evaporation (VTE) upon careful control of carrier gas temperature and dielectric surface properties (Yun et al. 2010). Several reports have been made on the application of OVJP for direct pixel printing of OLEDs (McGraw et al. 2011; Arnold et al. 2008; Yun et al. 2012). Digital-mode OVJP recently demonstrated by Yun et al. provides a point-by-point, jet-on-demand operation equivalent to digital inkjet printing, so that one can better deal with arbitrary discrete patterns such as pentile RGB pixel geometry (Fig. 6) (Yun et al. 2012). One challenge with the present OVJP technique is potential thermal damage to underlying layers. High printing resolution is achieved when the nozzle is sufficiently close to the layer on which the target material will be printed. As the nozzle is also heated at T > T_dep, heat transfer from the nozzle to the substrate is inevitable. A design ensuring heat shielding will thus be critical in applying OVJP to OLEDs, most of which are made in a multilayer configuration.

Fig. 6

(a) Schematic diagram showing the structure and operation of digital-mode organic vapor jet printing (D-OVJP) apparatus. (b) OLED pixels printed by D-OVJP. (c) Atomic force microscopy image showing the polycrystalline grains of a pentacene film printed by D-OVJP. Reproduced with permission. (Yun et al. 2012)

3 OLED Pixel Patterning in Vacuum Thermal Evaporation

Displays consist of multiple pixels, which can individually control the brightness or color. Each pixel consists of red (R), green (G), and blue (B) subpixels for additive color mixing. Since OLEDs are self-emissive, we can realize RGB subpixels by localized deposition of RGB emissive materials for each subpixel (RGB side by side) or by filtering pixelated WOLED light through RGB color filters (Spindler et al. 2006).

Displays with the resolution of ultrahigh definition (UHD) (3840 × 2160) contain over 24 million subpixels. The demands for high pixel density increase rapidly, and the high-end cell phones with state-of-the-art active-matrix OLED (AMOLED) displays are currently boosting their pixel density that is even higher than the 550 pixels per inch. These specifications are highly challenging to realize, and at the heart of this development, there have been many kinds of efforts and ingenious engineering toward the improvement of pixel patterning techniques in VTE.

3.1 Shadow, Fine Metal Mask (FMM) Technology for Top Emission OLEDs in Mobile Applications

Currently, the resolution of the mobile display is increasing very rapidly with excellent performances. To realize this, the top emission OLED (TEOLED)-based active-matrix display seems to be the ideal approach. Normally, the TEOLED can deliver a higher aperture ratio and longer lifetime feature compared to the bottom emission architecture (Son et al. 2016). Besides, TEOLED can also provide several advantages such as (i) high efficiency at the front viewing angle, (ii) saturated color purity, (iii) low-power consumption, and iv) good driving lifetime. Normally, the standard top emission structure comprises a strong micro-cavity effect between the highly reflective bottom anode and semitransparent cathode. The luminance and color characteristic of TEOLED structure largely depends on the optical characteristics of the semitransparent cathode, cavity length, and cavity modes. Hence, the cavity length and reflectivity of the semitransparent electrode are the performance-decisive factors. The distance between the reflective anode and semitransparent cathode as well as the wavelength of the emitted resonant light can have several precise cavity modes like first-order, second-order, and third-order micro-cavity conditions (Park et al. 2015). In TEOLEDs, the thickness of organic layers over 2000 Å is vital for the real fabrication of the device. Indeed, the second-order cavity mode is favorable for the mass production of large-size displays due to its considerable total thickness of OLED and good performances such as efficiency and viewing angle properties. Thus, it is presently considered for the mass production of AMOLED mobile display. On the other hand, first-order cavity mode is difficult to realize owing to its very less total device thickness, which may create several issues such as electrical short and charge unbalance. Normally, second-order cavity mode shows two probable locations of the emissive layer (EML), either near the anode side or near the cathode side (Park et al. 2014). Likewise, third-order cavity mode illustrates three different sites of EML, specifically, near the anode, cathode, and center of the device.

Figure 7 shows the theoretical calculation of the luminous intensity of red (606 nm), green (535 nm), and blue (470 nm) micro-cavity TEOLEDs according to the thickness of hole transport layer (HTL). Usually, variations in the thickness of the HTL does not produce a significant effect on the device driving voltage because of its high carrier mobility compared to the electron transporting layer (ETL). The calculated highest radiance values of blue, green, and red top emission devices are obtained at the HTL thickness of 85 nm, 120 nm, and 160 nm, respectively.

Fig. 7

The theoretical calculation of radiance distribution of (a) red, (b) green, and (c) blue devices as a function of HTL thickness

As shown in Fig. 7, the red device has a higher luminous intensity than those of green and blue devices. This indicates that the light intensity at a shorter wavelength is not easily enhanced in the device. Additionally, it is noted that only some wavelength of light is permitted in specific cavity modes. The micro-cavity-based top emission technology can enhance the color gamut and efficiency of OLEDs without changing the organic materials (Son et al. 2016). Similarly, the capping layer with a high refractive index on top of the semitransparent cathode is advantageous as it can further tune the transmittance or reflectance of the top electrode assembly. As a result, substantial enhancement in the luminous efficiency can be attained. Typically, the enhanced factor is twice for green and thrice for red emission devices. The current AMOLED display using top emission architecture has nearly 100% color gamut for mobile phone applications. In order to match the different cavity lengths for RGB emissions, normally common HTL and ETL are used with the same thickness, and then additional different HTLs for red and green (R′ and G′) are inserted. The RGB micro-cavity-based TEOLED architecture is shown in Fig. 8. Based on the luminous intensity calculations, Fig. 8 illustrates the different positions of the RGB emissive units according to the cavity length between cathode and anode.

Fig. 8

Schematic of RGB micro-cavity TEOLED devices

OLED manufacturing is a critical process, which requires good precision to make a high-resolution RGB color pattern. There are several methods reported for the deposition of organic semiconductor materials, for instance, organic materials such as inkjet, laser, and thermal evaporation (Kwon 2013). At present, AMOLED displays for mobile applications are mass produced by thermally evaporating organic materials through metal shadow mask (50 μm thick) under high vacuum conditions (10⁻⁷ to 10⁻¹⁰ torr) (Lih et al. 2006). However, when one light-emissive organic material is being deposited using a thermal evaporation system, the other color areas are precisely covered by the metal shadow mask. The schematic of the fabrication process of TEOLED including red, blue, and green subpixels patterning through a shadow mask with a linear source is shown in Fig. 9.

Fig. 9

Schematic of fabrication process of TEOLED with RGB subpixels using shadow mask and a linear source

Usually, the fabrication of full-color RGB TEOLED is performed by the sequential deposition of small molecules onto the ITO patterned (or reflective anode) glass substrates through open and closed shadow mask technique. In particular, the p-doped and pristine HTL, ETL, and optically transparent capping layer are successively deposited using thermal evaporation under high vacuum conditions through an open mask with a linear source. Similarly, the side-by-side red, green, and blue emissive unit patterns and additional HTLs (R′ and G′) are formed using FMM. It is noted that the metal mask should be situated near the glass surface to remove the shadow effect.

The thermal deposition of organic materials (for RGB pixels) through FMM with point source or linear source is a standard process for the manufacturing of AMOLED displays. However, the development of FMM with adequate accuracy is a critical and expensive process. Indeed, FMM is developed with a small aperture by utilizing lithography and chemical etching or electroforming or laser process on invar steel or stainless steel (Kumar et al. 2015; Heo et al. 2015). To form a high-quality pattern on invar steel, lithography combined with a two-step chemical etching process from the top and bottom surfaces of the invar steel can be performed. Figure 10 displays the cross-section of standard FMM after the developing and etching process. The opened pixel lines by the chemical etching process acquired with FMM can have a variation of about ±10 μm due to etching non-uniformity. But, this variation is very high compared to the other existing active matrix display technologies. To attain high quality in pixel patterning, few issues associated with the FMM need to be considered. The most important concern is the mask sagging, which is caused by the vast size, large thickness, and material properties of the mask. However, this problem can be avoided by incorporating a mask tension process. Besides, the other major issues related to the FMM and high pixel patterning are shadow effect (produced by mask thickness), total pitch deviation (generated by mask tension), and misalignment between FMM and device substrate. Indeed, these issues can create a dead space in the device area. Thus, the aperture ratio of the desired subpixels can be reduced. Moreover, further variations in the pixel window can be attained owing to the successive depositions of the organic materials; therefore, regular cleaning of FMM is needed to prevent such effects. Smooth and watchful mask cleaning is also required to inhibit any damage to the mask.

Fig. 10

Cross-sectional view of the opening area of shadow mask after one and two etching steps

Though the conventional FMM is a well-developed technology for the manufacturing of AMOLED displays, the development of high-resolution active matrix organic display is a challenging task owing to the expensive and complicated fabrication process of the high accuracy shadow mask. Undeniably, the resolution of AMOLED display is associated with the density and size of the pixels. Hence, a high-resolution AMOLED display can be realized by employing conventional shadow mask technology and increasing the density of pixels as well as incorporating new arrangements of RGB subpixels. Normally, the RGB stripe-type and RGB dot-type pixel configurations are already adapted in the commercialized AMOLED displays (Chen and Kwok 2012). The RGB subpixel arrangements are presented in Fig. 11. In recent times, pentile and diamond-type subpixel layouts are incorporated in the galaxy mobile displays. These subpixel layouts not only provide a high resolution but also show low power consumption. Generally, the pentile pixel pattern shows RGBG order with the small size of the green subpixel due to the sensitivity of the human eye to the green color. This pixel layout has been revealed an excellent brightness and resolution in the Samsung galaxy mobile display with enhanced lifetime yield for blue color. Likewise, the diamond pixel pattern presented in Fig. 11 indicates a very high resolution and brightness. This subpixel arrangement is very advantageous in terms of low power consumption and high resolution due to the wide space between two subpixels. To develop RGB TEOLED with extremely high accuracy, FMM should have an aperture (opening area) of less than 20 μm. Nevertheless, it is difficult to construct a shadow mask with an opening area of smaller than 20 μm including high yield because of several limitations, for instance, thickness, the distance between adjacent opening areas, and other major mask effects.

Fig. 11

RGB subpixel layout with different patterns like stripe, pentile, and diamond (Photo courtesy of Metalgrass LTD)

By considering the continuous improvement in the OLED technology, resolution and color quality including lifetime and operating performances of mobile displays are largely improved. Figure 12 shows the images of commercialized Samsung mobile display followed by its specifications. The table clearly demonstrates the advancement in the pixel pattern, resolution, and brightness of the mobile display. The resolution of Samsung mobile displays has been enhanced from 480x800 (Galaxy S) to 1440x2560 (Galaxy S7 edge) and brightness values from 250 cd/m² to 500 cd/m² with respect to the advancement in the pixel type, pixel area, as well as other device technologies.

Fig. 12

Samsung Galaxy S1-S7 mobile phone models made using FMM technology (Photo courtesy of Samsung Display)

3.2 Small Mask Scanning (SMS) Pixel Patterning for TV Applications

Existing shadow mask technology is well-developed and does not shows any severe issues when used on up to half-cut size of Generation 6 substrates. Nevertheless, this technology cannot be utilized for large-size glass substrates (Generation 8 substrates) because of the serious concern of the metal mask and glass substrate sagging. To inhibit this problem, Samsung has designed and established a new process technology entitled as SMS for large-area display manufacturing, especially TV applications. In this process technology, the only glass substrate moves during each pixel deposition, while both the FMM with the small opening area and the linear evaporation sources are fixed. The schematic of the full deposition process using the SMS method and the photograph of Samsung’s 55” TV developed using this method is shown in Fig. 13. The important advantage of this process is that essentially the same organic materials can be utilized for mobile as well as TV applications. Therefore, good synergy exists between the two applications. Besides, this technique also provides the benefits of low power consumption, long lifetime, and good color purity characteristics of the RGB side-by-side pixel configuration (Kwon 2013). However, a strong drawback of this technology is low pattern accuracy. Indeed, perfect pattern accuracy is more difficult for larger substrates. More specifically, any pattern misalignment could lead to color mixing and nonuniformity of the subpixels. Mask window variations with multiple patterning processes may also produce a serious issue in real mass production.

Fig. 13

Schematic illustration of SMS patterning process and Samsung 55” OLED TV (Photo courtesy of Samsung Display) fabricated using this process

The current AMOLED technology is utilizing large-size glass substrates, so called “Generation 8” for TV applications. After building low-temperature polysilicon (LTPS) thin-film transistor (TFT) backplane substrate, it is divided into six equal separate parts for OLED manufacturing process. Therefore, only 55” AMOLED is available with this current process. The Samsung OLED TV with this process technology using Generation 8 substrates demonstrates a very good resolution (1920x1080) and brightness (400 cd/m²) value. Further improvement in this patterning process is desired to upsurge the manufacturing yield as well as to advance scalability in substrate size and resolution.

3.3 Pixel Pattering Technique for White OLEDs Coupled with Color Filters

Recently, WOLED with color filters has received a lot of attention owing to its facile process and the associated advantage of utilizing the existing color filters. This method can simply prevent the main difficulty of RGB side-by-side patterning using the shadow mask. In this approach, the WOLED light has filtered through the RGB color filters. The schematic of the WOLED fabrication process and RGBW pixel layout structure is shown in Fig. 14. Similarly, this technique does not incorporate the FMM for the manufacturing of WOLED but uses an open mask system and linear source. Therefore, it can prevent the sagging of FMM. The important drawback of this approach (WOLED+ CF) is the low light output efficiency, which is reduced to 30% of the efficiency of RGB side-by-side method because of the light absorption by the color filter dyes (Fig. 15). Generally, very high-power consumption and a short lifetime can be an actual issue in the WOLED+CF-incorporated AMOLED display. Hence, the proper scheme is needed to apply to solve the issue of lifetime, power consumption, and color characteristics.

Fig. 14

Schematic of WOLED fabrication process using vacuum deposition with linear source and the representation of WOLED+CF to form RGBW subpixels

Fig. 15

Schematic of RGB and RGBW subpixels using WOLED

Theoretically the RGBW approach can shows about ~50% light output because of the mixing of white colors for bright images and thus compensate this for the light loss (Fig. 15). However, there is a trade-off between the reduction of power consumption and the degradation of color purity. Normally, AMOLED display with WRGB four-subpixel pattern can reduce the power consumption, and it consumes 40% less energy compared to the RGB side-by-side pixel pattern. The schematic of WOLED with color filters and output light is shown in Fig. 15. In the case of RGBW approach, the unfiltered white subpixel dominates the total emission of all subpixels because the majority of displayed colors are closely located to the Planckian locus. Consequently, a cool white emission with CIE color coordinates of (0.31~0.33, 0.31~0.33) is desired that additionally shows a stable angular dependence.

3.4 Device Structure of White OLED: Perspectives of Large OLED Displays

Generally, WOLEDs have a multilayer structure with two or more emitting layers in a single stack or tandem configurations. Figure 16 shows the different structures of WOLED such as single-EML, multiple-EML, and tandem OLEDs. Among them, the simplest form of generating white light is a single-EML light-emitting structure, where, multiple dopants such as red, green, and blue doped in the single host system (Wu et al. 2013; Koo et al. 2013) (Fig. 16). It is a simple and cost-effective technique for white light emission. However, the main drawbacks of this system are low efficiency, poor lifetime, and color controlling ability. Because of these issues, a single-EML structure is not well suited for the large-size display applications. On the other hand, in the multiple-EML structures, host-dopant systems for blue and yellow or green/red EML are successively stacked on each other to emit white light (Zhang et al. 2016). This structure provides the benefits of good efficiency and color control than that of the single-EML structure (Fig. 16). But, it leads to an issue of unbalanced energy transfer at the interface of two adjacent light-emitting layers or emission zone movement during operation. Hence, it creates a variation in the color spectrum during operation. Among the available WOLED structures, the tandem structure has various merits in terms of longer lifetime, higher efficiency, and good color stability although it normally requires a relatively high operating voltage and complicated manufacturing process (Hatwar et al. 2010; Liao et al. 2008; Tyan et al. 2009; Liu et al. 2013). In a tandem structure, multilayer OLED units are interconnected in series using the charge generation layer (CGL) (Kim et al. 2013a, b; Son et al. 2014). The main function of the CGL is to generate low-ohmic resistance between EMLs as well as to produce charge carriers. The tandem device architecture containing two light emission units is shown in Fig. 16. Most importantly, this structure can decrease the current level required to attain a target brightness, in principle by half. In this way, external quantum efficiency can be doubled at the expense of increased voltage. Likewise, power efficiency remains the same, but lifetime can be improved significantly compared to a conventional homo-junction device. Relatively good color stability can also be achieved in the tandem structure as long as each subunit is designed to have a good color stability. For these reasons, most of the companies are adopting the approach of hybrid tandem structure.

Fig. 16

Comparison of single-EML, multiple-EML, and tandem structures for the white light emission with their characteristics

Particularly, multi-stack tandem WOLED architecture is realized by interconnecting two or more OLED units of a single EML via CGL. This structure enables a fine-tuning of cavity length as well as charge carrier recombination zones. Normally, multi-stack hybrid WOLED structures are realized by using blue fluorescent and yellow or green/red phosphorescent layers (Hatwar et al. 2010). Figure 17 shows schematic structures of 2-stack and 3-stack tandem WOLEDs. Typically, fluorescent light-emitting materials have low quantum efficiency than those of the phosphorescent materials. However, highly stable and high-performance phosphorescent blue light-emitting materials are not commercially available. Hence, fluorescent blue material can be applied in multi-stack WOLED to ensure long lifetime driving characteristics. In this case, it is difficult to implement a cool white color coordinates with a high light intensity because of low-efficiency fluorescent blue and high-efficiency phosphorescent yellow dopants. Therefore, some sacrifices of yellow emission intensity are required. Indeed, a 2-stack structure with 3-peak spectrum produces 3-peak white color, but it has an issue of energy transfer due to the adjacent green- and red-emitting materials. To use red and green dopants together, less than 0.2% red doping concentration is needed to prevent the whole energy transfer from green to red. It is difficult to control such a small doping concentration in real production. Besides, color stability is indeed poor owing to the unbalanced red and green emissions during device operation. Therefore 2-stack structure with a 3-peak spectrum is not ideal for mass production. On the other hand, 2-peak 2-stack structure consists of phosphorescent yellow and fluorescent blue OLED units. It provides excellent quantum efficiency as well as good color stability (Cho et al. 2015; Lee et al. 2008). However, in the display application, this may generate an additional loss of light because yellow emission cannot give high transmittance for red and green color filters. Although there is some efficiency decreased in the 2-peak 2-stack (blue-yellow) structure through reducing yellow emission peak to match cool white color, this structure is one of the possible structures for mass production of large-size OLED display.

Fig. 17

Schematic of 2-stack and 3-stack tandem WOLED structures with their main characteristics

Alternatively, a 2-peak 3-stack tandem WOLED structure is very efficient for the cool white light and produces excellent efficiency (Kim et al. 2013a, b). The main disadvantage of this 3-stack architecture is a higher driving voltage and a complicated manufacturing process (Fig. 18). The number of deposition steps increases with the number of required layers. Additionally, the required amount of material is significantly higher compared with the 2-peak 2-stack design. Both production time and amount of material are the major factors that contribute to the final cost of the device and must be considered for an industrial realization.

Fig. 18

Schematic of 2-stack (blue-yellow) and 3-stack (blue-blue-yellow) tandem WOLEDs

In the 2-stack tandem structure (blue-yellow), the top phosphorescent yellow (or orange) unit matches well with the blue emission and successfully generates white light. However, the fluorescent blue emission system is a performance decisive component in the tandem especially, color gamut, lifetime, and efficiency. In order to fabricate 2-stack and 3-stack WOLEDs with a maximum external quantum efficiency, the emission from the blue unit needs to be as high as possible to balance the intensity ratio of the blue and yellow emissions. Indeed, yellow emission is more dominant because of its phosphorescent character. Hence, two blue emission systems in 3-stack WOLED could be ideal for balancing the intensity ratio of blue and yellow as well as for performance improvement (especially lifetime and efficiency). The ideal structure for 2-stack and 3-stack tandem WOLED is shown in Fig. 18. Another important and critical part of 2-stack or 3-stack tandem structure manufacturing is the CGL, which used to interconnect fluorescent blue and phosphorescent yellow emissive units. The charge generation material with low lowest unoccupied molecular orbital (LUMO) can be used to inject electrons into the n-type doped ETL layer. On the other hand, the highest occupied molecular orbital (HOMO) of HTL should properly match with the LUMO of charge generation material, so that the holes can easily drift toward the cathode of the device or into the next emissive unit (herein, phosphorescent yellow EML).

Figure 19 shows photographs of the 55″ and 77” OLED TV by incorporating 2- and 3-stack WOLED devices, which comprises fluorescent blue unit(s) and one phosphorescent yellow-green unit. LG 55” TV demonstrated an excellent high-definition resolution of 1920x1080 and a high peak brightness of more than 400 cd/m², which is also comparable to the Samsung 55” OLED TV. On the other hand, LG 77″ curved OLED TV has a resolution of UHD (3480x2160) and brightness of 150 (full white)/450 cd/m²(peak).

Fig. 19

Photographs of the 55-inch and 77-inch OLED TV manufactured by LG (Photo courtesy of LG Display)