Liquid Crystal Display (LCD) screens are a staple in the digital display marketplace and are used in display applications across every industry. With every display application presenting a unique set of requirements, the selection of specialized LCDs has grown to meet these demands.
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LCD screens can be grouped into three categories: TN (twisted nematic), IPS (in-plane switching), and VA (Vertical Alignment). Each of these screen types has its own unique qualities, almost all of them having to do with how images appear across the various screen types.
Related: OLED vs LCD
It's worth noting that although these screen types belong to the LCD screen type, they use thin-film-transistor ( TFT) technology which is a variant of the standard LCD screen type.
The main features that differentiate LCD screen types are brightness, viewing angles, color, and contrast.
TN vs VA vs IPS display comparison.This technology consists of nematic liquid crystal sandwiched between two plates of glass. When power is applied to the electrodes, the liquid crystals twist 90°. TN (Twisted Nematic) LCDs are the most common LCD screen type. They offer full-color images, and moderate viewing angles.
TN LCDs maintain a dedicated user base despite other screen types growing in popularity due to some unique key features that TN display offer. For one, TN LCDs have faster response times and refresh rates than other TFT LCDs.
TN TFTs remain very popular among competitive PC gaming communities, where accuracy and response rates can make the difference between winning and losing.
Refresh rates and response times refer to the time it takes pixels to activate and deactivate in response to user inputs; this is crucial for fast-moving images or graphics that must update as fast as possible with extreme precision.
TN displays remain popular due to its reliable performance and cost-effective price point.
Twisted nematic screens traditionally have been the most cost effect LCD option.
TN LCD screens have the highest refresh rates and response times.
TN LCD screens have average viewing angles of 45-65 degrees.
TN LCD screens are not bright enough for outdoor or direct sunlight viewing.
VA, also known as Multi-Domain Vertical Alignment (MVA) dislays offer features found in both TN and IPS screens. The Pixels in VA displays align vertically to the glass substrate when voltage is applied, allowing light to pass through.
Displays with VA screens deliver wide viewing angles, high contrast, and good color reproduction. They maintain high response rates similar to TN TFTs but may not reach the same sunlight readable brightness levels as comparable TN or IPS LCDs. VA displays are generally best for applications that need to be viewed from multiple angles, like digital signage in a commercial setting.
VA screens offer wider viewing angles than TN LCDs.
VA LCD screens have improved color and contrast compared to TN TFTs.
VA LCD screens tend to offer a lower brightness than an equivalent TN model TFT.
Sunlight Readable LCDs can consume more energy than standard LCD screens.
IPS (In-Plane Switching) technology improves image quality by acting on the liquid crystal inside the display screen. When voltage is applied, the crystals rotate parallel (or &#;in-plane&#;) rather than upright to allow light to pass through. This behavior results in several significant improvements to the image quality of these screens.
Related: What is an IPS display?
IPS outperforms TN displays in every major category.
IPS is superior in contrast, brightness, viewing angles, and color representation compared to TN screens. Images on screen retain their quality without becoming washed out or distorted, no matter what angle they&#;re viewed from. Because of this, viewers have the flexibility to view content on the screen from almost anywhere rather than having to look at the display from a front-center position.
IPS makes it possible to get colorful, accurate, and sharp images viewed from almost any angle.
IPS displays offer a slightly lower refresh rate than TN displays. Remember that the time for pixels to go from inactive to active is measured in milliseconds. So for most users, the difference in refresh rates will go unnoticed.
IPS displays are now more cost effective comparable to TN LCDs.
IPS screens have slower refresh rates and response times than TN LCD screens.
IPS LCD screens have the widest viewing angles of any TFT LCDs.
IPS LCD screens produce the most accurate, vivid colors of any TFT LCDs.
IPS LCD screens have high brightness backlights for sunlight readable environments.
Based on current trends, IPS and TN screen types will be expected to remain the dominant formats for some time. As human interface display technology advances and new product designs are developed, customers will likely choose IPS LCDs to replace the similarly priced TN LCDs for their new projects.
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To evaluate the performance of display devices, several metrics are commonly used, such as response time, CR, color gamut, panel flexibility, viewing angle, resolution density, peak brightness, lifetime, among others. Here we compare LCD and OLED devices based on these metrics one by one.
A fast response time helps to mitigate motion image blur and boost the optical efficiency, but this statement is only qualitatively correct. When quantifying the visual performance of a moving object, motion picture response time (MPRT) is more representative, and the following equation should be used53, 54, 55, 56, 57, 58:
where Tf is the frame time (e.g., Tf=16.67&#;ms for 60&#;fps). Using this equation, we can easily obtain an MPRT as long as the LC response time and TFT frame rate are known. The results are plotted in Figure 5.
Figure 5Calculated MPRT as a function of the LC (or OLED) response time at different frame rates.
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From Figure 5, we can gain several important physical insights: (1) Increasing the frame rate is a simple approach to suppress image motion blur, but its improvement gradually saturates. For example, if the LC response time is 10&#;ms, then increasing the frame rate from 30 to 60&#;fps would significantly reduce the MPRT. However, as the TFT frame rate continues to increase to 120 and 240&#;fps, then the improvement gradually saturates. (2) At a given frame rate, say 120&#;fps, as the LC response time decreases, the MPRT decreases almost linearly and then saturates. This means that the MPRT is mainly determined by the TFT frame rate once the LC response time is fast enough, i.e., τ&#;Tf. Under such conditions, Equation (1) is reduced to MPRT&#;0.8Tf. (3) When the LC response is <2&#;ms, its MPRT is comparable to that of an OLED at the same frame rate, e.g., 120&#;fps. Here we assume the OLED&#;s response time is 0.
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The last finding is somehow counter to the intuition that a LCD should have a more severe motion picture image blur, as its response time is approximately × slower than that of an OLED (ms vs. μs). To validate this prediction, Chen et al.58 performed an experiment using an ultra-low viscosity LC mixture in a commercial VA test cell. The measured average gray-to-gray response time is 1.29&#;ms by applying a commonly used overdrive and undershoot voltage method. The corresponding average MPRT at 120&#;fps is 6.88&#;ms, while that of an OLED is 6.66&#;ms. These two results are indeed comparable. If the frame rate is doubled to 240&#;fps, both LCDs and OLEDs show a much faster but still similar MPRT values (3.71 vs. 3.34&#;ms). Thus the above finding is confirmed experimentally.
If we want to further suppress image blur to an unnoticeable level (MPRT<2&#;ms), decreasing the duty ratio (for LCDs, this is the on-time ratio of the backlight, called scanning backlight or blinking backlight) is mostly adopted59, 60, 61. However, the tradeoff is reduced brightness. To compensate for the decreased brightness due to the lower duty ratio, we can boost the LED backlight brightness. For OLEDs, we can increase the driving current, but the penalties are a shortened lifetime and efficiency roll-off62, 63, 64.
High CR is a critical requirement for achieving supreme image quality. OLEDs are emissive, so, in theory, their CR could approach infinity to one. However, this is true only under dark ambient conditions. In most cases, ambient light is inevitable. Therefore, for practical applications, a more meaningful parameter, called the ACR, should be considered65, 66, 67, 68:
where Ton (Toff) represents the on-state (off-state) brightness of an LCD or OLED and A is the intensity of reflected light by the display device.
As Figure 6 depicts, there are two types of surface reflections. The first one is from a direct light source, i.e., the sun or a light bulb, denoted as A1. Its reflection is fairly specular, and in practice, we can avoid this reflection (i.e., strong glare from direct sun) by simply adjusting the display position or viewing direction. However, the second reflection, denoted as A2, is quite difficult to avoid. It comes from an extended background light source, such as a clear sky or scattered ceiling light. In our analysis, we mainly focus on the second reflection (A2).
Figure 6Schematic diagram of two types of reflections for an LCD (or OLED).
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To investigate the ACR, we have to clarify the reflectance first. A large TV is often operated by remote control, so touchscreen functionality is not required. As a result, an anti-reflection coating is commonly adopted. Let us assume that the reflectance is 1.2% for both LCD and OLED TVs. For the peak brightness and CR, different TV makers have their own specifications. Here, without losing generality, let us use the following brands as examples for comparison: LCD peak brightness= nits, LCD CR=:1 (Sony 75&#; X940E LCD TV); OLED peak brightness=600 nits, and OLED CR=infinity (Sony 77&#; A1E OLED TV). The obtained ACR for both LCD and OLED TVs is plotted in Figure 7a. As expected, OLEDs have a much higher ACR in the low illuminance region (dark room) but drop sharply as ambient light gets brighter. At 63&#;lux, OLEDs have the same ACR as LCDs. Beyond 63&#;lux, LCDs take over. In many countries, 60&#;lux is the typical lighting condition in a family living room. This implies that LCDs have a higher ACR when the ambient light is brighter than 60&#;lux, such as in office lighting (320&#;500&#;lux) and a living room with the window shades or curtain open. Please note that, in our simulation, we used the real peak brightness of LCDs ( nits) and OLEDs (600 nits). In most cases, the displayed contents could vary from black to white. If we consider a typical 50% average picture level (i.e., 600 nits for LCDs vs. 300 nits for OLEDs), then the crossover point drops to 31&#;lux (not shown here), and LCDs are even more favorable. This is because the on-state brightness plays an important role to the ACR, as Equation (2) shows.
Figure 7Calculated ACR as a function of different ambient light conditions for LCD and OLED TVs. Here we assume that the LCD peak brightness is nits and OLED peak brightness is 600 nits, with a surface reflectance of 1.2% for both the LCD and OLED. (a) LCD CR: :1, OLED CR: infinity; (b) LCD CR: 20 000:1, OLED CR: infinity.
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Recently, an LCD panel with an in-cell polarizer was proposed to decouple the depolarization effect of the LC layer and color filters69. Thus the light leakage was able to be suppressed substantially, leading to a significantly enhanced CR. It has been reported that the CR of a VA LCD could be boosted to 20 000:1. Then we recalculated the ACR, and the results are shown in Figure 7b. Now, the crossover point takes place at 16&#;lux, which continues to favor LCDs.
For mobile displays, such as smartphones, touch functionality is required. Thus the outer surface is often subject to fingerprints, grease and other contaminants. Therefore, only a simple grade AR coating is used, and the total surface reflectance amounts to ~4.4%. Let us use the FFS LCD as an example for comparison with an OLED. The following parameters are used in our simulations: the LCD peak brightness is 600 nits and CR is :1, while the OLED peak brightness is 500 nits and CR is infinity. Figure 8a depicts the calculated results, where the intersection occurs at 107&#;lux, which corresponds to a very dark overcast day. If the newly proposed structure with an in-cell polarizer is used, the FFS LCD could attain a :1 CR69. In that case, the intersection is decreased to 72&#;lux (Figure 8b), corresponding to an office building hallway or restroom lighting. For reference, a typical office light is in the range of 320&#;500&#;lux70. As Figure 8 depicts, OLEDs have a superior ACR under dark ambient conditions, but this advantage gradually diminishes as the ambient light increases. This was indeed experimentally confirmed by LG Display71. Display brightness and surface reflection have key roles in the sunlight readability of a display device.
Figure 8Calculated ACR as a function of different ambient light conditions for LCD and OLED smartphones. Reflectance is assumed to be 4.4% for both LCD and OLED. (a) LCD CR: :1, OLED CR: infinity; (b) LCD CR: :1, OLED CR: infinity. (LCD peak brightness: 600 nits; OLED peak brightness: 500 nits).
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Vivid color is another critical requirement of all display devices72. Until now, several color standards have been proposed to evaluate color performance, including sRGB, NTSC, DCI-P3 and Rec. , 74, 75, 76. It is believed that Rec. is the ultimate goal, and its coverage area in color space is the largest, nearly twice as wide as that of sRGB. However, at the present time, only RGB lasers can achieve this goal.
For conventional LCDs employing a WLED backlight, the yellow spectrum generated by YAG (yttrium aluminum garnet) phosphor is too broad to become highly saturated RGB primary colors, as shown in Figure 9a77. As a result, the color gamut is only ~50% Rec. . To improve the color gamut, more advanced backlight units have been developed, as summarized in Table 2. The first choice is the RG-phosphor-converted WLED78, 79. From Figure 9b, the red and green emission spectra are well separated; still, the green spectrum (generated by β-sialon:Eu2+ phosphor) is fairly broad and red spectrum (generated by K2SiF6:Mn4+ (potassium silicofluoride, KSF) phosphor) is not deep enough, leading to 70%&#;80% Rec. , depending on the color filters used.
Figure 9Transmission spectra of color filters and emission spectra of (a) YAG WLED, (b) KSF WLED, (c) QDEF and (d) Vivid Color LED. KSF, potassium silicofluoride; QDEF, quantum dot enhancement film; WLED, white light-emitting diode; YAG, yttrium aluminum garnet.
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Table 2 Comparison of different light sources in LCD backlightsFull size table
A QD-enhanced backlight (e.g., quantum dot enhancement film, QDEF) offers another option for a wide color gamut20, 80, 81. QDs exhibit a much narrower bandwidth (FWHM~20&#;30&#;nm) (Figure 9c), so that high purity RGB colors can be realized and a color gamut of ~90% Rec. can be achieved. One safety concern is that some high-performance QDs contain the heavy metal Cd. To be compatible with the restriction of hazardous substances, the maximum cadmium content should be under 100&#;ppm in any consumer electronic product82. Some heavy-metal-free QDs, such as InP, have been developed and used in commercial products83, 84, 85.
Recently, a new LED technology, called the Vivid Color LED, was demonstrated86. Its FWHM is only 10&#;nm (Figure 9d), which leads to an unprecedented color gamut (~98% Rec. ) together with specially designed color filters. Such a color gamut is comparable to that of laser-lit displays but without laser speckles. Moreover, the Vivid Color LED is heavy-metal free and shows good thermal stability. If the efficiency and cost can be further improved, it would be a perfect candidate for an LCD backlight.
The color performance of a RGB OLED is mainly governed by the three independent RGB EMLs. Currently, both deep blue fluorescent OLEDs87 and deep red phosphorescent OLEDs88 have been developed. The corresponding color gamut is >90% Rec. . Apart from material development89, the color gamut of OLEDs could also be enhanced by device optimization. For example, a strong cavity could be formed between a semitransparent and reflective layer. This selects certain emission wavelengths and hence reduces the FWHM90. However, the tradeoff is increased color shift at large viewing angles91.
A color filter array is another effective approach to enhance the color gamut of an OLED. For example, in , AUO demonstrated a 5-inch top-emission OLED panel with 95% Rec. . In this design, so-called symmetric panel stacking with a color filter is employed to generate purer RGB primary colors92. Similarly, SEL developed a tandem white top-emitting OLED with color filters to achieve a high color gamut (96% Rec. ) and high resolution density (664 pixels per inch (ppi) simultaneously93.
As mentioned earlier, TFT LCDs are a fairly mature technology. They can be operated for >10 years without noticeable performance degradation. However, OLEDs are more sensitive to moisture and oxygen than LCDs. Thus their lifetime, especially for blue OLEDs, is still an issue. For mobile displays, this is not a critical issue because the expected usage of a smartphone is approximately 2&#;3 years. However, for large TVs, a lifetime of >30 000&#;h (>10 years) has become the normal expectation for consumers.
Here we focus on two types of lifetime: storage and operational. To enable a 10-year storage lifetime, according to the analysis94, the water vapor permeation rate and oxygen transmission rate for an OLED display should be <1 × 10&#;6&#;g&#;(m2-day)&#;1 and 1 × 10&#;5&#;cm3&#;(m2-day)&#;1, respectively. To achieve these values, organic and/or inorganic thin films have been developed to effectively protect the OLED and lengthen its storage lifetime. Meanwhile, it is compatible to flexible substrates and favors a thinner display profile95, 96, 97.
The next type of lifetime is operational lifetime. Owing to material degradation, OLED luminance will decrease and voltage will increase after long-term driving98. For red, yellow and green phosphorescent OLEDs, their lifetime values at 50% luminance decrease (T50) can be as long as >80 000&#;h with a &#;cd&#;m&#;2 luminance99, 100, 101. Nevertheless, the operational lifetime of the blue phosphor is far from satisfactory. Owing to the long exciton lifetime (~μs) and wide bandgap (~3&#;eV), triplet-polaron annihilation occurs in the blue phosphorescent OLED, which generates hot polarons (~6&#;eV; this energy is higher than some bond energies, e.g., 3.04&#;eV for the C-N single bond), leading to a short lifetime. To improve its lifetime, several approaches have been proposed, such as designing a suitable device structure to broaden the recombination zone, stacking two or three OLEDs or introducing an exciton quenching layer. The operation lifetime of a blue phosphorescent OLED can be improved to &#;h (T50, half lifetime) with an initial luminance of nits. However, this is still ~20 × shorter than that of red and green phosphorescent OLEDs101, 102, 103.
To further enhance the lifetime of the blue OLED, the NTU group has developed new ETL and TTF-EML materials together with an optimized layer structure and double EML structure104. Figure 10a shows the luminance decay curves of such a blue OLED under different initial luminance values (, 10 000, and 15 000 nits). From Figure 10b, the estimated T50 at nits of this blue OLED is ~56 000&#;h (~6&#;7 years)104, 105. As new materials and novel device structures continue to advance, the lifetime of OLEDs will be gradually improved.
Figure 10(a) Luminance decay curves for the blue OLED with different initial luminance values. (b) Estimated T50 under different initial luminance values.
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Power consumption is equally important as other metrics. For LCDs, power consumption consists of two parts: the backlight and driving electronics. The ratio between these two depends on the display size and resolution density. For a 55&#; 4K LCD TV, the backlight occupies approximately 90% of the total power consumption. To make full use of the backlight, a dual brightness enhancement film is commonly embedded to recycle mismatched polarized light106. The total efficiency could be improved by ~60%.
The power efficiency of an OLED is generally limited by the extraction efficiency (ηext~20%). To improve the power efficiency, multiple approaches can be used, such as a microlens array, a corrugated structure with a high refractive index substrate107, replacing the metal electrode (such as the Al cathode) with a transparent metal oxide108, increasing the distance from the emission dipole to the metal electrode109 or increasing the carrier concentration by material optimizations110. Experimentally, external quantum efficiencies as high as 63% have been demonstrated107, 108. Note that sometimes the light-extraction techniques result in haze and image blur, which deteriorate the ACR and display sharpness111, 112, 113. Additionally, fabrication complexity and production yield are two additional concerns. Figure 11 shows the power efficiencies of white, green, red and blue phosphorescent as well as blue fluorescent/TTF OLEDs over time. For OLEDs with fluorescent emitters in the s and s, the power efficiency was limited by the IQE, typically <10&#;lm&#;W&#;1(Refs. 41, 114, 115, 116, 117, 118). With the incorporation of phosphorescent emitters in the ~&#;s, the power efficiency was significantly improved owing to the materials and device engineering45, 119, 120, 121, 122, 123, 124, 125. The photonic design of OLEDs with regard to the light extraction efficiency was taken into consideration for further enchantment of the power efficiency126, 127, 128, 129, 130. For a green OLED, a power efficiency of 290&#;lm&#;W&#;1 was demonstrated in (Ref. 127), which showed a >100 × improvement compared with that of the basic two-layer device proposed in (1.5&#;lm&#;W&#;1 in Ref. 41). A white OLED with a power efficiency >100&#;lm&#;W&#;1 was also demonstrated, which was comparable to the power efficiency of a LCD backlight. For red and blue OLEDs, their power efficiencies are generally lower than that of the green OLED due to their lower photopic sensitivity function, and there is a tradeoff between color saturation and power efficiency. Note, we separated the performances of blue phosphorescent and fluorescent/TTF OLEDs. For the blue phosphorescent OLEDs, although the power efficiency can be as high as ~80&#;lm&#;W&#;1, the operation lifetime is short and color is sky-blue. For display applications, the blue TTF OLED is the favored choice, with an acceptable lifetime and color but a much lower power efficiency (16&#;lm&#;W&#;1) than its phosphorescent counterpart131, 132. Overall, over the past three decades (&#;), the power efficiency of OLEDs has improved dramatically, as Figure 11 shows.
To compare the power consumption of LCDs and OLEDs with the same resolution density, the displayed contents should be considered as well. In general, OLEDs are more efficient than LCDs for displaying dark images because black pixels consume little power for an emissive display, while LCDs are more efficient than OLEDs at displaying bright images. Currently, a ~65% average picture level is the intersection point between RGB OLEDs and LCDs134. For color-filter-based white OLED TVs, this intersection point drops to ~30%. As both technologies continue to advance, the crossover point will undoubtedly change with time.
Flexible displays have a long history and have been attempted by many companies, but this technology has only recently begun to see commercial implementations for consumer electronics135. A good example is Samsung&#;s flagship smartphone, the Galaxy S series, which has an OLED display panel that covers the edge of the . However, strictly speaking, it is a curved display rather than a flexible display. One step forward, a foldable AM-OLED has been demonstrated with the curvature radius of 2&#;mm for 100 000 repeated folds136. Owing to the superior flexibility of the organic materials, a rollable AM-OLED display driven by an organic TFT was fabricated137. By replacing the brittle indium-tin-oxide with a flexible Ag nanowire as the anode, a stretchable OLED for up to a 120% strain was demonstrated138.
LCDs have limited flexibility. A curved TV is practical but going beyond that is rather difficult with rigid and thick glass substrates139. Fortunately, this obstacle has been removed with the implementation of a thin plastic substrate140, 141, 142. In , a 12.1&#; rollable LCD using organic TFT, called OLCD, was demonstrated, and its radius of curvature is 60&#;mm143. To maintain a uniform cell gap, a polymer wall was formed within the LC layer144. Additionally, it is reported that LCDs could be foldable with a segmented backlight. This is a good choice, but until now, no demo or real device has been demonstrated. Combining two bezel-less LCDs together is another solution to enable a foldable display, but this technology is still under development145.
In addition to the aforementioned six display metrics, other parameters are equally important. For example, high-resolution density has become a standard for all high-end display devices. Currently, LCD is taking the lead in consumer electronic products. Eight-hundred ppi or even >&#;ppi LCDs have already been demonstrated and commercialized, such as in the Sony 5.5&#; 4k Smartphone Xperia Z5 Premium. The resolution of RGB OLEDs is limited by the physical dimension of the fine-pitch shadow mask. To compete with LCDs, most OLED displays use the PenTile RGB subpixel matrix scheme146. The effective resolution density of an RGB OLED mobile display is~500&#;ppi. In the PenTile configuration, the blue subpixel has a larger size than the green and red subpixels. Thus a lower current is needed to achieve the required brightness, which is helpful for improving the lifetime of the blue OLED. On the other hand, owing to the lower efficiency of the blue TTF OLED compared with the red and green phosphorescent ones, this results in higher power consumption. To further enhance the resolution density, multiple approaches have been developed, as will be discussed later.
The viewing angle is another important property that defines the viewing experience at large oblique angles, which is quite critical for multi-viewer applications. OLEDs are self-emissive and have an angular distribution that is much broader than that of LCDs. For instance, at a 30° viewing angle, the OLED brightness only decreases by 30%, whereas the LCD brightness decrease exceeds 50%. To widen an LCD&#;s viewing angle, three options can be used. (1) Remove the brightness-enhancement film in the backlight system. The tradeoff is decreased on-axis brightness147. (2) Use a directional backlight with a front diffuser148, 149. Such a configuration enables excellent image quality regardless of viewing angle; however, image blur induced by a strong diffuser should be carefully treated. (3) Use QD arrays as the color filters20, 150, 151, 152. This design produces an isotropic viewing cone and high-purity RGB colors. However, preventing ambient light excitation of QDs remains a technical challenge20.
In addition to brightness, color, grayscale and the CR also vary with the viewing angle, known as color shift and gamma shift. In these aspects, LCDs and OLEDs have different mechanisms. For LCDs, they are induced by the anisotropic property of the LC material, which could be compensated for with uniaxial or biaxial films5. For OLEDs, they are caused by the cavity effect and color-mixing effect153, 154. With extensive efforts and development, both technologies have fairly mature solutions; currently, color shift and gamma shift have been minimized at large oblique angles.
Cost is another key factor for consumers. LCDs have been the topic of extensive investigation and investment, whereas OLED technology is emerging and its fabrication yield and capability are still far behind LCDs. As a result, the price of OLEDs is about twice as high as that of LCDs, especially for large displays. As more investment is made in OLEDs and more advanced fabrication technology is developed, such as ink-jet printing155, 156, 157, their price should decrease noticeably in the near future.
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