**2. Display resolution roadmap for various applications**

Resolution (number of pixels) and pixel pitch (size and spacing of pixels) are two main parameters defining the architecture of the display arrays. The first, expressed typically in megapixels, is standardized by the content type, resulting in different generations of TVs: VGA, full HD, 4K, and, most recently, 8K. The latter, expressed typically in pixels per inch (ppi) or pixels per degree (ppd), is used as a benchmark for smartphones, with high-end models featuring densities in the range of 600 ppi. This is a value that gives a good enough image quality for hand-held devices, with the viewing distance of approximately 30 cm (1 foot). At the same time, future near-to-eye augmented reality/virtual reality (AR/VR) displays impose ultrahigh definition, as the pixel density needs to be beyond the pattern resolving capabilities of the human retina (30 cycles per degree) [1, 2]. The resolution should be maximized to provide the highest possible output within the eye box in any given point of the 180° field of view (FOV), also to enable foveated rendering.

#### **Figure 1.**

*Color-by-white vs. RGB OLED frontplane can be fabricated on top of CMOS or TFT backplane. Each combination is suitable for different applications.*

**125**

**Figure 2.**

*Various display configuration options.*

*AMOLED Displays with In-Pixel Photodetector DOI: http://dx.doi.org/10.5772/intechopen.93016*

An aperture ratio close to unity will eliminate the screen door effect and ensure natural experience. Transparency is necessary to avoid sense of isolation from the real-world view and to diversify from the virtual reality (VR) headsets (**Figure 1**). To realize all of the above, we need both the microdisplay-like pixel pitch downscaling [3] and the flat-panel-display-like (FPD) backplane size up-scaling [4, 5]. Switching to advanced nodes in flat panel backplane manufacturing can result in ultrahigh-definition, direct-view AR displays fabricated in a cost-effective way.

In order to realize an ultrahigh resolution display, all elements of the system (backplane, frontplane, and driving) need to provide appropriate pixel density. On the frontplane side, several options for the light source can be chosen (**Figure 2**). OLED technology currently dominates the smartphone display industry not only with performance but also with the cost structure. In this case, the colors are defined by depositing separate device stacks for each color, which is typically referred to as side-by-side, red-green-blue (RGB) array. In OLED TVs, one common white OLED stack is combined with a color filter array (CFA). The limitation of the side-by-side RGB array is the pixel density, limited by the fine metal masking (FMM) technology, which uses deposition through a metal mesh. The white OLED array can achieve very small pixel pitch, which is only limited by the backplane and CFA resolution but imposes brightness loss due to CF transmission. Patterning multicolor OLEDs by photolithography can address the needs of ultralow pixel pitch for the future AR displays by realizing side-by-side OLED stacks with extreme density. Patterning OLEDs by photolithography is an emerging, disruptive fabrication technique. The main challenge is the extreme chemical sensitivity of OLED materials with solvent, moisture, air, and temperature exposure responsible for performance degradation. The choice of appropriate photolithography chemistry is crucial, with fluorinated [6] or non-fluorinated systems [7] as the dominant options. **Figure 3** shows the concept of using a negative-type photoresist to define patterns on top of OLED in a subtractive approach. First, the OLED stack is deposited as a plain layer over the entire substrate, on top of a pixel definition layer (PDL). This defines the active area of the light emitter. Second, photoresist is deposited on top of the entire substrate. Then, it is exposed through a lithography mask and developed to obtain the required pattern. Afterwards, the OLED layers that are not covered by the photoresist are etched away (typically with dry etching, such as reactive ion etch).

**3. Lithographic patterning of OLEDs to increase resolution**

In the end, the photoresist is stripped to achieve patterned OLED islands.

*AMOLED Displays with In-Pixel Photodetector DOI: http://dx.doi.org/10.5772/intechopen.93016*

*Liquid Crystals and Display Technology*

OLED and driving it in saturation.

detector for fingerprint and palmprint readout.

**2. Display resolution roadmap for various applications**

the ultralow leakage current of IGZO and a p-type LTPS transistor resulting in a hybrid complementary technology. An OLED is an organic LED emitting light directly proportional to its forward current. Therefore it requires a current source as driver in the pixel. In many cases, this is achieved by placing a TFT in series with the

In this chapter, we will investigate the potential to embed additional functionalities in the display. Therefore, several strategies will be discussed focusing on improving the resolution of the current displays, by technology optimization introducing photolithography patterning of the OLED and by design evaluating external compensation vs. internal compensation. The extra space in the pixel, due to the combination of photolithography and simple pixel circuit, provides opportunities to include extra functions at the same original area. The focus in this book chapter is to add a photosensitive

Resolution (number of pixels) and pixel pitch (size and spacing of pixels) are two main parameters defining the architecture of the display arrays. The first, expressed typically in megapixels, is standardized by the content type, resulting in different generations of TVs: VGA, full HD, 4K, and, most recently, 8K. The latter, expressed typically in pixels per inch (ppi) or pixels per degree (ppd), is used as a benchmark for smartphones, with high-end models featuring densities in the range of 600 ppi. This is a value that gives a good enough image quality for hand-held devices, with the viewing distance of approximately 30 cm (1 foot). At the same time, future near-to-eye augmented reality/virtual reality (AR/VR) displays impose ultrahigh definition, as the pixel density needs to be beyond the pattern resolving capabilities of the human retina (30 cycles per degree) [1, 2]. The resolution should be maximized to provide the highest possible output within the eye box in any given point of the 180° field of view (FOV), also to enable foveated rendering.

**124**

**Figure 1.**

*combination is suitable for different applications.*

*Color-by-white vs. RGB OLED frontplane can be fabricated on top of CMOS or TFT backplane. Each* 

An aperture ratio close to unity will eliminate the screen door effect and ensure natural experience. Transparency is necessary to avoid sense of isolation from the real-world view and to diversify from the virtual reality (VR) headsets (**Figure 1**). To realize all of the above, we need both the microdisplay-like pixel pitch downscaling [3] and the flat-panel-display-like (FPD) backplane size up-scaling [4, 5]. Switching to advanced nodes in flat panel backplane manufacturing can result in ultrahigh-definition, direct-view AR displays fabricated in a cost-effective way.
