**1. Introduction**

Active matrix OLED (AMOLED) displays are today's mainstream consumer displays available in various form factors, such as smart watches, mobile displays, and large area television. They are highly appealing because of their wide viewing angles, nice color saturation and great potential for curved, flexible and/or rollable format. There are several options for (flexible) backplane technologies based on thin-film transistors (TFTs), namely, metal-oxide TFTs (such as indium gallium zinc oxide or IGZO), low-temperature polycrystalline silicon (LTPS) TFTs, or a combination of IGZO and LTPS, more precisely LTPO or low-temperature polycrystalline silicon and metal oxide. All these technologies have their pros and cons. Among others, IGZO semiconductors are n-type only which is sufficient for a backplane driving an OLED but is less adequate for peripheral circuits. Another key important asset of IGZO is the ultralow source-drain leakage current due to the large bandgap of the semiconductor, enabling long retention times of data storing, i.e., pixels are not leaking. In contrary, LTPS has both n- and p-type devices and thus the capability of CMOS circuits. LTPS transistors can drive larger currents due to the intrinsically higher mobility, enabling complex in-pixel compensation schemes and peripheral circuits. However, with LTPS transistors the leakage will be larger. This is a key reason why LTPO has been developed: this technology combines 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 OLED and driving it in saturation.

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 detector for fingerprint and palmprint readout.
