**6. Conclusions**

In this book chapter, we have discussed a roadmap to include additional functionalities on displays by adding a fourth pixel to the display. The purpose of the fourth pixel in this work was a photodetector pixel to realize an in-panel fingerprint or palmprinting function. Several options have been discussed to enable this roadmap. At first, we have elaborated several techniques to realize higher-resolution frontplane (OLED) and TFT backplanes. For the frontplanes, photolitho patterning of the OLED was introduced as a disruptive technology impacting significantly the frontplane resolution. External compensation techniques for the backplane when driving OLEDs are proposed in this book chapter, resulting in a uniform and higherresolution display by using only 2T1C schemes. As photodetector pixel, we proposed a passive 1 T pixel, with included peripheral circuits to enable a lower system cost.
