An active matrix electro-wetting on dielectric (AM-EWOD) device has an optically black array element structure to enhance optical detection of constituents within a liquid droplet. The AM-EWOD device includes a thin film transistor (TFT) substrate assembly having a hydrophobic layer; thin film electronics having a plurality of array elements arranged in an array of rows and columns, each of the array elements including an array element electrode and a TFT device; and an optically black material disposed between a plane of the TFT device and the hydrophobic layer. The TFT substrate assembly further includes a planarization structure that includes a component having the optically black material. The planarization structure has a planarization component disposed between the TFT device and the array element electrode, and an ionic barrier disposed between the array element electrode and the hydrophobic coating. The planarization component or the ionic barrier includes the optically black material.