An OLED includes a substrate having a circuit region and a pixel region. At least one circuit Thin Film Transistor (TFT) and at least one pixel TFT are respectively arranged on the circuit region and the pixel region. Each TFT has a semiconductor layer, a gate electrode, a source electrode and a drain electrode. A pixel electrode is electrically connected to one of the source and drain electrodes of the pixel TFT. At least one silicon nitride layer is arranged between the source and drain electrodes and the substrate and is opened in the entire pixel region.