Organic light-emitting display apparatus

1. An organic light-emitting display apparatus comprising:

a thin film transistor comprising an active layer, a gate electrode, a source electrode, a drain electrode, a first insulating layer between the active layer and the gate electrode, and a second insulating layer between the gate electrode and the source and drain electrodes;

a pad electrode comprising a first pad layer at a same layer as the source electrode and the drain electrode, the first pad layer being at a surface of the second insulating layer facing away from the first insulating layer, and a second pad layer on the first pad layer;

a third insulating layer covering the source electrode and the drain electrode and an end portion of the pad electrode;

a pixel electrode comprising a semi-transmissive electrically conductive layer at an opening in the third insulating layer;

a protection layer between the pixel electrode and the first insulating layer;

a fourth insulating layer having an opening at a location corresponding to the opening formed in the third insulating layer, the fourth insulating layer covering an end portion of the pixel electrode;

an emission layer on the pixel electrode; and

an opposing electrode on the emission layer,

wherein a portion of the third insulating layer is located between the pixel electrode and the protection layer and covering a portion of a top surface of the protection layer in direct contact.

2. The organic light-emitting display apparatus of claim 1 , wherein the protection layer comprises a same material as that of the second pad layer.

3. The organic light-emitting display apparatus of claim 1 , wherein the second pad layer comprises a transparent conductive oxide.

4. The organic light-emitting display apparatus of claim 3 , wherein the transparent conductive oxide comprises one or more selected from the group consisting of indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), indium oxide (In 2 O 3 ), indium gallium oxide (IGO), and aluminum zinc oxide (AZO).

5. The organic light-emitting display apparatus of claim 1 , wherein a thickness of the protection layer is in a range of about 200 Å to about 800 Å.

6. The organic light-emitting display apparatus of claim 1 , wherein the source electrode and the drain electrode each have a stack structure of a plurality of heterogeneous electrically conductive layers having different electron mobility.

7. The organic light-emitting display apparatus of claim 6 , wherein the source electrode and the drain electrode each comprise a layer comprising molybdenum and a layer comprising aluminum.

8. The organic light-emitting display apparatus of claim 1 , wherein the source electrode and the drain electrode each comprise a first metal layer and a second metal layer on the first metal layer.

9. The organic light-emitting display apparatus of claim 1 , further comprising a capacitor comprising a first electrode at a same layer as the active layer and a second electrode at a same layer as the gate electrode.

10. The organic light-emitting display apparatus of claim 9 , wherein the first electrode of the capacitor comprises a semiconductor material doped with ion impurities.

11. The organic light-emitting display apparatus of claim 9 , wherein the second electrode of the capacitor comprises a transparent conductive oxide.

12. The organic light-emitting display apparatus of claim 9 , wherein the capacitor further comprises a third electrode at a same layer as the source electrode and the drain electrode.

13. The organic light-emitting display apparatus of claim 9 , further comprising: a pixel electrode contact unit electrically coupled between the pixel electrode and one of the source electrode and the drain electrode through a contact hole formed in the third insulating layer,

wherein the pixel electrode contact unit comprises:

a first contact layer comprising a same material as that of the source electrode and the drain electrode;

a second contact layer comprising a same material as that of the second pad layer; and

a third contact layer at the first insulating layer and the second insulating layer and comprising a same material as that of the second electrode of the capacitor,

wherein the first contact layer is electrically coupled to the third contact layer through a contact hole formed in the second insulating layer.

14. The organic light-emitting display apparatus of claim 13 , wherein the pixel electrode contact unit further comprises a fourth contact layer between the first insulating layer and the third insulating layer, the fourth contact layer comprising a same material as that of the gate electrode.

15. The organic light-emitting display apparatus of claim 1 , further comprising a capacitor comprising a first electrode at a same layer as the gate electrode and a second electrode at a same layer as the source electrode and the drain electrode.

16. The organic light-emitting display apparatus of claim 15 , wherein the first electrode of the capacitor comprises a same material as that of the gate electrode.

17. The organic light-emitting display apparatus of claim 15 , wherein the second electrode of the capacitor comprises a same material as that of the source electrode and the drain electrode.

18. The organic light-emitting display apparatus of claim 15 , wherein the second insulating layer is between the first electrode and the second electrode and the second electrode is at a trench formed in the second insulating layer.

19. The organic light-emitting display apparatus of claim 1 , wherein the first pad layer comprises a same material as that of the source electrode and the drain electrode.

20. The organic light-emitting display apparatus of claim 1 , wherein the semi-transmissive electrically conductive layer comprises silver (Ag) or a silver alloy.

21. The organic light-emitting display apparatus of claim 11 , wherein a first transparent conductive oxide layer is between the semi-transmissive electrically conductive layer and the protection layer.

22. The organic light-emitting display apparatus of claim 21 , wherein a second transparent conductive oxide layer is at an upper portion of the pixel electrode.

23. The organic light-emitting display apparatus of claim 1 , wherein an opening in the second insulating layer, an opening in the third insulating layer, and an opening in the fourth insulating layer overlap with each other, and

wherein a width of the opening in the third insulating layer is greater than a width of the opening in the fourth insulating layer, and smaller than a width of the opening in the second insulating layer.

24. The organic light-emitting display apparatus of claim 23 , wherein an end portion of the pixel electrode is at a top end of the opening in the third insulating layer.