A thin film transistor (TFT) array substrate for an X-ray detector that improves a fill factor is disclosed. According to one aspect, the substrate includes a plurality of pixel areas each including a transistor area in which a TFT is formed, and a photodiode area in which a photodiode is formed. A first wire is formed in a first layer disposed in a lower portion of a photodiode layer in which the photodiode is formed, in at least a portion of the transistor area of the photodiode layer, and in a second layer disposed in an upper portion of the photodiode layer. A second wire, insulated from the first wire, extends in the pixel areas and is formed in the second layer. At least one TFT is formed in the transistor area and electrically connected to at least one of the first and second wires.