[Abstrac]

Organic semiconductors are used in optoelectronic devices, such as organic light-emitting diodes, organic and perovskite solar cells, and organic field-effect transistors. The performance of such devices depends heavily on charge injection and transport. In many cases, organic semiconductors exhibit highly unipolar charge transport, meaning that they predominantly conduct either electrons or holes. A fundamental question is what causes this unipolarity. We demonstrate that an energetic window exists inside which organic semiconductors are not susceptible to charge trapping by water or oxygen, leading to trap-free charge transport of both carriers. The implication for devices such as OLEDs, organic solar cells and organic ambipolar transistors is that the energy levels of the organic semiconductors are ideally situated within this energetic window. However, for blue-emitting OLEDs with a large band gap this poses significant challenge to remove or disable charge traps.