**3.1. Hole generation at the interfaces of electron donor (CuPc) and acceptor (PTCDA) in OLEDs**

Pairs of organic donors and acceptors have been applied in organic solar cells to realize the efficient photo-to-electricity conversion since 1986 [17]. The underlying mechanism for organic photovoltaic devices is the photoinduced electron transfer from donor to acceptor, generating free holes in donor and electrons in acceptor. The donors and acceptors in organic photovoltaic devices constitute either planar interfaces or mixed interfaces. Recently, the planar donor/acceptor interfaces have been successfully used to realize the ambipolar transport in organic field effect transistors [18, 19]. The recent work shows that the planar and mixed interfaces of donor and acceptor can generate very efficient hole current in OLEDs [5-7].
