Interfacial electronic structures between fullerene and calcium for high performance n-type organic semiconducting devices

The electronic structure of fullerene (C60) deposited on calcium (Ca) was investigated using in-situ ultraviolet photoelectron spectroscopy. The energy level alignment at the C60/Ca interface was estimated by combining both shifts of the highest occupied molecular orbital (HOMO) level and of the vacuum level during the step-by-step deposition of C60 on Ca. The HOMO level of C60 shows shifts relating to band-bending, resulting in an electron injection barrier of 0.2 eV with accumulation contact with the Ca substrate. The vacuum level reveals an interface dipole of 1.11 eV with negative poles on the C60 side. Noticeably, gap states are formed at the interface region, which might pin the Fermi level and be responsible for the formation of the interface dipole. The complete interfacial energy level diagram of C60/Ca is presented.