Electrical doping of organic molecular semiconductors.

摘要

Electrical doping is perceived as a key to enhance the performance and versatility of organic molecular devices. Understanding the doping mechanism and the impact of doping on interface electronic structures is very important for better control of the doping.; We show that an efficient p-doping is a result of a good energy match between the host ionization energy and the dopant electron affinity, via a study of the electronic structure of host and dopant materials using direct and inverse photoemission spectroscopies (UPS/IPES). The hole transport materials zinc phthalocyanine (ZnPc) and N,N′-diphenyl-N,N ′-bis(1-naphthyl)-1,1′-biphenyl-4,4 ′-diamine (α-NPD) are used as the host materials, and the strong acceptor material tetrafluorotetracyanoquinodimethane (F₄ -TCNQ) is the p-type dopant. In p-doped films, E_F moves closer to the HOMO, analogous to inorganic semiconductors. The ultimate position of E_F with respect to the HOMO in highly doped film is limited by the large polarization and relaxation in molecular solids, especially in 3-D molecules like α-NPD.; The study of the impact of doping at metal-organic interfaces shows that the interface electronic structure, i.e. interface dipole, ionization energy and E_F-HOMO, is nearly independent of doping, although the bulk E_F-HOMO of the doped film is determined by the dopant concentration. A depletion region is formed at the interface with its width depending on the dopant concentration similarly as metal-inorganic semiconductor interfaces. This narrow space charge region greatly improves hole injection by several orders of magnitude via tunneling.; The impact of doping on the energy alignment at organic-organic heterojunction interfaces is found to be different compared to MO interfaces. Interface dipoles are generally seen upon doping of one organic material at these weakly interacting OO interfaces, and the electron and hole barriers at the interface are correspondingly modified. The interface dipole is found to be of the value that makes the E_F-HOMO in the other undoped organic material fixed. A modified induced density of interface states (IDIS) model is developed and found to give an appropriate understanding of the formation of interface dipole and energy alignment at both MO and OO interfaces.